Note: Descriptions are shown in the official language in which they were submitted.
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AN UPCONVERSION ACTIVE GAIN MEDIUM
AND A MICRO-LASER ON THE BASIS THEREOF
FIELD
The present invention relates generally to
upconversion solid-state lasers for producing blue or
ultraviolet lasing radiation from infrared pumping radiation,
and more specifically to an upconversion active gain medium
being made of a rare earth doped crystal host capable of
producing the blue or ultraviolet lasing radiation under
normal ambient temperature operating conditions as a result of
absorbing continuous or quasi-continuous single band infrared
pumping radiation, and to an upconversion micro-laser on the
basis of such active gain medium.
The present invention can be used for a variety of
applications in the fields of optical data storage and
computer CDROM (Compact Disk Read Only Memory), red-green-blue
multicolored display systems and optical information
processing. In these applications the higher resolutions
afforded by short optical wavelengths are important, the
continuous wave operation of visible or ultraviolet lasers at
room temperature and small sizes that the lasers can achieve
are crucial, and the simplicity of alignment or any other
adjustment of the packaged arrangement are highly desirable.
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2
BACKGROUND
Upconversion or excited state absorption based
conversion of pumping radiation to radiation that has a
shorter wavelength than the wavelength of the pumping
radiation has been known for many years.
The alternative conversion mechanism - nonlinear
wavelength conversion (frequency doubling, for example) that
employs nonlinear frequency conversion crystals requires
critical alignment and high maintenance of the lasers
realizing this mechanism. In order for doubling (or any
nonlinear conversion process) to be effected in each laser,
the optical axes of the laser's nonlinear crystal must be
precisely oriented at the phase-matching angle determined with
respect to the axis of the radiation beam to be converted and
polarization directions. These restrictions hinder
development of the nonlinear wavelength conversion mechanism
for applications such as optical data storage and computer
CDROM (Compact Disk Read Only Memory), red-green-blue
multicolored display systems and optical information
processing (the Applications). Upconversion lasers have no
similar restrictions.
Since the first demonstration of an upconversion
laser more than 25 years ago, a variety of rare earth-doped
crystal and glass hosts have been proposed to produce lasing
radiation in the red, green, blue and ultraviolet wavelengths.
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3
The peculiarities in producing blue or ultraviolet
lasing radiation are connected with the greater photon energy
in these wavelength ranges. This production makes use of a
rare earth ion ground state as a low state in a lasing
transition or a rare earth ion excited state as an upper state
in the transition. In both cases problems arise because, for
the specified host and specified pumping scheme, a population
inversion in the rare earth states can only be achieved in
continuous operation by increasing pumping radiation
intensities or by cooling the host.
Hitherto, the above problems were solved for a glass
host, having low thermal conductivity and unordered rare earth
ions in the host, by using a long length optical fiber with
low doping levels. The small core radius of the optical fiber
allows high pumping radiation intensities with modest pumping
powers. The guiding properties of the optical fiber allow a
good confinement of the pumping radiation giving efficient
excitation over its entire length and, therefore, minimizing
thermal problems. Thus, the continuous wave upconversion
lasing operation at one or more wavelengths in the blue or
ultraviolet regions has been achieved on a practical basis at
room temperature, using glass fiber.
The optical fibers appropriate for upconversion
lasers are usually fabricated from glasses with low phonon
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4
energy such as fluoride glasses. Booth et al. in his
publication entitled "Operation of Diode Pumped Tm3~ ZBLAN
Upconversion Fiber Laser at 482 nm"(IEEE Journal of Quantum
Electronics, Vo1.32, N 1, January 1996, p.118-123) disclosed a
continuous wave room-temperature operation at 482 nm of a Tm''-
doped fluorozirconate (ZBLAN) glass fiber laser. This laser
pumped with single-mode semiconductor diode lasers operated in
the 1110-1150 nm wavelength range. The fiber had a core
diameter of 3 Vim. The slope efficiency (versus launched
pumping power) of 13.8 was obtained in a 38 cm length of
fiber laser with 2,500 ppm-doping. The measurements indicated
that shorter, more heavily doped fiber has a higher output
because of the reduced effect of the background loss. From
the practical viewpoint this means that less fiber is required
for a more heavily doped fiber laser to achieve a desired
output power.
Fiber length is still the main restriction in
developing compact rare earth doped glass fiber lasers.
Further reducing fiber length by means of increasing its
doping levels is limited due to the increased influence of
rare earth pair interactions. The latter effect, which is
called photo-darkening, reduces the population of rare earth
ions excited states associated with lasing transition and,
therefore, the fiber laser output.
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In contrast to the glass fibers, crystal hosts are
much smaller in size, typically no more than several
millimeters in the direction of the lasing radiation. This
size reduction of crystal hosts is due to their higher thermal
conductivity and their capability of accepting more rare earth
ions in their crystal structures. The latter properties
facilitate the large separation of the rare earth active ions
in order to reduce non-radiative relaxation of their excited
states via the rare earth pair interactions. Crystal hosts
also have the advantages of high melting points (about 1000 to
1600 centigrade degrees for fluorides) and stability in an
ambient environment that is very important for commercial
Applications.
However, when rare earth crystals are used, there is
a problem that the temperatures of the upconversion laser
operation are much lower than room temperature. MacFarlane et
al. (Applied Physics Letters, vo1.52, No. l6, pp. 1300-
1302,1988) disclosed an upconversion laser operation at 380 nm
at temperatures up to 90°K in an Nd3'doped LaF3crystal pumped
simultaneously at 591nm and 788 nm. YLiF4:Tm''pumped at 649 nm
and 781 nm has produced a pulsed output at 450 nm. A fluoride
erbium doped bulk crystal has produced blue continuous laser
operation at 470 nm when mounted in a helium cryostat with
operating temperature variation between 15°K and 120°K (US
Patent No. 5,008,890 and Canadian Patent No. 2,040,557).
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6
Thrash et al. described rare earth ion upconversion
laser systems in patents US 5,299,215 and US 5,488,624, 1996.
Barium yttrium fluoride Tm:Yb codoped crystal hosts have
produced a quasi-continuous blue lasing radiation at a
wavelength of approximately 455 nm utilizing the upconversion
laser transition 1D2 -~ 3H4 . The output of the pumping source,
typically comprising one or more laser diodes, was made quasi-
continuous, such as with a chopper wheel, to prevent crystal
hosts from overheating. Thrash et al, in his article entitled
"Upconversion laser emission from Yb3'- sensitized Tm3' in
BaY2Fe" (Journal of the Optical Society of America Bulletin,
Vo1.11, N5, May 1994, pp.881-885) demonstrated a quasi-
continuous operation of the 456 nm upconversion laser
transition 1Dz -~ 'H9 at a temperature of 215°K in the ytterbium-
sensitized BaYlYbo_9gTmo.oZFe crystal. Using two 100 mW diode
lasers, polarization coupled to form a single pumping beam,
pulsed operation of the 456 nm laser transition was achieved
with a threshold of 190 mW at room temperature. At a
temperature of liquid-nitrogen this fluoride crystal displayed
a threshold of approximately 20 mW at the 482 nm for the
upconversion laser transition 1G4 ~ 3H6.
Reduced temperature operation and the lack of
availability of the corresponding equipment for supporting
such low temperatures are formidable obstacles in developing
compact lasers for the purposes of the Applications. The
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7
alternative variant of using pulsed operation of blue or
ultraviolet lasers at room temperature to prevent crystal
hosts from overheating seems to be of value only for
researching fluorescence spectra and upconversion excitation
mechanisms in various crystal hosts or for other similar
optical measurements. But this variant is entirely
unacceptable for the purposes of the Applications, because of
continuous or other specific operation of associated equipment
in such Applications not being suited to pulsed laser
operation.
Further developing compact blue or ultraviolet
lasers to achieve their continuous wave operation at room
temperature, therefore, requires a new approach in selecting a
more suitable crystal host as an active gain medium and a
related pumping scheme.
Until now, fluoride and fluoride based materials
have been the most preferred materials having been used as
crystal hosts for upconversion lasers. The same choice has
been made as a rule in respect of glass hosts for fiber
upconversion lasers. The only reason for such choice has been
the low phonon energies in fluorides despite the different
affect that crystal and glass hosts have on the upconversion
processes in rare earth ions contained in these hosts.
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In fact, the effect of the crystal host is
determined by a number of factors. So using only the values
of phonon energies as a basis for the choice of crystal host
for blue or ultraviolet lasers providing continuous wave
operation at room temperature has proven to be unacceptable
for the purposes of the Applications. Low ground state
splitting in a fluoride crystal host results in high
population of the low state in lasing transitions at room
temperature according to the Boltzmann distribution. This
high low state population reduces the population inversion for
the lasing transition and increases the lasing threshold. On
the other hand, the low thermal conductivity of fluorides
hinders the increase of pumping radiation intensities to
increase population inversion and is, in essence, the main
reason for fluoride-based crystal host overheating at the
required pumping radiation intensities. It is necessary,
therefore, to take into account both the values of the thermal
conductivity and energy splitting of the low state of the
lasing transition as well as the values of phonon energies in
order to make a better choice of the crystal host.
Another factor to be considered in selecting a
crystal host is related to the effect on the crystal host of
the photon energy of lasing radiation produced by the blue or
ultraviolet lasers having energies high enough to change the
structure of their crystal hosts themselves by creating in the
latter the color centers and vacancies or defect centers
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9
during the upconversion lasing operation. This leads to
increased loss at the wavelengths of lasing radiation and
correspondingly higher lasing thresholds that can reduce laser
outputs and even prevent their continuous wave operation. The
stability of the crystal host must therefore be taken into
account.
On the other hand, the presence of strong
ultraviolet radiation together with blue radiation in the
laser output could be absolutely unacceptable when only blue
radiation is required. Apart from this circumstance, lasing
operation in several wavelength ranges simultaneously reduces
the efficiency at the required wavelengths and stimulates a
decrease in stability and reproducibility of laser parameters
because of competition between the corresponding upconversion
mechanisms. This problem affects blue wavelength radiation to
the greatest extent. For these reasons, crystal hosts have to
be arranged to produce lasing radiation in only one of the
wavelength ranges, namely, either in the blue wavelength range
or in ultraviolet one. So a capability of providing spectral
purity of produced radiation (in visible radiation - color
purity) has to be taken into account as a further factor in
making a careful choice of a crystal host.
