JPS62226763A - Optical intensity control method in optical scanning system - Google Patents

Abstract

PURPOSE:To realize the light intensity for always correct optical scanning regardless of temperature rise or deterioration with the lapse of time by obtaining a correction voltage corresponding to a picture scanning clock frequency whose frequency is changed continously in response to the change in the scanning speed. CONSTITUTION:The reference value of the lighting intensity of a semiconductor laser 10 is set automatically by a reference signal from an output strength control circuit. A correction voltage Vc obtained from a picture scanning clock frequency control circuit 38 corresponds to a frequency of a picture scanning clock and is changed in response to the scanning speed. Thus, the variation of the I/P characteristic of the semiconductor laser 10 gives on effect to the correction voltage. Even if the I-P characteristic of the laser 10 is varied in case the scanning speed is an optional value in the speed region, the light intensity of the semiconductor laser is unchanged substantially.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a light intensity control method in an optical scanning system.

(Technical Background) An optical scanning method in which modulated light from a semiconductor laser is deflected using a rotating deflector such as a rotating polygon mirror or a hologram scanner is well known. Since a rotary deflector generally deflects a light beam at a constant angular velocity, an fθ lens is generally used to maintain a constant scanning speed on the surface to be scanned.

However, the fO lens is a special lens and its manufacturing cost is high. Therefore, there is a desire to avoid using an f0 lens if possible. In addition, recently, special polygon mirrors in which the scanning angular velocity of the light beam is not constant are being proposed (Japanese Patent Application No. 59-274324), and in such cases, even if an fθ lens is used, the scanning velocity is is not constant, so it is not possible to use fθ lenses.

In view of these circumstances, recently, an optical scanning method that performs optical scanning without using an fθ lens is being considered. For example, FIG. 3 shows an example of such an optical scanning type scanning device.

The light flux enters a rotating polygon mirror 82 which is a rotating deflector via a lens 80, is reflected by one of its reflecting surfaces, enters a photoconductive photoreceptor 84, and is exposed to light by the action of the lens 80. Focus on the body. When the rotating polygon mirror 82 is rotated at a constant speed in the direction of the arrow, the light beam is deflected from the left to the right in FIG. Light scan. In addition, the code|symbol 86 shows a light receiving element,
This light receiving element 86 is used to synchronize the starting point of optical scanning.

As the rotating polygon mirror 82 rotates, the reflecting surface that reflects the light beam is switched, and the deflection, that is, the optical scanning is periodically repeated.

Incidentally, when performing optical scanning, a clock having a frequency fK given by 1/T is called an image scanning clock, where T is the time allocated to writing information for one pixel.

In the optical scanning method that does not use an fθ lens, the scanning light
Since the scanning speed on the scanned surface is not constant,
If the frequency fK of the image scanning clock is kept constant, distortion will occur in the stored information. In order to remove such information distortion, it is necessary to change the frequency fK in accordance with changes in the scanning speed on the surface to be scanned. That is, where the scanning speed is large, accordingly,
The frequency fx of the image scanning clock must be made high, and the frequency fic must be made low where the scanning speed is low.

In this way, by changing the frequency fK of the image scanning clock according to the scanning speed, distortion of the written information image can be effectively reduced.

By the way, as mentioned above, the frequency fx is the reciprocal of the time T allocated to writing information for one pixel. Therefore,
A change in frequency fx corresponds to a change in time T. Then, when the intensity of the scanning light is constant during optical scanning, the amount of light used to write one pixel is When writing by optical scanning, there should be no difference in energy as the scanning speed changes.

The amount of exposure light per pixel changes, and the image density changes in the resulting information image in accordance with the change in scanning speed.

In order to solve this problem, the level of the light emission intensity of the light source suitable for optical scanning is changed with the scanning position, and the light intensity P for optical scanning is, for example, as shown in Fig. 4 (I). In this way, it may be set to be low at the center of the scanning area and to increase as it approaches both ends.

Incidentally, semiconductor lasers have recently come to be used as light sources for such optical scanning methods.

Since the emission intensity of a semiconductor laser changes depending on the operating current, the information signal can be directly converted into an optical signal by modulating the operating current with the information signal.

However, on the other hand, in terms of the characteristics that express the correspondence between the operating current and the emitted light intensity of a semiconductor laser, the so-called differential molecular efficiency, that is, the average rate of increase in light intensity per unit operating current, is Due to the tendency of the semiconductor laser to decrease due to temperature rise or deterioration of the laser over time, the following problems occur when a semiconductor laser is used as a light source in the optical scanning method as described above.

