Note: Descriptions are shown in the official language in which they were submitted.
CA 02604275 2007-09-26
TITLE OF THE INVENTION
Phosphate glass based optical device and method
CROSS-REFERENCE TO RELATED APPLICATIONS
This invention claims priority from U.S. Provisional Application Number
60/847,582, the contents of which are incorporated herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR
DEVELOPMENT
This invention was made with government support under contract F49620-02-1-
0380 awarded by USAF/AFOSR. The government has certain rights in the
invention.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a multicomponent phosphate glass based
optical
device and a method of producing the optical device.
The present invention includes the use of various technologies referenced and
described in references identified in the following LIST OF REFERENCES by the
author(s) and year of publication. The contents of these documents are
incorporated in
their entirety herein by reference.
LIST OF REFERENCES
[1] G. A. Ball and W. W. Morey, Opt. Lett. 17,420 (1992).
[2] J. L. Zyskind, V. Mizrahi, D. J. DiGiovanni, and J. W. Sulhoff, Electron.
Lett. 28,
1385 (1992).
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[3] P. Laporta, S. Taccheo, S. Longhi, O. Svelto, and C. Svelto, Opt.
Materials 11, 269
(1999).
[4] L. Dong, W. H. Loh, J. E. Caplen, J. D. Minelly, K. Hsu, and L. Reekie,
Opt. Lett. 22,
694 (1997).
[5] W. H. Loh, B. N. Samson, L. Dong, G. J. Cowle, and K. Hsu, J. Lightwave
Tech. 16,
114 (1998).
[6] Ch. Spiegelberg, J. Geng, Y. Hu, Y. Kaneda, S. Jiang, and N.
Peyghambarian, J.
Lightwave Tech. 22, 57 (2004).
[7] T. Qiu, S. Suzuki, A. Schulzgen, L. Li, A. Polynkin, V. Temyanko, J. V.
Moloney, and
N. Peyghambarian, Opt. Lett. 30, 2748 (2005).
[8] A. Schulzgen, L. Li, V. L. Temyanko, S. Suzuki, J. V. Moloney, and N.
Peyghambarian, Optics Express 14, 7087 (2006).
[9] S. Taccheo, G. Della Valle, K. Ennser, G. Sorbello and S. Jiang, Electron.
Lett. 42 594
(2006).
[10] J. Albert, A. Schulzgen, V. L. Temyanko, S. Honkanen, and N.
Peyghambarian, Appl.
Phys. Lett. 89, 101127 (2006).
[11] G. J. Spiihler, L. Krainer, E. Innerhofer, R. Paschotta, K. J.
Weingarten, and U. Keller,
Opt. Lett. 30, 263-265 (2005).
[12] S. Pissadakis, A. Ikiades, P. Hua, A. K. Sheridan and J. S. Wilkinson,
Opt. Express
12, 3131 (2004).
[13] L. Li, A. Schulzgen, V. L. Temyanko, M. M. Morrell, S. Sabet, H. Li, J.
V. Moloney,
N. Peyghambarian, Appl. Phys. Lett. 88, 161106 (2006).
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[14] A. Othonos and K. Kalli, Fiber Bragg gratings: fundamentals and
applications in
telecommunications and sensing, Artech House, Boston (1999).
(15] P. J. Lemaire, R. M. Atkins, V. Mizrahi and W. A. Reed, Electron. Lett.
29, 1191
(1993).
[16] S. Kannan, J. Z. Y. Guo, and P. J. Lemaire, J. Lightwave Technol. 15,
1478 (1997).
[17] J. Albert, S. Yliniemi, S. Honkanen, A. Andreyuk, and A. Steele, in
Proceedings of
the 2005 Topical Meeting on Bragg Gratings, Photosensitivity and Poling, B.
Eggleton,
ed., Sydney, Australia, pp. 402-404 (2005).
[18] O. M. Efimov, L. B. Glebov, L. N. Glebova, K. C. Richardson, and V. I.
Smimov,
Appl. Opt. 38, 619 (1999).
Discussion of the Background
Since the demonstration of the holographic side writing technique for
fabricating
gratings in silica based optical fibers, there has been sustained interest
toward the
development of high-performance single frequency fiber grating-based rare
earth-doped
fiber lasers and other optical devices based on gratings formed in a fiber.
