Agilent Technologies, Inc.

Barry John Vaughan

Charles Graeme Ritchie

Keith Everett

A driver circuit has a laser diode coupled in series between a supply voltage and a driving transistor collector electrode. The diode and transistor respectively have low input and high output impedances. Resonances are caused by the impedance mismatch and reactive components of the diode and transistor impedances. Feedback between the laser diode input and the transistor control electrode minimizes the effects of the resonances.

Driver circuit

In an optical communications system, it is necessary to control the extinction ratio of a laser device ( ) of an optical transmission unit. The laser device ( ) 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 ( ) using a photodiode ( ) in a control loop. Due to band limiting effects of the photodiode ( ) and parasitic circuit track capacitances, a feedback signal generated by the photodiode ( ) 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 ( ) 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 ( ) and hence the extinction ratio.

Control loop circuit and method therefor

A laser driver circuit includes a pair of transistors having base electrodes respectively coupled to a differential input signal. The respective collector electrodes are coupled to a first power supply rail and the respective emitter electrodes are coupled to a laser diode via respective impedances and to respective controllable current sources to provide an average modulation voltage at the more positive of the emitter electrodes. By having the transistors in an emitter follower configuration, rather than the usual collector open collector configuration, the problem of mismatch between the output impedance of the transistors and the input impedance of the laser diode is reduced.

Laser driver circuit

Known differential pair amplifier circuits can suffer from transistor saturation resulting in a reduction in switching speed. An alternative to the differential pair amplifier circuit is a common-emitter configuration, but the common-emitter configuration lacks differential operability and results in ground bounce. An amplifier circuit is provided for an output stage of a driver circuit comprising a pair of common emitter circuits cross-coupled by a pair of transistors. Slow operation is therefore overcome while providing a differential output signal in response to a differential input signal.

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Ipswich, United Kingdom

Barry John Vaughan