In view of the last factor, a crystal host selected
to study an upconversion mechanism which is in the specified
transitions of the crystal host or with a structure of energy
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levels near the specified state of rare earth active ions
could not be used as the active gain medium. In these or
other similar cases the crystal hosts are arranged (in their
compositions, rare earth doping levels and pumping schemes) to
produce a wide-ranging fluorescent emission that often covers
several wavelength ranges or for changing parameters of
pumping schemes over a wide range such as, for example,
changing the pumping radiation wavelengths.
An upconversion laser material comprising a micro-
sphere that is doped with an ion of a rare earth element and
made of crystal or glass was disclosed in U.S. Patent No.
5,684,815 issued to Kiolaka Miora et al. In particular, the
micro-spheres can be made of fluoride single crystals,
chloride single crystals, bromide single crystals or iodide
single crystals. But using a crystal that is small in phonon
energy is considered to be preferable. The rare earth element
can be erbium, holmium, praseodymium, thulium, neodymium or
dysprosium. When Tm3'-doped LiYFqsingle crystal was pumped
with a dye laser at 650 nm the occurrence of upconversion
lasing emission at 480 nm was ascertained. The upconversion
lasing radiation rotates within the micro-sphere repeatedly
totally reflecting on its outer surface. Furthermore, when
the thus totally reflected radiation has the same phase, this
radiation resonates within the micro-sphere. In other words,
the micro-sphere serves as a resonator. A series of total
internal reflections forms the radiation into a thin ring at
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the equator of the dielectric micro-sphere having a diameter
within a range 50 to 2,000 microns. The latter, however,
imposes the limitations on such laser output power at room
temperature, because the micro-sphere cannot be contacted with
a holder or cooler for extracting the heat. A fundamental
disadvantage of the micro-sphere resonator is the absence of
the spectral selectivity or spectral purity in lasing
emission. The upconversion red, green and blue radiation can
resonate within the micro-sphere and, as a result, lasing
operation in red, green and blue wavelengths begins
simultaneously with low efficiency at the blue wavelength. In
addition, such a scheme of direct excitation of rare earth
ions is not an efficient one because of wavelength sensitivity
that requires the pumping source to have a changeable
wavelength output. Such a pumping source is not acceptable
for the purposes of the Applications, where the stability and
reproducibility of optical parameters as well as the
simplicity of adjustment of the packaged arrangement are
highly desirable.
Zhang et al. reported a more complicated scheme of
the rare earth ion excitation in the article "Blue
Upconversion with Excitation into Tm Ions at 780 nm in Yb- and
Tm-Codoped Fluoride Crystals" (Physical Review B, Vo1.51, N
14, 1995, p.9298-9301). When only Tm'' ions are used, two
pumping sources (e.g. 780 nm and 650 nm) are required to
achieve upconversion blue lasing operation. Apart from such a
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pumping scheme suffering from wavelength sensitivity, there is
a definite inconvenience in having to align beams of both
pumping sources so as to couple these beams to the small-sized
upconversion crystal. This makes the adjustment of the
packaged arrangement using such blue laser more difficult.
The fluoride crystals, such as LiYFq, BaY2Fe and KYFQ doped only
with Tm3' ions having been excited at only 780 nm did not
produce any detectable blue emission. But the same crystals
codoped with Tm3~ and Yb'' produced the strong blue emission at
455 and 480 nm when excited into the Tm'' 'FQ state at 780
nm.
It is necessary to note that the efficient energy
transfer of Yb''~ Tm3', yb3'-~ Er3', Yb3'~ Ho'', etc. in laser
crystal and glass hosts has been known for more than 22 years
(see publication of A.M. Prokhorov entitled "Handbook of
laser", vol.l, Moscow, 1978, p.291). Among them rare-earth
Tm3+ and Pr'~ ions are particularly appropriate for upconversion
blue lasers due to their numerous long-lived metastable
levels, which store population during the upconversion
process. Pumping radiation energy may be applied to the
crystals codoped with two groups of rare earth active ions by
different ways. In laser systems (see U.S. Patent Nos.
5,299,215 and 5,488,624 supra.) fluoride Tm:Yb codoped crystal
hosts the Yb3'ions functioned as a sensitizer, at a wavelength
in the range of 830 nm to 1100 nm to stimulate upconversion
lasing transitions 1D2 --~ 3H4 in Tm'' ions, which functioned as an
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activator. The substantial interest in this pumping scheme is
motivated by the extremely broad and strong Yb absorption band
in the operating range of readily available high-power
semiconductor diode lasers. Thus, ytterbium-sensitized
upconversion lasers have the advantage of using a single
infrared pumping source (e. g., at 960 nm). Such diode lasers
proved to be the most practical pumping source taking into
account low cost, small size and simplicity of alignment of
the packaged arrangement.
In an alternative pumping scheme fluoride crystals
codoped with Tm3' and Yb'~ were pumped into Tm3' ions at 780 nm
(see Zhang's article above). Another pumping scheme was
described by A. Knupfer et al. in the article entitled "Two
step pumped YLF:Tm blue upconversion laser"(Journal de
PHYSIQUE IV Colloque C4, supplement au Journal de Physique
III, Vol. 4, 1994, C4-403). Several oxide and fluoride
crystals doped with Tm'' or Tm'' and Yb'' ions, such as Y3A15012,
YA103, YLiFa, GdLiFQ, were tested to obtain blue lasing emission
at the 1D2 ~ jFq transition at room temperature by two step
pulsed pumping into Tm3'ions at 780 nm and 650 nm. The proper
choice of the delay time between pulses of the dye laser and
the Ti: sapphire laser would be very crucial for such a pumping
process. Pulsed lasing operation at 453 nm could be obtained
in the fluoride crystals only, while the oxide Y3A15012 and YA103
crystals were destroyed before the lasing threshold was
reached.
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14
Therefore, the function of ytterbium ions in a
specified crystal host doped with two groups of rare earth
active ions and the effectiveness of the energy transfer
between the ions of these two groups could be changed
considerably depending on the composition and the structure of
the crystal host as well as the pumping scheme used. The
effectiveness of the energy transfer is determined also by the
concentration of the active ions due to the phenomena of
"concentration quenching" where the upconversion laser output
reduces as the concentration of the active ions exceeds a
definite limit for each of the two groups of rare earth active
ions.
Meanwhile, the choice of specific pumping scheme
should not be considered to be predetermined in advance
regardless of the choice of the crystal host or the specific
concentration level of rare earth active ions in the latter.
All of these schemes are participants in the complicated
upconversion process, which could occur in different ways
because of the presence of several mechanisms at the same time
affecting populating or depopulating the desired energy levels
of rare earth activator ions in competition with each other.
The constituents of the crystal host and its structure form an
environment for the activator ions affecting the position and
structure of the energy levels. The pumping scheme provides
that the pumping radiation should be used at specific
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wavelengths to realize conditions compatible with an energy
transfer from the sensitizer ions to the activator ions in the
crystal host by means of one of the preferred mechanisms and
to reach, as a result, the population inversion between the
desired energy levels. The energy transfer could be more
efficient in a crystal host where the phonon energies are
closer to the energy mismatch between the related transitions
in the sensitizer and activator ions. The latter presupposes
that phonon energies of the crystal host selected due to the
new approach could not be as low as in a fluoride-based one.
Also it is clear that these energies should not to be too high
so as to favor radiative lasing transitions as compared with
non-radiative ones.
Therefore, according to the new approach the choice
of the crystal host doped with two groups of rare earth active
ions, takes into account factors such as the choice of
concentration levels of the two groups of active ions, and the
choice of the related pumping scheme, provided the energy
transfer between the active ions of the two groups is
coordinated. This is, in essence, the basis of the identity
and purpose of the new approach and provides an opportunity to
achieve the purposes of the Applications. This approach could
allow the preferred upconversion mechanism that is the most
compatible with the pumping conditions to be realized in
providing spectral purity of produced lasing radiation and its
higher efficiency. Further, this approach enables crystal
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16
hosts other than fluorides to be tested and selected as active
gain media for compact blue or ultraviolet upconversion
lasers.
Until now, there were some crystal hosts selected in
the relevant art using a low phonon energy principle, namely,
fluoride single crystals, chloride single crystals, bromide
single crystals or iodide single crystals (see U.S. No.
5,684,815, supra.). As a rule, there were no oxide single
crystals among them. On the one hand, oxide single crystals
having been used as indicated in publications for only
fluorescence in several wavelength ranges to study the
physical aspects of the upconversion process or achieving
purposes other than those in the Applications. So these oxide
crystals could not be used as active gain media for producing
a specified radiation in the blue or ultraviolet ranges. On
the other hand, active gain media using oxide crystals are
well known. But they have been employed to produce lasing
radiation other than blue or ultraviolet (see, for example,
U.S. patent No. 5,682,397). Besides, pumping schemes such as
pumping into activator ions and/or operating conditions such as
impulse operation, low working temperature, etc. (see, for
example, A. Knupfer's article above or Silversmith's article
in Appl. Phys. Lett, 1987, v.51, N 24, pp. 1977-1979, U.S.
Patent No. 5,682,397) oxide crystals have not been considered
as suitable for use in upconversion lasers that have to be
used in the Applications.
r
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17
Therefore, such oxide single crystals (their
compositions, rare earth doping levels and pumping schemes)
could not be applicable as active gain media in compact
upconversion lasers for the purposes of the Applications.
SUN~1A,RY OF THE INVENTION
It is, therefore, an object of the present invention
to provide an improved upconversion active gain medium.
Another object of the present invention is to
provide an upconversion active gain medium being made of a
rare earth doped crystal host capable of producing either blue
or ultraviolet lasing radiation at room temperature (under
normal ambient temperature operating conditions) from
substantially single band infrared pumping radiation that is
continuous or quasi-continuous. The active gain medium can be
employed in an upconversion micro-laser, where a scheme of
pumping into rare earth sensitizer ions is realized, thus,
giving an opportunity for using a small size infrared pumping
source having simplicity in alignment or any adjustment in the
packaged arrangement.
A still other object of the present invention is to
provide a rare earth doped crystal host being made of a new
class of materials, the composition thereof being arranged to
be compatible with the preferred upconversion mechanism so as
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18
to produce blue or ultraviolet lasing radiation at the
required wavelengths efficiently. The crystal host being
employed in an upconversion micro-laser provides for the
stability and reproducibility of output parameters.
A further object of the present invention is to
provide a rare earth doped crystal host having a structure
arranged to produce the blue or ultraviolet lasing radiation
being polarized.
