Control loop circuit and method therefor

• US 20030043439A1
• Filed: 09/03/2002
• Published: 03/06/2003
• Est. Priority Date: 09/03/2001
• Status: Active Grant

• Current Assignee: Avago Technologies General IP PTE Limited (Broadcom, Inc.)
• Original Assignee: Agilent Technologies, Inc.
• Inventors: Everett, Keith, Ritchie, Charles Graeme, Vaughan, Barry John
• Granted Patent: US 6,829,267 B2
• Time in Patent Office: Days
• US Class Current: 398/197
• CPC Class Codes: H04B 10/504 using direct modulation

Abstract

In an optical communications system, it is necessary to control the extinction ratio of a laser device (192) of an optical transmission unit. The laser device (192) is typically used to transmit a signal that can be approximated by a pseudo-random bit stream. The bit stream is monitored by collecting light from a back facet of the laser device (192) using a photodiode (194) in a control loop. Due to band limiting effects of the photodiode (194) and parasitic circuit track capacitances, a feedback signal generated by the photodiode (194) becomes distorted, leading to difficulties in controlling the extinction ratio. The present invention therefore provides an apparatus that simulates a signal generated by the photodiode (194) and subsequent processing of the photodiode signal. The simulated signal and the processed photodiode signal are compared so as to generate a control signal for controlling modulation amplitude of the laser device (192) and hence the extinction ratio.

Claims

1. A control loop circuit apparatus (150) for compensating modulation amplitude control of a laser device (192), the apparatus comprising:

means for receiving a first signal for transmission to a receiver;

means for receiving a second signal generated by a sensor (194) for monitoring the laser device (192), the second signal corresponding to the first signal;

means for simulating signal generation (162, 172) arranged to use the first signal to generate a simulated signal corresponding to the second signal;

a processing unit (186, 187) arranged to receive the second signal and the simulated signal, and arranged to generate an output signal for compensating the modulation amplitude control of the laser device (192), the output signal constituting an indication of a difference between the second signal and the simulated signal.

2. An apparatus as claimed in claim 1, wherein the processing unit (186, 187, 208) comprises a first normalisation unit (208) for adapting the second signal to a predetermined level and/or a predetermined format.

3. An apparatus as claimed in claim 1 or claim 2, the processing unit (186, 187, 208) comprises a second normalisation unit (186) for adapting the simulated signal to the predetermined level and/or predetermined format.

4. An apparatus as claimed in any one of claims 1 to 3, wherein the sensor (194) has a frequency band associated therewith, and the means for simulating signal generation comprises a filter (172) having substantially a same frequency band as at least the frequency band of the sensor (194).

5. An apparatus as claimed in any one of the preceding claims, wherein the means for simulating signal generation (162, 172) comprises a first amplifier (182).

6. An apparatus as claimed in any one of the preceding claims, further comprising a second amplifier (200) for amplifying the second signal.

7. An apparatus as claimed in any one of the preceding claims, wherein the processing unit (186, 187, 208) comprises a differential subtractor (187).

8. An apparatus as claimed in claim 3 when dependent upon claim 2, wherein the first normalisation unit (208) is a rectifier or a peak detector, and the second normalisation unit is a rectifier or a peak detector.

9. A transmitter circuit for an optical transceiver unit, the circuit comprising the circuit apparatus as claimed in any one of the preceding claims.

10. A method of compensating modulation amplitude control of a laser device (192), the method comprising the steps of:

receiving a first signal for transmission to a receiver;

receiving a second signal generated by a sensor (194) for monitoring the laser device (192), the second signal corresponding to the first signal;

using the first signal to generate a simulated signal corresponding to the second signal;

receiving the second signal and the simulated signal, and generating an output signal for compensating the modulation amplitude control of the laser device (192), the output signal constituting an indication of a difference between the second signal and the simulated signal.

11. A control loop circuit apparatus (150) for compensating modulation amplitude control of a laser device (192), the apparatus comprising:

a first input for receiving a first signal for transmission to a receiver;

a second input for receiving a second signal generated by a sensor (194) for monitoring the laser device (192), the second signal corresponding to the first signal;

a simulation arrangement (162, 172) for using the first input signal to generate a simulated signal corresponding to the second signal;

a processing arrangement (186, 187) for receiving the second signal and the simulated signal, and arranged to generate an output signal for compensating the modulation amplitude control of the laser device (192), the output signal constituting an indication of a difference between the second signal and the simulated signal.