Laser diode monitoring apparatus for determining the decay of laser diode

A laser diode monitoring apparatus activates the laser diode in a manner of impulse, measures the sag of light output level, and judges the light emitting performance to be deteriorated if the sag is greater than the reference value. The value of sag comprehends the inequality of light output characteristics of individual laser diodes and the variation of ambient temperature, enabling the accurate decay determination.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a laser diode monitoring apparatus for 
determining the decay of laser diode, the apparatus being applicable 
suitably to a data recording and reproducing apparatus using 
amagneto-optical recording disk. Specifically, the apparatus is designed 
to be capable of a determining the decay of laser diode accurately even 
during the operation based on the monitoring of the light output level of 
the laser diode at a specific portion of sectors of tracks of the disk. 
2. Description of the Related Art 
A magneto-optical recording disk has its recording surface partitioned into 
an inner recording region for channel 1 and an outer recording region for 
channel 2, with these regions including tracks each divided into multiple 
sectors, e.g.,42 sectors, as shown in FIG. 6A. One sector consists of a 
preceded address area (ADD) and a data recording area (MO), as shown in 
FIG. 6B. 
The address area ADD begins with a sector mark (SM), which is followed by 
three repetitive address data of the same content, and ends with postamble 
data (PA). The three repetitive records of address data ADD1, ADD2, and 
ADD3 are intended for the reliable readout of address data in 
consideration of possible read error. The address data consists of VFO 
data, address marker (AM), and identification data (ID). The VFO (variable 
frequency oscillator) generates a reference clock signal of a fixed 
reference frequency used for the pulling of the clock generating PLL 
oscillator. 
The address data is preformatted in the form of pits on the disk surface. 
The address area ADD is followed by the data recording area MO, which 
begins with a test area. The test area has a record of ALPC (automatic 
laser power control) data used for the control of the power level of laser 
diode and VFO data VFO4 which has the same content as the VFO data 
VFO1-VFO3 of the address data area. 
The data recording area MO ends with a buffer area (blank area) which 
serves as the border to the address area ADD of the successive sector. It 
should be noted that the number of sectors of track and the number of 
bytes of a sector shown in FIGS. 6A and 6B are merely examples. 
Data is recorded on the magneto-optical disk 1 or data and address data 
recorded on the magneto-optical disk 1 are reproduced based on the 
projection of a laser beam onto the disk surface. A laser diode is used 
generally for the emission of the laser beam. The laser diode has its 
light emitting performance deteriorated with the time length of use, and 
FIG. 7 is a graph showing the characteristic of a typical laser diode. The 
laser diode is replaced when the excitation current for producing the 
prescribed light level following the screening period has increased to 1.2 
to 1.3 time the normal value as shown in FIG. 7. 
This nature suggests the feasibility of the detection of the decay of the 
laser diode through the monitoring of the excitation current. However, the 
laser diode has a temperature-dependent characteristic as shown in FIG. 8. 
Specifically, it requires a larger excitation current for producing the 
prescribed light level as the temperature rises. On this account, if the 
ambient temperature varies, the decay of the laser diode cannot be 
assessed accurately by simply monitoring the excitation current. Another 
disturbing factor to be taken into consideration is the inequality of the 
light emitting performance of individual laser diodes. 
Since the decay of a laser diode cannot be detected accurately during its 
use by simply monitoring the excitation current as mentioned above, it is 
conventionally replaced on expiration of a certain run time which is 
derived from the guarantee period of the appliance manufacturer. The laser 
diode dedicated to only the reproducing operation has a guarantee period 
of 10,000 hours or more, and it does not need to be subjected to the 
determination of decay. 
Whereas, the laser diode which operates at a high laser power for the 
recording and for the erasing operations has a relatively short guarantee 
period of 500 to 5,000 hours, and the user is required to replace it on 
expiration of a certain run time derived from the guarantee period in 
order to avoid the faulty recording due to a deficient light output. 
