1668 J. Opt. Soc. Am. B/Vol. 16, No. 10 /October 1999 Matsubara et al.

All-solid-state light source for generationof tunable continuous-wave

coherent radiation near 202 nm

Kensuke Matsubara, Utako Tanaka, Hidetsuka Imajo, and Masayoshi Watanabe

Kansai Advanced Research Center, Communications Research Laboratory,588-2, Iwaoka, Nishi-ku, Kobe 651-2401, Japan

Shinji Urabe

Department of Physical Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

Received March 15, 1999; revised manuscript received June 8, 1999

We have developed an all-solid-state tunable continuous-wave coherent light source near 202 nm by usingsum-frequency mixing of 266- and 850-nm radiation in b-barium borate (BBO). The 266-nm radiation wasgenerated by frequency doubling of a laser-diode-pumped frequency-doubled Nd:YVO4 laser. The power of the850-nm radiation, which was provided by a laser-diode-based master-oscillator power-amplifier system, wasenhanced in an external cavity. An output power of 59 mW was generated at 202.6 nm. We obtained ultra-violet radiation from 201.5 to 203.1 nm by tuning the wavelength of the master oscillator. © 1999 OpticalSociety of America [S0740-3224(99)00710-9]

OCIS codes: 140.3600, 140.3610, 140.4780, 190.2620.

1. INTRODUCTIONThere are strong and characteristic spectral lines of at-oms and ions in the short-wavelength region close to 200nm. Therefore many efforts to develop UV radiation inthis wavelength region have been made by several groupsof scientists working in research fields such as spectros-copy, laser cooling, and atom optics. In these researchfields a narrow spectral bandwidth and fine wavelengthtunability of the radiation are important properties re-quired of the light source. At present, however, such UVradiation is not available from the direct output of asingle laser. Hence the traditional radiation-generatingmethod, which is still employed in many laboratories, isfrequency conversion in a nonlinear crystal by use of anAr1-laser-pumped, single-mode dye or Ti:sapphire laseror occasionally by use of the single-mode Ar1 laser in it-self. Such light-source systems have the advantages ofhigh output power and wide wavelength tunability.However, they require a large space for installation,skilled techniques for maintenance, and periodic replace-ment of laser dye or Ar1-gas tubes.

In recent years the diode laser has been developed as asource of coherent light because it is compact and simpleto operate and maintain. The single-mode tunablecontinuous-wave (cw) type of diode laser is sometimes em-ployed for high-resolution spectroscopy.1 However, thewavelengths of such diode lasers are usually limited tothe red and near-infrared regions, and their output poweris relatively low. Therefore, blue and UV cw radiationwas produced from the output radiation of the diode laserby frequency doubling in a nonlinear crystal.2,3 Therethe power of the output radiation of the diode laser wasenhanced in an external resonant cavity to generate

0740-3224/99/101668-04$15.00 ©

frequency-doubled radiation efficiently. Moreover, thetechnique of two-stage frequency doubling by use of twoexternal cavities was developed to produce tunable cw ra-diation at wavelengths below 250 nm from IR radiation ofthe diode laser.4,5 Injection locking of a high-power diodelaser to a single-mode tunable diode laser, that is, a laser-diode- (LD-) based master-oscillator power-amplifier sys-tem, was employed to produce high-power IR cw radiationwhile maintaining the narrow bandwidth of single-modeoscillation and the fine wavelength tunability.5

In this paper we report on the sum-frequency genera-tion of tunable cw coherent radiation near 202 nm by useof an all-solid-state light source that is based on diode la-sers and a LD-pumped solid-state laser. One of our ob-jectives for this development is the application of a com-pact and reliable all-solid-state light source to high-resolution spectroscopy of Zn1 ions. The wavelength ofthe resonance transition between the ground level(4s2S1/2) and the 4p2P3/2 level of Zn1 ions is 202.6 nm.cw radiation that is tunable near 202 nm is produced bysum-frequency mixing of 266-nm radiation and tunableradiation near 850 nm. In this experiment, b-barium bo-rate (BBO) is the most suitable nonlinear crystal for fre-quency conversion to UV radiation near 202 nm. It canbe phase matched by frequency doubling to generate UVradiation at a wavelength as short as 204.8 nm.6 Sum-frequency mixing is required for generation of cw radia-tion below 204.8 nm in BBO. In earlier reports a single-mode Ar1 laser was used for sum-frequency generation ofcw radiation below 204.8 nm.7–10 As a result, large spaceand replacement of Ar1-gas tubes were needed for the ex-periments. Thus, to our knowledge, the 202-nm outputwavelength of our compact light source is the shortest

1999 Optical Society of America

Matsubara et al. Vol. 16, No. 10 /October 1999 /J. Opt. Soc. Am. B 1669

wavelength of cw coherent radiation generated by an all-solid-state tunable light source.

