U.S. patent number RE33,722 [Application Number 07/568,587] was granted by the patent office on 1991-10-22 for optical system with bright light output.
This patent grant is currently assigned to Spectra Diode Laboratories, Inc.. Invention is credited to Donald R. Scifres, D. Philip Worland.
United States Patent |
RE33,722 |
Scifres , et al. |
October 22, 1991 |
Optical system with bright light output
Abstract
An optical system producing bright light output for optical
pumping, communications, illumination and the like in which one or
more fiberoptic waveguides receive light from one or more diode
lasers or diode laser bars and transmit the light to an output end
where it is focused or collimated into a bright light image. The
input end of the fiberoptic waveguide may be squashed into an
elongated cross section so as to guide light emitted from an
elongated light source such as a diode laser bar. The waveguides
are preferably arranged at the output end into a tightly packed
bundle where a lens or other optical means focuses or collimates
the light. For diode laser bars much wider than 100 microns, a
plurality of waveguides may be arranged in a line to receive the
light, and then stacked at the output in a less elongated
configuration. In this manner, light from many diode lasers or
laser bars may be coupled through the bundle into the end of solid
state laser medium.
Inventors: |
Scifres; Donald R. (San Jose,
CA), Worland; D. Philip (San Jose, CA) |
Assignee: |
Spectra Diode Laboratories,
Inc. (San Jose, CA)
|
Family
ID: |
26720614 |
Appl.
No.: |
07/568,587 |
Filed: |
August 15, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
43612 |
Apr 28, 1987 |
04763975 |
Aug 16, 1988 |
|
|
Current U.S.
Class: |
385/33; 385/15;
385/115 |
Current CPC
Class: |
G02B
6/2856 (20130101); G02B 6/2552 (20130101); H01S
3/09415 (20130101); G02B 6/4203 (20130101); G02B
6/262 (20130101); G02B 6/425 (20130101); G02B
6/2808 (20130101); H01S 3/094057 (20130101); H01S
5/4031 (20130101); H01S 5/4025 (20130101); H01S
5/4062 (20130101); H01S 3/094053 (20130101) |
Current International
Class: |
G02B
6/28 (20060101); G02B 6/26 (20060101); G02B
6/255 (20060101); G02B 6/42 (20060101); H01S
3/0941 (20060101); H01S 5/40 (20060101); H01S
5/00 (20060101); G02B 006/26 () |
Field of
Search: |
;350/96.15,96.24
;372/6,22,27,34 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Chesler et al., Appl. Phys. Lett., vol. 23, No. 5, Sep. 1, 1973,
pp. 235-236, "Miniature Diode-Pumped Nd:YAG Lasers"..
|
Primary Examiner: Ullah; Akm
Attorney, Agent or Firm: Schneck & McHugh
Claims
We claim:
1. An optical system for producing a bright light output
comprising,
a plurality of light sources, each of said light sources emitting
light from a given emissive area with given lateral and transverse
divergences,
a plurality of fiberoptic waveguides, each of said waveguides
having an oblong input end and an output end, each oblong input end
positioned relative to one of said light sources for accepting
light therefrom, each oblong input end having core dimensions and
lateral and transverse numerical apertures corresponding
respectively to said emissive area and said lateral and transverse
divergences of said one of said light sources so as to guide most
of said light emitted by said light source, said fiberoptic
waveguides being arranged at said output ends to form a bundle.
2. The system of claim 1 wherein each of said light sources is a
diode laser.
3. The system of claim 1 wherein each of said light sources is a
laser diode bar, each input end of said waveguides having an oblong
core with a width at least as great as that of said laser diode
bar.
4. The system of claim 1 wherein each light source is a pulsed
laser.
5. The system of claim 4 wherein said light sources are operated
serially such that the total light intensity from all of said light
sources is substantially constant in time.
6. The system of claim 1 wherein each light source is a continuous
wave laser.
7. The optical system of claim 1 further comprising optical means
in proximity to each output end of said bundle of waveguides for
focusing said light into a bright light image.
8. The system of claim 7 wherein said optical means couples said
light into an end of a solid state active medium in a resonant
optical cavity.
9. The system of claim 8 wherein said optical means is a focusing
lens between the output ends of said bundle of waveguides and an
end of said medium, the lens matching said light to a desired mode
of said resonant optical cavity.
10. The system of claim 8 wherein said optical means is butt
coupling said bundle to an end of said medium.
