Lithium triborate picosecond optical parametric oscillator

Optics Communications 91 (1992) 93-96 North-Holland

OPTICS COMMUNICATIONS

Lithium triborate picosecond optical parametric oscillator V. K u b e c e k z, y . T a k a g i , K. Y o s h i h a r a Institutefor Molecular Science, Myodaot, Okazakt 444, Japan and G.C. Reali Department of Electromcs, Umversttyof Pavia, Via Abbtategrasso 209, 27100 Pavm, Italy Received 22 November 1991

We report on the first operation of a picosecond optical parametric oscillator (OPO) based on a hthlum tnborate (LBO) nonlinear crystal A singly resonant, non-colhnear OPO configuration was used, and the LBO was pumped synchronously by the third harmonic of a passive negative feedback mode-locked Nd YAG laser, obtaimng a conversion effioency into the idler wave ( @ 900 nm ) of up to 10% and an internal overall efficiency of up to 26%

Optical p a r a m e t r i c generation in a nonlinear crystal provides a most attractive m e t h o d to p r o d u c e widely tunable coherent radiation. In the past, insufficient nonhnearitles together with the low d a m age thresholds o f the available nonlinear crystals have p r e v e n t e d the c o m m o n use o f this scheme. The recent advances in material production, notably with the newly d e v e l o p e d nonlinear crystals KTP, betab a r i u m borate ( B B O ) and lithium trlborate ( L B O ) , and the great i m p r o v e m e n t s in stability o f the p u m p ing laser sources have renewed the interest in optical p a r a m e t r i c oscillators ( O P O ) . The work in pulsed systems has been mainly c o n c e n t r a t e d on the use o f BBO crystals, which often have been p u m p e d by an ultraviolet b e a m since in this case the output can cover a larger fraction o f the visible range in addition to the near infrared. Both O P O ' s generating nanosecond [ l ], picosecond [ 2 ], and femtosecond [ 3 ] pulses, and optical p a r a m e t r i c amplifiers ( O P A ' s ) generating picosecond pulses [4] have been d e m onstrated using BBO. Recently, we also r e p o r t e d the o p e r a t i o n o f a picosecond BBO O P O [6 ] synchronously p u m p e d by Present address Faculty of Nuclear Science and Physical Engineering, Czech Tech University, Prague, Czechoslovakia

the third h a r m o n i c o f a passive negative feedback mode-locked ( P N F M ) N d Y A G laser [5]. Such a laser p u m p is very attractive for synchronous p u m p ing o f laser-active m e d i a a n d nonlinear crystals for the generation o f ultrashort pulses, since it generates long trains o f short, energetic and stable pulses in several crystalline N d d o p e d materials. F o r example, from a P N F M N d ' YAG laser trams o f 50 to over 100 pulses, d e p e n d i n g upon the operating conditions, with energy up to l0 ~tJ per pulse and pulse d u r a t i o n less than 10 ps are directly generated from the oscillator. Using this source a signal wave, tunable within the range from 407 n m to 690 nm (range limited by the reflectlvlty o f m i r r o r s ) and generating pulses o f 3 ps time duration, was o b t a m e d . The LBO crystal has a n u m b e r o f advantages over the BBO [7]. The transmission extends further into the ultraviolet, it is not hygroscopic and its optical damage threshold is reported to be 2 times higher than that for BBO. LBO was used most often for generation o f higher harmonics and for O P O p u m p e d by n a n o s e c o n d pulses [ 8 ]. In the picosecond regime it was used as an efficient optical p a r a m e t r i c amplifier [9]. The work described in this letter reports on the first operation o f a LBO O P O for picosecond generation.

