Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
1999-09-07
2002-07-02
Font, Frank G. (Department: 2877)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S008000, C372S029010, C372S029015, C250S205000, C359S199200
Reexamination Certificate
active
06414974
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and to a control circuit for controlling the extinction ratio of a laser diode, and in particular, though not limited to a method and to a control circuit for controlling the extinction ratio of a laser diode of the type for use in optical transmission of digital data, for example, in telecommunications. The invention may also relate to a method and to a control circuit for controlling the average power output of a laser diode, and to a driver circuit for driving a laser diode.
BACKGROUND OF THE INVENTION
Laser diodes are extensively used in optical transmission systems as light emitters for transmitting data in digital form over telecommunications networks. Such laser diodes have an optical power output/current input characteristic curve, which has an initial inactive linear portion which is almost horizontal with a very slight upward slope. The curve then has a knee portion which is generally called the threshold point at which stimulated light emission commences. The characteristic curve then continues as a relatively linear portion with a relatively steep slope. This, is referred to as the linear operating portion of the slope. A bias current is applied to the laser diode of value sufficient to maintain a predetermined average optical power output. A switched modulation current is applied to the laser diode to establish a predetermined extinction ratio.
However, the power/current characteristic curve tends to vary with temperature and also varies as the laser diode ages. Three typical power/current characteristic curves of a laser diode, namely, curves A, B and C are illustrated in FIG.
1
. The curves A, B and C illustrate how the power/current characteristic curves vary with temperature. The curve A illustrates the power/current characteristic curve of the laser diode when operating at 0° C. The curve B illustrates the characteristic curve of the laser diode operating at 25° C., while the curve C illustrates the characteristic curve of the laser diode operating at 70° C. An operating range of 0° C. to 70° C. is not an unusual operating temperature range for a laser diode. In each curve A, B and C the inactive portion is illustrated by the letter e. The threshold point of the curve is illustrated by he letter f, while the operating portion of each curve is illustrated by the letter g.
The input current in milliamps (mA) to the laser diode is plotted on the X axis of the graph of
FIG. 1
while the optical power output in milliwatts (mW) of the laser diode is plotted on the Y axis. The bias current to the laser diode is indicated as I
b,
while the modulation current is indicated as I
m
, The power output of the laser diode when the bias current I
b
and the modulation current I
m
are applied to the laser diode is indicated as P
1
while the power output of the laser diode when the only current applied to the laser diode is the bias current I
b
is indicated as P
0
. The average power output of the laser diode is indicated as P
av
which is equal to half the sum of P
1
and P
0
, assuming an equal number of digital ones and zeros in the data stream. In order for the laser diode to operate efficiently, the bias current should be sufficient to operate the laser diode in the linear operating portion of the power/current characteristic curve just above the threshold point, in other words, the point h of the curve B, for example. Thereby the bias current operates the laser diode to produce a power output of P
0
. In this way, when the modulation current I
m
is applied to the laser diode on top of the bias current I
b
the laser diode operates in the linear operating portion of the curve, namely, between the point h and the point k on the curve B. By operating the laser diode so that the power output varies between the points h and k on the characteristic curve B in response to the modulation current the laser diode operates with the optimum extinction ratio, which is the ratio of the power output P
1
to the power output P
0
.
However, it will be clear from the curves A, B and C of
FIG. 1
that should the operating temperature of the laser diode vary, unless the bias current I
b
and the modulation current I
m
are varied to compensate for the change in operating temperature the laser diode will operate incorrectly. For example, if the bias current I
b
and the modulation current I
m
were set to operate the laser diode at an operating temperature of 25° C., an increase in the operating temperature would immediately cause the extinction ratio of the laser diode to drop, and also would result in a reduction in the average power output P
av
of the laser diode.
Accordingly, in order for a laser diode to provide an adequate extinction ratio over its life and over a typical range of operating temperatures, control circuitry is required for altering the bias current I
b
and the modulation I
m
to compensate for changes in operating temperature and as the laser diode ages. Typical control circuits which are known monitor the operating temperature of the laser diode, and alter the bias current and/or the modulation current in response to temperature change of the laser diode. A disadvantage of such circuits is that they tend to be inaccurate. They do not measure the extinction ratio directly. Measuring temperature gives only an indirect measure of the extinction ratio, and obviously, is not particularly accurate, since the extinction ratio as discussed may, in general, drift with age. Thus, any corrections made to correct the extinction ratio based on the operating temperature of the laser diode may be incorrect, thus leading to incorrect operation of the laser diode.
There is therefore a need for a method and a control circuit for controlling the extinction ratio of a laser diode.
SUMMARY OF THE INVENTION
According to the invention there is provided a method for controlling the extinction ratio of a laser diode, the method comprising the steps of
comparing a value of the power/current characteristic curve of the laser diode indicative of the slope of the operating portion of the power/current characteristic curve with a predetermined corresponding reference slope value, and
altering the modulation current to the laser diode in response to the slope value not comparing favourably with the reference slope value.
In one embodiment of the invention the slope value of the power/current characteristic curve is determined by altering the modulation current by a predetermined test amount, and determining the corresponding change in the average power output of the laser diode resulting from the alteration to the modulation current by the test amount.
Preferably, the predetermined test amount by which the modulation current is altered is of magnitude sufficiently small as not to affect normal operation of the laser diode.
Advantageously, the predetermined test amount by which the modulation current is altered is proportional to the modulation current when the modulation current is being altered. Ideally, the modulation current is increased by the predetermined test amount.
In one embodiment of the invention the predetermined test amount by which the modulation current is altered does not exceed 5% of the value of the modulation current when the modulation current is being altered, and preferably, the predetermined test amount by which the modulation current is altered does not exceed 1% of the value of the modulation current when the modulation current is being altered.
In another embodiment of the invention the modulation current to the laser diode is altered by a predetermined correcting amount in response to the slope value of the power/current characteristic curve not comparing favourably with the reference slope value. Advantageously, the correcting amount by which the modulation current is altered in response to the slope not comparing favourably with the reference slope value does not exceed 1% of the maximum value of the modulation current which may be applied to the laser diode.
In one embodiment of the invention th
Real Peter
Russell Brian Keith
Analog Devices Inc.
Rodriguez Armando
Wolf Greenfield & Sacks P.C.
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