Note: Descriptions are shown in the official language in which they were submitted.
1C~71745
This invention provides novel designs of laser resonators and laser
wavelength tuning arrangements.
A typical example of existing methods for tuning the wavelength of
a broad-band laser employs a laser resonator cavity consisting of a partially
~or totally) reflecting mirror and one grating.
; The grating is used in Littrow configuration at the oscillating
wavelength, by which the ray at the oscillating wavelength is reflected back
upon itself. For detailed description of a grating in Littrow configuration
and other properties of a grating see, for example: Principles of Optics by
Born and Wolf., Pergamon Press, New York (1959).
Such a resonator will provide regenerative feedback at the wave-
length for which the grating angle is in Littrow with respect to the resonator's
axis ~determined by thc direction normal to the resonator's fixed mirror). A
broad-band amplifying medium placed within this resonator produces laser
oscillation at the wavelength where the grating acts in Littrow, or in certain
cases at a few closely spaced wavelengths near the peak of the grating's
resolving band-width and determined by the various resonator modes which are
generally spaced in frequency by 2L. Keeping the resonator mirror fixed and
changing the grating angle changes the wavelength of the ray which will behave
in Littrow as it propagates along the resonator axis. This then provides a
means to wavelength tune a laser oscillator.
It is to be noted that such turning of the grating is in general
used to provide coarse wavelength tuning over a wide region. Fine tuning of
the laser is then obtained by keeping the grating angle fixed and changing
th0 spacing between the mirror and the grating by a small amount. There are
well known methods, such a piezoelectric tuning, where the latter can be
achieved stably.
In other examples of existing methods of tuning, a laser cavity is
employed in which a grating is fixed in non-Littrow position at least for some
frequencies, and mirrors or Littrow gratings are placed to reflect rays of
selected wavelengths diffracted by the grating, back upon themselves to the
original grating, thence to the first mirror, see Figure 2, Osgood, Sackett
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and Javan, Measurement of vibrational-vibrational exchange rates for excited
vibrational levels C2 < v S 41 in hYdrogen fluoride gas, The Journal of Chemical
Physics, Vol. 60, No. 4, 15 February 1974. See also United States Patent
3,928,817 and Friesem, Ganiel and Neumann, Simultaneous multiple wavelength
operation of a tunable dye laser, Appl. Phys. Lett., Vol. 23, No. 5,
1 September 1973.
Other arrangements for selection of wavelength or for simultaneous
oscillation at multiple wavelengths exist, for eample those shown in United
States patent 3,872,407 and in Lotem and Lynch, Double-wavelength laser,
Appl, Phys. Lett., Vol. 27, No. 6, 15 September 1975. These and other prior
art arrangements have disadvantages which the present invention overcomes.
According to one aspect of the invention there is provided, in a
laser comprising an optical cavity defined by first and second reflecting
optics, a first amplifying medium within the cavity and means to excite said
medium to produce lasing conditions, said laser including means ~o restrict
the laser beam to a co~mon path for all wavelengths in the region of said first
reflecting optics, the improvement comprising ~eans including spectral dis-
persion apparatus disposed within the optical cavity, positioned to receive a
beam of restricted cross section in said common path from said first reflect-
ing optics and to disperse~said beam to a plurality of paths in which rays of
different wavelength are laterally spaced apart, said paths passing through
said first amplifying medium whereby rays differing in wa~elengths can be sub-
jected to amplification in different spatially separated portions of said first
amplifying medium, said second reflecting optics constructed to reflect
regeneratively said rays back along said paths, in laterally spaced apart con-
dition through said portions of said first amplifying medium for further ampli-
fication, thence from said spectral dispersion apparatus along said common
` path to said first reflecting optics, said laser including a second amplifying
~ medium disposed in the common beam path between said first reflecting optics
- 30 and said dispersion apparatus, in which path said rays are in a non-resolved,
overlapping relation, and mean-s to excite both said first and said second
amplifying medium vhereby a given wavelength amplified in a portion of said
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; first amplifying medium can be present in said common beam path to cause
excitation of laser oscillation at said wavelength in said second amplifying
medium.