2. EXPERIMENTFigure 1 is a schematic of the experimental setup. Thelight source consists of a LD-based master-oscillatorpower-amplifier system, a LD-pumped frequency-doubledNd:YVO4 laser, and two external cavities. As much as 5W of single-mode cw radiation at 532 nm is provided bythe LD-pumped frequency-doubled Nd:YVO4 laser (Co-herent Verdi; linewidth, 3 MHz). This type of laser issuitable for frequency conversion as the fundamental la-ser because it provides stable single-mode radiation forthousands of hours without complicated readjustment.The output radiation at 532 nm is mode matched into anexternal resonant cavity, which we term the frequency-doubling cavity in this paper. Here, we use a modifiedmodel of a commercially available frequency-doublingsystem (Laser Analytical Systems, Wavetrain) with someimprovements to generate 266-nm radiation efficiently.The round trip length of the frequency-doubling cavity,into which a 7-mm long, phase-matched Brewster-cutBBO crystal is placed, is 300 mm.

Single-mode cw radiation at 850 nm is produced by theLD-based master-oscillator power-amplifier system. Inthis system the output beam of the master oscillator,which is an external-cavity tunable-diode laser (New Fo-cus 6226; linewidth, ,300 kHz), is circularized with aprism pair. After passing through two Faraday isolators(120-dB total isolation), the 850-nm beam is injected into

Fig. 1. Experimental setup of an all-solid-state light source forgeneration of cw radiation near 202 nm: PM, phase modulator;DBM, double-balanced mixer; PZT’s, piezoelectric transducers;CF, color filter; CL, cylindrical lenses; PBS, polarized beamsplitter.

a power amplifier. The power amplifier is ourmodification5 of a commercially available diode laser(SDL 8630). The Faraday isolators are used to preventfluctuation in the oscillation frequency caused by reflec-tion into the master oscillator.

For efficient sum-frequency generation, the amplified850-nm radiation is mode matched into the other externalcavity, which is the sum-frequency mixing cavity, afterpassing through another isolator (60-dB isolation). It isa bow-tie-type ring cavity consisting of two concave mir-rors with the same curvature radius of 150 mm and twoflat mirrors. The separation between the concave mir-rors is 160 mm, and the round-trip length of the cavity is776 mm. The reflectance of the input coupler is 97%, andthat of the other cavity mirrors is more than 99.5% for850 nm. A nearly circular beam waist with a 51-mm ra-dius is formed between the two concave mirrors. A7-mm-long, type I phase-matched 61.3°-cut BBO crystal isplaced at the beam waist. Both the input and the outputfacets of the crystal are antireflection coated for 850 nmat normal incidence. We optimize mode matching intothe cavity by adjusting two convex lenses. The cavity islocked to resonance with the 850-nm incident radiation bythe Hansch–Couillaud method.11

The transverse profile of the 266-nm output beam fromthe frequency-doubling cavity is nearly rectangular be-cause of walk-off of the BBO.12 A combination of cylin-drical lenses compensates for the walk-off, and then the266-nm beam is focused into the BBO crystal in the sum-frequency mixing cavity. The overlap between the beamwaists of the 266- and 850-nm radiation is adjusted by theuse of convex lenses to optimize the output power of thesum-frequency radiation. A dichroic beam splitter re-flects 95% of the radiation near 202 nm while transmit-ting more than 99% of the 850-nm radiation. The202-nm output beam is collimated by a lens and is sepa-rated from the fundamental beams by a prism. Thepower of the 202-nm radiation is measured by a cali-brated silicon photodiode (Hamamatsu S1226).

3. RESULTS AND DISCUSSIONThe output power of the 266-nm radiation produced fromthe frequency-doubling cavity, which was measured as afunction of the input power of the 532-nm radiation, isshown in Fig. 2. The output power was 790 mW at aninput power of 4.5 W. However, when we started this ex-periment with the modified frequency-doubling system asit was supplied by the manufacturer, the 266-nm outputpower was rather unstable. For example, when the532-nm input power was 4.5 W, the output power at 266nm decreased from 790 to less than 400 mW in severalminutes. This result was explained by absorption of asmall amount of the harmonically generated UV radiationinto the BBO.12 An anisotropic temperature gradientcaused by the absorption developed into a thermal lens inthe crystal. The thermal lens degraded the spatial modematching between the circulating radiation in the cavityand the incident radiation. This degradation decreasedthe circulating power at 532 nm. Therefore the holder ofthe BBO crystal was improved to stabilize the crystal atroom temperature by using a heat-sink block of alumi-

1670 J. Opt. Soc. Am. B/Vol. 16, No. 10 /October 1999 Matsubara et al.

num, a Peltier cooler, and a servo system. As a result, afairly stable output power, whose magnitude is shown inFig. 2, was obtained during the experiment over morethan 10 h.

