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T. Jensen, A. Diening, G. Huber, B. H. T. decreasing thresholds with longer pump

F. Heine, V. Ostroumov, E. Heumann, T. Jensen, G. Huber in Advanced Solid-state Lasers, Vol. 23, OSA Proceedings Series (Optical Society of America, Washington,

Chai, Opt. Lett. 21, 585 (1996). H. Voss, F. Massmann, in Advanced Solid State Lasers, Vol. I, Trends in Optics and Photonics Series (Optical Society of America, Washington, DC, 1996), paper

1 ~ 1 9 % . Ph;67mW pulses.

c q=9%, P,,,=loomw 1.

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"2 ME4. N. Hodgson, H. Weber, J. Mod. Opt. 35, 807 (1988). W. W. Rigrod, IEEE J. Quantum Electron. 14,377 (1978).

CTuE3 8:30 am

Diode-pumped cw and quasi-cw lasing in Yb-, Tm-doped fluorides at 1.5 pm and 2.3 pm

A. Diening, B.-M. Dicks, T. Jensen, F. Heine, E. Heumann, G. Huber, Institutfiir Laser- Physik, Universitat Hamburg, Jungiusstr. 9a, 20355 Hamburg, Germany; E-mail: diening@physnet. uni-hamburg.de Lasers in the eyesafe 1.5 and 2.3 pm spectral regions are of great interest in many applica- tions. We report on infrared cw and quasi-cw upconversion pumped lasing of Yb3+-, Tm3+- doped YLF (LiYF,) and GLF (LiGdF,) crystals in these wavelength ranges.

Both lasers start at the 3H, level of Tm3+ and end at the 3F, (1.5 pm) or 3H, (2.3 ym) level. The pump light at 970 nm is absorbed by the Yb3+ ions. The energy is transferred from the 2F,,, level of Yb3+ to the short living 3H, level of Tm3+ and then in a second transfer step from the 3F, state to the upper laser level (Fig. 1). Due to theYb3+ upconversion process the laser is not selfterminating as already re- ported for the 1.5-pm transition.' The second pump step ofthe upconversion process (3F, + 3H,) avoids the filling of the 3F, state, which is the lower laser level for the 1.5-ym laser.

For the cw experiments two polarization- coupled laser diodes with 1 -W output power at

v

Tm3+ yb3+ CTuE3 Fig. 1 Energy levels and transfer pro- cesses in the Yb3+-Tm3+ system.

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0 0 300 600 900 1200 1500

Pab, [mWI

CTuE3 Fig. 2 Input output characteristics of Yb3+, Tm3+:YLF (squares) andYb3+, Tm3':GLF (circles) (solid 1.5 pm, open: 2.3 pm wave- length).

0 50 100 150 200 250 300 350

PabbS [a CTuE3 Fig. 3 Quasi-cw operation of Yb3+, Tm3+:YLF at 1.5 pm.

970 nm were used. With this setup lasing at 1.5-pm in YLF' and for the first time we believe, in GLF was achieved. Flashlamp- pumpedlaser operation at 2.3 p m was demon- strated in Chromium co-doped Tm3+:YAG and Tm3+:YA103T3 We achieved diode-pumped lasing in Ytterbium co-doped Tm3+:YLF and Tm3+:GLF on this transition to our knowledge for the first time.

The maximum output power of 180 mW at 1.5 pm was achieved with a Yb(5%), Tm (0.5%):YLF crystal of 3-mm thickness. The crystal was HR-coated for the laser on one side and AR-coated on the other side. The AR coat- ing reflected 60% of the pump light back into the crystal thereby enhancing the pump power absorption.

The YLF and the GLF crystals for the 2.3-pm laser experiments were uncoated. For both wavelengths a 50-mm radius of curvature output coupler with a transmission of 2% was used. Input-output curves are shown in Fig. 2.

The 1.5-pm laser in YLF emitts on three wavelengths (1452 nm, 1500 nm and 1569 nm) with the major fraction of the output power in the center frequencies around 1500 nm. The 2.3-pm lasing occurs on a broad band from 2298-2318 nm.

For the quasi-cw experiments a diode array with a maximum pulse energy of 500 mJ at pulselengths of 1 ms was used. The pump light was imaged into the crystal via a lens duct with an output surface of 1.5 X 1.5 mm2. In these experiments the same coated YLF crystal as for the cw experiments was used. A maximum output energy of 20 mJ at repetition rates of 5 Hertz could be achieved (Fig. 3).

Experiments with different pulselengths show a linear increase of output energy and

De, 1995), pp.'276-278. L. M. Hobrock et al., presented at 7th In- ternational Quantum Electronics Confer- ence, May 1972, Montreal, P.Q., Canada. J. A. Caird, L. G. DeShazer, J. Nella, IEEE J. Quantum Electron. 11,874-881 (1992).

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CTuE4 8:45 am

High-power operation of an intracavity- pumped Ho:YAG laser at 2 .1 pm

C. Bollig, R. A. Hayward, M. Kern, W. A. Clarkson, D. C. Hanna, Optoelectronics Research Centre, University of Southampton, Southampton SO17 lBJ, U.K.

