Optical midpoint power control and extinction ratio control of a semiconductor laser

A method of controlling an optical midpoint power level of a semiconductor laser, the method comprising:

• (a) modulating the semiconductor laser at a first modulation level when the input data signal has a first logical state and modulating the semiconductor laser at a second modulation level when the input data signal has a second logical state;
• (b) transmitting an optical signal having a first optical power level in response to the first modulation level and having a second optical power level in response to the second modulation level, the first optical power level being greater than the second optical power level;
• (c) detecting the first optical power level and the second optical power level;
• (d) determining a measured optical midpoint power level as an arithmetic mean of the detected first optical power level and the detected second optical power level;
• (e) determining an optical midpoint error as a variance between the measured optical midpoint power level and a predetermined optical midpoint power level;
• and
• (f) using the optical midpoint error, adjusting the forward bias current of the semiconductor laser to generate the optical signal having substantially the predetermined optical midpoint power level, wherein using the optical midpoint error further comprises:
• integrating the optical midpoint error with a plurality of previous optical midpoint errors to form an integrated optical midpoint error;
• and
• adjusting the forward bias current of the semiconductor laser in response to the integrated optical midpoint error.

An apparatus comprising:

• a semiconductor laser capable of transmitting an optical signal having a first optical power level in response to a first modulation current level and having a second optical power level in response to a second modulation current level, the first optical power level being greater than the second optical power level;
• a modulator coupled to the semiconductor laser, the modulator capable of providing the first modulation current level to the semiconductor laser when the input data signal has a first logical state and providing the second modulation current level to the semiconductor laser when the input data signal has a second logical state, the first modulation current level being greater than the second modulation current level;
• a photodetector optically coupled to the semiconductor laser, the photodetector capable of generating a first photodetector current level in response to the first optical power level and a second photodetector current level in response to the second optical power level;
• a sampler coupled to the photodetector, the sampler capable of sampling the first photodetector current level to form a first photodetector current indicator and sampling the second photodetector current level to form a second photodetector current indicator, wherein the sampler is further capable of sampling the first photodetector current level to form a plurality of first photodetector current indicators and sampling the second photodetector current level to form a plurality of second photodetector current indicators;
• a forward bias current controller coupled to the sampler and to the semiconductor laser, the forward bias current controller capable of determining a measured optical midpoint power level as an arithmetic mean of the first photodetector current indicator and the second photodetector current indicator;
• determining a first variance between the measured optical midpoint power level and a predetermined optical midpoint power level and, based on the first variance, forming an optical midpoint error signal;
• and in response to the optical midpoint error signal, further capable of adjusting the forward bias current of the semiconductor laser to generate the optical signal having substantially the predetermined optical midpoint power level, wherein the forward bias current controller is further capable of determining the measured optical midpoint power level as an overall arithmetic mean of a first arithmetic mean of the plurality of first photodetector current indicators and a second arithmetic mean of the plurality of second photodetector current indicators;
• and
• a modulation current controller coupled to the sampler and to the modulator, the modulation current controller capable of determining a measured extinction ratio as a ratio of the first photodetector current indicator to the second photodetector current indicator;
• determining a second variance between the measured extinction ratio and a predetermined extinction ratio and, based on the second variance, forming an extinction ratio error signal;
• and in response to the extinction ratio error signal, further capable of adjusting the modulation current provided by the modulator to the semiconductor laser to generate the optical signal having substantially the predetermined extinction ratio, wherein the modulation current controller is further capable of determining the measured extinction ratio as a ratio of a first arithmetic mean of the plurality of first photodetector current indicators to a second arithmetic mean of the plurality of second photodetector current indicators.