HighEfficiency RoomTemperature 2.06μm Laser Using Sensitized Ho3+:YLFE. P. Chicklis, C. S. Naiman, R. C. Folweiler, D. R. Gabbe, H. P. Jenssen, and A. Linz Citation: Applied Physics Letters 19, 119 (1971); doi: 10.1063/1.1653837 View online: http://dx.doi.org/10.1063/1.1653837 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/19/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Energytransfer processes in high power Yb:Tm:YLF lasers emitting at 2.3 μm AIP Conf. Proc. 992, 386 (2008); 10.1063/1.2926892 A highly efficient room-temperature nanolaser Phys. Today 60, 24 (2007); 10.1063/1.2774088 Fast, high-efficiency, and homogeneous room-temperature cathodoluminescence of ZnO scintillator thinfilms on sapphire Appl. Phys. Lett. 89, 243510 (2006); 10.1063/1.2405392 Highefficiency 2.09 μm flashlamppumped laser Appl. Phys. Lett. 55, 1062 (1989); 10.1063/1.101705 Measurements and calculations of CO2 roomtemperature highpressure spectra in the 4.3 μm region J. Chem. Phys. 90, 2944 (1989); 10.1063/1.455894

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APPLIED PHYSICS LETTERS VOLUME 19, NUMBER 4 15 AUGUST 1971

High-Efficiency Room-Temperature 2.06-#lm Laser Using Sensitized H0 3+ :YLF

E. P. Chicklis, * C.S. Naiman, * and R.C. Folweiler* Electro-Optics Division, Sanders Associates, Inc., Nashua, New Hampshire 03060

and

D.R. Gabbe, t H. P. Jenssen, t and A. Linzt Center for Materials Science and Engineering and Department of Electrical Engineering,

Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 3 May 1971; in final form 10 June 1971)

Pulsed room-temperature laser action at 2.06 p.m has been achieved using Ho3+: YLF ILiF 4) sensitized with Er3+ and Tms+. An unoptimized cavity gave a threshold of 35 J and a slope efficiency of 1. 3% in normal mode with an output mirror of 62% T. A slope efficiency of 2% and a 65-J threshold were observed in Q-switched operation along with Q-switched en­ergy of> 500 mJ from a 0.9 x i-in. rod. The sensitizer ions have broad absorption bands and transfer times (to the active ion) of 150-200 p.sec.

Considerable effort has been directed in recent years at obtaining high-efficiency lasing operation in rare-earth ions through the use of energy transfer from sensitizing species to the laSing ions. Enhance­ment of pumping efficiency is a particularly acute problem in materials with nonvanishing terminal­state populations such as the first excited to ground manifold tranSition in H0 3+. We report very high­efficiency room-temperature lasing operation as­sisted by energy transfer in Ho3+-doped alphabet YLF (LiYF4) with the highest efficiency ever re-

0.4

'" ~ 0. E o

.~ "0 Q; a::

2.12 2.04

Sample 342

Excitat ion spectrum 300 0 K

Sample 342

Fluorescent output 300 0 K

1.96 1.88 1.80 Wavelength (}L )

(bl

1.72

FIG. 1. (a) Two-micron fluorescence excitation spectrum of Ho: alphabet YLF No. 342 at room temperature. Data are normalized for constant number of exciting photons. Optical slitwidth < O. 006 p.. (h) Room-temperature fluores­cence spectrum of alphabet YLF No. 342. Optical slitwidth <0.006 p..

