Greater than 12W of average output power has been generated from a diode-pumped YbYAG cladding-pumped planar waveguide laser. The laser radiation developed is linearly polarized and diffraction limited in the guiding dimension. A slope efficiency of 0.5WW with a peak optical optical conversion efficiency of 0.31WW is achieved. In a related structure, greater than 8W of Q -switched average output power has been generated from a NdYAG cladding-pumped planar waveguide laser by incorporation of a Cr 4+ YAG passive Q switch monolithically into the waveguide structure. Pulse widths of 3ns and pulse-repetition frequencies as high as 80kHz have been demonstrated. A slope efficiency of 0.28WW with a peak optical optical conversion efficiency of 0.21WW is achieved

Beach, R.J.

Bennett, W.J.

Krupke, W.F.

McMahon, J.M.

Meissner, H.E.

Meissner, O.R.

Mitchell, S.C.

Shepherd, D.P.

Southampton (e-Prints Soton)

June 15, 2001 / Vol. 26, No. 12 / OPTICS LETTERS 881Continuous-wave and passively Q-switched cladding-pumpedplanar waveguide lasersR. J. Beach, S. C. Mitchell, H. E. Meissner, O. R. Meissner, W. F. Krupke, J. M. McMahon, and W. J. BennettMaxios Corporation, 4749–A Bennett Drive, Livermore, California 94550D. P. ShepherdOptoelectronics Research Centre, University of Southampton, Highfield, Southampton SO17 1BJ, UKReceived January 25, 2001Greater than 12 W of average output power has been generated from a diode-pumped Yb:YAG cladding-pumpedplanar waveguide laser. The laser radiation developed is linearly polarized and diffraction limited in the guid-ing dimension. A slope efficiency of 0.5 WW with a peak optical–optical conversion efficiency of 0.31 WWis achieved. In a related structure, greater than 8 W of Q-switched average output power has been generatedfrom a Nd:YAG cladding-pumped planar waveguide laser by incorporation of a Cr41:YAG passive Q switchmonolithically into the waveguide structure. Pulse widths of 3 ns and pulse-repetition frequencies as highas 80 kHz have been demonstrated. A slope efficiency of 0.28 WW with a peak optical–optical conversionefficiency of 0.21 WW is achieved. © 2001 Optical Society of AmericaOCIS codes: 140.0140, 140.3580.Cladding-pumped planar waveguide lasers1 (CPPWLs)are the two-dimensional analogs of one-dimensionaldouble-clad fiber lasers. These f ive-layer structureswere developed several years ago at Maxios LaserCorporation as an outcome of recent developments inthermal bonding technology2 and have recently begunto appear in the scientif ic literature.3 The CPPWLreported here consists of f ive layers with either aNd:YAG or a Yb:YAG core layer. The CPPWL struc-ture is shown in Fig. 1. Using separate conf inementstructures for the laser radiation developed and thepump radiation delivered enables this class of laserto generate high gains and to be efficiently pumpedby use of ordinary laser diode bars. In particular, noradiance conditioning or other optical components arerequired for the diode light to be efficiently deliveredinto the waveguide structure (WS).Recently we demonstrated a cw Yb:YAG CPPWLthat serves to demonstrate the average power capabil-ity and eff iciency of this approach. In the cw Yb:YAGlaser the active core consists of an 8-mm-thick layerof 10 atm. % Yb:YAG. This core is clad on each sideby a 5-mm-thick undoped YAG layer. The individuallayers are joined by diffusion bonding.4 Owing to thepolarizability of the Yb in the central core layer,the refractive index of the core is slightly larger thanthe refractive index of the undoped YAG cladding lay-ers, by 1.4 3 1023. The weak waveguiding providedby this index step, combined with the gain guiding pro-vided by the core layer, is sufficient to ensure single-transverse-mode operation in the guided dimensionup to the 40 W of delivered pump power investigatedhere. The outermost cladding layers used in the WSare sapphire. The top sapphire layer is 300 mm thick,and the bottom layer is 2 mm thick and in contactwith a water-cooled copper heat sink. The refractiveindex of sapphire at 1.76 serves to confine the pumplight from the two proximity-located laser diode bars,0146-9592/01/120881-03$15.00/0which are positioned within 10 mm of the sidesof the structure. The cw diode bars used here are1 cm long, emit at the 940 nm peak of the Yb:YAGabsorption, and were procured from the CoherentSemiconductor Group.5 The overall footprint of theWS is 1 cm long by 0.5 cm wide. The high ref lector(HR) and the output coupler (OC) optical coatingsare monolithically applied to the WS. The OC hasa ref lectivity of 0.7 at 1030 nm, and both the HR’sand the OC’s ends are f lat. Figure 2 displays themeasured optical performance of this system. Theslope eff iciency achieved in cw operation is 0.5 WW,and the peak optical–optical efficiency is 0.31 WWat the peak optical output power of 12.4 W. In theguided dimension, the laser beam quality is measuredto be nearly diffraction limited at all pumping powers,with an M2 value of 1.07. In the unguided dimen-sion, the beam is observed to be highly multimode,as would be expected for the f lat–f lat resonatorFig. 