Analog Devices,inc.

Brian Keith Russell

Sean T Morley

Peter Real

A method and circuit for measuring the optical modulation amplitude in the operating region of a laser diode is described. The method utilises two measurements of OMA, each measurement being related to the slope in a specific portion of the operating region of the power/current characteristic curve of the laser diode. By combining the two measurement values, the invention provides a 1 measurement for OMA in the operating region of the laser diode that allows for the presence of a non-linear response in the region.

Method and circuit for measuring the optical modulation amplitude (OMA) in the operating region of a laser diode

A control circuit ( ) controls the operation of a laser diode ( ) for controlling the average power output (P ) and the extinction ratio. A state machine ( ) controls the control circuit ( ) which reads the current from a monitor photo diode ( ) which is coupled to the laser diode ( ). An amplifier ( ) determines the average power output of the laser diode ( ) which is fed to a first comparator ( ). The first comparator ( ) compares the average power output with a reference value set by a resistor (R ). The output from the comparator ( ) is fed to the up/down pin of a first counter ( ) which is clocked by the state machine ( ). In the event that the average power output is too high the first counter ( ) decreases the bias current to the laser diode ( ) outputted by a constant current source ( ), and vice versa. To determine the extinction ratio of the laser diode ( ) the slope of the operating portion of the power/current characteristic curve is determined by applying an incremental increase (&Dgr;I ) to the modulation current (I ) of the laser diode ( ), which is a small percentage of the modulation current (I ). The increase (&Dgr;P ) to the average power output (P ) of the laser diode ( ) resulting from the increase &Dgr;I to the modulation current I is determined through a latch ( ) and an adder ( ) and is fed to a second comparator ( ) which compares the increase &Dgr;P in the average power output with a reference value (&Dgr;P ) set by the resistor (R ). A second up/down counter ( ) increases the modulation current (I ) applied to the laser diode ( ) by a constant current source ( ) if the value of (&Dgr;P ) is less than (&Dgr;P ) and vice versa.

Method and a control circuit for controlling the extinction ratio of a laser diode

A resistive DAC ( ) comprises a digital input port ( ) and an analog output port ( ) on which analog resistance output values are outputted in response to corresponding digital input codes on the input port ( ). A decoding and control circuit ( ) selects appropriate resistors (R ) to (RN) from a resistor chain ( ) for providing the analog resistance output of the analog output port ( ). A register ( ) stores a transfer coefficient in binary code which can be read through the input port ( ) and by which each digital input code should be multiplied in order to produce an analog resistance output of predetermined value. The transfer coefficient in the register ( ) takes account of variations in internal circuit parameters which causes the analog resistance outputs on the output port ( ) to be less than they would in an ideal resistive DAC.

Digital-to-analog converter and a method for facilitating outputting of an analog output of predetermined value from the digital-to-analog converter in response to a digital input code

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Limerick, Ireland

Brian Keith Russell