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 μm, 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.
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