1016 OPTICS LETTERS / Vol. 28, No. 12 / June 15, 2003

50-mJ, Q-switched, 2.09-mm holmium laser resonantly pumpedby a diode-pumped 1.9-mm thulium laser

P. A. Budni, C. R. Ibach, S. D. Setzler, E. J. Gustafson, R. T. Castro, and E. P. Chicklis

BAE Systems, P.O. Box 868, Nashua, New Hampshire 03060-0868

High-enefor a vtions. Rremote2-mm wause of oprovidebe highlevels anear-diffdescribeby cw, d1.9-mm e5I7 mani

Eff iciepumped,advantagthe pumthe activvery higof oscillaaverage-becauselosses abetweenof thesethat sevlevels neIn additthe Q-sof theseaverage-system acapabilitfor Ho:YHo 5I7 m(using 1laser offTm–Hoeliminatreducedabsorbedefficienctempera2-mm la

Received December 5, 2002

We report .50-mJ Q-switched energy output from a 2.09-mm Ho:YAG oscillator, resonantly pumped at 1.9-mmby a diode-pumped Tm:YLF laser. We demonstrate a Q-switched-to-normal-mode extraction efficiency of100% at an output energy of 30 mJ and over 64% at the 50-mJ output level. Operating at 50 mJ (60 Hz)of output, a pulse width of 14 ns is achieved from the holmium oscillator, which corresponds to �3.6 MW ofpeak power. At the 50-mJ output level, a beam-propagation factor M2 � 1.2 is achieved. © 2003 OpticalSociety of America

OCIS codes: 140.0140, 140.3070, 140.3480, 140.3540, 140.3580, 140.5680.

rgy, Q-switched, 2-mm Ho lasers are usefulariety of scientif ic and technical applica-equirements for laser radar, ranging, and

sensing can be fulf illed with the eye-safevelength or by shifted wavelengths with theptical parametric oscillators (OPOs), whichvery broad wavelength-tuning ranges. To

ly efficient, OPOs generally require drivettained with high-peak-power pulses andraction-limited beams. In this Letter wethe performance of Ho:YAG lasers pumpediode-pumped Tm:YLF lasers, in which themission of the Tm:YLF laser pumps the Hofold directly.nt room-temperature operation with diode-sensitized Tm–Ho 2-mm lasers, which takee of a sensitizer ion (Tm) to strongly absorbp energy (diode pump) and transfer it toator or lasing ion (Ho), have demonstratedh Q-switched energy levels (600 mJ) by usetor amplifier chains.1 Efficient energy andpower scaling of these lasers is diff icultof the deleterious effects of upconversion

t high pump levels and reverse transferthe Tm and Ho ions.2 The combinationeffects produces a very large thermal load

erely limits performance at the high pumpeded to achieve gain at room temperature.

ion, because of the above-mentioned factors,witch-to-normal-mode extraction efficiencylasers is at best �30%. To circumvent the

power limitations of the sensitized Tm–Hond have access to the high-energy storage

y of Ho [long f luorescence lifetime of 8–16 msAG�YLF (Ref. 3)], direct laser pumping of the

anifold is employed. Direct laser pumping.9-mm radiation) of the Ho quasi-two-levelers several advantages over the sensitizedsystem: low quantum defect (1.9 2-mm),

ion of energy transfer (no sensitizer ion used),upconversion losses (linearity of gain versuspump intensity), high short-pulse extraction

y, and reduced sensitivity of gain versusture. The combined advantages produce aser capable of high-power scaling and high-

0146-9592/03/121016-03$15.00/0

energy output (lower lifetime reduction under intensepumping than sensitized Tm–Ho).

We have demonstrated a resonantly pumped Holaser at high average-power levels (�20 W ), operatingin the cw-pumped, repetitively Q-switched regimeand yielding low per-pulse energies (�1 mJ ) in thetens of kilohertz.4 Resonantly pumped Ho lasershave also been demonstrated by others5,6 who usedHo:LuAG and Ho:YLF lasers to generate high en-ergy (33 mJ) at very high quantum eff iciency (0.88)but with normal-mode operation and lamp-pumpedsystems. To date, no data have been obtained onhigh-energy, high-peak-power, Q-switched, resonantlypumped Ho lasers that use a diode-pumped sys-tem. Although high normal-mode eff iciencies havebeen achieved, their performance is based on high-concentration material, high output-coupler ref lec-tivity, and high pump f luences. These factors areparticularly problematic when one desires high-energyQ-switched operation. High-concentration materialis suitable for cw-pumped, repetitively Q-switchedoperation, where tight focusing configurations are re-quired for average-power-starved 1.9-mm pumps.7 Incontrast, low Ho concentrations are required forefficient normal-mode-to-Q-switched extraction, sincethe Ho-to-Ho upconversion effectively reduces theupper-state lifetime in high-concentration material.8