The potential
attraction of such optical devices is the simplicity of the fabrication,
involving just the
ultraviolet (UV) writing of grating(s) into a silica based fiber. In addition,
the wavelength
sensitivity to temperature is dictated by the sensitivity of the grating,
which is over an
order of magnitude lower than that for semiconductor lasers.
In the following, the problems facing the lasers that use a grating are
discussed.
However, other optical devices (for example a wave filter) that use the same
grating are
faced with similar problems. For a laser, to ensure robust single frequency
operation
without mode-hopping, these silica based lasers need to be short, a few cm in
length at
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most. While the earlier grating-based silica fiber lasers relied on the
availability of
conventional erbium-doped germanosilicate fibers, the laser efficiencies and
output
powers were however low, typically 0.1 % and in the mW regime respectively.
These
problems facing the silica based lasers are a direct consequence of the low
pump
absorption due to the short fiber cavity length. Increasing the erbium dopant
concentration
as a way of increasing the pump absorption is however problematical because
germanosilicate fibers, while having the merit of being photosensitive, are
particularly
prone to ion clustering, which not only leads to a degradation in efficiency
but gives rise to
instabilities in the laser as well. Thus, while the conventional lasers showed
good
characteristics in many respects, the need for amplification in order to boost
the low laser
powers to useful levels of a mW or more is a drawback that prevents the
existing silica
based lasers from achieving a high performance low noise source.
In the past few years, considerable effort has been put into investigating
possible
solutions for increasing the operating power of short cavity fiber lasers.
While the pump
absorption can be increased by over an order of magnitude simply by pumping at
an
appropriately shorter wavelength, this is unlikely to be a fully practical
solution until the
arrival of reliable green laser diodes. On the other hand, Er3+:Yb3+ codoped
fibers are an
immediate possibility. In this scheme, a 980 nm pumped light is mainly
absorbed by the
Yb3+ ions and then transferred to the Er3+. In addition to the large 980 nm
absorption
cross-section of Yb3+ (by an order of magnitude greater than that of Er3+), a
higher Yb3+
ion concentration is also attainable without detrimental side-effects. The 980
nm pump
absorption can therefore be typically increased by up to two orders of
magnitude with this
approach, with a corresponding increase in laser efficiency and output power.
However, two problems remain for obtaining a practical single frequency Er:Yb
grating-based fiber laser. One is the lack of photosensitivity in the
phosphosilica and
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phosphate glass based fibers, the glass hosts that enable large Er:Yb doping
levels and
efficient Yb3+ to Er3+ energy transfer. While tin-codoping can provide some
enhancement in photosensitivity, the UV exposure time required to reach a
suitable grating
strength is still very long. Reasonable writing times can be achieved with the
incorporation of hydrogen loading, but substantial losses are incurred at the
shorter (pump)
wavelengths and lead to a loss in device efficiency. In addition, such lasers
were observed
to operate in both orthogonally polarized modes, which is unattractive for
many
telecommunications applications.
Another approach could surmount these problems. By adopting an Er3+:Yb3+
fiber with a photosensitive annular region surrounding the phosphosilicate
core, strong
gratings could be written with relative ease in spite of the non-
photosensitive core,
enabling efficient lasers to be realized (see for example L. Dong, W. H. Loh,
J. E. Caplen,
J. D. Minelly, K. Hsu, and L. Reekie, "Efficient single-frequency fiber lasers
with novel
photosensitive Er/Yb optical fibers," Opt. Lett., vol. 22, pp. 694-696, 1997,
the contents of
which are entirely incorporated herein by reference).
In addition, the resulting silica based fiber lasers were observed to lase
only in a
single polarization state and slope efficiencies of 25% were reported.
However, the
known lasers have a low power output, on the order of a few mW, which makes
these
lasers unsuitable for the needs of CATV for example, which would require more
than a
few mW of output power.
