WIDE-SPAN OPTICAL FREQUENCY COMB GENERATOR FOR ACCURATE OPTICAL FREQUENCY DIFFERENCE MEASUREMENT

An optical frequency comb (OFC) generator was realized for accurate op tical frequency difference measurement of 1.5 mum wavelength semicondu ctor lasers by using a high frequency LiNbO3 electrooptic phase modula tor which was installed in a Fabry-Perot cavity. It was confirmed that the span of the OFC was wider than 4 THz. By using semiconductor lase rs whose spectrum linewidths were narrowed to 1 kHz and a sensitive op tical balanced-mixer-receiver for measuring beat signal between the si deband of the comb and the laser, we demonstrated a frequency differen ce measurement up to 0.5 THz with a signal-to-noise ratio higher than 61 dB, and a heterodyne optical phase locking with a heterodyne freque ncy of 0.5 THz in which the residual phase error variance was less tha n 0.01 rad2. The maximum measurable frequency difference, which was de fined as the sideband frequency with the signal-to-noise ratio of 0 dB , was estimated to be 4 THz.