Mode partition noise of a 1.55 µm DFB laser is suppressed by an optical injection locking technique for the first time. The effectiveness is confirmed in a 2.4 Gbit/s, 40 km transmission experiment. This technique completely eleminates the bit-error-rate floor of 10-4.

Buried-heterostructure lasers (BH lasers), having a narrow active region and a low threshold current, oscillate stably in a single transverse mode in a wide range of driving currents above threshold. In addition, no significant kinks can be observed in the light output vs. current characteristics. Sinusoidal modulation characteristics of BH lasers are measured especially with respect to frequency response, spectrum broadening and nonlinear distortion. From experiments, it is exhibited that BH lasers have a flat frequency response up to 2 GHz, with less spectrum broadening than conventional wide stripe lasers, and a low 2nd-order distortion of about 50 dB below the fundamental level. This distortion is favourable when compared to LED's. These results demonstrate the feasibility of BH lasers as promising optical sources with linear and wideband modulation capabilities.

This letter proposes an improved polarization diversity method for heterodyne/coherent optical transmission systems. It is theoretically clarified that this method can reduce the noise caused by polarization fiuctuations and can receive a binary FSK or PSK modulated signal with high sensitivity.

Novel optical FSK modulation/demodulation schemes with AMI coding are proposed to overcome degradation due to the non-uniform frequency modulation characteristic of DFB lasers. It is shown that an NRZ signal can be demodulated without a decoder as opposed to other coding schemes proposed earlier.