A still further object of the present invention is
to provide a rare earth doped crystal host being arranged to
produce the blue or ultraviolet lasing radiation at one of the
wavelengths, preferably, using, in addition, a red or an
infrared exciting radiation having also been produced by the
crystal host itself during the upconversion process.
In accordance with the present invention, there is
provided an upconversion active gain medium capable of
producing blue or ultraviolet lasing radiation from
substantially single band infrared pumping radiation that is
continuous or quasi-continuous. The active gain medium
consists of an oxide crystal host doped with two groups of
active ions, one of the groups being ytterbium ions which
function as a sensitizer, the second of the groups being
selected from other than ytterbium rare earth ions which
function as an activator. The sensitizer ions are capable of
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19
absorbing the pumping radiation energy in the wavelength range
of 915 nm to 1080 nm and transferring at least part of this
pumping radiation energy to the activator ions. The activator
ions are excited to at least one of their upper states so as
to produce blue or ultraviolet lasing radiation when these
excited activator ions relax from the upper state into one of
their related low states.
The active gain medium may be employed in an
upconversion micro-laser comprising a chip being made of an
upconversion active gain medium having an oxide crystal host
doped with two groups of active ions, one of the groups being
ytterbium ions which function as a sensitizer, the second of
the groups being selected from other than ytterbium rare earth
ions which function as an activator. The sensitizer ions may
be capable of absorbing the pumping radiation energy in the
wavelength range of 915 nm to 1080 nm and transferring at
least part of this pumping radiation energy to the activator
ions having been excited to at least one of their upper
states. As a result blue or ultraviolet lasing radiation is
emitted when these excited activator ions relax from the upper
state into one of their related lower states.
The micro-laser further includes a pumping source
for generating the continuous or quasi-continuous single band
infrared pumping radiation having at least one wavelength in
the range of 915 nm to 1080 nm, the pumping source being
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optically coupled to the chip of upconversion active gain
medium for applying the pumping radiation energy to the chip
by means of a beam or a set of beams of the pumping radiation.
The micro-laser may have an optical cavity
consisting of the chip of upconversion active gain medium for
resonating at least the blue or ultraviolet lasing radiation
produced by the chip.
The present invention is based on the results of the
careful choice of a crystal host being more suitable as an
upconversion active gain medium of the upconversion micro-
laser for achieving the purposes of the Applications. The
choice of oxide crystal host is most unexpected based upon the
relevant art, where oxides have not been considered as a
prospective upconversion material for producing blue or
ultraviolet lasing radiation up to now.
It is only this new approach, which is responsible
for revealing all the properties of an oxide crystal host that
are the most essential and preferable to accomplish the
foregoing objects. With respect to fluorides, oxides have
higher thermal conductivity and its (greater affecting of)
constituents (atoms or ions) more greatly affect the position
and structure of the activator and sensitizer ions energy
levels, the constituents forming an environment for the two
groups of rare earth active ions. Higher thermal conductivity
CA 02314316 2000-07-21
21
of an oxide crystal host makes for increasing pumping
radiation intensities to increase population inversion between
the desired activator ions energy levels without overheating
such a host. So in all senses, proposed oxide crystal hosts
proved to be of interest as a new class of materials providing
realization of the preferable upconversion mechanism in
producing the blue or ultraviolet lasing radiation at the
desired wavelengths.
The greater environmental effect of an oxide crystal
host on said sensitizer ions results in the greater overlap of
their energy levels. On the one hand, this considerably
reduces wavelength sensitivity when pumping into sensitizer
ions as compared with pumping into activator ions and provides
stability and reproducibility of said micro-laser parameters
that are very important for commercial arrangements using such
lasers in mentioned Applications. On the other hand, this
provides more success probability in selecting related
transitions in the activator and sensitizer ions that are most
compatible with each other for increasing the effectiveness of
energy transfer between them. The selecting of such related
transitions is carried out by choosing the specific
wavelengths in the wavelength range of the pumping radiation.
The greater environmental effect of an oxide crystal
host on said activator ions results in a greater splitting of
their states such as, for example, the ground state. As a
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22
result, some of the energy levels having a higher energy and
so lower population at room temperature could be used as a low
state for the lasing transition providing the population
inversion in the case when the low state could not be reached
from the levels having a lower energy. Therefore, using an
oxide crystal host provides more beneficial conditions for
increasing the population inversion for desired lasing
transition and reducing the lasing threshold.
The increased effect an oxide crystal host has on
the upconversion process is attributable to its higher phonon
energies that may be sufficient to cover possible energy
mismatch between the selected transitions in the sensitizer
and activator ions. These higher. phonon energies could
sustain continuous lasing operation either in blue or
ultraviolet wavelength ranges at room temperature due to one
of the preferred upconversion mechanisms. Therefore, the
possibility of providing spectral purity of produced radiation
arises not only due to resonating either blue or ultraviolet
radiation in an upconversion laser optical cavity, but also
due to using such oxide crystal host as an upconversion active
gain medium. A chip being made of such active gain medium may
be employed in an upconversion micro-laser where a scheme of
pumping into rare earth sensitizer ions is realized. This
provides stability and reproducibility of said micro-laser
parameters as a result of reducing the influence of other
CA 02314316 2000-07-21
23
upconversion mechanisms on upconversion process at the
required wavelengths in said blue or ultraviolet range.
The peculiarities of affecting an oxide crystal host
really display that the choice of the pumping scheme have to
be made in a favor of pumping into rare earth sensitizer ions
as being the most coordinated with the energy transfer from
the sensitizer ions to the activator ions for increasing the
effectiveness of the desired lasing transition and sustaining
continuous either blue or ultraviolet lasing operation. It is
very important to know that the concentration levels of
ytterbium ions may be far higher in the oxide crystal host
than for any activator ions without concentration quenching.
Besides, the extremely broad and strong ytterbium absorption
band in infrared wavelength range and the availability of
high-power semiconductor diode lasers that can operate in this
range were taken into account in making the choice.
Thereupon, pumping source operating in the
wavelength range of 915 nm to 980 nm could comprise a
semiconductor infrared laser diode for generating a beam of
the pumping radiation or a laser diode array for generating a
set of beams of the pumping radiation. A small size and
simplicity in alignment or any adjustment of the packaged
arrangement inherent to such a pumping source are the
desirable characteristics for using this source in the
upconversion micro-laser to reach the objectives of the
CA 02314316 2000-07-21
24
Applications. Pumping with diode lasers directly is the
simplest and most convenient way to realize the pumping
scheme. The scheme is implemented by applying the pumping
radiation energy to the chip by means of the beam or the set
of beams of pumping radiation. However, it is possible to use
other pumping sources, for example, a diode pumped solid state
laser for generating a beam of the pumping radiation.
In a preferred embodiment, a gallium-based oxide
crystal host displays the properties of an oxide crystal host
to the utmost.
The gallium-based oxide crystal host may be a garnet
structure single crystal or a perovskite structure single
crystal and the rare earth activator ion may be thulium. The
garnet single crystal structure, or the perovskite single
crystal structure is capable of producing the blue or
ultraviolet lasing radiation at one or more wavelengths in the
ranges of 450 nm to 495 nm or 350 nm to 380 nm when pumped in
the wavelength range of 915 nm to 980 nm.
The gallium-based oxide crystal host may be a garnet
single crystal structure or a perovskite single crystal
structure and the rare earth activator ion may be
praseodymium. Thus, the garnet single crystal structure, or
the perovskite single crystal structure is capable of
producing the blue lasing radiation at a wavelength in the
CA 02314316 2000-07-21
range 480 nm to 495 nm when pumped in the wavelength range of
915 nm to 980 nm.
According to the present invention, the chip made of
gallium-based oxide single crystal having a perovskite
structure has the advantage of being able to produce the blue
or ultraviolet lasing radiation to be polarized. In this case
the chip of such a uniaxial single crystal should be oriented
so its axis is transverse (perpendicular) to the upconversion
micro-laser axis.
Use of an oxide crystal host also provides an
opportunity to vary its composition in a wide range. This
provides greater effectiveness of the energy transfer between
the sensitizer and the activator ions by changing the
constituents of the oxide crystal host (and their
concentrations) forming the environment for the rare earth
active ions. Among these constituents may be one or more
elements selected from:
- the group consisting of Y, La, Gd, Lu and Yb, occupying
dodecahedral sites of said garnet structure;
- the group consisting of Ga, Sc and A1, occupying octahedral
sites of said garnet structure;
- the group consisting of Y, La, Gd, Lu and Yb, occupying non-
octahedral sites of said perovskite structure;
CA 02314316 2000-07-21
26
- the group consisting of Ga, Sc and Al, occupying octahedral
sites of said perovskite structure.
So greater effectiveness for a specified lasing transition
could be achieved by producing the blue or ultraviolet lasing
radiation at a required wavelength preferably by selecting the
following constituents: the main elements forming along with
oxygen the oxide crystal host, and their contents in the
latter.
Moreover, there is also an opportunity to finely
regulate the spectral purity of an oriented upconversion
process of an oxide crystal host by introducing a small amount
of the following additional elements into the composition of
the oxide crystal host:
- one or more metal elements selected from the group
consisting of Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, T1, Pb
and/or auxiliary elements selected from Ce, Er, Eu, Sc, Tb
and Bi on dodecahedral sites of said garnet structure;
- one or more metal elements selected from the group
consisting of Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W and/or
said subsidiary elements selected from B and Be on
octahedral sites of said garnet structure;
- one or more metal elements selected from the group
consisting of Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, T1, Pb
CA 02314316 2000-07-21
27
and/or ancillary elements selected from Ce, Er, Eu, Sc, Tb
and Bi on non-octahedral sites of said perovskite structure;
- one or more metal elements selected from the group
consisting of Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W and/or
said subsidiary elements selected from B and Be on
octahedral sites of said perovskite structure.
This fine regulation could allow continuous wave operation of
either blue or ultraviolet micro-lasers at room temperature to
be realized in the case when it seems to be impossible using
only the constituents mentioned above.
Besides, some of said additional metal elements or
fluorine could be used to provide a stability of the crystal
host structure to the effect of its own radiation and,
therefore, a continuous lasing operation in the blue or
ultraviolet wavelength range.
