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NASA CASE NO. 


, ' / 

NPO-18611-1-CU 


PRINT FIG. 5 


NOTICE 



The invention disclosed in this document resulted from 
research in aeronautical and space activities performed under 
programs of the National Aeronautics and Space Administration. The 
invention is owned by NASA and is, therefore, available for 
licensing in accordance with the NASA Patent Licensing Regulation 
(14 Code of Federal Regulations 1245.2). 

To encourage commercial utilization of NASA— Owned inventions, 
it is NASA policy to grant licenses to commercial concerns. 
Although NASA encourages nonexclusive licensing to promote 
competition and achieve the widest possible utilization, NASA will 
consider the granting of a limited exclusive license, pursuant to 
the NASA Patent Licensing Regulations, when such a license will 
provide the necessary incentive to the licensee to achieve early 

practical application of the invention. 

Address inquiries and all applications for license for this 
invention to NASA Patent Counsel, NASA Resident Office-JPL, Mail 
Code 180-801, 4800 Oak Grove Drive, Pasadena, CA 91109. 

Approved NASA forms for application for nonexclusive or 
exclusive license are available from the above address. 


Serial Number: 08/044 , 668 

Filed Date: March 30. 1993 


NRO-JPL 


June 11, 1993 

( N AS A-Case-NPQ— 18611— 1-CU) TUNABLE N93-30415 

CW 0IO0E-PUMPED Tm, Ho : YL i F4 LASER 
OPERATING AT OR NEAR ROOM 

TEMPERATURE Patent Application Unclas 

(NASA) 29 p 


G3/36 0171670 


Serial No. 08 / 044 , 668 


Filin * Date Marnh in r iqq^ 

Contract No. NAS7-918 
Contractor: Caltech/JPL 

Pasadena, CA 91109-8099 


AWARDS ABSTRACT 


Inventor: Brendan T. McGuckin 

Robert T. Menzies 
Contractor: Jet Propulsion 
Laboratory 


JPL Case No. 18611 

NASA Case No. NPO-18611-1-CU 

Dated: March 18, 1993 


TUNABLE CW DIODE-PUMPED Tm, HOsYLiF^ LASER 
OPERATING AT OR NEAR ROOM TEMPERATURE 


The invention relates generally to precise control of diode 
laser pumped Tm,Ho:YLiF 4 laser emission wavelength by means of 
angle tuning of an intracavity, uncoated quartz etalon and by 
control of Tm,Ho:YLiF 4 laser crystal temperature. A quartz 
etalon of thickness 0.25mm in the laser cavity, is rotated 
relative to the optical axis of the laser to tune wavelength. 
Laser crystal temperature may also be used to change the 
emission wavelength. Stabilization of the emission wavelength 
is obtained relative to an external cavity using a reference 
etalon. 

FIG. 1 is a block diagram of the optical train used in an 
embodiment of the invention; FIG. 2 is a graph of output power 
versus absorbed pumped power for the laser of the invention; 
FIG. 3 is a video digitizer-produced graph of the spatial 
energy distribution output beam profile of the laser hereof; 
FIG. 4 is a spectral graph of output of the inventive laser at 
two different temperatures; FIG. 5 is a graphical 
representation of the tunability of the inventive laser; FIG. 

6, comprising FIGs. 6a and 6b, is a pair of spectral recordings 
of the laser output illustrating the effect of the intracavity 
etalon used in the laser of the invention; and FIG. 7 is a 
block diagram illustration of an arrangement for stabilizing 
the laser frequency relative to the transmission characteristic 
of an external reference confocal cavity. 

The novelty of the invention resides in providing a CW diode- 
pumped Tm,Ho:YLiF 4 laser operating at or near room temperature 
with a high output power and with emission spectrum etalon 
tunable over 7nm centered at 2.067ym using an intracavity 
uncoated quartz etalon rotatable relative to the optical axis 
of the laser. Novelty is also found in stabilization of the 
emission wavelength by a closed loop employing an external 
confocal cavity reference etalon. 



PATENT APPLICATION 


JPL Case No. 18611 
NASA Case No. NPO-18611-1-CU 
Attorney Docket No. JPL-38 

Serial No. 08/044,668 

Filing Date. Marnh ^ 0 TQQ^ 

Contract No. NAS7-918 
1 Contractor: Caltech/JPL 

Pasadena, CA 91109-8099 


TUNABLE CW DIODE- PUMPED Tm#Ho:YLiF 4 LASER 
OPERATING AT OR NEAR ROOM TEMPERATURE 

ORIGIN OP 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 not to 
retain title. 


