Tunable solid state laser

An electronically tuned, optically pumped, transition metal based, solid-state laser is disclosed. Tuning is achieved by using resonant reflecting means comprising an output mirror, a mirror located between the output mirror and the source of laser light, a polarizer located in the light path between the two mirrors, and an electronically tuned, voltage controlled, variable, liquid crystal waveplate for receiving light passing through the polarizer. The components of the resonant reflecting means are selected to have a longitudinal mode that is anti-resonant with the optical cavity of the laser.

Technical Field 
This invention relates to the general subject of solid state lasers and, in 
particular, to lasers which can be operated over a range of wavelengths. 
BACKGROUND OF THE INVENTION 
Interest in the development of tunable solid-state lasers has increased 
significantly in recent years. The potential for storing energy in the 
population inversion and the long, maintenance-free operating lifetimes of 
solid-state lasers make the current generation of laser- and lamp-pumped, 
transition-metal-doped, insulating crystal lasers superior to dye lasers 
for many scientific and technical applications including laser 
spectroscopy and remote sensing. Unfortunately, widespread commercial use 
of these lasers will require significant reductions in complexity, size, 
and cost. 
These lasers are characterized by a broad gain spectrum (which, in some 
cases, is several hundred nanometers wide) and typically have an 
intra-cavity tuning element which can be used to tune the output 
wavelength over a portion of this curve. Earlier devices (e.g., ones using 
Co:MgF.sub.2 or Ti:Al.sub.2 O.sub.3) used a mechanical Lyot Filter which 
was inserted inside a laser cavity and rotated to shift the output 
wavelength of the laser. See Lovold et al., IEEE J.Q.E., QE-21(3) page 202 
(March, 1985). Galvanometer controlled etalons or Lyot Filters 
incorporating electro-optic crystals have also been used for this purpose. 
See Otsuka et al., Optics Communications, 63(1), p. 37 (July 1987). One 
disadvantage of using a diffraction grating is that it cannot be tuned 
electronically. 
Tuning elements, like the mechanically-tuned Lyot Filter and galvano-driven 
etalon, in which an optical component is physically translated are 
comparatively slow and bulky. The electro-optic Lyot Filter is faster and 
smaller but requires a voltage of several hundred volts to tune it. For 
these reasons, the tuning techniques described in the prior art are not 
well suited to compact, diode-pumped tunable laser systems, such as 
Cr:LiSAF. 
A liquid crystal Lyot Filter is described by Shin-Tson Wu, Applied Optics, 
28,48, (1989): J. R. Andrews, IEEE Photonics Technology Letters, 2, 334 
(1990) and in U.S. Pat. No. 4,394,069 to W. I. Kaye entitled "Liquid 
Crystal Tuned Birefringent Filter." As with any conventional birefringent 
tuner, light that is transmitted through the polarizer passes through the 
variable waveplate, is reflected by the mirror and again transmitted 
through the waveplate and polarizer. The return beam is attenuated if the 
retardation of the variable birefringent plate is not an integral number 
of half-waves. The liquid crystal waveplate will only act like a half- or 
full-waveplate for a band of wavelengths. Wavelengths lying outside this 
band are attenuated by the polarizer as they return through it. 
Unfortunately, the maximum single-pass transmission of the 
polarizer/waveplate combination is between 90% and 98%, making it useless 
as an intra-cavity tuning element in a low-gain, diode-pumped system. It 
is useful, however, in high-gain systems and compound liquid crystal 
birefringent filters have been used as an intra-cavity tuning element in 
external cavity laser diodes (see "Electronically tunable single-mode 
external-cavity diode laser", J. R. Andrews, Optics Letters, 16, 732-734 
(1991)). The development of AlGaInP semiconductor lasers with output 
wavelengths near 670 nm has led to the demonstration of CW, diode-pumped 
lasers in Alexandrite or (Cr.sup.3+ : BeAl.sub.2 O.sub.4) (See R. Scheps, 
et al., Appl. Phys. Lett. 56, 2288 (1990)), in Cr.sup.+3 :LiSrAlF.sub.6 or 
(Cr:LiSAF) (See G. J. Dixon, et al., Digest of Sixth Interdisciplinary 
Laser Science Conference, (American Physical Society, New York, 1990), 
paper B3-1; Q. Zhang et al., Digest of Conference on Lasers and 
Electro-Optics (Optical Society of America, Washington, D.C., 1991), paper 
CTHR6; and R. Scheps, et al., Opt. Lett. 16, 820 (1991)) and in Cr.sup.3+ 
:LiCaAlF.sub.6 or (Cr:LiCAF) (See R. Scheps, IEEE J. Quantum Electron., 
27, 1968 (1991)). Because of a broad, intense absorption near 670 nm and 
an advantageous combination of emission cross section and lifetime, 
diode-pumped operation of 10%-doped Cr:LiSAF lasers with threshold powers 
as low as 3 mW have been demonstrated. With dye-laser excitation, a slope 
efficiency of 41% was measured for broadband operation near 850 nm. 