0030-4018/92/$05.00 © 1992 Elsevier Science Publishers B V All rights reserved

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The experimental arrangement of the system is shown schematically in fig. 1. The pulse train from P N F M Nd YAG laser, with duration of 600 ns and contaming approximately 80 pulses wxth total energy of 1 m J, was amplified by three Nd YAG single-pass amphfiers to an energy of 20 m J, and the third harmonic was generated using two K D P type II crystals with conversion efficiency of approximately 10% The 355 n m beam, having total energy m excess of 2 mJ and single pulse width of 9 ps, was focussed, using a 1 m focal length spherical lens, into a 5 × 5 × 10.5 m m 3 LBO crystal (supphed by Fujlan Castech Crystals, Inc ) cut for type I phase-matching at 0 = 9 0 ° and 0 = 3 9 ° The p u m p beam at the LBO crystal was detected by a CCD TV camera, and from tts bell-shaped profile the 10% intensity level diameter was measured to be 300 pm. Similarly to BBO, when the 355 n m p u m p beam was focussed into the LBO crystal, a bright parametric fluorescence (signal wave) was observed, which did not propagate m the same direction as a p u m p beam, but at a certain angle This angle was measured to be 35 mrad m the present case, which was 2.5 times less than that with BBO Due to the lower n o n l i n e a r coefficients (type I phase-matching, xy-plane: d32(LBO) ~ 0 66 dzz(BBO) [ 10] ), we observed para m e t n c fluorescence from the 10.5 m m long LBO crystal at a p u m p i n g power 2.4 t~mes higher than that from a 7 m m long BBO crystal The non-colhnearaty of the p u m p and parametric fluorescence was a reason for the n o n - c o l h n e a r p u m p i n g geometry, used

1 July 1992

with an angle of 35 mrad between the p u m p beam and the resonator axis The resonator of the OPO consisted of two identical dtelectric mirrors M3 and M4, each havmg a radius of curvature of 1 m and high reflectlvaty ( > 9 9 % ) from 405 to 690 nm. The mirror M4 was m o u n t e d on a precision translation stage to allow the fine tuning of the cavity length. The beam waist of this confocal resonator had a dmmeter

I IPN}-\I

~ X(;

....

-i

Fig 1 Schematic of singly resonant OPO with PNFM Nd YAG laser AMPL, amphfiers, SHG, second harmomc generator, THG, third harmomc generator, F, uv transmitting filter, L, spherical lens f= 1 m, M1, M2, flat mtrrors, M3, M4, spherical mirrors, RoC = 1 m, LBO, hthlum trlborate crystal 94

50

ns/div

Fig 2 0 s c d l o s c o p e traces of the undepleted pump (a), depleted pump (b), and signal wave parametric pulse (c) trams Pumpmg level was three times above the oscdlatlon threshold

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matsu, Model H T V C1370-01, with a resolution o f 1.8 ps. W h e n the pulse width of the p u m p i n g pulse was 9 ps, the pulse width, measured at the signal wavelength o f 575 nm, was found to be 6 ps The tuning o f the O P O was achieved by tilting the LBO crystal with respect to the axis o f the resonator. The measured tuning curve giving the d e p e n d e n c e o f the wavelength of the signal wave on the internal phase-matching angle is shown in fig. 3 The experimental points are represented by e m p t y circles. The shortest measured signal wavelength was 452 nm, corresponding to an internal angle between the p u m p wave and the LBO crystallographic axis of 29.8 °. This wavelength limit was due to the geometrical dimensions o f our crystal Near degeneracy we were able to generate a signal wave up to 690 nm, where the h m l t was set by the reflectlvity o f our mirrors. The sohd line represents the tuning curve calculated using the following Sellmeler equations for LBO [ 11 ]

o f 400 g m which a p p r o x i m a t e l y m a t c h e d the p u m p b e a m d i a m e t e r at the focus o f spherical lens L. The p u m p i n g threshold for p a r a m e t r i c generation was 500 ~tJ, corresponding to a peak p u m p i n g intensity o f ~ 1 G W / c m 2, m o r e than an order o f magnitude lower than LBO damage threshold [ 11 ]. At a p u m p energy 3 times above this threshold, 150 laJ o f energy was o b t a i n e d at 900 nm, corresponding to a conversion efficiency into the infrared idler wave of 10%. For this case, fig. 2a shows the u n d e p l e t e d p u m p t r a m detected after one pass through the LBO crystal when the p a r a m e t r i c oscillation was blocked and there was no generation. Figure 2b shows the depleted p u m p at the same position in the case o f parametric oscillation, while the generated signal wave train is shown in fig. 2c. These oscillographs show an optical p a r a m e t r i c oscillation b u i l d up t i m e o f 150 ns, measured at the time o f half the peak intensity. This build up time is longer at lower p u m p levels. F r o m this figure we also know that single pulse internal conversion efficiency gets as high as 70%, while the overall internal efficiency, integrated over the whole train, is 26%. The pulse width o f the p u m p and signal pulse from O P O was measured using a streak c a m e r a H a m a -

