U.S. patent number RE34,729 [Application Number 07/791,891] was granted by the patent office on 1994-09-13 for method and apparatus for efficient operation of an optically pumped laser.
This patent grant is currently assigned to California Institute of Technology. Invention is credited to Donald L. Sipes, Jr..
United States Patent |
RE34,729 |
Sipes, Jr. |
September 13, 1994 |
Method and apparatus for efficient operation of an optically pumped
laser
Abstract
An optically pumped single mode laser, e.g. Nd:YAG crystal (20)
with planoconcave mirrors is increased in efficiency by an order of
magnitude to about 8% by optics (25, 27) for focusing the high
power multimode output of laser diode arrays (21, 22) into the mode
volume (20') of the laser medium (20). A plurality of these
optically pumped single mode lasers (1-4) may be cascaded in a ring
with dichroci mirrors (M.sub.1 -M.sub.4) at the corners for
coupling in the laser diode arrays, each having its own means for
spatially tailoring its beam to concentrate pump distribution
inside the lasing mode volume of the medium. An InGaAlAs pump diode
(30) with its wavelength of the same as a lasing medium makes the
ring unidirectional. The questions raised in reexamination request
No. 90/002,473, filed Oct. 10, 1991, have been considered and the
results thereof are reflected in this reissue patent which
constitutes the reexamination certificate required by 35 U.S.C. 307
as provided in 37 CFR 1,570(e).
Inventors: |
Sipes, Jr.; Donald L. (Lisle,
IL) |
Assignee: |
California Institute of
Technology (Pasadena, CA)
|
Family
ID: |
25126949 |
Appl.
No.: |
07/791,891 |
Filed: |
November 12, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
782711 |
Oct 1, 1985 |
04710940 |
Dec 1, 1987 |
|
|
Current U.S.
Class: |
372/75;
372/69 |
Current CPC
Class: |
H01S
3/07 (20130101); H01S 3/083 (20130101); H01S
3/09415 (20130101); H01S 3/0606 (20130101); H01S
3/08095 (20130101); H01S 3/094038 (20130101); H01S
3/094069 (20130101); H01S 3/09408 (20130101); H01S
3/10092 (20130101) |
Current International
Class: |
H01S
3/0941 (20060101); H01S 3/081 (20060101); H01S
3/083 (20060101); H01S 3/094 (20060101); H01S
3/23 (20060101); H01S 003/093 () |
Field of
Search: |
;372/69.70,75 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
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|
Primary Examiner: Epps; Georgia Y.
Attorney, Agent or Firm: Leydig, Voit & Mayer
Government Interests
ORIGIN OF INVENTION
The invention described herein was made in the performance of work
under a NASA contract, and is subject to the provisions of Public
Law 96-517 (35 USC 202) in which the Contractor has elected to
retain title.
Claims
What is claimed is:
1. .[.An optically.]. .Iadd.A Laser diode pumped .Iadd.single mode
.Iaddend.laser comprising an optical resonator cavity, a laser
medium in said resonator cavity, said laser medium having an
optical axis and two ends, one end at each of two opposite sides of
said medium intersected by said optical axis, an array of laser
diodes positioned for pumping said laser medium in the direction of
the axis of said resonator cavity, .Iadd.said laser diode array
being a source of light of poor spatial and spectral qualities
.Iaddend.and means for causing .[.the pump.]. distribution .Iadd.of
the light .Iaddend.from .[.said.]. .Iadd.the .Iaddend.array of
laser diodes to be concentrated inside .[.the.]. .Iadd.a
.Iaddend.lasing mode volume of said medium .Iadd.at an imput pump
power of at least 100 milliwatts.Iaddend.. .[.
2. An optically pumped laser as defined in claim 1 wherein an array
of laser diodes and said means for concentration is provided at
both ends of said medium, and including a mirror for separating the
output of said medium as an output beam from the pump distribution
at one end of said medium..]. .[.
3. An optically pumped laser as defined in claim 2 including a
second array of laser diodes at one end of said medium, and means
for combining at said one end the output of the second array of
laser diodes with the output of the first array of laser diodes for
a combined pumping beam into said lasing mode volume through
separate means for each array of laser diodes for concentration of
the pump distribution form each of said array of laser diodes
inside the lasing mode volume of said medium..]. .[.4. An optically
pumped laser as defined in claim 2 including a plurality of
cascaded laser mediums arranged with separate mirrors to form a
closed optical polygon for each laser medium, each laser medium
having at least one laser diode array and means for concentration
of pump distribution of light from the laser diode array into the
lasing mode volume of the laser medium through one of said separate
mirrors coupling said laser mediums in said polygon, an injection
locking laser diode operating at the wavelength of said medium, and
means for combining the output of said injection locking laser
diode with the output of one array of laser diodes for pumping one
of said mediums in a predetermined direction around said closed
optical polygon, and means for extracting from said closed optical
polygon output laser beam emanating from one medium in said
predetermined direction..]. .[.5. An optically pumped laser as
defined in claim 2 including a plurality of cascaded laser mode
volumes arranged in a zig-zag pattem with the optical axis of
adjacent optical mode volumes intersecting at a corner of said
zig-zag pattern, a mirror at each corner, each mode volume having
at each end a laser diode array and means for concentration of
light from said laser diode array into the mode volume for pumping
and means at one end of said zig-zag pattern for extracting an
output laser
beam emanating from one laser mode volume..]. 6. .[.An optically.].