High-power, quasi-CW diode-pumped operation from a laser incorporating 
lower-doped Cr:LiSAF has also been reported (See R. Scheps, et al., Digest 
of the Advanced Solid State Laser Conference (Optical Society of America, 
Washington, D.C., 1991), p. 291). 
Although the cross-section lifetime product (i.e., product of stimulated 
emission cross section and lifetime) of Cr:LiSAF is higher than that of 
other tunable chromium-doped materials, it is approximately 50 times 
smaller than that of Nd:YAG (and many other rare-earth doped hosts). As a 
result, increasing the intra-cavity losses of a diode-pumped laser through 
the addition of intra-cavity tuning elements significantly increases its 
threshold. 
It would be most advantageous if a relatively simple, low cost, compact 
apparatus and method were available which would allow the use of materials 
like Cr:LiSAF. It would be especially advantageous if such a laser could 
be efficiently tuned over a range of frequencies. 
SUMMARY OF THE INVENTION 
A general object of the invention is to provide a tuning technique which 
has minimized intra-cavity losses in a low gain system. 
Yet another object of the invention is to provide a coupled-cavity tuning 
element which can be operated at low voltage. 
Still another object of the invention is to provide a liquid crystal Lyot 
Filter which can be used to tune a low-gain diode-pumped solid-state 
laser. 
One specific object of the invention is to disclose a coupled-cavity liquid 
crystal tuner for a thulium fiber laser. 
Another object of the invention is to provide a tuning technique which is 
relatively fast, consumes little power, has no moving parts, and is 
relatively small in size. 
In accordance with the present invention, a tunable laser is provided 
comprising: an optically pumpable lasant material; an input mirror; and 
resonant reflecting means for forming with said input mirror an optical 
cavity for said lasant material. The resonant reflecting means comprises: 
an output coupler component; a mirror component, located between said 
output coupler component and said lasant material, for transmitting light 
from said lasant material to said output coupler component; a polarizer 
component located in the light path between said mirror and said output 
coupler; and an electronically tuned, voltage controlled, variable, liquid 
crystal waveplate component for receiving light passing through said 
polarizer, said components of said resonant reflecting means being 
selected to have a longitudinal mode that is anti-resonant with said 
optical cavity. 
The invention provides a tuning technique which is relatively fast (e.g., 
it can be swept through a full tuning range at KHz rates) and consumes 
very little power. By means of a liquid crystal variable retarder, a 
low-gain laser can be quickly tuned through a retardation of several 
thousand nanometers with an applied potential of a few volts. This tuning 
method is well-suited for use in a compact, diode-pumped laser because it 
has no moving parts, is very compact and consumes very little power. 
Numerous other advantages and features of the present invention will become 
readily apparent from the following detailed description of the invention, 
the embodiments described therein, from the claims, and from the 
accompanying drawings.

DETAILED DESCRIPTION 
While this invention is susceptible of embodiment in many different forms, 
there is shown in the drawings, and will herein be described in detail, 
two specific embodiments of the invention. It should be understood, 
however, that the present disclosure is to be considered an 
exemplification of the principles of the invention and is not intended to 
limit the invention to the specific embodiments illustrated. 
Turning to the drawings, FIG. 1 is a schematic representation of a liquid 
crystal Lyot Filter 10 of the type used in the present invention. The 
general operation of this type of filter is described by S. T. Wu, Applied 
Optics, 28,48,(1989); J. R. Andrews, IEEE Photonics Technology Letters, 2, 
334 (1990); and by W. I. Kaye in U.S. Pat. No. 4,394,069 "Liquid Crystal 
Tuned Birefringent Filter." The Lyot Filter comprises a polarizer P, a 
liquid crystal/variable waveplate 12, and a mirror M. When used to tune a 
standing-wave laser, the birefringent filter of FIG. 1 replaces one of the 
end-mirrors of the laser resonator. The principle axes of the liquid 
crystal waveplate 12 are tilted at 45 degrees relative to the polarization 
direction which is transmitted by the polarizer P. Light L that is 
transmitted through the polarizer P passes through the variable waveplate 
12, is reflected by the mirror M and again transmitted though the 
waveplate and polarizer. The return beam R is attenuated if the 
retardation of the variable birefringent waveplate 12 is not an integral 
number of half-waves. The liquid crystal waveplate 12 will only act like a 
half- or full-waveplate for a band of wavelengths. Wavelengths lying 
outside this band are attenuated by the polarizer P as they return through 
it. 
