Method and system for closed loop control of an optical link
A method of closed loop control for an optical link is presented, utilizing a copper feedback connection between the optical transmitter and optical receiver, suitable for short distance applications. An architecture is provides that may be used to define and maintain an optimum optical launch power for a defined bit error rate, guaranteeing extinction ratio and absolute optimum operating power. The invention also includes the use of such a loop in achieving fast link initialization and dynamic optimization to compensate for all effects of time and temperature for all components within the link.
1. A system for providing closed loop control of an optical link, comprising:
a transmitter for generating an optical signal;
an optical fiber for transmitting the optical signal, having a first end for receiving the optical signal from the transmitter and a second end for outputting the optical signal;
a receiver for receiving the optical signal from the second end of the optical fiber;
a sensor for measuring the mean power of the optical signal received by the receiver for a predetermined desired level of the optical signal generated by the transmitter;
feedback means for providing a signal from the receiver to the transmitter in the form of a current which is proportional to the mean power of the optical signal received at the input of the receiver after transmission by the optical fiber;
a comparator for comparing the mean power of the signal received by the receiver to the predetermined desired level of the optical signal transmitted by the transmitter; and
a controller for determining the threshold voltage of the laser diode and the modulation levels to be applied to the laser diode such that the extinction ratio and signal swing of the launched power signal result in a received signal to noise ratio within the receiver which achieves a predetermined bit error rate.
2. The system of claim 1 , wherein the feedback means comprises a copper interconnect from the receiver to the transmitter.
3. A method for determining the transfer function of an optical link having a transmitter including a laser diode which generates an optical signal and a receiver which receives the optical signal, comprising:
measuring the mean power of the optical signal received by the receiver for a plurality of predetermined desired levels of the optical signal generated by the laser diode;
providing a feedback signal from the receiver to the transmitter in the form of a current which is proportional to the mean power of the optical signal received by the receiver;
comparing the mean power of the signal received by the receiver to the plurality of predetermined desired levels of the optical signal; and
determining the threshold of the laser diode and the modulation levels to be applied to the laser diode such that the Extinction Ratio and signal swing of the launched power signal result in a received signal to noise ratio within the receiver which achieves a predetermined Bit Error Rate.
4. The method of claim 3 , wherein comparing the mean power of the signal received by the receiver to the plurality of predetermined desired levels of the optical signal comprises:
feeding back a current from the receiver which is proportional to the current received at the input of the receiver; and
comparing the current fed back from the receiver to a plurality of reference currents which are proportional to the desired levels of the optical signal.
5. A method of operating an optical link having a transmitter including a laser diode which generates an optical signal and a receiver which receives the optical signal, comprising:
performing a calibration of the optical link with no data signal present; and
determining the transfer function of the optical link by:
measuring the mean power of the optical signal received by the receiver for a plurality of predetermined desired levels of the optical signal generated by the laser diode;
providing a feedback signal from the receiver to the transmitter in the form of a current which is proportional to the mean power of the optical signal received by the receiver;
comparing the mean power of the signal received by the receiver to the plurality of predetermined desired levels of the optical signal; and
determining the threshold of the laser diode and the modulation levels to be applied to the laser diode such that the Extinction Ratio and signal swing of the launched power signal result in a received signal to noise ratio within the receiver which achieves a predetermined Bit Error Rate.
6. The method of claim 5 , wherein performing a calibration of the optical link with no data signal present further comprises forming an analog closed loop of direct current levels.
7. The method of claim 5 , wherein comparing the mean power of the signal received by the receiver to the plurality of predetermined desired levels of the optical signal comprises:
feeding back a current from the receiver which is proportional to the current received at the input of the receiver; and
comparing the current fed back from the receiver to a plurality of reference currents which are proportional to the desired levels of the optical signal.
8. The method of claim 7 , wherein the level of the optical signal is determined by a drive voltage applied to the laser diode, and further comprising performing an additional calibration with a data signal present by:
trimming any differences between the current fed back from the receiver and the plurality of reference currents;
slowly shifting the level of the optical signal by increasing the drive voltage of the laser diode from a first level corresponding to a minimum desired level of the optical signal to a second level corresponding to a maximum desired level of the optical signal;
comparing the current fed back from the receiver to the first one and second one of the plurality of reference currents; and
if necessary, adjusting the first one and second one of the plurality of reference currents to correspond to the minimum and maximum desired levels of the optical signal.
9. The system of claim 1 , wherein the feedback current is mirrored from a direct current restore loop contained in the receiver.
10. The system of claim 9 , wherein the direct current restore loop is operated at a very low frequency to minimize low frequency jitter.