TECHNICAL FIELD 


The invention relates generally to precise control of 
diode laser pumped Tm# Ho:YLiF^ laser emission 
wavelength by means of angle tuning of an intracavity, 
uncoated quartz etalon and by control of Tm# Ho: YLiF^ 
laser crystal temperature. A quartz etalon of 
thickness 0.25mm in the laser cavity# is rotated 
relative to the optical axis of the laser to tune 
wavelength. Laser crystal temperature may also be used 
to change the emission wavelength. Stabilization of 
the emission wavelength is obtained relative to an 
external cavity using a reference etalon. 


20 


2 


BACKGROUND ART 


Eyesafe 2um holmium lasers are of considerable topical 
interest, having potential use in diverse applications 
such as altimetry, ranging, low altitude wind shear 
5 detection and avoidance, and in atmospheric remote 

sensing including Doppler lidar wind sensing and water 
vapor profiling by differential absorption lidar. For 
next generation instruments, a diode-pumped solid-state 
transmitter would proffer extended lifetime and higher 
10 efficiency as potential benefits. The thulium, holmium- 
doped yttrium lithium fluoride (or Tm,Ho:YLiF 4 ) laser 
potential for atmospheric remote sensing depends on the 
ability to tune it to either atmospheric window regions 
or to regions where gases of interest selectively absorb 
15 the laser radiation. CC> 2 and H 2 0 absorption features 
overlap the emission spectrum of the laser. These 
attenuate the beam when the laser frequency overlaps one 
of these features and reduce the effective range of the 
laser beam through the atmosphere. However, it is 
20 possible with this laser to tune the wavelength into the 
high transmissivity regions between these absorption 
features if the application (such as ranging) so 
requires . 



STATEMENT OF THE INVENTION 


Disclosed herein is efficient performance from a 

diode laser-pumped thulium holmium yttrium lithium 

fluoride (Tm,Ho:YLiF 4 ) laser operating near 2.067um on 

. 5 5 

the I 7 - I 8 transition in holmium. An absolute 
conversion efficiency of 42% and slope efficiency of 
60% relative to absorbed pump power are recorded with a 
peak continuous wave (CW) output power of 84mW at a 
temperature of 275K. The emission spectrum is etalon 
tunable over 7nm centered on 2.067pm with a 
-3cm-i . 100K-1 thermal fine-tuning capability of the 
transition. The measured effective emission cross- 
section of SkIO” 21 cm 2 and other aspects of the laser 
performance will be discussed. 

The observed gain and efficiency are noteworthy 
characteristics for both lidar applications and compact 
spectroscopic sensor applications. The tuning range 
spans atmospheric carbon dioxide and water vapor lines 
with high transmissivity regions between the lines. 
Notwithstanding the slightly higher pump thresholds 
when compared to yttrium aluminum garnet (YAG) , 
advocacy of YLF as a suitable host is based upon 
salient factors such as the high slope conversion 



4 


ef f icienc ies , and reduced up-conversion losses when 
compared to other hosts. Further, it has been 
demonstrated that Tm,Ho:YLiF 4 exhibits a stored 
excitation energy distribution conducive to efficient 
5 energy extraction when operated in Q-switched mode. 

Furthermore, an insensitivity to thermally induced beam 
depolarization due to the uniaxial crystalline nature 
and minimal thermal lensing in YLF are important 
attributes in a material with potential for multi-joule 
10 laser development. 



5 


OBJECTS OP THE INVENTION 


It is therefore a principal object of the present 
invention to provide an efficient tunable continuous 
wave (CW) diode-pumped thulium holmium yttrium lithium 
' fluoride laser operating at or near room temperature. 

It is another object of the present invention to 
provide a tunable frequency stabilized diode-laser- 
pumped Tm,Ho:YLiF 4 laser operating at or near room 
temperature with precise control of emission 
10 wavelength. 

It is still an additional object of the present 
invention to provide a Tm,Ho:YLiF 4 CW laser having 
precise emission wavelength control by means of angle 
tuning of an intracavity, uncoated quartz e talon and by 
15 control of Tm,Ho:YLiF 4 laser crystal temperature. 

It is still an additional object of the invention to 
provide a diode laser pumped single frequency thulium 
holmium yttrium lithium fluoride laser having a tunable 
output wavelength and frequency stabilization relative 
20 t0 an external reference etalon and exhibiting an 

output power of around 25mWatts at or near room 
temperature. 