According to another aspect of the invention there is provided,
in a laser comprising an optical cavity defined by first and second reflect-
ing optics and amplifying medium within the cavity, said laser including means
to restrict the laser beam to a common path for a band of wavelengths for the
output of said laser, the improvcment comprising means including spectral
dispersion apparatus disposed within the optical cavity, positioned to receive
a beam of restricted cross section in said common path and to disperse said -
beam, said first and second reflecting optics constructed to reflect regenera-
tively said rays back and forth in said cavity along a regenerative path for
amplification by said amplifying ~edium, a second source of radiation separate
from said amplifying medium for producing radiation at a selected frequency
and disposed to transmit said radiation at a relatively weak intensity to
follow the said regenerative path, and means to excite both said second source
; and said first mentioned amplifying medium whereby a given wavelength originat-
ing in said second source can be present in said amplifying medium to determine
the wavelength of radiation amplified by said amplîfyîng medium, (at relative-
ly weak intensity) whereby radiation at the determined frequency and at a
relatively high level of intensity can be produced at said output.
According to one aspect of the invention a pair of similar spectral
dispersion means, e.g., diffraction gratings, are provided within ~he laser
cavity, the first dispersion means arranged to disperse a beam of restricted
cross-section coming from first reflection optics to a first path with rays
at angles dependent upon their wavelength, the second spectral dispersion
means arranged to receive rays from the first path, and to disperse the rays
to a second path in which rays of different wavelength coextend substantially
parallel to each other with distance of separation dependent upon wavelength,
and second reflecting optics constructed to regeneratively reflect the rays
back to the second and first spectral dispersion means and first reflecting
optics. In preferred embodiments a single reflecting optics unit serves the
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function of regenerative reflection for spatially separate rays of various
wavelengths.
According to another aspect of the invention, a laser amplifying
medium is provided in an optical cavity following first reflecting optics
and a spectral dispersion means, whereby rays of different wavelengths pass-
ing through the medium are displaced spatially, thereby enabling their ampli-
fication to occur substantially in different regions of the amplifying medium
and thus limiting their coupling via the amplifying medium.
In certain embodiments the amplifying medium produces multiple
wavelengths from different transitions of a gas, e.g., from a given molecular
rotation-vibration band, and the spatial separation of the rays serves to
limit collisional coupling. In other embodiments the amplifying medium pro-
duces multiple wavelengths sufficiently close in frequency to be coupled to
the same transition by homogeneous broadening and the spatial separation of
rays serves to limit coupling of the wavelengths via such brnadening, e.g., as
in pressure-broadened gas lasers and in dye lasers. In all such cases,
parallelism of the spatially separated rays while propagating through the
amplifying medium, achieved by placing the amplifying medium following both
first and second spectral dispersion means as abovementioned, facilitates the
design and provides uniformity in the conditions for rays at the various
wavelengths.
According to another aspect of the invention, two volumes of ampli-
fying madium are employed, one disposet in the common beam between first
reflecting optics and first spectral dispersion means, and the other provided
in a path where the ~ays of different wavelengths are spatially separate,
thus to avoid coupling via the amplifying medium. By exciting both amplifying
media, a wavelength produced in the amplifying medium în the path can be
present in the common beam path to cause further excitation of laser oscil-
lation at the wavelength in the sacond amplifying medium there. In certain
such embodiments the amplifying medium in the dispersed path is adapted to
produce multiple wavelengths. Advantageously, e.g., where the amplifying
medium in the dispersed path and its excitation system îs pulsed at low power,
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~o7~745
its excitation is triggered first in time, followed with a predetermined
delay by triggering of excitation of the lasing process in the common
path according to an injection phenomenon caused by the weak radiation
from the amplifying medium in the dispersed path.
In the embodiments mentioned, in certain instances, a modulating
means, either active or passive, is disposed in the common beam path
between the first mirror means and the first spectral dispersion means,
arranged to modulate in unison the different wavelengths or to provide
one or multiple injection frequencies for amplifying medium elsewhere in
the resonator.
~ The invention also features one or more apertures disposed in a
; dispersed path, positioned to transmit a selected ray or rays whereby the
wavelength of laser oscillation is determined. In preferred embodiments
- a set of these apertures is adjustably positioned across the dispersed
path for wavelength tuning.
In various embodiments, colinear output of the numerous wave-
lengths is provided through the first reflecting optics of the cavity or
by zero order diffraction from a diffraction means within the cavity that
receives colinear beams from the first reflecting optics.
In various embodiments adapted for tuning or multiple wavelength
selection, the second reflecting optics employed to define the laser
cavity comprises an extended plane mirror constructed to reflect substan-
tially parallel rays back upon themselves to the mentioned pair of
dispersion means while in other embodiments the second reflecting optics
may be a long focal length concave reflector, e.g., 10 times longer than
the optical path in the resonator. In still other embodiments, variable
optics, e.g., a rotary mirror, is employed to select wavelength or to
achieve rapid frequency scanning across a predetermined wavelength band
or bands.
