Abstract

A semiconductor laser source, whose frequency is stabilized and resettable to a definite value, is necessary to realize an optical heterodyne communication. We have developed such a laser system using an optical frequency comb (OFC) generator and two semiconductor lasers at 1.5 µm wavelength. Figure 1 shows a schematic explanation of this laser source. The OFC generator is a Ti-LiNbO₃ waveguide type of OFC generator.¹ An optical waveguide (Anritsu SC3B01A) working at 1.5 µm was used as a phase modulator and an optical resonator was formed by coating high reflection films (refrectivities = 97%) on the two facets of the waveguide. The linewidths of the master and slave lasers were reduced to 1 MHz by employing an external cavity structure using a grating. Figure 2 shows a spectral envelope of the comb, i.e., modulation sidebands, which was generated by the waveguide type OFC generator. This OFC generator could generate modulation sidebands whose power was higher than –80dBm in the frequency span as wide as 4.3 THz (=35 nm). The modulation frequency and the modulation index were 13 GHz and 2 π, respectively. Figure 3 shows a heterodyne signal between a 47th modulation sideband and a slave laser. A frequency difference between a master laser and the slave laser was 0.6 THz. A frequency offset locking between the 47th modulation sideband and the slave laser was carried out by controlling the injection current of the slave laser so as to nullify the phase difference between the heterodyne signal and a microwave local oscillator signal.

© 1995 IEEE