.Iadd.A laser diode .Iaddend.pumped single mode laser comprising an
optical resonator cavity, a laser medium in said resonator cavity,
said laser medium having an optical axis and two ends .Iadd.and
being selected from the group (Nd:YAG, ND:GGG and Nd:YLF),
.Iaddend.one end at each of two opposite sides of said laser medium
intersected by said optical axis, an array of .[.multimode.]. laser
diodes .Iadd.that provides multimode light .Iaddend.positioned for
pumping said laser medium in the direction of the axis of said
resonator cavity, and means for concentration of .[.the pump.].
.Iadd.a .Iaddend.distribution .Iadd.of the multimode light
.Iaddend.from said .[.multimode.]. laser diodes inside .[.the.].
.Iadd.a
.Iaddend.lasing mode volume of said laser medium. .[.7. An
optically pumped singlemode laser as defined in claim 6 wherein an
array of multimode laser diode and concentration means is provided
at both ends of said laser medium, and including a mirror at one
end of said laser medium for separating the singlemode output of
said laser medium as an output beam..]. .[.8. An optically pumped
singlemode laser as defined in claim 7 including a second array of
multimode laser diodes at one end of said medium, and means for
combining the output of said second array with the output of said
first array of multimode laser diodes at said one end for a
combined pumping beam directed into said means for concentration of
the
pump distribution from said laser diodes..]. .[.9. An optically
pumped singlemode laser as defined in claim 6 including a plurality
of cascaded single-mode laser mediums arranged in a closed optical
polygon with a separate mirror between each pair of cascaded laser
mediums, each medium having at each end a multimode laser diode
array and concentration means for pumping through said separate
mirrors between cascaded laser mediums into the mode volume of said
laser medium between said separate mirrors, an injection locking
laser diode operating at the wavelength of said medium, and means
for combining the output of said injection locking laser diode with
the output of one laser medium in a predetermined direction, and
means for extracting from said closed optical polygon an output
laser
beam emanating from one medium in said desired direction..].
.Iadd.10. A laser diode pumped laser comprising an optical
resonator cavity, a laser medium in said resonator cavity, the
laser medium having an optical axis and two ends, one end at each
of two opposite sides of the laser medium intersected by the
optical axis, and array of laser diodes positioned for pumping the
laser medium in the direction of the axis of the resonator cavity,
the laser diode array being a source of light having poor spatial
and spectral qualities, and means for concentrating a distribution
of the light from the laser diodes inside a lasing mode volume of
the laser medium such that the laser exhibits an overall
electrical-to-optical efficiency of approximately five (5) percent
or more. .Iaddend. .Iadd.11. A laser diode pumped laser comprising
an optical resonator cavity, a laser medium in the resonator
cavity, the laser medium having an optical axis and two ends, one
end at each of two opposite sides of the medium intersected by the
optical axis, first and second arrays of laser diodes, wherein one
of said arrays is positioned at each of the two ends of said laser
medium for pumping the medium at each end in the direction of the
axis of the resonator cavity, each of the laser diode arrays being
a source of light of poor spatial and spectral qualitites and means
for causing a distribution of the light from each array of laser
diodes to be concentrated inside a lasing mode volume of the
medium. .Iaddend.
.Iadd. A laser diode pumped laser comprising an optical resonator
cavity, a laser medium in the resonator cavity, the laser medium
having an optical axis and two ends, one end at each of two
opposite sides of the medium intersected by the optical axis, first
and second arrays of laser diodes positioned at one end of the
medium for pumping the laser medium in the direction of the axis of
the resonator cavity, the laser diode arrays each being a source of
light of poor spatial and spectral qualities and a focusing
subsystem including a coupling between the arrays of the laser
medium other than a fiberoptical coupling for combining the light
from the arrays to provide a distribution of the light concentrated
inside a lasing mode volume of the medium. .Iadd.13. A laser diode
pumped laser comprising a plurality of cascaded laser mediums
forming a closed optical polygon in an optical resonator cavity,
each laser medium having an optical axis and two ends, one end at
each of two opposite sides of the medium intersected by the optical
axis, each of the laser mediums being associated with an array of
laser diodes positioned for pumping the laser medium in the
direction of the optical axis, each of the laser diode arrays being
a source of light of poor spatial and spectral qualities, means for
causing a distribution of the light from each array of laser diodes
to be concentrated inside a lasing mode volume of the associated
laser medium, and means for extracting from the closed optical
polygon an output laser
beam. .Iaddend. .Iadd.14. A laser diode pumped laser as defined in
claim 13 including a separate mirror between each pair of cascaded
laser mediums. .Iaddend. .Iadd.15. A laser diode pumped laser as
defined in claim 13 including an injection locking laser operating
at the wavelength of the laser mediums and means for combining an
output of the injection locking laser with the output of one array
of laser diodes for pumping one of the laser mediums in a
