JPH01254911A - Picture element clock generator of laser scanning optical system - Google Patents

Abstract

PURPOSE:To lessen the deterioration of dot arrangement accuracy by a fluctuation in scanning speed at a low cost and to generate stable picture element clocks for picture element signal modulation by providing an adder circuit which adds the signals from the photoelectric transducers of plural photodetecting parts. CONSTITUTION:A lens array 22 for split condensing is provided and the photodetecting parts 25 consisting of the plural photoelectric transducers which are of the same number as the number of the lens elements receiving the beams condensed by the respective lens elements of said lens array and are arranged to the respective lens condensing positions are provided. The adder circuit 27 which adds the signals from these photoelectric transducers is provided. A PLL circuit 28 which outputs the picture element clocks by receiving the reference pulse signal input from the adder circuit is provided. Further, grating scanning length by the beam for generating the picture element clocks is set longer than the recording scanning length by a laser beam for recording and the reference pulse signal is generated earlier than the start of scanning. The stable picture element clocks are thereby obtd.

Description

TECHNICAL FIELD The present invention relates to a pixel clock generation device in a raster type laser scanning optical system.

Prior Art Conventionally, this type of laser scanning optical system has been used in laser printers,
Laser plate making machine, laser facsimile, digital copying machine,
It is well known in connection with writing optical signals and reading document information in devices such as flying spot scanners.

In such a scanning optical system, a laser beam for scanning is generally deflected and scanned by a deflector such as a rotating polygon mirror. In order to properly perform such optical scanning, a synchronization signal is required to time the optical scanning.

Therefore, in general, the laser beam is received by a single light-receiving element placed at a position on the scanning optical path of the laser beam on the scanning start side and outside the image range, and a synchronization signal is obtained by this light-receiving element. The laser beam is modulated by image information in synchronization with the signal. but,
Since this method only synchronizes at the start of scanning,
On the image terminal side, the speed or timing of optical scanning is not necessarily constant for each scan due to uneven rotation of the deflector for laser beam scanning, uneven processing accuracy, and the like. As a result, the dot arrangement accuracy, that is, the printing quality, etc. deteriorate.

In this regard, as a method of generating a pixel clock using a grating (also referred to as a slit, a grid, or a scale), for example, there is a method disclosed in Japanese Patent Laid-Open No. 10967/1983. In this method, a scattering surface is placed behind the grating (scale), and the scattered light is received by a light-receiving element placed on the side and photoelectrically converted to obtain a reference pulse signal, which is synchronized in phase with the pixel clock. It is something that generates. However, since this method uses scattered light, the amount of light received by the light-receiving element is small, and a special light-receiving element (for example, Photomaru) with high sensitivity is required.

In other words, it is expensive and has low sensitivity to semiconductor lasers.

Furthermore, Japanese Patent Laid-Open No. 60-75168 discloses a method using a concave mirror array and a plurality of small diameter light receiving elements (for example, bin photodiodes). Then, the signals from these light receiving elements are added together and used directly as a pixel clock to drive the AOM. This method requires fine gratings with the same pitch as the recording density, but such gratings are difficult to manufacture and expensive.

Purpose The present invention has been made in view of the above points, and provides a grating-type laser that generates a pixel clock for stable pixel signal modulation, which can reduce the deterioration of dot arrangement accuracy due to scanning speed fluctuations at low cost. The object of the present invention is to obtain a pixel clock generation device in a scanning optical system.

Structure In order to achieve the above object, the present invention provides a deflector for deflecting the recording laser beam and the pixel clock generation laser beam in the optical path of the recording laser beam and the pixel clock generation laser beam. A grating in which transparent parts and non-transparent parts are formed at a pitch of N times the recording density is arranged on the scanning line, and a photoelectric conversion element is provided to photoelectrically convert the transmitted light from the grating, and a reference pulse signal from the photoelectric conversion element is provided. In a pixel clock generation device in a laser scanning optical system, which is provided with a PLL circuit that generates a pixel clock multiplied by N in phase synchronization with the grating, the laser beam transmitted through the grating is arranged in a line in a length corresponding to the scanning line length. A lens array for dividing light condensing in which a plurality of lens elements are arranged is provided, and the same number of lens elements as the number of said lens elements receiving the beam condensed by each lens element of this lens array are arranged at each lens condensing position. A light receiving section including a plurality of the photoelectric conversion elements is provided, an addition circuit is provided for adding up signals from these photoelectric conversion elements, and a P is configured to output a pixel clock upon receiving a reference pulse signal input from the addition circuit.
LL circuit is provided, and further, the grating scanning length by the pixel clock generation beam is set longer than the recording scanning length by the recording laser beam, and the reference pulse signal is generated before the start of recording. do.

