We investigate, both theoretically and experimentally, spectral, temporal, and coherence properties of a degenerate synchronously pumped optical parametric oscillator (SPOPO) as a divide-by-2 subharmonic generator. Periodically poled lithium niobate was used as the nonlinear gain medium and 180 fs pulses from a mode-locked Ti:Sapphire laser as the pump. A regime of stable SPOPO operation at degeneracy was achieved, where the SPOPO longitudinal modes were phase-locked to the pump, even without active cavity-length stabilization. Phase locking was confirmed by interference measurements between the pump and the frequency-doubled optical parametric oscillator output, as well as by beat frequency measurements using an independent continuous-wave laser. We have found that the stability range of such a phase-locked state, with respect to external perturbations, increased with the pump power and decreased with the cavity Q at a constant number of times above threshold, in excellent agreement with our model based on coupled nonlinear wave equations. At degeneracy (around 1550 nm), the SPOPO produced 70 fs output pulses with the full width at half-maximum spectral width of , which manifests significant pulse compression and spectral broadening with respect to the pump laser.
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