predetermined direction around the closed
optical polygon. .Iaddend. .Iadd.16. A laser diode pumped laser
comprising an optical resonator cavity, a laser medium in the
cavity, a plurality of cascaded laser mode volumes in the laser
medium each having first and second ends and collectively arranged
in a zig-zag pattern, each laser mode volume having an optical axis
such that the optical axes of adjacent optical mode volumes
intersect at a corner of the zig-zag pattern, a mirror at each
corner, a plurality of said mode volumes having a laser diode at
one or both of said first and second ends for pumping the mode
volume and means for concentration of light from each of the laser
diodes into the mode volume being pumped and means at one end of
the zig-zag pattern for extracting an output laser beam emanating
from one of
the laser mode volumes. .Iaddend. .Iadd.17. A laser diode pumped
laser as defined in claim 16 wherein each mode volume has at each
of its first and
second ends a laser diode array. .Iaddend. .Iadd.18. A laser diode
pumped laser comprising an optical resonator cavity, a laser medium
in said resonator cavity, the laser medium having an optical axis
and two ends, one end at each of two opposite sides of the laser
medium intersected by the optical axis, an array of laser diodes
positioned for pumping the laser medium in the direction of the
axis of the resonator cavity, the laser diode array being a source
of light having poor spatial and spectral qualities, and means for
concentrating a distribution of the light from the laser diodes
inside a lasing mode volume of the laser medium such that the laser
medium exhibits an operating efficiency (.eta..sub.o) of
approximately 90 percent or more. .Iadd.19. A laser diode pumped
laser comprising an optical resonator cavity, a laser medium in
said resonator cavity, said laser medium having an optical axis and
two ends and being selected from the group (Nd:YAG, Nd:GGG and
Nd:YLF), one end at each of two opposite sides of said medium
intersected by said optical axis, an array of laser diodes
positioned for pumping said laser medium in the direction of the
axis of said resonator cavity, the laser diode array being a source
of light of poor spatial and spectral qualities and means for
causing a distribution of the light from the array of laser diodes
to be concentrated inside a lasing mode volume of said medium.
.Iaddend. .Iadd.20. A laser diode pumped laser comprising an
optical resonator cavity, a laser medium in said resonator cavity,
said laser medium having an optical axis and two ends, one end at
each of two opposite sides of said medium intersected by said
optical axis, an array of laser diodes positioned for pumping said
laser medium in the direction of the axis of said resonator cavity,
the laser diode array being a source of light of poor spatial and
spectral qualities and a focusing subsystem including a coupling
between the array of laser diodes and the laser medium other than a
fiberoptical coupling for concentrating a distribution of the light
from the array of laser diodes inside a lasing mode volume of said
medium.
.Iaddend. .Iadd.21. An optically pumped laser comprising an optical
resonator cavity, a laser medium in said resonator cavity, said
laser medium having an optical axis and two ends, one end at each
of two opposite sides of said laser medium intersected by said
optical axis, an array of laser diodes positioned for pumping said
laser medium in the direction of the axis of said resonator cavity,
and means for causing the pump distribution from said array of
laser diodes to be concentrated inside a lasing mode volume of said
medium at an input pump power of at least 100 milliwatts. .Iaddend.
.Iadd.22. An optically pumped laser comprising an optical resonator
cavity, a laser medium in said resonator cavity, said laser medium
having an optical axis and two ends, one end at each of two
opposite sides of said laser medium intersected by said optical
axis, an array of laser diodes positioned for pumping said laser
medium in the direction of the axis of said resonator cavity, and
means for causing the pump distribution from said array of laser
diodes to be concentrated inside a lasing mode volume of said
medium to provide an
output power of at least 80 milliwatts. .Iaddend. .Iadd.23. A laser
as defined in any one of claims 1, 6, 10, 11, 12, 13, 16, 18, 19,
20, 21 having an output power of at least 80 milliwatts. .Iaddend.
.Iadd.24. A laser as defined in any one of claims 1, 6, 10, 18, 19,
20, 21 or 22 wherein the light from the array of laser diodes has
more than one lobe and all of the lobes are directed into the
lasing volume by the means for concentrating the distribution of
the light. .Iaddend. .Iadd.25. A laser as defined in claim 24
wherein the means for concentrating the distribution of the light
from the array of laser diodes is a focusing subsystem having a
coupling efficiency (.eta..sub.o) of approximately 90 percent or
more. .Iaddend. .Iadd.26. A laser as defined in any one of claims
1, 10, 11, 12, 16, 18, 20, 21 or 22 wherein the laser medium is
selected from the group (Nd:YAG,Nd:GGG and Nd:YLF). .Iaddend.
.Iadd.27. A laser as defined in any one of claims 1, 6, 10, 11, 12,
13, 16, 19, 20, 21 or 22 having an operating efficiency
(.eta..sub.o) of approximately 90 percent or more. .Iaddend.
.Iadd.28. A laser as defined in any one of claims 1, 6, 10, 11, 12,
13, 16, 18, 19, 20, 21 or 22 having a total optical conversion
efficiency (.eta..sub.opt) and a quantum efficiency (.eta..sub.q)
whose ratio (.eta..sub.opt /.eta..sub.q) is approximately
0.65. .Iaddend. .Iadd.29. A laser as defined in any one of claims 1
or 21 wherein the input pump power is at least 200 milliwatts.
