Laser beam recording method and apparatus

A laser beam recording method and apparatus are constituted for recording an image by obtaining a laser beam whose optical intensity is modulated by controlling a drive current for a semiconductor laser on the basis of a light emission level instructing signal corresponding to an image signal, and scanning the laser beam on a photosensitive material. The optical output is stabilized by detecting the optical intensity of the laser beam and feeding back a feedback signal corresponding to the detected optical intensity to the light emission level instructing signal. A filter circuit formed so that the gain gradually increases from near to a cutoff frequency of a circuit for the optical output stabilization toward a higher frequency side is disposed in a stage prior to the circuit for the optical output stabilization. Or, a bias current of a value smaller than the value of the drive current that produces the laser beam of the minimum optical intensity capable of exciting the photosensitive material is always fed to the semiconductor laser.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a laser beam recording method for recording a 
continuous tone image on a photosensitive material by scanning the 
photosensitive material with a laser beam modulated in accordance with an 
image signal, and an apparatus for carrying out the method. This invention 
particularly relates to a laser beam recording method for recording an 
image of high gradation by analog modulation of the optical intensity of 
the laser beam, and an apparatus for carrying out the method. 
2. Description of the Prior Art 
Light beam scanning recording apparatus wherein a light beam is deflected 
by a light deflector and scanned on a photosensitive material for 
recording an image on the photosensitive material have heretofore been 
used widely. A semiconductor laser is one of the means used for generating 
a light beam in the light beam scanning recording apparatuses. The 
semiconductor laser has various advantages over a gas laser or the like in 
that the semiconductor laser is small, cheap and consumes little power, 
and that the laser beam can be modulated directly by changing the drive 
current. 
FIG. 2 is a graph showing the optical output characteristics of the 
semiconductor laser with respect to the drive current. With reference to 
FIG. 2, the optical output characteristics of the semiconductor laser with 
respect to the drive current change sharply between a LED region (natural 
light emission region) and a laser oscillation region. Therefore,it is not 
always possible to apply the semiconductor laser to recording of a 
continuous tone image. Specifically, in the case where intensity 
modulation is carried out by utilizing only the laser oscillation region 
in which the optical output characteristics of the semiconductor laser 
with respect to the drive current are linear, it is possible to obtain a 
dynamic range of the optical output of only approximately 2 orders of ten 
at the most. As is well known, with a dynamic range of this order, it is 
impossible to obtain a continuous tone image of high quality. 
Accordingly, as disclosed in, for example, Japanese Unexamined Patent 
Publication Nos. 56(1981)-115077 and 56(1981)-152372, an attempt has been 
made to obtain a continuous tone image by maintaining the optical output 
of the semiconductor laser constant, continuously turning on and off the 
semiconductor laser to form a pulsed scanning beam, and controlling the 
number or the width of pulses for each picture element to change the 
scanning light amount. 
However, in the case where the pulse number modulation or the pulse width 
modulation as mentioned above is carried out, in order to obtain a density 
scale, i.e. a resolution of the scanning light amount, of 10 bits 
(approximately 3 order of ten) when the picture element clock frequency is 
1 MHz for example, the pulse frequency must be adjusted to a very high 
level (at least 1 GHz). Though the semiconductor laser itself can be 
turned on and off at such a high frequency, a pulse counting circuit or 
the like for control of the pulse number or the pulse width cannot 
generally be operated at such a high frequency. As a result, it becomes 
necessary to decrease the picture element clock frequency to a value 
markedly lower than the aforesaid value. Therefore, the recording speed of 
the apparatus must be decreased markedly. 
Also, with the aforesaid method, the heat value of the semiconductor laser 
chip varies depending on the number of the widths of the pulses which are 
emitted during the recording period of each picture element, so that the 
optical output characteristics of the semiconductor laser with respect to 
the drive current change, and the exposure amount per pulse fluctuates. As 
a result, the gradation of the recorded image deviates from the correct 
gradation, and a continuous tone image of a high quality cannot be 
obtained. 
