Optics & Laser Technology 33 (2001) 111–115www.elsevier.com/locate/optlastec

Tunable solid state UV laserFeng Huang ∗, Qihong Lou, Tianyan Yu, Jingxing Dong, Bo Lei, Yunrong Wei

Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-216, Shanghai 201800, People’s Republic of ChinaReceived 21 June 2000; received in revised form 30 October 2000; accepted 30 November 2000

Abstract

A tunable Ti:sapphire laser pumped by an intracavity frequency-doubled YAG laser was designed, and third-harmonic generation ofthe tunable Ti:sapphire laser was achieved by rotating the cardinal plane of the crystal through a multi-crystal cascade method. Theparameters such as crystal length, time predelay, and intensity ratio related to the e8ciency of third-harmonic generation are analyzed bynonlinear equations. c© 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Third-harmonic generation; Ti:sapphire laser; UV laser

In recent years, the UV laser has attracted consider-able attention as one of the most promising candidates forthe lithography light source [1] for next-generation 1Gbitdynamic random access memory (DRAMs) modules, whichare expected to have design rules under 0:2�m. Anotherpromising UV laser application [2] is Inertial ConAnementFusion (ICF) which produces clean energy for the next gen-eration. To obtain a UV laser, one approach is the excimerlaser, another harmonic generation of a solid state laser.Excimer lasers operate in the UV region but at a number ofAxed wavelengths and its optical quality is poor. In order besuitable for the ICF, Raman shaping system or MOPA sys-tem is needed [3]. Frequency conversion of solid state laserby nonlinear crystal such as BBO [4], LBO, CLBO, KDPor KLN has become the most promising way of realizinglight source in the UV region. Tunable 187.9–196 nm UVradiation has been generated at room temperature in betabarium borate (BBO) crystal by sum-frequency mixing ofNd:YAG laser radiation and the second harmonic of a dyelaser pumped by the second harmonic of the same Nd:YAGlaser [5].

1. Tunable Ti:sapphire laser

Ti:sapphire laser is a type of solid tunable laser with largetunable range and long operation life. A Ti:sapphire laser

∗ Corresponding author. Tel.: +86-21-5953-4890-530; fax: +81-21-599-16703.

E-mail address: jxdong@mail.shcnc.ac.cn (F. Huang).

with folded three-mirror cavity and longitudinal pumping byintracavity frequency doubling Nd:YAG is shown in Fig. 1.In Fig. 1, the left part is Nd:YAG laser, the cavity is

90◦ folded, and the length of the cavity is 30 cm long, andthe 15mm Nd:YAG crystal is used. The repetition rateof the Nd:YAG laser is 10KHz, the modulation crystal isan acousto-optic crystal with 1–10 kHz frequency, and a5mm× 5mm× 15mm BBO crystal is used for intracavityfrequency doubling, and single mode at 532 nm with 2–8Wis generated. Ti:sapphire laser cavity is on the right part ofFig. 1. The total length of the cavity is 25 cm, and a Brest-wer cut 18mm long Ti:sapphire crystal is used. In order tocompensate the dispersion in Ti:sapphire crystal, a foldedcavity is needed, the fold angle is 12◦. Furthermore, aspecial birefringent Alter is designed to obtain laser with250MHz narrow band. Three groups of mirrors areused to obtain wavelength 700–800 nm, 800–900 nmand 900–1000 nm, respectively. The tunable curve ofthe Ti:sapphire laser is shown in Fig. 2, when thepumped power is 2W and the maximum power is 350mWat 780 nm.

2. E�ciency of third harmonic generation (THG)

240–260 nm UV laser is obtained by THG in BBO crys-tals. The e8ciency of THG is not only related to the lengthof the crystal but also to the parameters of Ti:sapphire laser.The e8ciency of THG is calculated by nonlinear couplingwave equations [6].

0030-3992/01/$ - see front matter c© 2001 Elsevier Science Ltd. All rights reserved.PII: S 0030 -3992(00)00128 -6

112 F. Huang et al. / Optics & Laser Technology 33 (2001) 111–115

Fig. 1. Setup of the Nd:YAG–Ti:sapphire laser system. O.C: outputcoupler, B.F: birefringent Alter. The cavity of the Nd:YAG laser is 30 cmlong plane–plane cavity and the cavity of Ti:sapphire laser is a folded 25cm long concave-plane cavity. The Beam-splitter reNects 1064 nm waveand is transparent to 532 nm wave.