.Iaddend. .Iadd.30. A laser as defined in any one of claims 1, 6,
10, 18, 19, 20, 21 or 22 wherein the concentration of the
distribution from the array of laser diodes provides an input
intensity of approximately 100 w/cm.sup.2
or more. .Iadd.31. A laser as defined in any one of claims 1, 6,
10, 18, 19, 21 or 22 wherein the means for concentrating the
distribution from the array is a focusing subsystem that includes a
coupling between the array of laser diodes and the laser medium
other than a fiberoptical coupling. .Iaddend. .Iadd.32. A laser as
defined in either claim 11 or 13 wherein the means for
concentrating the distribution of the light from at least one of
the arrays is a focusing subsystem that includes a coupling between
the array of laser diodes and the laser medium other than a
fiberoptical coupling. .Iaddend. .Iadd.33. A laser as defined in
any one of claims 1, 6, 10, 18, 19, 20, 21 or 22 wherein the axis
of said resonator cavity forms a closed polygon. .Iaddend.
.Iadd.34. A laser as defined in any one of claims 1, 6, 10, 18, 19,
20, 21 or 22 wherein said optical resonator cavity is a ring
cavity, and said laser additionally comprises means for insuring
unidirectional power flow within the cavity. .Iaddend. .Iadd.35. A
laser as defined in any one of claims 1, 6, 10, 18, 19, 20, 21 or
22 wherein said laser medium comprises a single crystal, and the
axis of said resonator cavity in the crystal is a zig-zag path.
.Iaddend. .Iadd.36. A laser as defined in claim 35 which comprises
a plurality of pump volumes along said zig-zag path, wherein each
pump volume is pumped by at least one laser diode of the array.
.Iaddend.
Description
.Iadd.
This application is a reissue of 06/782,711 filed Oct. 1, 1985,
U.S. Pat. No. 4,710,940. .Iaddend.
BACKGROUND OF THE INVENTION
This invention relates to optically pumped lasers, and more
particularly to such lasers as laser diode pumped Nd:YAG
lasers.
Many applications require an efficient reliable laser source having
a high peak to average power capability, and capable of emitting a
stable radiation pattern. Optical communication over .[.the.]. deep
space is such an application. ND:YAG lasers pumped by semiconductor
laser diodes have figured prominently among potential sources for
such applications. In this arrangement many GaAlAs/GaAs laser
diodes can be combined to optically pump the ND:YAG laser. Recent
work reported by the inventor in Appl, Phys, Lett. 47(2), Jul. 15,
1985, pp 74-76, has indicated that by utilizing the proper pump
geometry, nearly half of the output of the GaAlAs/GaAs diode laser
can be converted to ND:YAG laser light.
The ND:YAG laser can be thought of as a means for converting
incoherent light from laser diodes to coherent light. In that
manner, many laser diodes with poor spatial and spectral qualities
may be converted into a single coherent source with vastly improved
spatial and spectral properties. Thus, as compared to simply
combining incoherent laser diode sources, a powerful, extremely
bright, coherent laser source can be realized with such a converter
while sacrificing little in size or efficiency. Moreover, the
increased power .[.allow.]. .Iadd.allows .Iaddend.the system
designer of a deep space communication system the added freedom to
trade for increased data rate or decreased aperture or pointing
requirements, thus reducing size mass and complexity of the
communications system.
In prior-art devices, the geometry conventionally used to
accomplish this conversion of incoherent light from laser diodes
into .[.a.]. coherent light is the side-pumped geometry in which
the diodes are placed along the length of the laser medium, such as
a crystal rod (e.g., ND:YAG) or liquid column (e.g., dye laser).
The medium is thus pumped perpendicular to the direction of
propagation of the laser resonator mode. As more power is required,
more diodes can be added along and around the laser medium.
However, this prior-art arrangement is relatively inefficient, and
thus requires large numbers of pump diodes to achieve a respectable
output power level.
SUMMARY OF THE INVENTION
In accordance with the present invention, a laser medium is end
pumped by an array of laser diodes through means for focusing the
output of the laser diodes into the small cross section of the
resonator mode volume of the laser medium in order to produce
TEM.sub.oo mode lasing. Thus, by spatial tailoring of the pump
distribution for mode control, i.e., for sufficient
.[.conoentration.]. .Iadd.concentration .Iaddend.inside the lasing
mode volume of the medium along the axis of propagation, the laser
operates in a single transverse lasing mode, a property desired to
make the pumped laser useful.
In one embodiment, the output of a laser diode array is focused
into the small cross section of the resonator mode volume of the
laser medium. The resonator cavity mirrors are planoconcave with
one having the concave mirror surface coated for high reflection at
the wavelength of the lasing medium, which is distinct from the
wavelength of the laser diode array, and at the other having the
concave mirror surface coated with a reflecting material that will
reflect about 95% of the light at the wavelength of the lasing
medium and transmit as a coherent output the balance. All of the
light at the wavelength of the laser diode array is preferably
reflected by these concave mirror surfaces. In a variation of this
embodiment, a second laser diode array with its own spatial
tailoring means is end coupled by suitable means into the laser
medium, such as by a dichroic mirror or a polarizing beam-splitting
cube. The variation lends itself very well to end pumping a
plurality of lasing mediums and means for coupling each lasing
medium in the ring to the next. At each corner of the ring, a pair
of laser diode arrays are coupled to end pump each adjacent lasing
medium with its own spatial tailoring means. The output of an
injection locking diode is coupled with the output of a laser diode
array by suitable means to provide unidirectional operation of the
ring.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a illustrates schematically the side pumping geometry of the
prior art.