On the other hand, as disclosed in Japanese Unexamined Patent Publication 
No. 56(1981)-71374 for example, it has been proposed to record a 
high-gradation image by combining pulse number modulation or pulse width 
modulation with optical intensity modulation. However, also with the 
proposed method, the heat value of the semiconductor laser chip varies 
depending on the number of the widths of the pulses which are emitted 
during the recording period of each picture element, so that the exposure 
amount per pulse fluctuates. 
In view of the above, in order to record a high-gradation image of a 
density scale of approximately 10 bits, i.e. approximately 1024 levels of 
gradation, it is desired that a dynamic range of the optical output be 
adjusted to approximately 3 orders of ten by carrying out optical 
intensity modulation over the LED region and the laser oscillation region 
as shown in FIG. 2. However, the optical output characteristics of the 
semiconductor laser with respect to the drive current are not linear over 
the two regions. Therefore, in order to control the image density at an 
equal density interval for a predetermined difference among the image 
signals so that a high-gradation image can be recorded easily and 
accurately, it is necessary to make linear the relationship between the 
light emission level instructing signal and the optical output of the 
semiconductor laser by compensation of the optical output characteristics 
of the semiconductor laser with respect to the drive current. 
As a circuit for making linear the relationship between the light emission 
level instructing signal and the optical output of the semiconductor 
laser, it has heretofore been known to use an optical output stabilizing 
circuit (an automatic power control circuit, hereinafter abbreviated to 
the APC circuit) for detecting the optical intensity of a laser beam and 
feeding back a feedback signal, which corresponds to the detected optical 
intensity, to the light emission level instructing signal for the 
semiconductor laser. FIG. 3 is a block diagram showing an example of the 
APC circuit. The APC circuit will hereinbelow be described with reference 
to FIG. 3. A light emission level instructing signal Vref for instructing 
the optical intensity of a semiconductor laser 1 is fed to a 
voltage-to-current conversion amplifier 3 via an addition point 2. The 
amplifier 3 feeds a drive current proportional to the light emission level 
instructing signal Vref to the semiconductor laser 1. A laser beam 4 
emitted forward by the semiconductor laser 1 is utilized for scanning a 
photosensitive materia via a scanning optical system (not shown). On the 
other hand, the intensity of a laser beam 5 emitted rearward from the 
semiconductor laser 1 is detected by a pin photodiode 6 disposed for 
optical amount monitoring, for example in a case housing the semiconductor 
laser 1. The intensity of the laser beam 5 thus detected is proportional 
to the intensity of the laser beam 4 actually utilized for image 
recording. The output current of the pin photodiode 6 which represents the 
intensity of the laser beam 5, i.e. the intensity of the laser beam 4, is 
converted into a feedback signal (voltage signal) Vpd by a 
current-to-voltage conversion amplifier 7, and the feedback signal Vpd is 
sent to the addition point 2. From the addition point 2, a deviation 
signal Ve representing a deviation between the light emission level 
instructing signal Vref and the feedback signal Vpd is output. The 
deviation signal Ve is converted into a current signal by the 
voltage-to-current amplifier 3 and is utilized for operating the 
semiconductor laser 1. 
In the case where the loop gain of the APC circuit constituted by the loop 
passing through the addition point 2, the voltage-to-current conversion 
amplifier 3, the semiconductor laser 1, the photodiode 6, and the 
current-to-voltage conversion amplifier 7 and then returning to the 
addition point 2 is adjusted to a substantially high level, the 
relationship between the light emission level instructing signal and the 
optical output of the semiconductor laser becomes linear. 
In the APC circuit constituted by the feedback loop as mentioned above, the 
light emission response characteristics of the semiconductor laser become 
higher the wide the band is, and become lower the narrower the band is. 
Also, the LD is a gain change element, so that the band of the APC circuit 
becomes wider and the response characteristics increase the higher the 
optical output is. That is, at the time of a low output, problems with 
regard to low response characteristics arise and the sharpness 
deteriorates. Though no problem would be caused if the band of the APC 
circuit could be increased on the overall optical amount level, an 
increase of the band is actually limited by the high-frequency 
characteristics of the operational amplifier, the junction capacitance of 
the photodetector, and other factors. 
