JPH05232414A - Laser recording device - Google Patents

Abstract

PURPOSE:To obtain an image of high quality whose jag is reduced. CONSTITUTION:The laser beam emitted by a semiconductor laser 1 is collimated by a collimator lens 2 into a parallel beam, which is made incident on a variable aperture element. The projection beam from the variable aperture element can have its projection beam diameter and subscanning-directional center position selected. The projection laser beam is reflected by a rotary mirror 3 and imaged as a fine spot on a photosensitive body 5 through an image forming lens 4. A lens 7 which is arranged between the image forming lens 5 and photosensitive body 5 is a cylinder or toroidal lens which makes the incidence surface 8a of the variable aperture element and the photosensitive body surface conjugate in the subscanning direction. Electrodes of the variable aperture element are applied with a voltage according to screen information are selected to vary the subscanning-directional beam diameter on the photosensitive surface.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a laser recording device. For example, it is applied to a digital copying machine, a digital color copying machine, a laser printer, a laser fax, and the like.

[0002]

2. Description of the Related Art A laser printer combining electrophotographic technology and laser scanning technology can use plain paper and can obtain high-speed and high-quality images. Therefore, it has rapidly spread to computer output devices or digital copying machines. There is. FIG. 19 is a diagram showing a conventional laser scanning optical system.
1 is a semiconductor laser (LD), 52 is a collimating lens,
Reference numeral 53 is a rotary polygon mirror (polygon scanner), 54 is an imaging lens (fθ lens), 55 is a photoconductor, and 56 is a light receiving element (synchronous detection element). A laser beam emitted from a semiconductor laser 51, which is a light source modulated in accordance with an image signal, is collimated by a collimator lens 52 into a parallel beam, reflected by a rotary polygon mirror 53, and formed as a minute spot on a photoconductor by an imaging lens 54. It is imaged. The minute spot scans and exposes the photoconductor 55 by the rotation of the rotary polygon mirror 53 and the photoconductor 55, and forms an electrostatic latent image of an image. The light receiving element 56 placed outside the image area on the scanning start side on the scanning line is for controlling the image writing start position in the main scanning direction.

In this type of image output apparatus, when an image is digitally reproduced, for example, when an oblique line is recorded, a normal recording method causes a step (jaggy) for one dot as shown in FIG. , It doesn't become a smooth straight line. To reduce this jaggy, it is necessary to increase the recording density. However, in order to achieve high density, it is necessary to reduce the beam diameter on the photoconductor, which has a high frequency of the modulation clock of the laser as the light source,
The rotation speed of the rotary polygon mirror, which is a deflector, increases, and a non-contact air bearing or the like must be used instead of a normal ball bearing, resulting in a very high cost.

For example, in the "recording optical system" proposed in Japanese Patent Laid-Open No. 62-275214, it is possible to reduce jaggies by making the beam diameter in the sub-scanning direction on the photosensitive member variable. The dot arrangement of the diagonal lines in this case is shown in FIG.
As described above, it is possible to reduce the jaggies to some extent, but it is not possible to relatively change the dot recording position between the dots and the positions are constant, so it is not possible to effectively reduce the jaggies.

The "variable aperture element" proposed in Japanese Patent Laid-Open No. 3-75725 has a groove formed in an electro-optic crystal, and an electrode is attached to a microaperture element partitioned by the groove to form a polarizer. It is placed between analyzers and a voltage is applied to turn the light on and off. Also,
The "laser recording device" proposed in Japanese Patent Application Laid-Open No. 3-75716 makes the dot diameter variable by using an aperture element which has a plurality of aperture elements and whose aperture amount for an incident laser beam is variably controlled. It was done.

[0006]

An object of the present invention is to provide a high-quality image in which jaggies (jagged lines) are reduced, and to make the dot position variable in the sub-scanning direction. With this, it is possible to reduce density unevenness caused by uneven scanning pitch on the recording medium due to tilting of the reflecting surface of the rotary polygon mirror with respect to the rotation axis, and to realize high-definition halftone image expression, and diversify expression patterns on the screen. The purpose of the present invention is to provide a laser recording device that can be easily improved in quality.