FIG. 1b is an end view of the lasing medium showing the small cross
section of the resonator mode of the lasing medium.
FIG. 2a illustrates schematically the end pumping geometry of the
present invention.
FIG. 2b is a graph of power absorbed per unit length of the end
pumped lasing medium of FIG. 2a.
FIG. 2c is an end view illustrating the mode matching properties of
the geometry of FIG. 2a.
FIG. 3 is a plot of calculated values of semiconductor-laser-pumped
ND:YAG efficiency as a function of mirror reflectivity for several
values of input pump intensity (for the case .[.or.]. .Iadd.of
.Iaddend.perfect mode matching).
FIG. 4 is a plot of calculated ND:YAG efficiency as a function of
input pump power for several values of minimum pump beam radii.
FIG. 5 illustrates the spectral properties of .Iadd.a
.Iaddend.diode-pumped ND:YAG laser; graph (a) is for the 0.81 .mu.m
absorption band in a 1-cm sample of 1% ND:YAG; graph (b) is for the
emission spectrum of a semiconductor laser pump array.
FIG. 6 is a plot of power collection efficiency for standard laser
diodes.
FIG. 7 is a schematic diagram of a unidirectional ring laser
comprised of four end pumped lasers pumped by eight laser diode
sources.
FIG. 8 is a schematic diagram of a variant of FIG. 7.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to the drawings, FIG. la illustrates a laser medium 10
(such as an ND:YAG crystal) with conventional side-pumping
geometry. In this configuration, the incoherent diodes 15
(.[.AlCaAs/GaAs.]. .Iadd.AlGaAs/GaAs, .Iaddend..lambda.=0.81 .mu.m)
are placed along the length of the laser medium and pumped
perpendicularly to the direction of propagation of the laser
resonator mode volume 10'. As more power is required, more laser
diodes can be added along and around the laser rod. The result is a
incoherent-to-coherent lasing converter which is only about 0.5%
efficient. That is much too low for a deep-space optical
transmitter which requires about 5% to 10% efficiency.
The large inefficiencies of the side-pumped geometry for a crystal
or liquid laser result from a small absorption length (usually
.about.3 mm), relatively large pumped volume (so the pumping
density is low), and the small cross section of the resonator mode
volume 10'. There are pumped regions of the total volume where
energy is wasted because of this mode mismatch.
The resonator configuration is planoconcave with one end 11 having
a mirror 12 coated for high reflection at the lasing wavelength
(.lambda..[.d.]. =1.06 .mu.m for ND:YAG) and the other end 13
having a mirror 14 coated for about 95% reflection at that
wavelength; the balance is transmitted as an output beam. FIG. 1b
shows in an end view of the lasing medium 10 the total volume
pumped by an array of the laser diodes 15, and the lasing mode
volume 10'. It can be readily appreciated that only a very small
fraction of the pump light is directed into the mode volume.
In one example of the present invention illustrated in FIG. 2a,
nearly half the light from a .[.CaAlAs/GaAs.]. .Iadd.GaAlAs/GaAS
.Iaddend.laser diode array is converted to ND:YAG laser radiation
using a tightly focused end-pumping configuration. In this
configuration the ND:YAG medium 20 acts as an efficient
incoherent-to-coherent converter of the laser diode light pumped
through both ends from laser diodes or diode arrays. Experimentally
80 mW CW power in a single mode was achieved with a single
semiconductor laser array pump. This corresponds to an overall
efficiency of 8.0%. With two laser diode arrays 21, 22 pumping, one
at each end, even greater power may be achieved out of the ND:YAG
laser. It is even possible to couple yet another laser diode array
23 such as by a polarizing beam splitting cube 24 which reflects
light from the diode array 23, and transmits light from the diode
array 21 to laser optics which includes a focusing lens 25 and
planoconcave mirror 26. The laser diode array 22 at the other end
has its focusing lens 27 and planoconcave mirror 28. It also has
means for separating the laser diode wavelength from the ND:YAG
wavelength to provide an ND:YAG output, such as a dichroic mirror
29 that reflects 1.06 .mu.m and transmits 0.81 .mu.m
wavelengths.
In this end-pumping geometry shown in FIG. 2a, the pump light is
collected and focused to a small spot (typically 50 to 100 .mu.m)
that matches the ends of the resonator mode volume 20'. It is
immediately apparent that this geometry .[.reotifies.].
.Iadd.rectifies .Iaddend.virtually all the inefficiencies that
plague the side-pumped scheme. First, the absorption length can be
made as long as necessary to absorb practically all of the pump
light, as illustrated in FIG. 2b. Second, the pump light can be
focused to provide the intensities needed for efficient lasing.