Because of the infeasibility of accurate assessment of the decay of the 
laser diode as mentioned above, it is probable that it may be replaced 
prematurely and wastefully. 
When an excitation pulse current of around 1-.mu.s width as shown in FIG. 
9A is fed to a new laser diode, it produces a light output having a 
generally rectangular waveform as shown by the solid line in FIG. 9B. As 
the laser diode decays and demands more input power, it produce a sagging 
light output waveform as shown by the dashed line in FIG. 9B. 
FIG. 9C compares the waveforms of small, medium and large light powers 
resulting from a decaying laser diode. Waveform Pa is of a small light 
power (1 to 2 mW measured on the magneto-optical disk surface) and 
waveform Pb is of a medium light power (around 5 mW), and even the 
decaying laser diode does not virtually vary the light output performance 
in these cases. Waveform Pc exhibits a degraded characteristic, known as 
"droop" characteristic, resulting from a larger light power (7 to 9 mW). 
OBJECT AND SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide a laser diode 
monitoring apparatus capable of determining the decay of the laser diode 
accurately by monitoring the light output level for detecting the droop 
characteristic. 
According to a first aspect of the present invention, there is provided a 
laser diode monitoring apparatus which operates to judge the decay of the 
laser diode based on the variation of light output level resulting from 
the activation by an excitation pulse current. 
According to a second aspect of the present invention, there is provided a 
laser diode monitoring apparatus comprising a laser diode, a photodiode 
which receives the light output of the laser diode, a selector which 
receives the results of comparison of the laser diode light output with 
reference values set for individual operation modes and selects a 
comparison result depending on the operation mode, a laser diode driver 
which receives the output of the selector, means for determining the decay 
of laser diode, and a controller which receives the result of 
determination. 
According to a third aspect of the present invention, there is provided a 
laser diode monitoring apparatus for determining the decay of a laser 
diode which is used with a magneto-optical recording disk, based on the 
light output level of the laser diode measured immediately after the ALPC 
period. 
According to a fourth aspect of the present invention, there is provided a 
laser diode monitoring apparatus comprising, a laser diode, a photodiode 
which receives the light output of the laser diode, a selector which 
receives the results of comparison of the laser diode light output with 
reference values that are set for individual operation modes and selects 
the comparison result depending on the operation mode, a laser diode 
driver which receives the output of the selector, an error holding 
amplifier which holds the error level of the ALPC period, a level shift 
circuit located at the front of the error holding amplifier, means for 
determining the decay of laser diode, and a controller which receives the 
result of determination, with the laser diode being excited immediately 
after the APC period at the error level that is rendered the level 
shifting so that the decay of laser diode is judged based on the resulting 
light output characteristic. 
In the invention described above, the laser diode is driven at a high power 
(recording or erasing mode), and the light output level of the laser diode 
resulting from the application of an excitation pulse current is 
monitored. The light output maintains a virtually constant level 
throughout the period from rising to falling unless the light emitting 
performance of the laser diode is deteriorated. 
However, as the laser diode decays, the light output sags, exhibiting the 
droop characteristic. Since the droop characteristic comprehends the 
inequality of characteristics of individual laser diodes and the variation 
of ambient temperature, the difference of light output levels at time 
points near the rising and falling edges is measured. The decay of laser 
diode which jeopardizes the recording and erasing operations is predicated 
if the differential light output level exceeds the prescribed value, and 
it can be indicated to the operator so as to replace the laser diode. 
The droop characteristic can be detected instead of using the excitation 
pulse current, but based on the monitoring of the light output levels 
across the same time span as of the case of excitation pulse current 
immediately after the ALPC period, with the erasing mode being set. In any 
case, the decay of laser diode can be judged accurately based on the 
differential light output level without being affected by the variation of 
ambient temperature and the inequality of light output characteristics of 
individual laser diodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The laser diode monitoring apparatus of this invention embodied in the 
application to the magneto-optical recording and reproducing apparatus 
will be explained in detail with reference to the drawings. 