If the filter 10 of FIG. 1 is used in place of one of the end-mirrors of a 
standing-wave, linear cavity laser, the gain medium sees a reflectivity 
that is wavelength dependent. The reflectivity is maximum at those 
wavelengths where the retardation is an integral number of half waves and 
approaches zero at those where it is an odd integral multiple of a quarter 
wave. The wavelengths that are attenuated the least by the 
polarizer/waveplate/mirror combination can be changed by varying the 
voltage V applied to the waveplate 12. Thus, the assembly 10 shown in FIG. 
1 is a voltage-controlled tuning element. 
While it is only necessary to vary the retardation of the variable 
waveplate 12 by one half of the center wavelength of the tuning range to 
tune it over its maximum range, a large value of total retardation is 
often required to narrow the bandwidth of the filter. This can be achieved 
by inserting a fixed birefringent plate WP, with axes parallel to those of 
the variable waveplate 12, between the polarizer P and the mirror M. Such 
an element WP is shown with a dotted outline in FIG. 1. Unfortunately, the 
maximum single-pass transmission of the polarizer/waveplate combination is 
between 90% and 98%, making it impractical as an intra-cavity tuning 
element in a low-gain, diode-pumped system. 
The liquid crystal birefringent filter 10 of FIG. 1 can be used with 
low-gain laser systems if it is placed inside a resonant reflector as 
shown in FIG. 2. By placing the lossy elements inside a coupled-cavity, it 
is possible to tune the laser without increasing the losses seen by the 
gain medium to an unacceptably high level. 
Turning to FIG. 2, there is illustrated a primary laser cavity formed by 
two end mirrors M1 and M2 in which a lasant material LM is located, and a 
Lyot Filter 10 which forms a coupled-cavity. Although coupled-cavity 
tuning techniques in which a diffraction grating is used in place of M3 
are described in the literature they are not known to have used in 
connection with an electronically-tuned element and a liquid crystal 
waveplate in laser systems described by this invention. If we consider the 
equivalent reflectivity of the cavity formed by mirrors M2 and M3 of FIG. 
2 (without the variable waveplate and polarizer), those skilled in the art 
know that the reflectivity varies with wavelength (See Siegman, A. E., 
Lasers, University Science Books, 1986, pages 413-426). On resonance 
(i.e., when the frequency of the input is a multiple of 
##EQU1## 
where "c" is the velocity of light, "n.sub.i " is index of refraction of 
the "i" th optical element, "L.sub.i " is its length, and 
##EQU2## 
is the total length of the cavity), the reflectivity is a minimum. Between 
these points are broad regions where the reflectivity is maximum. More 
importantly, it follows that the "effective reflectivity" of a two-mirror 
resonant reflector (i.e., considering actual reflectivity plus the 
round-trip losses inside the resonator) can be significantly larger than 
the reflectivity of either of the mirrors from which it is made. As an 
example, we can consider the effect of intra-cavity losses on a cavity 
with an input reflectivity of 98% and output reflectivity of 96%. As loss 
is added into the cavity, the effective reflectivity of the output mirror 
decreases from 96%. If we assume that the round-trip losses of the 
coupled-cavity are equal to 20% (e.g., a good approximation to the losses 
of a liquid crystal waveplate/polarizer combination), then its 
anti-resonant reflectivity (calculated using Siegman's analysis on page 
424) for a beam incident from the 98% mirror is significantly greater than 
99%. In fact, calculations show that the effective reflectivity of the 
resonant reflector is greater than 99% for any effective reflectivity of 
M3 greater than 20%. Thus, a resonant reflector makes it possible to 
tolerate very high intra-cavity losses without a significant reduction in 
anti-resonant reflectivity. 