According to the invention, the critical resonator components (the
gratings, mirrors, etc.) can however be kept at a fixed angle with respect
to the laser axis and locked in position. Coarse frequency tuning is then
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accomplished by moving an aperture within the resonator, e.g., by trans-
lation, in a way which is considerably less critical than turning a
grating about its axis. Such configuration lends itself to a rugged
mechanical design free from microphonic and jitter effects.
The invention provides a new way to tune the frequency of a broad-
band laser oscillator over a wide region. In one embodiment, the ~;~
invention makes it possible to operate the broad-band laser on a number -
of frequencies simultaneously, relatively strongly, with each of the
frequencies controllable independently, while the laser beams corres-
ponding to the various simultaneously oscillating frequencies can all
leave the laser oscillator colinearly. In this case, these independently
controllable laser beams will be overlapping at the laser output along
their propagation paths--optimally, the basic dispersion spread will
determine the extent of their overlap.
The colinearity of such output beams of many different frequencies
is an important feature. This will obviate in certain cases the need to
use an alternate method in which several lasers, each oscillating at the
different frequencies, are used and their outputs combined into a colinear
direction through cumbersome use of several beam splitters. The indepen-
dent controllable multifrequency operation of the device, with a colinear
output, will make it possible to excite or probe simultaneously several
transitions of an atomic or molecular system. Multi-quantum excitation
has important application in laser initiated chemistry, molecular photo-
dissociation, molecular or atomic photoionization, isotope separation, and
others.
These and other objects, features, and advantages of the invention
will be understood from the following description of preferred embodiments,
taken in conjunction with the drawings wherein:
Figure 1 is a diagrammatic view of a multiple grating optical path
employed in the invention and Figures 2, 3, 4 and 5 are diagrammatic views
of alternative lasers employing the optical path of Figure l;
Figure 6 is a diagrammatic view, similar to the foregoing, of an
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embodiment employing a single grating and reflective optics;
Figure 7 is a diagrammatic view employing a pair of oppositely
directed prisms for creating the different paths;
Figure 8 is a diagrammatic view of another embodiment of the
separated path amplification feature of the invention;
Figure 9 is a diagrammatic view of an embodiment featuring chirping
across a frequency band; and
Figure 10 is a diagrammatic view of an injection locked laser
according to the invention.
Referring to Figure 1, consider a ray at a wavelength ~1 incident
on a grating along the AB direction. The ray will be diffracted from the
grating in various orders. As an example, consider grating 1 which is
blazed so that most of the energy is diffracted in the first order. Suppose
this grating is to diffract the ray at the wavelength ~1 in the direction
BC1. ~Note that AB direction is not in Littrow at ~1 ~ Consider now a
second ray at an appreciably different wavelength ~2' to be incident on the
same grating, again along the same AB direction. For this ray, the
diffracted ray will be along a path BC2 different from BCl. A second grat-
ing 2 which may be an exact replica of the first grating may then be
placed at some distance from the first grating and parallel to the first
grating. The separation between the two gratings is selected so that,
for a given beam size, the ~1 and ~2 rays incident on the second grating
are resolved and non-overlapping. Inspection shows that, for the two
gratings parallel to each other, the two rays diffracted from the 2nd
grating will follow directions ClDl and C2D2 which are parallel to one
another.
Consider now another beam at an intermediate wavelength ~i (between
~1 and ~2' say~1>~ >~2)' to be again incident on grating 1 along the common
path AB. The diffracted ray at the weavelength ~i will follow the paths AB,
BCi, CiDi. Note that CiDi is parallel to the other two rays in the CD
region.
Consider now a plane N perpendicular to the paths of the rays
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diffracted from grating 2, The intercept of the Al,Ai~ and A2rays on this
plane follow a direction perpendicular to the CD path, defined as the y
axis. If the wavelength of a ray incident along the fixed AB path is
continuously tuned from ~1 to ~2' after diffraction from the first and
the second gratings, its intercept with the fixed plane will continuously
move along the y direction from al to a2.
Referring to Figure 2, a resonator is constructed by placing a long
planar reflecting mirror 3 perpendicular to the CD path, and another
reflecting mirror 4 perpendicular to the common AB path. An aperture
` 10 member 5 defining aperture 6 bounded by blocking walls is disposed in
front of the long mirror, adjustable by micrometer screw 9. The wave-
length region where the resonator can provide high-Q regenerative feedback
now depends upon the position of the aperture 6 along the y direction.
Further, by moving the aperture along the y direction from, say, al
position to a2 position in Figure 1, the resonator is coarse frequency
tuned from ~1 to ~2~ the extended mirror 3 regeneratively reflecting the
wavelength back upon itself wherever the aperture 6 is positioned.