1. Schematic drawing of our CPPWL system. Thelaser is pumped by two laser diode bars located within10 mm of either side of the WS. All components aremounted upon a copper base plate that is water cooled.© 2001 Optical Society of America882 OPTICS LETTERS / Vol. 26, No. 12 / June 15, 2001Fig. 2. cw output power from a Yb:YAG CPPWL as a func-tion of the delivered pump power. The pump power iscorrected for the Fresnel losses that are incurred at theuncoated sides of the WS and so corresponds to the pumppower actually delivered into the structure.used. Under all pumping conditions, the output fromthe system is TE polarized with greater than 90%purity.The Nd:YAG CPPWL used in our Q-switchinginvestigations is similar in design to the Yb:YAGdescribed above, except that the central core layer is20 mm thick and consists of 1 atm. % of Nd:YAG. Allother layer thickness are identical of those of theYb:YAG WS described above. Into this WS we haveincorporated a Cr41:YAG passive Q switch6 in thecenter of the structure by replacing a section of the20-mm thick Nd:YAG layer, extending across the 5-mmwidth, with a section of Cr41:YAG that in the directionof the laser cavity is 540 mm long. The Cr41:YAGused has a measured absorption coefficient of 4.34cmat 1064 nm. The resonator design for this Nd:YAGpassively Q-switched CPPWL is similar to that usedfor the Yb:YAG system above; it uses a f lat mirrorsurface at each end of the WS with a 50% ref lectiveOC coating, and an HR coating applied monolithically.Two Coherent cw diode bars emitting at 808 nmserve as the pump source. Figure 3 displays both theoutput power and the pulse-repetition frequency (PRF)for this system as a function of the optical pump powerdelivered into the WS. The slope efficiency of thislaser is 0.28 WW, and, at the maximum deliveredpump power of 39 W, the average output power ofthe laser is 8.1 W, leading to a peak optical–opticalefficiency of 0.21 WW. The PRF of the Q-switchedoutput pulses was observed to increase linearly withdelivered pump power at low pump powers and theneventually to roll over at higher pump powers, asdisplayed in Fig. 3, achieving a maximum value of80 kHz. In addition to suffering roll over, the PRFwas observed to become progressively more unstableas the pump power increased. This is indicated bythe error bars in Fig. 3, which represent the standarddeviation of the multiple PRF measurements thatwere made for each data point. The instability inthe PRF is not completely understood but may beattributed partially to the 4.1-ms excited-state lifetimeof the Cr41:YAG (Ref. 7); at 80 kHz the time betweenextraction pulses is only 12.5 ms. Additionally, thelarge number of transverse modes supported in theunguided dimension of our f lat– f lat cavity, alongwith their varying loss, may contribute to jitter intheir buildup time and so contribute to the observedPRF instability. The Q-switched pulse width wasobserved to f luctuate from 2.5 to 3.5 ns (FWHM)over the range of pump powers investigated, leadingto peak output pulse powers of 28 kW. The linescurves in Fig. 3 are taken from an energetics modelbased on a recently developed passive Q-switchingformalism.8 In this model fit to our data, both thestorage lifetime of the Nd31:YAG and the ground-stateabsorption cross section of the Cr41:YAG at 1064 nmwere treated as adjustable parameters. The bestagreement between data and model were obtained for90 ms and 1.55 3 10218 cm2 for the storage lifetimeand absorption cross section, respectively. ThisCr41:YAG absorption cross section value is generallyin agreement with recently published literature valuesthat range from 1.2 3 10218 to 1.9 3 10218 cm2.9 Thesmall value for the Nd31:YAG storage lifetime is notsurprising for the high specif ic gains and the high-numerical-aperture gain regions that are charac-teristic of our WS and corresponds to an amplifiedspontaneous emission multiplier of 2.86, assum-ing a 257-ms lifetime in the absence of amplifiedspontaneous emission. Suppression of amplifiedspontaneous emission and parasitic modes are impor-tant design considerations in the planar WS developedhere and are critical for achieving the performancereported herein. We have found it necessary togrind vertical grooves into the polished sides of theWSs between which the pump light is transmittedto suppress parasitic lasing modes. In general, thenecessity to groove the sides of the WS presents noproblem with pump delivery because of the sub-filleddiode-bar apertures that characterize commerciallyavailable cw operating laser diode bars. For example,the Coherent diode bars used here for our pumpsources have a 30% fill and 150-mm wide emitterscentered on 500-mm centers. Using a dicing saw,we placed parasitic suppression grooves along thepump-input sides of the WS, between diode emitters,without affecting pump delivery. The output of thislaser was observed to be TM polarized with greaterFig. 