11. The system of claim 1 wherein said waveguides are arranged in a
substantially circular cylindrical bundle of hexagonal-close-packed
waveguides.
12. The system of claim 1 wherein said input end of said waveguide
has a curved lens surface.
13. The system of claim 1 wherein the output ends of said
waveguides terminate in a bead with a curved lens surface.
14. The system of claim 1 wherein the output end of said bundle is
fused to form a homogeneous mixing rod.
15. The system of claim 1 wherein the output end of said bundle is
tapered to a smaller diameter than the bundle diameter.
16. The system of claim 1 wherein a homogeneous mixing rod is
butted to the output end of said bundle of waveguides.
17. The system of claim 16 wherein a partially reflecting mirror is
disposed on an end of the mixing rod.
18. An optical system comprising,
a laser diode bar emitting a plurality of light elements from an
oblong emissive area having a width and a height, the light
elements radiating with characteristic lateral and transverse
divergences, the emissive area being divided into a plurality of
segments, each segment having at least one light element,
a plurality of fiberoptic waveguides, each of said waveguides
having an oblong input end and an output end, the oblong input end
of each waveguide being positioned relative to said laser diode bar
for guiding light elements from one of said segments, the elongated
input end of each waveguide having core dimensions and lateral and
transverse numerical apertures corresponding respectively to a
segment and said characteristic lateral and transverse divergences
so as to guide most of said light emitted from said segment, said
waveguides being arranged at said output end to form a bundle.
19. The optical system of claim 18 further comprising,
optical means in proximity to said output ends of said bundle of
waveguides for coupling said light elements into an end of a solid
state active medium in a resonant optical cavity.
20. The system of claim 19 wherein said optical means is a focusing
lens between said output ends and said medium.
21. The system of claim 19 wherein said bundle is butt coupled to
an end of said medium.
22. An optical system for producing a bright light output
comprising,
a plurality of light sources, each of said light sources emitting
light from a given emissive area with given lateral and transverse
divergences,
a plurality of fiberoptic waveguides, each of said waveguides
having an oblong input end and an output end, each oblong input end
positioned relative to one of said light sources for accepting
light therefrom, each oblong input end having core dimensions and
lateral and transverse numerical apertures corresponding
respectively to said emissive area and said lateral and transverse
divergences of said one of said light sources so as to guide most
of said light emitted by said light source, said fiberoptic
waveguides being arranged at said output ends to form a bundle,
and
optical means in proximity to each output end of partially
reflecting a portion of said light emerging from said waveguides
back into said waveguides thereby coupling said portion of said
light into said light sources to induce coherent emission of said
light sources.
23. The optical system of claim 22 wherein said waveguides are
single mode waveguides. .Iadd.
24. The system of claim 1 wherein said oblong input end of each of
said fiberoptic waveguides is rectangular. .Iaddend. .Iadd.25. The
system of claim 18 wherein said oblong input end of each of said
fiberoptic waveguides is rectangular. .Iaddend. .Iadd.26. The
system of claim 22 wherein said oblong input end of each of said
fiberoptic waveguides is rectangular. .Iaddend. .Iadd.27. An
optical system for producing a bright light output comprising:
a plurality of light sources, each of said light sources emitting
light from a given emissive area with given lateral and transverse
divergences,
a plurality of fiberoptic waveguides, each of said waveguides
having an oblong input end and an output end, each oblong input end
positioned relative to one of said light sources for accepting
light therefrom, each oblong input end having core dimensions
corresponding to said emissive area and a numerical aperture
capable of guiding most of said light emitted by said light source,
said fiberoptic waveguides being arranged at said output ends to
form a bundle. .Iaddend. .Iadd.28. The system of claim 27 wherein
said oblong input end of each of said fiberoptic waveguides is
rectangular. .Iaddend. .Iadd.29. The system of claim 27 wherein
said numerical aperture of each input end corresponds to the larger
of said lateral and transverse divergences of said one of said
light sources.
