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We demonstrate the mode spacing multiplication of optical frequency combs (OFCs) using interleaving technique. The pulse train of an OFC after interleaving is phase modulated, and phase demodulation is necessary for mode spacing multiplication. This scheme enables mode spacing multiplication without power loss in principle, which is in contrast to the conventional mode filtering technique. During demonstration, the mode spacing of the OFC of a mode-locked Er-doped fiber laser (repetition rate of 97 MHz) is quadrupled by the interleaving process and successive phase demodulation.

Laser frequency stability locked to D2 transition of 85Rb at 780 nm by the Sagnac-loop stabilization scheme was evaluated using the heterodyne measurement with an optical frequency comb. This scheme does not require modulation and magnetic field to generate the feedback error signal. Therefore, the setup for the laser frequency stabilization is simple. Because of the mechanical stability of the Sagnac interferometer, the dispersion of atom transition, which is used as an error signal for feedback loop, can be observed with negligibly small effects of mechanical fluctuations. In the experiment, the Allan variance for relative frequency uncertainty was 2.6×10−12 over 64 s averaging.

Polarization of an optical frequency comb is electrically controlled using a waveguide electro-optic phase modulator (WG-EOM). Owing to the low operation voltage and wide electric bandwidth of the WG-EOM, fast polarization control is possible. It is found that birefringence of the WG-EOM and polarization-maintaining optical fibers causes polarization-dependent pulse separation, which makes polarization control of the optical frequency comb impossible. Therefore, compensation of the birefringence is required for polarization control. In the experiment, a delay line in free space is used for birefringence compensation, and pulse-to-pulse polarization control of an optical frequency comb (with a repetition rate of 100 MHz) is demonstrated.