Another route to increase the power output of silica based lasers is the
fabrication
of hybrid phosphate/silicate fiber devices. In these conventional hybrid
devices, as shown
in Figure 1, the active fiber is made from highly doped phosphate glass 10
while the fiber
gratings 20 are written into a photosensitive silicate fiber section 30 that
is fusion spliced
to the phosphate fibers, such that a splice interface 40 is formed. In these
spliced devices,
CA 02604275 2007-09-26
both optical losses and mechanical instabilities at the splicing points
present inherent
challenges due to large differences in thermal properties, such as melting
temperature and
thermal expansion coefficient, between the different glasses. In addition this
method is
only suitable to make short cavities with Bragg reflectors, while more robust
and better
tunable distributed feedback (DFB) fiber laser schemes can not be implemented.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided an optical
device including an optical fiber having a core including multicomponent
phosphate
glasses, and a cladding surrounding the core, and a first fiber Bragg grating
formed in the
first portion of the core of the optical fiber and having an index modulation
amplitude
greater than 2x 10-5.
According to another aspect of the present invention, there is provided a
method of
modulating an index of refraction in an optical fiber that includes ,
providing the optical
fiber having a core including multicomponent phosphate glasses, irradiating a
first portion
of the core of the optical fiber via a phase mask with laser pulses in the
ultraviolet range to
form a fiber Bragg grating within the first portion of the core, and heating
the irradiated
core to increase an index modulation amplitude within the core to above 2x 10-
5.
According to still another aspect of the present invention, there is provided
a
method for generating a laser signal in a laser device having an optical fiber
having a core
that includes multicomponent phosphate glasses, the core having first and
second portions
surrounded by a cladding, and at least two fiber Bragg gratings in which an
index
modulation amplitude is greater than 2X 10"5, the at least two fiber Bragg
gratings disposed
in the first portion of the core at a predefined distance from each other, the
method
including pumping from a multimode pump a multimode wave into the cladding
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corresponding to the second portion of the core, transmitting the pumped
multimode wave
from the cladding to the at least two fiber Bragg gratings, generating the
laser signal
between the at least two fiber Bragg gratings based on the pumped multimode
wave, and
outputting the generated laser signal through one of the at least two fiber
Bragg gratings.
According to another aspect of the invention, there is provided a laser device
including an optical fiber having a core including multicomponent phosphate
glasses, and
a cladding surrounding the core, first and second fiber Bragg gratings formed
in a first
portion of the core of the optical fiber and having an index modulation
amplitude greater
than 2x 10-5, and an optical cavity between the first and second fiber Bragg
gratings and
configured to amplify an electromagnetic wave reflected by the first and
second fiber
Bragg gratings to output a laser signal.
According to another aspect of the invention, there is provided an optical
filter
including an optical fiber having a core including multicomponent phosphate
glasses, and
a cladding surrounding the core, and a first fiber Bragg grating formed in a
first portion of
the core of the optical fiber and having an index modulation amplitude greater
than 2x 10"5.
An input signal provided in the optical fiber is partially reflected and
partially transmitted
by the first fiber Bragg grating.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the attendant
advantages thereof will be readily obtained as the same becomes better
understood by
reference to the following detailed description when considered in connection
with the
accompanying drawings, wherein:
Figure 1 is a schematic diagram of a hybrid phosphate/silicate fiber device;
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Figure 2 is a schematic diagram of an arrangement for making a phosphate fiber
according to one embodiment of the present invention;
Figures 3(a) and 3(b) are schematic diagrams of a phase mask according to one
embodiment of the present invention;
Figures 4(a)-(f) show SEM and AFM images of the phase mask and a section
analysis of the phase mask according to one embodiment of the present
invention;
Figure 5 is a graph showing a reflectivity versus time of the phosphate fiber
Bragg
grating according to one embodiment of the present invention;
Figure 6 is a graph showing UV-induced refractive index modulation amplitude
and average value of the fiber according to one embodiment of the present
invention;
Figure 7 is a graph that shows the gratings amplitude modulation growth during
thermal treatment;
Figures 8(a) and (b) are graphs showing phosphate fiber Bragg grating
transmission indicating high reflectivity and a partial decrease in
reflectivity of the grating
due to exposure to high temperature and also a reflection spectrum of the
grating after
thermal treatment according to one embodiment of the present invention;
Figures 9(a) to (c) are schematic diagrams of a DFB laser pumped with a single-
mode pump diode, multimode pump diodes and a multiple DFB laser
implementation;
Figure 10 is a graph showing the emission spectrum of the cladding pumped DFB
fiber laser according to one embodiment of the present invention;
Figure 11 is a graph showing an output power of the cladding pumped DFB fiber
laser;
Figure 12 is a flow chart illustrating a method for making a fiber Bragg
grating in a
phosphate glass fiber according to an embodiment of the present invention; and
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Figure 13 is a flow chart illustrating a method of using the fiber Bragg
grating in
the phosphate glass fiber according to an embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A novel spliceless optical device that is capable of both (i) outputting a
light signal
having a high power output (in the orders of tens to hundreds of mW), and (ii)
exhibiting
photosensitive properties is discussed next.
