JPH04328512A - Laser light source device - Google Patents

Abstract

PURPOSE:To reduce the defocusing of a scanning optical system until no practical problem is left by combining a laser diode and a condenser lens which has diffraction effect with each other and specifying the focal length of a condenser lens. CONSTITUTION:The laser beam emitted by the laser diode 23 is transmitted through a condenser lens 24 with the diffraction effect and projected outward. The focal length of the condenser lens 24 satisfies an inequality I. In the inequality I, lambda is the oscillation wavelength of the laser diode at reference temperature, lambda' the oscillation wavelength of the laser diode at the time of a temperature rise, lambda the quantity of wavelength variation of the laser diode, fCO the focal length of the condenser lens corresponding to the wavelength lambda, D0 the beam diameter on an image plane at the time of the wavelength lambda, D1 the beam diameter on the image plane at the time of wavelength lambda+ lambda, and fH the focal length of the scanning optical system in the main scanning direction. Namely, variation in the focal length of the condenser lens 24 due to variation in the oscillation wavelength of the laser diode 23 is reduced lastly to an extent where there is no practical problem by making the focal length fCO satisfy the inequality I.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a laser light source device, particularly a laser light source that uses a laser diode as a light source and whose emitted laser beam can be used as a light source for a sensor or a printer. Regarding equipment. BACKGROUND OF THE INVENTION Conventionally, in electrophotographic laser printers, a laser diode as a light source has been widely used as a laser beam scanning optical system for writing an image on a photoreceptor. Since the laser beam emitted from the laser diode is diffused light having a certain spread angle, a condensing lens (collimator lens) is provided in front of the laser diode to condense the light into parallel or convergent light. On the other hand, a Fresnel lens has been developed which is a set of lattice-like concentric circular patterns having a period on the order of microns and whose cross section is sawtooth-like. This Fresnel lens uses refraction and diffraction phenomena, and when parallel light enters it, the light bends at each part of the grating and focuses the incident light on a single point. Conversely, the diffused light emitted from the focal point is collimated at each part of the grating. [0004] Therefore, it is conceivable to construct a laser light source unit by replacing the Fresnel lens with a conventional collimator lens. However, the biggest problem here is defocus caused by changes in the oscillation wavelength of the laser diode. That is, a Fresnel lens that utilizes a diffraction phenomenon is unstable with respect to changes in wavelength, and its focal length changes sensitively to slight changes in wavelength. In the laser beam scanning optical system of a printer, a slight change in focal length is magnified hundreds of times through the scanning optical system, causing defocus on the image plane (photoreceptor). [0005] Incidentally, the wavelength of the laser beam emitted from the laser diode shifts to the longer wavelength side due to an increase in the amount of heat generated by the light emitting section and an increase in the environmental temperature. OBJECTS, STRUCTURE, AND OPERATIONS OF THE INVENTION Therefore, the objects of the present invention are as follows:
By making the Fresnel lens used in combination with a laser diode have predetermined characteristics, fluctuations in the focal length of the Fresnel lens can be suppressed to a minimum, and defocus of the scanning optical system can be reduced to an extent that does not pose a practical problem. An object of the present invention is to provide a laser light source device. In order to achieve the above object, a laser light source device according to the present invention consists of a combination of a laser diode and a condensing lens having a diffraction effect, and the condensing lens comprises:
The focal length substantially satisfies the following formula. [Equation 2] λ: Laser diode oscillation wavelength at reference temperature λ': Laser diode oscillation wavelength when temperature rises Δλ: Laser diode wavelength change fCO: Light condensation at wavelength λ Focal length of the lens D0: Beam diameter on the image plane due to the wavelength of λ (1/e2 value) D1: λ+Δ
Beam diameter on the image plane according to the wavelength of λ (1/e2 value) (tolerable value) fH: Focal length in the main scanning direction of the scanning optical system using a laser light source device The condenser lens has a thin flat plate shape, and its Focal length is 1