.Iaddend. .Iadd.30. An optical system comprising:
a laser diode emitting a plurality of light elements from an oblong
emissive area having a width and a height, the light elements
radiating wih characteristic lateral and transverse divergences,
the emissive area being divided into a plurality of segments, each
segment having at least one light element,
a plurality of fiberoptic waveguides, each of said waveguides
having an oblong input end and an output end, the oblong input end
of each waveguide being positioned relative to said laser diode bar
for accepting light elements from one of said segments, the oblong
input end of each waveguide having core dimensions corresponding to
a segment and a numerical aperture capable of guiding most of said
light emitted from said segment, said waveguides being arranged at
said output ends to form a bundle. .Iaddend. .Iadd.31. The system
of claim 30 wherein said oblong input end of each fiberoptic
waveguide is rectangular. .Iaddend. .Iadd.32. The system of claim
30 wherein said numerical aperture of each waveguide corresponds to
the larger of said lateral and transverse divergences of said light
elements. .Iaddend. .Iadd.33. The system of claim 30 wherein said
bundle has width and height dimensions such that the ratio of width
to height of said bundle is less than the ratio of width to height
of said emissive area of said laser diode bar. .Iaddend. .Iadd.34.
The system of claim 30 further comprising optical means in
proximity to said output ends of said bundle of waveguides for
coupling said light elements into a solid state laser medium.
.Iaddend.
Description
TECHNICAL FIELD
The present invention relates to optical systems for producing a
bright light output, and in particular to fiberoptic systems having
a laser light source.
BACKGROUND ART
Many different methods are available for pumping solid state
lasers, such as neodymium doped yttrium aluminum garnet (Nd:YAG)
lasers. One common technique is to place a rod of solid state
material at one focus of a tubular reflector having an elliptical
cross section and a flash lamp or other bright light source at the
other focus. In such an arrangement, light emitted by the flash
lamp and reflector from the reflector walls will impinge on the
rod. One problem with this arrangement is that the rod must have a
diameter large enough to absorb a substantial portion of the
pumping radiation during passage through the rod. If during this
initial traverse the pumping illumination is not absorbed, it is
likely to be reflected by the reflector walls back to the light
source, where it will be reabsorbed, generating heat and reducing
the lifetime of the source. Another problem is that much of the
optical energy produced by flash lamps and other broadband light
sources is wasted, because it does not match the absorption
spectrum of the laser medium.
In U.S. Pat. No. 3,982,201, Rosenkrantz et al. disclose a solid
state laser in which a Nd:YAG laser rod is end pumped by an array
of semiconductor laser sources. The wavelength of the pump light
from the diode lasers is selected for optimum absorption, while the
increased optical path length of the pump light within the rod from
end pumping relative to that from side pumping ensures more
complete absorption. The lasers are cryogenically cooled and
operate in a pulsed mode with a low duty cycle to enable heat
generated by the diodes to dissipate between pulses and thereby
maintain the array at a temperature which provides the desired pump
wavelength.
Individual diodes and diode lasers, as well as arrays of diode
lasers just described, have been directly coupled to the end of
solid state rods to achieve low to medium power laser output.
However, the power outputs of these solid state lasers are limited
by the brightness of the pump light. In order to achieve higher
power, brighter pump sources are required along with efficient
means for coupling light from these sources into the solid state
medium.
In U.S. Pat. No. 4,575,854, Martin discloses a Nd:YAG laser which
is side pumped by a plurality of diode laser bars. Each bar
contains many diode lasers which in turn circumferentially envelope
a Nd:YAG rod or other suitable solid state medium. The bars are
driven by a high frequency pulse which is switched so as to drive
the bars in any desired combination, but not all at the same time.
The laser operates in a continuous wave mode even though any given
laser bar is pulsed at a very low duty factor so that the array may
operate uncooled.
Diode laser bars with 30 W peak power output, a 50 Hz repeat rate
and a 150 .mu.sec pulse length have been demonstrated, and it would
be desirable to use laser bars to end pump a solid state laser.
However, if one were simply to butt couple a diode laser bar to the
end of a solid state rod, the laser would not be efficiently
pumped, because of the elongated diode laser bar geometry. Laser
bars may have a lateral dimension or width of up to 1 cm, which is
too great to form a small enough image to fit within the
fundamental mode volume of a solid state rod. Typical rods are 3 mm
in diameter and have a fundamental mode volume about 100 .mu.m in
diameter.
In U.S. Pat. No. 4,653,056, Baer et al. disclose an intra-cavity
frequency-doubled Nd:YAG laser which allows efficient coupling by a
high power laser diode array, despite the fact that the diode array
has an output beam with too much spatial structure and limited
focus-ability. Baer et al., achieve this result by expanding the
lasing volume to match the focused image of the laser diode array.
A combination of a concave output coupler mirror and a lens-shaped
end at the front end of the Nd:YAG rod enables the beam size within
the YAG rod to be adjusted to the appropriate volume. For efficient
pumping, the pumping volume must overlap and preferably match
closely the lasing volume of the rod.
