Improvement in the Extinction Ratio of Output Optical PCM Signals From an Uncooled External Cavity Diode Laser by Using a Semiconductor Saturable Absorption Device

This paper describes the increase in the extinction ratio (ER) of an optical signal achieved by using a semiconductor saturable absorption device (SAD). We investigate a method for optimizing the SAD length in order to increase the ER, where we deal with the SAD characteristics phenomenologically. We then undertake a theoretical analysis of the bit error rate dependence on the ER increase. We clarify experimentally that by using a SAD with an effective length of 350 , the ER of the output optical signals from an uncooled directly modulated light source can be increased from 6.8 to 8.2 dB and, thus, conform to the synchronous optical network/synchronous digital hierarchy standard.