Finally, the laser diode beams can be adjusted to completely fill
the lasing mode volume as illustrated in FIG. 2c, and thus avoid
loss of light in the lasing medium outside of the lasing mode
volume. This is particularly useful with high power laser diode
arrays which inherently produce a beam with more than a single
lobe, such as two lobes. The focusing system can direct both lobes
into the lasing mode volume.
End pumping was the subject of considerable interest in the
mid-1970s for use as transmitters in optical fiber communications.
R. B. Chester and D. A. .[.Drawgert.]. .Iadd.Draegart .Iaddend.,
"Miniature diode-pumped .[.ND:YAIG.]. .Iadd.Nd:YAlG
.Iaddend.lasers," Appl. Phys. Lett. 23, 235 (1973); M. Saruwateri,
T. Kumura, and K. .[.Otuka.]. .Iadd.Otsuka, .Iaddend."Miniaturized
CW .[.LiNdP4012.]. .Iadd.LiNd.sub.4 O.sub.12 .Iaddend.laser pumped
with semiconductor laser," Appl. Phys. Lett. 29, 291 (1976); and K.
Washia, K. Iwanto, K. Inoue, I. Hino, S. Natsumato and S. Saito,
"Room-temperature cw operation of an efficient miniaturized ND:YAG
laser end-pumped by a superluminescent diode," Appl. Phys. Lett.
29, 720 (1976). Such arrangements using low power, single mode
laser diodes were primarily for achieving a low lasing threshold,
and were not concerned with high power and high efficiency
operation. Laser diodes of sufficient output power have been
available only recently to fully exploit this highly efficient
regime of operation. However, high power laser diodes are multimode
because they are comprised of an array of diodes in a chip which
tend to operate multimode unless operation of the array is
tailored, such as by gain tailoring the array. Such gain tailored
diode arrays are generally of less power output and furthermore are
not yet commercially available with high power output.
To accurately estimate the overall efficiency of an end-pumped
lasing medium, all factors that give rise to energy loss must be
identified. First, there is the quantum efficiency, .eta..sub.q,
which is the ratio of the lasing photon energy to the pumping
photon energy. The quantum efficiency represents the maximum
theoretical limit for laser efficiency. Next, .[..eta..sub.o M.].
.Iadd..eta..sub.o, .Iaddend.the operating efficiency, includes the
resonator losses and conversion efficiency of pump photons into
lasing photons. The mode-matching efficiency, .eta..sub.m, is the
fraction of the pumped cross-section area that lies within the
oscillating mode volume. The fraction of light incident at the
laser rod end that is absorbed in the gain medium (assuming all of
the laser diode light falls within the pump absorption bands) is
designated .eta..sub.abs, while .eta..sub.i and .eta..sub.c
describe the interface and pump light coupling efficiencies,
respectively. Finally, .eta.LD iS the electrical-to-optical
laser-diode efficiency. These factors are separately discussed in
greater detail.
OPERATION EFFICIENCY (.eta..sub.o)
The efficiency with which pump photons are converted to lasing
photons can be calculated as a function of input pump power, cavity
loss, and beam radius. We start with the steady-state rate
equations describing the spatial evolution of the inversion and
photon energy densities: ##EQU1## where S.sup.+ and S.sup.- are the
forward and backward propagating photon energy densities
respectively (J/cm.sup.3), N is the inversion energy density
(J/cm.sup.3), a is the loss coefficient per unit length of material
(cm.sup.-1), .beta. is the stimulated emission coefficient
(.beta.=.sigma./h.nu..sub.1 where .sigma. is the stimulated
emission cross section (cm.sup.2)),.tau..sub.s is the spontaneous
emission lifetime (s), .nu. is the group velocity of the wave in
the medium, and R.sub.p is the pumping power density (W/cm.sup.3).
Any radial dependence is included in the mode matching efficiency
.eta..sub.m and so is neglected here.
These equations can be solved numerically to find the output power
efficiency as a function of the mirror reflectivity for various
input power intensities. FIG. 3 illustrates that for an input
intensity of 10 kW/cm.sup.2, the photon-to-photon conversion
efficiency exceeds 90% (where a single pass loss of 1% and
.sigma.=7.6.times.10.sup.-9 cm.sup.2 given by M. .[.Birnbam.].
.Iadd.Birmbaum .Iaddend., A. Tucker and C. Fincher, "Laser Emission
Cross Section of ND:YAG at 1064 .[.HM.]. .Iadd..mu.m.Iaddend.," J.
Appl. Phys. 52 Mar. 1981, pp 1212-1214 were assumed). FIG. 3 shows
efficiency as a function of output mirror reflectivity for various
input power intensities, and FIG. 4 relates efficiency to input
power for various modal radii.
MODE MATCHING EFFICIENCY (.eta..sub.m)
For the efficient use of pump light for stimulated emission, the
pump light must fall within the Gaussian mode of the resonator and
not be wasted as spontaneous emission. Because the pump beam cross
section may be elliptical and not circular, and because the gain
along the laser rod is nonuniform, laser efficiency varies with
pump and laser mode parameters in a complex way. In D. C. Hall, R.