For example, the light intensity P when scanning both ends of the optical scanning area
Let us consider the case where the light intensity P is changed by the maximum ΔPo during optical scanning, using <Pa as a reference as shown in FIG. 4G).

49th proviso) is a semiconductor laser operating current engineering Op,
A straight line 4-1 shows the characteristics with respect to the light intensity P, and a straight line 4-1 shows the characteristics under normal conditions, and a straight line 4-2 shows the characteristics when the temperature rises or deteriorates. Differential quantum efficiency is nothing but the slope of these characteristic curves. The above characteristics are as follows I-
This is called the P characteristic.

Now, during optical scanning, using the light intensity Po as a reference, the maximum light intensity is Po and the minimum light intensity is Po - ΔPo. This means that when the semiconductor laser is normal, the reference light intensity Pa
, the operating current I o [a) is set, and the light intensity P
For G-ΔP, the operating current is set to be smaller than Iop(a) by ΔI op (a).

In conventional light intensity control, ΔI op (a) is set as a constant, and control is performed such that the operating current Iop is set to the reference light intensity Po. With this kind of control, there is no problem as long as the I-P characteristics of the semiconductor laser are normal, but if the above characteristics of the semiconductor laser change due to temperature rise or deterioration, it will not be possible to produce proper light. It becomes impossible to control the intensity.

For example, the I-P characteristics of a semiconductor laser are @4 Figure 01)
Let us consider a case where the straight line 4-1 (at normal time) changes to straight line 4-2 (at the time of temperature rise or deterioration).

In this case, the operating current for the reference light intensity Pa is automatically changed to IOp(b) under control. However, in characteristic 4-2, in order to change the light intensity P by ΔPG, an operating current change of ΔI o p (b) is required, but as described above, the operating current change width is Δl0P
If (a) is set as a constant, the maximum change in light intensity P in characteristic 4-2 is 8P1.

It is not possible to correctly provide the desired amount of change ΔPo.

(Purpose) The present invention has been made in view of the above-mentioned circumstances, and its purpose is to eliminate changes in the knee P characteristics of a semiconductor laser due to temperature rise or deterioration over time. An object of the present invention is to provide a novel light intensity control method that can always achieve correct light intensity for optical scanning.

(composition) The present invention will be explained below.

The light intensity control method of the present invention controls the light intensity of light emitted from a semiconductor laser in an optical scanning method in which modulated light from a semiconductor laser is deflected by a rotary deflector and a scanned surface is scanned without using an fθ lens. This is the way to do it.

The output intensity control circuit obtains a digital reference value signal for setting the emission intensity of the semiconductor laser to a reference value during optical writing scanning.

In addition, the image scanning clock frequency control circuit generates an image scanning clock whose frequency changes continuously according to changes in the scanning speed on the scanned surface, and also generates a correction voltage value corresponding to the frequency of the image scanning clock. get.

This correction voltage value is multiplicatively processed by the reference signal to obtain an analog signal that controls the optical output intensity proportionally to changes in scanning speed.

The semiconductor laser is driven while modulating this analog signal with a modulation signal.

According to this control method, even if the knee P characteristic of the semiconductor laser varies, the light intensity of optical scanning can always be maintained at an appropriate value. This will be explained below with reference to FIG.

In FIG. 5, straight lines 4-1 and 4-2 indicate the I-P characteristics of the semiconductor laser, as in FIG. Characteristic 4-1 is the I-P characteristic at normal times, and characteristic 4-2 is the I-P characteristic at the time of temperature rise and deterioration over time.

Regarding these characteristics 4-1.4-2. Differential quantum efficiency,
Let η(a) and ηΦ) be respectively.

In addition, Pr (a), Ps (b) & Ma indicate the intercept of the vertical axis of each I-P characteristic 4-1.4-2, and Ps (a)
+ Ps (b).

Po indicates the reference value of the emission intensity of the semiconductor laser. This reference value Po is set by a reference value signal from an output intensity control circuit.

Therefore, the operating current ■Op is determined by this reference value signal.
It is automatically set according to the I-P characteristics, and the above characteristics are set as characteristic 4.
-1, it is automatically set to Iop(a), and when the characteristic is 4-2, it is automatically set to Iop(b).

Now, using the symbols in Figure 6, I-P characteristics 4-1.4
-2 can be expressed as linear.

Next, consider the correction voltage value VC obtained from the image scanning clock frequency control circuit.