Fig. 2. Ti:sapphire laser’s output power versus wavelength.

Fig. 3. Energy conversion e8ciency dependence on the BBO crystallength d, with diPerent input pulse time pre-delay.

We have analyzed the e8ciency of the THG related toparameters such as the pre-delay, power ratio. We foundthat if the parameters such as pre-delay and power ratio areappropriately chosen, the e8ciency of THG can be highlyimproved.The relationship between the energy conversion e8ciency

of THG and the crystal length dwith diPerent time pre-delayis shown in Fig. 3. When the laser intensity I1 and I2 is3Gw=cm2; �1 and �2 is 500 fs, the time pre-delay t1 − t2 is0 fs, the energy conversion e8ciency of THG was very low,

Fig. 4. Energy conversion e8ciency dependence on the BBO crystallength d, with diPerent ratio of I(2!)=I(!).

Fig. 5. THG laser duration dependence on BBO crystal length d withdiPerent duration of fundamental laser.

but when the time pre-delay increased to 200 fs, the energyconversion e8ciency of THG increased rapidly. In addition,the saturation in THG process existed with an optimal crystallength.In addition, taking account of the ratio of I2 and I1 on the

energy conversion e8ciency, the result is shown in Fig. 4with duration �1 and �2 are all 500 fs, and the time pre-delayt1− t2 is 0 fs. When laser intensity ratio I2=I1 is equal to 0.5,the curve of energy conversion e8ciency of THG related tocrystal length is almost the same as the result when I1=I2 is 1.But, when laser intensity I2=I1 is 2, the energy conversione8ciency of THG is higher than that when I2=I1 is 1, whichshowed that when the I2 increased, the e8ciency also in-creased.The relationship between the output pulse duration of the

THG laser and the crystal length d is shown in Fig. 5, whenlaser intensity I1=I2 equals to 3Gw=cm2, and time pre-delayt1−t2 is 0. When the duration � equals to 500 fs but less thanR�u (R�u = 1=u2 − 1=u1 = 1:25× 10−12 s), the output pulse

F. Huang et al. / Optics & Laser Technology 33 (2001) 111–115 113

Fig. 6. Arrangement of two BBO crystals and the distributing of the o-polarization and the e-polarization.

duration of THG laser increased while the crystal lengthincreases, but when �=1ns�R�u, the output pulse durationof THG laser decreases with the crystal length d. From thisresult, pulse compression is possible by using THG BBOcrystals that is shown in latter experiment.

3. THG by rotating the cardinal plane of the crystal

The output laser is horizontal polarization because thatthe Ti:sapphire crystal is cut of Brestwer angle. So typeI frequency doubling is easily accomplished. But thepolarization of fundamental frequency laser and the secondharmonic generation (SHG) is vertical to each other. In therange of 240–260 nm, the matching angle of type II is 90◦.This requires type I frequency summing to process THG,which results in a half-wave plate or a rotation plate beingneeded. To remedy that disadvantage, THG by rotating thecardinal plane of the crystal is designed as shown in Fig. 6.The intensity of third harmonic generation is expressed

as follow:

I3(3!) = K × cos4(’) sin2(’)

In which,’ is rotated angle,K is a constant related to crystalsquality as follows:

K =16�4L21L22 2ePd2eP I 3

n1n2n3n′21 n′2�

23�

21c2�

20;

where, L1 and L2 are the length of the SHG and THG crys-tal, respectively, I is the intensity of the fundamental laser,n1; n2 and n3 is the index of the crystal at fundamental wave,SHG wave and THG wave, respectively. When the rotatedangle is 35:1◦, experimental data matches the calculation asshown in Fig. 7.Two pieces of crystal are used, one is 4mm × 4mm ×

7mm shaped, and 28◦ cut, the other is 5mm × 5mm ×7mm shaped, 48◦ cut. The curve of the power of THGoutput versus the wavelength is shown in Fig. 8, the THGpower is 3mW when the THG wavelength is 248 nm andthe power of 744 nm Ti:sapphire laser is 140mW, which is

Fig. 7. The relationship between THG output power and BBO crystalrotated angle.