BRIEF DESCRIPTION OF THE DRAWINGS 


The aforementioned objects and advantages of the 
present invention, as well as additional objects and 
advantages thereof will be more fully understood 
hereinafter as a result of a detailed description of a 
preferred embodiment when taken in conjunction with the 
following drawings in which: 

FIG. 1 is a block diagram of the optical train used 
in an embodiment of the invention; 

FIG. 2 is a graph of output power versus absorbed 
pumped power for the laser of the invention; 

FIG. 3 is a video digitizer-produced graph of the 

spatial energy distribution output beam profile of the 
laser hereof; 

FIG. 4 is a spectral graph of output of the inventive 
laser at two different temperatures; 

FIG. 5 is a graphical representation of the 
tunability of the inventive laser; 



7 


FIG. 6, comprising FIGs. 6a and 6b, is a pair of 
spectral recordings of the laser output illustrating 

the effect of the intracavity etalon used in the laser 
of the invention; and 

FIG. 7 is a block diagram illustration of an 
arrangement for stabilizing the laser frequency 
relative to the transmission characteristic of an 
external reference confocal cavity. 



8 


detailed description op a preferred embodiment 


10 


15 


20 


Tm,Ho.3fLiF 4 laser crystal was grown along the 
a-axls with 6%Tm and 0.4»Ho composition with 
corresponding Impurity In concentrations of 
(Tm’+J-8.39xlO i!, cm- 3 and [Ho 3+ )- 5.59x10 ‘’cm’ 3 . The 
Optimum crystal length for use at 275K was previously 
determined, the 2.18mm long crystal selected being the 
best compromise between minimising re-absorption losses 
and optimal gain length for this composition. 

The optical train Is Illustrated in pig. 1. The 

laser crystal Is axially pumped by a single Spectra 

Diode Labs (SDL- 820) 500raW GaAlAs diode laser tuned to 

792nm. The elliptical pattern of the collimated output 

beam is near circularised by the anamorphic prism pair 

affording Improved alignment stability and output 

performance (at the expense of a 9» transmission loss). 

The beam Is focussed onto the laser crystal by the 5cm 

focal length doublet lens. The near hemispherical 

Ho- laser resonator is formed between the crystal 

"input" face and a 10cm radius of curvature mirror. 

A dichroic coating on the Input face Is 88% 

transmitting at 792nm but has unity reflectivity near 
2pm. 



The crystal "output" face is anti-reflection (AR) 
coated at both pump and generated wavelengths. The 
output coupler is 99% reflecting at 2um. The 
7T -spectrum absorption coefficient at 275K is 3.88cm 

Temperature control of the laser crystal is achieved 
by a single-stage thermo-electric (TE) cooler and 
variable D.C. current supply. An Omega 44030 precision 
thermistor , bonded onto the laser crystal, is used for 
temperature measurement. The integrity of the bond and 
readings are periodically verified by a temperature 
probe brought into direct contact with the crystal at a 
point adjacent to the thermistor. Condensation on the 
crystal surfaces is obviated by maintaining the 
cry stal— cooler assembly in a flowing dry nitrogen gas 
atmosphere. The pump power and the filtered output 
power from the holmium laser are measured using 
calorimetric power meters. 

For a crystal temperature of 27 5K, the Ho-laser 
output as a function of absorbed pump power is 
illustrated in FIG. 2. The extrapolated threshold 
absorbed pump power is ~60mW. The maximum output power 
is 84mW for an absorbed power of 200mW, corresponding 
to 42% conversion efficiency. Typically, 51% of the 



10 


pump power is absorbed in the crystal , therefore the 
slope efficiency relative to the absorbed pump power 

( n , ) is 60% and is believed to be the highest 

' 'slope 

reported to date from a Tm,Ho:YLiF 4 laser operating 
5 near room temperature. When operating the laser on a 
single frequency, the laser crystal was maintained at 
slightly higher temperatures. Upon raising the crystal 
temperature to 29 8K, the output power was measured to 
be ~ 3 5 mW . However, when using the intracavity etalon 
10 as a frequency discriminant, this fell to 25mW due to 

intracavity losses associated with reflections from the 
etalon surfaces. The influence of the resonant Tm-Ho 
energy transfer in reducing the Stokes losses is 
manifest here also as an effective quantum efficiency 
15 ri p = 1.57 for the pumping process. Evidently this is 
being moderated from the Tip~ 2 anticipated from 
theoretical analysis of the performance of these 
materials. This is considered to be due to the 
deleterious influence of up-conversion processes on the 
20 pumping dynamics and also the relatively high operating 


temperature. 