119

ported for room-temperature laser operation in a rare-earth ion other than Nd3 ••

Stimulated emiSSion in Hos, was first reported1 in singly doped H03+:CaW04 at 77 OK. Observation of energy transfer between Er-H02 led to the first3 re­port of efficient cw operation at 77 OK in Ho-doped alphabet YAG. Ho laSing action assisted by energy transfer at 77 OK has also been reported in glass, 4

yttrium iron garnet, 5 ErZ0 3 , 6 LiYF4 , 7 and HoF3• 8

Low-threshold room-temperature Ho operation in multiply sensitized hosts has been reported in Ca;,ErsF199 defect structure10 and YAG. 11 Our ex­perience with the latter host, to be reported else­where, 12 has indicated low over-all efficiency and considerable susceptibility to damage in this mate­rial. The performance of Ca2Er5F19 appears to be limited by material problems. 10

We have observed high-efficiency pulsed laser action in alphabet YLF (LiYo.41SEro.5Tmo.067Hoo.o17F4) at 2.065 11m at room temperature with a normal-mode threshold of 35 J and slope efficiency of 1. 3%. Q-switched slope efficiency of -2% and a Q-switched output in excess of O. 5 J has also been obtained. It should be noted that these values of threshold and slope effiCiencies are obtained by measurement of the total electrical energy into a 3-in. flashlamp pumping considerably shorter rods.

The rods were grown by a top-seeded solution tech­nique from a LiF-rich melt in a helium atmosphere in a modified NRC 2804 crystal-pulling furnace. 13

Rods were cut along the length of the boule perpen­dicular to the optic axis. YLF crystallizes in the tetragonal Scheelite (CaW04) structure14 and ac­commodates rare-earth dopants at the y+3 site with­out charge compensation.

The room-temperature excitation spectrum (nor­malized for constant number of exciting photons) for the 2.06-l1m Ho3+ fluorescence [Fig. l(a)] shows broad excitation throughout the region of high emis­sion of Xe flashlamps. Peaks above 0.5 11m are not

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120 CHICKLIS et al.

