Bollig, C.

Botha, R. C.

Collett, O.

Combrinck, W. L.

Esser, Daniel

Koen, W.

Kuleshov, N. V.

Strauss, H. J.

von Bergmann, H. M.

English

A 1314 nm Nd:YLF laser was designed and operated both CW and passively Q-switched. Maximum CW output of 10.4 W resulted from 45.2 W of incident pump power. Passive Q-switching was obtained by inserting a V:YAG saturable absorber in the cavity. The oscillator delivered a maximum of 825 mu J energy per pulse, with a pulse duration of 135 ns at a pulse repetition frequency of 6.3 kHz, effectively delivering 5.2 W of average power. (C) 2013 Optical Society of America</p

Heriot Watt Pure

    Heriot-Watt University Research Gateway                                                          Heriot-Watt UniversityHigh average power 1314 nm Nd:YLF laser, passively Q-switched with V:YAGBotha, R. C.; Strauss, H. J.; Bollig, C.; Koen, W.; Collett, O.; Kuleshov, N. V.; Esser, MatthewJan Daniel; Combrinck, W. L.; von Bergmann, H. M.Published in:Optics LettersDOI:10.1364/OL.38.000980Publication date:2013Link to publication in Heriot-Watt Research GatewayCitation for published version (APA):Botha, R. C., Strauss, H. J., Bollig, C., Koen, W., Collett, O., Kuleshov, N. V., ... von Bergmann, H. M. (2013).High average power 1314 nm Nd:YLF laser, passively Q-switched with V:YAG. Optics Letters, 38(6), 980-982.10.1364/OL.38.000980General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.High average power 1314 nm Nd:YLF laser,passively Q-switched with V:YAGR. C. Botha,1,2,* H. J. Strauss,2,3 C. Bollig,2,4 W. Koen,2,3 O. Collett,2,3 N. V. Kuleshov,5M. J. D. Esser,3 W. L. Combrinck,1 and H. M. von Bergmann21HartRAO, P.O. Box 443, Krugersdorp 1740, South Africa2Stellenbosch University, P/Bag X1, Matieland 7602, South Africa3National Laser Centre, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, South Africa4Physics Department, Carl von Ossietzky University, Oldenburg 26111, Germany5Centre for Optical Materials and Technologies, Belarus National Technical University, Minsk, Belarus*Corresponding author: roelf@hartrao.ac.zaReceived November 22, 2012; revised February 8, 2013; accepted February 8, 2013;posted February 11, 2013 (Doc. ID 180380); published March 14, 2013A 1314 nm Nd:YLF laser was designed and operated both CW and passively Q-switched. Maximum CW output of10.4 W resulted from 45.2 W of incident pump power. Passive Q-switching was obtained by inserting a V:YAG satur-able absorber in the cavity. The oscillator delivered a maximum of 825 μJ energy per pulse, with a pulse duration of135 ns at a pulse repetition frequency of 6.3 kHz, effectively delivering 5.2 W of average power. © 2013 OpticalSociety of AmericaOCIS codes: 140.0140, 140.3070, 140.3480, 140.3530, 140.3540.High-power 1.3 μm lasers have a wide range of applica-tions, which include communications, sensing, timingsystems, and monitoring techniques. The 1.3 μm outputcan be Raman-shifted to the 1.5 μm region, which isuseful for applications requiring eye-safe operation athigh powers, such as Lidar and free-space optical com-munication [1]. Furthermore, 1314.0 nm (specifically the657.0 nm harmonic) is required to probe the relevanttransition [2] for optical calcium clocks.The main 1.3 μm emission lines of Nd:YLF are at1314 nm for the σ polarization and 1321 nm for the πpolarization. To our knowledge, the highest 1.3 μmC Woutput power reported from a diode-end-pumpedNd:YLF laser is 6.2 W [3], which leaves room for powerscaling as was done for 1.0 μm Nd:YLF [4,5], or 1.3 μmNd:YAG, and Nd:GdVO4 [6,7] lasers. Operating an end-pumped Nd:YLF laser at 1.3 μm is attractive because ofthe weak thermal lens when it is operated on the σpolarization [8,9]. This results in excellent beam qualityover a wide range of output powers.Nd:YLF also has a long upper-laser-level (4F3∕2)lifetime of τ ∼ 520 μs compared to ∼250 μs for Nd:YAGand ∼100 μs for Nd:YVO4 [10,11]. The resulting highenergy storage capability makes Nd:YLF suitable to gen-erate high pulse energies during Q-switched operation.However, the emission cross section σem at 1.3 μm forNd:YLF is ∼2–2.5 × 10−20 cm2 (for