[0007]

In order to achieve the above object, the present invention provides (1)
A laser light source that is modulated according to an image signal, a variable aperture element that receives a laser beam from the laser light source, and a deflection element such as a rotating polygon mirror for deflecting and scanning an outgoing beam from the variable aperture element, The variable aperture element includes an imaging optical system that forms a deflection laser beam by the deflection element on the recording medium as a minute spot, and a recording medium that moves in a direction orthogonal to the scanning direction of the deflection laser beam by the deflection element. A one-dimensional variable aperture element whose aperture amount can be changed in the sub-scanning direction of the incident laser beam (direction orthogonal to the scanning direction of the deflected laser beam),
In a laser recording device in which the surface of the variable aperture element and the surface of the recording medium are in a conjugate relationship of image formation, the variable aperture element can change the aperture amount asymmetrically with respect to the optical axis of the incident laser beam in the sub-scanning direction. 1D variable aperture element, and (2) the variable aperture element is an optical microshutter array using the electro-optical effect in which a plurality of variable aperture elements are arranged in the sub-scanning direction, and each optical microshutter is independent. Or (3) the light source is a semiconductor laser, the light output of the semiconductor laser is detected by a light receiving section, and a light receiving signal and a light emission level command proportional to the light output of the semiconductor laser obtained from the light receiving section are provided. An optical / electrical negative feedback loop for controlling the forward current of the semiconductor laser so that the signal becomes equal to the signal, the received light signal and the emission level command The light output / forward current characteristics of the semiconductor laser, the coupling coefficient between the light receiving section and the light output of the semiconductor laser, and the light input / light receiving signal characteristics of the light receiving section so that The semiconductor laser is controlled by a conversion means for converting a level command signal into a forward current of the semiconductor laser, and a current which is the sum or difference of the control current of the optical / electrical negative feedback loop and the current generated by the conversion means. And a semiconductor laser control unit that operates. Hereinafter, an explanation will be given based on an embodiment (claim 1) of the present invention.

First, the variable aperture element will be described. FIG. 2 is a diagram showing a variable aperture element, and is a diagram showing a case where an electro-optical effect is used. In the figure, 9 is the incident surface of the variable aperture element, 10 is an electro-optic crystal, 11 is a common transparent electrode, 12a to 12e are a plurality of transparent electrodes, 13 is a polarizer, and 14 is an analyzer. LiNbO ₃ , KH ₂ P
A common transparent electrode 11 is provided on one surface of an electro-optic crystal 10 such as O ₄ , LiTaO ₃ , and PLZT, and a large number of insulated transparent electrodes 12 a to 12 e are provided adjacent to the other surface. The laser beam, which is linearly polarized or linearly polarized by the polarizer 13 or the like, is used to enter the electrode surface. Analyzer 14 on the output side
Is arranged so that no laser beam is emitted when no voltage is applied between the electrodes. When a voltage is selectively applied to each electrode with such a configuration, the polarization direction of only the voltage application portion of the incident laser beam is rotated by the electro-optical effect, and is blocked by the analyzer 14.

For example, when a voltage is applied to the electrodes 12a and 12e, the beam of D ₂ is transmitted with respect to the incident beam D ₁ as shown in FIG. Further, by applying a voltage electrode 12a, to 12b, the beam offset D ₃ of the optical axis with respect to the incident beam D _1, as shown in FIG. 4 (beam center) is transmitted.
However, as can be easily understood, the laser beam diameter and the optical axis in the direction orthogonal to the arrangement of the electrodes 12a to 12e are not changed. In this way, the variable aperture element can change the beam diameter of the emitted beam or move the beam center by selectively applying a voltage to each electrode.

FIG. 1 is a block diagram for explaining an embodiment of a laser recording apparatus according to the present invention, in which a variable aperture element is arranged in a laser scanning optical system. In the figure, 1 is a semiconductor laser (LD), 2 is a collimator lens, 3 is a rotary polygon mirror (polygon scanner), 4 is an imaging lens (fθ lens), 5 is a photoconductor, and 6 is a light receiving element (synchronous detection element). ,
Reference numeral 7 is a lens, 8 is a variable aperture element, and 8a is an incident surface of the variable aperture element.

A laser beam emitted from a semiconductor laser 1 which is a light source modulated in accordance with an image signal is collimated by a collimator lens 2 and enters a variable aperture element 8. As described above, the output beam from the variable aperture element 8 can be selected in the output beam diameter and the center position of the output beam in the sub-scanning direction. The emitted laser beam is reflected by the rotary polygonal mirror 3 and is imaged as a minute spot on the photoconductor 5 by the imaging lens 4. Here, the imaging lens 4
The lens 7 disposed between the photosensitive member 5 and the photoconductor 5 conjugates the incident surface 8a of the variable aperture element and the photosensitive member surface in the sub-scanning direction (the incident surface 8a of the variable aperture element is set to the photosensitive member surface in the sub-scanning direction). It is a cylinder or toroidal lens (as long as it forms an image).

