Agilent Technologies, Inc.

Avago Technologies General IP Pte, Ltd.

Charles Graeme Ritchie

Peter Henry Mahowald

Michael Allen Robinson

Clifford Neil Didcock

Michael Geoffrey Andrew Wilson

Barry John Vaughan

Keith Everett

Charles Ronald Cook

In one aspect, the invention features a fiber optic receiver that includes a preamplifier circuit incorporated together with an opto-electronic transducer in a receiver optical sub-assembly (ROSA), and an adjustable bandwidth post-amplifier that is located outside the ROSA to allow the overall size of the receiver package to be reduced. The ROSA is mounted on a substrate and is fitted with a fiber optic connector for coupling to a mating connector of a fiber optic cable. The opto-electronic transducer is incorporated within the ROSA and is configured to generate an electrical data signal in response to a received optical data signal. The preamplifier circuit is incorporated within the ROSA and is operable to linearly amplify an electrical data signal generated by the opto-electronic transducer. The adjustable bandwidth post-amplifier circuit is mounted on the substrate and is coupled to an output of the preamplifier circuit.

Fiber optic receiver with an adjustable bandwidth post-amplifier

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 DC feedback control circuit for compensating for a DC voltage level shift in a transimpedance amplifier circuit having a transistor as a high speed switching device. The DC feedback control circuit includes a filter for determining a DC voltage level at an output of the amplifier circuit and a differential pair of transistors for comparing the DC voltage level with a reference voltage. A pair of current mirrors mirror a current dependent on the comparison of the DC voltage level with the reference voltage and apply the mirrored current to an emitter of the transistor so as to maintain a substantially constant bias current through the switching transistor.

DC feedback control circuit

A variable gain amplifier stage has first and second input terminals for receiving a differential input signal and first and second emitter follower transistors having their base electrodes coupled to the input terminals and their emitter electrodes coupled to a pair of matched voltage controlled current sources. A differential pair of transistors has their base electrodes coupled to the emitter electrodes of the first and second emitter follower transistors and their emitter electrodes coupled together and to a third voltage controlled current source. The voltage controlled current sources are controlled by the same voltage source so that a decrease in the differential pair tail current increases the bandwidth and decreases the gain of the stage, but a decrease in the current through the emitter follower transistors decreases the bandwidth of the stage so that by carefully controlling the relationship between these currents, a constant bandwidth can be maintained.

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Ipswich, United Kingdom

Charles Graeme Ritchie