~10 mm, and by installing a laser diode at or near the focal point, a light source unit that is densely packaged in one package can be obtained. The diffused light emitted from the laser diode is focused into parallel or convergent light by the refraction and diffraction effects of the condenser lens. The wavelength of the beam emitted from the laser diode tends to shift toward longer wavelengths as the temperature of the heat generating part increases, and this causes the focal length of the condensing lens to vary. However, since the focal length fCO of the condenser lens substantially satisfies the above equation, the defocus of the scanning optical system can be suppressed to a level that does not pose a practical problem. [Embodiments] Hereinafter, embodiments of the laser light source device according to the present invention will be described with reference to the accompanying drawings. [First Embodiment, See FIGS. 1 to 4] FIG. 1 shows a laser printer incorporating a laser beam scanning optical system 20 having a built-in light source unit, which is a first embodiment of the present invention. In this laser printer, a photoreceptor drum 2 is installed approximately at the center of a main body 1 so as to be rotatable in the direction of arrow a, and surrounding it are a charger 3, a developer 4, a transfer charger 5, and a cleaner for residual toner. 6 is arranged. A laser beam scanning optical system 20 is installed above the photoreceptor drum 2, and irradiates a laser beam onto the surface of the photoreceptor drum 2, which has been uniformly charged to a predetermined potential by the charger 3, to form a predetermined image as a latent image. Form. This latent image is developed by a developing device 4 to form a toner image. On the other hand, recording sheets are automatically fed one by one from a paper feed cassette 10 installed at the bottom of the main body 1.
It is conveyed to the transfer section via the timing roller 11. Here, the toner image is transferred to the sheet, and after the toner is fixed in a fixing device 12, the sheet is discharged onto the upper surface of the main body 1 from a discharge roller 13. FIG. 2 shows a laser beam scanning optical system 20. As shown in FIG. The optical system 20 includes a light source unit 21, a cylindrical lens 30, a polygon mirror 31, an fθ lens 32, a plane mirror 33, and an image writing start position detection sensor 45 (hereinafter referred to as an SOS sensor).
Mirrors 41 and 42 that guide the laser beam to the OS sensor 45
is attached to a housing (not shown). A laser beam emitted from the light source unit 21 (the configuration of which will be described later) passes through the cylindrical lens 30 and is focused near the reflective surface of the polygon mirror 31 in a straight line that coincides with its deflection surface. The polygon mirror 31 is driven to rotate at a constant speed in the direction of arrow b, and deflects and scans the laser beam continuously at a constant angular speed. The scanned laser beam passes through the fθ lens 32, is reflected by the plane mirror 33, and forms an image on the photoreceptor drum 2 through a slit in the housing (not shown). At this time, the laser beam is scanned at a constant speed in the axial direction of the photoreceptor drum 2, and this is called main scanning. Furthermore, scanning based on the rotation of the photosensitive drum 2 in the direction of arrow a is referred to as sub-scanning. In the above configuration, an image (electrostatic latent image) is formed on the photosensitive drum 2 by turning on and off the laser beam from the light source unit 21 and by the main scanning and sub-scanning. The fθ lens 32 corrects (distortion aberration) so that the scanning speed of the laser beam in the main scanning direction is equalized from the center to both ends of the scanning area. The cylindrical lens 30 works together with the fθ lens 32 to correct the surface tilt error of the polygon mirror 31. On the other hand, a part of the laser beam deflected and scanned by the polygon mirror 31 enters the SOS sensor 45 from the mirrors 41 and 42 via the cylindrical lens 46, and an image is generated line by line based on the detection signal. The writing start position is controlled. Here, the light source unit 21 will be explained. As shown in FIG. 3, the light source unit 21 includes:
Base 22, laser diode 23, Fresnel lens 2
4. Consists of a metal cover 25 and a photodiode 28 for monitoring the amount of light. The cover 25 has an exit window 25a provided with a protective glass 26. The laser diode 23 emits diffused light from its junction surface by supplying a predetermined current. The Fresnel lens 24 is a collection of lattice-like concentric circular patterns having a period on the order of microns, and has a sawtooth cross section. This Fresnel lens 24 has a refraction effect and a diffraction effect, and light is bent at each part of the grating. When parallel light is incident, it is converged to one point (focal point), and the diffused light emitted from the focal point is made into parallel light (see FIG. 4). Therefore, by placing the light emitting part of the laser diode 23 at the focal point of the Fresnel lens 24, the diffused light emitted from the laser diode 23 is condensed into parallel light by the Fresnel lens 24, and is transmitted from the light source unit 21 to the cylindrical The light is emitted toward the lens 30. The Fresnel lens 24 used here is made of polycarbonate and is designed to accommodate a laser beam with a wavelength of 780 nm. The Fresnel lens 24 is extremely small and lightweight, and is compatible with the laser diode 23 and the monitor photodiode 2.