Most glasses that are used in optical devices (fibers, lasers, etc.) that have
a high
solubility for erbium and ytterbium, such as phosphate and phosphosilicate
glasses, also
show a lack of photosensitivity that is necessary for fabrication of efficient
fiber gratings.
For laser emission to occur, the active medium is placed inside a resonant
cavity. An
optical feedback can be provided by the reflectivity of the end facets, by
mirrors, by
distributed feedback (DFB) Bragg gratings that act as mirrors, or by
distributed Bragg
reflectors (DBR), or by constructing a ring cavity structure. The laser
emission occurs
when the total gain overcomes the losses in the cavity. Hence, a minimum gain
has to be
achieved to reach the laser threshold condition.
As discussed above, conventional phosphate based lasers do not have good
photosensitive properties and thus, good quality DFB or DBR cannot be made in
this type
of glasses.
Robust single mode (single wavelength) performance can be achieved using a
very
short cavity of less than about 5 cm together with a wavelength selective
reflector. The
output power is dictated by the total absorbed pump power, which is generally
proportional to the number of active ions and therefore, proportional to the
doping level, to
the length of the doped fiber inside the cavity as well as to the
crossectional area of the
active material that is contained in the core of the doped fiber inside the
fiber laser cavity.
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While longer cavities lead to more stringent requirements on the longitudinal
mode
selector, a larger crossectional area typically sacrifices single transverse
mode operation of
the laser.
Thus, output power must typically be traded off against single frequency and
single-mode performance in conventional devices. The spectral linewidth of
single
frequency lasers, defined as the wavelength interval over which the magnitude
of all
spectral components is equal to or greater than a specified fraction of the
magnitude of the
component having the maximum value, is in general determined by a variety of
noise
contributions from the pump laser, the active medium itself, or the laser
cavity.
According to one embodiment of the present invention, fiber Bragg gratings
(FBGs) are formed in a multicomponent phosphate glass fiber as explained next.
Figure 2
shows the phosphate glass fiber 100 being placed under a phase mask 102. In
one
embodiment, preforms were drawn into single mode fibers with core diameters of
13.5
microns, outer diameters of 125 microns and a numerical aperture of 0.08.
Preforms and
fibers were made from multicomponent phosphate glasses that in addition to its
main
constituent phosphate (PZO5) also may contain various amounts of at least one
of BaO,
A1203, and B203. In one embodiment, the BaO, A1203, and B203 materials are not
included into the phosphate glass fiber. In another embodiment, fiber Bragg
gratings were
written in phosphate glass fiber without active ions in the core. In another
embodiment,
fiber Bragg gratings were written in phosphate glass fiber with I wt% Er203
and 8 wt%
Yb2O3 added to the core glass.
The FBG 20 was formed by irradiating the fiber 100 with 193 nm-wavelength,
high intensity pulses from an ArF excimer laser 104 through the phase mask
102. A wave
having a wavelength between 190 and 196 nm is also possible. As shown in
Figure 8,
fiber Bragg gratings with more than 99% reflectivity and stable at high
temperatures were
CA 02604275 2007-09-26
obtained after following the described UV exposure with a thermal treatment
process. As
discussed in more detail later, the reflectivity grows when the gratings are
exposed to an
increased temperature. A small thermal decay occurs when exposing the grating
to 400 C
for one minute and the reflectivity decreases from 99.9% to 99.7%. The FBG 20
may be
formed in one embodiment in an active fiber to make a monolithic fiber laser
that does not
require any fiber fusion splicing.
