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Picosecond Ti:sapphire-pumped optical parametric oscillator based on periodically-poled LiNbO3
S.D.Butterworth, P.G.R.Smith and D.C.Hanna
A low threshold OPO has been developed with tuning from 1.15 to 2.45μm as the Ti:sapphire pump laser is tuned from 790 to 810nm. Output powers up to 130mW have been achieved at 75% pump depletion.
The attraction of bulk periodically poled lithium niobate (PPLN) for synchronously pumped optical parametric oscillators (OPO) has been demonstrated in recent experiments, pumping in both the visible and IR [1, 2]. Tuning of the signal/idler output was achieved by varying the PPLN temperature. An alternative approach is to use a tunable pump wavelength and fixed crystal temperature. Features of our PPLN OPO are (a) the freedom from photorefractive damage in the PPLN and (b) tunable visible radiation can be generated, via simultaneous 3rd order quasi-phase-matched sum frequency generation.
The PPLN sample, which we fabricated by electric-field poling of a 500μm thick lithium niobate substrate, contained two domain-inverted gratings with periods of 19μm and 20μm suitable for pumping around 800nm. The sample length of 6mm was chosen as the maximum that avoids significant temporal walk-off of the interacting waves for the 2psec pump pulses provided by a mode-locked Ti:sapphire laser. This operated at 76MHz with a maximum average power of 550mW. The plane/plane PPLN sample had a single MgF2 layer as AR coating, centred at 1.38μm. The PPLN oven was fixed at 170°C to ensure negligible photorefractive effects.
Spot sizes in the crystal for the pump (signal) wave were 20μm (27μm) respectively. A threshold pump power of 18mW (at the crystal) was observed for λs = 1.35μm with all highly reflecting mirrors, increasing to 38mW when an output coupler of T = 6% was used. The tuning behaviour of the OPO is shown in fig.#1. With λ = 20μm a tuning range of 1.15 to 1.53μm (signal) and 1.68 to 2.4μm (idler) was obtained as the pump tuned from 784 to 804nm. This tuning range was limited by the bandwidth of our mirror set. The more limited tuning for λ = 19μm was set by the short wavelength limit of the pump, ~780nm. Typical output powers and pump depletions are shown in fig.#2. The pump depletion saturates at the 75% level at over 4 times threshold. From the measured spectral bandwidth (Δλ = 1.46nm at 1.3μm) and pulse duration (1.4psec assumed sech2) a time-bandwidth product of 0.36 was obtained. The beam quality of the signal was measured to be M2 = 1.2, showing no significant beam distortion from photorefraction.
Besides signal and idler five visible light outputs were simultaneously generated - identified as 2ωp, 2ωs, ωp + ωi, ωp + ωs, and 3ωs having average power of typically 10s to 100s of μW. The ωp + ωs process was particularly efficient since with suitable pump wavelength and temperature it is possible to satisfy 3rd order QPM exactly. Thus at 170°C, a pump wavelength of 803.2nm gave a signal wavelength of 1488nm and a sum-frequency output at 522nm. The measured average power at 522nm was 1.5mW, however only 10% was transmitted by the resonator mirror, so, the actual generated power was nearer 15mW.
Further significant extension of the tuning range could be obtained with suitable mirrors for both the OPO and Ti:sapphire laser, see fig.#3. Convenient and compact all-solid-state sources of broadly-tunable pulses are now a prospect as the demonstrated threshold power levels are now accessible to diode-pumped Cr:LiSAF lasers, and are beginning to be approached by mode-locked diode laser.
Wavelength tuning of the Ti:sapphire pumped PPLN OPO with two separate gratings
Figures 2 (a, b).
Output power and pump depletion versus pump power at the crystal for two different signal/idler wavelengths
Theoretical tuning behaviour of signal/idler versus pump wavelength for various grating periods at a fixed crystal temperature of 170°C, also shown are the conditions for exact 3rd order QPM of the sum-frequency process (circles).
CLEO '97 Baltimore Maryland 18-23 May (1997)
Southampton ePrint id: 76817
Copyright University of Southampton 2006