It can be mounted in high density together with 8 etc. in one package. Conventionally, a glass-molded single aspherical lens was used as a collimator lens, but compared to this, the light source section has become smaller, and the positions of the laser diode and Fresnel lens must be adjusted relative to each other when incorporated into the optical system housing. There will be no need. Further, Fresnel lenses have the advantage that they can be mass-produced by a molding method and do not require a polishing process. Furthermore, today, as the speed of laser printers has become slower and the sensitivity of photoreceptors has improved, the amount of light required on the image plane is sometimes sufficient to be about 0.2 mW. In this case, since the light transmittance of a normal optical system is about 25 to 30%, the output of the laser diode is about 0.8 mW. However, in this case, the laser diode only has a threshold output in the region where LED light emission is switched to LD light emission, resulting in poor response. However, a Fresnel lens with a light transmission efficiency of 50% or less can be manufactured, and a laser diode can be driven in the LD emission region to improve responsiveness. However, a laser diode has a characteristic that its oscillation wavelength changes due to an increase in the amount of heat generated by the light emitting part or a rise in environmental temperature. A Fresnel lens that utilizes the diffraction effect is unstable with respect to changes in wavelength, and its focal length changes sensitively to slight changes in wavelength. Considering the laser beam scanning optical system as a whole, a slight change in focal length is magnified several hundred times through the aforementioned optical elements 30, 31, 32, and 33, and defocusing on the image plane (photoreceptor drum surface) is prevented. generate. This problem will be analyzed below. A Fresnel lens that utilizes the diffraction effect has a focal length that varies with respect to the oscillation wavelength of the laser diode according to the relationship shown in the following equation. fλ=f'λ'
…
...(1) f'=(λ/λ')f

...(1a) λ: Laser diode oscillation wavelength λ': Laser diode oscillation wavelength after change f: Fresnel lens focal length f': Defect on the image plane due to a change in focal length greater than or equal to the Fresnel lens focal length after change. The focus is the following formula (2)
, (3). Main scanning direction: ΔXH=(fH/fCO)2Δx
...(
2) ΔXH: Image plane defocus amount in the main scanning direction Δx: Light source defocus amount fH: Scanning optical system focal length in the main scanning direction fCO: Fresnel lens focal length in the sub-scanning direction ΔXV=β2(fCY/fCO)2Δx
……
(3) ΔXV: image plane defocus amount in the sub-main scanning direction β: scanning optical system lateral magnification in the sub-scanning direction fCY: cylindrical lens focal length Furthermore, the relationship between the defocus amount and the beam diameter is expressed by the following equation (4). It will be done. [Equation 3] DO: Beam diameter on the image plane due to the wavelength of λ (
1/e2 value) D: Beam diameter on the image plane when defocused by ΔXH (1/e2 value) λ=780nm, fCO=6mm, fH=150mm,
β=3, fCY=4.0mm, beam diameter on the image plane (1
/e2 value) of 100 μm in the main scanning direction and 150 μm in the sub-scanning direction.
Table 1 shows the influence of changes in wavelength λ in terms of μm. [0025] As is clear from Table 1, the change in wavelength affects the main scanning direction more than the sub-scanning direction. An increase in the beam diameter causes a decrease in energy density on the photoreceptor, resulting in thinner lines and a decrease in image density in the final image that has undergone the electrophotographic process. Possible solutions to this problem include a method of suppressing the change in the oscillation wavelength of the laser diode itself, and a method of suppressing defocus on the image plane due to the wavelength change. In this example, by selecting a Fresnel lens to be used that has a predetermined focal length, defocusing on the image plane can be practically achieved even if the wavelength changes due to a rise in the temperature of the laser diode. We decided to keep it to a value that would not cause any problems. As is clear from Table 1, consideration should be given so that the defocus (beam diameter) in the main scanning direction does not become large. Therefore, in the above equation (4), if the upper limit of the beam diameter that is practically allowable is D1, it is sufficient that D<D1 is satisfied even when the wavelength changes. That is, 002
On the other hand, when the oscillation wavelength of the laser diode changes from λ to λ', the defocus amount Δx at the light source section is expressed by the following equation (5). Δx=(λ'-λ)/λ'・fCO
...(5) When formula (5) is substituted into the above formula (2), the following formula (2a) is obtained.