As described above, by making the incident surface 8a of the variable aperture element and the photosensitive member surface conjugate with each other in the sub-scanning direction, FIG.
If a voltage is applied to the electrodes 12a and 12e as shown in,
The beam on the surface of the photoconductor has a smaller laser beam diameter in the sub-scanning direction than in the case where no voltage is applied (shown by the broken line in FIG. 5) as shown in FIG. By selecting the electrode to which the voltage is applied according to the image information, the beam diameter in the sub-scanning direction on the surface of the photoconductor can be changed, so that it is possible to select a plurality of gradations for one dot. Further, when a voltage is applied to the electrodes 12a and 12b as shown in FIG. 4, the beam on the surface of the photosensitive member is more subordinate than that when no voltage is applied (shown by a broken line in FIG. 6) as shown in FIG. The beam center in the scanning direction will shift.

As described above, by selecting the electrode to which the voltage is applied according to the image information, the center of the beam on the surface of the photosensitive member can be shifted, so that the recording pitch in the sub-scanning direction is not fixed, and FIG. It is possible to record diagonal lines as shown in FIG. 6 and reduce jaggies. Similarly,
It is also possible to correct the fluctuation of the pitch between the scanning lines due to the variation of the normal line of each reflecting surface of the rotary polygon mirror in the sub-scanning direction (reflecting surface tilt) to reduce the density unevenness. Although the case where the number of electrodes is 5 has been described above, the present invention is not limited to this number.

Next, the invention according to claim 2 will be described. FIG. 10 is a diagram showing an optical micro-shutter array using a plurality of electro-optical effects arranged in the sub-scanning direction (direction orthogonal to the scanning direction of the deflected laser beam), in which 20 is an optical micro-shutter array and 21 is an optical micro-shutter array. Electro-optic crystal, 22 is a groove, 23a and 23b are recesses, and 24a to 24g are light transmitting regions. The hatched portions in the figure show separated electrodes. Recesses 23a and 23b are formed in the PLZT electro-optic crystal 21 whose both surfaces are optically polished by a dicing saw, and the light transmitting regions 24a to 23b are formed between the recesses 23a and 23b.
Leave 24g. Next, an electrode film is formed in the recess by sputtering, and the groove 22 is dug in the direction orthogonal to the recesses 23a and 23b to separate the electrode film. Through these steps, a plurality of shutters (here, seven) for turning on and off the light are formed. The operation will be described below.

FIG. 11 is a diagram for explaining the operation of the optical micro-shutter array, in which 28 is a deflector and 29 is an analyzer, which are arranged so that their polarization directions are orthogonal to each other, and are applied to the optical micro-shutter. It is installed at an angle of 45 ° with the direction. 30 shows an example of emitted light. 1
In each of the micro shutters, the intensity of the incident light is Ii,
When the intensity of the emitted light is I ₀ , I ₀ = Ii sin ² (Γ / 2) (1) where Γ = π · t · n ₀ ³ · Rc · E ² / λ λ: wavelength, t: light Micro shutter thickness, n ₀ :
Refractive index, E: electric field strength, Rc: secondary electro-optic constant.

In the equation (1), I ₀ is maximum when the phase difference Γ is mπ (m is an odd number), and the optical micro shutter is in an open state. When the phase difference Γ is m'π (m 'is an even number), I ₀ becomes the minimum and the optical micro shutter is closed.
Light on / off area (optical micro shutter) 24a-2
An arbitrary shutter can be opened and closed by independently applying a voltage to 4 g. Therefore, it is possible to change the beam diameter of the outgoing beam and move the beam center in the same manner as described in the first aspect of the invention. Further, by disposing the optical micro-shutter array 20 at the position of the variable aperture element 8 in FIG. 1, it becomes possible to change the beam diameter in the sub-scanning direction and shift the beam center on the photoconductor surface.

FIG. 12 shows light on / off regions 24a-2.
Light intensity distribution in the sub-scanning direction on the photoconductor surface when all 4g are ON, FIG. 13 is a light intensity distribution in the sub-scanning direction on the photoconductor surface when 24c, 24d, and 27e are on, FIG. , 24d, 24e, 24f, and 24g are on, the light intensity distribution in the sub-scanning direction on the photoconductor surface is shown.