400r-----.-----.--.--,-----,-----,

~~~o:a~~ing jA 360

320

~ 280

:::J

g. 240

E -;:, 200 ~ <L> c: <L> 160 :::J

~ :::J

o 120

80

40

slope eff. ~ 1.3% 0.94 • 1/8"

o

A

YLF 71.2 I 62 % coupling +/ slope eft . .v I % / ,~. ",,"

+

/ 1:,: :,;,~, /Je slope,.. 0.4 %

o

",.

..... / o ./.

/..-.- YLF 71.2

/e 4% coupling

+ slope NO.2 %

O~~--~~~~------~----~----~ 30 50 70 90 110 130

Input energy (Joules)

FIG. 2. Normal-mode output energy vs input energy for rods 71. 2 and 71. 6. 71. 6, although shorter, is of higher crystal quality. Uncorrected for mismatch with 3-in. flashlamp.

shown due to the difficulties of accurate normaliza­tion. The unpolarized emission spectrum is shown in Fig. 1(b). Energy transfer appears to be efficient but not complete. Er and Tm fluorescence has been observed and will be reported elsewhere. 12 Room­temperature pulsed fluorescence measurements re­vealed a 12 msec ± 1~ lifetime for the 5[7 - 5[S

transition independent of the pumping band.

Lasing action was observed with low threshold and high efficiency with a number of rods with various pumping conditions. Best results to date were ob­tained with a nonimaging silvered cylinder. The pump was a 3-in. PEK XE-14C-3 xenon lamp with a flash duration of 500 /J.sec. The rods and lamp were convection cooled with no uv filtering employed. No uv or laser-induced damage has been observed in any of the YL F rods.

Input energies were determined by measuring the capacitor bank voltage and the output energies with TRG model 102 thermopiles. Q-switched outputs were detected with a Philco L4530 lnAs diode (band­width - 5 GHz) and the energy meter. A plane paral­lel resonator configuration was used with external and in some instances attached reflectors. Q

switching was obtained with a rotating mirror. In Fig. 2, the normal-mode output vs input energy is plotted for rods 71. 2 and 71. 6. The abscissa is the total electrical energy input to the lamp uncorrected

for the length mismatch between the rods and the flashlamp. With a 250-/J.sec flashlamp pulse, oscil­lations were observed at about 150 /J.sec after initia­tion of the flashlamp trigger pulse indicating rapid energy transfer from the senSitizing ions. The wavelength of the laser emission was measured with a i-m grating monochromator. Normal-mode oscil­lations were observed only at 2.0654 ± o. 0004 /J.m up to the highest level investigated, 30% above thresh­old. Measurements at higher pump levels will be made in the future.

Figure 3 shows the results of Q-switched behavior with a rotating mirror for rod 71. 2. The higher slope efficiency observed for Q-switched vs normal­mode operation is probably the result of the long storage time of this material. Because of the high gain of the material and the slow switching speed of the rotating mirror (-1 /J.sec), multiple Q-switched pulses were observed. Experiments with faster switches are underway. Q-switched outputs greater than 500 mJ were obtained from a rod of only 0.24 cms without evidence of damage or saturation.

The 5 Is ground state of Hos- with a multiplicity of 17 is split into 13 levels in 54 symmetry, four of them being twofold degenerate. Based on this multiplicity and the splitting of the ground-state manifold (- 340 cm- 1), the terminal state population at room tem­perature is estimated to be 2% of the total Hos- pop­ulation, or -4. 6x 1019/cms.

An estimate of the stimulated emission cross sec­tion was obtained by direct measurement of the ab­sorption cross section between what is believed to be the initial and final lasing states. The computed value is 1. 5x 10-19 cm2 compared to 8x 10-19 cm2

for Nd3-: YAG 15 and 2 x 10-20 for Nd silicate glass. 16

The degeneracy of the upper level state is not un-

600 1 I I I I

Alphabet YLF

Rod 71.2. 1 5/16' 1/8 inches I Rotating mirror, 62 % coupling • mirror /

V> • - ) -~ 400 :::J

:g. E

-

SO.".21 -

( ,

>. e' <L> c: <L>

:; 200 ~ :::J

0

o I 1 ./, I

o 20 40 60 80 100 120 Input energy (Joules)

FIG. 3. Q-switched output energy vs input energy for rod 71. 2. Uncorrected for mismatch with 3-in. flashlamp.

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HIGH-EFFICIENCY ROOM-TEMPERATURE LASER 121

ambiguously known, precluding precise determina­tion of the emission cross section. An independent check of the value using a dynamic technique16 is presently underway.

A study of the threshold energy vs coupling mirror reflectivity17 yielded a computed total loss coeffi­cient greater than 20%/cm. The threshold condition for oscillation is R 1R z e-2/(o<-6)= 1, where Rl and R2 are the mirror reflectivities, 1 is the rod length, and Ci and 5 are the gain and loss coefficient per cm. For unit reflectivity (zero output coupling), the threshold condition is simply 5= Ci = t::..Na, where a is the emission cross section and t::..N is the popula­tion difference between the upper and lower states of the laSing transition. Using the measured values of a and 5, we obtain an upper-state population of 7 x 1018 ions/cm3 required for inversion. With crys­tals of improved optical quality, considerable reduc­tion in threshold inversion would be expected. A typical value of 5 for high-quality laser material is 2%/cm or less.

Alphabet YLF appears to be a very favorable H03•

laser material with apparent high resistance to uv and laser-induced damage. The broad excitation spectrum, efficient sensitizer-activator energy transfer, moderately high-emission cross section, and quasi-4 level character result in high over-all gain and low threshold for room-temperature pulsed operation. The long H03• fluorescent lifetime in this material provides for high-energy storage for effi­cient Q-switched operation. The long lifetime to­gether with projected reduction in threshold with rods of lower H03• concentration and improved opti­cal properties point to the possibility of cw opera­tion at room temperature.

We gratefully acknowledge the experimental assis­tance of J. C. Doherty with the laser measurements

and R. Mills with the crystal growth. We wish to thank members of the Laser Physics Branch at the Naval Research Laboratory for fruitful technical discussions.

*Supported in part by the Air Force Office of Scientific Research and Sanders ASSOCiates, Inc. Independent Re­search and Development Program.

tCrystal growth supported by the Office of Naval Research and Advanced Research Projects Agency.

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