the two polarizations),which is a factor of three less than that of Nd:YAG andan order of magnitude less than that of Nd:YVO4 [11],which implies lower gain.Power scaling Nd lasers at 1.3 μm are more difficultthan at 1.0 μm because of the lower σem. The relativelylonger lifetime τ of Nd:YLF partially compensates for this.The σem and τ values of Nd:YLF also necessitate a carefuldesign of the pump beam radius where a trade-off hasto be made between a reasonably low threshold andthe risk of thermal fracture [5]. Thermal effects are espe-cially problematic under 1.3 μm operation (comparedto 1.0 μm operation) due to the larger quantum defect.By using a relatively low Nd doping, one can reduceupconversion and spread out the thermal load longitud-inally in the crystal, which increases the thermal fracturepump limit [4,5,12].Nd-doped active media are suitable for an efficientQ-switched operation at 1.3 μm using a V:YAG saturableabsorber because they have a high ratio α  σGSA∕σem,with σGSA, the ground-state absorption cross section ofthe saturable absorber and σem the emission cross sec-tion of the laser crystal [13]. It has been demonstratedthat V:YAG operates efficiently as a saturable absorberfor Nd lasers operating in the 1.0 and 1.3 μm bands.Nd:YAG, Nd:YVO4, Nd:YAP, Nd:KGW, Nd:GdYVO4,, andNd:GGG have previously been passively Q-switched withV:YAG at 1.3 μm[ 13–21]. To the best of our knowledge,V:YAG has not been used to passively Q-switch Nd:YLFlasers at either 1.0 or 1.3 μm. Considering other diodeend-pumped 1.3 μm Nd lasers passively Q-switched withV:YAG, the highest average power was 2.1 W for Nd:YVO4[13] while Nd:YAG delivered the highest energy per pulseof 126 μJ and peak power of 6.1 kW in different setups[18]. Nd:YLF has the potential to significantly increasethe pulse energy due to its longer upper-laser-levellifetime [5,22].Here we demonstrate high-power 1314 nm operationof a diode end-pumped Nd:YLF laser in both CW andpassively Q-switched modes.The linear two mirror resonator (Fig. 1) consists of a300 mm concave input coupler (IC) mirror which ishighly reflective at 1314 nm and is highly transmissiveFig. 1. (Color online) Experimental resonator layout for CWand a passively Q-switched (with V:YAG) operation.980 OPTICS LETTERS / Vol. 38, No. 6 / March 15, 20130146-9592/13/060980-03$15.00/0 © 2013 Optical Society of Americaat 805 nm. Two 200 mm convex output couplers (OCs)with transmissions of 5% and 10% at 1.3 μm were avail-able and placed ∼165 mm from the IC. Operation wasforced onto the 1.3 μm line by specifying both the ICand OC to be highly transmissive at ∼1 μm. The sizeof the fundamental laser mode in the crystal was adjustedat full pump power to match the pump beam by adjustingthe position of the OC.The laser crystal consisted of a single 0.5% doped a-cutNd:YLF rod which was 5 mm in diameter and 30 mm inlength. It was mounted in a water-cooled copper blockwith its c-axis horizontally and was placed next to theIC. The laser crystal was end-pumped from one side witha fiber-coupled diode laser module (Jenoptik JOLD-140-CAXF, 0.6 mm, 0.22 NA fiber, ∼805 nm) with the pumppower limited to ∼45 W to avoid thermal fracture. Thepump beam was focused to a waist radius of ∼700 μmin the center of the gain medium and had a roughlybell-shaped energy distribution. This waist radius wasdetermined with a gain optimization method similar tothe one described in [5] because both the pump andlaser beam radii have a strong influence on the gain aswell as on the thermal load. In this method, we also setthe parameters so that the threshold would be at about25% of the maximum pump power.The CW slope efficiencies of the oscillators with thetwo OCs are shown in Fig. 2. The most efficient CWoperation, as well as highest output power, was achievedwith a 5% OC with a resulting slope efficiency of 29%.This laser had an incident pump power threshold of9.25 W and a maximum power output of 10.4 W, whichis 1.7 times higher than recently reported [3]. The beamhad a symmetrical Gaussian profile, but at the maximumpump power the beam became slightly elliptical with