Coherent light generators – Particular component circuitry – Having fault protection circuitry
Reexamination Certificate
1999-03-23
2001-05-08
Healy, Brian (Department: 2874)
Coherent light generators
Particular component circuitry
Having fault protection circuitry
C372S031000, C372S034000
Reexamination Certificate
active
06229833
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates a laser diode protecting circuit in a laser diode drive having an automatic current control circuit (ACC circuit) for performing control in such a manner that laser diode current attains a set value, as well as to a laser driving current control circuit in the above-mentioned ACC circuit. More particularly, the invention relates to a laser diode protecting circuit for protecting a laser diode by preventing an excessive emission from the laser diode when the laser diode is started up at low temperatures, as well as to a laser driving current control circuit applicable also to laser diodes of both the common-anode and common-cathode types.
A deterioration in transmission characteristics due to wavelength fluctuation (chirping) cannot be ignored in high-speed optical communications. In addition, wavelength stability is extremely important in wavelength division multiplexing. For these reasons the laser diode drive is constructed by combining an ACC circuit and an ATC (Automatic Temperature Control) circuit and control is performed in such a manner that the laser diode current will attain a constant current value and the laser diode chip temperature (laser diode temperature) a constant temperature.
FIGS. 24A
,
24
B are block diagrams illustrating optical transmitters used in digital optical communication, in which
FIG. 24A
shows an optical transmitter using a laser diode of the common-anode type, and
FIG. 24B
shows an optical transmitter using a laser diode of the common-cathode type. Numeral
1
in these Figures denotes a laser diode drive,
1
a
a common-anode laser diode and
1
b
a common-cathode laser diode. Also shown are an ACC circuit
2
, which is constituted by an operational amplifier (OP amp) for performing control in such a manner that the laser diode current attains a set current value, an ATC circuit
3
for performing control in such a manner that the laser diode temperature attains a set value, optical fibers
4
,
5
, a D-type flip-flop (D-FF)
6
for storing a data signal DATA in response to a clock CLK, and a drive circuit (DRV)
7
for a light intensity modulator (IM)
8
, which modulates light intensity in accordance with the “1”, “0” logic of the data. The laser diodes are of common-anode type and common-cathode type, the driving currents of which have different directions. The laser diode
1
a
of common-anode type (
FIG. 24A
) has its anode connected to ground, and it is required that a driving current id be expelled from the laser diode
1
a.
The laser diode
1
b
of common-cathode type (
FIG. 24B
) has its cathode connected to ground, and it is required that a driving current id be drawn in by the laser diode
1
b.
FIGS. 25A
,
25
B show examples of the ACC circuit
2
, in which
FIG. 25A
shows an ACC circuit of common-anode type, and
FIG. 25B
shows an ACC circuit of common-cathode type.
In
FIG. 25A
, the laser diode (LD) of common-anode type is indicated at
1
a.
The ACC circuit includes resistors R
1
-R
3
having resistance values r
1
-r
3
, respectively, a transistor TR
1
and a comparator (current control circuit) IC
1
constituted by an operational amplifier. The laser diode
1
a,
transistor TR
1
and resistor R
1
are serially connected and provided between ground and a negative power source −Vee. If id represents a current that flows through the laser diode
1
a,
then id•r
1
will enter the inverting input terminal of the comparator IC
1
. On the other hand, a reference voltage V
REF
, obtained by voltage division by the resistors R
2
, R
3
, enters the non-inverting input terminal of the comparator IC
1
. The ACC circuit
2
brings the laser diode current id into line with the set current value by controlling the on/off operation of the transistor TR
1
in such a manner that the terminal voltage id•r
1
across the resistor R
1
becomes equal to the reference voltage V
REF
. More specifically, the voltage V
REF
obtained by voltage division by the resistors R
2
, R
3
becomes the voltage across the resistor R
1
and a value obtained by dividing this voltage by the resistance value r
1
becomes the current id that flows through the laser diode
1
a
. In other words, the base of the transistor TR
1
is controlled by the comparator IC
1
in such a manner that the resistor R
1
will serve as a constant-current source the current value of which will be V
REF
/r
1
at all times, thereby making it possible to obtain a constant current value even when the temperature varies.
In
FIG. 25B
, the laser diode (LD) of common-cathode type is indicated at
1
b
. The ACC circuit includes resistors R
4
-R
6
having resistance values r
4
-r
6
, respectively, a transistor TR
2
and a comparator (current control circuit) IC
2
constituted by an operational amplifier. The laser diode
1
b
, transistor TR
2
and resistor R
4
are serially connected and provided between ground and a positive power source +Vcc. If id represents a current that flows through the laser diode
1
b
, then id•r
4
will enter the inverting input terminal of the comparator IC
2
. On the other hand, a reference voltage V
REF
, obtained by voltage division by the resistors R
5
, R
6
, enters the non-inverting input terminal of the comparator IC
2
. This ACC circuit brings the laser diode current id into line with the set current value by controlling the on/off operation of the transistor TR
2
in such a manner that the terminal voltage id•r
4
across the resistor R
4
becomes equal to the reference voltage V
REF
. More specifically, the voltage V
REF
obtained by voltage division by the resistors R
5
, R
6
becomes the voltage across the resistor R
4
and a value obtained by dividing this voltage by the resistance value r
4
becomes the current id that flows through the laser diode
1
b
. In other words, the base of the transistor TR
2
is controlled by the comparator IC
2
in such a manner that the resistor R
4
will serve as a constant-current source the current value of which will be V
REF
/r
4
at all times, thereby making it possible to obtain a constant current value even when the temperature varies.
FIG. 26
illustrates an example of the ATC circuit. The laser diode chip is shown at
1
a
. The ATC circuit includes a Peltier device
3
a
for heating or cooling the laser diode chip
1
a
depending upon the direction of the current, and a thermister
3
b
having a negative resistance characteristic for detecting the temperature of the laser diode chip
1
a.
The laser diode
1
a,
Peltier device
3
a
and thermister
3
b
are accommodated in a package
3
c.
The ATC circuit further includes resistors
3
d,
3
e,
PNP, NPN transistors
3
f
,
3
g
and a comparator
3
h.
A voltage Vt (which conforms to the laser diode temperature) resulting from voltage division by the thermister
3
b
and resistor
3
d
is applied to the inverting input terminal of a comparator
3
h
, and a reference voltage V
REF
is applied to the non-inverting input terminal of the comparator
3
h.
The output terminal of the comparator is connected to the bases of transistors
3
f
,
3
g
. The emitter of the PNP transistor
3
f
is connected to V+, the emitter of the NPN transistor
3
g
is connected to V−, and the collectors of these transistors are connected to the Peltier device
3
a.
When the laser diode chip is at a low temperature, the resistance of the thermister
3
b
increases, the voltage Vt decreases to establish the inequality Vt<Vref and the output of the comparator
3
h
becomes positive. As a result, the transistor
3
f
is turned off and the transistor
3
g
is turned on so that a current flows in a direction that causes the heating of the Peltier device
3
a,
thereby heating the interior of the package
3
c
and raising the temperature of the laser diode. When the temperature of the laser diode chip rises, the resistance of the thermister
3
b
decreases and the voltage Vt increases to establish the inequality Vt>Vref so that the output of the comparator
3
g
becomes negative. As a result, t
Kiyonaga Tetsuya
Misaizu Setsuo
Nagakubo Yasunori
Noda Mitsuharu
Sato Nobuaki
Fujitsu Limited
Healy Brian
Staas & Halsey , LLP
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