J. Smith and R. R. Rice, "Pump Size Effects in ND:YAG Lasers,"
Appl. Opt., 19, 1980, pp 3041-3043, it was shown that efficiency is
maximized for the matched mode case: i.e., R.sub.o
.congruent.W.sub.o, where R.sub.o and W.sub.o are the pump- and
laser-mode Gaussian beam waist radii, respectively. Although their
analysis did not take into account high-gain operation, nonuniform
gain distribution, or the divergent nature of Gaussian beams,
R.sub.o .congruent.W.sub.o is still a good design starting point.
Further analysis and experimentation are being conducted to
determine exact conditions for optimum performance.
ABSORPTION EFFICIENCY (.eta..sub.abs)
FIG. 5 shows the absorption spectrum of a 1-cm sample of 1%
Nd.sup.+3 in YAG and the corresponding emission spectrum of the
laser diode pump source. It can be seen from this figure that the
laser diode source spectrum falls well within the main ND:YAG pump
band centered at 807 mn. As illustrated, the ND:YAG absorption
spectrum is fine structured, so experimentation is needed to
determine how precisely the laser diode spectrum needs to be
controlled. From FIG. 5 one can calculate the length of the YAG rod
needed to absorb virtually all of the pump light. For example, a
1-2-cm-long crystal will absorb over 90% of the incident pump
light.
INTERFACE EFFICIENCY (.eta..sup.i)
To achieve sufficient feedback, the gain medium in a laser must be
placed between two mirrors of high reflectivity. The high
reflectivity is usually obtained by coating the mirrors with a
multilayer dielectric coating. Since the pump light must pass
through this coating (or interface) to pump the medium, it is
important that the coating be highly reflective at the lasing
wavelength yet highly transmissive at the pump wavelength. By using
properly designed dielectric coatings, one can produce a mirror
that reflects 99.8% of the light at 1.06 .mu.m, yet transmits over
95% of the pump light at 0.810 .mu.m.
COUPLING EFFICIENCY (.eta..sub.c)
For the device to operate efficiently, pump light must be collected
and focused onto the gain medium efficiently. The focusing system
must be small, have a short working distance, and have a minimum
number of optical components to ensure high throughput and less
sensitivity to motion (displacements). FIG. 6 shows how collection
efficiency varies with the f number of the collecting lens for
standard laser diodes. Systems with over 90% efficiency (collection
and transmission) are available commercially.
TOTAL OPTICAL CONVERSION EFFICIENCY
The total optical conversion efficiency can now be calculated by
simply taking the product of all the subsystem efficiencies
previously mentioned:
quantum efficiency: .[..eta..sub.c .]. .Iadd..eta..sub.q
.Iaddend.=76.7%
operation efficiency: .eta..sub.o .gtorsim.90%
mode matching efficiency: .eta..sub.m .apprxeq.100%
absorption efficiency: .eta..sub.abs .gtorsim.90%
interface efficiency: .eta..sub.1 .gtorsim.95%
collection efficiency: .eta..sub.c .gtorsim.90%
.thrfore..eta..sub.opt =.eta..sub.q .eta..sub.o .eta..sub.m
.eta..sub.abs .eta..sub.i .eta..sub.c .gtorsim.50% The laser is
therefore expected to convert over half of the pump power into
laser light at the ND:YAG fundamental wavelength.
OVERALL ELECTRICAL EFFICIENCY
The total overall electrical-to-optical efficiency is just the
optical efficiency .eta..sub.opt times the power efficiency of the
laser diode pump source, .eta..sub.LD. For commercially available
diode lasers, .eta..sub.LD is 10% or more, so overall efficiencies
of up to and greater than about 5% can be expected. This value is
over 10 times better than the efficiency of previous side-pumped
lasers. It is also to be noted that laser diode efficiency becomes
the limiting factor for highly efficient operation.
RESONATOR PARAMETERS
To achieve the small spot sizes necessary for efficient operation,
the dimensions of a simple (i.e., 2-mirror) and stable resonator
must be small. For example, the confocal resonator, the most
stable, requires mirrors with radii .[.or.]. .Iadd.of
.Iaddend.curvature of 5 cm and a separation of 5 cm to achieve beam
waist radii of approximately 50 .mu.m. This .[.isadvantageous.].
.Iadd.is advantageous .Iaddend.in that it reduces the overall size
of the optical transceiver package-a prime consideration in space
optical communications systems development.
FOCUSING SUBSYSTEM
The focusing subsystem needs to have an f number smaller than 1 to
efficiently collect and deliver the pump light (see FIG. 6). Since
only on-axis performance is required, commercially available
aspheric lenses with f numbers as low as 0.6 can be used for this
purpose. A working distance (i.e., the distance from the optics to
the focal plane) of 1 to 2 cm is required. For a given working
distance, there is a minimum diameter that the incident beam
possesses in order to be focused to the desired size. For Gaussian
beams, the minimum focused spot size is given by: ##EQU2## where
W.sub.o is the radius of the focused spot, d is the diameter of the
incident beam, f is the focal length of the lens, and .lambda. is
the wavelength. Hence, for W.sub.o =50 .mu.m and f=2 cm, d equals
0.02 cm. What this shows is that extremely small optics can be used
to collect and deliver the pump light, thus keeping the overall
size and weight of the laser small.