In one embodiment, an optical filter includes an optical fiber having a core
including multicomponent phosphate glasses, and a cladding surrounding the
core, and a
first fiber Bragg grating formed in a first portion of the core of the optical
fiber and having
an index modulation amplitude greater than 2x 10-5. An input signal provided
in the
optical fiber is partially reflected and partially transmitted by the first
fiber Bragg grating.
The grating shown in Figure 2 constitutes an example of such optical fiber.
In one embodiment, short lengths (3-10 cm) of the multicomponent phosphate
fibers were spliced to standard telecommunications fiber pigtails (Corning SMF-
28) and
positioned immediately behind the silica phase mask used to define the grating
pattern.
The mask 102 had a period of 976.3 nm, corresponding to a fringe pattern
period of
488.15 nm in the fiber. In another embodiment, the diffraction efficiency of
the mask was
controlled by modifying the depth of the grooves of the phase mask, thus
controlling the
fringe contrast and the amount of zero order light that reached the fiber
without diffraction.
The improved diffraction efficiency has been achieved by modifying the depth
of the
grooves in the phase mask. A GSI Lumonics PM-8481aser equipped with an
unstable
resonator cavity and filled with an ArF mixture to generate the 193 nm wave
was used for
the excimer laser 104. However, other lasers with similar characteristics are
also possible.
The laser was generating, in one embodiment of the invention, 80 mJ pulses at
100
Hz, and the pulses had durations of approximately 14 ns. However, these
numbers are
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CA 02604275 2007-09-26
exemplary and values of 60 to 100mJ, 80 to 120 Hz, and 10 to 18 ns for the
energy of the
pulse, its frequency and time duration, respectively, are also possible. The
UV irradiation
of the optical fiber via the mask is performed between 10 to 700 seconds. The
FBG are
formed by interference between the first orders diffraction beams.
After the UV illumination, the fibers are heated (in an oven for example) for
0.5
hours to 10,000 hours. During the heating treatment, an index modulation
amplitude
becomes greater than 2x 10"5. It was observed that the index modulation
amplitude can be
even greater than 104 . Also, the method described above is independent of a
specific
material composition of the optical fiber and is different from conventional
methods in
which, similar to a photographic recording process, chemical reactions are
involved. The
thermal treatment process may use a temperature between 100 C and 400 C.
In one embodiment of the invention, an aperture 106 was used to select the
most
homogeneous part of the excimer laser beam pattern 108, which was then
expanded and
imaged onto the fiber over a length (L) of 14 mm and a fluence per pulse of
400 mJ/cm2.
Optionally, a mirror 110 and lenses (for example cylindrical lens 112) can be
used to
direct the laser beam 108. The reflectivity (R) of the grating was monitored
in situ during
the irradiation by launching a broadband light from a pumped Er-doped fiber
Amplified
Spontaneous Emission source (not shown) and measuring the reflected or
transmitted light
spectra with an Optical Spectrum Analyzer (ANDO AQ6317B) (not shown).
Following the irradiation with UV, the fiber gratings were placed in a
temperature
controlled oven and re-measured at regular intervals over 1000 hours. The
fibers were
removed from the oven and allowed to cool to room temperature for each
measurement.
According to one embodiment, Figure 3(a) shows an exemplary phase mask 102
having grooves with a depth "d" and a period "A." The grooves are formed in
two
sections Al and A2, both having a period of 982.0 nm but different lengths. In
one
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embodiment, the lengths may be the same. The length of section A1 may be 20 mm
and
the length of section A2 may be 15 mm. Other values are also possible. In one
embodiment, a distance "a" between the sections Al and A2 may be 0.05 mm, a
width of
the phase mask grating may be 2 mm, and the depth "d" of the grooves may be
172 nm.
Figure 3(b) shows the numerical values discussed above in a graphical form.
Figures 4(a) and (b) are SEM (scanning electron microscopy) pictures at
different
magnifications of a mask according to one embodiment of the present invention.
Figure
4(c) shows an AFM (atomic force microscopy) image of the same mask and Figure
4(d)
shows a dimensional measurement of the mask. Figures 4(e) and (f) shows high
resolution
SEM images of the same mask and indicate a gap of 50 m between the two
grating
sections AI and A2.
