Ultra-low threshold current laser for optical parallel data communication

The drive current of a directly modulated semiconductor laser for optical parallel data communication was investigated by taking into account both the threshold current and the differential quantum efficiency which are defined from the cavity length and the mirror reflectivities. Then the drive current required for an error free data communication (bit error rate: BER<10^-9) with the bit rate of 1 Gb/s was shown as a function of the threshold current with parameters of the cavity length and the mirror reflectivities. It was found that the laser cavity design which yields the minimum drive current is almost the same condition to achieve the minimum threshold current operation when the total system loss is low (<10 dB), whereas the cavity design for higher differential quantum efficiency is required when the total system loss is much higher. When the cavity length is 100 μm and the rear mirror reflectivity R_r is 0.99 for an example, the front mirror reflectivity R_f which gives the minimum drive current was obtained to be 0.94, 0.91, and 0.8 for the total system loss of 6, 10, and 20 dB, respectively