Patent ID: 7443896

Claim:
An integrated circuit, the integrated circuit couplable to a semiconductor laser and to a photodetector, the photodetector optically couplable to the semiconductor laser, the semiconductor laser capable of transmitting an optical signal in response to a modulation current, and the photodetector capable of converting the optical signal into a photodetector current, the integrated circuit comprising: a modulator couplable to the semiconductor laser, the modulator capable of providing the modulation current to the semiconductor laser, the modulation current corresponding to an input data signal, wherein the modulator is further capable of providing a first modulation current level to the semiconductor laser when the input data signal has a first logical state and providing a 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; wherein the semiconductor laser is further capable of providing the optical signal having a first optical power level in response to the first modulation current level and having a second optical power level in response to the second modulation current level, the first optical power level being greater than the second optical power level; wherein the photodetector is further 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; and an optical midpoint controller couplable to the photodetector and couplable to the semiconductor laser, the optical midpoint controller capable of: in response to the photodetector current, adjusting a forward bias current of the semiconductor laser, so that the semiconductor laser generates the optical signal with a power level approximate to a predetermined optical midpoint power level, said predetermined optical midpoint power level determined by calculating an arithmetic mean of a plurality of optical power levels; sampling the first photodetector current level to form a plurality of first photodetector current indicators; sampling the second photodetector current level to form a plurality of second photodetector current indicators; determining a measured optical midpoint power level as an overall 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 determining a variance between the measured optical midpoint power level and the predetermined optical midpoint power level, and based on the variance, forming an optical midpoint error signal.