An embodiment of the present invention will be described below with reference to FIGS. 1 to 4. FIG. 3 shows an example of a recording apparatus using a laser scanning optical system using a semiconductor laser as a light source to which the present invention is applied. First, two semiconductor lasers (LD) 11 and 12 are provided. The semiconductor laser 11 emits a recording laser beam P1 modulated by a pixel signal. The semiconductor laser 12 emits a laser beam P for generating a pixel clock. Beams P, . After the beam diameter in the sub-scanning direction is shaped by the cylinder lens 16, these beams enter one reflecting surface of a rotating polygon mirror 17, which is a deflector, and are deflected as the beam rotates. Both beams P, , P, deflected by this rotating polygon mirror 17 are transmitted through an fθ lens 18
incident on . The fθ lens 18 has an anamorphic structure that has a function of correcting the inclination of each reflecting surface of the rotating polygon mirror 17, that is, it has a structure in which the focal length is different in the main scanning direction and the sub-scanning direction. The recording beam P1 is an fθ lens 18
The light is narrowed down to a minute spot on a photosensitive recording medium 19 such as a photoconductive photoreceptor, and the recording medium 9 is exposed and scanned in the main scanning direction.

On the other hand, the pixel clock generation beam P transmitted through the fθ lens 18 is reflected by the separation mirror 20, and the recording beam P
It is separated from 2. For this reason, first, the semiconductor laser 12
The optical axis of the pixel clock generation beam P8 emitted from the recording beam P, as shown by broken lines in FIGS. 3 and 4, is
It has a slight angle in the sub-scanning direction with respect to the optical axis (shown by a solid line), and after being deflected by a rotating polygon mirror 17, it can be separated from the recording beam P1 by this separation mirror 20. That is, both beams P, , P are incident on the reflective surface of the rotating polygon mirror 17 at slightly different angles, and are reflected in slightly different directions. However, when projected onto a plane perpendicular to the rotation axis of the one-rotation polygon 17, the reflected lights of both beams P I IF5 overlap each other (that is, they are at the same position in the main scanning direction).

The pixel clock generating beam P2 separated and reflected by the separation mirror 20 enters the grating 21 and scans the grating 21 as the rotating polygon mirror 17 rotates. Although this grating 21 is not particularly shown, as is well known, transparent areas (bright areas) and opaque areas (dark areas) are arranged at a constant pitch, specifically, at a pitch that is N times the recording pixel density (N is an integer). It is formed by forming a large number of them in succession alternately. When the pixel clock generation beam P passes through the grating 21 while scanning, its light intensity is modulated according to the arrangement of the transparent portions and opaque portions.

The beam transmitted through this grating 21 is incident on a lens array 22. This lens array 22 includes a plurality of (
In this embodiment, condensing lenses 23a to 23n (n pieces) as condensing elements are arranged in an array in close contact over the entire length of the scanning line. Further, on the opposite side of the grating 21 with respect to the lens array 2, there is a light receiving system 25 in which the same number of light receiving elements 24a to 24n (specifically, pin photodiodes) as the number n of the condensing lenses 23 of the lens array 22 are arranged in an array. It is deployed. The light-receiving surface of each light-receiving element 24 serves as a light-receiving part of the light-receiving system, and these light-receiving parts are provided in a one-to-one relationship with each condenser lens 23 in the lens array 22, and The light receiving section corresponding to the lens is set to receive the condensed light. Therefore,
When the pixel clock generation beam P2 that has passed through the grating 21 is incident on one of the condensing lenses 23, this light is condensed on the light receiving element 24 corresponding to this condensing lens 23. That is, the beam transmitted through the grating 21 is split and received by the light receiving system 25 by the lens array 22.