In the absence of these elements the oxygen vacancy
or the color center defects could be created in the oxide
crystal host by the absorption of ultraviolet photons
producing by the excited activator ions of this host. The
oxygen vacancy causes the radiation absorption in the range of
320 nm to 500 nm to be increased and, therefore, the losses at
the wavelengths of lasing radiation to be increased. This
leads to higher lasing thresholds that could be the reason for
the falling off in the micro-laser parameters (for example,
CA 02314316 2000-07-21
28
efficiency) during the upconversion process or even preventing
their continuous wave operation. Such a phenomenon was also
called photo-darkening. The experiments performed by the
authors showed that a micro-laser having gallium-based oxide
crystal host of perovskite structure with these color center
defects demonstrates low blue or ultraviolet lasing emission
when pumped in the range of 915 nm to 980 nm. The same
experiments performed for gallium-based oxide crystal host of
garnet structure with these color center defects demonstrated
no emission in blue or ultraviolet range (i.e. laser threshold
was not reached) until a pumping power of about 1.6 W at 960
nm.
Apart from this, said gallium-based oxide crystal
host of garnet or perovskite structure co-doped with other
metal elements, such as Mn, Fe, Co, showed yellow color
centers after ultraviolet irradiation and demonstrated low
blue or ultraviolet lacing emission when pumped in the range
of 915 nm to 980 nm.
On the other hand, the availability some of said
additional metal elements or fluorine in a composition of said
gallium-based oxide crystal host of the garnet or perovskite
structure showed no additional absorption (or coloration)
after ultraviolet irradiation and demonstrated strong blue or
ultraviolet lacing radiation when pumped in the range of 915
nm to 980 nm.
CA 02314316 2000-07-21
29
Therefore, by selecting constituents and additional
metal elements or fluorine for a composition of said gallium-
based oxide crystal host of the garnet or perovskite
structure, all its properties for obtaining an upconversion
could be realized to obtain said upconversion active gain
medium can be accomplished and hence the foregoing objects can
be achieved. Its full compositions are described as follows.
According to the one embodiment of the present
invention, said oxide crystal host may be a garnet structure
single crystal selected from the groups of such crystals
having in each of these groups one of the following general
formulae:
~ A 3_x_yH XYbY } f B ~Ga2_Z l Ga,~l2 ,
{A' 3_X_y_tH' xYbYMet} ~B ~ zGa2-Z_"Me' ~~ Ga3012 ,
{A' 3_x_y_tH' xYbyMet} ( B' sGaz_Z_~Me' "] Ga301z_qFq,
where H' is said rare earth activator ion;
A' is an element, a pair of the elements or a combination of
more than two elements selected from the group consisting of
Y, La, Gd, Lu and auxiliary elements, said auxiliary elements
being capable of occupying a portion not exceeding 0.05 of all
dodecahedral sites of said garnet structure; and
3~10'<x<0.45, 1~10-2<y<2.94, 0~t<0.01;
B' is an element, a pair of the elements or a combination of
more than two elements selected from the group consisting of
Al, Sc and subsidiary elements, said subsidiary elements being
CA 02314316 2000-07-21
capable of occupying a portion not exceeding 0.05 of all
octahedral sites of said garnet structure; and 0~z<2;
Me is an element, a pair of the elements or a combination of
more than two elements selected from the group consisting of
Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, Tl, Pb;
Me' is an element, a pair of the elements or a combination of
more than two elements selected from the group consisting of
Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W; and 0~v<0.004; 0~q<0.95.
The rare earth activator ion may be thulium. The
garnet single crystal structure is capable of producing blue
or ultraviolet lasing radiation at one or more wavelengths in
the ranges of 450 nm to 495 nm or 350 nm to 380 nm when pumped
in the wavelength range of 915 nm to 980 nm.
The rare earth activator ion may also be
praseodymium and 3~10-3<x<0.09, thereby the garnet single
crystal structure is capable of producing blue lasing
radiation at a wavelength in the range 480 nm to 495 nm when
pumped in the wavelength range of 915 nm to 980 nm.
According to the other embodiment of the present
invention, said oxide crystal host may be a perovskite
structure single crystal selected from the groups of such
crystals having in each of these groups one of the following
general formulae:
~ A ~_x_YH xYbY } Ga~_zB ~03
CA 02314316 2000-07-21
31
{A' 1_x_y-tH' XYbyMet} Gal_Z_~B' zMe' ~~3
{A' 1_X_Y_tH' xYbyMet}Gal_z_~B' ZMe' ~O3_qFq,
where H' is said rare earth activator ion;
A' is an element, a pair of the elements or a combination of
more than two elements selected from the group consisting of
Y, La, Gd, Lu and ancillary elements; said ancillary elements
being capable of occupying a portion not exceeding 0.05 of all
non-octahedral sites of said perovskite structure and
1~103<x<0.15, 1~102<y<0.95, 0~t<0.005;
B' is an element, a pair of the elements or a combination of
more than two elements selected from the group consisting of
Al, Sc and subsidiary elements, said subsidiary elements being
capable of occupying a portion not exceeding 0.05 of all
octahedral sites of said perovskite structure; and 0~z<0.5;
Me is an element, a pair of the elements or a combination of
more than two elements selected from the group consisting of
Zn, Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs, Tl, Pb;
Me' is an element, a pair of the elements or a combination of
more than two elements selected from the group consisting of
Ti, V, Ge, Zr, Nb, Mo, Sn, Hf, Ta, W; and 0~v<0.003; 0<q<1.5.
The rare earth activator ion may be thulium. The
perovskite single crystal structure is capable of producing
the blue or ultraviolet lasing radiation at one or more
wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380
nm when pumped in the wavelength range of 915 nm to 980 nm.
CA 02314316 2000-07-21
32
The rare earth activator ion may also be
praseodymium and 1~10-3<x<0.03. The perovskite single crystal
structure is capable of producing the blue lasing radiation at
a wavelength in the range 480 nm to 495 nm when pumped in the
wavelength range of 915 nm to 980 nm.
In particular, the selection of constituents and
additional elements or fluorine for a specified composition of
said oxide crystal host doped with Tm and Yb ions makes it
possible achieving continuous lasing operation only at the 1G4
3H6 transition (blue radiation at a wavelength of about 485
nm) or at the 1D2 -~ 3F4 transition (blue radiation at a
wavelength of about 455 nm) at room temperature. Besides, it
is possible to realize continuous lasing operation only at the
1D2 ~ 3H6 transition (ultraviolet radiation at a wavelength of
about 365 nm) at room temperature. Such specified
compositions are discovered beneath in the detailed
description of the preferred embodiment. Lasers producing
said blue or ultraviolet radiation in said conditions for the
purposes of applications in mentioned Applications are not
known in the relevant art. In said A. Knupfer's article
lasing at the 1D2 ~ 3FQ transition in the fluoride crystal host
at room temperature was described but only for pulsed
operation.
CA 02314316 2000-07-21
33
Further capabilities of said gallium-based oxide
crystal host employed in said upconversion micro-laser
according to the present invention are related with producing
said blue or ultraviolet lacing radiation at one of the
wavelengths preferably using an additional red or an infrared
exciting radiation having been produced by said crystal host
itself during the upconversion process. This exciting
radiation used at the specified wavelengths creates conditions
affecting the upconversion process (on the activator ions) to
realize the preferred upconversion mechanism in producing the
blue or ultraviolet lacing radiation. Such external
conditions could be very useful in the case when the
environmental affect on the oxide crystal host activator ions
proves to be insufficient to sustain continuous wave operation
at room temperature. It is particularly remarkable that this
exciting radiation represents itself as in-cavity radiation
only and doe not form any essential part of the upconversion
micro-laser output radiation.
For realizing these capabilities according to a
further embodiment of the present invention, the optical
cavity may be made for resonating both the blue or ultraviolet
lacing radiation in the wavelength ranges of 450 nm to 460 nm
or 350 nm to 380 nm and a red exciting radiation in the
wavelength range of 630 nm to 695 nm produced also by the
chip. The red exciting radiation is associated with
relaxation of relatively low energy upper states into their
CA 02314316 2000-07-21
34
related intermediate states, in order to provide an additional
excitation of a high upper state using the red exciting
radiation and therefor to produce the blue or ultraviolet
lacing radiation preferably at one of the wavelengths
associated with relaxation of said relatively high upper state
of the activator ions into its related low state.
According to another version of the present
invention, the optical cavity may be made for resonating both
the blue or ultraviolet lacing radiation in the wavelength
ranges of 450 nm to 460 nm or 350 nm to 380 nm and an infrared
exciting radiation in the wavelength range of 1850 nm to 2150
nm produced also by the chip, the infrared exciting radiation
being associated with relaxation of one of the low states into
its related ground state of the activator ions, in order to
provide an additional depletion of such a low state and
thereby to produce said blue or ultraviolet lacing radiation
preferably at one of the wavelengths associated with
relaxation of a relatively higher one of an upper state of the
activator ions into the depleted low state.
In particular, the selection of said optical cavity
resonating both the ultraviolet lacing radiation and the red
exciting radiation makes it possible to achieve continuous
lacing operation at the 1Dz ~ 3H6 transition (ultraviolet
radiation at a wavelength of about 365 nm only) at room
temperature. Accordingly, it is possible to realize
CA 02314316 2000-07-21
continuous lasing operation at the 1D2 --~ 3F4 transition (blue
radiation at a wavelength of about 453 nm only) at room
temperature by selecting said optical cavity resonating both
said blue lasing radiation and said infrared exciting
radiation. Lasers producing said blue or ultraviolet
radiation in said conditions for the purposes of applications
in mentioned Applications are not known in the relevant art.