This correction voltage value Vc corresponds to the frequency of the image scanning clock, and the frequency of the image scanning clock changes depending on the scanning speed. Therefore, the correction voltage value Vc is also a function of the scanning speed υ, and can be written as Vc(υ). This corrected voltage value Vc (υ) is generated independently of the I-P characteristics of the semiconductor laser. Therefore, variations in the I-P characteristics of the semiconductor laser have no effect on the corrected voltage value Vc (υ).

Now, when carrying out the control method of the present invention, the light emission intensity of the semiconductor laser when the scanning speed of optical scanning is υ is set to P (a) in the normal characteristic (characteristic 4-1), It is assumed that when the I-P % characteristic changes to I-P characteristic 4-2, it changes to this or P(b). These P (
Let the operating currents IOp corresponding to a) and P(b) be Iop(a) and Iop(b), respectively.

Then, a linear relationship clearly holds.

Now, the correction voltage value Vc (υ) is for giving the emission intensity of the semiconductor laser at the scanning speed υ, and this correction voltage value VC is multiplicatively processed by the reference value signal.

Since the reference value signal is a signal intended to give the reference light intensity Po, it is set to V for characteristic 4-1.

Ca), characteristic 4-2, Vo (b”l),
Obviously, vo(a) : Vo(b) = Iop(a) : I
op(b) and vo(a), VOΦ) as VC(
When processed multiplicatively with υ), Vo(a) −Vc(υ)
: Vo(b) −Vc(υ)= Iop(a) :
Iop(b). However, since vc(υ) does not depend on the I-P characteristics, Vo(a):vo('b)=Vo(a)・VC(υ):
VoCb)・VC(υ), so in the end, Iop(a): Iop(b)= Iop(a):
Iop(b) (3) holds true. α), (
Applying equation (3) to equation 2), in that case, PI (a) =
-pt (b), we end up with P(
a)=i-pΦ) (4)
is obtained. That is, when the scanning speed is an arbitrary value υ within the speed range, the light intensity of the light emitted from the semiconductor laser remains substantially unchanged even if the I-P characteristics change.

Hereinafter, description will be given based on specific examples.

FIG. 1 shows one example of a circuit for implementing the present invention.

In the circuit example shown in FIG. 1, the portions other than the image scanning clock frequency control circuit 38 constitute an output intensity control circuit.

Therefore, with reference to FIG. 1, the setting of the emission intensity of the semiconductor laser to a reference value and the generation of the reference value signal will be explained first.

Now, the laser light emitted backward from the semiconductor laser 1o is received by the photosensor 14. The photosensor 14 outputs a current proportional to the intensity of the received light, and this current is converted into a stain pressure by an amplifier 16 and a comparator 18.
The voltage value VM is applied to the voltage value VM and compared with the reference voltage Vref. The output voltage of the comparator 18 is the voltage VM and Vre
Depending on the magnitude of f, the level will be high or low,
The count mode of the up/down counter 20 (hereinafter simply referred to as counter 2o) is controlled. for example,
When VM<Vref, that is, the semiconductor laser 1
When the output strength of o does not reach the reference value, comparator 1
8 becomes a low level, and the counter 2o enters the count mode, that is, the up mode, in which it operates as an up counter.When VM>Vref, the counter 2o enters the count mode snare, that is, the down mode, in which it operates as a down counter.

The edge detection circuit 32 detects the rising edge of the frame synchronization signal FSYNC, and the detection signal passes through an OR circuit 34 to an AND circuit 3o.
The AND is taken.

Flip-flop 28 is set at the beginning of the system standby mode by the output of AND circuit 30 to produce an output signal which is ANDed with the non-scanning signal in AND circuit 26.

The counter 20 is operated by the output signal of the AND circuit 26.
The disabled state is released and the clock pulses from clock pulse generator 24 are counted up or down.

The count output of the counter 20 is converted into an analog quantity by a D/converter 40 and applied to the semiconductor laser drive circuit 12. The same circuit 12 is.

The semiconductor laser 10 is driven by the modulation signal, and its operating current is changed depending on the output from the converter 40.

Therefore, as the count value of the counter 20 gradually increases (or decreases), the intensity of the laser light from the semiconductor laser 10 gradually increases (or decreases), and the voltage VM applied to the comparator 18 becomes , gradually increases (but still decreases).

When the voltage VM gradually changes and the magnitude relationship with Vref is reversed, the output of the comparator 18 is also color-inverted from a low level to a low level (high level and low level). At this time, edge detection circuit 22 detects the rising (or falling) edge of the output of comparator 18, resets flip-flop 28, and returns counter 20 to the disabled state. Therefore, the counter 20 holds the count value when the output of the comparator 18 is inverted, and therefore, the magnitude of the driving current of the semiconductor laser 10 is maintained as it is.