An amplifying medium 7 is provided with a broad amplification band-
width extending at least from ~1 to ~2. For alaser to be oscillated on a
` 20 single tunable frequency, the amplifying medium can be placed in either
the common arm AB, or in the region BC or in the CD region. A more
convenient location for this is the AB region. The frequency tuning of
the tunable wavelength ~t is then obtained by moving the aperture 6.
Referring to the resonator of Figure 3, regenerative feedback is
provided simultaneously at several wavelengths, by providing several
separate apertures along the y axis; specifically the figure shows a
system tunabl-e at two different wavelengths chosen by two apertures,
5a, 5b.
The cross-section of the common arm beam AB is restricted, as by
limiting aperture member 10, to restrict the point of incidence of rays
from mirror 4, to ensure well defined multi-wavelength operation. (In
place of the aperture member, the beam aperture may be similarly
10717~5
restricted by limiting the length of the grating 1, or limiting the size
of mirror 4.)
A basic feature of this multi-wavelength resonator is that it
provides regions, such as BC and CD, where the directions for regenerative
feedback at two different wavelengths are spatially resolved and non-
overlapping. By placing the amplifying medium 7a, 7b in such regions,
highly troublesome coupling of twa (or several) oscillating wavelengths by
the amplifying medium is avoided. Such coupling effects arise from a
variety of nonlinear efects, for instance homogeneous broadening of a
single transition as in dye lasers or high pressure gas lasers, or
collisional coupling of different transitions in a given rotation vibra- -
tion band of a gas. In either case there is a tendency for the energy to
be concentrated mainly in one wavelength and deprived from another, an
effect which can be diminished or entirely avoided by causing ~as in
Figure 3~, the rays at different wavelengths to occupy different regions
in the amplifying medium placed within the resonator. Placement of the
amplifying medium 7a in the path CD has the further advantage that the
various wavelengths are parallel, and of equal path length through the
medium. (In contrast, with the coupling effects mentioned, it is realized
that diffusion coupling between spaced points in the medium, being
relatively time dependent, will not defeat the isolation here achieved,
; particularly if relatively short pulses are employed.)
Another advantage of the embodiment of Figure 3 is that the
simultaneously oscillating frequencies can all be coupled out of the
resonator colinearly by partially transmitting mirror 4 in the common AB
arm. The coupling can also be obtained colinearly via zero-order
diffraction from the first grating, via the arrow in dotted lines. The
zero order diffraction is one for which the angle of diffracted ray with
respect to the normal to the grating is exactly the same as the incident
angle but it occurs on the opposite side of the normal to the grating, i.e.,
the diffracted angle is exactly the negative of the incident angle. Since,
in this embodiment, the angle of incidence of common arm AB is the same
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irrespective of the wavelengths (i.e., the rays corresponding to the
different wavelengths are all incident along the AB path), the zero order
diffraction from the first grating occurs colinearly for all wavelengths,
along the dotted line path in Figure 3.
The above resonator i5 used to obtain an independently controllable
multi-frequency laser, using a molecular rotation-vibration band. For
this, the amplifying medium is placed in the BC or CD region. At a low
gas pressure, the independent frequencies will consist of oscillations at
the different rotation-vibration transitions within the band. At elevated
pressures where collision broadening in the amplifying medium results in
overlapping of all of the transitions within the band, continuous frequency
tuning can be obtained over the entire band.
For modulation of all frequencies a modulator 18 is placed in the
common arm AB, either an active modulator, e.g., an electro-optic modulator,
or a passive modulator, e.g., a saturable absorbing medium, for forming
short pulses.
Referring to Figure 4, in this embodiment the amplifying medium is
a dye laser 7c ~e.g., rhodamine 6g (R6g) pumped by a nitrogen laser 12) and
the different wavelengths ~ 2~ colinear in the output 0, are spatially
separate and parallel in the dye laser with the advantages of avoiding
coupling by homogeneousbroadening via the amplifying medium.
Referring to Figure 5, here amplifying medium 7d with low power
pulsed excitation source 20, is provided in the parallel CD arm, while an
additional amplifying medium 7e provided with high power pulsed excitation
source 22 and subject by itself to coupling difficulties is placed in the
common arm AB. By predetermined delay 24 it is ensured that excitation
source 22 for the common arm fires after pulsing excitation source 20 for
the CD arm, but while radiation produced by excitation 20 persists in the
resonator. The injecting effects of Al and ~2 produced separately in arm
CD force oscillation at both ll and ~2 in the high power medium 7e, despite
tendencies to couple via the amplifying medium.
