Self-referencable frequency comb from a 170-fs, 1.5-μm solid-state laser oscillator

Abstract

We report measurement of the first carrier-envelope offset (CEO) frequency signal from a spectrally broadened ultrafast solid-state laser oscillator operating in the 1.5 μm spectral region. The f-to-2f CEO frequency beat signal is 49 dB above the noise floor (100-kHz resolution bandwidth) and the free-running linewidth of 3.6 kHz is significantly better than typically obtained by ultrafast fiber laser systems. We used a SESAM mode-locked Er:Yb:glass laser generating 170-fs pulses at a 75 MHz pulse repetition rate with 110-mW average power. It is pumped by one standard telecom-grade 980-nm diode consuming less than 1.5 W of electrical power. Without any further pulse compression and amplification, a coherent octave-spanning frequency comb is generated in a polarization-maintaining highly-nonlinear fiber (PM-HNLF). The fiber length was optimized to yield a strong CEO frequency beat signal between the outer Raman soliton and the spectral peak of the dispersive wave within the supercontinuum. The polarization-maintaining property of the supercontinuum fiber was crucial; comparable octave-spanning supercontinua from two non-PM fibers showed higher intensity noise and poor coherence. A stable CEO-beat was observed even with pulse durations above 200 fs. Achieving a strong CEO frequency signal from relatively long pulses with moderate power levels substantially relaxes the demands on the driving laser, which is particularly important for novel gigahertz diode-pumped solid-state and semiconductor lasers.