Enhancing the signal-to-noise ratio of optical frequency beating using open-loop photonic crystal fiber

A method to enhance the signal-to-noise ratio (SNR) of the optical frequency beat note (fb) with the femtosecond optical frequency comb is demonstrated. The SNR of fb and carrier-envelop offset frequency (f0) can be optimized separately by using two pieces of photonic crystal fiber (PCF). While the SNR of f0 is maintained more than 40 dB (300 kHz resolution bandwidth), the SNR of the beat note between the comb and the iodine stabilized He-Ne laser at 633 nm is promoted to 38 dB. The beat note can be directly counted by a commercial frequency counter without tracking oscillators. The frequency fluctuation due to the open-loop property of this method is below 2.8×10-15 at 1 s integration time and no obvious drift is observed at longer integration time. The open-loop fast-changing phase noise is measured to be sensitive to the acoustic noise from the environment.