Figure 5 is a graph showing the growth of the grating reflectivity and central
wavelength with increasing the UV dose during formation of the gratings in the
fiber. The
central wavelength QIBragg) shift gives a direct measure of the modal
effective index (Neff)
of the fiber since XBmgg = 2 Neff xA, where A is the grating period in the
fiber, a fixed
parameter determined by the phase mask as discussed above. A shift in XB,agg
reveals the
average change of the refractive index of the fiber induced by the
irradiation. The gratings
are formed by changing (modulating) the index of refraction of the grating at
desired
locations. In one embodiment, the gratings are formed in a first portion of
the core and no
gratings are formed in a second portion of the core of the same fiber.
Moreover, in one
embodiment, the fiber is a single mode fiber.
The reflectivity of the gratings is directly related to the refractive index
modulation
amplitude (An) of the gratings through the equation R = tanh2(7rOnLrj), where
rj is an
overlap factor between the core mode and the cross section of the refractive
index
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modulation. In addition, the transmission spectra shown in Figure 5 indicate
no coupling
to cladding modes and hence a good overlap.
Measurements of the UV-induced index change in the fiber are shown in Figure
6.
The results of Figure 6 show that the average index change (the average of
highest and
lowest values of the index of refraction in the irradiated portion of the
optical fiber)
induced by the irradiation is positive and larger than the index modulation
amplitude. In
the case of a perfect grating with 100% fringe contrast and strictly positive
index change,
the modulation amplitude is equal to the average induced index change.
The formation of the refractive index modulation can be observed during
illumination as shown in Figure 6. The index modulation amplitude is defined
as being
one half of the difference between the highest and the lowest value of the
index of
refraction of the optical fiber in the region irradiated with UV light. It is
noted that both
the lowest and highest values of the index of refraction in the irradiated
optical fiber is
larger than the index of refraction of the optical fiber prior to UV
irradiation.
Large refractive index modulations greater than 2x 10"5 and in one embodiment,
greater than 104 , are obtained after exposure to the UV light of the order of
tens to
thousands of seconds and heating between 0.5 and 10,000 hours.
One concern in the conventional devices with photoinduced refractive index
changes is the thermal stability of the changes. The induced changes in the
fiber,
according to one embodiment, were investigated by placing several samples in
an oven
maintained at 100 C to monitor the development of the induced changes. In
contrast to
conventional gratings fabricated in silica fiber, the grating reflectivities
increased instead
of decaying, as shown by the growth in refractive index modulation plotted in
Figure 7 for
two gratings with different initial reflectivities. A further increase in
temperature to 170 C
led to even larger growth of the refractive index modulations. In the two
cases shown in
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Figure 7, the refractive index modulation amplitude more than doubled with an
increase in
temperature, a feature not exhibited by any of the known conventional devices.
The first
grating FBG I had an initial reflectivity of 52% at 100 C and the second
grating FBG 3
had an initial reflectivity of 87% at 170 C.
Then, a quick anneal at approximately 400 C for about one minute was performed
on the portion of the core having the gratings and it was observed a minimal
thermal decay
of the strongest grating as shown in Figure 8(a). For this grating, lowering
the reflectivity
by 0.2% corresponds to a decrease of the refractive index modulation amplitude
by 13%.
The spectral quality of the gratings in transmission and reflection as shown
in Figure 8(b)
is high and corresponds to shapes for gratings of uniform strength along their
length. In
particular, as noted above, the absence of features on the short wavelength
side of the
transmission spectrum indicates both very good alignment of the grating
fringes
perpendicular to the fiber axis, and very good uniformity of the fringe
pattern across the
depth of the fiber cross-section.
In another embodiment, similar results were obtained in fibers made from the
same
materials but using microstructured cladding for optical mode confinement and
also in
fibers with rare-earth dopants (Er and Yb) in the core to provide gain in the
C-band. Thus,
the fabrication of short, monolithic cavity phosphate glass fiber lasers with
improved
spectral purity and stability and lower fabrication costs can be achieved
based on the
above disclosed materials and methods.
For many applications such as fiber optic sensing, coherent optical
communication,
or as seed laser for laser ranging and LIDAR (Light Detection and Ranging)
applications,
high power (>10 mW and preferably greater than >25 mW), narrow linewidth (<10
kHz)
single mode lasers that operate in the eyesafe spectral region of the
telecommunication
CA 02604275 2007-09-26
band around 1550 nm are in demand. These lasers include fiber, waveguide and
microchip lasers.
