Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
2001-10-11
2004-05-18
Ip, Paul (Department: 2828)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S038030
Reexamination Certificate
active
06738401
ABSTRACT:
BACKGROUND OF THE INVENTION
Laser diodes are typically used to transmit data information over fiber optic networks. To achieve higher speed data rates, a laser diode can be biased with a drive current so it is ‘ON’ and produces at least a minimal optical output. While the diode is biased, the diode can be driven with additional current so that the light output of the diode varies over time between two power output levels. One power output level of the diode can represent a logic low while another power output level of the diode can represent a logic high.
SUMMARY OF THE INVENTION
One aspect of the present invention is directed towards a system and method for providing high speed switching to transmit data information. This can be achieved by switching current through a diode or alternate path rather than adjusting a setting of a current source between 0.0 mA (to turn off the diode) and I
BIAS
(to bias the diode). Complex circuitry often used in precision current sources ordinarily do not lend themselves to high speed adjustments that would be necessary to rapidly shut off, bias or modulate a diode for transmitting.
In an illustrative embodiment, a current is switched through a diode or shunted through an alternate path to selectively bias a diode for transmitting. When the current is switched through the diode, the diode can be biased for generating a light output. A second current can be selectively switched through the diode to modulate a light energy output transmitted from the diode. Based on this configuration, fast bias-switching for generating a modulated laser diode output can be achieved by selectively switching the bias current through the diode or alternate path. Consequently, a solid state device can be controlled to transmit data information after a relatively minimal setup time delay to bias the diode.
A light output of the diode can be coupled to a fiber to transmit information over an optical network such as a passive optical network or cable television (CATV). In such applications, hi-speed biasing of the diode or multiple diodes can be used to more efficiently transmit information via time division multiplexing. That is, one of multiple diodes can be selectively activated in minimal time to transmit a modulated signal over a fiber network shared by multiple transmitters. The other diodes not transmitting can be almost completely shut down so that they do not interfere with the activated diode.
A resistor can be disposed in series with the diode to dissipate energy and reduce chirping of the circuit during the “switch over” process of biasing the diode. In one application, the resistor is greater than 20 ohms so that the power output of the diode makes a smooth transition when it is switched or modulated. This resistor can be split into two separate resistors, one in each of the modulation and bias current paths.
A current switched to bias or modulate the diode also can be adjusted to compensate for aging of the diode. For example, when a light energy output of the diode decays due to aging or the system changes temperature, the bias or modulation current can be increased or decreased so that the diode still generates a bias and modulation output at particular power levels.
One method for selectively switching the bias current involves disposing a first switch in series with the diode and a second switch along an alternate path so that the bias current can be switched either through the diode or the alternate path depending on which of the two switches is activated. In such an application, switching can be controlled so that one switch has a lower impedance than the other. Accordingly, the current can be directed through the switch (and path) having the lower impedance.
In one application, the first and second switches are driven with a differential signal to selectively bias the diode. More specifically, part of the differential signal can be used to control one switch while another part of the differential signal can be used to control the other switch. When the differential signal is binary, one of the pair of switches is typically active except during a transition period between changing states. This transition period can be minimal when hi-speed differential drivers are used to control a state of the switches. In this instance, the bias current is generally either selectively switched to bias the diode or is shunted through an alternate path. To provide fast switching times, bipolar transistor switches can be driven with a differential signal based on ECL (Emitter Coupled Logic) or other suitable voltage levels.
In a similar manner as discussed for the bias current, the modulation current can be switched through the diode or another alternate path using a pair of switches such as bipolar transistors driven by a differential voltage signal to modulate the diode for transmitting.
To ensure that the bias current is set properly, a power level of light transmitted by the diode can be measured while the diode is biased. Based on a measured output of the diode, feedback can be used to adjust the bias current so that the diode produces a light output within a specified range when biased.
The diode also can be calibrated for storing setting information in memory. Typically, a bias and modulation current are selected so that a receiver of the optical signal generated by the diode can detect corresponding logic levels of each data bit transmitted in a packet or time frame.
The previously discussed aspects of the present invention have advantages over the prior art. For example, one application of the present invention involves transmitting data information from one of multiple laser diodes to a target device in a shared network. Since the laser diode can be biased to transmit data information in minimal time, the bandwidth of the shared network can be increased.
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Bowler David B.
Colella Barry D.
Dill, III Colby
Pratt Bruce C.
Hamilton Brook Smith & Reynolds P.C.
Nguyen Phillip
Quantum Bridge Communications Inc.
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