3. Average output power and output pulse PRF froma passively Q-switched Nd:YAG CPPWL as a function ofthe delivered pump power. The pump power is correctedfor the Fresnel losses that are incurred at the uncoatedsides of the WS.June 15, 2001 / Vol. 26, No. 12 / OPTICS LETTERS 883Fig. 4. Solid curve, measured far-f ield intensity in theunguided dimension from the strip unstable resonator.Dashed curve, expected far-field from a uniformly filledstrip unstable resonator aperture but with a 100% ref lec-tive central spot on the OC, as described in the text.than 90% purity. This polarization is perpendicularto the observed polarization from the cw Yb:YAGsystem. It is known that both the stress inducedin the YAG layers during fabrication of the WS andthe crystallographic orientation of the Cr41:YAG(Ref. 10) affect the polarization of the developed laserradiation; however, we have not yet fully analyzedthese mechanisms.Based on our energetic models of the systemsdescribed above, we expect the output power to beoptimal with OC ref lectivities near 0.2. These largeoutput-coupling values are compatible with the re-quirements of strip unstable resonators, which shouldpermit good beam quality to be generated in theunguided dimension. To illustrate the potential forcleaning up the beam in the unguided dimension,we have demonstrated a strip unstable resonatorby polishing a concave 20-cm radius of curvatureinto the output end of a cw Nd:YAG WS, i.e., aNd:YAG WS as described above but without a passiveQ switch. The HR end of the structure was keptf lat. These choices for mirror curvature result inan unstable resonator with a geometric cavity mag-nification of 1.56. The optical coating applied to thecurved OC end of this structure consisted of a 0.70ref lectivity coating over the central 64% of the endface 0.64  1magnification and an antiref lectioncoating everywhere else. The solid curve in Fig. 4shows the measured far-field beam intensity in theunguided dimension from this structure. The deepmodulation in this intensity pattern demonstratesthe high degree of coherence across the 5-mm-wideaperture. The dashed curve in Fig. 4 represents theexpected far-field intensity distribution that would beseen from this strip unstable resonator if the outputaperture were uniformly f illed and there were a100% ref lectivity spot in the center of the OC. Theoptimum OC ref lectivity of 0.2 for these systemscorresponds to a cavity magnif ication of 53 for a stripunstable resonator with a 100% ref lectivity spot in thecenter of the OC. Going to a cavity magnification of53 should result in a far f ield characterized by onlythree dominant lobes and an M2 value of 2.In conclusion, we have demonstrated a diode-pumped Yb:YAG CPPWL generating greater than12 W of cw output power. This represents improve-ments by a factor of 3.5 in the average power and bya factor of 2.2 in optical–optical eff iciency comparedearlier demonstrations of CPPWL systems.3 For thefirst time to our knowledge, we have demonstrated apassively Q-switched CPPWL by using a Nd:YAG gainelement and a monolithically incorporated Cr41:YAGQ switch. This system has demonstrated greaterthan 8 W of repetitively pulsed Q-switched averageoutput power in short 2.5-ns duration pulses. Finally,we have for what we believe is the f irst time demon-strated the use of an unstable strip resonator witha CPPWL, showing the potential for achieving highbeam quality in both the guided and the unguideddimensions from CPPWL systems. The results re-ported here have demonstrated the high efficiencythat is possible from compact and robust packageswith the CPPWL approach as well as the ability togenerate linearly polarized output radiation. Suchsystems will have relevance to applications that in-volve both marking and nonlinear optical generation.The authors acknowledge many useful discussionswith J. M. Scott and his help with various aspectsof the research presented here. This research wassupported by Eglin Air Force Base under contractF08630-99-C-0016. D. P. Sheppard is supported byEngineering and Physical Sciences Research Council(UK) grant GR/M98449.References1. H. E. Meissner and O. R. Meissner, “Laser-pumpedcompound waveguide lasers and amplif iers,” U.S.patent 6,160,824 (December 12, 2000).2. H. C. Lee, P. L. Brownlie, H. E. Meissner, and E. C.Rea, Proc. SPIE 1624, 2 (1992).3. C. L. Bonner, T. Bhutta, D. P. Shepherd, and A. C.Tropper, IEEE J. Quantum Electron. 36, 236 (2000).4. Onyx Optics, Inc., Dublin, Calif., fabricates ourCPPWL components.5. Coherent Semiconductor Group, Santa Clara, Calif.6. Y. Gui, G. Yao, T. Ji, Y. Gu, S. Lo, and L. Zhang,in Digest of Conference on Lasers and Electro-Optics(Optical Society of America, Washington, D.C., 1984),p. 166.7. S. Kuck, K. Petermann, U. Pohlmann, and G. Huber,Phys. Rev. B 51, 17,323 (1995).8. F. D. Patel and R. J. Beach, IEEE J. Quantum Electron.37, 707 (2001).9. G. Xiao, J. H. Lim, S. Yang, E. Van Stryland, and M.Bass, IEEE J. Quantum Electron. 35, 1086 (1999).10. N. N. Il’ichev, E. S. Gulyamova, and P. P. Pashinin,Quantum Electron. 27, 972 (1997).

Continuous-wave and passively Q -switched cladding-pumped planar waveguide lasers

Continuous-wave and passively Q -switched cladding-pumped planar waveguide lasers

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