The light receiving signals received by these light receiving elements 24a to 24n and subjected to photoelectric conversion are amplified by amplifiers 26a to 26n, respectively, and then added by an adding circuit 27. As a result, the signal becomes a pulse train (reference pulse) signal over the entire scanning length according to the bright and dark arrangement of the grating 21, and after being waveform-shaped by the waveform-shaping circuit 28 as necessary, the signal is processed by the PLL.
A pixel clock is generated by processing by a (phase locked loop) circuit 29.

Here, the configuration and operation of the second PLL circuit 29 will be explained with reference to FIG. First, the reference pulse generated by the adder circuit 27 is
A phase comparator 32 compares the phase difference with a comparison pulse obtained by dividing the pixel clock output from a voltage controlled oscillator (VCO) 30 by 1/N by a frequency divider 31. The output from this phase comparator 32 is output to the voltage controlled oscillator 30 via a low pass filter (LPF') 33 that removes noise and high frequency components, so that the phases of the reference pulse and comparison pulse match. Voltage controlled oscillator 30
is controlled by feedback. As a result, the voltage controlled oscillator 30 generates a pixel clock that is phase synchronized with the reference pulse and multiplied by N. Changes in scanning speed (changes in frequency of reference pulse) by such a PLL circuit 29
A pixel clock that follows can be obtained. Therefore, recording is performed while modulating the pixel-corresponding recording information output from the printer controller or host machine 34 to the drive modulation circuit 35 for the recording semiconductor laser 11 in synchronization with the pixel clock from the PLL circuit 29. This enables exposure recording with high dot arrangement accuracy. That is,
Even if the scanning speed fluctuates due to uneven rotation of the rotating polygon mirror 17 during recording, the modulation timing of the semiconductor laser 11 is controlled by the pixel clock accordingly, making it possible to perform proper optical writing.

Note that the semiconductor laser 1 for the pixel clock generation beam P8
2 is driven to emit light by a drive circuit 36.

As described above, according to this embodiment, since the lens array 22 is used for condensing light, it is possible to use small-diameter light-receiving elements with small junction capacitance, such as the bin photodiodes 24a to 24n, which improves high-speed processing. It can be done at low cost. In addition, the grating 21 has a pitch between transparent parts and non-transparent parts.
Since it has a coarse pitch structure N times the recording density, it is easy to manufacture and is not easily affected by contamination due to dust or the like.

Note that in this embodiment, semiconductor lasers 11 and 12 are used as light sources.
was used, but other light sources such as He-Ne laser,
When a gas laser such as an Ar laser is used, the configuration may be as shown in FIGS. 5 and 6. First, in the case of the gas laser 41, unlike a semiconductor laser, it is a continuous wave laser, so one light source is sufficient. When considering the recording beam P1, the printer controller or host machine 34
a modulator driver 42 driven according to recorded information from
Accordingly, the laser from the gas laser 41 is modulated and output to the rotating polygon mirror 17 side through an A10 modulator 43 such as an AOM. Therefore, the laser beam from the gas laser 41 is split into two by a beam splitter 44 at a position before the modulator 43 to separate the pixel clock generation beam P2. Then, while changing the separated pixel clock generation beam P to the same optical path as in the case of the beam from the semiconductor laser 12 of the previous embodiment using the three mirrors 45, 46, and 47, Together with the recording laser beam P, the laser beam is made to enter the collimator lens 16 side. The rest is the same as in the previous embodiment.

Next, a second embodiment of the present invention will be described with reference to FIGS. 7 and 8. First, P L L as shown in Figure 2
In the circuit 29, after the reference pulse signal is input, the PLL
A certain amount of time (pull-in time) as shown by L8 in FIG. 8(b) is required until the circuit 29 itself stably outputs a pixel clock whose phase is synchronized with this reference pulse signal. On the other hand, in this type of laser scanning optical system, the reference pulse is input as an intermittent signal for each scan, as shown in FIG. 8(a). Therefore, the pixel clock becomes unstable at pull-in clock t1 immediately after the start of scanning.