Low output-coupling ref lectivities are needed inQ-switched operation to reduce the high circulatingintracavity f luences and peak-power densities onintracavity components for damage-free operation.Low pump f luences are needed because of pump-induced damage and bleaching encountered with theresonantly pumped Ho quasi-two-level system. Thelong effective-storage lifetime of Ho lends itself wellto the use of diode-pumped 1.9-mm lasers. Laserdiodes are inherently low-peak-power devices, so wetake advantage of the long storage lifetime of Ho byintegrating the 1.9-mm Tm radiation. The Tm laseroperates by providing an output consisting of a fewrelaxation oscillation spikes followed by a quasi-cwpulse, producing �5-ms pulses (the diode pump pulsewidth is �5.5 ms).

The Ho:YAG laser conf iguration utilizes an end-pumped geometry for both the Tm (1.9-mm) and

© 2003 Optical Society of America

June 15, 2003 / Vol. 28, No. 12 / OPTICS LETTERS 1017

Ho (2.09-mm) lasers. We chose this approach forits inherent high efficiency and ability to producegood beam quality in addition to its capability toallow for very compact laser systems. The inherenthigh brightness available with the Tm pump allowslong Ho laser crystals to be used. This results inincreased energy storage and eff iciency, provided thatabsorption of the pump is high enough to achievehigh inversion and transparency along the laser modeaxis. Because the performance of the Tm:YLF laserhas been described elsewhere,4,7 we will focus on theQ-switched, resonantly pumped Ho:YAG oscillator.The Ho laser resonator is shown in Fig. 1. We used afolded cavity because it allowed us to easily adjust thecavity length (for good Tm-to-Ho mode match), accom-modate a Brewster-cut acousto-optic (AO) Q-switch,and polarization couple the Tm pump. Polariza-tion coupling was accomplished by using a dichroicthat is highly transmissive to p-polarized, 1.9-mmpump radiation and highly ref lective for s-polarizedradiation at 2.09 mm. The geometry of the foldedHo resonator we used was semihemispherical with aplano output coupler and a concave high ref lector (5-mconcave). The length of the Ho resonator’s physi-cal cavity was �33 cm. Evaluated output couplersranged in ref lectivity from 30% to 95%, with bestlong-pulse performance achieved with a 50% ref lector.Best Q-switched performance was obtained with a30% ref lective mirror; although this increased thethreshold, the low intracavity f lux achieved with thismirror allowed sustained damage-free operation. Toincrease absorption efficiency we double passed thepump by ref lecting the unabsorbed radiation with theHo oscillator’s high ref lector. Under intense pumpinga quasi-two-level system such as resonantly pumpedHo (and Yb) will exhibit a reduction in the absorptioncoeff icient as a result of a decrease in the number ofavailable ions in the ground state as they are liftedto the 5I7 energy level (ground-state depletion orbleaching). Hence performance optimization must in-clude this saturable effect by carefully calculating therequired crystal length and concentration to maximizepump absorption. In this work we did not make anyattempt to optimize the absorption; ongoing work willaddress this matter. The Tm pump’s radiation wasfocused to fill the mode volume of the Ho resonator.As a result of the high brightness of the Tm pumpbeam (and close wavelength proximity), good overlapof the pump-to-Ho-resonator mode was achieved.The 1.9-mm pump spot diameter was approximately1.58 mm with M2 � 3, so the pump beam’s diameterwas nearly unchanged over the length of the Ho crys-tal (35 mm). The Ho concentration used was 0.5%.The Ho laser crystal was encapsulated in a windowedhousing. The crystal was mounted in a conductivelycooled Cu cold finger, which in turn was attached tothermoelectric coolers. This was done to evaluate theperformance of the Q-switched Ho oscillator over abroad temperature range.

Q-switched operation was achieved by using aBrewster-cut AO Q-switch. The physical length ofthe Q-switch was 45 mm, and the acoustic aperturewas 6 mm. Operating the switch at 27 MHz and

100-W peak power, we were able to achieve .50%diffraction eff iciency. Normal-mode laser hold-offwas accomplished by tuning the AO Q-switch throughthe Bragg angle and monitoring the onset and extinc-tion of long-pulse laser emission with a photodiode.In addition, we were able to achieve good dynamichold-off by monitoring the Q-switched pulse width forthe shortest, most energetic pulses as the Q-switchwas tuned. The Q-switch rf on-time (low-cavity Q)was approximately 5 ms and was delayed 0.5 msrelative to the diode-drive waveform.