Fig. 8. Tunable output of UV laser.

used to be the seed source of KrF excimer laser. And theduration is also measured, the fundamental wave is 44.1 ns,the harmonic wave is 22.8 ns, but the third-harmonic waveis 16.2 ns, which indicates wave is compressed which hasbeen calculated in Fig. 5 of Part 2.

114 F. Huang et al. / Optics & Laser Technology 33 (2001) 111–115

Fig. 9. Experiment setup of THG of Ti:sapphire laser by cascademulti-crystals.

Fig. 10. The relationship between the SHG e8ciency and the pump laserenergy.

4. THG by multi-crystal cascading method

The fundamental Ti:sapphire laser is partly not completelyused by BBO crystal in THG by rotating the cardinal planeof crystal method. According to the theory in Part 2, in orderto increase the e8ciency of THG, the e8ciency of SHGmust be improved, then the multi-crystal cascading methodis designed as shown in Fig. 9. Another advantage of themulti-crystal cascading method is that the pre-delay betweenthe SHG and fundamental wave can improve the e8ciency.From the experiment, the e8ciency of SHG, when

two pieces of short BBO crystals used is higher thanwhen one piece of long BBO crystal used which isshown in Fig. 10. Parameters of the crystals is as fol-lows: BBO I: 4mm × 7mm × 7mm; � = 28◦, BBO II:4mm × 4mm × 7mm; �= 28◦. To indicate the advantageof the multi-crystal cascading method, a single BBO III:4mm × 7mm × 14mm; � = 28◦ is used to generate thesecond harmonic of the Ti:sapphire laser. We found thatthe e8ciency of SHG when multi-crystals are used is higherthan that when one long crystal is used. It is well knownthat the e8ciency of THG is greatly related to the length ofthe crystal, but when the length of the crystal increases, thefundamental wave is out from the side of the crystal because

Fig. 11. Power of UV laser relate to wavelength.

of the walk-oP angle. Walk-oP angle is expressed as [7]:

tg �=12

(n2e − n2o)n2o sin �+ n2e cos �

× sin 2�

� is 4:32◦ when the fundamental wavelength is 744 nm, andwhen the length of the crystal is longer than 13mm, thefundamental wave is out from the side of the crystal anddoesnot interact with each other in the crystal, which de-creases the e8ciency of the THG.A beam-splitter system is used to depart the SHG wave

and the fundamental wave. Furthermore, it can pre-delaythe fundamental wave. The distance from the HR mirror toanother HR mirror is 6 cm, then the pre-delay time is 40ns. The middle high reNection wavelength of the HR mir-ror is 248 nm. A half wave-plate of fundamental wave isused to rotate the polarization of the fundamental wave. Thehalf wave-plate is installed on piezoelectricity quartz, whichcan be adjusted slightly. A 4mm× 4mm× 7mm; �= 31◦

BBO crystal is used to sum the THG laser and the funda-mental laser, the tunable curve of the UV laser is shown inFig. 11, the power is stronger than that obtained by rotatingthe cardinal plane of crystals.

5. Conclusion

A new 240–260 nm tunable UV laser is designed byTHG of Ti:sapphire laser, and two methods are used, oneis rotating the cardinal plane of the crystal, the other ismulti-crystals cascading. In addition, the e8ciency of THGrelates to the crystal length, time pre-delay and intensityratio are calculated. The e8ciency of THG can be im-proved by adjusting the time pre-delay, enhancing intensityof second-harmonic wave and selecting an optimal crystallength.

F. Huang et al. / Optics & Laser Technology 33 (2001) 111–115 115

Acknowledgements

This work is supported by state key lab of laser technologyof HUST and National Science Foundation.

References

[1] Ozaki Y, Kawai Y, Yoshikawa A. Jpn J Appl Phys 1990;29:2553.[2] Shaw MJ, Bailly-Salins R, Edwards B. Laser and Particle Beams

1993;11:331.

[3] Huang F, Lou Q. Chin J Laser B 1999;8:15.[4] Bhar GC, Chatterjee U, Kumbhakar P. Opt Lett 1997;22(21):1606.[5] Seifert F, Ringling J, Kittelmann O. Opt Lett 1994;19(19):1538.[6] Wang Y, Dragila R. Phys Rev A 1990;41(10):5645.[7] Eimerl D, Auerbach JM, Milam D. Opt Lett 1997;22(16):1208.