One approach to elimination of the aforesaid problems is to design the 
circuit so that the cutoff frequency of the APC circuit is adjusted to be 
as high as possible to increase the response characteristics at a low 
output. However, in this case, the loop gain of the APC circuit cannot be 
adjusted to a high level, and it is not always possible to make linear the 
relationship between the light emission level instructing signal and the 
optical output of the semiconductor laser. 
With the aforesaid APC circuit, the intensity of the lser beam 5 is 
proportional to the light emission level instructing signal Vref in the 
case where ideal linearity compensation is effected. Specifically, the 
intensity Pf of the laser beam 4 (i.e. the optical output of the 
semiconductor laser 1) utilized for image recording is proportional to the 
light emission level instructing signal Vref. 
However, when analog modulation of the optical intensity of the laser beam 
is carried out over the LED region and the laser oscillation region of the 
semiconductor laser by use of the APC circuit as mentioned above, there 
arises the problem that the rise response of the optical output of the 
semiconductor laser slows down at the time when, for example, a sharp 
light emission instruction is given for activating laser oscillation from 
the condition when no light is being emitted. Specifically, as shown in 
FIG. 9 for example, the normalized gain of the semiconductor laser which 
is one of the factors affecting the loop gain of the APC circuit becomes 
very low in the low output region of the semiconductor laser. As the 
normalized gain of the semiconductor laser becomes very low, the loop gain 
of the APC circuit decreases markedly. For this reason, with respect to a 
pulsed light emission level instructing signal as shown in FIG. 10A, a 
response delay arises with the forward current of the semiconductor laser 
as indicated by the solid line in FIG. 10B. Therefore, a comparatively 
long time is taken for the forward current of the semiconductor laser to 
increase up to a threshold current Is at which the laser oscillation 
begins, and the rise response of the optical output of the semiconductor 
laser is delayed as shown in FIG. 10C. 
In the case where the rise of the optical output of the semiconductor laser 
is delayed as mentioned above, even through the duty ratio of the pulsed 
light emission level instructing signal is adjusted to 50%, for example in 
the case of high-speed modulation, and the exposure amount at each picture 
element is controlled based on said duty ratio, the duty ratio of the 
light pulse actually irradiated onto the photosensitive material does not 
come up to 50%, and the line of the recorded image becomes thin. Also, the 
rise time taken for the optical ouput to come up to a level P1 from the 
condition when the laser is emitting light to some extent and the time 
taken for the optical output to come up to the level P1 from the condition 
when the laser is off are different from each other, and therefore the 
recording start position deviates and a gap is caused in the recorded 
image. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is to provide a laser beam 
recording method wherein the light emission response characteristics of a 
semiconductor laser are increased without the loop gain of an APC circuit 
being decreased, thereby to enable recording of an image of high sharpness 
to take place. 
Another object of the present invention is to provide a laser beam 
recording method wherein the response characteristics of a semiconductor 
laser at the rise of optical output is enhanced, thereby to make possible 
the recording of an image with good image reproducibility. 
The specific object of the present invention is to provide an apparatus for 
carrying out the laser beam recording method. 
The present invention provides a laser beam recording method which 
comprises the following steps, in the course of obtaining a laser beam 
whose optical intensity is modulated by controlling a drive current for a 
semiconductor laser on the basis of a light emission level instructing 
signal corresponding to an image signal, and recording an image on a 
photosensitive material by scanning the laser beam on the photosensitive 
material: 
(i) stabilizing the optical output by detecting the optical intensity of 
said laser beam and feeding back a feedback signal corresponding to the 
detected optical intensity to said light emission level instructing 
signal, and 
(ii) gradually increasing the gain in a stage prior to the stage for said 
optical output stabilization from near to a cutoff frequency in said 
optical output stabilization toward a higher frequency side. 
The present invention also provides a laser beam recording apparatus 
provided with a semiconductor laser for emitting a laser beam, a beam 
scanning system for scanning the laser beam on a photosensitive material, 
and a laser operation control circuit for producing a light emission level 
instructing signal corresponding to an image signal, and controlling a 
drive current for the semiconductor laser on the basis of the light 
emission level instructing signal, thereby to modulate the optical 
intensity of the laser beam, 
wherein the improvement comprises providing said laser operation control 
circuit with: 
(i) an optical output stabilizing circuit for detecting the optical 
intensity of said laser beam, and feeding back a feedback signal 
corresponding to the detected optical intensity to said light emission 
level instructing signal, and 
(ii) a filter circuit disposed to allow the passage of said light emission 
level instructing signal therethrough in a stage prior to said optical 
output stabilizing circuit, and formed so that the gain gradually 
increases from near to a cutoff frequency of said optical output 
stabilizing circuit toward a higher frequency side. 
