We report a tunable, narrow-linewidth UV laser based on intracavity second-harmonic generation in a red semiconductor disk laser. Single-frequency operation is demonstrated with a total UV output power of 26 mW. By servo-locking the fundamental frequency to a reference Fabry–Pérot cavity, the linewidth of the UV beam has been reduced to 16 kHz on short timescales and 50 kHz on a 1 s timescale, relative to the reference

Hastie, Jennifer

Paboeuf, David

Schlosser, Peter

Optics Letters

English

David Pabœuf

Peter J. Schlosser

Jennifer E. Hastie

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Frequency stabilization of an ultraviolet semiconductor disk laser

Name not available

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryPaboeuf, David and Schlosser, Peter and Hastie, Jennifer (2013) Frequency stabilization of anultraviolet semiconductor disk laser. Optics Letters, 38 (10). pp. 1736-1738. ISSN 0146-9592Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/Frequency stabilization of an ultravioletsemiconductor disk laserDavid Pabœuf,* Peter J. Schlosser, and Jennifer E. HastieInstitute of Photonics, SUPA, University of Strathclyde, Wolfson Centre, 106 Rottenrow, Glasgow G4 0NW, UK*Corresponding author: david.paboeuf@strath.ac.ukReceived March 7, 2013; revised April 12, 2013; accepted April 13, 2013;posted April 18, 2013 (Doc. ID 186639); published May 15, 2013We report a tunable, narrow-linewidth UV laser based on intracavity second-harmonic generation in a redsemiconductor disk laser. Single-frequency operation is demonstrated with a total UV output power of 26 mW.By servo-locking the fundamental frequency to a reference Fabry–Pérot cavity, the linewidth of the UV beam hasbeen reduced to 16 kHz on short timescales and 50 kHz on a 1 s timescale, relative to the reference. © 2013 OpticalSociety of AmericaOCIS codes: (140.7270) Vertical emitting lasers; (190.2620) Harmonic generation and mixing; (140.3610) Lasers,ultraviolet; (140.3425) Laser stabilization.http://dx.doi.org/10.1364/OL.38.001736During the last decade, optically pumped semiconductordisk lasers (SDLs) have become an attractive means toachieve tunable, high power, and near-diffraction-limitedcw laser sources. The combination of bandgap engineer-ing and efficient intracavity nonlinear conversion hasenabled almost complete spectral coverage from theultraviolet (UV) to the near IR [1–3]. Moreover, single-frequency operation is readily achieved in a simple stand-ing wave cavity, without spatial hole burning effects, dueto the use of resonant periodic gain [4], and the externalcavity accommodates mode-filtering elements such asbirefringent filters (BRFs) or etalons, which enablesingle-frequency emission together with relatively broadtuning. Most of the demonstrations of single-frequencySDLs have been done in the near IR [4–6], but the visiblehas also been reached either directly [7] or via nonlinearconversion [8–10], including commercial SDLs withlinewidths in the megahertz range [11]. Achieving highspectral purity under normal laboratory conditionsusually requires the use of active frequency stabilization.Holm et al. demonstrated a linewidth of 3 kHz at 870 nmover 2 s [4] and Rösener et al. a linewidth of 9 kHz at2 μm over a short sampling time of 100 μs [5] by lockingtheir respective SDLs to a Fabry–Pérot (FP) reference cav-ity. Narrow linewidths, <100 kHz, can also be achievedin free-running operation via careful engineering and iso-lation of the external cavity [12].Here we describe the realization of a single-frequencyUV-emitting laser based on intracavity second-harmonicgeneration in an AlGaInP-based SDL. Reduction of thelaser linewidth is obtained by servo-locking the funda-mental frequency to a reference FP cavity [13]. Com-pared with other frequency-stabilized UV laser systemsthat require successive external enhancement cavities,each with separate stabilization [9,10,14], we benefitfrom a simpler setup due to the short fundamental wave-length and high finesse cavity