11 


The primary up-conversion loss mechanism has been 
identified as the removal of Ho- ions from the 5 1 ? upper 
laser level by up-conversion with a Tm-ion in the 3 F 4 
manifold. The Ho- ion is excited to the 5 1 level from 
where it then relaxes back to the Tm-Ho manifold via 
phonon emission. A secondary and less influential up- 
conversion process giving rise to the distinctive 
visible fluorescence accompanying lasing is also 
active. A 0.5m path length monochromator and 
photomultiplier tube were used to record this 
fluorescence spectrum between 400nm and 800nm. The 
peak wavelengths observed and probable transition 
assignments are: 486nm ( 5 F^ 5 l e ), 514-530nm 

( s 2 ^ V' 6 00-63 Onm ( 5 F 5 -*- 5 I 8 ) and 700-770nm ( 5 S 2 + 5 I 7 ), 
the most distinctive of which is the 5 S 2 * 5 I 8 green 
fluorescence. 

The spatial energy distribution of the Ho-laser was 

recorded using a vidicon T.V. camera and video 

digitizer system. The output beam profile is shown in 

FIG. 3. From this it is apparent that the laser is 

operating on a single transverse mode (TEM ). TEM 

00 00 

operation is routinely achieved, despite the original 
lobed pattern of the pump laser, and is also attributed 
to the circularization of the pump beam shape by the 



12 


anamorphic prism pair. The improved pump beam pattern 
also enables a smaller focussed spot size onto the 
laser crystal allowing more pump energy to be coupled 
into the fundamental mode with resultant improvements 
5 in laser efficiency. The plane of polarization of the 

2 ym emission was determined by monitoring the spectrum 
through an optic oriented at Brewster's angle and is 
p-polarized with respect to the pump beam E-vector. 

Tunability is of interest in a laser with possible 
10 applications in remote sensing. Accordingly, the laser 

output spectrum and its sensitivity to crystal 
temperature, and control by an intracavity etalon, have 
been investigated. Wavelength measurements were made 
using a Spex 1700-11 spectrometer. The 0.75m unit has 
15 a first order theoretical resolution of 0.035nm using a 

600 g.mnr 1 ruled grating, although the typical 
resolution attained in practice was 0.042nm (0.098cm -1 ) 
at the emission wavelength. A liquid nitrogen-cooled 
mercury cadmium tellur ide detector monitored the energy 
transmitted through the spectrometer and phase 
synchronous detection was used to improve the signal to 
noise ratio. 


20 


13 


A typical spectrum is shown in FIG. 4. Recorded at 
275K, this consists of 6 distinct peaks between 2.064 
and 2 . 07 ym separated by 0.6-0.7nm. The laser emission 
calibration included/ in addition to the monochromator 
calibration with several He—Ne orders/ the observation 
of transmission spectra through a 3 atm-m path of C0 2 / 
where line positions in the 2 uj + u 3 band are known to an 
accuracy of better than 0.005cm -1 . The discontinuous 
emission spectrum is attributed to Fabry-Perot action 
in the laser crystal, the spacing corresponding to that 
expected from the refractive index and length of 
crystal used. The crystal is susceptible to this 
behavior due to the large product of the surface 
reflectivities at the resonant wavelength. The 0.25mm 
etalon is chosen to have a free spectral range greater 
than the laser crystal so as to restrict oscillation to 
one fringe. Due to the homogeneously broadened 
transition, the effect of the etalon is to extend the 
width of the spectrum by concentrating the available 
gain into one mode, thereby accessing regions further 
from the center of the gain bandwidth. As illustrated 
in FIG. 5, for a temperature of 256K the laser is 
tunable over 7nm (16cm-i). This requires an etalon 
rotation of 7 of the 0.25mm etalon used here. This 
tuning range is consistent with previous observations 


25 



14 


of this transition in diode-laser pumped Tm, Ho: YAG and 
HojYSGG and in flashlamp excited Tm, HorYLiF^ . The 
insertion losses due to the etalon reduce the output 
power by 60%, primarily due to the short, highly 
divergent cavity design used. Extending the length of 
the resonator and locating the etalon close to the beam 
waist will effect an improvement. 


