Methods and apparatus for monitoring and controlling optical modulator signal quality

A method of automatically adjusting the operating voltage levels of an optical modulator for optimum response over a range of monochromatic signal wavelengths and temperatures of operation comprising the steps of:

• automatically determining the operating voltage levels for a first wavelength and temperature of operation, such that the optical modulator operating at said operating voltage levels produces a maximum difference in modulated light transmitted between a non-absorbed optical carrier signal and an absorbed optical carrier signal;
• automatically adjusting the operating voltage levels as the wavelength and or temperature of operation change, such that the optical modulator operating at said adjusted operating voltage levels produces a maximum difference in modulated light transmitted between a non-absorbed optical signal and an absorbed carrier signal;
• wherein the step of automatically determining comprises the sub-steps of:
• employing a first monitor photodiode and a second monitor photodiode;
• applying a first predetermined voltage to the first monitor photodiode;
• applying the first predetermined voltage plus a second voltage to the second monitor photodiode;
• exposing the first and second monitor photodiodes to an optical source of light of a first wavelength;
• measuring the difference between the absorbed photocurrents for the first and second monitor photodiodes;
• varying the second voltage;
• measuring the difference between the absorbed photocurrents as the second voltage is varied;
• and
• determining the value of the second voltage corresponding to the maximum difference between the absorbed photocurrents.

An apparatus for automatically adjusting the operating voltage levels of an optical modulator for optimum response over a range of monochromatic signal wavelengths and temperatures of operation comprising:

• means for automatically determining the operating voltage levels for a first wavelength and temperature of operation, such that the optical modulator operating at said operating voltage levels produces a maximum difference in modulated light transmitted between a non-absorbed optical carrier signal and an absorbed optical carrier signal;
• means for automatically adjusting the operating voltage levels as the wavelength and or temperature of operation change, such that the optical modulator operating at said adjusted operating voltage levels produces a maximum difference in modulated light transmitted between a non-absorbed optical signal and an absorbed carrier signal;
• wherein the means for automatically determining comprises:
• a first monitor photodiode and a second monitor photodiode;
• a supply for applying a first predetermined voltage to the first monitor photodiode;
• a supply for applying the first predetermined voltage plus a second voltage to the second monitor photodiode;
• an optical source of light of a first wavelength applied to the first and second monitor photodiodes;
• circuitry for measuring the difference between the absorbed photocurrents for the first and second monitor photodiodes;
• circuitry for varying the second voltage;
• a circuitry for measuring the difference between the absorbed photocurrents as the second voltage is varied;
• and
• circuitry for determining the value of the second voltage corresponding to the maximum difference between the absorbed photocurrents.

A method of automatically adjusting the operating voltage levels of an optical modulator for optimum response over a range of monochromatic signal wavelengths and temperatures of operation comprising the steps of:

• automatically determining the operating voltage levels for a first wavelength and temperature of operation, such that the optical modulator operating at said operating voltage levels produces a maximum difference in modulated light transmitted between a non-absorbed optical carrier signal and an absorbed optical carrier signal;
• automatically adjusting the operating voltage levels as the wavelength and or temperature of operation change, such that the optical modulator operating at said adjusted operating voltage levels produces said maximum difference in modulated light transmitted between a non-absorbed optical carrier signal and an absorbed optical carrier signal;
• wherein the step of automatically determining includes the sub-steps of:
• employing a first monitor photodiode and a second monitor photodiode;
• applying a first direct current (DC) voltage plus an alternating current (AC) voltage to the first monitor photodiode;
• applying the first direct current (DC) voltage plus the alternating current (AC) voltage plus a second direct current (DC) voltage to the second monitor photodiode;
• exposing the first and second monitor photodiodes to an optical source of light of a first wavelength;
• measuring a photocurrent difference between the absorbed photocurrents for the first and second monitor photodiodes;
• increasing the second direct current (DC) voltage if the phase of the photocurrent difference is in phase with the alternating current (AC) voltage;
• and
• decreasing the second direct current (DC) voltage if the phase of the photocurrent difference is not in phase with the alternating current (AC) voltage.