© 2003 OSA/ASSP 2003
Continuously tunable visible compact laser source using optical parametric generation in microlaser-pumped periodically poled lithium niobate Emilie Hérault, Sébastien Forget, Gaëlle Lucas-Leclin and Patrick Georges Laboratoire Charles Fabry de l’Institut d’Optique, UMR 8501 du CNRS Université Paris-Sud 91403 Orsay cedex e-mail
[email protected]
Abstract : We demonstrated a very compact laser system providing continuous tunability at high-repetition rate in the visible range by pumping an optical parametric generator (OPG) of periodically poled lithium niobate (PPLN) with a frequency-doubled microchip laser operating at 532 nm. The 660-nm pulses were 0.55 µJ (with 40% of slope efficiency) and 500 ps, and the threshold was only 1.3 µJ (75 MW.cm-2). OCIS codes : (140.0140) Laser and laser optics, (140.3480) Diode-pumped laser, (190.0190) Non linear optics, (190.4410) parametric process, (190.2620) frequency conversion.
Tunable visible laser operation is important in many applications, such as spectroscopy or fluorescence excitation. There is a strong need for efficient and very compact laser sources that can be easily tuned in a relatively large range of wavelengths. With the rapid development of passively Q-switched microchip lasers [1], it is now possible to design all-solid-state laser sources that combine good efficiency and important peak powers with a high level of compactness and simplicity. Such sources have consequently attracted wide attention for pumping nonlinear frequency conversion stages [2,3]. Another progress was made with the use of quasi-phase matched non-linear optical materials as PPLN [4,5,6] to enhance the efficiency of the non-linear processes. A particularly interesting process is the optical parametric generation that provides in a very simple single-pass design a widely tunable radiation. We report in this paper on an optical parametric generator pumped by a frequency doubled microchip laser operating at high-repetition rate (40 kHz) to provide continuous tunability in the spectral range of 640 to 685 nm (for the signal wave). The pump laser was a high-repetition rate passively Q-switched microchip Nd:YAG laser operating at 40 kHz in the near-infrared (1064 nm). The saturable absorber consisted of a Cr4+ layer embedded monolithically in the Nd:YAG gain medium. The laser beam is then send in a multipass Nd:YVO4 diode-pumped amplifier to reach high peak power (10 kW). A bulk LBO crystal was then used to convert the wavelength from 1064 nm to 532 nm. The output green laser beam is diffraction-limited and the temporal jitter was 0.5%. The energy per pulse was 3 µJ while the width of each pulse was 900 ps. The complete system (microchip laser, amplifier and frequency converter) forms a very compact commercial product (DualChip®, by JDS Uniphase -formerly Nanolase). The periodically-poled lithium niobate crystal was 3 cm-long and consisted in 5 parallel gratings. The five periods were 9, 9.25, 9.5, 9.75 and 10 µm, each with a width of 1 mm and a height of 0.5 mm (see figure 1a). The end faces of the crystal were uncoated, giving rise to 14 % Fresnel reflection from each
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face. The crystal is placed in an oven to avoid photorefractive damage and to control precisely the temperature. The nominal temperature was 90 °C to obtain phase-matching at 660 nm (the idler being 2743 nm in this case) for the 9.5 µm-period grating.
(a)
(b)
Figure 1 (a) : schematic representation of the five gratings and dimensions of the PPLN crystal. (b) Tuning range of the OPG with the temperature for the 5 gratings. The solide curve represents the signal wavelength calculated with the Sellmeier equation given in Ref. 7
The experimental setup is described on figure 2. The pump power was controlled via a polarizing beam splitter coupled to a half-wave plate at 532 nm. The infrared radiation was removed by the 45°-532-nm high reflectors used to bring the pump to the PPLN. The 532-nm pump laser was then focused at the center of the non-linear crystal to a 25 µm waist radius by a 80-mm lens. The oven containing the PPLN could be easily pushed sideways via a micrometric screw to choose one of the five gratings. The temperature of the oven could also be tuned from 30°C to 150 °C to complete continuous tunability for the signal from 640 to 685 nm (3.15 µm to 2.38 µm for the idler) as shown on figure 1b. The slight discrepancy between the experimental data and the theoretical fit based on the published Sellmeier equation [7] is Figure 2 : Experimental setup. The amplification and SHG stages are not probably attributable to the represented here. HW : Half-Wave plate. PBS : Polarization Beam-Splitter. precision of the grating DM1 : Dichroïc Mirror HR at 532 nm / HT at 1064 nm. DM2 : Dichroïc periods or of the temperature Mirror HT at 660 nm / HR at 532 nm. L : focusing lens. measurements.