FIGS. 15A and 15B are views showing another embodiment of the optical micro-shutter array, in which 31a and 3a are shown.
1b is a cylinder lens, 32 is a beam waist position, 3
Reference numeral 3 denotes a beam position, and other parts having the same functions as those in FIG. 11 are designated by the same reference numerals. Two cylinder lenses 31a and 31b having a curvature in the main scanning direction are provided, and a light on / off region (optical micro shutter) 24a is provided.
˜24 g is the beam waist position of the incident beam by the cylinder lens 31 a (the position where the beam is most narrowed)
32. The incident beam is the cylinder lens 3
By 1a, the laser beam on the optical micro-shutter surface is vertically elongated in the array direction, as indicated by the broken line 33 in FIG. Therefore, the light utilization efficiency is improved as compared with the case where the cylinder lens is not used (shown by the one-dot chain line 32 in FIG. 15B). Further, since the size of the optical micro-shutter in the main scanning direction can be reduced, there is an advantage that the speed can be increased. The array having seven optical micro shutters has been described above, but the present invention is not limited to this number.

Next, the invention according to claim 3 will be described. FIG. 16 is a block diagram of a semiconductor laser control circuit, in which 40 is a driven semiconductor laser (corresponding to 1 in FIG. 1), 4
Reference numeral 1 is a comparison amplifier, 42 is a current converter, and 43 is a light receiving element. The emission level command signal is input to the comparison amplifier 41 and the current converter 42, and a part of the optical output of the driven semiconductor laser 40 is monitored by the light receiving element 43. The comparison amplifier 41, the semiconductor laser 40, and the light receiving element 43 form an optical / electrical negative feedback loop.
The received light signal proportional to the photocurrent (proportional to the optical output of the semiconductor laser 40) induced in the laser and the light emission level command signal are compared, and the forward current of the semiconductor laser 40 is compared with the light received signal and the light emission level according to the result. Control so that it becomes equal to the command signal. Further, the current converter 42 sets a current (light output / forward current characteristic of the semiconductor laser, a light receiving element and a semiconductor laser) preset according to the light emission level so that the light reception signal and the light emission level command signal become equal to each other. It outputs a preset current based on the coupling coefficient and the light input / light receiving signal characteristics of the light receiving element. The sum of the output current of the current converter 42 and the control current output by the comparison amplifier 41 is the forward current of the semiconductor laser 40.

When the crossover frequency in the open loop of the optical / electrical negative feedback loop is f0 and the gain is 10,000, the step response characteristic of the optical output Pout of the semiconductor laser 40 can be approximated as follows. Pout = PL + (PS−PL) exp (−2πf0t) PL: Optical output at t = ∞ PS: Light intensity set by current converter 42 DC gain in open loop of optical / electrical negative feedback loop is 1000
Since 0 is set, PL is considered to be equal to the set light amount when the allowable range of the setting error is 0.1% or less. Therefore, if the light quantity PS set by the current converter 42 is equal to PL, the semiconductor laser 40 is instantaneously released.
Light output of PL becomes equal to PL. Also, due to disturbances, PS
Even if the value fluctuates by 5%, if f0 = 40 MHz, the error of the optical output of the semiconductor laser 40 with respect to the set value becomes 0.4% or less after 10 ns. By using the high-speed, high-precision, and high-resolution semiconductor laser control circuit realized in this way, the exposure light amount can be controlled accurately and at high speed.

Therefore, the fluctuation of the light quantity which occurs when the opening amount is changed by the variable aperture element (compared to the case where the voltage shown by the broken line in FIGS. 5 and 6 is not applied, the total light quantity is lowered when the opening amount is changed. Yes) can be corrected, and dots can always be recorded with the same light amount. For example, diagonal lines can be recorded as shown in FIG. Further, since the light quantity can be controlled with high precision, the size of the dots to be recorded can be determined and finely controlled, as compared with the case where only the variable aperture element is used, and the halftone image expression can be made highly precise. Although the inventions described in claims 1 to 3 have been described above, by combining the pulse width modulation of the semiconductor laser, it is possible to change the dod size in the main scanning direction as shown in FIG. It is possible to reduce the amount of noise and improve the definition of halftone images.

[0022]

