Two-crystal, synchronously pumped, femtosecond optical parametric oscillator

Abstract

We demonstrate a femtosecond optical parametric oscillator based on two nonlinear crystals synchronously pumped by a single ultrafast laser for efficient intracavity signal amplification and output power enhancement. By deploying two identical MgO:PPLN crystals in a single standing-wave cavity, and two pump pulse trains of similar average power from the same Kerr-lens-mode-locked Ti:sapphire laser, a minimum enhancement of 56% in the extracted signal power is achieved, with un-optimized output coupling, when temporal synchronization between the two intracavity signal pulse trains is established, resulting in a corresponding enhancement of 49% in pump depletion. Using intracavity dispersion control, near-transform-limited signal pulses with clean spectrum are obtained. , and in picosecond operation, we used a single optical cavity for the two-crystal OPO [12]. In the cw regime, we demonstrated the concept using both a single optical cavity [13,14] and an ARR to couple the two OPOs [15]. While the primary goal of our earlier experiments on twocrystal OPOs has so far been arbitrary dual-wavelength generation, it would be desirable to also explore the feasibility of exploiting the concept to achieve further improvements in the OPO performance, for example, with regard to gain, efficiency, and output power enhancement. This can, in principle, be attained through intracavity optical parametric amplification (IOPA) of the circulating signal field in both crystals when operating at identical wavelengths, as observed in our earlier experiments on a two-crystal cw OPO in a single optical cavity [13,14]. In femtosecond OPOs, the exploitation of IOPA can be particularly advantageous for gain and efficiency enhancement because of the restrictions imposed by the spectral acceptance bandwidth (SAB) and group velocity mismatch (GVM) on the maximum usable interaction length of the nonlinear crystal for efficient parametric conversion. Given the relatively broad spectral bandwidth and short duration associated with femtosecond pump pulses, only crystals of short interaction length (typically <1-2 mm) can be utilized to minimize the effects of SAB and GVM. In critically phase-matched configurations, spatial walkoff is another factor restricting the useful interaction length, since the relatively low intensities associated with high-repetition-rate pump pulses demand tight focusing. On the other hand, by deploying multiple thin nonlinear crystals, one can circumvent such limitations and maintain the same SAB, GVM and spatial walkoff as that of a single crystal of short length [14], while increasing the total effective interaction length for parametric gain, thus enhancing the efficiency and output power through IOPA. Here, we demonstrate this concept in a femtosecond OPO deploying a single optical cavity with two thin identical MgO:PPLN nonlinear crystals that are pumped in synchronization with two input pulse trains from the same Kerr-lens-mode-locked (KLM) Ti:sapphire laser. We show that such a scheme can provide major improvemen...

Dynamical modeling and experiment for an intra-cavity optical parametric oscillator pumped by a Q-switched self-mode-locking laser

“…That is, when the fundamental laser makes a round trip between M 1 and M 5 , the signal laser can make about 17 round trips between M 4 and M 5 . Then the synchronous pump can be satisfied [23][24][25].…”