Coherent light generators – Particular component circuitry
Reexamination Certificate
2000-07-07
2003-01-07
Ip, Paul (Department: 2828)
Coherent light generators
Particular component circuitry
C372S038020
Reexamination Certificate
active
06504857
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a circuit for detecting deterioration of a laser diode, and particularly to a circuit that detects deterioration of the laser diode based on change of differential efficiency thereof.
In devices employing laser diodes as a light source, it is necessary to monitor operational status of the laser diodes and to detect deterioration thereof in order to ensure the operation of the devices. That is, if the laser diode is deteriorated, even if a predetermined current flows in the laser diode, the optical output power of the laser diode is lower than the designed value.
The main reasons why the laser diode is deteriorated are variation of semiconductor junctions with time, electrostatic destruction of the semiconductor structure, and the like.
A relationship of current to optical output of a laser diode is shown in FIG.
4
A. In
FIG. 4A
, a curve S
1
represents a characteristic of a normal (i.e., not deteriorated) laser diode, S
2
and S
3
represent characteristics of deteriorated ones. As shown in
FIG. 4A
, when the laser diode is deteriorated, the threshold Ith increases, and differential efficiency (i.e., a ratio of current to optical output power when the current is greater than the threshold Ith) decreases. It should be noted that for stimulated emission, the laser diode should be driven with the current greater than the threshold Ith. In
FIG. 4A
, thresholds for the characteristics S
1
-S
3
are indicated by Ith(S
1
), Ith(S
2
) and Ith(S
3
), respectively.
The deterioration may be detected based on the threshold Ith. However, the threshold Ith varies depending on the temperature around the laser diode as shown in FIG.
4
B. Therefore, the deterioration cannot be detected only by the value of the threshold Ith.
Accordingly, there has been suggested an LD (Laser Diode) deterioration detecting circuit based on differential efficiency thereof. As shown in
FIG. 4B
, although the threshold Ith varies depending on the temperature, the differential efficiency (i.e., &Dgr;P/&Dgr;iL) remains substantially the same regardless of the temperature. While, as shown in
FIG. 4A
, as the laser diode is deteriorated, the differential efficiency becomes lower. Therefore, based on the differential efficiency, the deterioration of the laser diode can be detected regardless of the temperature.
FIG. 5
shows a block diagram of an example of a conventional LD deterioration detection circuit. The circuit shown in
FIG. 5
is a redrawing of a circuit disclosed in Japanese Patent Publication HEI8-4163. In this LD deterioration detection circuit, a laser beam emitted by a laser diode
201
is received by a photo diode
202
, which generates a photocurrent ID corresponding to the intensity of the received laser beam. An I-V (current-to-voltage) converter
203
converts the photocurrent ID into a voltage VL, which is input to an inverted input terminal of a differential amplifier
204
. The differential amplifier
204
outputs an amplified value of a difference between the voltage VL and a reference voltage VO generated by a reference voltage generator
205
. The output of the differential amplifier
204
is input to a hold circuit
206
, output of which controls a drive current supply circuit
207
to output a driving current IX for driving the laser diode
201
.
When a deterioration detection command signal E is OFF (not issued), the circuit performs an APC (Automatic Power Control) process, and the current IX is controlled such that the voltage VL output by the I-V converter
203
is equal to the reference voltage VO.
When the detection command signal E is ON (issued), the circuit performs the detection of deterioration of the laser diode
201
. That is, when the command signal E is ON, a switch
208
of the hold circuit
206
is switched such that the output of the differential amplifier
204
is not transmitted to the drive current supply circuit
207
. The hold circuit
206
holds the output voltage of the differential amplifier
204
just before the switch
206
is turned OFF with a capacitor
213
, thereby the driving current IX is maintained as a fixed value. Further, when the command signal E is ON, a switch
209
is switched so that a predetermined current &Dgr;I from a biasing circuit
210
is added to the drive current IX and then supplied to the laser diode
201
.
A deterioration determining circuit
211
, which includes a comparison circuit and an inverting circuit, compares the voltage VL output by the I-V converter
203
with a deterioration detection reference voltage VT for determining whether the laser diode
201
is deteriorated. The comparison result of the deterioration determining circuit
211
and the command signal E are applied to an AND gate
212
, which outputs a deterioration detection signal Sout.
In this conventional circuit, in accordance with the detection command E (when the command E is ON), the current &Dgr;l from the biasing circuit
210
is added to the driving current IX, which flows in the laser diode
201
, thereby output power of the laser diode
201
temporarily increases. The output photocurrent ID of the photo diode
202
increases, and accordingly, the output voltage VL of the I-V converter
203
also increases. The deterioration determining circuit
211
compares the output voltage VL with the reference voltage VT. If the voltage VL is greater than the reference voltage VT, it is determined that the differential efficiency of the laser diode
201
is greater than a predetermined value, and thus, the deterioration detection signal Sout will not be output. If the voltage VL is not greater than the deterioration detection voltage VT, it is detected that the differential efficiency has decreased, and therefore, the deterioration detecting signal Sout is output.
In the conventional LD deterioration detecting circuit as described above, the driving current IX is held by the hold circuit
206
, and then a current &Dgr;I from the biasing circuit
210
is added to the driving current IX. Then the sum of the current (IX+&Dgr;I) is supplied to the laser diode
201
. Thus, the voltage VL corresponding to the current (IX+&Dgr;I) is output by the I-V converter
203
, which voltage VL is compared with the reference voltage VT by the deterioration detecting circuit
211
. If the optical output power of the laser beam is to be selectively set to one of a plurality of levels (either stepwise or continuously), it should be ensured that the optical output of the laser diode
201
is adjusted to correspond to the reference voltage VO, which is the reference voltage for examining the laser diode
201
, before the deterioration detection command E is issued. Therefore, control is complicated.
Further, since the hold circuit
206
is connected in series with the APC circuit, when the deterioration detection is not performed, the sample-hold circuit
206
composed of the switch
208
and the capacitor
213
functions as a low-pass filter, which lowers a frequency characteristic of the APC. Specifically, the APC functions to maintain the optical output of the laser diode
201
to be predetermined quantity when it is driven. At the time when the laser diode
201
is started to be driven, or when the optical output is changed and then returned to the predetermined quantity, the frequency characteristic of the APC affects the response of the APC system. Generally, if the frequency characteristic becomes higher, the response of the APC system is improved.
Furthermore, since the deterioration detection circuit
211
continuously compares the voltage VL with the reference voltage VT, even if the detection command E is OFF, i.e., even when the bias current &Dgr;I is not added to the drive current IX, if the voltage VL temporarily increases and exceeds the reference voltage VT, the detection signal Sout is issued. In order to avoid such a misdetection, the AND gate
212
, which prevents output of the detection signal Sout when the detection command E is OFF, must be provided, and therefore the circuit configuration i
Greenblum & Bernstein P.L.C.
Ip Paul
Pentax Corporation
Zahn Jeffrey N
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