As is apparent from the above description, the present invention has the following effects. (1) In the laser recording apparatus according to the first aspect, the beam diameter in the sub-scanning direction on the photoconductor surface can be changed by selecting the electrode to which the voltage is applied according to the image information. Since the gradation of can be selected and the center of the laser beam in the sub-scanning direction on the photoconductor can be selected, the recording pitch in the sub-scanning direction is not fixed and jaggies are reduced. A laser recording device capable of high-quality image output can be provided. (2) In the laser recording apparatus according to the second aspect, the beam diameter in the sub-scanning direction on the photoconductor surface can be changed by selecting the electrode to which the voltage is applied according to the image information. Since the gradation of can be selected and the center of the laser beam in the sub-scanning direction on the photoconductor can be selected, the recording pitch in the sub-scanning direction is not fixed and jaggies are reduced. High-quality image output is possible. In addition, since the optical micro-shutter array is used, each electro-optical effect element becomes minute and the electrostatic capacity becomes small, so that it is possible to provide a laser recording device capable of low voltage driving and high speed. (3) In the laser recording apparatus according to the third aspect, the beam diameter in the sub-scanning direction on the photoconductor surface can be changed by selecting the electrode to which the voltage is applied according to the image information. Since the gradation of can be selected and the center of the laser beam in the sub-scanning direction on the photoconductor can be selected, the recording pitch in the sub-scanning direction is not fixed and jaggies are reduced. High-quality image output is possible. In addition, since the amount of light can be controlled with high accuracy, the size of the dots to be recorded can be determined and controlled more finely than in the case of using only the variable aperture element, and a laser recording device capable of outputting high-definition halftone images is provided. it can.

[Brief description of drawings]

FIG. 1 is a configuration diagram for explaining an embodiment of a laser recording apparatus according to the present invention.

FIG. 2 is a diagram showing a variable aperture element.

FIG. 3 is a diagram showing states of an incident beam and an outgoing beam in a variable aperture element.

FIG. 4 is a diagram showing states of an incident beam and an outgoing beam in a variable aperture element.

FIG. 5 is a diagram showing a light intensity distribution of a beam on a surface of a photoconductor.

FIG. 6 is a diagram showing a light intensity distribution of a beam on a surface of a photoconductor.

FIG. 7 is a diagram showing dot recording.

FIG. 8 is a diagram showing dot recording.

FIG. 9 is a diagram showing dot recording.

FIG. 10 is a diagram showing an optical micro shutter array.

FIG. 11 is a diagram for explaining the operation of the optical micro shutter array.

FIG. 12 is a diagram showing a light intensity distribution in the sub-scanning direction on the surface of the photoconductor.

FIG. 13 is a diagram showing a light intensity distribution in the sub-scanning direction on the photoconductor surface.

FIG. 14 is a diagram showing a light intensity distribution in the sub-scanning direction on the surface of the photoconductor.

FIG. 15 is a diagram showing another embodiment of the optical micro shutter array.

FIG. 16 is a configuration diagram of a semiconductor laser control circuit.

FIG. 17 is a diagram showing dot recording.

FIG. 18 is a diagram showing dot recording.

FIG. 19 is a diagram showing a conventional laser scanning optical system.

[Explanation of symbols]

1 ... Semiconductor laser (LD), 2 ... Collimating lens, 3
... rotary polygon mirror (polygon scanner), 4 imaging lens (fθ lens), 5 photoconductor, 6 light receiving element (synchronous detection element), 7 lens, 8 variable aperture element, 8a
Incident surface of variable aperture element.

Claims (3)

[Claims] 1. A laser light source that is modulated according to an image signal, a variable aperture element that receives a laser beam from the laser light source, a rotary polygon mirror for deflecting and scanning an outgoing beam from the variable aperture element, and the like. A deflecting element, an imaging optical system for forming a deflected laser beam by the deflecting element as a minute spot on a recording medium, and a recording medium moving in a direction orthogonal to the scanning direction of the deflected laser beam by the deflecting element. The variable aperture element is a one-dimensional variable aperture element whose opening amount is variable in the sub-scanning direction of the incident laser beam, and the laser recording device in which the variable aperture element surface and the recording medium surface have a conjugate relationship of image formation. In the above, the variable aperture element is a one-dimensional variable aperture element capable of asymmetrically changing the aperture amount with respect to the optical axis of the incident laser beam in the sub scanning direction. A laser recording device characterized by being a aperture element. 2. The variable aperture element is an optical micro shutter array using a plurality of electro-optical effects arranged in the sub-scanning direction, and each optical micro shutter can be independently driven. Laser recorder. 3. The light source is a semiconductor laser, and a light receiving portion which detects the light output of the semiconductor laser by a light receiving portion and is proportional to the light output of the semiconductor laser obtained from the light receiving portion is equal to a light emission level command signal. So that an optical / electrical negative feedback loop for controlling the forward current of the semiconductor laser,
The light output / forward current characteristics of the semiconductor laser, the coupling coefficient between the light receiving unit and the light output of the semiconductor laser, the light input of the light receiving unit, so that the light receiving signal and the light emission level command signal become equal. A conversion unit that converts the emission level command signal into a forward current of the semiconductor laser based on a light reception signal characteristic, and a sum or difference of a control current of the optical / electrical negative feedback loop and a current generated by the conversion unit. And a semiconductor laser control unit that controls the semiconductor laser according to the current of 1.