Single-mode semiconductor diodes are limited to a maximum optical power of a
few hundred mW by the occurrence of higher order transverse modes above
leading above
a certain level of injection currents. In contrast, multimode semiconductor
laser diodes
can generate several Watts of output power and can be combined to deliver
hundreds of
Watts of optical power through multimode fiber.
However, the deployment of fiber optic sensing require compact low-cost
continuous single-mode lasers that can deliver greater than 50 mW of output
power with a
narrow linewidth. According with an embodiment of this invention, a novel
optical device
is provided that is capable of achieving this power as discussed next.
In one embodiment of the present invention, a distributed feedback Bragg fiber
laser that is optically pumped by multimode diode lasers is discussed. The
laser resonator
is formed by a symmetric or asynnnetric grating structure that provides
distributed
feedback for a signal light that is propagating in the single mode core of the
active fiber.
The grating structure is written directly into a doped single mode fiber by
varying
exposure to UV light, as discussed above with reference to Figures 2 and 6,
and
subsequently thermally treated, as discussed above with reference to Figures 7
and 8. This
fabrication process results in a refractive index variation along the
propagation direction
inside the active fiber.
The core of the active fiber may be doped with various rare earth ions that
absorb
pump light at specific wavelengths and provide optical gain or signal
amplification at
other wavelength, specific for any particular rare earth ions. In one
embodiment, the core
of the phosphate glass is doped with erbium and ytterbium ions that provide
absorption of
pump light and optical gain. The spectral maximum of the optical gain is
around the
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wavelength of 1535 nm and the distributed feedback grating structure has been
designed to
provide feedback for this wavelength, in one embodiment.
As shown in Figure 9(a), in conventional DFB fiber lasers the DFB grating
cavity
200 is pumped with a single-mode pump diode 202. However, the efficiency of
the
single-mode pumping process is low. According to one embodiment as shown in
Figure
9(b), the DFB grating cavity 200 is inside an active single mode phosphate
fiber 208.
Multimode pump light generated by multimode pump diodes 204 is delivered
through a
multimode optical fiber 206 that can be either spliced or mechanically coupled
to the
single mode fiber 208 that contains the DFB grating laser cavity 200.
In one embodiment, the DFB fiber laser cavity 200 may be formed in phosphate
glass fiber with the method described in Figure 2 using a phase mask as
described in
Figures 3 and 4. In one embodiment, all fibers are circular step index fibers.
The active
single mode phosphate fiber 208 has an outer diameter of 125 microns and a
core diameter
of 9 microns appropriate for single-mode guiding (usually less than 20 m core
diameter)
and the multimode delivery fiber 208 has an outer diameter of 125 microns and
a core
diameter of 105 microns, to support a multimode optical signal as generated by
the
multimode pump diodes 204 (multimode pumping scheme).
In another embodiment, both the single mode fiber 208 and multimode fiber 206
are made of the similar or same phosphate glass and the two fibers are fusion
spliced
together.
In one embodiment, as shown in Figure 9(c), multiple DFB grating cavities
200a,
200b, and 200c are formed in corresponding sections of the active phosphate
single mode
fiber 208. The number of gratings and corresponding portions can be between
one and
twenty. Each region may work as a laser. In another embodiment, some gratings
in the
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CA 02604275 2007-09-26
single mode-fiber 208 may act as other known optical devices, for example as a
wavelength filter.
Thus, according to the embodiments shown in Figures 9(b) and (c), one or more
lasers and/or other optical devices can be formed in a monolithic single
optical fiber
without the need to splice together different optical components, thus
improving the
characteristics of the system. In other words, multiple optical components can
be formed
in a single given optical fiber and also, there is no mechanical connection
(interface)
between the various optical components and the optical fiber that guides the
optical signal
as all the optical components are formed in the single optical fiber.
Figure 10 shows the DFB fiber laser emission spectrum of the system 200 shown
in Figure 9(b) measured by an optical spectrum analyser. The cladding pumped
DFB fiber
laser (multimode pumping scheme) emits a narrow laser line located at the
grating
structure design wavelength. The width of the measured line in Figure 10 is
limited by the
resolution of the spectrum analyzer (0.07 nm) and the true emission linewidth
is narrower
on the order of 10 to 100 kHz.
