Coherent light generators – Particular component circuitry – Having fault protection circuitry
Reexamination Certificate
2002-08-05
2004-09-14
Wong, Don (Department: 2828)
Coherent light generators
Particular component circuitry
Having fault protection circuitry
C372S038020, C372S038070
Reexamination Certificate
active
06792020
ABSTRACT:
BACKGROUND
A primary concern for an opto-electrical transmitter such as a vertical cavity surface emitting laser (VCSEL) is control of the light intensity entering an optical fiber from the opto-electrical transmitter. A driver for the opto-electrical transmitter typically controls a drive current to the opto-electrical transmitter to control the emitted light intensity. The drive current generally has a level set according to a data signal and the performance characteristics of the opto-electrical transmitter, but for safety, the driver current should be limited so that the light intensity entering the optical fiber does not exceed eye safety levels.
FIG. 1
shows a conventional driver
100
that controls the output power level of a transmitter
110
. As illustrated in
FIG. 1
, driver
100
provides a current having two components Ibias and Imod. Current Imod is modulated between 0 and a maximum current IMOD according to the logic value of a data signal DATA and maintains an average value of IMOD/2 under normal operating conditions. Current Ibias controls the current level for a logic value 0 (i.e., when Imod=0) and contributes to the average current level (Ibias+IMOD/2), which controls the average emitted light intensity and has a level controlled by an analog feedback loop in driver
100
.
For the feedback loop, driver
100
includes a monitor diode
120
, an amplifier
130
, and a current source
140
. Monitor diode
120
is a photodiode that generates a voltage Vmon according to an intercepted portion of the light emitted from transmitter
110
. Monitor diode
120
is a slow device (relative to variations in a data signal DATA) so that voltage Vmon is directly related to the average intensity of the light emitted from transmitter
110
. Alternatively, monitor diode
120
could comprise a “fast ” device with the addition of a low-pass-filter such that Vmon is related to the average intensity of the light emitted from transmitter
110
.
Amplifier
130
is a differential amplifier having a negative input terminal coupled to monitor diode
120
, a positive terminal connected to receive a reference voltage REF, and an output terminal coupled to control the bias current Ibias through current source
140
. Accordingly, amplifier
130
is in a feedback loop that limits the average intensity of light emitted from transmitter
110
. In particular, when the light intensity increases, monitor voltage Vmon increases causing amplifier
130
to reduce bias voltage Vbias and causing current source
140
to reduce bias current Ibias. When the light intensity decreases, monitor voltage Vmon decreases causing amplifier
130
to increase bias voltage Vbias and causing current source
140
to increase bias current Ibias.
The feedback loop drives bias current Ibias towards an equilibrium level that depends on reference voltage REF. In turn, the average intensity of emitted light depends on the current Ibias+IMOD/2 and the performance of transmitter
110
in converting current to emitted light. In the embodiment of
FIG. 1
, a calibration process for transmitter
110
selects the resistance of a resistor
132
and thereby selects reference voltage REF according to the performance of transmitter
110
and monitor diode
120
. The setting of the resistance
132
compensates for permanent or structural variations between opto-electrical transmitters such as a variation in VCSEL efficiency and the feedback loop can compensate for temporary variations in the operation of transmitter
110
.
Driver
100
also requires a mechanism to shut down transmitter
110
in the event of a permanent unsafe condition. A permanent unsafe condition can arise, for example, when a short in monitor diode
120
to ground causes the feedback loop to increase the drive current so that the average emitted intensity remains above the eye safety level.
Driver
100
includes a mechanism to shut down transmitter
110
if monitor voltage Vmon rises to a level indicating that the output power of light from transmitter
110
is unsafe. In particular, a differential amplifier
150
compares monitor voltage Vmon to a maximum voltage MAX. A resistor
152
has a resistance selected to set maximum voltage MAX at the appropriate level according to the fraction of the light monitor diode
120
receives and an eye safety level for the total intensity. If monitor voltage Vmon rises above maximum voltage MAX, the output voltage from amplifier
150
sets a latch
154
, which in turn shuts off the currents Imod and Ibias via switches
160
and
161
such that no current flows to transmitter
110
.
A disadvantage of driver
100
is that driver
100
shuts down and becomes inoperable as soon as the voltage Vmon from monitor diode
120
rises above voltage MAX. However, if the unsafe condition is transient, e.g., if transmitter
100
is functioning properly but some external transient effect caused the laser power to temporarily rise above the eye safety level, driver
100
shuts down, breaking any communication link through transmitter
110
. Generally, it would be desirable not to assert a fault signal so that the communication link can remain intact, as a transient high light-output-level is eye-safe as long as the transient time is short.
Another disadvantage of driver
100
is the requirement of analog components such as resistors
132
and
152
that must be calibrated according to the specific performance of transmitter
110
. The analog components are difficult to fabricate in a small device package. Additionally, the analog time constant may make such analog circuits unable to meet all timing requirements.
A digital system can overcome many of the drawbacks of analog drive circuits. U.S. pat. No. 5,019,769 describes a digital laser drive system that uses a digital data processor. While this digital system avoids many of the drawbacks of analog drivers, the requirement of a digital data processor increases the complexity and cost of the driver. Accordingly, a digital driver circuit is sought that avoids the drawbacks of analog driver circuits and distinguishes between permanent and temporary unsafe conditions but does not require the cost or complexity of a digital data processor.
SUMMARY
In accordance with an aspect of the invention, an opto-electrical transmitter such as a VCSEL has a driver with a digital feedback loop and digital fault detection. The fault detection allows the driver and opto-electrical transmitter to continue operating in the event where the average light intensity exceeds the maximum eye-safe level allowable for continuous exposure. Henceforth, this maximum allowed continuous exposure level will be referred to as CESL, for continuous emission safe level. The digital feedback loop includes an up/down counter having an output count that controls the bias current for the opto-electrical transmitter. In response to a clock signal, the counter counts up or down if a monitor current from a monitor diode indicates the average emitted light intensity is less than or greater than a desired intensity. If the count reaches a maximum (overflow) or minimum (underflow) value, a fault condition is detected.
Additional fault detection circuitry in the driver includes a second counter. The second counter counts up if the monitor current indicates the average power of the emitted light is outside a target range and counts down or resets if the monitor current indicates the average emitted light intensity is in the target range. If the count from the second counter reaches a trigger level a fault is detected. Accordingly, if the laser power temporarily exceeds the CESL but returns to a level below the CESL before the second count reaches the fault trigger, the fault signal is not activated and the driver can continue to operate. But, if the laser power is persistently outside the CESL, the second count will reach the fault threshold, and the fault signal is activated.
One specific embodiment of the invention is a driver circuit for an opto-electrical transmitter. The driver circuit includes a monitor diode, a coun
Agilent Technologie,s Inc.
Menefee James
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