In this embodiment, in order to avoid the unstable recording operation at the pull ink t1, the image recording width (main scanning direction) on the photosensitive recording medium 19 is changed as shown in FIG.
The pixel clock generation layer beam P for the grating 21 is set to have a grating scanning width (main scanning direction) G which scans wider, that is, wider, on the scanning start side than W. That is, O in the scanning start direction.
In contrast to the recording laser beam P which starts the recording operation from a deflection angle of l, the pixel clock generation beam P2 starts scanning the grating 21 at a deflection angle 02 with a relationship of θ, minus θ2 in the scanning start direction. It is. As a result, as shown in FIG. 8, a stable lock region t of the pixel clock
The recording operation will be performed at step 8.

By the way, if the condensing lenses 23a to 23n, which are the lens elements of the lens array 22 used in these examples, are made of glass, they may be single condensing lenses 23a' to 23n, as shown in FIG. 9(a). 23n' are combined in an array. However, the glass condensing lenses 23a' to 23n
′, the edges are likely to be chipped during processing. As a countermeasure to this problem, it is possible to chamfer the edges.

However, when the condenser lenses 23a' to 23n' having chips or chamfers 50 are combined as shown in FIG. 9(a), it becomes impossible to transmit light through the grating at the lens boundary 51.

As a result, the reference pulse is lost at the lens joint, causing a discontinuous portion in the reference pulse, making it impossible to obtain a stable pixel clock.

In this regard, as shown in FIG. 9(b), the condenser lenses 23a to
The lens array 22 is made of a plastic lens in which 23n is integrally molded, so that no chipping occurs and 5.
There is no chamfering, and there is no loss of reference pulses between the respective condensing lenses 23. As a result, a stable and continuous reference pulse 3-pixel clock can be obtained. Also, compared to glass,
It is also low cost.

Effects The present invention is constructed as described above, and first, since a lens array is used for condensing light, a small-diameter light-receiving element with a small junction capacitance can be used as a photoelectric conversion element, which is low cost and suitable for high speed. In addition, since the grating was made with a coarse pitch N times the recording density,
It should be less susceptible to dirt from dirt, etc.
As a result, it is possible to obtain a stable and continuous pixel clock for highly accurate pixel signal modulation, and by making the grating scan length long, it is not affected by the pull-in time of the PLL circuit. A stable pixel clock can be obtained within the recording range.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a first embodiment of the present invention, and FIG.
The figure is a block diagram of the PLL circuit, Figure 3 is a schematic perspective view of the scanning optical system, Figure 4 is a side view of the optical system, Figure 5 is a schematic perspective view showing a modified example, and Figure 6 is its block diagram. , FIG. 7 is a plan view showing a second embodiment of the present invention, FIG. 8 is a timing chart thereof, and FIG. 9 is an explanatory diagram of the lens array configuration. 17... Deflector, 21... Grating, 22...
... Lens array, 23a to 23n... Condensing lens,
24a to 24n... Photoelectric conversion element, 25... Light receiving section, 27... Addition circuit, 29... PLL circuit, Pl
... Laser beam for recording, P... Beam for pixel clock generation, G... Grating scanning length, W...
Recording scan length, 716 Figure 17

Claims (1)

[Claims] 1. A deflector is provided in the optical path of the recording laser beam and the pixel clock generation beam to deflect these laser beams, and recording is performed on the scanning line of the deflected pixel clock generation laser beam. A grating with transparent parts and non-transparent parts formed at a pitch N times the density is arranged, and a photoelectric conversion element is provided to photoelectrically convert the transmitted light from this grating, and the reference pulse signal and phase from the photoelectric conversion element are Sync N
In a pixel clock generation device in a laser scanning optical system that is provided with a PLL circuit that generates a multiplied pixel clock, a plurality of lens elements are arranged in a row with a length corresponding to the scanning line length to transmit the laser beam that has passed through the grating. A split focusing lens array is provided, and a plurality of said photoelectrons arranged at each lens focusing position are provided, and the number of said lens elements is the same as the number of said lens elements that receive the beam focused by each lens element of said lens array. The present invention is characterized by providing a light receiving section using conversion elements, an addition circuit for adding signals from these photoelectric conversion elements, and a PLL circuit for receiving a reference pulse signal input from the addition circuit and outputting a pixel clock. A pixel clock generator in a laser scanning optical system. 2. Laser scanning according to claim 1, characterized in that the grating scanning length by the pixel clock generation beam is set longer than the recording scanning length by the recording laser beam, and the reference pulse signal is generated before the start of recording. Pixel clock generator in optical system.