It is not always possible or desirable to change the shape and size
of the mode volume to match the pump light of a diode laser bar.
Further, it may be desired to further increase the power by using a
plurality of diode laser bars for pumping, as taught for example by
Martin. Thus, ways of pumping a bright source that fits the
available mode volume of a solid state laser are sought. It may
also be desired to operate a plurality of diode laser bars
continuously, but still provide the necessary heat dissipation.
An object of the present invention is to produce an optical system
producing a bright light output.
Another object of the present invention is to produce an optical
pumping system for end pumping a solid state laser.
DISCLOSURE OF THE INVENTION
The above objects have been met with optical systems in which one
or more fiberoptic waveguides receive light from one or more diode
lasers or diode laser bars and transmit the light to an output end
where it is formed into a bright light image. The input end of a
fiberoptic waveguide, which in the prior art is typically round
with a diameter substantially equal to that at the output end, has
in the present invention elongated core dimensions and lateral and
transverse numerical apertures corresponding respectively to the
typically elongated emissive area and to the lateral and transverse
divergences of the light source to which it is coupled, so as to
guide most of the light emitted by that source. In other words, the
fiber may be squashed, flattened or otherwise elongated at the
input end to match the light output by the diode laser or diode
laser bar, and may be tapered to a smaller core area at the output
end to match the solid state rod or slab. The optical power emitted
at the output end of a fiber waveguide is greater than 50% of the
total diode laser power output, and waveguide outputs as high as
88% of total laser output have been demonstrated. However, because
the light emitted at the output end of an elongated fiber has a
higher power density than that obtained by butt coupling a laser
array to a circular core fiber of the same diameter as the laser
array, the overall pumping efficiency is improved. Further, because
the fiber coupled laser diode can be remotely located, thermal
dissipation in the region of a solid state laser medium is not a
problem.
In the case in which a plurality of diode laser or laser bar light
sources are used, a plurality of fiberoptic waveguides like those
just discussed are arranged at their output ends to form a bundle.
Focusing optics proximate the output end may be used to image the
light into a very bright spot forward of the end of the bundle, or
into a collimated beam. Alternatively, butt coupling as an input to
another device may be used. Each input end of a fiber waveguide is
positioned proximate to one of the light sources for accepting
light therefrom. In this manner, the combined optical power from
many diode lasers or diode laser bars may be coupled through the
fiber bundle into the end of a solid state laser medium.
In another embodiment, light from a fiber bundle is made to impinge
on a partially reflecting mirror surface formed or positioned so
that some of the light is reflected back into the fibers and
ultimately back into the laser diodes. This serves to lock all
laser diodes in-phase and creates a multiple laser coherent source.
In this embodiment, single mode fibers are preferred and
polarization preserving fibers could be used. Fiber ends could also
be anti-reflection coated to maximize the throughput.
In a further embodiment, a laser diode bar emitting an array of
light elements from an elongted emissive area is coupled into a
plurality of fiberoptic waveguides. Each waveguide is positioned
proximate to the diode laser bar and has a flattened input end so
as to accept light elements from one segment of the array. The
waveguides may then be stacked at the output end to emit a less
elongated light beam made up of the individual light elements. A
lens or other optical means may then couple the light into the
cavity mode volume of a solid state laser medium if desired.
The present invention solves the problem of coupling elongated
light sources, such as diode laser bars, into the mode volume of a
solid state laser. The invention also produces a bright light
source, especially where light divergence precludes lenses and
where spacing and thermal dissipation problems preclude use of
multiple sources. Using fiberoptic waveguides, the problem of
increasing brightness is separated from usually related problems of
spacing and heat dissipation. In addition to being used to
optically pump solid state lasers, the optical system of the
present invention, producing bright light output, may be used for
surgery, scribing, welding, cutting, illuminator systems, such as
beacons, communications links, rangefinders and the like.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a simplified side view of an optical system of the
present invention with a solid state laser.
FIG. 2 is a perspective view of a laser light source and the input
end of a fiberoptic waveguide in the optical system of FIG. 1.
FIG. 3 is a front end view of a fiberoptic bundle of the optical
system of FIG. 1.
FIG. 4 is a side plan view of the fiberoptic bundle in FIG. 3 butt
coupled to an end of a solid state laser medium.
FIG. 5 is a side plan view of a second optical system and solid
state laser embodiment of the present invention.
FIG. 6 is a side plan view of a third optical system and solid
state laser embodiment of the present invention.
