We present in this paper a dc-coupled in-line optical detector based on a multisection semiconductor optical amplifier (SOA). The key principle is to use a voltage reference correlated with the bias voltage level by the way of a two-section or a three-section SOA. So, by means of the differential detection, the signal dc-component is kept with reduced sensitivity to temperature and bias current fluctuations. Experimental and theoretical results are presented when a two-section SOA is used and performance predictions when a three-section is employed. The obtained responsivity is 63 V/W at 40 mA and over 110 V/W at 50 mA when the input optical power, measured within the fiber, is 13 dBm. When the SOA is biased at 40 mA, the detection bandwidth is over 1 GHz when the input optical power is 4 dBm.
Thierry Rampone, Hong-Wu Li, and Ammar Sharaiha, "Semiconductor Optical Amplifier Used as an In-Line Detector with the Signal DC-Component Conservation," J. Lightwave Technol. 16, 1295- (1998)