BRIEF DESCRIPTION OF THE DRAH1INGS
The objects, advantages and features of the present
invention will become more apparent from the following
detailed description of the preferred embodiment thereof in
connection with the accompanying drawings, in which:
FIG.1 is a simplified schematic block diagram of an
upconversion micro-laser using a chip of an upconversion
active gain medium embodying the present invention;
FIGS.2A and 2B show energy state diagrams of Tm, Yb
ions and FIG. 2C show energy state diagrams of Pr and Yb ions
[and accordingly schematic views of different mechanisms of
the energy transfer there between in an upconversion active
gain medium according to the present invention];
FIG.3A is the room temperature fluorescence spectra
of {Y2.695Pro.ooSYbo.3} [SW.89,Gao.o6Alo.o4Tlo.oo3~Ga301z crystal host when
CA 02314316 2000-07-21
36
pumped into Yb ions in the wavelength range of 920 nm to 958
nm;
FIG.3B shows room temperature spectral curves
describing the dependence of the upconversion fluorescence
intensity of blue (488 nm) and green (541 nm) radiation from
the crystal host of FIG.3A as a function of the pumping
radiation wavelength;
FIG.4A is the room temperature blue fluorescence
spectra of {Y1.82Luo,~Tmo.o3Ybo.4s~ ~Sc~.9Gao.osAlo.oa]Ga3G~z crystal host
when pumped in the wavelength range of 918 nm to 946 nm;
FIG.4B is the room temperature ultraviolet
fluorescence spectra of {Y1.3,6Tmo.lzYb,.SCao.oo4} ~SW.9Gao.~]Ga3011.992F0.008
crystal host when pumped in the wavelength range of 918 nm to
946 nm;
FIGS.5A, 5B, and 5C depict simplified schematic
block diagrams of upconversion micro-lasers embodying the
present invention and using a semiconductor infrared laser
diode provided with a lens at its output and different optical
cavity configuration;
FIGS.6A and 6B depict simplified schematic block
diagrams of upconversion micro-lasers embodying the present
CA 02314316 2000-07-21
37
invention and using the chip of an upconversion active gain
medium in a form of crystal ball of different sizes;
FIG.7 is a simplified schematic block diagram of an
upconversion micro-laser according to the present invention
using the chip of an upconversion active gain medium in a form
of double convex lens;
FIG.8 is a simplified schematic block diagram of an
upconversion micro-laser according to the present invention
using the chip of an upconversion active gain medium in a form
of optical waveguide having a core with a greater refractive
index and pumping with a semiconductor infrared laser diode;
FIGS.9A, and 9B depict simplified schematic block
diagrams of upconversion micro-lasers according to the present
invention using the chip of an upconversion active gain medium
in a form of optical waveguide pumping with a laser diode
array.
DETAILED DESCRIPTION OF THE PREFERRED EN~ODIMENT
With reference to FIG. 1, a simplified schematic
block diagram of an upconversion micro-laser 1 used, according
to the present invention, for producing blue or ultraviolet
lasing radiation from single band infrared pumping radiation
will be described in the following. The micro-laser 1 has a
chip 10 being made of an upconversion active gain medium, an
CA 02314316 2000-07-21
38
optical cavity 20 comprises chip 10 for resonating at least
the blue or ultraviolet lasing radiation produced by the chip
and a pumping source 30 for generating said single band
infrared pumping radiation that is continuous or quasi-
continuous.
The pumping source 30 should be optically coupled to
the chip 10 for applying to the chip 10 the pumping radiation
energy by means of a beam 31 of the pumping radiation.
Accordingly, the chip 10 has an input surface 11 optically
coupled with the pumping source 30 to pass the pumping
radiation through the input surface 11 into the chip 10 and an
output surface 12.
The optical cavity 20 is defined by a first mirror
21 and a second mirror 22 opposing each other on a common
laser axis 23 along the axis of the beam 31. The chip 10 is
arranged in optical cavity 20 on the common laser axis 23 so
its input surface 11 and output surface 12 are disposed
adjacent the first mirror 21 and the second mirror 22,
respectively. The size of the chip 10 along the laser axis 23
may be within the range of 0.1 mm to 20 mm while its size in
the transverse (perpendicular) direction to this axis 23 may
be of about 0.4 - 2 mm. The first mirror 21 is designed to be
transmitting the pumping radiation while reflecting the blue
or ultraviolet lasing radiation. The second mirror 22 is
CA 02314316 2000-07-21
39
designed to be reflecting the pumping radiation and also the
blue or ultraviolet lasing radiation while transmitting a
portion of the blue or ultraviolet lasing radiation from the
micro-laser 1. In other words, the second mirror 22 serves as
an output coupler.
It is expedient that the pumping source 30 be
provided with a lens 32 at its output to focus the beam 31
into a pumping region 13 of the chip 10 and to match the beam
31 size within the chip 10 with a required size of the pumping
region 13 in the transverse direction to the axis 23. This
provides increasing the pumping radiation intensity in pumping
region 13 and optimizing the micro-laser 1 output efficiency.
This pumping region's form and size in the transverse
direction to the axis 23 are determined by a desired mode (in
particular, TEMoo mode) of lasing radiation being produced by
micro-laser 1. Usually, matching the size of the beam 31
provides for its overlapping the desired mode. The lens 32
may be a single cylindrical or spherical lens (with a focal
length of about 30-90 mm), a pair of lenses having a
possibility of changing the distance between them or a
combination of more than two lenses, if it is necessary to
realize such a matching more precisely. When the overlapping
is too much, other transverse modes of the lasing radiation
could be excited. The latter may be unacceptable if the
single mode operation of the micro-laser 1 is required.
CA 02314316 2000-07-21
The pumping source 30 operating in the wavelength
range of 915 nm to 980 nm could comprise a semiconductor
infrared laser diode 33 (see FIG.5A, for example) for
generating the beam 31. The laser diode 33 has usually a
substrate 34 for heat abstracting and making connections to a
power supply unit (not shown). The other variant of the
pumping source 30 may comprise a diode pumped solid state
laser such as a Nd:YV04 laser operating at 1.064 ~.lzn and pumping
with a diode (diode-808 nm) or similar for one for generating a
beam 31 of said pumping radiation in the far part of said
wavelength range of 915 nm to 1080 nm.
Said chip 10 of an upconversion active gain medium
has preferably a gallium-based oxide crystal host doped with
ytterbium ions which function as a sensitizer and thulium (or
praseodymium) ions which function as an activator.
FIGS.2A, 2B, 2C show energy state diagrams of Tm, Yb
ions (FIGS.2A, 2B) and Pr, Yb ions (FIGS.2C) and accordingly
schematic views of different mechanisms of the energy transfer
there between in an upconversion active gain medium according
to the present invention.
The pumping radiation energy from the pumping source
30 in the wavelength range of 918 nm to 946 nm preferably
(FIGS.2A, 2B) is absorbed by ytterbium ions to be excited, as
CA 02314316 2000-07-21
41
a result, to their 2F5,2 state. Upconversion mechanisms
depicted in FIGS.2A, 2B include transferring at least part of
this pumping radiation energy to the 3H5 state of thulium ions.
The relaxation of the latter to their 'F4 state is followed by
the first Yb-Tm upconversion energy transfer, where the 2F5,2 -j
zF~,z transitions in Yb ions cause the Tm ions to be excited to
their 3Fz,3 states . Then relaxation of Tm ions to their 3H4
state is followed by the second Yb-Tm upconversion energy
transfer that populates one of Tm ions upper states - 1G4 due
to the zFs,2 --~ ZF~,z transitions in Yb ions .
When Tm ions relax from their lGQupper state into one
of their relating low states - 'H6, blue lasing radiation at a
wavelength of about 485 nm is produced (FIG.2A). Continuous
room temperature lasing operation only at the 1G4 ~ 3H6
transition could be achieved in the following conditions:
selecting a specified composition for said gallium-based oxide
crystal host of the chip 10, using small thulium concentration
(because of concentration quenching) and low Yb concentration,
resonating only blue lasing radiation in the wavelength range
of 480 nm to 495 nm by means of optical cavity 20.
When Tm ions relax from their lGQupper state into
their relating intermediate 3F4state, said red exciting
radiation at a wavelength of about 650 nm is produced
(FIG.2B). Said red exciting radiation stimulates increasing
CA 02314316 2000-07-21
42
efficiency of a cross-relaxation process involving two thulium
ions in their 'F3state, one Tm ion being excited to a
relatively higher 1D2 state of its upper states while the other
Tm ion relaxing to its ground state - 'H6. The energy of both
transitions in Tm ions conforms to photon energy at the
wavelength of about 695 nm. Besides, the 1G4 ~ 3H6 transition
in Tm ions is effectively quenched, providing single-line
lasing operation in blue (or ultraviolet) range at the
wavelength of nearby 455 nm (or nearby 365 nm accordingly).
Continuous room temperature lasing operation only at the
'F4 (or 1D2 -~ 3H6 accordingly) transition could be achieved in
the following conditions: selecting a specified composition
for said gallium-based oxide crystal host of the chip 10,
using higher thulium concentration and higher Yb concentration
(as compared with the case of FIG.2A), resonating both said
blue (or ultraviolet) lasing radiation in the wavelength range
of 450 nm to 460 nm (or 350 nm to 380 nm accordingly) and said
red exciting radiation in the wavelength range of 630 nm to
695 nm by means of optical cavity 20.
The further capabilities in producing blue lasing
radiation preferably at the wavelength of nearby 455 nm arise
with an additional depletion of its relating low state - 3F4 of
Tm ions using an infrared exciting radiation in the wavelength
range of 1850 nm to 2150 nm (not shown in FIG.2B) produced
also by the chip 10. Said infrared exciting radiation is
CA 02314316 2000-07-21
43
associated with relaxation of said 3F4 state into the 'H6 state
of Tm ions. Continuous room temperature lasing operation only
at the 'D2 -~ 'F4 transition could be achieved in the following
conditions: selecting a specified composition for said
gallium-based oxide crystal host of the chip 10, resonating
both said blue lasing radiation in the wavelength range of 450
nm to 460 nm and said infrared exciting radiation in the
wavelength range of 1850 nm to 2150 nm by means of optical
cavity 20.
An upconversion mechanism illustrated in FIG.2C
includes exciting ytterbium ions to their zF5,2 state by
absorbing the pumping radiation energy from the pumping source
30 in the wavelength range of 918 nm to 958 nm preferably and
transferring at least part of this pumping radiation energy to
1G4 state of praseodymium ions. The latter stimulates Yb-Pr
upconversion energy transfer, where the zFs,2 -~ ZF"2 transitions
in Yb ions cause the Pr ions to be excited to their lI6state.
Then relaxation of Pr ions to their 'Po state populates this
upper state. Tn~hen Pr ions relax from their 'Poupper state into
one of their relating low states - 3H4, blue lasing radiation at
a wavelength of about 488 nm is produced. The 3Po -~ 'H5
transitions of Pr ions are also possible producing the green
emission at the wavelength of nearby 541 nm.
CA 02314316 2000-07-21
44
The fluorescence spectra from the chip 10 of
gallium-based oxide crystal host -
Yz.s9sPro.oosYbo.s} ~SW .as~C'ao.osAlo.o9Tl.p.oo3]Ga3Glz selected according to
the
new approach is depicted in FIG.3A. It shows, in particular,
that continuous blue lasing radiation at a wavelength of
nearby 488 nm could be produced by said chip 10 at room
temperature when pumped in the wavelength range of 920 nm to
958 nm. The peaks at 488 nm and at 541 nm were identified as
being related to the 'Po -~ 3HQ transition and the 3Po ~ 3H5
transition of the Pr3~ ions correspondingly (see FIG.2C).