FIG. 7 is a side plan view of a fourth optical system and solid
state laser embodiment of the present invention.
FIG. 8 is a perspective view of a diode laser bar and a plurality
of fiberoptic waveguides for another optical system of the present
invention.
FIG. 9 is an end view of the diode laser bar of FIG. 8 taken along
the line 9--9.
FIG. 10 is an end view of the stacked output end of the fiberoptic
waveguides of FIG. 8.
FIG. 11 is a perspective view of the output end of FIG. 10 coupling
to a solid state laser slab.
FIGS. 12-16 are side perspective views of embodiments of the output
end of a fiberoptic waveguide bundle in accord with the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
With reference to FIG. 1, an optical system for producing a bright
light output useful for pumping a solid state laser includes a
plurality of semiconductor laser light sources, such as diode
lasers or diode laser bars 11, 12 and 13. Typically, there are
seven laser light sources, although the number may vary. Each of
the laser light sources 11, 12 and 13 emits a light beam 25 which
couples into one of a plurality of fiberoptic waveguides, such as
waveguides 17, 18 and 19. The waveguides 17, 18 and 19 are arranged
to form a bundle 23, and emit the light 25 guided by waveguides 17,
18 and 19 from light sources 11, 12 and 13 and emitted at an output
end 27 of the waveguide bundle 23. Optics, such as the lens of lens
system 29 in FIG. 1, may be disposed in front of output end 27 to
focus the light 25 into a bright light image which may, for
example, couple into the end 31 of a solid state laser medium 33.
Alternatively, the fiber bundle may be butt coupled to the rod.
Note that the solid state laser rod may act as a light guide via
total internal reflection in order to confine the diverging pump
light rays, thereby allowing for more efficient light
absorption.
Each light source 11, 12 and 13 is typically a high power
semiconductor diode laser bar, such as any of the (GaAl)As phased
array lasers or broad area lasers known in the art. Alternatively,
semiconductor diodes and individual diode lasers may be used.
Lasers and laser arrays composed of light emitting semiconductor
materials other than (GaAl)As, such as InGaAsP, may also be used.
Typically, each light source emits with a continuous wave optical
power output of at least 200 mW, and preferably at least 500 mW.
Semiconductor laser arrays with facet windows fabricated by silicon
impurity induced disordering have been demonstrated with continuous
power outputs of up to 3 Watts before exhibiting catastrophic facet
damage. One such laser is disclosed in an article by R. L.
Thornton, et al. in Applied Physics Letters, vol. 49, no. 23, Dec.
8, 1986, pp. 1572-1574. In short pulse operation, i.e. shorter than
1 .mu.s pulse lengths, the catastrophic power limit increases
inversely as the square root of the pulse length. Lasers with 100
nsec pulses and 10 kHz repeat rate can be used in the present
invention to produce an optical system with 20 W peak power output
from the fiber bundle 23. The lasers can be modulated up to
gigahertz rates by simply varying the drive current so that the
system can be used as part of a communication link.
A power supply 35, in electrical communication with laser light
sources 11, 12 and 13, as indicated by arrow 37, supplies
electrical power to drive the light sources. Typically, the light
source operate at about 25% electrical to optical overall
efficiency. Efficiencies of up to 50% have been achieved.
Accordingly, each laser light source producing 500 mW continuous
optical power output draws about 1 A current and 2 V. Thus, power
supply 35 may be a lightweight portable battery unit delivering at
least 14 W. The light sources 11, 12 and 13 may be connected in any
of various series-parallel combinations depending on the most
convenient power supply. The remaining electrical power drawn by
the light source results in heat which is removed by heat sinks 39,
40 and 41. Temperature control may be required in some optical
pumping applications to ensure that the wavelength of the laser
output coincides with the absorption band of lasing material 33.
The system typically operates at the ambient temperature, i.e.
about 15.degree. C. to 25.degree. C.
With reference to FIG. 2, each laser light source 11 has an
elongated emissive area. For example, the preferred diode laser bar
11, seen in FIG. 2 typically has a substantially rectangular
emissive area with a lateral dimension or width of about 100 .mu.m
and a transverse dimension or height of about 1 .mu.m. The far
field profile of the emitted light 45 from such a laser is
characterized by lateral and transverse divergences, typically
about 7.degree. and 30.degree. respectively.