One advantage of the novel multimode pumping scheme is the availability of
pump
sources with much higher optical power at much lower cost compared to single
mode laser
diodes required for conventional core pumping of DFB fiber lasers. As shown in
Figure
11, a multimode pump light over ten Watts (hundreds of Watts are also
possible) can be
launch into the cladding of the active fiber shown in Figures 9(b) and (c) for
example.
The DFB fiber laser was found to operate stable up to - 160 mW of output
power.
This power level is already amongst the highest reported output powers for any
DFB fiber laser indicating the stability of the novel laser. The continuous
performance
over a ten hour period at an output power level of 150 mW was analyzed. The
variations
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CA 02604275 2007-09-26
, , .
in output power and emission wavelength were below 2% and 0.05 nm,
respectively
during this ten hour period.
The novel multimode pumping scheme discussed above can be optimized for low
price, high power, low amplitude and phase noise, stability of operation,
wavelength
tunability, or any combination of the above. It has a simple and robust
structure and in the
applied forward pumping geometry it does not require wavelength multiplexing
structures
(e.g. for pump and signal wavelength as shown in Figure 9(a)) or optical
isolators.
Another advantage of this novel scheme is the easy implementation of a laser
cascading
scheme, as shown in Figure 9(c).
A single high power multimode pump light source can be utilized to pump a
cascade of DFB resonators that can be imprinted into the active fiber and can
be designed
to emit at desired wavelengths.
To summarize, a method of modulating an index of refraction in an optical
fiber,
as shown in Figure 12, includes a step S 1200 of providing the optical fiber
having a core
including multicomponent phosphate glasses, a step S1202 of irradiating a
first portion of
the core of the optical fiber via a phase mask with laser pulses in the
ultraviolet range to
form a first fiber Bragg grating within the first portion of the core, and a
step S 1204 of
heating the irradiated core to increase an index modulation amplitude within
the core to
above 2x 10'5.
The method may also include irradiating the first region of the core with
ultraviolet
light between 10 and 700 seconds and heating the first region of the core
between 0.5 to
10.000 hours, after the irradiating is performed. The method also may include
irradiating
a wave having a wavelength between 190 and 196 nm onto the mask, using a
silica phase
mask having a period of 976.3 run, irradiating pulses having between 60 and
100 mJ at 80
to 120 Hz, each pulse having a time period between 10 and 18 ns, and
irradiating the
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CA 02604275 2007-09-26
pulses over a portion of the core that has a length between 10 and 18 mm and a
fluence per
pulse between 360 and 440 mJ/cm2. The method also may include heating the
optical
fiber at temperatures between 80 C and 400 C, forming a plurality of optical
gratings
within the core at predetermined distances, forming a fiber Bragg grating,
doping the
optical fiber with 1020 to 2x1021 Yb ions/cm3 and 1020 to 2x1021 Er ions/cm3,
and
combining P205 with at least one of BaO, A1203, and B203 to produce the
optical fiber.
A method for generating a laser signal in a laser device including an optical
fiber
having a core that includes multicomponent phosphate glasses, the core having
first and
second portions surrounded by a cladding, and at least two fiber Bragg
gratings in which
an index modulation amplitude is greater than 2x 10"5, the at least two fiber
Bragg gratings
disposed in the first portion of the core at a predefined distance from each
other, is shown
in Figure 13. The method includes a step S 1300 of pumping from a multimode
pump a
multimode wave into the cladding corresponding to the second portion of the
core, a step
S 1302 of transmitting the pumped multimode wave from the cladding to the at
least two
fiber Bragg gratings, a step S1304 of generating the laser signal between the
at least two
fiber Bragg gratings based on the pumped multimode wave, and a step S 1306 of
outputting the generated laser signal through one of the at least two fiber
Bragg gratings.
The method may also include outputting the generated laser signal into a
second
portion of the core, the third portion being spliceless with the first
portion, and generating
multiple laser signals by multiple pairs of fiber Bragg gratings formed in the
first portion
of the core.
Numerous modifications and variations of the present invention are possible in
light of the above teachings. It is therefore to be understood that within the
scope of the
appended claims, the invention may be practiced otherwise than as specifically
described
herein.