It is not altogether clear whether or not the pump beam needs to be
anamorphically transformed from its elliptical shape at the laser
diode source to the circular beam of the resonator mode. The
analysis of D. G. Hall, "Optimum Mode Size Criterion for Low Gain
Lasers," Appl. Optics, 20, May 1 1981, pp 1579-1583, seems to
indicate that pump profile shape does not matter much as long as
all of the pump light falls within the resonator mode. There is a
problem with applying this analysis because it does not take into
account the divergent nature of Gaussian beams. However, if
experience shows that it is advantageous to manipulate the cross
section of the pumping beam, it would be a matter of simply the
addition of a cylindrical lens.
PUMPING CONFIGURATIONS
In all the analyses presented thus far, the problem of
concentrating the diode pump power to produce sufficient laser
output power has been neglected. The area into which the laser
diodes themselves must be packed to ensure proper mode matching
over the length of the rod is govemed by conservation of
brightness: A.sub.1 .OMEGA..sub.1 =A.sub.2 .OMEGA..sub.2, where
A.sub.1 and A.sub.2 are the object and image source areas
respectively, and .OMEGA..sub.1 and .OMEGA..sub.2 are the divergent
and convergent solid angles respectively. Since the diode pump
source is anamorphic, the packing requirements for directions
perpendicular to the junction are different than those for
directions parallel to the junction. The relationship between pump
beam and laser-mode size is quite complicated, but estimates based
on .[.convertion.]. .Iadd.conservation .Iaddend.of brightness seem
to indicate that pump diodes must be placed within 100 .mu.m for
efficient operation. An alternative to simple stacking, which has
been used in pumping very short Nd.sup.+3 lasers, is fiberoptical
pump coupling. In this scheme, the pump light is transmitted from
the diodes to the gain medium through multimode fibers. Such fiber
couplers have been built with only 1.5-dB insertion loss. It would
be preferable to increase the ND:YAG power output by a
unidirectional ring arrangement of a plurality of end-pumped
lasers, each as shown in FIG. 2a.
Since the single pass gain of the laser is the integral of the gain
per unit length, several ND:YAG lasers can be cascaded together so
that the packing requirement goes down by a factor .[.or.].
.Iadd.of .Iaddend.1/N compared to a simple double-ended pump (N is
the number of lasers, assuming each laser can be pumped from both
ends, cascaded together). This novel concept is illustrated in FIG.
7. A traveling wave laser consisting of four ND:YAG lasers 1-4
pumped by eight laser diode sources P.sub.1 -P.sub.8 of 200 mW
each, which together produce 1.6 W of pump power, and 800 mW of
output power at 1.06 .mu.m. Unidirectional power flow in this ring
cavity can be insured in several ways; for example by an InGaAlAsP
laser operating at 1.06 .mu.m acting as an injection locking
device. The scale in FIG. 7 is drawn to show that it is possible to
achieve high output powers in very small packages for free-space
optical communications.
Each pair of laser diode pump sources (each with its focusing
optics) at a comer of the laser ring is coupled into the ring by a
dichroic mirror M.sub.1 -M.sub.4 that reflects the wavelength of
the ND:YAG lasing mediums 1-4 at 1.06 .mu.m. A similar mirror
M.sub.5 is used to couple the locking diode laser beam at 1.06
.mu.m into the ring to give it unidirectionality. A mirror M.sub.6
used just for the purpose of positioning the laser diode source 30
may be a plain mirror. Similarly, a dichroic mirror M.sub.7 is used
to couple the output beam at 1.06 .mu.m, and a plain mirror M.sub.8
is used simply to direct the output beam in a desired direction
with respect to the total system that will measure about
4.5.times.4.5 cm, including packaging. Such a small size and high
power output will lend the system to efficient use in space
communications.
Although a highly efficient TEM.sub.oo ND:YAG laser end-pumped by
GaAlAs/GaAs laser diodes has been disclosed as an example, it is
recognized that the concept of the invention may be applied to
lasing mediums other than ND:YAG, such as ND:GGG, ND:YLF, or even a
liquid as used for dye lasers. The concept may also be extended to
other analogous arrangements, such as disclosed in FIG. 8 wherein a
medium 32, such as Nd:YAG crystal, is provided with dichroic
mirrors 33, 34 on opposite sides and planoconcave mirrors 35 and 36
through which the medium is pumped. Each beam path in the medium
constitutes a pump volume as though contained in separate crystals
arranged in a zig-zag pattern. Laser diode arrays 37 with focusing
optics pump from the ends, while similar laser diode arrays 39 with
focusing optics pump at the corners of the lasing beam path in the
medium. An advantage of such an arrangement over the ring
arrangement of FIG. 7 is that it will not require an injection
laser. The reflectivity .[.miorror.]. .Iadd.mirror .Iaddend.36 is
optimized for maximum power output for a given pump power input as
in the single mode volume of FIG. 2a. An output beam is reflected
by a dichroic mirror 40. Consequently, it is intended that the
claims be interpreted to cover such modifications and
equivalents.
* * * * *