The Stanford Photonics Research Center

September 17, 2008

We demonstrated a stable femtosecond synchronously pumped optical parametric oscillator (SPOPO) that self-phase-locked when operating at degeneracy without any need for active stabilization.  The nonlinear gain element was a 1-mm-long type-I periodically poled MgO:LiNbO_3 crystal phase-matched for 1550 nm.  A mode-locked Ti:sapphire laser generating 180-fs pulses at 80 MHz was employed as the pump at 775 nm.  The spectral bandwidth of the degenerate signal/idler was 50 nm around 1550 nm or 200 cm^-1 and thus exhibited an output comb broadening of about 3.  The corresponding pulse duration was 70 fs, which was transform-limited for Gaussian pulses.  Phase-coherence between the degenerate SPOPO output and the pump was confirmed using separated beat note measurement techniques with respect to the pump laser and with respect to an independent CW laser serving as an external phase reference.  The frequency locking range of the degenerate state was measured to vary as a square root of the number of times above pump threshold (i.e., parametric gain) and as a linear function of the output coupling (i.e., inverse cavity Q).  The data appeared consistent with steady-state injection locking.  The self-phase-locked SPOPO operated stably for up to 1 hour provided that there was minimal environmental noise.  As a divide-by-2 sub-harmonic generator, the degenerate SPOPO is a novel source for extending phase-stable frequency combs to longer wavelengths.  Cascading these broadband phase-locked systems and implementing phase-preserving optical parametric amplifiers (OPA) can produce high-peak-power combs at mid-IR wavelengths unavailable to solid-state lasers and useful for high-precision metrology and high-resolution spectroscopy.
 
 
Sam Wong received both his B.S. and M.Eng. degrees in Electrical Engineering and Computer Science in 2001 at the Massachusetts Institute of Technology, where he worked on fiber optics and dispersion-managed solitons with the late Prof. Hermann Haus.  He is currently a Ph.D. candidate in the Electrical Engineering department at Stanford University, where he works with Prof. Robert Byer on ultrafast lasers and nonlinear optics as well as novel laser material.