Download Red and blue light generation by frequency doubling and tripling in

Red and blue light generation by frequency doubling and tripling in
periodically-pole_4LiNbO,
G.W.Ross,N.S.Felgate,W.A.Clarkson, P.G.R.Smith,P.E.Britton and D.C.Hanna
OptoelectronicsResearchCentre,University of Southampton,
Highfield, SouthamptonSO17 lBJ, United Kingdom.
Tel : +44 (0)1703 593144 Fax : +44 (0)1703 593142 Email : [email protected]
Abstract
Efftcient frequency-doublingand tripling of a diode-pumpedNd:YAG laser (670mW at
1320~1) generates360mW red (660nm) and 35mW blue (440~1) light in a single passthrough
two cascadedperiodically-poledLiNbO, gratings.Photorefractiveeffects are negligible.
Red and blue light generation by frequency doubling and tripling in
periodically-poled_LiNbO,
G.W.Ross,N.S.Felgate,W.A.Clarkson, P.G.R.Smith,P.E.Britton and D.C.Hanna
OptoelectronicsResearchCentre,University of Southampton,
Highfield, SouthamptonSO17 lBJ, United Kingdom.
Tel : +44 (0)1703 593144 Fax : +44 (0)1703 593142 Email : [email protected]
Periodically-poled LiNbO, (PPLN), with its high optical nonlinearity and non-critical
phase-matching,is an attractive nonlinear material for generatinghigh-power visible output via
frequencyup-conversionof existing infrared sources.In this work we demonstratesecondand
third harmonic generation of a Nd:YAG laser (1320nm) to produce red (660nrn) and blue
(440nm)wavelengthswith high spatialbeamquality,in a simplesingle-passarrangement.Samples
of PPLN were preparedfor this experimentusing the electric-field poling technique [ 11.
The fundamental infrared laser source was a Nd:YAG laser end-pumpedby a single,
beam-shaped20W diode-bar [2]. The experimentalarrangementis shown in figure 1. A folded
resonatordesignwas usedwith an intracavityetalon to selectthe 132Onmlasing wavelength, and
an acousto-optic modulator to Q-switch the laser.The laserwas operatedat a pulse repetition
rate of 17kH2, producing Q-switched pulses of energy 6OuJ and 170ns duration (FWHM)
corresponding to a peak power of 353W in a linearly polarised fundamentaltransversemode
(beam quality factor, MZ < 1.2). After collimation and focusing, an averagepower of 78OmW
(275W peak) was incident on the first PPLN sample.
Second harmonic generation (SHG) was achievedusing PPLN of period 12.34um,
thickness 0.5mm and length 20mrn. The grating period was chosento allow first-order quasiphase matched (QPM) SHG at an elevated temperature of 140°C in order to suppress
G. ITRoss et al., “Red and blue light generation... ”
photorefractiveeffects.The fundamentalinfrared(1320nm)beamwas focusedto a spot of radius
42um inside the uncoatedcrystal sample.An average(internal) fundamentalpower of 670mW
resultedin the generationof 360mW (14 1W peak) of secondharmonic red (660nm) in a singlepass,representinga conversionefficiency of 54%. Figure 2 showshow the conversionefficiency
dependsupon the fundamentalpower. The output red beamhad a circular profile with M2 = 1.1
indicating successfIr suppressionof photorefiactive effects and no optical damage at these
operating power levels.
The generatedsecondharmonic and the remainingunconvertedfundamentalwere then
combined in a second(uncoated)PPLN sampleto achievesum frequencygeneration(SFG) of
440~1 blue light - the third harmonicof the Nd:YAG source.This PPLN samplehad a period of
11.62ym, thickness 0.3rnm and length 25mm, designedfor third-order QPM SFG, again at
140°C. The use of a third-order grating through a 0.3mm samplepermitted the optimisation of
a 50:50 mark-to-space domain inversion ratio both to maximise the third-order conversion
efficiency to the blue and to minimise photorefiactive effects.With averageinternal powers of
174mW at 132Onmand 246mW at 66Onm,35mW ( 13.7Wpeak) of blue light was generatedin
a single-pass(conversionefficiency8%) figure 3. The output bluebeamalso had a circular spatial
profile with M2 = 1.1.
These results demonstratethe capability of SFG, even in a third-order interaction, to
generate significant powers of blue (44Onm) light in PPLN. Additionally, results show that
photorefiactiveeffectscanbe successfullyeliminatedin undopedLiNbO, at this short wavelength.
Provisionof anti-reflectioncoatings for optics and crystals,and optimisation of beamoverlap in
the frequency-mixing process,should all offer a significantimprovementin performance.
References
1. J.Webjorn,V.Pruneri,P.St.J.Russell,
J.R.M.Barr& D.C.Hanna,Electron.Lett. 30, 894 (1994)
2. W.A.Clarkson and D.C.Hanna,Opt.Lett. 21, 869 (1996)
G. W.Rosset al., “Red and blue light generation... ”
Figure Captions
Figure 1
The experimentalarrangement.M1,3,4 are plane mirrors. MZ is a curved mirror
ROC 5Omm E is an lOOurnthick etalon. Q-SWis acousto-optic modulator QswitchandLl-3 arelenseswith focal lengths3OOmm,15Omn1,
50mm respectively.
Figure 2
Red (660~1) light generationin the first PPLN crystal by SHG. Graph of SHG
conversionefficiency vs averagefundamentalpower.
Figure 3
Blue (44Onm)light generationin the secondPPLN crystal by SFG. Graph of blue
vs (f?.mdamental)x(second
harmonic) averagepowers.
G. W.Rosset al., ‘Red and blue light generation... ”