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1(~7~745
In another mode of operation the excitation of amplifying medium 7d -
by itself can be kept below the threshold for oscillation. The mere
presence of small gain in that medium and the very weak radiation
associated with it will be sufficient to trigger the amplifying medium
7e at wavelength determined by the gain characteristics of the 7d amplify-
ing medium. In still another embodiment both the 7d and 7e amplifying
- medium can be placed in a path where the diffracted rays are spatially
resolved according to their wavelength. `
As shown, the laser of Figure 5 is constructed as a C02 laser for
operation in the 10.6 u band. The adjustable apertures Sa and Sb are
translated parallel to plane mirror 3 to positions corresponding for ~ ;
instance to wavelengths of the p(l8) and p(20) transitions (blocking the
wavelength of the p(l6) transition).
Amplifying medium 7d may be a gas laser at a low pressure and of low
power and the high power system 7e may comprise a high pressure gas laser
employing a photoionization method to produce a uniform high density
plasma gain medium. In other embodiments the amplifying medium 7d can
operate in CW, or the amplifying medium 7e may be pulsed so that gain -
exists in both gain media 7d and 7e simultaneously.
Referring to Figure 6, similar effects to those of Figure 3 are
- obtained employing a single grating lb, two sections of which are employed
` by reflection, e.g., by corner cube 12 as shown. Thus the cavity extends
from mirror 4 to grating lb, thence diffracted to corner cube 12, then
reflected back to grating lb, then diffracted with parallel paths,
~ through amplifying medium to mirror 3. Here, as well as in Figures 2, 3,
- etc., an extensive concave mirror of relatively large focal length, e.g.,
focal length of 30 meter in comparison to a cavity length of 1 to 3 meters,
can be employed in lieu of the plane mirror, with advantages in ease of
alignment, but in some cases with sacrifice in breadth of band width, or
requirement of a smaller common arm beam cross-section.
Referring to Figure 7, other dispersive means can be employed, e.g.,
the parallel prisms 41 and 42, which are oppositely directed, the first
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prism 41 refracting the common beam to the first refracted path Pl and the
second prism refracting the beam to refracted path P2, thence to mirror 3
for regenerative reflection. The amplifying medium in one or both
diffracted paths Pl and P2 can be employed in accordance with principles
mentioned above.
Referring to Figure 8, here a single grating 51 is employed,
serving to diffract varîous wavelengths ~ 2~ ~3~ of a broad-band beam
of restricted cross-section in arm AB. A laser medium 57 is provided in
the diffracted path whereby laser amplification of each wavelength occurs
while the rays are separated. The rays are returned back upon themselves,
by a mirror arrangement 53~ through the amplifying medium, to the grating,
thence colinearly to partially transmissive mirror 54 through which a
colinear output at the various frequencies is obtained. The advantages of
physical separation of the rays in the amplifying medium are present here
too.
Referring to the embodiment of Figure 9, the laser here shown is
similar to that of Figure 3, except that the two substantially identical
gratings 71, 72 are offset slightly from parallel, angle ~, so that the
rays in the second diffracted path P2 are slightly Ollt of parallel. Also,
the mirror 73 is mounted to rotate, c.g., by constant speed drive 80 or by
a limited rotation, oscillating motor. By feedback of the position of
the mirror, employing light beam B reflected from the back of the mirror
to sensor 83, laser excitation source 84 is triggered as the rotating mirror
approaches perpendicular relation to the first wavelength ¦l. Thereby,~ 1
is regeneratively reflected and laser oscillation occurs at~ 1 As the
mirror progresses to perpendicular relation to other rays, in sequence,
regenerative reflection shifts to those wavelengths. Thus the laser is
chirped to produce a laser pulse over which the frequency changes during
time due to rotation of the mirror. Here, the amplifying medium can be
placed in the non-common arms CD or BC and apertures can be employed in
the diffracted path to restrict laser oscillation to selected frequencies.
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107174S
An external source of radiation can also be employed to advantage
with the novel resonator of the invention. According to the embodiment
of Figure 10, a laser cavity similar to that of Figures 3 or 4 is employed.
The output of an external laser 90, preferably after passing through ;~
isolator 92, enters the cavity through the first mirror and locks laser ` `~
oscillation produced by amplifying medium 94 or 96. Output is obtained
through beam splitter 87.
One advantage offered by the resonator cavity for injection
purposes lies in the many resonator modes offered by the arrangement.
; 10 Even further resonator modes can be obtained in certain cases by using
an unstable laser construction~ e.g., by use of convex mirrors. The many
resona~or modes assure that a resonant path is found by rays of the
desired wavelength despite variations in the optical properties of the
resonator, e.g., variation in the refractive index of the amplifying
medium, etc.