In order to efficiently couple light 45 from laser bar 11 into a
fiberoptic waveguide 17, the waveguide is squashed at one end into
an elongated shape. Waveguide 17 comprises a transparent core 47
characterized by a first index of refraction, and a cladding 49
surrounding core 47 and characterized by a second lower index of
refraction. The cladding 49 is preferably thin, i.e. about 2 .mu.m
thick. The numerical aperture (N.A.) of a fiberoptic waveguide,
i.e. the sine of the half-angle within which the fiber can accept
or radiate light guided by the fiber, is a function of the indices
of refraction of core 47 and cladding 49 and other factors, and is
indicative of the ability of the fiber or waveguide to couple light
from laser light sources 11, 12 and 13. Typically waveguide 17 is
an 0.3 N.A. waveguide with a 50 .mu.m core diameter circular cross
section, which is then elongated at an input end 51 to an
elliptical cross section with 120 .mu.m by 20 .mu.m core
dimensions. Transition from circular to elliptical cross section is
about 0.5 cm but the fiber could be several meters in length.
Squashing the fiber waveguide causes the core material to emerge
slightly from the cladding in the form of a semiellipsoidal lens.
This lens may be retained to further improve coupling into the
fiberoptic waveguide 17 or the waveguide end 51 may be mechanically
polished flat for butt coupling to laser bar 11. Fiberoptic
waveguides with rectangular cross sections may also be used.
The fiberoptic waveguide 17 is shown in FIG. 2 to be spaced
proximate to laser bar 11. Typically, however, waveguide 17 abuts
laser bar 11 with the major elliptical axis of the waveguide core
aligned with the lateral direction of laser bar 11. The waveguide
17 efficiently couples light from laser bar 11 because the
elongated core dimensions correspond closely to the dimensions of
the laser bar's emissive area. Further, the input numerical
apertures measured in both the lateral and transverse directions
are altered by the flattening process from the intrinsic numerical
aperture of the circular core fiberoptic waveguide from which
waveguide 17 is formed. Typically, waveguide 17 has a lateral
numerical aperture of about 0.125, corresponding to acceptance of
light with up to 14.degree. lateral divergence. Waveguide 17 also
has a transverse numerical aperture of about 0.75, corresponding to
acceptance of light with up to 97.degree. transverse divergence.
These acceptance angles are at least twice the lateral and
transverse divergences of the laser bar 11, sufficient to couple
most of the light emitted by laser bar 11. Typically, the coupling
efficiency is at least 50% and typically about 75%. Coupling
efficiencies as high as 88% have been achieved.
With reference to FIG. 3, the plurality of fiberoptic waveguides
17, 18 and 19 in FIG. 1, are arranged into a bundle 23 of
waveguides, here indicated by the reference numerals 55a through
55g. Typically, the waveguides 55a-55h are arranged in a hexagonal
close packed array for maximum brightness. Seven waveguides, having
core diameters of about 50 .mu.m and cladding thicknesses of about
2 .mu.m, produce a bright light source with an effective diameter D
of about 170 .mu.m. For lasers with 500 mW continuous output, and a
75% coupling efficiency, the bundle 23 produces a continuous bright
output of approximately 2.5 watts. Higher power outputs up to about
7.5 watts continuous wave can be achieved if the individual lasers
operate up to their catastrophic limits, or sequentially in a
pulsed mode.
With reference to FIG. 4, an output end 27 of a bundle 23 of
fiberoptic waveguides is butt coupled to an end 31 of a solid state
active lasing medium 33. The medium 33 may be in the form of a
cylindrical rod, as shown, or a rectangular slab or other
convenient shape. The medium 33 may be any solid material which has
been developed for lasers, including neodymium doped yttrium
aluminum garnet (Nd:YAG), Nd:glass, Nd:YLF, Nd:YALO, Er:YAG, ruby,
and alexandrite, provided the output wavelength of the fiberoptic
bundle 23 matches an absorption band of the medium 33. For example,
for Nd:YAG pumping, the light 25 output by laser light sources 11,
12 and 13 and transmitted by fiberoptic bundle 23 has a wavelength
of about 0.8 .mu.m. End 31 of medium 33 typically has a coating
which is highly reflective at the lasing wavelength, 1.06 .mu.m for
Nd:YAG lasers, and which is antireflective and highly transmissive
to light 25 at the pumping wavelength, 0.8 .mu.m for Nd:YAG lasers.
Butt coupling, as seen in FIG. 4, is adequate for multimode laser
operation, but may waste considerable optical energy for single
transverse mode laser operation.