The laser power in the data recording and erasing operations is controlled 
automatically to follow the reference level by utilization of the ALPC 
period mentioned previously. The laser diode monitoring apparatus is 
included within the APC loop. 
In the laser diode monitoring apparatus shown in FIG. 1, a laser diode LD 
is energized by the output current of a driver 12, and the light output is 
detected by photodiodes PDa and PDb which function as opto-electric 
transducers. The photodiode PDa has its output amplified by an amplifier 
14 and then received by a comparator 16, by which it is compared with a 
reference voltage RFa provided by a reference voltage source 18. The 
comparator output controls the driver 12 such that the output from the 
amplifier 14 is equal to the reference voltage RFa. 
An error holding amplifier 22 is used to keep the comparator output, which 
has been controlled to follow the reference laser power in the ALPC 
period, for the subsequent period of a data recording area. The comparator 
output in the ALPC period (level P of waveform B of FIG. 2) is held in 
response to a sampling pulse (not shown) which is produced in the latter 
half section of the ALPC period, and the laser diode LD is controlled to 
produce the reference laser power. 
The magneto-optical disk 1 has a recording format which includes an address 
field ADD, a data recording field MO, and an ALPC field which precedes the 
data recording field MO, as shown by A in FIG. 2. Different values of 
laser power (the light power measured on the magneto-optical disk) are 
used for these fields. The laser power also differs depending on the 
operation mode. Specifically, the laser power used for retrieving the 
address data and the data recorded on the magneto-optical disk 1 is as low 
as around 1.2 mW, whereas the laser power used for recording or erasing 
data is 8 to 9 mW. 
In the reproducing mode, a comparator 24 is used to compare the light 
output with the reference voltage RFb provided by a reference voltage 
source 26, and the comparator output is held by an error holding amplifier 
28. The driver 12 is activated in accordance with the held value. 
The error holding amplifiers 22 and 28 and selector 20 which hold the 
comparator outputs and selects a comparator output depending on the 
operation mode are supplied with sampling pulses and a switching pulse SW 
that depends on the operation mode from a controller 30. 
In addition to the foregoing arrangement, there is included a decay 
determining means 32 for the laser diode LD, which consists of a pair of 
sample-holding circuits 34 and 36 which receive the output of the 
amplifier 14 and a comparator 38 which receives the outputs of these 
sample-holding circuits. With the APC loop being open, determination of 
laser diode decay takes place, and the comparator output Cx is delivered 
to the controller 30 and also to an adder 44, by which it is added to the 
reference voltage RFa. 
The photodiode PDb has its detection output received by an ID detector 40, 
by which ID data recorded in the address area ADD is detected and 
delivered to the controller 30. Only after the detection of this ID data, 
it becomes possible to control the emission of the laser diode LD in the 
ALPC period. If the controller 30 detects ID data, the controller 30 
provides a pulse signal which has a predetermined width for the ALPC 
period to the driver 12. 
The controller 30 has an associated display 42, by which the determination 
of decay of the laser diode LD is indicated to the operator. An alarm 
means (not shown) may be added to the display 42 so that the operator is 
prompted acoustically to replace the laser diode LD. Alternatively, a 
voice synthesizer may be provided in place of the alarm means so that the 
operator is prompted by a voiced message to replace the laser diode LD. 
The laser diode monitoring apparatus 10 arranged as described above 
operates in the normal reproducing mode to drive the laser diode LD in 
accordance with the output of the error holding amplifier 28 received on 
terminal c of the selector 20. At this time, the light output is 1.2 mW in 
terms of the laser power detected by the photodiode PDa. 
In the erasing mode, the selector 20 is controlled to select the input 
terminal c so as to set the reproducing mode only for the address area 
ADD. After that, selection is switched to the a-terminal so as to set the 
APC mode for the ALPC period, and it is switched to the b-terminal at the 
end P of the ALPC period so that the laser diode LD is driven thereafter 
in accordance with the output of another error holding amplifier 22 (refer 
to waveform B of FIG. 2) based on the closed-loop control so that the 
light output is 8.5 mW in terms of the laser power detected by the 
photodiode PDa. 