If we now insert a variable waveplate or tuning element 12 and polarizer P 
in the cavity (between mirrors M2 and M3), these elements and the mirror 
M3 form a birefringent tuner of the type shown in FIG. 1. The reflectivity 
of the coupled cavity is then modulated by, or is a function of, 
wavelength (i.e., the effective reflectivity of the output mirror M3 is 
given by the actual reflectivity of the mirror minus the round-trip losses 
of the polarizer/waveplate combination). For wavelengths transmitted by 
the birefringent tuner, the effective reflectivity is equal to the actual 
reflectivity minus the possessive losses of the birefringent filter. In a 
typical crystal liquid waveplate birefringent tuner this reflectivity is 
in the range of 20% to 40%. For the wavelengths attinuated by the filter 
the effective reflectivity is equal to zero (if the polarizer is good). 
The maximum reflectivity of the coupled cavity formed by the two mirrors M2 
and M3 is therefore a function of wavelength. For wavelengths transmitted 
by the birefringent filter, it can be quite high, as described above. 
However, for wavelengths which are not transmitted by the filter, the 
effective reflectivity is given by the reflectivity of one mirror M2 
alone. This variation in anti-resonant reflectivity of the two mirror 
combination can be used to tune the laser output. 
These properties of the coupled-cavity birefringent tuner make it 
well-suited for tuning low-gain, diode-laser-pumped solid-state lasers, 
like Cr:LiSAF. Cr:LiSAF has an advantageous combination of spectroscopic 
and material properties. (See Qi Zhang and G. J. Dixon, et al., 
"Electronically Tuned Diode-Laser-Pumped Cr:LiSrAlF.sub.6 Laser", Optics 
Letters, 17(1), 43-45 (Jan. 1, 1992)). In addition to a cross-section 
lifetime product that is greater than that of the other Cr-doped tunable 
materials, Cr:LiSAF can be doped at concentrations exceeding 10% without 
appreciable lifetime quenching or degradation of crystal quality. In 
10%-doped material, the 670-nm absorption coefficient is approximately 50 
cm.sup.-1 ; this makes it possible to use submillimeter-thick platelets of 
grain material without sacrificing pump absorption. Short absorption depth 
makes it possible for the material to absorb a tightly focused pump beam 
before it can expand appreciably. Since the threshold power is 
proportional to the sum of the areas of the pump and laser modes, averaged 
across the active length of the gain medium, it is possible to minimize 
the laser threshold by tightly focusing the pump into a small mode volume 
in a highly doped Cr:LiSAF crystal. In this application, the input mirror 
M2 of the coupled-cavity has a transmission of a few percent, just large 
enough to keep the laser from operating off of that mirror alone. This 
mirror is coupled to a liquid crystal birefringent filter 10 of the type 
shown in FIG. 1. The coupled resonator is designed to mode-match the 
fundamental transverse mode of the primary resonator which contains the 
laser gain medium. Its output transmission is between 2% and 25% and is 
chosen to optimize the output coupling of the laser. Varying the 
birefringence of the liquid crystal waveplate 12 will then tune the output 
of the laser over a broad spectral range. In our initial experiments, we 
have been able to tune the output over a range of 60 nm with an applied 
potential difference of less than 2 volts. 
While placing the birefringent tuner in a coupled-cavity minimizes its 
losses, it also makes it necessary to control the relative cavity lengths 
of the coupled-cavity and the primary resonator. The coupled-cavity tuning 
arrangement will not operate properly if the frequency of the input is not 
anti-resonant with the cavity. To first approximation, the resonant 
frequency of the primary cavity formed by mirrors, M1 and M2 in FIG. 2 
determines the input frequency to the coupled-cavity. In order to assure 
anti-resonant operation, its optical path length must be adjusted so that 
the frequencies of its longitudinal modes are anti-resonant with the 
coulped-cavity. In practice, this means that the cavity length of both 
cavities must be stabilized to a small fraction of a wavelength. The 
output power of these lasers can be used as a control signal for the 
coupled-cavity length control. 
FIG. 3 shows a preferred embodiment of a tunable, coupled-cavity Cr:LiSAF 
laser 30 in which a liquid-crystal wave plate 12 and a dielectric 
polarizer P are used as an electronically controlled tuning element. A 
thin (i.e., preferrably 0.5-mm-thick or less) crystal 14 of 10% 
Chromium-doped (preferably exceeding 5%) Cr:LiSAF (e.g., grown from a 
chromium-doped melt at the CREOL Crystal Growth Laboratory at the 
University of Central Florida) was polished flat and parallel and coated 
with dielectric reflectors M1 and M2 to form a cube laser resonator R. The 
pump face M1 was highly reflective (HR) at 865 nm (preferably between 820 
nm and 880 nm) and highly transmitting (HT) at a pump wavelength of 670 
nm. The opposite face M2 was coated for 90% to 98% reflectivity at 865 nm 
(preferably between 820 nm and 880 nm). The Cr:LiSAF crystal 14 was pumped 
with the output of an InGaAlP laser diode D which is focused by a lens F 
using conventional techniques (cf., U.S. Pat. No. 5,105,434) to Krupke and 
Payne). 