that allow us to reach theUV with a single intracavity doubling step. High-spectral-purity UV emission opens new applications for SDLssuch as interference lithography, UV spectroscopy [10],and metrology.The experimental setup is described in Fig. 1. The SDLgain structure contains an AlGaAs distributed Braggreflector on top of which are 20 GaInP quantum wells(QWs) within a resonant subcavity for fundamental emis-sion around 675 nm, with the QWs being distributedevenly between the 10 available field antinodes. Thestructure is very similar to the one reported previouslyin [3]. It was inserted as an end mirror in a four-mirrorcavity including two mirrors (M1 and M2) with radii ofcurvature of 100 mm and the end mirror (M3) having aradius of curvature of 25 mm. A 500-μm-thick single crys-tal diamond heatspreader with low birefringence wasbonded onto the intracavity surface of the SDL structurefor efficient thermal management [15]. The gain mediumwas then clamped into a water-cooled brass mount, withthe water temperature set to 0°C. It was optically pumpedby a Coherent Inc. Verdi laser with up to 4 W at 532 nm.The losses due to reflection by the heatspreader were20%, so that the actual maximum pump power incidentat the SDL was 3.2 W. The intracavity frequency doublingof the red beam was achieved with a 7-mm-thick beta-barium borate (BBO) crystal cut for type I phase match-ing at 674 nm (φ  35.2°), inserted at the beam waistposition between M2 and M3. The crystal was held in analuminium mount with indium foil, and the end facetswere antireflection coated for both red (R < 0.1%) andUV (R < 0.2%) wavelengths. All cavity mirrors werecoated to have high reflectivity (R > 99.9%) for the redwavelengths, with M2 and M3 highly transmitting in theFig. 1. Schematic of the single-frequency UV emitting lasercavity and of the servo-locking loop. M1, M2, M3, cavity mirrors;BRF, birefringent filter; BBO, beta-barium borate crystal; PZT,piezoelectric transducer; FP, Fabry–Pérot; PI, proportionalintegrator; HVA, high voltage amplifier.1736 OPTICS LETTERS / Vol. 38, No. 10 / May 15, 20130146-9592/13/101736-03$15.00/0 © 2013 Optical Society of AmericaUV (T ∼ 98%) so that two UV beams with equal powersexited the cavity. These beams are sometimes recom-bined using a cavity mirror with high reflectivity at thesecond harmonic to reflect one beam back through thecrystal; however, for the single-frequency beam with acoherence length much longer than the laser cavity, in-terference effects would be detrimental. The distancesbetween the SDL and M1, M1 and M2, and M2 and M3were, respectively, 72, 310, and 93 mm. The beam waistdiameters at the gain structure were calculated as 80 μmin the tangential plane and 87 μm in the sagittal, and inthe BBO crystal, as 71 and 80 μm, respectively.In order to achieve single-frequency emission, a 2-mm-thick quartz plate at Brewster’s angle, acting as a BRF, anda 1 mm thick uncoated FP glass etalon (Suprasil 311)were inserted in the cavity. The diamond heatspreader,being uncoated, also behaved as an etalon and contributedto the mode filtering. The single-frequency behaviorwas monitored by coupling a leak of the fundamentalbeam through M1 into a confocal FP cavity with a 750MHzfree spectral range and finesse of 10. Figure 2(a) showsthe resulting spectrum obtained for an absorbed pumppower of 3.2 W and a total UV output power of 26 mW,clearly demonstrating single-frequency operation. Thenonlinear output coupling was ∼0.1%. By rotating theBRF, tuning of the fundamental wavelength was possible,which resulted in a tuning of the UV wavelength; how-ever, as the heatspreader is a fixed etalon, tuning is notcontinuous in the current setup. Single-frequency opera-tion was regained by slightly adjusting the tilt of the glassetalon and changing the orientation of the BBO crystal tooptimize the phase matching, and hence the UV outputpower, at the new wavelength. Continuous tuning maybe achieved in a future setup via the use of a wedged,antireflection-coated heatspreader [16]. An overall singlefrequency tuning range of 5 nm in the UV was achieved[Fig. 2(b)]. Maximum output power was obtained for afundamental wavelength of 676.4 nm (338.2 nm inthe UV).In order to isolate the laser cavity from outside pertur-bations, the optical table was floated and the SDL wasenclosed in a box and mounted on a breadboard, whichwas insulated from the optical table by 25.4 mm thickSorbothane feet at each corner. To evaluate the free-running frequency variations of this source, we used theFP cavity as a frequency reference. An optical isolatorwas used to prevent perturbing feedback into the laser.The length of the FP cavity was set with a piezoelectrictransducer (PZT) so that the laser frequency fluctuatedon the side of the cavity resonance peak. The transmittedsignal was then recorded for 1 s. Due to the rather largefrequency fluctuations, this measurement can only givean estimation of the laser linewidth. The free-runninglinewidth was estimated to be ∼3 MHz for the red beam,which resulted in a linewidth around 6 MHz for the UVlaser beam. Computing the Fourier transform of the sig-nal leads to the frequency noise power spectral density(PSD) shown in Fig. 3, indicating that the bandwidth offrequency fluctuations is limited to a few kilohertz, whichis, as for most solid-state lasers, several orders of magni-tude lower than that of a typical dye laser or laser diode.To stabilize the frequency, we used the so-called “sideof fringe” technique [13] using the setup shown in Fig. 1.The signal detected at the output of the reference FPcavity was amplified and sent to a commercial propor-tional integrator servo-controller (Precision Photonicsmodel LB1005). The locking reference was set at halfthe maximum of the FP cavity fringe. After amplificationby a high-voltage amplifier (PI model E-503), the correc-tion signal was applied to a PZT (PI model P-010.00H)carrying cavity mirror M1.With this arrangement, the laser frequency could belocked for several minutes, the limiting factor being thedrift of the reference cavity, which induced mode hops ofthe laser or multimode behavior. From the residual errorsignal, one can compute the laser frequency noise PSDrelative to the reference cavity. Figure 3 shows the result-ing PSD measured over 1 s when the laser was locked. Atthe time of the locking experiment, the total UV outputpower was 20 mW. One can clearly see that the frequencynoise was significantly reduced as compared to thefree-running case over the full noise bandwidth. At lowfrequencies the noise level is reduced by a factor of 106.With the exception of some localized peaks between200 Hz and 1 kHz, the PSD is almost flat, with a noiselevel around 105 Hz2∕Hz. From the integration of the PSD,a standard deviation of 25 kHz is deduced for the funda-mental frequency.Fig. 2. (a) Transmission of the scanned FP cavity showingsingle-frequency operation for a total UV output power of26 mW and (b) total single frequency UV output powermeasured as the BRF is rotated to tune the fundamentalwavelength.Fig. 3. Frequency noise power spectral densities (PSDs) ofthe red laser beam measured over a 1 s long sampling timefor the free-running laser (gray curve) and the locked laser(black curve).May 15, 2013 / Vol. 38, No. 10 / OPTICS LETTERS 1737A closer look at the PSD shows that more than half ofthe overall noise level is contained in the 100 Hz–1 kHzrange. We suspect that these noise components originatefrom mechanical resonances in the laser cavity. The rel-ative increase of the noise level for frequencies above100 Hz is related to the correction bandwidth of ourservo-locking system, which is currently limited by thePZT to only 250 Hz. We expect that by reducing theweight of the cavity mirror, currently a standard 6 mmthick optic, the correction bandwidth will be increased,thus further decreasing the laser linewidth. The broadpeak around 6 kHz is due to an electronic oscillation ofthe servo loop. The gain of the loop must be carefully setto minimize this