The present invention further provides a laser beam recording method which 
comprises the following steps, in the course of obtaining a laser beam 
whose optical intensity is modulated by controlling a drive current for a 
semiconductor laser on the basis of a light emission level instructing 
signal corresponding to an image signal, and recording an image on a 
photosensitive material by scanning the laser beam on the photosensitive 
material: 
(i) stabilizing the optical output by detecting the optical intensity of 
said laser beam and feeding back a feedback signal corresponding to the 
detected optical intesity to said light emission level instructing signal, 
and 
(ii) always feeding a bais current of a value smaller than the value of 
said drive current, that produces said laser beam of the minimum optical 
intensity capable of exciting said photosensitive material, to said 
semiconductor laser. 
The second-mentioned laser beam recording method in accordance with the 
present invention is carried out by an apparatus provided with a 
semiconductor laser for emitting a laser beam, a beam scanning system for 
scanning the laser beam on a photosensitive material, and a laser 
operation control circuit for producing a light emission level instructing 
signal corresponding to an image signal, and controlling a drive current 
for the semiconductor laser on the basis of the light emission level 
instructing signal, thereby to modulate the optical intensity of the laser 
beam, 
wherein the improvement comprises providing said laser operation control 
circuit with: 
(i) an optical output stabilizing circuit for detecting the optical 
intensity of said laser beam, and feeding back a feedback signal 
coresponding to the detected optical intensity to said light emission 
level instructing signal, and 
(ii) a bias current feed means for always feeding a bias current of a value 
smaller than the value of said drive current, that produces said laser 
beam of the minimum optical intensity capable of exciting said 
photosensitive material, to said semiconductor laser. 
As the filter circuit in the first-mentioned laser beam recording apparatus 
in accordance with the present invention, a lead-lag filter or the like 
may be used. 
With the first-mentioned laser beam recording method and apparatus in 
accordance with the present invention, the band of the system including 
the filter circuit and the APC circuit becomes wider than the band of the 
APC circuit alone. Therefore, the light emission response characteristics 
of the semiconductor laser can be enhanced, and a continuous tone image of 
a high image quality with high sharpness can be recorded. Also, the 
effects of improving the light emission response characteristics are 
obtained by the filter circuit disposed in the stage prior to the APC 
circuit, and there is no risk of the loop gain of the APC circuit being 
decreased for the purpose of obtaining said effects. Accordingly, with the 
first-mentioned laser beam recording method and apparatus in accordance 
with the present invention, the relationship between the light emission 
level instructing signal and the optical output of the semiconductor laser 
can be kept linear, and a fine continuous tone image having a high density 
resolution can be recorded. 
With the second-mentioned laser beam recording method and apparatus in 
accordance with the present invention wherein the bias current is always 
fed to the semiconductor laser, the time required for the forward current 
of the semiconductor laser to increase to a predetermined value when the 
pulsed light emission level instructing signal is given to the laser 
operation control circuit is reduced, and the rise response of the optical 
output becomes quicker. Specifically, at the time an instruction for 
high-level light emisison is given to the semiconductor laser in the 
condition when no light is being emitted, the optical output of the 
semiconductor laser rises quickly with only a short response lag. 
Accordingly, with the second-mentioned laser beam recording method and 
apparatus in accordance with the present invention, the duty ratio of the 
image recording laser beam approaches the duty ratio of the light emission 
level instructing signal, and an image can be recorded with good image 
reproducibility.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will hereinbelow be described in further detail with 
reference to the accompanying drawings. 
With reference to FIG. 1, an imaqge signal generator 10 generates an image 
signal S1 representing a continuous tone image. By way of example, the 
image signal S1 is a digital signal representing a continuous tone image 
of a density scale of 10 bits. The image signal generator 10 changes over 
the signal for a single main scanning line on the basis of a line clock S2 
as will be described later, and emits the image signal S1 at each picture 
element based on a picture element clock S3. In this embodiment, the 
picture element clock frequency is adjusted to 1MHz. In other words, the 
recording time for a single picture element is adjusted to 1.mu.sec. 