Detection of the decay of laser diode is implemented by utilization of the 
ALPC period. Since the laser diode LD needs to be driven at a high laser 
power for the detection of decay as mentioned previously in connection 
with FIG. 9, the selector 20 is controlled to set the erasing mode (or 
recording mode) during the ALPC period. The operation mode for periods 
other than the ALPC period is arbitrary, and it is set to the reproducing 
mode in this embodiment (refer to waveform H of FIG. 2). 
At decay determination, a certain period that precedes the beginning of 
ALPC is used to apply a test pulse current having a width of about 1 
.mu.s. For this implementation, the write data to be fed to the driver 12 
is made high for a prescribed duration (1 .mu.s) at the beginning of ALPC, 
and it is made low invariably for the entire period of the data recording 
area MO, as shown by waveform C in FIG. 2. Consequently, a light power PL 
derived from the test pulse current LP is produced as shown by waveform D 
in FIG. 2. The laser diode LD is rendered the APC control immediately 
after the test pulse current LP as shown by waveform D, and thereafter the 
low laser power drive mode is restored for data retrieval. 
There arises virtually no difference in level at the beginning and end of 
the light power PL for the test pulse current LP unless the light emitting 
performance of the laser diode LD is deteriorated, as explained previously 
in connection with FIG. 9. If, on the other hand, the light emitting 
performance becomes to deteriorate, the sag of laser power emerges 
progressively as shown for its ultimate state by waveform E in FIG. 2. 
A first sampling pulse Pa (shown by waveform F in FIG. 2) is supplied to 
the first sample-holding circuit 34 of FIG. 1 at a time point close to the 
rising edge of the light power PL, and the level Va at time point "a" is 
sampled and held. Similarly, a second sampling pulse Pb (shown by waveform 
G in FIG. 2) is supplied to the second sample-holding circuit 36 of FIG. 1 
at a time point close to the falling edge of the light power PL, and the 
level Vb at time point "b" is sampled and held. 
The outputs of these sample-holding circuits are delivered to the 
comparator 38, and their differential value .DELTA. V (Va-Vb) is compared 
with the reference value Vf. If the differential value .DELTA.V exceeds 
the reference value Vf, indicative of the advanced decay of the laser 
diode LD, it is delivered to the controller 30, which then activates the 
display 42 thereby to prompt the operator to replace the laser diode LD. 
The droop characteristic caused by the deteriorated light emitting 
performance of the laser diode comprehends the inequality of 
characteristics of individual laser diodes and the variation of ambient 
temperature. Accordingly, by monitoring the droop characteristic, it is 
possible to detect accurately the decay of laser diode which could 
possibly impair the operation in the recording mode and the erasing mode 
in a state of comprehension of the inequality of characteristics of 
individual laser diodes and the variation of ambient temperature. 
In addition to the arrangement for the foregoing operation, the comparison 
output .DELTA.V is further supplied to the adder 44 as shown in FIG. 1 so 
that the reference value is shifted, i.e., lowered to RFa-.DELTA.V, 
following the determination of laser diode decay by the reason explained 
in the following. 
Generally, the laser diode LD has its erasing laser power set higher by 
about 25% in order to cope with negative factors including the 
contamination of optical system and the fluctuation of the reflectivity of 
disk surface. For this marginal setting, the AV is set to a value that is 
equal to 15% of the above-mentioned setup laser power. 
In consequence, the data erasing operation based on the laser power of 15% 
reduction at the determination of decay can meet the erasing condition, 
and it becomes possible for the operator to replace the laser diode on 
completion of data recording instead of suspending the recording 
operation. For this implementation, the comparison output Cx for the 
differential value .DELTA.V is supplied to the adder 44. 
