Injection laser diode digital transmitter having signal quality

Abstract

An injection laser diode transmitter for a fiber optic link contains a detector that monitors transmitted optical pulses for signal quality. The transmitted signal component is extracted from the back facet of the ILD for feedback stability control and is coupled to signal quality measuring circuitry from a wideband current-to-voltage converter that is coupled to the output of a fast photo diode that extracts the optical output signal from the rear facet of the ILD for the purposes of generating a feedback control voltage to be used for ILD bias control. The bias current feeds a reference amplifier, the output of which is coupled to a lowpass filter to remove the pulse signal component. The output of the lowpass filter is a DC voltage level which is proportional to the average optical output of the ILD. The voltage level output of the lowpass filter is applied to a current sink coupled to the ILD. The bias current for the ILD is controlled to drive the differential to zero. Because the bias current control feedback loop is responsive to the optical output of the ILD, the bias current is automatically compensated for changes in temperature. The current-to-voltage converter output is further coupled to a buffer amplifier to supply an output voltage to downlink signal quality monitoring circuitry to facilitate fault isolation.