FIG.3B shows room temperature spectral curves describing the
dependence of the upconversion fluorescence intensity of blue
(488 nm) and green (541 nm) radiation from said crystal host
as a function of the pumping radiation wavelength. A 1 Watt
Ti:sapphire laser tunable over the range of 880 nm to 980 nm
was used as a pumping source 30 in this case. Its output
radiation was focused into the chip 10 with the lens 32 of
focal length 50 mm. FIG.3B demonstrates also the possibility
of choosing the specific wavelengths in said range of pumping
radiation to increase the effectiveness of Yb-Pr energy
transfer in a favor of blue (488 nm) upconversion mechanism.
Meanwhile, taking this into account, it is worth paying
attention that more successful possibilities of providing
spectral purity of radiation in the blue or ultraviolet range
are related with a more careful choice of composition for the
gallium-based oxide crystal host.
CA 02314316 2000-07-21
Apart from this, it is important to note that
spectroscopic characteristics of Yb:Pr fluoride crystals and
Yb:Pr oxide crystals are different. Accordingly, the optimal
pumping of the Yb:Pr YLiF4 fluoride crystal host occurred at a
wavelength of about 850 nm whereas for said Yb:Pr gallium-
based oxide crystal host - at a wavelength of about 925 nm
(see FIG.3B). This is what could be expected due to results
of direct comparing the properties of the YLFQ fluoride crystal
host and a gallium-based oxide crystal host with a composition
closed to that of pointed out above with reference to FIG.3A.
This oxide crystal host displayed the greater ground state
spl i t t ing ( DEYLiF = 415 cm 1, DEoX;de = 5 9 5 cm 1 ) and phonon energy
(560 ciriland 640 crrilaccordingly). So these facts, on the one
hand, illustrate the practical importance of the new approach
in choosing composition of crystal hosts. On the other hand,
these facts demonstrate the difficulties in this way because
the known data obtained in the relevant art for other crystal
hosts couldn't be often used without special researching. The
latter is explained by existence of numerous effects of the
crystal on the position and structure of the energy levels of
rare earth active ions doping this crystal host.
It is noteworthy that the variation of composition
of said gallium-based oxide crystal host in a wide range
provides also an opportunity of further optimization of
crystal host's properties when using in said micro-laser
CA 02314316 2000-07-21
46
according to the present invention. This optimization may be
related with providing spectral purity of produced radiation,
reaching continuous wave lacing operation at one of the
wavelength preferably or increasing effectiveness in the
wavelength range (blue or ultraviolet) as a whole and so on.
The specified composition of the gallium-based oxide
crystal host may include the various combinations of said
constituents and additional elements as well as fluorine
depending on the crystal structure and the purposes of
applications in mentioned Applications.
The garnet structure single crystal may consist of
{A' 3_X_yTmXYbY} [B' zGa2_z] Ga3012, where A' is Y or Lu, or Y+Lu in the
ratio being about 4:1, or Y+Lu+La+Ce in the ratio being about
20:5:5:1; B' is Sc, or Sc+Al in the ratio being about 5:1,
or Sc+A1+Be in the ratio being about 30:9:1; and the
particular values of subscript parameters are at nearby:
0.03 for x; 0.45 for y; 1.8 for z.
The garnet structure single crystal may also consist
of {Y3_X_y_tTmxYbyMet} (SczGa2_~_~Me' "] Ga3012, where Me is Ca and Ma' is
Ti, or Me is Ca+Mg in the ratio being about 3:1 and Me' is
Ti+Zr in the ratio being about 3:1, or Me is Ca+K in the ratio
being about 3:1 and Me' is Hf+Ta in the ratio being about 3:1,
or Ma is Ca+Mg+K in the ratio being about 3:1:1 and Ma' is
Ti+Nb+W in the ratio being about 5:2:1; and the particular
CA 02314316 2000-07-21
47
values of subscript parameters are at nearby: 0.06 for x; 0.3
for y; 0.0005 for t; 1.6995 for z; 0.0005 for v.
The garnet structure single crystal may also consist
of {A' 3_X_Y_tPrXYbYCat} [B' ZGa2_ZTi~] Ga3012, where A' is Y or Lu, or
Y+Lu in the ratio being about 5:1, or Y+Lu+Gd+Sc in the ratio
being about 30:15:14:1; B' is Sc, or Sc+A1 in the ratio being
about 2:1, or Sc+Al+B in the ratio being about 1:1:1; and the
particular values of subscript parameters are at nearby: 0.005
for x; 0.1 for y; 0.002 for t; 0.06 for z; 0.002 for v.
The garnet structure single crystal may also consist
of {A' 3_X_y_tTmXYbYCat} [B' ZGa2_z_~Me' ~~ Ga3012_qFQ, where A' is Y or Lu,
or
Y+Lu in the ratio being about 1:1, or Y+Ce+Tb in the ratio
being about 116:3:1; or Y+Lu+Gd+Er+Bi in the ratio being about
40:24:24:1:1; B' is Sc, or Sc+Al in the ratio being about
19:1, or Sc+Al+Be in the ratio being about 38:1:1; and the
particular values of subscript parameters are at nearby: 0.12
for x; 1.5 for y; 0.004 for t; 1.0 for z; 0 for v; 0.008 for
q.
The perovskite structure single crystal may consist
of {A' 1_X_YTmxYbY}Gal_zB' Z~3, where A' is La or Lu, or La+Lu in the
ratio being about 4:1, or La+Lu+Y+Ce in the ratio being about
20:5:1:1; B' is Sc, or Sc+Al in the ratio being about 5:1, or
Sc+Al+Be in the ratio being about 30:9:1; and the particular
CA 02314316 2000-07-21
48
values of subscript parameters are at nearby: 0.01 for x; 0.15
for y; 0.28 for z.
The perovskite structure single crystal may also
consist of {Lal_x_Y_tTmXYbYMet}Gal_Z_~Sc2Me'~03, where Me is Ca and Me'
is Ti, or Me is Ca+Mg in the ratio being about 3:1 and Ma' is
Ti+Zr in the ratio being about 3:1, or Me is Ca+K in the ratio
being about 3:1 and Me' is Hf+Ta in the ratio being about 3:1,
or Me is Ca+Mg+K in the ratio being about 3:1:1 and Me' is
Ti+Nb+W in the ratio being about 5:2:1; and the particular
values of subscript parameters are at nearby: 0.02 for x; 0.1
for y; 0.00017 for t; 0.34 for z; 0.00017 for v.
The perovskite structure single crystal may also
consist of {A' 1_x-y-tPrxYbYCat}Gal_z-"B' zTl 03, where A' is La or Lu,
or La+Lu in the ratio being about 5:1, or La+Lu+Gd+Sc in the
ratio being about 30:15:14:1; B' is Sc, or Sc+Al in the ratio
being about 2:1, or Sc+A1+B in the ratio being about 1:1:1;
and the particular values of subscript parameters are at
nearby: 0.0017 for x; 0.1 for y; 0.0006 for t; 0.02 for z;
0.0006 for v.
The perovskite structure single crystal may also
consist of {A' 1_x_Y_tTmxYbYCat}Gal_z_~B' ZMe' 03_qF''q, where A' is La, or
La+Y in the ratio being about 15:1, or La+Ce+Tb in the ratio
being about 116:3:1; or La+Lu+Gd+Er+Bi in the ratio being
about 40:4:4:1:1; B' is Sc, or Sc+A1 in the ratio being about
CA 02314316 2000-07-21
49
19:1, or Sc+A1+Be in the ratio being about 38:1:1; and the
particular values of subscript parameters are at nearby: 0.04
for x; 0.5 for y; 0.0003 for t; 0.2 for z; 0 for v; 0.0006 for
q.
The results of a direct realization of the purposes
of applications in mentioned Applications in respect of
providing spectral purity of produced radiation are
demonstrated in FIGS.4A, 4B.
FIG.4A shows the room temperature blue fluorescence
spectra of {Yl,ezLuo.~Tmo,~jYb0.45~ ~SCi.sGao_osAlo.oa~Ga3G~2 crystal host
when pumped in the wavelength range of 918 nm to 946 nm. There
was examined that the peak at 455 nm is caused by the 1D2~ 3F4
lacing transition while the peak at 485 nm is related to the
1G4 -~ 3H6 lacing transition ( see thulium energy level diagram in
FIG.2B).
FIG.4B shows the room temperature ultraviolet
fluorescence spectra of {Y1.3~6Tmo.izYbi.sCao.ooa} ~SW
.9~'ao.nC'a30m.99aFo.oos
crystal host when pumped in the wavelength range of 918 nm -
946 nm. The peak at 365 nm was identified as being related to
the 1Dz --~ 'H6 lacing transition of Tm3~ ions (see FIG.2B) . This
crystal host revealed no additional coloration and stability
of its composition in respect of its own ultraviolet radiation
produced.
CA 02314316 2000-07-21
Well-known techniques can be used to obtain the
specified composition of gallium-based oxide crystal host. In
one technique fluoride crystals are prepared starting from the
highest purity oxides commercially available. The host oxide
is doped by diffusion at an elevated temperature (700°C) to
give a host/dopant oxide mixture for subsequent conversion to
fluorides (see, for example, Canadian patent Ca 2,040,557 and
references in its description). It is clear, of course, that
for preparing oxide crystals the last conversion is not
required except for, perhaps, the case of obtaining the
specific composition of gallium-based oxide crystal host using
fluorine (described above in relation with FIG.4B, in
particular).
The raw materials were high-purity powders
characterized by the level of 99,995 for the oxide
constituents (Gaz03, Scz03, Yz03, La20,, GdZ03, Luz03) , additional
elements (Ce02, Erz03, Tb203, Biz03 etc. ) and dopants (Yb203,
Tmz03). The purity levels of other oxide and fluoride chemical
materials were of 99,95 and 99,995. All of the oxide
crystals used may be grown by the Czochralski technique. The
required raw materials are contained in an iridium crucible 40
mm in diameter and 40 mm high in a furnace for optimizing the
doping levels of rare earth ions. An iridium crucible 80 mm
in diameter and 80 mm high is used for growing crystals. A
furnace atmosphere of about 2 percent oxygen in dry nitrogen
CA 02314316 2000-07-21
51
is maintained for crystal growth. The temperature of the
charge is raised to the melting temperature over a period of
several hours. A crystal boule pulling rate was about 1.5
mm/h using a pull boule rotation rate of 10-35 rpm to obtain
crystal boules up to 40 mm in diameter and 100 - 120 mm in
length. The melt fraction crystallized was between 10 and 30
percent. Such a boule could be used for preparing many chips
for micro-lasers 1. It is possible to develop the
technology of growing gallium-based oxide crystals up to 100
mm in diameter by using a crucible 20 cm in diameter and 20 cm
high. The large size crystals are the basis of a low price
commercial production of said upconversion blue or ultraviolet
micro-lasers.