Suitable optical pumping means include, but are not limited to, laser 
diodes, light-emitting diodes (including superluminescent diodes and 
superluminescent diode arrays) and laser diode arrays, together with any 
ancillary packaging or structures. For the purposes hereof, the term 
"optical pumping means" includes any heat sink, thermoelectric cooler or 
packaging associated with said laser diodes, light-emitting diodes and 
laser diode arrays. For example, such devices are commonly attached to a 
heat resistant and conductive heat sink and are packaged in a metal 
housing. For efficient operation, the pumping means D is desirably matched 
with a suitable absorption band of the lasant material. The heat sink can 
be passive in character. However, the heat sink can also comprise a 
thermoelectric cooler or other temperature regulation means to help 
maintain laser diode at a constant temperature and thereby ensure optimal 
operation of laser diode at a constant wavelength. It will be appreciated, 
of course, that during operation the optical pumping means D will be 
attached to a suitable power supply. Electrical leads from laser diode D, 
which are directed to a suitable power supply, are not illustrated in the 
drawings for simplicity. 
Conventional light-emitting diodes and laser diodes are available which, as 
a function of composition, produce output radiation having a wavelength 
over the range from about 630 nm to about 1600 nm, and any such device 
producing optical pumping radiation of a wavelength effective to pump a 
lasant material can be used in the practice of this invention. For 
example, the wavelength of the output radiation from InGaAsP based devices 
can be used to provide radiation in the wavelength range from about 1000 
to about 1600 nm. 
If desired, the output facet of semiconductor light source D can be placed 
in butt-coupled or close coupled relationship to input surface of the 
lasant material without the use of optics F, thereby reducing the size and 
complexity of the device. (See U.S. Pat. No. 4,847,851 to Dixon for a 
description of close-coupled pumping of high-concentration laser 
materials). As used herein, "butt-coupled" is defined to mean a coupling 
which is sufficiently close such that a divergent beam of optical pumping 
radiation emanating from semiconductor light source D or laser diode will 
optically pump a mode volume within the lasant material 14 with a 
sufficiently small transverse cross-sectional area so as to support 
essentially only single transverse mode laser operation (i.e., TEM.sub.00 
mode operation) in the lasant material. 
Returning to FIG. 3, a lens F, located to the left of the laser diode 
source D, is used to focus pumping radiation into lasant material 14. This 
focusing results in a high pumping intensity and an associated high photon 
to photon conversion efficiency in lasant material. Focusing means F can 
comprise any conventional means for focusing light such as a gradient 
index lens, a ball lens, an aspheric lens or a combination of lenses. The 
output from the primary resonator R is sent by means of another lens 16 to 
the wavelength-tuning element. 
The wavelength-tuning element, comprising of a dielectric polarizer P and a 
variable wave plate 12, is located between the lens 16 and the output 
mirror M3. The axes of the fixed waveplate WP and variable birefringent 
waveplate 12 are preferably oriented at 45 degrees with respect to the 
direction of maximum transmission through the polarizer P. The output 
mirror M3 is a flat having a reflectivity between 90% and 98% over the 
range from 820 nm to 880 nm on the intra-cavity surface and having an AR 
coating good for the same wavelength range, on the output. The output 
mirror M3 is mounted on a piezoelectric translator PZT for active cavity 
length control and the gain element is preferrably housed in a temperature 
controlled enclosure. In this configuration, the reflectivity of the 
resonant cavity formed by the output face M2 of the Cr:LiSAF cube 14 and 
the output mirror M3 determines the threshold of the laser. 
Using the well-known expression for the reflectivity of an anti-resonant 
optical cavity, it can be shown that the effective reflectivity of the 
tuning cavity is greater than 99% for all cases in which the round-trip 
losses (i.e., output coupling plus absorption and scatter) are less than 
88%. Thus, relatively lossy tuning elements can be placed inside the 
coupled-cavity without increasing the threshold of the laser to an 
unacceptably high value. Because it is a monolithic design, the cavity R 
is stable against vibrations and its length stabilized with a simple 
temperature control. 