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The OPG threshold was found to be 1.3 µJ - or 75 MW.cm-2 - (see figure 3) and the slope efficiency was 40 %. Near the OPG threshold, the single pass gain is 122 dB. Those results were obtained with the 9.5µm grating, but the thresholds and efficiencies were the same for the five different gratings, since the idler’s wavelength remained always under 4 µm were the lithium niobate absorbs laser energies. We obtained an average output power of 18 mW for 100 mW of incident pump power corresponding to an internal conversion efficiency of 21 % and a gain of 122 dB if considering one photon for the input.
Figure 3 : Signal-pulse energy for the 9.5 µmperiod grating at 90 °C corresponding to a signal wavelength of 660 nm
Figure 4 : Spectral linewidth of the 660-nm OPG pulses at 90°C with the 9.5-µm grating. The resolution of the spectrum analyser was 0.07 nm.
The spatial quality of OPG beam was quite good as the M² factor was 1.5 and the signal FWHM pulse width was reduced to 500 ps because of the OPG exponential gain expression in the high-gain regime. The spectral width of the 660-nm pulse was about 0.1 nm, limited by the acceptance bandwidth of the PPLN (see figure 4). The pump intensity Ip,t at threshold (defined by the detection of an OPG output pulse of 0.1 µJ) is given in agreement with reference 8 by
ξI p ,t
1 4λ s E s , t = ln 2l hc
2
where :
ξ=
8π 2 d eff
2
ε 0 cλ s λi n s ni n p
h is the Planck’s constant, c is the speed of light, ε0 is the dielectric constant of free space, deff is the effective non-linear coefficient, Es,t is the output pulse energy at threshold, λ is the wavelength and n the refractive index. The subscripts p, s and I refer to the pump, the signal and the idler respectively, and t stands for threshold. We can then estimate in a plane wave approximation the value of deff : we found 12 pmV-1, well below the theoretical value of 17 pmV-1. This difference can be explained by a numerous defects and relatively strong inhomogeneities in the poling of the gratings. We also perform optical parametric amplification with the same PPLN gratings : 1 mW of a CW 660-nm laser diode was injected in the PPLN, corresponding to 0.9 pJ per pulse. As expected, the threshold was reduced from 1.3 to 0.6 µJ and the output power slightly increased up to 25 mW (0.9 pJ).A great 3
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reduction of the spectral bandwidth is also expected but the resolution of our spectrum analyser is too high to make an accurate measurement. In conclusion, we have demonstrated the generation of a short-pulse visible laser at high repetition rate (40 kHz) by using optical parametric generation in PPLN. The continuous tunability over a 45 nm range is easily obtained by simply translating the crystal and controlling its temperature. The threshold and slope efficiency were noticeably improved relatively to previously published comparable works [4,5]. The compactness of this source is also very interesting as the whole system could be put in a only 50 x 30 x 15 cm3 box. The authors thanks gratefully JDS Uniphase for the supplying of the Dualchip laser. This work has been partially supported by the research program POLA from the Contrat Plan Etat Région (2000-2006) (French State and Conseil Général de l'Essonne).
References 1. 2. 3. 4. 5. 6. 7. 8.
J.J Zayhowski and C. Dill III, “Diode-pumped passively Q-switched picosecond microchip lasers” in Opt. Lett. 19, 1427-1429 (1994). F.Druon, F.Balembois, P.Georges and A.Brun, “High repetition-rate pulsed ultraviolet source using diode-pumped microchip laserr and a multipass amplifier“, in Opt. Lett. 24, 499-501 (1999). K.W Aniolek, R.L Schmitt, T.J. Kulp, B.A Richman, S.E Bisson and P.E Powers, “Microlaser-pumped periodically poled lithium niobate optical parametric generator-optical parametric amplifier” in Opt. Lett. 25, 557-559 (2000). U.Bäder, J.P Meyn, J.Bartschke, T.Weber, A. Borsutzky, R. Wallenstein, R.G Batchko, M.M Fejer and R.L Byer, “Nanosecond periodically poled lithium niobate optical parametric generator pumped at 532 nm by a single-frequency passively Q-switched Nd:YAG laser”, in Opt. Lett. 24, 1608-1610 (1999). A.C Chiang, Y.C Huang, Y.W Fang and Y.H Chen, “Compact, 220-ps visible laser employing single-pass, cascaded frequency conversion in monolithic periodically poled lithium niobate” in Opt. Lett. 26, 66-68 (2001). J. J. Zayhowski, "Periodically poled lithium niobate optical parametric amplifiers pumped by high-power passively Qswitched microchip lasers," Opt. Lett. 22, pp. 169-171 (1997). D. Jundt, “temperature-dependant equation for the index of refraction, ne, in congruent lithium niobate” in Opt. Lett. 22, 1553-1555 (1997). R.L. Byer, in Quantum Electronics : A Treatise, H.Rabin and C.L Tang, eds. (Academic, New York, 1975), pp.587-702.
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