In the absence of the etalon, the temperature 
sensitivity of the emission spectrum reveals a central 
10 fringe shift of ~4.55xl0“ 2 nra.Krl (0 .106cm" 1 ,K _1 ) . 

This motion is the result of two distinct effects: The 

envelope under which the fringes move exhibits a red- 
shift with increasing crystal temperature at a rate of 
-1.3x10-* nm . K - 1 (0 .03cm ) , and is considered to 

6 ^ Ue I® ser transition tuning with temperature. 

The secondary effect involves thermally induced changes 
in the crystal length. The thermal expansion 
coefficient of YLF is 13xl0‘ 6 K _1 , which at 
2 .067ym would effect a fringe movement of 
20 2. 7xl0- 2 nm.K- 1 (0 .063cm -1 .K' 1 ) . This in conjunction 

with the transition motion largely accounts for the 
total observed movement of the central fringes. A 
transition temperature sensitivity of this magnitude 
would facilitate thermal tuning of the emission 



15 


wavelength through atmospheric C0 2 absorption lines 
(2 ui + u 3 band) which are predominant in this spectral 
region. 

Analysis of the threshold pump power with different 

output coupler reflectivities at a crystal temperature 

of 256K reveals an effective n-spectrum absorption 

coefficient at the resonant wavelength a * 0.065cm" 1 , 

L 

considering absorption to be the sole loss mechanism. 
Using the energy level scheme of Castleberry and, 
assuming a Boltzmann approximation for the population 
distribution of the Tm, 3 F 4 -Ho 5 I 7 coupled system and the 
h ° 5 I 8 ground state, the occupation factor of the lower 
laser level (317cm- 1 ) is f L « 0.019. This results in an 
emission cross-section of a g - 6xlO~ 20 cm 2 , which is 
slightly higher than that calculated from the 
a -spectrum in the same material. The effective 
emission cross-section, a gff , can be obtained from 
the threshold small-signal gain coefficient 
(g 0 - 0.065 cm -1 ). At this temperature 23% of the 
Ho-concentration must be inverted to achieve threshold, 
translating into o fiff » 5x 10 -21 cm 2 . This should be 
compared to 17% and a o eff « 9xl0 -21 cm 2 measured at 
2. 091 urn in Tm,Ho:YAG of the same relative Tm/Ho 
composition as that used here. In Q-switched 



16 


oscillator amplifier systems, the lower c eff in 
Tm,Ho:YLiF 4 may facilitate higher energy storage 
densities, the material being less susceptible to 
parasitic osc illations# 

For many potential applications the control of the 
laser axial mode control is important. The laser 
output spectrum as recorded using a scanning confocal 
Fabry-Perot etalon with a 3 GHz free spectral range 
(FSR) is shown in FIG. 6. Evident in FIG. 6 is a 
fringe pattern consistent with multi-axial mode 
oscillation. Next to the main confocal etalon fringes, 
separated from them by 0.15 GHz, are auxiliary lower 
amplitude modes. These are in fact paired with the 
next order etalon fringe located 3 GHz away for a total 
mode separation of 3.15 GHz, corresponding to the laser 
resonator free spectral range. The fringe pattern 
exhibits rapid amplitude fluctuations and general 
instability consistent with mode competition. 



17 


The effect of the intracavity etalon is manifest in 
FIG. 6, where a fringe pattern consistent with single 
axial mode oscillation is depicted. Coarse angle 
tuning of the etalon and tuning of the resonator length 
by means of a PZT element on the cavity output coupler 
are required to create the most favorable balance 
between the gain as modulated by the etalon fringe and 
the location of an axial mode relative to the gain 
curve maximum, resulting in single frequency output. 

Once attained, the amplitude stability is very good, 
fluctuations being <5%, and no evidence of mode hopping 
is observed. Aware of the fact that the analyzing 
etalon and laser resonator free spectral ranges are 
relatively close (3.0 and 3.15 GHz, respectively) any 
ambiguity so caused in deducing the spectral purity of 
the single frequency output was addressed by analyzing 
an individual etalon fringe at the limit of resolution 
of the instrument. This is achieved by progressively 
reducing the ramp voltage applied to the scanning 
etalon PZT at a fixed bias voltage while monitoring the 
laser mode spectrum with progressively higher 
resolution, with the shortest possible scan period of 

tr ical mode shape was maintained 


20ms. The symme 



throughout and no evidence of a second mode (which 
would cause an asymmetry or inflection on either side 
of the fringe) was observed. An uncompensated drift 
rate of 2.5 MHz min -1 is deduced from observations of 
the fringe pattern drift over many hours, highlighting 
the need for external stabilization. 