The image signal S1 is corrected as will be described below by a correction 
table 40 comprising a RAM via a multiplexer 11, and is converted into a 
light emission level instructing signal S5 of, for example, 16 bits. The 
light emission level instruction signal S5 is fed to a multiplexer 15 and 
then to a D/A converter 16, and is converted by the D/A converter 16 into 
a light emission level instructing signal Vref composed of an analog 
voltage signal. The light emission level instructing signal Vref is fed to 
an addition point 2 of an APC circuit 8. A voltage-to-current conversion 
amplifier 3, a semiconductor laser 1, a photodiode 6, and a 
current-to-voltage conversion amplifier 7 of the APC circuit 8 operate in 
the same manners as the ones in the circuit mentioned above with reference 
to FIG. 3. Therefore, a laser beam 4 of an intensity corresponding to the 
light emission level instructing signal Vref, i.e. to the image signal S1, 
is emitted by the semiconductor laser 1. In this embodiment, a lead-lag 
filter 50 through which the light emission level instructing signal Vref 
is to be passed is disposed in the stage prior to the addition point 2. 
Operations of the leadlag filter 50 will be described later. 
The laser beam 4 is collimated by a collimator lens 17, and is then 
reflected and deflected by a light deflector 18 constituted by a polygon 
mirror or the like. The laser beam 4 thus deflected is passed through a 
converging lens 19 normally constituted by an f.theta. lens, is converged 
into a minute spot on a photosensitive material 20, and scans the 
photosensitive material 20 in a main scanning direction as indicated by 
the arrow X. The photosensitive material 20 is moved by a movement means 
(not shown) in a sub-scanning direction as indicated by the arrow Y 
approximately normal to the main scanning direction X, and thus is scanned 
with the laser beam 4 in the sub-scanning direction Y. In this manner, the 
photosensitive material 20 is two-dimensionally scanned with and exposed 
to the laser beam 4. Since the laser beam 4 is intensity modulated based 
on the image signal S1 as mentioned above, the continuous tone image which 
the image signal S1 represents is recorded as a photographic latent image 
on the photosensitive material 20. When the laser beam 4 scans on the 
photosensitive material 20, passage of the laser beam 4 over a start point 
of the main scanning is detected by a photodetector 21, and a start point 
detection signal S6 generated by the photodetector 21 is fed to a clock 
generator 36. The clock generator 36 emits the aforesaid line clock S2 and 
the picture element clock S3 in synchronization with the input timing of 
the start point detection signal S6. 
Then, the photosensitive material 20 is sent to a developing machine 22 and 
is subjected to development processing. Thus the continuous tone image is 
developed as a visible image on the photosensitive material 20. 
Correction of the image signal S1 by the correction table 40 will be 
described hereinbelow. The correction table 40 comprises a gradation 
correction table 12, an inverse logarithmic conversion table 13, and a 
correction table 14 (hereinafter referred to as the V-P characteristics 
correction table) of making linear the optical output characteristics of 
the semiconductor laser 1 with respect to the light emission level 
instructing signal. The gradation correction table 12 is of the known type 
for correcting the gradation chracteristics of the photosensitive material 
20 and the development processing system. The gradation correction table 
12 may be of the fixed correction characteristics type. However, in this 
embodiment, by taking into consideration changes in the gradation 
characteristics of the photosensitive material 20 among the lots thereof, 
changes in the characteristics of the developing solution in the 
developing machine 22 with the passage of time, or the like, the gradation 
correction table 12 is constituted for changing the correction 
characteristics when necessary in accordance with the actual gradation 
characteristics. Specifically, a test pattern signal S4 representing some 
steps (e.g. 16 steps) of image density on the photosensitive material 20 
is generated by a test pattern generating circuit 26, and is fed to the 
multiplexer 11. At this time, the multiplexer 11 is changed over from the 
condition at the time of image recording for feeding the image signal S1 
to the correction table 40 as mentioned above to the condition for feeding 
the test pattern signal S4 to the correction table 40. The semiconductor 
laser 1 is operated in the manner mentioned above on the basis of the test 
pattern signal S4, and therefore the laser beam 4 is intensity modulated. 