Figure 2 shows the performance of the Ho oscillatorQ-switched versus the Tm pump drive. We demon-strate 50.6 mJ of Ho:YAG output in a near-TEM00beam with only 269 mJ of input energy from theTm:YLF laser at 60 Hz, which corresponds to �19%optical-to-optical conversion. Normal-mode energyoutput at the corresponding 50.6-mJ Q-switchedoutput level was �80 mJ (not shown in the f ig-ure). This unoptimized result represents a �64%Q-switch-to-normal-mode extraction efficiency. Theabove Q-switched-to-normal-mode extraction effi-ciency is more than a factor of two greater than acodoped Tm–Ho system. At the 30-mJ output level,100% extraction efficiency was observed. The roll-offin the energy output versus drive is indicative ofthe saturation of the absorption coeff icient for thequasi-two-level system.

To determine the area-overlap factor between theTm pump and Ho resonator mode, we measured thebeam-propagation factor �M2� and far-field divergence.This allowed us to calculate the diameter of the beamin the resonator. The resulting beam diameter in thecavity was �1.1 mm, yielding a resonator-to-pumparea-overlap factor of only 48%. Applying this factorto the performance achieved yields .39% conversionefficiency. This shows that better area overlap cangreatly improve the good performance obtained here.In addition, beam forming the Tm pump to a f lat-topprofile may also improve eff iciency. We obtaineddata or performance over a range of temperaturesby varying the current drive with the thermoelectriccooler and monitoring the laser’s energy output. Fig-ure 3 shows the output versus Ho crystal temperaturefor the range 215 20 ±C. We note a very mild slope

Fig. 1. Folded-cavity Q-switched Ho:YAG oscillator.1.9-mm pump radiation was injected through a thin-f ilmpolarizer (TFP) in a p-polarized state, the 2.1-mm resonantfield was s polarized. To increase absorption efficiencythe 1.9-mm pump was recycled through the Ho gainmedium by ref lection from the Ho high ref lector (HR).

1018 OPTICS LETTERS / Vol. 28, No. 12 / June 15, 2003

Fig. 2. The resonantly pumped Q-switched Ho:YAG oscil-lator demonstrated over 50 mJ per pulse at 60 Hz with anoptical-to-optical efficiency of �19%. Dotted curve, bestfit to the data.

Fig. 3. Q-switched Ho:YAG output average power versustemperature. Solid line, best fit to the data, with a slopeof �10 mW�±C corresponding to �0.17 mJ�±C.

of approximately 0.17 mJ�±C or 10 mW�±C. Our pre-vious work7 yielded a 35-mW�±C slope under intensepumping �.10 kW�cm2� of high-concentration (2%Ho) material. We believe the three-times-lower slopeobserved here is a result of reduced thermal loadingthat was due to less upconversion caused by the use ofmaterial with a lower dopant concentration.

To determine the beam-propagation factor �M2�,we routed the Ho laser’s radiation through a 50-cm-focal-length lens and measured the beam diameterby use of apertures along the propagation directionat full Q-switched laser energy output. Apertureswere used because the beam profile exhibited verygood circular symmetry on a pyroelectric cameraand on film burns. Derived from standard beam-propagation expressions, a curve f it to the data yielded

a near-diffraction-limited beam with M2 � 1.23. Thepulse width was measured with a high-bandwidthphotoelectromagnetic HgCdTe detector and a 1-GHzoscilloscope. At the 50-mJ output level we achieved a14 ns (FWHM) pulse width.

In this Letter we have demonstrated resonantlypumped Ho, delivering Q-switched .50-mJ pulsesat 60 Hz with a near-diffraction-limited beam�M2 � 1.2�. In addition, Q-switched-to-normal-modeoperation achieves �64% efficiency with an optical-to-optical eff iciency of �19% at full drive; at re-duced drive ��30 mJ of output) 100% extraction ofQ-switched-to-normal-mode efficiency is achieved.For applications requiring high-peak-power, near-diffraction-limited beams, and energetic Q-switched2-mm pulses, the resonantly pumped Ho system isa feasible transmitter or pump source for nonlinear-frequency conversion with an OPO in the 3–5- and8 12-mm regions. (The OPO driven by this pumplaser will be described elsewhere.) In addition, be-cause of the high gain available and the relatively lowsaturation f luence ��5.5 J�cm2 at 300 K), the reso-nantly pumped Ho system is also a feasible system foramplification.

We acknowledge the support of Gregory J. Vansuchof the Defense Advanced Research Projects AgencySpecial Projects Off ice, John F. Carr of the Air ForceResearch Laboratory Sensors Directorate, Donald J.Smith of the Air Force Research Laboratory DirectedEnergy Directorate, and BAE Systems Informationand Electronic Warfare research and development.

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