0.01150 n ~ = 2 . 4 5 3 1 6 + 2 2 _ 0 . 0 1 0 5 8 - 0 01123 22 ,

(la)

0.01249 ny= 2.53969+ 22_0.01339

(lb)

2

00202922 ,

750 I

700 -

o/,

/

650~

600 r i i

T-%

550 i I .7

500 -

450

J

--

I

400 28

o

o

30

32

34

36

' 38

' 40

42

phase-matchmg angle [degrees] Fig. 3 Dependence of the parametric signal wave wavelength on the internal phase-matching angle in LBO crystal Sohd curve: calculated, empty circles measured

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l July 1992

~ 6 at 4 6 2 n m . T h i s is a c o m m o n b e h a v i o r o f a t y p e I interaction.

I nm

Acknowledgements T h e a u t h o r s w o u l d h k e to t h a n k Dr. A. Agnes1, o f D e p a r t . o f Electr., P a v i a U n i v . , for t h e h e l p w i t h t h e computanon of the tuning curve and for many helpful d i s c u s s i o n s .

2, = 4 6 2 n m A;, =08nm

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)~ 6"~0 n m A~=]3nm

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References

Fig 4 Spectral wtdths of the OPO signal wave for different wavelengths at twice above threshold pumping 0.01412 n~ = 2 5 8 5 1 5 + 2 2 _ 0 . 0 0 4 6 7

0 . 0 1 8 5 0 22 ,

(lc)

w h e r e n~ v._- are t h e r e f r a c t i v e i n d i c e s a n d t h e w a v e l e n g t h 2 is in lam. T h e t u n i n g c u r v e was c a l c u l a t e d a s s u m i n g s c a l a r p h a s e - m a t c h i n g , a n d t h e slight disc r e p a n c y f r o m t h e m e a s u r e d p o i n t s c o u l d b e d u e to the non-colllnearlty of our pumping scheme. The calculated idler wave tunabdlty corresponding to m e a s u r e d signal w a v e l e n g t h s IS f r o m 731 n m to 1689 n m . T h e s p e c t r a l w i d t h o f t h e signal w a v e for p u m p i n g t w i c e a b o v e t h e t h r e s h o l d , as m e a s u r e d using a m o n o c h r o m a t o r a n d C C D T V c a m e r a , is s h o w n f o r d i f f e r e n t w a v e l e n g t h s in fig. 4 W e c a n see t h a t n e a r t h e d e g e n e r a c y t h e s p e c t r a l w i d t h is 6 n m , b u t further from this point the spectral width decreases below 1 nm, reaching a time-bandwidth product of

96

[ l ] Y X Fan, R C Eckardt, R L Byer, J Noltmg and R Wallensteln, Appl Phys Lett 53 (1988) 2014 [2] S Burduhs, R Grlgoms, A Plskarskas, G Slnkevlcms V Slrutkaltls, A Fix, J Noltlng and R Wallenslem, OpILlCS Comm 74 (1990) 398 [3] R Laenen, H Graener and A Lauberau, OpUcs Lett 15 (1990) 971 [ 4 ] J Y Huang, J Y Zhang, Y R Shen, C Chen and B Wu, Appl Phys Lett 57 (1990)1961 [5 ] A Del Corno, G Gabetta, G C Reah, V Kubecek and J Marek, Optics Lett 15 (1990) 734 [ 6 ] V Kubecek, Y Takagl, K Yoshlhara and G C Reah, CLEO Technical Digest, paper CThRI0 8 ( 1991 ) 434 [7]C Chen, Y Wu, A Jlang, B Wu, G You, R LlandS Lm, J Opt Soc Am B 6 ( 1 9 8 9 ) 6 1 6 [8] A Fix, C Huang, T Schroder and R Wallenstem, CLEO Technical Dtgest, paper CWE8 7 (1990) 248 [ 9 ] J Y Zhang, J Y Huang, Y R Shen, C Chen and B Wu Appl Phys Lett 58 (1991)213 [10] V G Dmltrlev, G G Gurzadyan and D N Nlkogosyan, Handbook of nonhnear optical crystals (Springer, Berhn, 1991 ) [ 11 ] Fujmn Castech Crystal, lnc LBO Brochure, Technical specifications