As a result, a step wedge (test pattern) whose density changes step-wise, 
for example in 16 steps, is recorded as a photographic latent image on the 
photosensitive material 20. The photosensitive material 20 is sent to the 
developing machine 22, and the step wedge is developed. After the 
development is finished, the photosensitive material 20 is sent to a 
densitometer 23, and the optical density at each step of the step wedge is 
measured. The optical density thus measured is sent to a density value 
input means 24 in conformity with each step of the step wedge, and a 
density signal S7 representing the optical density of each step of the 
step wedge is generated by the density value input means 24. The density 
signal S7 is fed to a table creation means 37. On the basis of the density 
signal S7 and the test pattern signal S4, the table creation means 37 
creates the gradation correction table such that a predetermined image 
density is obtained with a predetermined value of the image signal S1. As 
mentioned above, the gradation correction table makes the image signal 
values of approximately 16 steps correspond respectively to predetermined 
image density values. A signal S8 representing the gradation correction 
table is fed to a signal interpolation means 38, which carries out 
interpolation processing to obtain a gradation correction table adapted to 
the image signal S1 of 1024 steps (i.e. 10 bits). The aforesaid gradation 
correction table 12 is created on the basis of a signal S9 representing 
the gradation correction table thus obtained. 
In the course of image recording based on the image signal S1, the image 
signal S1 fed to the gradation correction table 12 via the multiplexer 11 
is converted to a signal S1.varies. by the gradation correction table 12, 
and is then converted by the inverse logarithmic conversion table 13 into 
a light emission level instructing signal S1". 
The V-P characteristics correction table 14 will now be described below. 
Even though the feedback signal Vpd is fed back to the addition point 2 in 
the APC circuit 8, it is not always possible to obtain the ideal 
relationship between the light emission level instructing signal and the 
intensity of the laser beam 4 as indicated by the solid line in Figure 4. 
Specifically, in order to obtain the ideal relationship, it is necessary 
to adjust the loop gain of the APC circuit to a very high value 
(approximately 70dB). However, it is not always possible to realize such a 
high loop gain with the present technique. The V-P characteristics 
correction table 14 is provided for obtaining such as ideal relationship. 
Specifically, the ideal relationship between the light emission level 
instructing signal Vref and the optical output of the semiconductor laser 
1 is indicated by a straight line "a" in FIG. 5, the actual relationship 
therebetween is indicated by a curve "b" in FIG. 5, and the voltage value 
at the time the light emission level instructing signal S1" is directly 
D/A converted is assumed to be equal to Vin. In this case, the V-P 
characteristics correction table 14 is constituted to convert the voltage 
value Vin to a voltage value V. When the value of the light emission level 
instructing signal Vref is equal to Vin, only the optical intensity equal 
to P' can be obtained. However, in the case where the voltage value Vin is 
converted to the voltage value as mentioned above, the optical intensity 
equal to Po can be obtained with respect to the voltage value Vin. Thus 
the relationship between the voltage value Vin corresponding to the light 
emission level instructing signal S1" and the optical output Pf becomes 
linear. 
With the aforesaid configuration, density on the photosensitive material 20 
can be controlled at equal density intervals by changing the level of the 
image signal S1 by a predetermined amount. Also, as mentioned above, the 
characteristics curve "b" shown in FIG. 5 is for the case where the 
semiconductor laser 1 is operated over the LED region and the laser 
oscillation region. Therefore, with the aforesaid embodiment, an optical 
output dynamic range of approximately 3 orders of ten can be obtained, and 
consequently a high-gradation image of approximately 1024 levels of 
gradation can be recorded easily and accurately as mentioned above. 