Tm'+- and Ho3+-doped lasers have attracted much interest, as they operate in the 'eyesafe' 2-pm region, and, with their long lifetime, have the potential for high-energy cw-pumped Q-switched operation, as required for applica- tions such as wind monitoring for airport safety.

The better atmospheric transmission around 2.1 pm favors the Ho3+ laser over the Tm3+ laser at -2.0 pm, whereas the latter offers a more convenient wavelength for diode-pumping. The standard approach to diode-pumped Ho3+ lasers has been to use laser crystals with Tm3':Ho3+ co-doping, the Tm3+ being diode-pumped at 785 nm fol- lowed by energy transfer to the Ho3+ ions upper laser level. This process works efficiently in cw lasers, but much less well in Q-switched lasers where upconversion effects substantially shorten the storage lifetime.

This problem can be avoided using an intra- cavity pumping scheme with Tm3+ and Ho3+ ions separated into two different rods in the same cavity. The Tm3+ material can be directly diode-pumped at 785 nm, while the Ho3+ ma- terial is pumped by the 2-pm laser emission of the Tm3+. The weak absorption of the Ho3+ (typically a few percent) acts as the 'output coupler' for the Tm3+ laser.

Recently such an intracavity pumped laser has been reported,' which had an output of 120 mW with the medium Tm:YAG being pumped by a TkSapphire laser. Here we report a diode-pumped system with multi-watt out- put.

Recently we have reported efficient room- temperature operation of aTm:YAG laser end- pumped by a 20-W diode-bar.' An output power of 4 W TEM,, had been obtained for 13.5 W of incident pump power at a mount temperature of 20°C. To achieve this we used the two-mirror beam-shaping technique re-

2w @ 2.lum

Tm:YAG Ho:YAG I ] , 9w 0

5mm 10" n -

Rc = -100mm Rc = 50"

90%R B 2 . l w HR 0 2.0-2.lpm HR 0 Z.Opm

CTuE4 Fig. 1 Cavity design ofthe intracavity- pumped Ho:YAG laser operating at 2.1 pm.

76 / CLEO’97 / TUESDAY MORNING

cently r e p ~ r t e d , ~ to reconfigure the output beam from a high-power diode bar so as to allow intense end-pumping with an essentially circular beam.

The setup for the intracavity-pumped laser described here is similar to the above Tm:YAG laser. The 45-mm-long linear cavity has a pump input mirror having high reflectivity from 2.0-2.1 pm. The output coupler is highly reflective from 2.00-2.02 pm with 10% trans- mission around 2.1 pm. The convex (- 10 cm) input mirror compensated for the strong ther- mal lensing in the 5-mm-long, 6%-doped Tm:YAG rod, while the output coupler had a 5-cm radius of curvature. Both the Tm:YAG rod and the 10-mm-long 0.5%-doped Ho:YAG rod were mounted on the same water cooled heat sink maintained at 10°C. The shaped beam from the diode pump was fo- cused to a spot of 300 pm diameter. For 9.2 W of diode power incident on the Tm:YAG rod the maximum output was 2.1 W at 2097 nm corresponding to a slope efficiency of 28%. At 1.9 W of output, M2 values of 1.6 were mea- sured in both planes. The Tm:YAG laser itself operated at 2012 nm. The threshold for Tm:YAG lasing was 1.5 W, while the Ho:YAG started lasing at the slightly higher diode power of 1.7 w.

Further work is now aimed at Q-switched operation of this laser. 1. R. C. Stoneman and L. Esterowitz, in Ad-

vanced Solid State Lasers, Vol. 13, OSA Proceedings Series (Optical Society of America, Washington, DC, 1992), p. 114. C. Bollig, W. A. Clarkson, D. Schmundt, D. C. Hanna, in Conference on Lasers and Electro-Optics/Europe, 1996, p. 56. W. A. Clarkson and D. C. Hanna, Opt. Lett. 21, 375 (1996).

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CTuE5 9:00 am

Ho:Tm:YLF diode-end-pumped 2-pm amplifier

Waldo J. Rodriguez, Norman P. Barnes,* Center for Materials Research, Norfolk State University, 2401 Corprew Avenue, Norfolk, Virginia 23504 Space-based Doppler lidars require high en- ergy and efficient laser systems. High energies are better achieved by master oscillator-power amplifier systems. Quasi four-level lasers have shown to operate more efficiently with end- pumped schemes.’-3 This stems from the high fluences necessary to invert the lasing ion. A diode-end-pumped Ho:Tm:YLF laser ampli- fier system has been designed and evaluated. Gains of 2.8 have been achieved in the small signal regime translating into 0.15 optical to optical extractable energy efficiencies. Double- passing the pump improves the performance by 20%. At higher Ho concentrations, higher gains are achievable with the available pump energy of this amplifier module design.

The amplifier design is shown in Fig. 1. Two, thirteen 100-watt bar stacks are mounted side to side at the end of a lens duct assembly. The lens duct concentrates the light to a -3.3-mm spot into a Ho:Tm:YLF amplifier disk. The amplifier diskis sandwiched between

CTuE5 Fig. 1 Layout of the laser amplifier module.