The arrangement shown in FIG. 1 is capable of determining the decay of 
laser diode LD even during the use of the magneto-optical recording disk. 
Since the droop characteristic of laser diode does not vary with the 
ambient temperature, determination of decay is accurate irrespective of 
the ambient temperature variation. The operator is allowed to use the 
apparatus continuously even after the detection of laser diode decay and 
to replace it at a proper occasion. 
FIG. 3 shows another embodiment of this invention. This circuit arrangement 
is intended to judge the decay of laser diode without using an excitation 
pulse current, and it is specifically designed to judge the decay of laser 
diode by detecting the variation of light output immediately after the 
ALPC period. 
The circuit arrangement differs from that of FIG. 1 in the provision of a 
level shift circuit 46 at the front of the error holding amplifier 22. The 
level shift circuit 46 is activated in the erasing mode to shift (raise in 
this embodiment) the ALPC level by .DELTA.V immediately after the ALPC 
period, as shown by waveform A in FIG. 4. The offset value .DELTA.V is 
equivalent to 1.0 mW. 
The light output characteristic in the normal state is shown by the solid 
line on waveform A, the light emitting performance of the laser diode is 
deteriorated due to the droop characteristic as shown by the dashed line. 
The first sample-holding circuit 34 is supplied with the first sampling 
pulse Pc shown by waveform C in FIG. 4, and it holds the laser power level 
Va at time point "a". On expiration of about 1.mu.s, the second 
sample-holding circuit 36 receives the second sampling pulse Pd shown by 
waveform D and holds the laser power level Vb at time point "b". The 
differential value .DELTA.V of these levels is evaluated. 
The differential value .DELTA.V is compared with the reference value Vf, 
and if .DELTA.V exceeds Vf, the comparison output Cx indicative of the 
detection of laser diode decay is delivered so that an alarming operation 
similar to that explained in connection with FIG. 1 takes place. As a 
result, determination of the decay of laser diode can be accomplished 
without using a test pulse current. This decay determination operation can 
be carried out also in the data erasing mode. 
FIG. 5 shows a specific circuit arrangement of the above-mentioned portion 
of FIG. 3, in which a differential amplifier 16A that constitutes the 
comparator 16 is supplied on its non-inverting input terminal with the 
monitor output provided by the amplifier 14 and the reference voltage RFa 
on its inverting input terminal. The comparison output is delivered to the 
level shift circuit 46 which also serves as a buffer amplifier. 
The level shift circuit 46 includes a transistor Qa, which is connected on 
the emitter path with an emitter resistor Ra. The emitter output produced 
on the node "r" is fed back through an RC feedback circuit 48 to the 
inverting input terminal of the differential amplifier 16A and also 
delivered to the selector 20 as the comparison output of the APC mode. 
Another resistor Rb for level shifting is connected in series to the 
emitter resistor Ra, and it provides the comparison output of the APC mode 
for the selector 20. The node "s" between the emitter of Qa and the 
resistor Rb has a voltage level higher than that of the node "r" by the 
amount of I.times.R (where I is the emitter current and R is the 
resistance of Rb), resulting in an increased drive current in the APC 
mode. The resistance values of these resistors are selected properly such 
that the voltage difference I.times.R results in an increase of laser 
power by 1.0 mW. 
According to this invention, as described above, it becomes possible to 
judge the deterioration of the light emitting performance of laser diode 
accurately even during its operation by monitoring the light output level 
of the laser diode at the specific portion of record. The inventive 
circuit is capable of determining the decay of laser diode accurately 
during the use irrespective of the inequality of light output 
characteristics of individual laser diodes and the variation of ambient 
temperature, and such improprieties as the abortion of data recording in 
the recording mode can be prevented. 
With a sufficient marginal operating condition being set for the driving of 
laser diode, the inventive circuit enables the continuous use of the laser 
diode even after the detection of decay, allowing the operator to prepare 
the replacement of the laser diode.