The Czochralski technique allows continuous viewing
of the crystal growing, and gives the opportunity of aborting
this process and restarting it again if the problems of
changing parameters or developing a polycrystalline structure
are arisen. For the Czochralski growing the gallium-based
oxide crystals of specific composition using fluorine, the raw
materials have to contain additionally HF as the source of F-.
The alternative variant using only oxide raw materials in the
melt but a reactive atmosphere containing anhydrous HF, as the
source of F-, and CO with helium as a carrier gas is possible
too. This variant was also discovered in detail in mentioned
patent Ca 2,040,557.
CA 02314316 2000-07-21
52
It is particularly remarkable that the chip 10 made
of said gallium-based oxide crystal may be shaped most
diversely and used in micro-lasers 1 having various optical
cavity configurations. This allows reaching the specific
purposes of applications in mentioned Applications and
displays the great practical value of the present invention.
In one embodiment of the present invention the chip
may be made in a form of parallelepiped (FIG.5A), polished
facets of which that are transversal (perpendicular) to said
laser axis 23 being said input 11 and output 12 surfaces of
the chip 10.
In other embodiment of the present invention the
chip 10 may be made in a form extended along said laser axis
and provided with a flat and a convex polished end surfaces as
said its input 11 and output 12 surfaces respectively
(FIG.5B).
For both embodiments the first 21 and the second 22
mirrors may comprise dielectric coatings applied directly to
the input 11 and output 12 surfaces of the chip 10
respectively therefore forming a monolithic chip-optical
cavity structure. The latter improves the stability and
robustness of the optical cavity 20, which would be important
in a commercial micro-laser. The dielectric coating as the
first mirror 21 may be made to have a transmissivity in the
CA 02314316 2000-07-21
53
range of 70~ to 95~ for the pumping radiation (in the
wavelength range of 915 nm to 980 nm) and a reflectivity in
the range of 99,8 to 99,99 for the blue or ultraviolet
lacing radiation (at one or more wavelengths in the ranges of
450 nm to 495 nm or 350 nm to 380 nm respectively).
Accordingly, the dielectric coating as the second mirror 22
may be made to have a reflectivity in the range of 70g to 95~
for said pumping radiation (in the wavelength range of 915 nm
to 980 nm) and in the range of 60~ to 99,8 for said blue or
ultraviolet lacing radiation (at one or more wavelengths in
the ranges of 450 nm to 495 nm or 350 nm to 380 nm
respectively). Such dielectric coatings are well known in the
art and commercially available.
Such a monolithic chip-optical cavity structure is a
simplest resonator configuration providing an efficient lasing
operation for said pumping radiation power of about 0.5 - 2 W
being applied to the chip 10 by means of the beam 31 of said
pumping radiation from the pumping source 30. The latter
comprises a semiconductor infrared laser diode 33 on the
substrate 34 and is provided with the lens 32.
In case of the plane-plane cavity structure shown in
FIG.5A a generated mode of lacing radiation is determined by
the thermal lens induced in the chip 10 of said upconversion
active gain medium. When the chip 10 is made of {Y3_X_
yTmxYbY} [SczGa2_z]Ga301z single crystal, this thermal-induced lens
CA 02314316 2000-07-21
54
acts as a mirror with a radius of about 30-35 mm for pumping
radiation power of 2 W. Such a thermal lens provides a stable
resonator configuration with good overlapping the pumping
region 13 of said generated mode by the beam 31 within the
chip 10. For a generated mode waist of 70 ~m and the pumping
radiation power of 1W the thermal lens gives a constant beam
diameter for a cavity length up to 6 mm. So an optimum size
of the chip 10 along the laser axis 23 in this variant may be
within the range of about 0.1 mm to 6 mm. The chip 10 may be
longer if it is made doped with said active ions only in its
middle region to be pumped by said infrared pumping radiation
remaining undoped the rest parts of the chip 10, between which
said middle region being disposed along said laser axis 23.
This chip 10 configuration may be useful for the micro-laser
of a higher power where the greater heat abstraction from the
chip 10 is required.
The plane-concave cavity structure shown in FIG.5B
has a stable resonator configuration and differs from that of
in FIG.5A by the forms of the chip 10 and the second mirror 22
used. The convex output end surface 12 of the chip 10 has the
radius of curvature of 30 mm to 40 mm.
A still further embodiment of the present invention
comprises, on the contrary, a concave-plane cavity structure
shown in FIG.5C. The first 21 and the second 22 mirrors are
CA 02314316 2000-07-21
configured to have a spherical and a flat surfaces
respectively therefore forming a stable resonator (optical
cavity) configuration. The chip 10 is made with flat polished
end surfaces being said its input 11 and output 12 surfaces
and disposed on the laser axis 23 separately with respect to
the mirrors 21 and 22. The better heat abstraction may have
that variant, where the chip 10 is disposed between two
undoped crystals 14 and 15 along the laser axis 23. Each
undoped crystal 14 (or 15) has a flat polished end surface to
provide a good thermal and optical contact with the input
surface 11 (output surface 12) of the chip 10. A lens cement
may be used to improve said optical contact by reducing the
unwanted reflections at the interfaces between them and to
integrate the chip 10 with undoped crystals 14 and 15 into a
single construction. The other polished end surface of
undoped crystal 14 (or 15) is made spherical (flat) for
applying thereto directly a dielectric coating as said first
mirror 21 (second mirror 22). The latter has the same
parameters as the corresponding mirror in FIG.5B mentioned
above.
In other variant the chip 10 may be disposed between
such undoped crystals 14 and 15 along the laser axis 23 but
separately from them. The same mirrors 21 and 22 (as in the
previous variant) are applied to said undoped crystals 14 and
15 respectively, at least one of which serving as a polarizer.
CA 02314316 2000-07-21
56
This allows producing said blue or ultraviolet lasing
radiation to be polarized.
Yet another embodiment of the present invention
comprises a monolithic chip-optical cavity structure with the
stable resonator (optical cavity) configuration shown in
FIGS.6A, 6B. The chip 10 is made in a form of polished
crystal ball disposed on the laser axis 23. Such crystal ball
may have a diameter within the range of 0.1 mm to 6 mm. The
first 21 and the second 22 mirrors comprise dielectric
coatings applied directly to the parts of said crystal ball
surface opposing each other on the laser axis 23 along the
beam 31 axis, said parts being the input 11 and output 12
surfaces of the chip 10 respectively. The first mirror 21 is
designed to be transmitting said pumping radiation while
reflecting said blue or ultraviolet lasing radiation as well
as said red exciting radiation. The second mirror 22 is
designed to be reflecting said pumping radiation and said blue
or ultraviolet lasing radiation as well as said red exciting
radiation while transmitting a portion of said blue or
ultraviolet lasing radiation, i.e. the second mirror 22 serves
as an output coupler.
The variant of FIG.6B is structurally and
functionally similar to that of FIG.6A except for the fewer
size of the chip 10. So the semiconductor infrared laser
diode 33 may be directly coupled to the chip 10 without
CA 02314316 2000-07-21
57
focusing lens 32 due to the greater surface curvature of said
crystal ball. The emission facet of the laser diode 33 may be
as close as 30-50 ~m from the crystal ball in this case. On
the other hand, a suitable heat sink 16 may be used for the
small ball 10, if desired, to provide abstracting the
excessive heat.
This embodiment differs from the upconversion laser
material described in US patent No 5,684,815 in using:
- other crystal host - gallium-based oxide single crystal of a
garnet structure;
- the composition of {A' 3_X-Y-~TmxYbyMet} [B' ZGa2_Z] Ga301z_qFq being
arranged to be compatible with the upconversion mechanism to
produce said blue or ultraviolet lasing radiation at one of
the wavelengths preferably;
- other pumping scheme with pumping into Yb sensitizer ions as
being the most coordinated with said energy transfer instead
of pumping into Tm ions;
- the semiconductor infrared laser diode as the pumping source
operating in the wavelength range of 915 nm to 980 nm
instead of a dye laser at 650 nm;
- the outer resonator (optical cavity) defined by the first
mirror 21 and the second mirror 22 opposing each other on
the laser axis 23 along the beam 31 axis;
- the first mirror 21 that has a transmissivity in the range
of 70~ to 95~ for said pumping radiation (in the wavelength
CA 02314316 2000-07-21
58
range of 915 nm to 980 nm) and a reflectivity in the range
of 99,8 to 99,99 for said blue or ultraviolet lasing
radiation (at one of the wavelengths in the ranges of 450 nm
to 460 nm or 350 nm to 380 nm respectively) as well as in
the range of 99,8 to 99,99 for said red exciting radiation
(in the wavelength range of 630 nm to 695 nm); and
- the second mirror 22 that has a reflectivity in the range of
70~ to 95~ for said pumping radiation (in the wavelength
range of 915 nm to 980 nm), in the range of 60~ to 99,8 for
said blue or ultraviolet lasing radiation (at one of the
wavelengths in the ranges of 450 nm to 460 nm or 350 nm to
380 nm respectively) and in the range of 99,8 to 99,99 for
said red exciting radiation (in the wavelength range of 630
nm to 695 nm).
All of this allows providing spectral purity of
produced radiation in contrast to the micro-sphere resonator
construction used in US patent No 5,684,815.
A further embodiment of the present invention shown
in FIG.7 uses the same laser diode 33 - chip 10 configuration
as that of FIG.6B. At the same time, the chip 10 having
polished spherical surfaces as said input 11 and output 12
surfaces are made in a form of double convex lens. The chip
is disposed on the laser axis 23 separately with respect to
the first 21 and second 22 mirrors that are configured both to
have spherical surfaces, therefore forming a stable optical
CA 02314316 2000-07-21
59
cavity configuration. The chip 10 being arranged in the
optical cavity 20 so its input 11 and output 12 surfaces to be
disposed adjacent the first 21 and second 22 mirrors
respectively. The latter has the same parameters as the
corresponding mirrors in FIG.6A mentioned above. The chip 10
has a size along the laser axis 23 within the range of 0.3 mm
to 6 mm. The first 21 and second 22 mirrors have the same
radius of curvature as the corresponding surfaces of the chip
in the range of 26 mm to 46 mm.
