Coherent multi-order stimulated Raman generated by two-frequency pumping of hydrogen-filled hollow core PCF

The development of hollow-core photonic crystal fiber (HC-PCF) has led to a number of interesting experiments in the gas-phase nonlinear optics, an example being stimulated Raman scattering (SRS) in hydrogen gas. Multi-octave Raman frequency comb generation has been observed in H2-filled HC-PCF using a single-line pump laser. If such frequency combs could be made coherent, they would find a wide range of practical uses, ranging from sub-fs pulse synthesis to optical clocks and control of carrier-envelope phase. Although the results of theoretical study of higher order SRS, under conditions similar to those reported suggest that Stokes noise can be filtered out if the Raman gain is high enough, so far no direct evidence of mutual coherence between the individual comb lines has been reported. Here we describe an effective two-frequency-pumping technique for producing mutually-coherent multi-order Raman sidebands in hydrogen-filled HC-PCF.