Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
2002-10-30
2004-04-27
Ip, Paul (Department: 2828)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S038030
Reexamination Certificate
active
06728276
ABSTRACT:
BRIEF DESCRIPTION OF THE INVENTION
The present invention relates generally to semiconductor lasers, and particularly to operational amplifiers configured to switch semiconductor lasers on and off.
BACKGROUND OF THE INVENTION
Passive optical networks enable a plurality of optoelectronic transceivers to share one or more optical fibers while transmitting and receiving data in an optical form. Typically, passive optical networks employ a time division multiplexing access (TDMA) scheme to make this possible. In such schemes, the data transmission capabilities of the plurality of optoelectronic transceivers are operational only during separate, non-overlapping periods of time.
When the turn-on and turn-off times of the optoelectronic transceivers decrease, the amount of time available to each optoelectronic transceiver in a passive optical network to transmit optical data increases. Prior art optoelectronic transceivers are able to turn a laser diode on and off within 100 microseconds to 1 millisecond.
Persons skilled in the art, moreover, recognize that turning a laser diode on and off is a time consuming aspect of turning an optoelectronic transceiver on and off. Passive optical networks, therefore, require laser diodes to be turned on and off quickly to make efficient use of network bandwidth.
A laser diode is typically embedded in a feedback loop of an optoelectronic transceiver. This feedback loop turns the laser diode on, and then maintains the laser diode in a linear operating range so that it is able to transmit data efficiently. Maintaining the operational efficiency of the laser diode includes adjustments to the output of an operational amplifier, which is a portion of the feedback loop. Persons skilled in the art recognize that the optical output power of a given laser diode may fluctuate in ways that are inconsistent with electrical input that is intended to modulate the optical output of the laser diode. The purpose of the feedback loop is to counteract these unwanted fluctuations.
In particular, the operational amplifier produces a bias current to maintain the operational efficiency of the laser diode. However, feedback loops (e.g., operational amplifiers) with a large bandwidth tend to null out the electrical input that is intended to modulate the optical output of a corresponding laser diode. This is so because the bandwidth of a feedback loop may overlap some or all of the bandwidth of the electrical input. This is problematic in the context of passive optical networks because feedback loops with a large bandwidth are ideal for turning a laser diode on and off quickly (and thus enable optoelectronic transceivers in a passive optical network to transmit more optical data or the inclusion of additional optoelectronic transceivers in the passive optical network). More specifically, feedback loops with a large bandwidth are able to modulate the optical output strength of a given laser diode at a relatively high frequency.
What is needed in the art, therefore, is an optoelectronic transceiver capable of turning a laser diode on and off within 0.1 to 1.0 microseconds that does not adversely affect electrical input.
SUMMARY OF THE INVENTION
An embodiment of the present invention is directed to a three stage operational amplifier for controlling bias current in a laser diode. The first stage includes a differential amplifier configured to receive as input a reference voltage and a laser diode voltage. The laser diode voltage represents an optical output strength of a laser diode and the reference voltage corresponds to a desired magnitude of the laser diode voltage. A second stage includes a capacitor and is configured to integrate an output current produced by the first stage to generate a first output voltage. A third stage includes an output buffer configured to receive as input the first output voltage to generate a second output voltage that is approximately equal to the first output voltage. The second output voltage is applied to a voltage controlled current source to control the magnitude of a bias current for the laser diode. The differential amplifier of the first stage if formed from a symmetrical assembly of transistors such that a transconductance of the differential amplifier approaches a constant when a difference between the desired magnitude of the laser diode voltage and the actual laser diode voltage is substantially zero volts and increases exponentially as this difference increases.
Another embodiment of the present invention is directed to a three stage operational amplifier for controlling bias current in a laser diode. The first stage includes a differential amplifier configured to receive as input a reference voltage and a laser diode voltage. The laser diode voltage represents an optical output strength of a laser diode and the reference voltage corresponds to a desired magnitude of the laser diode voltage. The second stage includes a capacitor configured to integrate an output current produced by the first stage to produce a first output voltage. The third stage includes an output buffer configured to receive as input the first output voltage to produce a second output voltage that is approximately equal to the first output voltage. The second output voltage is applied to a voltage controlled current source to control the magnitude of a bias current for the laser diode. The operational amplifier also includes a voltage comparator to compare the laser diode voltage to the reference voltage. The voltage comparator directs a boosting current from a current source to the second stage when a difference between the laser diode voltage and the reference voltage is greater than or equal to a predefined amount.
Yet another embodiment of the present invention is directed to a three stage operational amplifier. The first stage includes a differential amplifier configured to receive as input a reference voltage and a laser diode voltage. The laser diode voltage represents an optical output strength of a laser diode and the reference voltage corresponds to a desired magnitude of the laser diode voltage. The second stage has a plurality of stages. Each of the stages includes at least a capacitor for integrating an output current produced by the first stage to produce a first output voltage. The third stage includes an output buffer configured to receive as input the first output voltage to produce a second output voltage that is approximately equal to the first output voltage. The second output voltage is applied to a voltage controlled current source to control the magnitude of a bias current for the laser diode. The operational amplifier also includes a voltage comparator to compare the laser diode voltage to the reference voltage. The voltage comparator selects one of the stages from the second stage by reference to a relationship between the laser diode voltage and the reference voltage.
In still other embodiments, the present invention includes a plurality of optoelectronic transceivers, a coordinator, a controller, an optical combiner, and a shared communication line in a passive optical network. The coordinator is configured to assign each of the plurality of optoelectronic transceivers to a separate portion of a cyclical time period. The controller is configured to turn optical data transmit capabilities of the plurality of optoelectronic transceivers on and off during their respective separate portions of the cyclical time period. The optical combiner is configured to relay optical data received from the plurality of optoelectronic transceivers to the shared communication line. And each of the plurality of optoelectronic transceivers includes an operational amplifier consistent with one of the embodiments described in the preceding paragraphs.
REFERENCES:
patent: 5726965 (1998-03-01), Hajjar et al.
patent: 6345062 (2002-02-01), Taguchi et al.
Case Daniel K.
Nguyen The′ Linh
Shapiro Philip D.
Finisar Corporation
Ip Paul
Vy Hung
Workman Nydegger
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