Broadband Phase-Coherent Optical Frequency Synthesis With Actively Linked Ti:Sapphire and Cr:Forsterite Femtosecond Lasers

Abstract

Extremely broadband femtosecond pulse sources with well defined carrier-envelope phase evolution have been a field of intense research over the past years [1–3]. The time-domain motivation for the push towards increased bandwidth is the generation of optical pulses that approach the single-cycle regime and can be used for phase-sensitive nonlinear experiments [4]. The frequency-domain motivation is the interest in extending the bandwidth over which optical frequencies can be precisely synthesized and subsequently used for absolute optical frequency measurements or comparisons [1, 5, 6]. Nonlinear frequency conversion is one route to overcome the bandwidth limitations of current systems based on one femtosecond laser. A second attractive route is linking two femtosecond lasers with overlapping emission spectra. Phase-locking the repetition rates of two femtosecond lasers with different gain media has recently been demonstrated. Here, we use a Ti:sapphire laser and a Cr:forsterite laser to extend this approach and additionally equalize the rate at which the carrier-waves move underneath their pulse envelopes. In other words, we link the absolute position of the two frequency combs, thereby establishing a true phase-coherence between all modes of their combined spectrum. The laser repetition rates are phase-locked by using a nonlinear cross-correlation technique.