As mentioned above, nonlinearity of the laser beam optical output 
characteristics with respect to the light emission level instructing 
signal, which nonlinearity is caused by nonlinearity of the optical output 
characteristics of the semiconductor laser 1 with respect to the drive 
current, is eliminated and said characteristics are made linear by the V-P 
characteristics correction table 14. Accordingly, the loop gain of the APC 
circuit 8 constituted by the system passing through the voltage-to-current 
conversion amplifier 3, the semiconductor laser 1, the photodiode 6, and 
the current-to-voltage conversion amplifier 7 and then returning to the 
addition point 2 need not include the gain necessary for the correction 
for eliminating the aforesaid nonlinearity. Thus it is not only necessary 
that the loop gain be of a value required for the compensation for 
deviations of the optical output characteristics of the semiconductor 
laser 1 with respect to the drive current which are caused by transitional 
changes in the temperature arising in the course of the operation of the 
semiconductor laser 1 or by error and/or hunting in the control for making 
constant the temperature in the case of the semiconductor laser 1, and for 
the compensation for drifts of the amplifiers or the like. Specifically, 
in the case where the picture element clock frequency is 1MHz and the 
semiconductor laser 1 is operated to generate an optical output of 3mW, it 
is only necessary that the aforesaid loop gain be approximately 30dB. The 
loop gain of this order can be achieved easily with the present technical 
level. 
Creation of the V-P characteristics correction table 14 will be described 
hereinbelow. To the apparatus shown in FIG. 1, a table creation device 35 
can be connected when necessary. The table creation device 35 comprises a 
test signal generating circuit 27, a table creation circuit 28 and a 
memory 29. When the V-P characteristics correction table 14 is to be 
created, a variable-level digital test signal S10 is generated by the test 
signal generating circuit 27 and is fed to the multiplexer 15. At this 
time, the multiplexer 15 is changed over from the condition for feeding 
the light emission level instructing signal S5 to the D/A converter 16 in 
the course of image recording to the condition for feeding the test signal 
S10 to the D/A converter 16. Also, the table creation circuit 28 is 
connected so that it receives the feedback signal Vpd from the 
current-to-voltage conversion amplifier 7 of the APC circuit 8. The test 
signal S10 is emitted such that the signal level increases or decreases 
step-wise. At this time, the table creation circuit 28 activates a 
variable-level signal generator built therein to generate a reference 
signal corresponding to the minimum optical output, and compares the 
feedback signal Vpd with the reference signal. The reference signal has 
the voltage value Vin as shown in FIG. 5. The table creation circuit 28 
latches the value of the test signal S10 at the time the feedback signal 
Vpd and the reference signal coincide with each other. The voltage value 
represented by the latched test signal S10 corresponds to the voltage 
value V as shown in FIG. 5, and therefore the relationship between the 
voltage value Vin and the voltage value V can be found. The table creation 
circuit 28 changes the value of the reference signal in 1024 steps, and 
detects the relationship between the voltage value Vin and the voltage 
value V for each reference signal value. In this manner, the correction 
table for converting 1024 levels of the voltage value Vin to the voltage 
value V is created. The creation table thus created is stored in the 
memory 29, and is then set as the V-P characteristics correction table 14. 
After the V-P characteristics correction table 14 is created in this 
manner, the table creation device 35 is disconnected from the APC circuit 
8. 
The effect of the lead-lag filter 50 will now be described below. The 
lead-lag filter 50 is constituted by, for example, a passive filter having 
a circuit configuration as shown in FIG. 6. As shown in FIG. 7B, the gain 
of the lead-lag filter 50 is adjusted so that it is flat up to a frequency 
f1, gradually increases at frequencies above the frequency f1, and then 
becomes flat at frequencies above a frequency f2. In the circuit 
configuration as shown in FIG. 6, the following formulas hold. 
##EQU1## 
On the other hand, the gain of the APC circuit 8 is as shown in FIG. 7A. 
Specifically, the band is limited on the high frequency side for the 
reasons mentioned above. In the case where the cutoff frequency of the APC 
circuit 8 thus formed is designated by fc, the lead-lag filter 50 is 
constituted so that f1 is approximately equal to fc. Though the cutoff 
frequency of the APC circuit 8 is caused to fluctuate by fluctuations of 
the differential quantum efficiency as the gain of the semiconductor laser 
1 in accordance with the optical output of the semiconductor laser 1, the 
frequency f1 is adjusted to be approximately equal to the lowest cutoff 
frequency, i.e. the cutoff frequency fc at the time the optical output of 
the semiconductor laser 1 is the lowest. 