At this time, VM=Vref substantially, and the output intensity of the semiconductor laser 10 is set to a reference value set through the reference voltage Vref. In this manner, the digital signal output from the counter 20 is the reference value signal in a state where the emission intensity of the semiconductor laser 10 is set to the reference value.

Note that the edge detection circuit 22 detects the counter 2 only when the output of the comparator 18 is inverted from low level to high level.
0 may be configured to be disabled.

In this way, when the output level of the comparator 18 is inverted from a low level to a high level, it is the same as the above case, but when the output level is inverted from a high level to a low level, the following is done. become. That is, when the high level is reversed to the low level, the counter 20 operates as an up counter while remaining in the disabled state. Then, when the driving current of the semiconductor laser 10 increases and the output of the comparator 18 is reversed from a low level to a high level, the edge detection circuit 21 detects the rising edge and disables the counter 20 to perform its calculation. It keeps the numerical value.

Further, the counter 20 operates as a down counter when the output of the comparator 12 is at a low level, and operates as an up counter when the output is at a high level, so that the count value and the driving current of the semiconductor laser 10 are inversely proportional. You may also do so.

Note that the output control timing generation circuit 36 operates in standby mode in response to the frame synchronization signal FSYNC, generates an output control timing signal at a constant cycle, and outputs the output control timing signal to the OR circuit 3.
4, the power setting of the semiconductor laser 10 is performed at a constant cycle.

When scanning the photoreceptor, the non-scanning signal disappears, the AND circuit 26 turns off, the counter 20 becomes disabled, the semiconductor laser 10 is not driven during scanning in the standby state, and the power set of the semiconductor laser 10 is set as follows. If it is not completed, it will be interrupted.

Then, during non-scanning, the power set is restarted. As described above, when the emission intensity of the semiconductor laser is set to the reference value, a reference value signal is obtained from the counter 20. This reference value signal may change each time a power set is performed, but once a power set is performed, it does not change until the next time.

Next, generation of the correction signal will be explained.

FIG. 2 shows a specific example of the image scanning clock frequency control circuit 38 in FIG. 1.

In this image scanning clock frequency control circuit 38, a phase detection circuit 58, a low pass filter 60, a voltage controlled oscillator 621, and a frequency divider 64 constitute a phase locked loop circuit (hereinafter referred to as a PLL circuit).

The reference clock of frequency fo generated by the oscillator 54 is frequency-divided by the frequency divider 56 to become a position control clock of frequency fo/N, which is input to the control circuit 16 and the phase detection circuit 58 of the PLL circuit.

The phase detection circuit 58 compares the phases of the position control clock and the clock input from the branching device 64, and outputs the phase difference to the low-pass filter 62 as a pulse signal. Information on the phase difference is input to the voltage controlled oscillator 62 via the low-pass filter 60.
outputs a clock whose frequency corresponds to the output voltage of the low-pass filter 60.

This clock becomes the image scanning clock. The image scanning clock is frequency-divided by the frequency divider 64, applied as a clock to the phase detection circuit 58, and compared in phase with the position control clock.

The frequency division ratio of the frequency divider 64 is a fixed value, if this is M, and the clock applied from the frequency divider 64 to the phase detection circuit 58 and the position control clock (image scanning which is also emitted from the frequency divider 62) The clock frequency fK is fx=fo・−
It is.

In this state, the frequency division ratio of the frequency divider 56 is changed from N to N1, and the frequency fK of the image scanning clock is changed from fo.- to fo.
・Changes continuously and monotonically up to -.

l Therefore, by switching the frequency division ratio of the frequency divider 56 stepwise, an image scanning clock whose frequency changes continuously can be obtained.

Now, the control circuit 50 sets the preset value of the frequency division ratio in the frequency divider 56 to the up/down counter 52 (hereinafter referred to as
A clock CK for outputting from the counter 52 (simply referred to as a counter 52), a signal (2) for releasing the disabled state, and a signal U/D for setting up/down modes are generated. Note that the control circuit 50. A synchronizing signal obtained from the light receiving element 86 (FIG. 3) is applied to the frequency divider 56.

The up/down mode changes from up mode (or down mode) to down mode (
A signal U/D is generated so as to switch to the up mode.

When the clock CK is input manually, the counter 52 updates the preset value and switches the frequency division ratio of the frequency divider 56. The switching width ΔN is constant.

