Electro-optic system controller and method of operation

A circuit for calibrating a photodiode, the circuit comprising:

• a photodetector coupled through a load line resistor to a summing junction of an amplifier;
• an analog to digital converter connected to the amplifier for accepting measurement signals from the photodetector and converting the measurement signals to a digital measurement signal;
• and
• a processor for accepting the digital measurement signal, the processor having means for subtracting a dark current value from the digital measurement signal.

A method for calibrating a photodiode, the method comprising the steps of:

• coupling a photodiode to an amplifier;
• converting an analog signal from the amplifier into a digital signal;
• and
• utilizing digital circuitry to subtract a pre-defined compensation value from the digital signal.

A circuit for calibrating a photodiode for monitor light power of a laser diode, the circuit comprising:

• a power sense measurement circuit for sensing a threshold power level PLTH of the laser diode and a second power level PLM;
• the photodiode coupled through a load line resistor to a summing junction of an amplifier;
• an analog to digital converter connected to the amplifier for accepting a threshold photodiode current IPTH at the threshold power level PLTH and a second photodiode current IPM at the second power level PLM, the analog to digital converter for converting the threshold and second photodiode currents to an initial and second digital measurement signal;
• and
• a processor for accepting the initial and second digital measurement signals and the threshold and second power levels, the processor for calculating an effective responsivity Reff of a photodiode module comprising the laser diode and the photodiode according to the equation Reff=(PLM−PLTH)/(IPM−IPTH).

An electro-optic control system apparatus comprising:

• a laser module having inputs comprising a low frequency DC bias current and a signal current;
• a photodetector circuit for monitoring the laser module, the photodetector circuit having means for compensating for dark current;
• a temperature sensor coupled to the laser module;
• a low frequency VCCS circuit for driving the low frequency DC bias current;
• a high frequency VCCS circuit for driving the signal current;
• an amplifier connected to the high frequency VCCS circuit, the amplifier having adjustable offset and gain;
• a test system switch for connecting a plurality of input signals to the amplifier, the plurality of signals comprising calibration signals;
• a plurality of digital to analog converters for providing control voltages comprising:
• offset and gain control voltages for controlling the amplifier;
• and
• low frequency VCCS control voltages for controlling the low frequency DC bias current;
• a plurality of analog to digital converters for providing a digital representation of analog variables comprising laser temperature, photodiode current, laser current, and laser voltage;
• and
• a controller comprising at least one integrated circuit, the controller having at least one input coupled to the plurality of analog to digital converters, and at least one output couple to the plurality of digital to analog converters, the controller utilizing the at least one input and at least one output for controlling the electro-optic control system.

A method for setting a laser threshold of a laser, the method comprising the steps of:

• coupling a photodiode through a load line resistor to a summing junction of an amplifier;
• converting an analog signal from the photodiode into a digital signal;
• subtracting a pre-defined compensation value from the digital signal;
• measuring a laser output power PL as a laser bias current IP is changed;
• calculating a derivative dPL/dIP;
• identifying a laser threshold current ITH in a region of linear operation of the laser, wherein the threshold current is a point at which the derivative dPL/dIP has reached a stable value after changing in a transition region between spontaneous emission and stimulated emission;
• and
• setting the laser bias current IP to an operating point current ILQ above laser threshold current ITH by utilizing the analog signal from the photodiode as a feedback according to the equation ILQ=ITH+ΔIL, where ΔIL comprises a margin to prevent operation below the threshold current.

A circuit for compensating for a slope efficiency of a laser having a driver input, the circuit comprising:

• a processor for outputting a plurality of control signals, the processor having a plurality of measurement inputs and a storage means;
• a signal amplifier for amplifying a signal to the driver input of the laser;
• a D/A converter connected to a variable resistor for setting a gain of the signal amplifier, the D/A converter receiving at least one resistor control signal of the plurality of control signals from the processor;
• a lookup table in the storage means in the processor, the lookup table for relating the at least one resistor control signal to a resistance, the lookup table for storing expected predetermined slope efficiencies, amplifier gains, and at least one predetermined laser output power;
• a laser bias circuit connect to the driver input of the laser, the laser bias circuit set to a threshold current;
• and
• a means for measuring an actual value of laser output power connected to a measurement input of the plurality of measurement inputs of the processor;
• wherein the processor sends the at least one resistor control signal to the D/A converter to change the gain of the signal amplifier until the at least on predetermined laser output power is obtained for the threshold current.