Since the lead-lag filter 50 as mentioned above is disposed in the stage 
prior to the APC circuit 8, the gain of the system from the lead-lag 
filter 50 to the APC circuit 8 becomes as shown in FIG. 7C. Specifically, 
the cutoff frequency of said system becomes equal to fc' (=f2), which 
value is on the frequency side higher than the cutoff frequency fc of the 
APC circuit 8, and thus the band of said system becomes wider than the 
band of the APC circuit 8. Therefore, the light emission response 
characteristics of the semiconductor laser 1 is improved by the extent 
corresponding to the widening of the band as compared with the case where 
no lead-lag filter 50 is provided. As the light emission response 
characterisitics of the semiconductor laser 1 is improved in this manner, 
the sharpness of the image recorded on the photosensitive material 20 in 
the manner mentioned above increases. 
In the aforesaid embodiment, the V-P characteristics correction table 14 
for making linear the relationship between the light emission level 
instructing signal S1" and the optical output Pf is provided. However, in 
the case where the gain of the APC circuit 8 is adjusted to a 
substantially high level (for example, approximately 70dB), the ideal 
relationship as indicated by the solid line in FIG. 4 can be obtained by 
the APC circuit 8 alone, and the V-P characteristics correction table 14 
mentioned above need not necessarily be provided. 
Also, the beam scanning system for scanning the laser beam 4 is often 
provided with an optical element whose optical transmittance 
characteristics with respect to the incident light intensity are 
nonlinear, for example a polarizing filter, an interference filter, or an 
aperture stop plate. In such a case, the V-P characteristics correction 
table 14 should preferably be formed to compensate also for the 
nonlinearity of such characteristics. 
The second laser beam recording apparatus in accordance with the present 
invention will be described hereinbelow with reference to FIG. 8. In FIG. 
8, similar elements are numbered with the same reference numerals with 
respect to FIG. 1. 
With reference to FIG. 8, a bias current feed circuit 51 is connected to 
the semiconductor laser 1, and a bias current Ib of a predetermined level 
is always fed from the bias current feed circuit 51 to the semiconductor 
laser 1 in the course of operation of the apparatus. The bias current Ib 
is adjusted to a value smaller than the value of the semiconductor laser 
drive current that produces the laser beam 4 of the minimum optical 
intensity capable of exciting the photosensitive material 20. As the bias 
current Ib is being fed to the semiconductor laser 1, in the case where 
the light emission level instructing signal Vref for instructing the 
optical output in the laser oscillation region is given sharply from the 
condition when no light is being emitted as shown in FIG. 10A, the forward 
current of the semiconductor laser rises as indicated by the curve "e" in 
FIG. 10B. Specifically, the rise start point of the current is adjusted in 
advance to be higher by the level of the bias current Ib as compared with 
the case where no bias current is being fed as indicated by the solid line 
in FIG. 10B. Therefore, the rise response to the forward current of the 
semiconductor laser up to the predetermined level which the light emission 
level instructing signal Vref represents becomes quicker than in the case 
where no bias current is being fed. Accordingly, the rise response delay 
of the optical output of the semiconductor laser 1 is decreased as 
indicated by the curve "f" in FIG. 10C. 
Besides the provision of the bias current feed circuit 51 as in the 
aforesaid embodiment, the correction table 40 may be adjusted to always 
apply a bias current of a predetermined level to the semiconductor laser 
1. 
When the rise response characteristics of the optical output of the 
semiconductor laser 1 are improved in the manner mentioned above, the duty 
ratio of the pulsed laser beam 4 emitted for each picture element becomes 
closer to the duty ratio of the light emission level instructing signal 
Vref. Accordingly, thinning of the line of the recorded image and 
deviation in the recording start position are minimized, no gap is caused 
in the recorded image, and the image, reproducibility is raised. 
In the embodiment shown in FIG. 8, the V-P characteristics correction table 
14 is provided for compensation for nonlinearity of the optical output 
characteristics of the semiconductor laser with respect to the light 
emission level instructing signal. However, in the second laser beam 
recording apparatus in accordance with the present invention, the V-P 
characteristics correction table 14 need not necessarily be provided.