Referring again to FIG. 1, in order to efficiently pump medium 33
for single transverse mode operation, it is preferable that the
pump light 25 emitted from bundle 23 be focused to a spot size
which substantially matches the mode volume of the desired lasing
mode. This mode volume is determined from the resonant optical
cavity of the solid state laser defined between end 31 and laser
output mirror 57. The mode volume 59 in active medium 33 is a
function of the curvatures of end 31 and mirror 57 and the distance
between them. Typically, for a 3 mm diameter rod, the lowest order
mode volume has a diameter of about 100 .mu.m. A lens 29 or
multiple lens system may be used to focus the light from bundle 23.
Typically, lens 25 has a focal length of about 6 cm.
Applications other than optically pumping a solid state laser, such
as communications, illumination, beacons and the like, require a
collimated beam output from bundle 23. To produce a beam divergence
comparable to a 1 to 10 mW helium-neon laser, i.e. a 1 mrad
divergence, a 17 cm focal length lens 29 is required. Lens 29
should have a diameter of about 10 cm for a bundle of 0.3 N.A.
fiberoptic waveguides. Such a system produces an optical power
density of about 0.35 .mu.W/cm.sup.2 at a distance of 30 km, which
is easily detectable by conventional silicon photodetectors.
With reference to FIGS. 5-7, an optical pumping system for pumping
a solid state laser includes a semiconductor diode laser bar 61,
broad area laser or other elongated light source. Laser bar 61
emits a light beam 62 which is coupled into a fiber optic waveguide
62. The input end of waveguide 63 is elongated, as in FIG. 2, so as
to have core dimensions and lateral and transverse numerical
apertures which correspond respectively to the emission area and
lateral and transverse divergences of laser bar 61. Thus, waveguide
63 butt coupled or otherwise positioned proximate to laser bar 61
guides most of the light 62 emitted by laser bar 61. Various
optical means are shown for coupling light 64 emitted from the
output end of waveguide 63 into an end 66 of solid state laser
material 67. In FIG. 5, a focusing lens 65 couples light 64 into
medium 67. In FIG. 6, waveguide 63 is butt coupled to the end 66 of
medium 67. In FIG. 7, one or both of end 75 of waveguide 63 or
surface 77 of end 66 is curved, thereby effectively focusing light
64 into medium 67. These optical means cooperate with the output
end of waveguide 63 to match the light 64 to a desired mode of the
resonant optical cavity of the laser. Solid state medium 67 may be
in the form of a cylindrical rod, rectangular slab or other
convenient shape. Reflective output mirror 69 may have a concave
surface 73 or be planar. Medium 67 is typically provided with a
coating on end 66 which is highly reflective at the lasing
wavelength and antireflective at the pump wavelength. With any of
these embodiments, the light from the elongated emissive area of a
diode laser bar is efficiently coupled into the mode volume of a
solid state laser.
With reference to FIG. 8, a semiconductor diode laser bar 81 may
have a width greater than 100 .mu.m up to 1 centimeter or more. The
laser bar 81 may thus emit a plurality of light elements ranging
from a few to several thousand or more. To date, 100 .mu.m core
diameter fibers have been squashed to 460 .mu.m by 18 .mu.m cross
sections with a coupling efficiency greater than 50 percent. It may
be somewhat difficult to squash a single fiber to dimensions of 1
cm width.
In FIG. 9, laser bar 81 comprises a plurality of contiguous
semiconductor layers 83, 85, 87, 89, 91 and 93, of which at least
one layer 87 forms an active region for lightwave generation and
propagation under lasing conditions. Laser bar 81 may be
constructed in any of the known ways for producing a laser bar 81,
preferably with stable phase locked output. For example, laser bar
81 may have a plurality of conductive contact stripes 95 for
introducing lateral gain guiding. Laser arrays with real refractive
index guiding may also be used. Active region 91 together with
cladding layers 85 and 89 form an emissive area at reflective facet
97, emitting light elements 99, seen in both FIG. 8 and FIG. 9.
For the purpose of coupling this emissive area to fiberoptic
waveguides, the emissive area formed by layers 85, 87 and 89 is
divided into a plurality of segments demarcated by dashed lines
101. The divisions may be made anywhere in the emissive area,
provided that each segment emits at least one light element 99 in
the array of light elements, and provided that the width of each
segment does not exceed 200 .mu.m at most and preferably does not
exceed 150 .mu.m. In FIG. 8, a plurality of fiberoptic waveguides
103, 105, 107, etc. are disposed in front of the emissive area of
laser bar 81 for accepting and guiding light elements 99 from the
laser segments. Preferably, the core of each waveguide 103, 105,
107, etc. has a rectangular cross section, although elliptical and
circular cross-section waveguides may also be used. The input end
of each waveguide has squashed or elongated core dimensions
corresponding to the dimensions of the laser segment proximate
thereto, and should also have lateral and transverse numerical
apertures corresponding to respective lateral and transverse
divergences characteristic of the lasing elements 99.