Electronic tuning was accomplished by using a fixed wave plate WP and a 
liquid-crystal variable retarder 12 (obtained from Meadowlark Optics of 
Longment, Colorado). The liquid-crystal retarder 12 was chosen as the 
electronically controlled tuning element since its operating voltage is 
approximately two orders of magnitude smaller than that of variable 
retarders based on electro-optic insulating crystals. The fixed waveplate 
WP had seven waves of retardation at 865 nm, while the retardation of the 
liquid-crystal retarder 12 could be varied from 2000 to 2900 nm by varying 
the 2-kHz AC control voltage V from 0 to 20 volts. Although the round-trip 
loss of the liquid-crystal tuning element was greater than 5%, an incident 
power threshold of 22 mW was observed when a mirror M3, coated for 96% 
reflectivity at 920 nm, was used as an output. At an input power of 42 mW 
the laser output consisted of several longitudinal modes, separated by the 
free spectral range of the cube resonator R.sub.o, which spanned 
approximately 5 nm. By using a voltage-controlled liquid-crystal variable 
wave plate, the laser output was tunable over a range of 65 nm with an 
applied voltage V of only 3 volts. The output power tuning curve, 
corresponding to an applied potential difference of 3 volts, had an 
asymmetric shape which is believed to be due to a rapid decrease in the 
transmission of the dielectric polarizer below 860 nm. A maximum output 
power of 4.3 mW corresponded to an optical slope efficiency of 22%. 
Improvements in output efficiency and tuning range are expected with 
optimization of the coating reflectivities and a reduction of losses in 
the tuning elements. It is reasonable to expect optical efficiencies 
approaching 40% and a tuning range of 80 to 100 nm from a properly 
designed laser. Single-frequency operation over a continuous range of 
output wavelengths should be possible in a laser that incorporates 
multiple liquid-crystal filters and a temperature-controlled gain element. 
Without cavity length control, significant amplitude instability was 
observed in the output of the coupled-cavity laser owing to fluctuations 
in the optical path length between the cube resonator R and the output 
mirror M3. It is clear that the lengths of both the cube resonator R and 
the cavity containing the tuning elements P, WP and 12 must be controlled 
in a practical device. This can be accomplished by designing a stable 
temperature-controlled resonator and/or mounting the output coupler M3 on 
a piezoelectric translator PZT and using a control 18 to adjust its 
position to maintain constant output power. 
The coupled-cavity, liquid crystal waveplate tuner can be used to tune any 
laser which requires a low-loss optical resonator for its operation. 
Clearly, low gain, diode-pumped tunable solids state lasers like Cr:LiSAF, 
Cr:LiCAF, Cr:LiSCAF, etc. fall into this category. It may also be used to 
tune a titanium sapphire laser. Another use is to tune a Thulium (Tm) 
fiber laser (e.g., a 2.mu. Tm fiber laser). Thulium in a silica fiber has 
a very broad gain spectrum which spans a wavelength range from 1.8 to 
beyond 2.0 microns. In contrast to many fiber lasers, the Tm laser 
operates best in a high finesse resonator which will minimize the excited 
state population needed for laser operation. This is due to up-conversion 
from the upper laser level which significantly decreases the efficiency of 
the laser at population densities. The coupled-cavity liquid crystal tuner 
just described is well-suited to this device, since it can tune the fiber 
laser over a broad spectral range without significantly increasing the 
intra-cavity losses. 
From the foregoing description, it will be observed that numerous 
variations, alternatives and modifications will be apparent to those 
skilled in the art. Accordingly, this description is to be construed as 
illustrative only and is for the purpose of teaching those skilled in the 
art the manner of carrying out the invention. Various changes may be made, 
materials substituted and features of the invention may be utilized. 
Pumping at 760 nm using a GaAlAs device is suggested. For example, the 
precise geometric shape of lasant material can vary widely. Similarly the 
lasant material can be doped with a variety of rare earth. The lasant 
material can be rod-shaped or rhombohedral in shape if desired, and 
lens-shaped ends can be used if desired. An end-pumped fiber of lasant 
material can also be used. In particular, optical fibers, which are doped 
with Thulium, for example, are suggested. The length of such a fiber is 
easily adjusted to result in absorption of essentially all of the optical 
pumping radiation. If a very long fiber is required, it can be coiled, on 
a spool for example, in order to minimize the overall length of the laser 
apparatus. Thus, it will be appreciated that various modifications, 
alternatives, variations, etc., may be made without departing from the 
spirit and scope of the invention as defined in the appended claims. It 
is, of course, intended to cover by the appended claims all such 
modifications involved within the scope of the claims.