3 I

I I

0 5 1 1 k 6 a Ib 1’2

0

Incident Pump Ener.gy Fluence (Jlcm’)

CTuE5 Fig. 2 Gain vs. incident pump energy fluence for the 0.5%Ho:4%Tm:YLF and l.O%Ho:6%Tm:YLF amplifier disks in the c-axis.

a 2-pm, high-reflector, 793-nm high-trans- mittance window at the pump end and a 793- nm, high-reflector, 2-pm high-transmittance window at the output end. The first window bounces the 2-pm amplified light out of the gain media and the latter window provides a second pass of the pump light. The pump- probe experiments were performed utilizing a single-mode Ho:Tm:YLF microchip laser op- erating at 2.05 pm with (1 MHz bandwidth. The AlGaAs bar stacks were tuned to the Tm 3H, absorption line and were operated at 1.1 msec pulses.

Amplifier disks of two different dopant concentrations have been evaluated to date. The c-axis gain vs. incident pump energy flu- ence is shown in Fig. 2. The 0.5%Ho:4%Tm: YLF sample showed a maximum double pass c-axis gain of 1.64 reaching saturation at -10 J/cm2. Gains of 2.8 were achieved with the 1 .O%Ho:6%Tm:YLF disk reaching saturation at around 14 J/cm2. Saturation of the l.O%Ho disk suggest that higher gains can be achieved in this system utilizing higher Ho concentra- tions?’* Double-passing the pump improved the gain by a factor of 1.6.

Using the measured small-signal gain, the extractable energy efficiency of the Ho:Tm: n F laser amplifier was calculated to be 0.15: Details on the amplifier system design, its ca- pabilities and limitations, and experimental results are discussed.

We acknowledge Julie A. Williams-Byrd (NASA) and George E. Miller I11 (CMWNSU) for facilitating equipment for this work and Jasper R. Lewis, Jr. and Tamika D. Smith (NSU/CMR) for assisting in the data collec- tion.

This work was supported by NASA grant #NAG-1-1453 and DOE grant #DEFG0194EW1149. *MS 474, NASA Langley Research Center, Hampton, Virginia 23681

1. R. J. Beach, S. B. Sutton, J. A. Skidmore, M.A. Emanuel, in Conference on Lasers and Electro-Optics, OSA Technical Digest Series (Optical Society of America, Wash- ington, DC), paper CWN1. C. Bibeau, I. L. Bass, R. J. Beach, L. K. Smith, C. D. Marshall, S. C. Mitchell, S.A. Payne, in Advanced Solid State Lasers, Vol. I, 1996 OSATrendsin Optics and Photon- ics Series (Optical Society of America, Washington, DC, 1996) pp. 19-22. W. J. Rodriguez, Mark E. Storm, Norman P. Barnes, in Advanced Solzd-State Lasers, Vol. 24, OSA Proceedings Series (Optical Society of America, Washington, DC,

N. P. Barnes, W. J. Rodriguez, B. M. Walsh, “Ho:Tm:YLF Laser Amplifiers,” J. Opt. Soc. Am. B, in press.

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1995) pp. 392-395. 4.

CTuE6 9:15 am

Reliable, single-pulse @switched laser output at the 1535nm eyesafe wavelength

Ruikun Wu, Scott J. Hamlin, J Andrew Hutchinson,* Lawrence T. Marshall,* Kzgre, Inc., 100 Marshland Road, Hzlton Head Island, South Carolina 29926, E-mad kzgre@aol com Recently we have been under contract mth the United State Army Nigh Vision & Electronic Sensors Directorate (NVESD) to investigate laser diode pumped, passively Q-switched Er. Glass lasers. Under this project, various aspects which influence the passive Q-switching of ErGlass lasers are under study. Parameters in- clude resonator geometry and configurations, resonator losses, gain media geometry, and gain media dopant concentrations

The input pump energy dynamic range for single-pulse operation of a saturable absorbing Q-switched laser is a very important specifica- tion for rangefinders. Typically, the double- pulse threshold is about 120% of the single- pulse energy threshold. Operation in the double-pulse regime can cause difficulties in ranging or cause the laser to become non- eyesafe. The pump-energy range for single- pulse operation must compensate for ambient conditions and degradation. A grater input en- ergy tolerance will help improve the opera- tional characteristics of rangefinders.

In our radial pumping geometry, the Er: Glass gain media is in the form of a right cylin- drical rod with a polished barrel. In such a geometry, it is relatively easy to produce para- sitic ring laser modes inside the rod if the gain is sufficient. These so-called “whisper modes” are polygonal in shape and may exist in two or three dimensions. In high cross-section gain media such as NdYAG, these whisper modes can be a real problem.

With the low cross section of Er.Glass, we originally did not expect to observe parasitic oscillations Calculations indicate, in a 3-mm diameter laser rod, these modes exist outside of the 1-mm-core diameter where the oscilla- tor mode extracts energy In this annulus area, the gain will drop due to parasitic oscillation, if the diameter of the rod is of reasonable optical

Figure 1 lllustrates the 1535-nm fluores- quality.