A still further embodiment of the present invention
shown in FIG. 8 uses the same laser diode 33 - chip 10
configuration as that of FIG.6B. Meanwhile, the chip 10 is
made in a form of an optical waveguide extended along the
laser axis 23 and has polished flat end faces being the input
11 and output 12 surfaces of said chip. The first 21 and
second 22 mirrors comprise dielectric coatings applied
directly to the input 11 and output 12 surfaces of the chip 10
respectively therefore forming a monolithic chip-optical
cavity structure. The first 21 and second 22 mirrors have the
same parameters as the respective mirrors in FIG.5A mentioned
above. The optical waveguide 10 may be extended and has a
core (not shown in FIG.8) with a greater refractive index than
that of an outer part of the optical waveguide 10 surrounding
said core. The small core radius allows high pumping
radiation intensities with modest pumping powers from the
CA 02314316 2000-07-21
semiconductor infrared laser diode 33. A heat sink 16 may be
used, if desired, to provide abstracting the excessive heat.
A yet further embodiment of the present invention is
shown in FIGS.9A, 9B where the optical cavity 20 is defined by
the first mirror 21 and the second mirror 22 opposing each
other on the common laser axis 23. The chip 10 of an
upconversion active gain medium is made in a form of an
optical waveguide extended along the laser axis 23 and has a
side face optically coupled with the pumping source 30 to
serve as an input surface 11 through which said pumping
radiation pass into the chip 10. The latter has also the
first polished flat end face 17 and the second polished flat
end face being the output surface 12 of the chip 10. The chip
10 is arranged in the optical cavity 20 so its first 17 and
second 12 end faces to be disposed adjacent the first 21 and
second 22 mirrors respectively.
The pumping source 30 for generating a set of beams
of said pumping radiation in the wavelength range of 915 nm to
980 nm comprises a linear infrared laser diode array 35
arranged along the side face 11 of the chip 10 at a specified
distance from this face 11. The emission facet of each laser
diode 33 of the array 35 may be as close as 30-100 Elm from the
input surface 11. So each laser diode 33 may be optically
coupled with the specified part of the input surface 11 by
CA 02314316 2000-07-21
61
means of the corresponding beam 31, the axis of which being
transversal or almost transversal (within the range of several
degrees) to the laser axis 23. All of said parts of the input
surface 11 are disposed along the laser axis 23 with
overlapping each other, if necessary. The laser diode array
35 has usually a single substrate 36 for heat abstracting and
making connections to a power supply unit (not shown in
FIGS.9A, 9B). A heat sink 16 may be used, if desired, to
provide abstracting the excessive heat.
In the variant of FIG.9A of this embodiment the
first 21 and second 22 mirrors comprise dielectric coatings
applied directly to the first 17 and second 12 end faces of
the chip 10 respectively therefore forming a monolithic chip-
optical cavity structure.
In the variant of FIG.9B of this embodiment the
first mirror 21 comprises dielectric coating applied directly
to the first end face 17 of the chip 10, while the second
mirror 22 is disposed separately with respect to the chip 10
and configured to have a spherical surface therefore forming a
stable optical cavity configuration.
For both variants of this embodiment the
reflectivity of the first 21 and second 22 mirrors for said
blue or ultraviolet lasing radiation (at one or more
wavelengths in the ranges of 450 nm to 495 nm or 350 nm to 380
CA 02314316 2000-07-21
62
nm respectively) is in the range of 99,8 to 99,99 and in the
range of 60~ to 99,8 respectively.
The additional dielectric coatings 18 and 19 may be
applied directly to the input surface 11 and an opposite side
face of the chip 10. These coatings 18 and 19 have a
transmissivity in the range of 70~ to 95~ and a reflectivity
in the range of 70~ to 95~ respectively for said pumping
radiation (in the wavelength range of 915 nm to 980 nm) to
increase its absorbing by the chip 10. At the same time both
coatings 18 and 19 may have a reflectivity in the range of
99,8 to 99,99 for the blue or ultraviolet lacing radiation
(at one or more wavelengths in the ranges of 450 nm to 495 nm
or 350 nm to 380 nm respectively) for improving efficiency in
continuous wave lacing operation.
One more embodiment of the present invention differs
from that of shown in FIG.9A only in the form of the chip 10
and the properties of the mirror 21 and 22. The references
concerning this embodiment may be made also with respect to
this FIG.9A.
The chip 10 of an upconversion active gain medium is
made in a form extended along the laser axis 23. The chip 10
has a side face optically coupled with the pumping source 30
to serve as an input surface 11, a first polished flat end
CA 02314316 2000-07-21
63
face 17 and a second polished flat end face 12 being an output
surface of the chip 10.
The first mirror 21 has a reflectivity in the range
of 99,8 to 99,99 for said blue or ultraviolet lasing
radiation (at one of the wavelengths in the ranges of 450 nm
to 460 nm or 350 nm to 380 nm respectively) as well as in the
range of 99,8 to 99,99 for said infrared exciting radiation
(in the wavelength range of 1850 nm to 2150 nm). The second
mirror 22 has a reflectivity in the range of 60~ to 99,8 for
said blue or ultraviolet lasing radiation (at one of the
wavelengths in the ranges of 450 nm to 460 nm or 350 nm to 380
nm respectively) and in the range of 99,8 to 99,99 for said
infrared exciting radiation (in the wavelength range of 1850
nm to 2150 nm).
The herein-proposed upconversion micro-laser 1
functions as follows. Single band infrared pumping radiation
having at least one wavelength in the range of 915 nm to 1080
nm (preferably in the range of 915 nm to 980 nm) is generated
by the pumping source 30 in the form of a beam 31 or a set of
beams 31. When only one beam 31 is used, said pumping
radiation passes through the focusing lens 32 (for micro-
laser's embodiments of FIGS.1, 5A-6A) or directly (for micro-
laser's embodiments of FIGS.68, 7,8) along the laser axis 23
and penetrates the optical cavity 20 through the first mirror
21 for applying to the input surface 11 of the chip 10. When
CA 02314316 2000-07-21
64
a set of beams 31 is used (for micro-laser's embodiments of
FIGS.9A, 9B), said pumping radiation passes through the
coating 18 (FIG.9B) or directly (FIG.9A) into the optical
cavity 20 for applying to the input surface 11 of the chip 10.
Then, the pumping radiation that has transmitted through the
input surface 11 into the chip 10 is absorbed to a greatest
extent by ytterbium ions, which function as a sensitizer, to
provide the energy transfer to the activator ions as was
described above in connection with FIGS.2A-3B, for example.
With this, said blue or ultraviolet lacing radiation at one or
more wavelengths in the ranges of 450 nm to 495 nm or 350 nm
to 380 nm produced by the chip 10 appears from the second
mirror 22 being an output coupler of the optical cavity 20,
when the lacing threshold is exceeded.
The foregoing and furher variants of the embodiments
of the the present invention stated before are by no means
exhaustive. Modifications and changes, without, doubt are
possible. Thus, for instance, there are practicable
embodiments similar to that of FIG.5A except the composition
of the chip's single crystal and the properties of the mirror
21 and 22. The garnet structure single crystal doped
additionally with fluorine to have the composition of the
general formula {A' 3_X_y_tTmXYbYMet} [B' zGa2_Z] Ga3012_qFq has demonstrated
the lowest room temperature threshold when producing said blue
or ultraviolet lacing radiation at one wavelength preferably
using said red or infrared exciting radiation. The latter
CA 02314316 2000-07-21
permits achieving continuous wave upconversion lasing
operation and high efficiency at the desired wavelength
without self-terminating for the following examples describing
properties of the mirror 21 and the mirror 22.
Example 1
The first mirror 21 has a transmissivity in the
range of 70~ to 95~ for the pumping radiation (in the
wavelength range of 915 nm to 980 nm) and a reflectivity in
the range of 99,8 to 99,99 for the ultraviolet lasing
radiation (in the wavelength range of 350 nm to 380 nm) as
well as in the range of 99,8 to 99,99 for said red exciting
radiation (in the wavelength range of 630 nm to 695 nm). The
second mirror 22 has a reflectivity in the range of 70~ to 95~
for the pumping radiation (in the wavelength range of 915 nm
to 980 nm), in the range of 60~ to 99,8 for the ultraviolet
lasing radiation (in the wavelength range of 350 nm to 380 nm)
and in the range of 99,8 to 99,99 for said red exciting
radiation (in the wavelength range of 630 nm to 695 nm). In
this case the ultraviolet lasing radiation at 366 nm can be
obtained with the room temperature lasing threshold of about
140 mW.
Example 2
Both mirrors' properties are similar to that of
example 1 with respect to the pumping radiation and said red
exciting radiation. The first mirror 21 has a transmissivity
CA 02314316 2000-07-21
66
in the range of 70~ to 95~ for the pumping radiation and a
reflectivity in the range of 99,8 to 99,99 for the blue
lacing radiation (in the wavelength range of 450 nm to 460 nm)
as well as in the range of 99,8 to 99,99 for said red
exciting radiation. The second mirror 22 has a reflectivity
in the range of 70~ to 95~ for the pumping radiation, in the
range of 60~ to 99,8 for the blue lacing radiation (in the
wavelength range of 450 nm to 460 nm) and in the range of
99,8 to 99,99 for said red exciting radiation. Then, the
blue lacing radiation at 455 nm can be obtained with the room
temperature lacing threshold of about 50 mW.
Examp 1 a 3
Both mirrors' properties are similar to that of
example 1 with respect to the pumping radiation and blue
lacing radiation.
The first mirror 21 has a transmissivity in the
range of 70~ to 95~ for the pumping radiation and a
reflectivity in the range of 99,8 to 99,99 for the blue
lacing radiation as well as in the range of 99,8 to 99,99
for said infrared exciting radiation (in the wavelength range
of 1850 nm to 2150 nm). The second mirror 22 has a
reflectivity in the range of 70~ to 95~ for the pumping
radiation, in the range of 60~ to 99,8 for the blue lacing
radiation and in the range of 99,8 to 99,99 for said
infrared exciting radiation (in the wavelength range of 1850
CA 02314316 2000-07-21
67
nm to 2150 nm). Then, the blue lasing radiation at 455 nm can
be obtained with the room temperature lasing threshold of
about 70 mW.
It is understood that the above-described
embodiments of the proposed invention can by no means be
regarded as limiting the present invention, but are to be
interpreted as illustrative to promote understanding of its
essence, and that various changes and improvements may be
effected therein by those skilled in the art without departing
from the scope or spirit of this invention as defined in the
appended claims.