Referring to FIGS. 10 and 11, the fiberoptic waveguides 103, 105,
107, etc. are stacked or otherwise arranged at an output end 109 to
form a bundle with less elongated dimensions than the laser bar's
emissive area. This bundle may be coupled to an end of a solid
state laser medium 110, here shown as a rectangular slab. For
example, if laser bar 81 has an emission area 1 mm wide and 10
.mu.m high emitting 100 light elements, the emissive area may be
divided into 10 laser segments each 100 m wide and each containing
10 light elements. Each fiberoptic waveguide 103, 105, 107, etc.
may then be elongated at the input end to 100 .mu.m wide by 20
.mu.m high with core dimensions of about 96 .mu.m wide and 16 .mu.m
high. When stacked or arranged into a bundle at the output end as
shown in FIG. 10, the bundle measures 100 .mu.m wide by 200 .mu.m
high. The elongation of the light emitted by laser bar 81 has thus
been reduced in this example from a width to height ratio of 100 to
1 to a ratio of 2 to 1. Other dimensions and other stacking
configurations may be used to produce a less elongated light output
for optical pumping or other application.
In FIGS. 12-16, various ways of forming the output end of a fiber
bundle 111 are seen. In FIG. 12, the end of bundle 111 is fused so
as to form a bead 113 of molten core material. When the bead 113
cools and hardens, all of the fiber waveguides of bundle 111 are
fused together. Further, the surface 115 of bead 113 is curved to
as to function as a convergence lens to light output from the
waveguide bundle. In FIG. 13, a greater length of bundle 111 is
heated, preferably to the melting point of the fibers. As a result,
the fibers are not only fused together, but also air gaps, such as
gaps 117 in FIG. 4, between the fibers are removed and a
homogeneous rod 119 is formed on the end of bundle 111. Rod 119
functions as a mixing rod to blend the light coming from the
individual fiber waveguides of bundle 111 and thereby produce a
more spatially uniform light output. Further, because the air gaps
have been removed in forming rod 119, the rod 119 has a reduced
diameter and greater optical power density than bundles within this
rod. The original diameter of bundle 111 is indicated by dashed
lines 121 representing the original fiber waveguides from which rod
119 was formed. In FIG. 14, the fiber bundle 111 is formed
processed by a combination of heating and pulling. A tapered output
rod 123 results. Tapered rod 123 is characterized by a diameter
which is typically reduced by about one-half compared to mixing rod
119 in FIG. 13. This produces a greater optical power density but
also with greater light divergence. For example, a 200 .mu.m
diameter bundle of 0.3 NA fibers, when terminated in a rod 123
tapered to 100 .mu.m diameter emits light with a 0.6 NA.
In FIGS. 15 and 16, the output end of fiber bundle 111 is not
melted, but instead butted against a glass mixing rod 125. In FIG.
15, rod 125 performs the same function as melted rod 119 in FIG.
13, namely blending the light from the fiber waveguides of the
bundle to produce a uniform light output. If desired, an
antireflection coating may be applied to the end 126 of rod 125. In
FIG. 16, a partial reflector 127 is added to the end of rod 125.
Portions of the light rays 129 diverging from fiber waveguides in
bundle 111 is reflected by mirror 127 back into the bundle 111.
This light is then coupled back into laser light sources on the
input ends of the fiber waveguides. Since the light diverges and is
mixed in rod 125, a portion of the light originating from one laser
ends up being coupled back into another laser, and the lasers can
become phase locked. Thus, the embodiment in FIG. 16 produces
coherent emission from all the lasers in the bundle. Preferably,
the fiberoptic waveguides are polarization preserving single mode
fibers. A reflectivity of at least 5% for mirror 127 is sufficient
to produce coherent light output.
The present invention produces a bright light output derivative
from a plurality of light sources of an extremely elongated light
source suitable for optical pumping a solid state laser. The
invention is also applicable as a communications link for
ground-to-ground, ground-to-air, ground-to-space, ship-to-shore
communications, as a designator, range finder, illuminator, beacon,
welding, cutting, scribing, or any other application requiring a
bright collimated light source.
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