A method for continuous compensation of laser current threshold variations due to a plurality of temperature changes of a laser, the method comprising the steps of:

• recording an initial value of threshold current ILTH upon power up or after calibration of the laser;
• recording a corresponding value for the laser temperature T1 at the initial value of threshold current ILTH;
• recording a changed temperature T2 of the plurality of temperature changes;
• calculating an adjustment to the threshold current using the formula IL=ILTH+DTH(T)ΔT, where IL is an adjusted laser DC bias current, where ΔT is a temperature change between T1 and T2, where DTH(T) is a function of temperature representing a temperature drift coefficient of a bias current of the laser, the temperature drift coefficient approximating a drift behavior of the laser;
• and
• setting a Voltage Controlled Current Source to output the adjusted laser DC bias current;
• wherein the adjusted laser DC bias current is changed in a gradual manner so as to emulate a temperature drift.

A method for continuous compensation of laser gain drift of a laser and a signal amplifier due to variations in temperature, the method comprising the steps of:

• recording an initial gain G1 of the signal amplifier and an initial gain GL of the laser during a calibration process;
• recording a corresponding initial temperature T1 of the laser;
• continuously monitoring the laser for changes in temperature to a second temperature T2;
• calculating a new gain using the formula G2=G1+G1DG1GL(T)ΔT where G2 is an second amplifier gain at the second temperature T2, DG1GL(T) is a function of temperature representing a temperature drift coefficient of the laser and the amplifier gain, where ΔT is a temperature change between T1 and T2;
• and
• utilizing a high frequency variable resistor and a high resolution D/A converter to change the initial gain G1 of the signal amplifier to the second amplifier gain G2;
• wherein changing the initial gain G1 to the second amplifier gain G2 is gradual so as to emulate a temperature drift.

A circuit for determining laser temperature TJ of a laser module comprising:

• a means for measuring a temperature TS of a sensor thermally coupled to the laser module;
• a high impedance circuit for measuring a voltage VLD across the laser;
• a high impedance circuit for measuring a signal current ILS driving the laser;
• a means for calculating a DC bias current ILQ driving the laser;
• a processor for approximating an instantaneous power PL of the laser, where PL=VLD(ILQ+ILS), and an average calculation of power is obtained by averaging data taken in periodic samples;
• and
• calculating the temperature TJ using an equation representing the laser temperature in terms of the instantaneous power PL, temperature TS, and characteristic thermal resistances of the mechanical system that comprises the laser module.

A circuit for eliminating offset of an amplifier for a laser signal driver of a laser, comprising:

• a drive controller;
• a D/A converter controlled by the drive controller and connected to a summing junction of the amplifier;
• a resistor for applying an offset voltage to the summing junction;
• a test system switch for providing a common connection to the amplifier;
• and
• a means for measuring an offset of the amplifier utilizing a spontaneous emission from an output of the laser;
• wherein the D/A converter applies current to the summing junction to cancel the offset of the amplifier.

A method for calibrating a gain of a laser and amplifier driver circuit, comprising the steps of:

• providing an input voltage reference to the amplifier driver circuit;
• calibrating the gain for the laser and amplifier driver circuit utilizing the input voltage;
• detecting a change in a slope efficiency of the laser due to aging;
• and
• compensating for the aging of the laser using the change of the slope efficiency.

An electro-optic control system apparatus comprising:

• a laser module having inputs comprising a low frequency DC bias current and a signal current;
• a photodetector circuit for monitoring the laser module, the photodetector circuit having means for compensating for dark current;
• a temperature sensor coupled to the laser module;
• a low frequency VCCS circuit for driving the low frequency DC bias current;
• a high frequency VCCS circuit for driving the signal current;
• an amplifier connected to the high frequency VCCS circuit, the amplifier having adjustable offset and gain;
• a test system switch for connecting a plurality of input signals to the amplifier, the plurality of signals comprising calibration signals;
• a plurality of digital to analog converters for providing control voltages comprising:
• offset and gain control voltages for controlling the amplifier;
• and
• low frequency VCCS control voltages for controlling the low frequency DC bias current;
• a plurality of analog to digital converters for providing a digital representation of analog variables comprising laser temperature, photodiode current, laser current, and laser voltage;
• a controller comprising at least one integrated circuit, the controller having at least one input coupled to the plurality of analog to digital converters, and at least one output couple to the plurality of digital to analog converters, the controller utilizing the at least one input and at least one output for controlling the electro-optic control system;
• and
• an input protection circuit coupled to the amplifier for protecting the electro-optic control system from over voltages, power surges and electrostatic discharge on a signal current input.