An optical efficiency of 0.052 under normal mode operation for diode-pumped Ho:Tm:YLiF4 at a pulse repetition frequency of 10 Hz has been achieved. Laser output energy of 30 mJ in single Q-switched pulses with 600-ns pulse length were obtained for an input energy of 3 J. A diffusion-bonded birefringent laser rod consisting of Ho:Tm-doped and undoped pieces of YLF was utilized for 10-Hz operation

Barnes, Norman P.

Jani, Mahendra G.

Lockard, George E.

Murray, Keith E.

Naranjo, Felipe L.

English

NASA Technical Reports Server

872
NASA-TM-III850
a reprint from Optics Letters
Diode-pumped long-pulse-length Ho:Tm:YLiF4 laser at 10 Hz
Mahendra G. Jani and Felipe L. Naranjo
Science and Technology Corporation, 101 Research Drive. Hampton, Virginia 23666
Norman P. Barnes, Keith E. Murray, and George E. Lockard
NASA Langley Research Center, Hampton, Virginia 23681
Received September 22, 1994
An optical efficiency of 0.052 under normal mode operation for diode-pumped Ho:Tm:YLiF4 at a pulse repetition
frequency of 10 Hz has been achieved. Laser output energy of 30 mJ in single Q-switched pulses with 600-ns
pulse length were obtained for an input energy of 3 J. A diffusion-bonded birefringent laser rod consisting of
Ho:Tm-doped and undoped pieces of YLF was utilized for 10-Hz operation.
Diode-pumped 2-#m solid-state lasers have received
considerable attention recently for detection of wind
velocities as well as for remote sensing of carbon diox-
ide and water vapor in the atmosphere. High-power
Q-switched holmium lasers are also useful as pump
sources for optical parametric oscillators for generat-
ing tunable mid-infrared radiation for optical remote
sensing and/or infrared countermeasure systems.
In addition, medical applications for 2.0-#m lasers
include arthroscopic surgery, laparoscopic cholecys-
tectomy, and refractive surgery with laser thermal
keratoplasty methods.
A Ho:Tm:YLiF4 (Ho:Tm:YLF) laser, operating nor-
mal mode at a pulse repetition frequency of 10 Hz,
has achieved a laser output energy of 130 mJ/pulse
at an optical conversion efficiency of 0.052. Previ-
ously reported pulsed diode-pumped 2-_m lasers ei-
ther have used Ho:Tm:Y3A15012 as the laser material
or have been operated at cryogenic temperatures, a-4
High-efficiency end-pumped Ho:Tm:YLF lasers have
been demonstrated, which tend to be either continu-
ous wave or produce modest energies per pulse. 5-s
The laser performance reported in this study was ob-
tained with a water-cooled laser rod and transverse
pumping geometry.
We achieved operation of this laser at pulse repe-
tition frequencies up to 10 Hz by using a composite
Ho:Tm:YLF laser rod fabricated from three pieces of
this birefringent material. We fabricated the laser
rod by diffusion bonding two undoped pieces of YLF
to a piece of Ho:Tm:YLF, one at each end. Because
YLF is a birefringent material, extra precautions
were taken during the fabrication and the diffusion-
bonding processes to ensure precise alignment of
the c axes of all the components and thus avoid de-
polarization. When the diffusion-bonded rod was
placed between crossed polarizers, an extinction ra-
tio of 1500:1 was measured with a He-Ne laser.
The diffusion-bonding process involves precision pol-
ishing, assembly by optical contacting, and heat
treatment. Such a diffusion-bonded laser rod with
undoped end pieces allows pumping of the full length
of the doped YLF piece while avoiding ground-state
absorption losses in the unpumped ends of the laser
0146-9592/95/080872- 0356.00/0
rod. In two previous studies diffusion bonding of
isotropic YAG single crystals was demonstrated. 9'_°
When operated in a Q-switched mode, this laser
produces the 600-ns-long pulse lengths required for
accurate wind velocity measurements. A 4-m ring
resonator was used to yield 600-ns-long pulses. Sev-
eral approaches, such as relaxation oscillations or
Q switching with resonators designed for pulse
stretching, can be utilized to generate long pulse
lengths, n The approach used in our study has the
advantages of simplicity and efficiency.
A narrow spectral bandwidth is required so that
the smallest Doppler shift of a wind-sensing sys-
tem is commensurate with the spectral bandwidth
of the laser pulse. In turn, the long pulse length
results from Fourier-transform limits on the narrow-
spectral-bandwidth pulse. Long pulse lengths are
required for the narrow, transform-limited band-
widths needed to measure wind velocity to -+1.0 m/s.
For our experiments, a 42-mm-long diffusion-