When the 850-nm master-oscillator radiation of 7.5mW and a current of 1.75 A were injected into the tapereddiode chip of the power amplifier, the 850-nm radiationpower was amplified to 500 mW.5 It was decreased to410 mW at the input coupler of the sum-frequency mixingcavity because of loss at the Faraday isolator. When westarted the measurement of the output power of the202-nm radiation, the temperature of the BBO crystalwas not stabilized in the sum-frequency mixing cavity.The output power, which was measured as a function ofthe 266-nm input power into the sum-frequency mixingcavity, is shown by open circles in Fig. 3. The 850-nm in-put power was 410 mW throughout the measurement. A202.6-nm power of 24 mW was measured at a 266-nm in-put power of 455 mW. When the 266-nm input powerwas more than 500 mW, we could not obtain stable outputpower. The 266-nm input power was measured beforethe sum-frequency mixing cavity. The 266-nm incidentpower into the BBO crystal was 74% of the input powershown in Fig. 3 because of reflection losses at the inputmirror and the crystal facet.

Until we introduced the 266-nm radiation into the sum-frequency-mixing cavity for the first time, the enhance-ment factor of the 850-nm radiation was more than 40.However, while the 266-nm radiation was passingthrough the crystal, the enhancement factor gradually de-creased. When we measured the 202.6-nm output powerin Fig. 3, the enhancement factor was ;30 at low 266-nminput powers, probably as a result of the UV-induced deg-radation of the crystal facet.13 In addition, as the266-nm input power was increased, the enhancement fac-tor further decreased. When the crystal temperaturewas not stabilized in the sum-frequency mixing cavity,the enhancement factor was less than 20 at the 266-nminput power of 455 mW. This decrease consequently re-strained the 202.6-nm output power at high 266-nm inputpowers. Because the enhancement factor did not dependon the 850-nm input power, we concluded that it was de-creased by the thermal lens caused by absorption of the266-nm radiation. In the case of sum-frequency genera-

Fig. 2. 266-nm output power measured as a function of 532-nminput power into the frequency-doubling cavity.

tion, not only did the thermal lens decrease the circulat-ing power in the external cavity but it also probablycaused the overlap between the two fundamental beamsto deteriorate.

Therefore we measured the 202.6-nm output powerwhile stabilizing the crystal temperature in the sum-frequency-mixing cavity to avoid the temperature rise, inthe same way as we used BBO in the frequency-doublingcavity. The resultant output power is shown by filledcircles in Fig. 3. A 202.6-nm output power of 42 mW wasachieved at a 266-nm input power of 740 mW. The850-nm input power was 410 mW. When we accountedfor reflection losses at the crystal facet, the beam splitter,the lens, and the prism, the maximum power of the202.6-nm radiation generated in the crystal was esti-mated to be 59 mW. As the result of stabilizing the crys-tal temperature, when the 266-nm input power was 480mW the 202.6-nm output power was increased by ;40%.When we measured the maximum output power of 42mW, the enhancement factor of the 850-nm radiation was19. By tuning the wavelength of the master oscillator,we observed a tuning region of the sum-frequency radia-tion of 201.5–203.1 nm. The range was limited by thewavelength of the tunable master oscillator used in thissetup, which was 831–859 nm.

By stabilizing the crystal temperature in the sum-frequency mixing cavity, we increased the maximum out-put power at 202.6 nm to approximately double the maxi-mum power obtained without stabilizing thetemperature. However, the output power was still re-strained as the 266-nm input power was increased to 740mW, probably because of the thermal lens in the crystal.If further improvements are performed to dissipate thetemperature gradient in the crystal, an output power ofmore than 42 mW could be obtained at a 266-nm inputpower of 740 mW. Moreover, it has been reported thatthe power of sum-frequency radiation was increased byuse of a double-enhancement cavity.9,10 If the power ofthe 266-nm beam is enhanced in addition to that of the850-nm beam by use of a double-enhancement cavity, it is

Fig. 3. 202.6-nm output power measured as a function of266-nm input power into the sum-frequency mixing cavity.Open circles, the output power measured when the crystal tem-perature is not stabilized; filled circles, the output power mea-sured when the temperature is stabilized. The 850-nm inputpower was 410 mW throughout all measurements.

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possible that the 202-nm output power is increased by anorder of magnitude.

4. SUMMARYWe have generated tunable cw radiation near 202 nm bysum-frequency mixing of 266- and 850-nm radiation withan all-solid-state light source. The maximum generatedpower was 59 mW at 202.6 nm in the crystal. In this ex-periment the power of the 850-nm radiation, whose opti-cal path was arranged to overlap the optical path of the266-nm radiation in BBO, was enhanced in the sum-frequency mixing cavity. The enhancement factor of the850-nm radiation was considerably decreased because ofthe presence of a thermal lens, which was caused by ab-sorption of the 266-nm radiation into BBO. As the resultof stabilizing the crystal temperature in the sum-frequency mixing cavity, we increased the maximum202.6-nm power to approximately double the power ob-tained without stabilizing the temperature. The202.6-nm power is sufficient for the high-resolution spec-troscopy of Zn1 ions that we are planning. By tuning thewavelength of the master oscillator, we observed UV ra-diation from 201.5 to 203.1 nm.

K. Matubara’s e-mail address is matubara@crl.go.jp.

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