Self-phase-locked divide-by-2 optical parametric oscillator as a broadband frequency comb source

Abstract

We investigate coherence properties of a degenerate synchronously-pumped optical parametric oscillator (SPOPO) as a divide-by-2 subharmonic generator. Type 0 (e-ee) periodically-poled MgO : LiNbO₃ was used as a nonlinear gain crystal and a femtosecond mode-locked Ti:Sapphire laser at 775 nm - as a pump source. We observed that the SPOPO longitudinal modes at degeneracy were phase-coherent with that of the pump. The self-phase-locking and self-stabilization effect can be explained in terms of mutual injection locking between the signal and the idler frequencies of the SPOPO. We confirmed the phase-locking effect by performing interference between pump and frequency-doubled output, as well as beat frequency measurements between the SPOPO output and (i) the pump laser and (ii) an independent continuous-wave (CW) laser. A frequency locking range around SPOPO degeneracy &Delta;<i>f</i> was measured as a function of pump power, when the SPOPO operated in the phase-locking regime. We have found that &Delta;<i>f</i> increased monotonically with the pump power and decreased with the cavity Q, in good accord with our theoretical model based on coupled nonlinear optical wave equations. When the proper regime was chosen, the SPOPO remained phase-locked to the pump without any active stabilization even in the presence of environmental noise. At degeneracy (around 1550 nm), the SPOPO produced 70-fs output pulses with the FWHM spectral width of 210 cm^-1 that is 2.6 times broader than the spectrum of the pump laser.