Now, the scanning area where optical scanning is performed is made up of a plurality of blocks BLI, BL2,...r BL' t in advance.
..., divided into BLK, each block BLi
For each (i=1 to K), the numbers Mi and nl (i=1 to K)
is stipulated.

Then, in the i-th block BLi, the position control clock is sent to the control circuit 50 every Mi pulses manually, and the control circuit 50 generates the clock CK, so that the frequency division ratio of the 1 frequency divider 56 is increased by ΔN. Switch. Block 13
1i, the clock CK is generated by ni regeneration f. Therefore, the block BLi uses the position control clock Mt a nl
Corresponds to the individual. Then, while optically scanning the block BLi, the frequency division ratio changes by ni·ΔN.

The number of blocks and the value of Milni are the voltage controlled oscillator 2
2, so that the frequency fx of the image scanning clock generated from 2 closely approximates the ideal frequency change as the scanning speed changes.
It is determined experimentally or theoretically depending on the design conditions of the optical scanning device.

An example is shown in FIG. In the figure, the curve represents the ideal image scanning clock fKo (as a rotating deflector, the
A special polygon mirror proposed in No.-274324 is envisaged. In this rotating polygon mirror, depending on the rotation angle α of the polygon mirror, the polarization angle θ of the light beam is sinθ=
The relationship (1--sin α) is satisfied. A and R are morphological constants of the polygon mirror. ), and the stepped graph line indicates one frequency Mfx+=-.fo, respectively. By switching the frequency division ratio N in stages, the frequency changes in a stepwise manner.

The numbers 5, 6, 10°16 below this fold gfK+ are ■, M2., respectively, with the right end of the figure as the scanning start side. M3
．． Compatible with M4. As is clear from the figure, nl
= 6, n2 = 9, n3 = 3,
n4 = 5. This figure shows only half of the symmetric figure, and as is clear from the symmetry, the number of blocks includes 7, M5 = 10, n5 = 3, M
6 = 6, n6 = 9, M7-5. n7
=6. Further, the switching width ΔN of the frequency division ratio N is 1.

As the frequency division rate is switched stepwise, the image scanning clock changes continuously to closely approximate the ideal frequency change fK. By the way, the frequency division ratio N is 69 at both ends of the scanning area.
, 89 in the center.

The above is the explanation of the image scanning clock frequency control circuit.

Now, returning to Figure 2 again, the low-pass filter 6
The output of 0 is taken out via the amplifier 68 and becomes a corrected voltage value.

This correction voltage value changes continuously in response to changes in scanning speed.

This corrected voltage value is q as shown in FIG.

The signal is applied to the converter 40, and the counter 20 is also multiplied by the reference value signal applied to the converter 40.

Therefore, the converter 40 outputs a signal corresponding to the product of the reference value signal and the correction voltage value, and this output signal is applied to the semiconductor laser drive circuit.

(Effects) As described above, according to the present invention, a novel light intensity control method can be provided in the optical scanning method.

Since this method is configured as described above, it is possible to always realize optical scanning with an appropriate amount of light, regardless of changes in the I-P characteristics due to temperature changes or deterioration over time of the semiconductor laser.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of a circuit for implementing the present invention, FIG. 2 is a block diagram showing an image scanning clock frequency control circuit, and FIG. 3 is a block diagram showing an example of an image scanning clock frequency control circuit. FIG. 4 is a diagram for explaining the method, and FIG. 5 is a diagram for explaining the problem to be solved by the present invention.
FIG. 6, which is a diagram for explaining the present invention, is a diagram for explaining the function of the image scanning clock frequency control circuit. 10...Semiconductor laser, 14...Photo sensor, 40...4 units, converter.

Claims (1)

[Claims] Deflecting modulated light from a semiconductor laser with a rotating deflector,
A method of controlling the emission intensity of a semiconductor laser in an optical scanning method that scans a surface to be scanned without using an fθ lens, in which the emission intensity of the semiconductor laser during optical writing scanning is set as a reference by an output intensity control circuit. A digital reference value signal is obtained for setting the value, and an image scanning clock frequency control circuit generates an image scanning clock whose frequency changes continuously according to changes in the scanning speed on the surface to be scanned. By obtaining a correction voltage value corresponding to the frequency of this image scanning clock, and processing this correction voltage value in a multiplicative manner using the reference value signal, the light output intensity is controlled proportionally to changes in the scanning speed. A light intensity control method, comprising: obtaining an analog signal, and driving the semiconductor laser while modulating the analog signal with a modulation signal.