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.

Candidate plans for adaptive digital transmission are briefly reviewed. It is suggested that adaptive format transmission with asynchronous multiplexing capability is attractive and promising from the standpoint of practical application. Feasibility and efficiency of this new transmission plan are investigated. A formula for predicting the minimum required transmission rate is derived. Upper and lower bounds of transmission efficiencies are analysed. A signal distributing technique for improving transmission efficiency is also developed. Investigation of memory requirements anticipates the possibility of simple implementations for format conversion and reconversion. Results of simulations prove the reliability of theoretical predictions. Fiber optic transmission is expected to be suitable for realizing this new communications concept.