Stabilization of the laser frequency relative to an 
external reference confocal cavity was achieved using 
the arrangement illustrated in FIG. 7. The reference 
etalon is a Burleigh temperature stabilized confocal 
instrument of the same type as that used above but with 
a 150 MHz FSR. The etalon optics were originally 
specified for use at another wavelength; however, the 
coating on the ZnSe substrates allowed sufficient 2pm 
transmission and formation of fringes of sufficient 
finesse ( * 5 ) with which to achieve lock. Locked status 
is immediately apparent on the 3 GHz etalon fringe 
pattern with the absence of long-term drift and 
significantly reduced short term fluctuations. The 
output frequency is tunable by adjustment of the bias 
to the PZT of the controlling etalon and a continuous 
single-mode tuning range of 800 MHz is routinely 
possible. By mounting the laser cavity elements on an 
invar base, improved frequency stability is possible. 



19 


Over a period of many seconds , short term frequency 
jitter of approximately 1 MHz is observed with a PZT 
ramp period of 20ms. In this configuration the error 
signal is derived from the dither signal applied to the 
5 150 MHz confocal etalon. (Due to the low finesse 

fringes it initially proved difficult to set up the 
servo— control electronics, a situation alleviated by 
reversing the roles of the two etalons in the set-up. 
The higher finesse fringes (-166) obtained using the 3 
10 GHz device are more conducive to realizing stable lock, 

which facilitated establishment of proper control 
settings on the lock-in amplifier and dither signal 
generator.) In an alternate configuration, frequency 
lock was obtained when the dither voltage was applied 
15 directly to the laser PZT. However, although locked 

status was again achieved, the modulation on the laser 
output was considered to be less desirable. 


20 


CONCLUSIONS 


In conclusion, efficient, tunable operation of a 
Tm,Ho:YLiF 4 laser has been demonstrated at temperatures 
in the 250-275K range. Output power of 84mW 
5 corresponding to slope conversion efficiencies of 60% 

relative to the absorbed pump power is obtained. The 
emission spectrum is etalon tunable over 7nm centered 
on 2.067ym. A measured transition temperature 
sensitivity of 0.03cm -1 .K-i is recorded facilitating 
10 fine tuning through atmospheric C0 2 and H 2 0 

absorption lines overlapping the emission spectrum. An 
effective emission cross-section of 

5xl0" 21 cm 2 is measured which points to improved energy 
storage capacity over other hosts. Stabilization of 
15 the single frequency output relative to an external 

cavity is also obtained with the demonstration of a 
continuous single-mode tuning range of 800 MHz. 
Closed-loop short term stability of 1 MHz was obtained, 
with the maintenance of single mode long term stability 
20 over many hours. 

Having thus described a preferred embodiment of the 
invention, what is claimed is: 



JPL Case No. 18611 

NASA Case No. NPO-18611-1-CU 

Attorney Docket No. JPL-38 


PATENT APPLICATION 



TUNABLE CW DIODE- PUMPED Tm, Ho: YLiF^ LASER 
OPERATING AT OR NEAR ROOM TEMPERATURE 

ABSTRACT OF THE DISCLOSURE 

A conversion efficiency of 42% and slope efficiency 
of 60% relative to absorbed pump power are obtained 
from a continuous wave diode-pumped Tm,Ho:YLiF 4 laser 
at 2 ym with output power of 84mW at a crystal 
temperature of 275K. The emission spectrum is etalon 
tunable over a range of 7nm (16.3cm - ^) centered on 
10 2.067um with fine tuning capability of the transition 

frequency with crystal temperature at a measured rate 
of -0.03cm 1 .K *. The effective emission cross-section 
is measured to be 5xl0 -21 cm 2 . These and other aspects 
of the laser performance are disclosed in the context 
15 of calculated atmospheric absorption characteristics in 
this spectral region and potential use in remote 
sensing applications. Single frequency output and 
frequency stabilization are achieved using an 
intracavity etalon in conjunction with an external 
reference etalon. 


20 



WAVENUMBER (cttH ) 


r .SA C: 


/g-W/S-ck 

f •' 



2062 2064 2066 2068 2070 

AIR WAVELENGTH (nm) 











: • . /eC//-/<Ct 




/■)■&//■/ f( : t 





STABILIZATION CONTROL 



FIG.