oscillation while maintaining an efficientnoise correction.Figure 4 shows the fundamental laser linewidth mea-sured over different time scales ranging from 10 μs to10 s. The laser linewidth remains between 25 and30 kHz over long sampling times and steeply decreasesbelow 10 ms to reach a value of 8 kHz at 10 μs. Thisdecrease of the linewidth when measured on short time-scales is to be expected with noise dominated by lowfrequencies [17]. Moreover, the fact that the linewidthpeaks around 10 ms and remains nearly constant overlonger timescales confirms, as observed with the PSD,the majority of the frequency noise is contained withina 1 kHz bandwidth.In conclusion, we have demonstrated a tunable, nar-row-linewidth UV laser based on intracavity frequencydoubling of a single-frequency red SDL. By stabilizingthe frequency to a reference FP cavity, a linewidth asnarrow as 16 kHz was achieved for the UV beam. Overa 1 s long sampling time the relative linewidth of theUV beam has been measured as 50 kHz. To the best ofour knowledge, this is the narrowest linewidth achievedfrom an SDL with nonlinear frequency conversion.Further improvements of both the UV output power andthe linewidth are expected via optimization of the nonlin-ear output coupling and increasing the bandwidth of theservo-locking system to lock the frequency to a higherfinesse cavity, respectively. We also aim to achieve con-tinuous tuning by using a wedged heatspreader.This work is supported by the UK Engineering andPhysical Sciences Research Council (EPSRC) undergrant EP/I022791/1. The SDL gain structure was grownby Dr. A. B. Krysa at the EPSRC National Centre forIII-V Semiconductors, University of Sheffield, UK.References1. S. Calvez, J. E. Hastie, M. Guina, O. G. Okhotnikov, andM. D. Dawson, Laser Photon. Rev. 3, 407 (2009).2. N. Schulz, J.-M. Hopkins, M. Rattunde, F. Bussières, andJ. Wagner, Laser Photon. Rev. 2, 160 (2008).3. J. E. Hastie, L. G. Morton, A. J. Kemp, M. D. Dawson, A. B.Krysa, and J. S. Roberts, Appl. Phys. Lett. 89, 061114 (2006).4. M. A. Holm, D. Burns, A. I. Ferguson, and M. D. Dawson,IEEE Photon. Technol. Lett. 11, 1551 (1999).5. B. Rösener, S. Kaspar, M. Rattunde, T. Töpper, C. Manz,K. Köhler, O. Ambacher, and J. Wagner, Opt. Lett. 36,3587 (2011).6. A. Rantamäki, A. Chamorovskiy, J. Lyytikäinen, and O. G.Okhotnikov, IEEE Photon. Technol. Lett. 24, 1378 (2012).7. L. G. Morton, H. D. Foreman, J. E. Hastie, M. D. Dawson,and E. Riis, in Advanced Solid-State Photonics (OpticalSociety of America, 2007), paper WB7.8. M. Jacquemet, M. Domenech, G. Lucas-Leclin, P. Georges,J. Dion, M. Strassner, I. Sagnes, and A. Garnache, Appl.Phys. B 86, 503 (2007).9. Y. Kaneda, J. M. Yarborough, L. Li, N. Peyghambarian, L.Fan, C. Hessenius, M. Fallahi, J. Hader, J. V. Moloney, Y.Honda, M. Nishioka, Y. Shimizu, K. Miyazono, H. Shimatani,M. Yoshimura, Y. Mori, Y. Kitaoka, and T. Sasaki, Opt. Lett.33, 1705 (2008).10. J. Paul, Y. Kaneda, T. Wang, C. Lytle, J. V. Moloney, andR. J. Jones, Opt. Lett. 36, 61 (2011).11. Coherent Inc. Press Release No. 1277 (2011).12. A. Laurain, M. Myara, G. Beaudoin, I. Sagnes, and A.Garnache, Opt. Express 18, 14627 (2010).13. R. L. Barger, M. S. Sorem, and J. L. Hall, Appl. Phys. Lett 22,573 (1973).14. S. Vasilyev, A. Nevsky, I. Ernsting, M. Hansen, J. Shen, andS. Schiller, Appl. Phys. B 103, 27 (2011).15. A. J. Kemp, G. J. Valentine, J.-M. Hopkins, J. E. Hastie, S. A.Smith, S. Calvez, M. D. Dawson, and D. Burns, IEEE J.Quantum Electron. 41, 148 (2005).16. A. J. Maclean, A. J. Kemp, S. Calvez, J.-Y. Kim, T. Kim, M. D.Dawson, and D. Burns, IEEE J. Quantum Electron. 44, 216(2008).17. S. Kaspar, M. Rattunde, T. Töpper, C. Manz, K. Köhler, andJ. Wagner, Appl. Phys. Lett. 100, 031109 (2012).Fig. 4. Fundamental laser linewidth relative to the FP cavitymeasured over different sampling times. PtotalUV  20 mW.1738 OPTICS LETTERS / Vol. 38, No. 10 / May 15, 2013

Frequency stabilization of an ultraviolet semiconductor disk laser

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