Actively mode-locked optical parametric oscillator

CTuM2.pdf

Active Mode-locked Optical Parametric Oscillator N. Forget1, J.-M. Melkonian3, C. Drag2, F. Bretenaker2,4, M. Lefebvre3, and E. Rosencher3,4 1 Fastlite, Ecole Polytechnique bât. 404, Palaiseau, France Laboratoire Aimé Cotton, CNRS, Bâtiment 505, Campus universitaire, 91405 Orsay CEDEX, France 3 Office National d'Etudes et de Recherches Aérospatiales, Chemin de la Hunière, 91761 Palaiseau CEDEX, France 4 Département de Physique, Ecole Polytechnique, Palaiseau, France 2

Abstract: continuous-wave active mode-locking of near degenerate singly and doubly resonant OPOs is reported. Transient and steady-state regimes are explored. ©2006 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers, (190.7110) Ultrafast nonlinear optics

1. Introduction Despite the large tunability of broadband femtosecond oscillators, much effort has been made to increase their wavelength coverage by means of frequency mixing in nonlinear crystals or by using synchronously pumped optical parametric oscillators1. These systems however require a preexistent ultrashort laser source and an extra optical cavity with a carefully matched repetition rate and even, in some cases, an intermediate stage is needed to convert the wavelength of the mode-locked laser before pumping the OPO. An alternative to these indirect and complex schemes would be to generate short pulses directly by mode-locking an optical parametric oscillator pumped by a continuous wave pump laser. As it can be easily demonstrated, there is a strong theoretical analogy between laser and OPO mode-locking in the undepleted regime and the formalism originally developed by Haus for active or passive mode-locked laser can be directly applied to OPOs2. Thanks to the very large gain bandwidth of near-degenerate OPOs, it is possible to predict that an actively mode-locked OPO must be able to sustain oscillation of picosecond pulses in the undepleted regime and we indeed recently demonstrated active mode-locking of a quasi-continuous-wave near-degenerate doubly resonant OPO and recorded pulses of 700ps3. In this talk, we demonstrate that mode-locking of a continuous-wave OPO can indeed be obtained in doubly and singly resonant configurations. In both cases, we investigate the transient and steady-state regimes and explore the mechanisms limiting the reduction of the pulse duration. 2. Experimental setup The experimental setup we used is schemed in Fig. 1. The doubly-resonant OPO (DRO) cavity is a 1.2-m-long linear cavity (free spectral range, 114.5 MHz). Mirrors M1 and M2 have a 100 mm radius of curvature while mirrors M3 and M4 are flat.

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Fig. 1. a) Experimental setup. b) Gain bandwidth in SRO (dashed line) and in DRO (solid line) when clusters are taken into account.

All mirrors are designed to provide a maximum reflectivity (R > 98 %) between 1044 nm and 1084 nm and a high transmission (T > 95 %) at 532 nm. The nonlinear crystal is a 0.5-mm-thick, 30-mm-long, 5 % MgO doped PPLN crystal (deff = 14 pm/V) with a single poling period of 6.92 µm. The OPO is pumped at 532 nm by a 10 W cw laser. The pump beam is focused to a 47 µm waist inside the crystal, as measured by the knife-edge method. No photorefractive damage was observed in the crystal up to 2.8 W. Direct active mode-locking is achieved with an acousto-optic modulator cut at Brewster angle to minimize insertion losses. The diffraction efficiency of this modulator is 50 % at the nominal driving power. A mechanical chopper providing 800 µs pump pulses can be used to study the transient behavior of the OPO. The calculated single-pass gain bandwidth for PPLN:MgO is 3.9 THz,

CTuM2.pdf

which is large enough to sustain ultrashort pulses. The singly resonant OPO (SRO) has a 2.5-m-long ring cavity, with mirrors M1 and M2 having a 200 mm radius of curvature. All mirrors exhibit maximum reflectivity between 900 and 980 nm, and maximum transmission between 1160 and 1300 nm. The crystal temperature is set to 93° C so that the signal lies at 973 nm and the idler at 1174 nm, ensuring a singly-resonant operation. At these wavelengths, the gain bandwidth is 0.4 THz, which is still large enough to sustain picosecond pulses. 3. Mode-locked operation Once the acousto-optic modulator is active and tuned to match the cavity roundtrip period (RF frequency of 57.355 MHz for the DRO and 60.782 MHz for the SRO), fast oscillations appear on the output intensity of the idler and signal waves in the cw pumping regime. The pulses were then measured with a fast InGaAs photodiode (rise time 70 ps) and a 2.5 GHz bandwidth oscilloscope. Fig. 2 shows the mode-locked pulses of the idler wave recorded at the end of a chopped pump pulse. At this time the pulses have the same duration as in the cw regime, about 800 ps FWHM for the SRO and DRO at ~1.5 times above the threshold and up to 2 ns when the OPO was pumped at ~4 times above threshold. The pulses period are 8.2 ns and 8.7 ns for the SRO and DRO respectively, matching the corresponding cavity round-trip times. As expected for active mode-locking, the pulse duration depends on the relative strengths of the modulation and intracavity power, i.e pump power. However, we could not observe pulse shorter than ~600 ps by increasing the modulation power without stopping completely the oscillation. 2.0

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Fig. 2. Left: experimental pulse duration as a function of the time elapsed since the experimental detection of oscillation when the OPO were pumped at ~1.5 times above threshold. Center and right: experimental pulse trains measured in SRO and DRO in steady-state regime when pumped far above threshold.

When pumped far above threshold, the mode-locked pulse duration increases from a minimal value (~800ps) up to ~2 ns almost linearly in 100 µs and then reaches its steady-state value. Clearly, this transient behavior does not match the usual build up of the mode-locking process, for which the pulses duration are first expected to decrease with time. What is more the cw duration is much larger than the values of ~100 ps and ~13 ps calculated for a DRO and a SRO in the undepleted regime. 3.0 Pulse duration (ns)

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Fig. 3. Numerical simulations showing the evolution of the pulse duration as a function of the time elapsed since the beginning of oscillation.

Numerical simulation of the pulse width evolution as a function of time (Fig. 3) brings some evidence that such a behavior is to be attributed to the saturation process. What is more we show that the pulse duration at steady-state can be analytically predicted. Paths to improve mode-locking in DRO or SRO will also be addressed. References 1. 2. 3.

G. J. Hall, M. Ebrahimzadeh, A. Robertson, G. P. A. Malcolm, A. I. Ferguson, “Synchronously pumped optical parametric oscillators using all-solid-state pump lasers,” J. Opt. Soc. Am. B 10, 2168 (1993). H. Haus, “A Theory of Forced Mode Locking,” IEEE J. Quantum Electron. QE-11, 323 (1975). N. Forget, S. Bahbah, C. Drag, F. Bretenaker, M. Lefebvre, E. Rosencher, “Actively mode-locked optical parametric oscillator,” Opt. Lett. 31, 972 (2006).