Bias point control circuit for externally modulated transmitter
Methods and apparatuses for controlling a bias point voltage for an external optical modulator are provided. A control loop is used to adjust a bias signal applied to an external modulator by determining unwanted signals at a predetermined frequency received by an optical receiver of the control loop and accounting for these unwanted signals when determining the bias point voltage.
1. A bias voltage controller for an external optical modulator comprising:
a signal generator configured to produce a first signal having a first frequency based on a first control input and a second signal having a second frequency;
a combiner configured to produce a bias voltage control signal based on the first signal and a bias point voltage;
an external optical modulator configured to operate at a bias point based on the bias voltage control signal and produce a modulated signal;
a detector configured to produce a first error signal based on the modulated signal and the second signal when the first control input is a first value and produce a second error signal based on the modulated signal and the second signal when the first control input is a second value; and
a processor configured to produce the bias point voltage based on the first error signal and second error signal.
2. The bias voltage controller of claim 1 wherein:
the first signal has a zero magnitude when the first control input is a first value and has a non-zero magnitude when the first control input is a second value; and
the bias point voltage is based on a difference between the first error signal and the second error signal.
3. The bias voltage controller of claim 2 wherein the processor is configured to set the bias point voltage to a predetermined value if the difference between the first error signal and the second error signal exceeds a threshold value.
4. The bias voltage controller of claim 2 wherein the processor is configured to maintain a previously computed bias point voltage if the difference between the first error signal and the second error signal exceeds a threshold value.
5. The bias voltage controller of claim 1 wherein the second frequency is twice the first frequency.
6. A bias voltage controller for an external optical modulator comprising:
a signal generator configured to produce a first signal having a first frequency based on a first control input and a second signal having a second frequency;
a combiner configured to produce a bias voltage control signal based on the first signal and a bias point voltage;
an external optical modulator configured to operate at a bias point based on the bias voltage control signal and configured to modulate an optical carrier with a third signal to produce a modulated signal;
an optical receiver to receive the modulated signal;
a switch configured to output the modulated signal from the optical receiver or a reference voltage when a second control input is a third value or fourth value respectively;
a detector configured to receive the switch output and the second signal wherein and produce a first, second, third, and fourth error signal based on the switch output and the second signal and a first, second, third, and fourth predetermined setting of the first and second control input, respectively;
a processor configured to produce the bias point voltage based on the first error signal and second error signal; and
a optical receiver monitor configured to compute the difference between the third error signal and the fourth error signal to determine whether the difference exceeds a threshold value.
7. The bias voltage controller of claim 6 wherein:
the first signal has a zero magnitude when the first control input is a first value and has a non-zero magnitude when the first control input is a second value.
8. The bias voltage controller of claim of claim 7 wherein the detector is configured to produce a first error signal based on the switch output when the second control input is a third value and the first control input is a first value and the second signal and configured to produce a second error signal based on the switch output when the second control input is a third value and the first control input is a second value and the second signal and configured to produce a third error signal based on the switch output when the second control input is a fourth value and the first control input is a first value and the second signal and configured to produce a fourth error signal based on the switch output when the second control input is a third value and the first control input is a first value and the second signal.
9. The bias voltage controller of claim 6 wherein the second frequency is twice the first frequency.
10. A method for controlling a bias point voltage for an external optical modulator, the method comprising:
producing a first signal having a first frequency based on a first control input;
producing a second signal having a second frequency;
producing a bias voltage control signal based on the first signal and a bias point voltage;
receiving a modulated signal from an external optical modulator wherein the external optical modulator operates at a bias point based on the bias voltage control signal;
producing a first error signal based on the modulated signal and the second signal when the first control input is a first value;
producing a second error signal based on the modulated signal and the second signal when the first control input is a second value; and
producing the bias point voltage based on the first error signal and second error signal.
11. The method of claim 10 wherein:
the first signal has a zero magnitude when the first control input is a first value and has a non-zero magnitude when the first control input is a second value; and
the bias point voltage is based on a difference between the first error signal and the second error signal.
12. The method of claim 11 further comprising setting the bias point voltage to a predetermined value if the difference between the first error signal and the second error signal exceeds a threshold value.
13. The method of claim 10 wherein the second frequency is twice the first frequency.
14. The method of claim 10 further comprising:
producing a third error signal based on a reference voltage and the second signal;
producing a fourth error signal based on the modulated signal when the first control input is a first value and second signal; and
computing the difference between the first error signal and the second error signal.