thehorizontal radius being ∼25% larger than the vertical.This was due to the YLF’s astigmatic thermal lensing[23]. Wavelength measurements showed oscillation onlyat 1314 nm on the σ polarization due to the oscillatorbeing unstable for the stronger negative thermal lensesassociated with the π polarization [8,9].The 5% OC oscillator was subsequently passivelyQ-switched by inserting a 3 mm thick V:YAG saturableabsorber with an initial single pass absorption of∼3% in the cavity between the gain medium and OC(Fig. 1). The V:YAG crystal was cut along [111] to avoidanisotropy. Initial experiments indicated that the pulsedperformance at various input powers was relatively in-sensitive to the position of the V:YAG Q-switch andthe Q-switch was subsequently placed ∼12 mm fromthe Nd:YLF crystal. The incident pump-power thresholdof the Q-switched laser was 17.5 W, and the maximumaverage output power was 5.2 W at an incident pumppower of 45.2 W(Fig. 3). As the incident pump power wasincreased from threshold to 27 W, the pulse repetitionfrequency (PRF) increased from 650 Hz to 5.9 kHz, afterwhich the PRF stayed nearly constant at ∼6.3 kHz withincreasing incident power (Fig. 3). Thermal lensing ofboth the Nd:YLF and V:YAG crystals change the laser-beam size within the resonator, which in turn influencesthe bleaching properties of the V:YAG crystal. This givesrise to a nonlinear behavior in the pulse repetition rate[24]. The constant PRF over a range of high powersmay be a desirable characteristic for some applications.Pulse duration at full width at half-maximum de-creased from 162 to 136 ns with increasing pump power(Fig. 4). The increase in average output power andalmost constant PRF above 27 W resulted in an increasein pulse energy up to 825 μJ (Fig. 4) with a peak power of6.1 kW. Energy per pulse and average power was higherthan previously reported with other diode-end-pumpedFig. 2. (Color online) Slope efficiencies of the CW laser for thedifferent OCs used.Fig. 3. (Color online) Passively Q-switched behavior: PRF(left axis) and average output power (right axis).Fig. 4. (Color online) Passively Q-switched behavior: pulseduration (left axis) and energy per pulse (right axis).March 15, 2013 / Vol. 38, No. 6 / OPTICS LETTERS 981passively Q-switched 1.3 μm Nd lasers with V:YAG: by 6.5times (126 μJ with Nd:YAG) [18]) and 2.5 times (2.1 Wwith Nd:YVO4 [13]), respectively.In conclusion, high average power 1314 nm oscillationof an Nd:YLF laser was demonstrated for CW operationand delivered up to 10.4 W of output power. PassivelyQ-switched operation of an Nd:YLF laser using a V:YAGsaturable absorber was demonstrated for the first time.Pulsed operation delivered 825 μJ of energy per pulsewith a pulse duration of 135 ns and an average powerof 5.2 W. These results are, to the best of our knowledge,the highest reported values for all diode-end-pumpedpassively Q-switched 1.3 μm Nd lasers with V:YAG.References1. J. T. Murray, R. C. Powell, N. Peyghambarian, D. Smith, W.Austin, and R. A. Stolzenberger, Opt. Lett. 20, 1017 (1995).2. Y. Louyer, M. D. Plimmer, P. Juncar, M. E. Himbert,F. Balembois, and P. Georges, Appl. Opt. 42, 4867 (2003).3. Y. Wei, S. Xu, C. Huang, F. Zhuang, W. Chen, L. Huang, X.Wang, and G. Zhang, Laser Phys. 22, 1029 (2012).4. W. A. Clarkson, P. J. Hardman, and D. C. Hanna, Opt. Lett.23, 1363 (1998).5. C. Bollig, C. Jacobs, M. J. D. Esser, E. H. Bernhardi, andH. M. von Bergmann, Opt. Express 18, 13993 (2010).6. C. Du, S. Ruan, H. Zhang, Y. Yu, F. Zeng, J. Wang, andM. Jiang, Appl. Phys. B 80, 45 (2005).7. D. Sauder, A. Minassian, and M. J. Damzen, Opt. Express15, 3230 (2007).8. M. Pollnau, P. J. Hardman, M. A. Kern, W. A. Clarkson, andD. C. Hanna, Phys. Rev. B 58, 16076 (1998).9. P. J. Hardman, W. A. Clarkson, G. J. 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Resonatorinterne Frequenzverdopplung von diodengepumpten Neodym-Lasern mit hohen Ausgangsleistungen im blauen Spektralbereich,” Ph.D. dissertation (University of Hamburg,

High average power 1314 nm Nd:YLF laser, passively Q-switched with V:YAG

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