bonded laser rod with a 4-mm diameter was uti-
lized. The a axis laser rod consists of 20-mm-long
Ho:Tm:YLF and two ll-mm-long undoped YLF pieces
diffusion bonded at each end of the doped YLF piece.
The reflection losses at the bonded interface for YAG
composite laser rods were measured by Lee et al. to
be less than 0.0012 Similar reflection losses are
also expected for bonded YLF laser rods. The laser
rod has 30-min opposing wedged ends that are an-
tireflection coated at 2.06 #m. The concentrations
of Ho and Tm were 0.005 and 0.04, respectively, for
the doped YLF piece.
The laser pump head design, consisting of two sets
of five laser diode arrays (LDA's) placed symmetri-
cally around the perimeter of the laser rod, is shown
in Fig. 1. The 4-mm-diameter laser rod at the cen-
ter of the pump head is encased by a fused-silica
glass flow tube with a 5-mm inner diameter and a
7-mm outer diameter, respectively. The laser rod is
directly cooled by the flow of water. Because the
O-ring seals occur on an unpumped section of the
laser rod, minimal thermal stress on the laser rod will
occur at the seals. The temperature of the coolant
flow was maintained at 16°C in our experiments.
© 1995 Optical Society of America
https://ntrs.nasa.gov/search.jsp?R=19970007218 2020-06-16T02:39:09+00:00Z
April 15,1995/ Vol 20, No. 8 / OPTICS LETTERS 873
I I
END VIEW
Water Water
Fig. 1. Design of the water-cooled pump head.
Nominal coolant flow rate around the laser rod was
1 L/min. This flow rate corresponds to laminar flow
in our system. In total, 10 diode arrays were used
to pump a 20-mm length of Ho:Tm:YLF. The LDA's
were six-bar-stack devices with 300-W peak power.
The pulse repetition frequency was varied from 1 to
10 Hz, and the current pulse length was 1 ms. We
temperature tuned the LDA's by controlling the tem-
perature of the circulating cooling water. A center
wavelength of 0.792 #m was selected for optimum ab-
sorption of the pump light by the a-axis Ho:Tm:YLF
laser rod.
A maximum optical-to-optical efficiency of 0.052,
corresponding to 130 mJ of laser output energy for
2.526-J optical input energy, was achieved with a
0.90-reflecting output coupling mirror. The slope ef-
ficiencies and threshold energies were measured in
a 50-cm-long standing-wave resonator composed of
an 80-cm radius-of-curvature high reflector and vari-
ous flat output coupling mirrors. Results of these
experiments at 10 Hz, with various output mirror
reflectivities, are shown in Fig. 2. We did not ob-
tain data at high pump energies for the 0.98- and
0.94-reflecting output mirrors so we could avoid op-
tical damage that is due to high circulating fluences
in the resonator. The slope efficiency is 0.144 for
the 0.90 output coupling mirror and is 0.134 of the
0.94 output coupling mirror. Laser output wave-
length was centered at 2.065 #m for various out-
put coupling mirrors. These results obtained at a
pulse repetition frequency of 10 Hz are comparable
with the results previously reported for 1-Hz opera-
tion of Ho:Tm:YLF. ]2 The significant increase in
pulse repetition frequency is attributed to pumping
all the doped length of gain medium as well as to bet-
ter extraction of heat from the laser rod. Higher ef-
ficiencies or higher pulse repetition frequency opera-
tion can be obtained by decreasing the temperature
of coolant flow around the laser rod. The observed
laser output was multitransverse mode for this
resonator.
The efficiency of the diode-pumped Ho:Tm:YLF
laser is limited by the available pump energy.
Flash-lamp-pumped Ho:Tm:Cr:YAG lasers can have
a comparable efficiency because they can be pumped
by a factor of 2.0 over optical transparency. In ad-
dition, Cr is an efficient absorber of the flashlamp
radiation, and the quantum efficiency can be high.
In the diode-pumped Ho:Tm:YLF case, pumping is
limited to a factor of 1.3 over optical transparency.
Thus, even though the quantum efficiency can also
be high for the diode-pumped case, the overall effi-
ciency is comparable with that of flash-lamp-pumped
Ho:Tm:Cr:YAG. Higher efficiencies should also be
possible by closer coupling of the laser diode arrays
to the laser rod.
We have obtained the requisite 600-ns-long
Q-switched pulses by using a 4-m ring resonator in
a figure-eight configuration. This ring resonator
consists of two flat high reflectors, a 4.5-m radius-
of-curvature high reflector, and a 0.82-reflecting
output coupler. An acousto-optic Q switch made
of water-free fused silica having Brewster faces
was used for Q-switched operation. In our experi-
ments the acousto-optic Q switch was adjusted fi)r
a particular pump energy to produce unidirectional
operation of this ring resonator. The Q-switching
approach used in a ring resonator permits long
photon lifetimes, yielding longer pulse lengths,
and, because spatial hole burning is eliminated,
narrow-frequency laser outputs are possible. We
measured laser output energies and pulse lengths of
Q-switched pulses by varying the input optical pump
energies for various pulse repetition frequencies.
Pulse lengths were measured with a fast-response
gold-doped germanium detector.
We have achieved maximum laser output energies
of 30 mJ in a single Q-switched TEMoo-mode pulse
with a 590-ns pulse length tbr an input energy of
3 J, corresponding to a 0.01 optical-to-optical effi-
ciency. Results of this study for a pulse repetition
frequency of 10 Hz are shown in Fig. 3. We achieved
unidirectional operation by adjusting the Q switch
around Bragg angle, as reported in earlier studies by
Clarkson et al. _:_ For the results shown in Fig. 3, the
acousto-optic Q switch was adjusted to produce uni-
directional operation with a laser output energy of
30 mJ. At energies below 15 mJ the ring laser op-
erated in both directions.
0.15
>,
L_(1)
:3
O
0.12
0.09
0.06
0.03
0.00
1.0
o 98%
"_94%
90%
A 82%
[]
0
._7 A.
[]
0
0
V
8
[] A
0 A i
1.5 2.0 2.5 3.0
Input optical energy (J)
Fig. 2. Slope efficiencies and threshold energies ibr a
50-cm-long resonator at a pulse repetition frequency of
10 Hz and a coolant flow temperature of 16 °C.
874 OPTICSLETTERS/ Vol. 20, No. 8 / April 15, 1995
4000
(_ 3500
C
3000
¢.- •
"" 2500
03
r- 20000
i
(_ 1500
m 1000
Q- 500
0
0
Unidirectional
I I l 1 I I
5 10 15 20 25 30 35
Output energy (md)
lengths and output energies of singleFig. 3. Pulse
Q-switched pulses for a 4-m ring resonator at a pulse
repetition frequency of' 10 Hz and a coolant flow tempera-
ture of 16 °C.
0.05
.--j
""" 0.04
03
L_(_ 0.03
•*" 0.02
"t
O.
--t 0.01
0
0.00
2.2
i ! i i
o Normal mode o
• Q-switched
0
0 •
go
I 0 I I I
2.4 2.6 2.8 3.0
Input optical energy (J)
3.2
Fig. 4. Slope efficiencies and threshold energies for a
4-m ring resonator at a pulse repetition frequency of
10 Hz and a coolant flow temperature of 16 °C.
60c2: of the normal mode energy was extracted
into single Q-switched pulses for this ring resonator.
Q-switched and normal mode performances with the
same resonator conditions are shown in Fig. 4. The
beam quality of our laser output is 1.4 times diffrac-
tion limited, and the variation of the beam quality
as a function of input average power is less than
0.1. This observation is in agreement with results
obtained for Nd:YLiF4 by Murray. TM
The laser output wavelength was centered at
2.052 #m, with a FWHM of 220 pm, which is the
resolution limit of our detection system. By insert-
ing an 6talon into the resonator we were able to vary
the output laser wavelength from 2.052 to 2.066 pm.
Such a laser system would be useful for differential
absorption lidar measurements. Experiments are in
progress on injection seeding of this laser to achieve
narrow optical bandwidths needed for velocity
resolution.
For the water-cooled laser system no parasitic las-
ing was observed, as expected, even at the highest
pump energies available. Parasitic lasing can oc-
cur on total internal reflection modes of the laser
rod. Lasing on these modes reduces the laser out-
put energies and thus the efficiency of the laser
system. Parasitic lasing will make it impossible to
increase the stored energy, which limits efficiency of
the laser system. Once parasitic lasing starts, all
further pump energy is converted into parasitic las-
ing modes and therefore is not available in the single
Q-switched pulses.
In conclusion, we have shown an optical-to-optical
efficiency of 0.052 for normal mode operation of a
diode-pumped Ho:Tm:YLF laser at a pulse repeti-
tion frequency of 10 Hz. Maximum output energy
of 30 mJ was obtained in single Q-switched pulses
with 590-ns pulse lengths corresponding to an optical-
to-optical efficiency of 0.01. We achieved operation
of this laser system at 10 Hz by using a diffusion-
bonded birefringent laser rod consisting of doped and
undoped pieces of YLF.
We gratefully acknowledge cooperation and sup-
port from H. Meissner of ONYX Optics with the dif-
fusion bonding of the YLF material.
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Diode-Pumped Long-Pulse-Length Ho:Tm:YLiF4 Laser at 10 Hz

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