is obtained. ΔXH=(fH/fCO)2(λ'-λ)/λ
'·fCO...(2a) By substituting equation (2a) into equation (4a), the following equation (6) is obtained. [0031] Since the problem in this embodiment is the focal length fCO of the Fresnel lens 24, it is sufficient to select a Fresnel lens whose fCO is larger than the value on the right side of equation (6). . Note that the practically acceptable beam diameter D1 is the wavelength λ
This is about 1.25 times the beam diameter D0. However, it should be noted that if a lens with a focal length fCO larger than necessary is selected, the laser diode drive current must be increased to compensate for the decrease in transmittance, which will lead to an increase in the amount of change in the oscillation wavelength. There is. Incidentally, if the amount of wavelength change Δλ of the laser diode can be suppressed to about 1 nm, and if the allowable value of the beam diameter on the image plane is 25%, then D1/D0 is 1.
It becomes 25. That is, the above equation (6) can be rewritten as follows. ##EQU6## The beam diameter D0 is a value determined from the image density, and is determined by the following equation (7). D0/P≦2.0

...(7) P: Image pitch 240DPI=0.106 300DPI=0.085 400DPI=0.0635 The relationship between fCO and fH is determined by equations (6a) and (7). For example, at 400DPI, D0 is 1.7P (0.10
795), fCO>1.45×10-4fH2
……
(8) Similarly, at 300DPI, fCO>8.08×10-5fH2
……
(9) At 400DPI, fCO>5.2×10-5fH2
...(
10) Now, if fH is 170 mm and the light source unit 21 is used in a printer with an image density of up to 400 DPI, fC
A Fresnel lens with O>4.2 may be selected. The focal length fCO of the Fresnel lens is preferably set slightly longer than this lower limit value. The analysis in the first embodiment is based on the case where the laser diode 23 is installed with its light emitting part at the focal point of the Fresnel lens 24, and parallel light is emitted from the light source unit 21. When the light emitting part of the laser diode 23 is set at a position slightly farther than the focal point of the Fresnel lens 24, convergent light is emitted from the light source unit 21. In this case, the right side of the equation (6) is different. Further, the same applies when an fθ mirror system is installed in place of the fθ lens system after the polygon mirror 31. [Second embodiment, see FIGS. 5 to 7] Therefore,
As a second embodiment, a laser beam scanning optical system 20' that emits convergent light from a light source unit 21' and is provided with an fθ mirror system will be described. In FIG. 5, the laser beam scanning optical system 20' focuses the laser beam deflected and scanned by the polygon mirror 31 onto the photoreceptor drum 2 through a toric lens 35, a spherical mirror 36, and a plane mirror 37 provided after the polygon mirror 31. It is configured to image. One mirror 4 for SOS sensor 45
3 to guide the laser beam. [0038] Here, the toric lens refers to a lens in which either the entrance side or the exit side is a toroidal surface and the other surface is a spherical, flat, or cylindrical surface. In this embodiment, the toric lens 35 has a toroidal surface on the incident side and a spherical surface on the exit side. A toroidal surface is a surface whose two principal meridians have different centers of curvature. The spherical mirror 36 replaces the fθ lens,
Together with the toric lens 35, it corrects (distortion aberration) so that the scanning speed in the main scanning direction is equal from the center of the scanning area to both ends thereof, and also corrects the curvature of field on the photosensitive drum 2 in the main scanning direction. . Further, the toroidal surface of the toric lens 35 corrects the surface tilt error of the polygon mirror 31 and also corrects the curvature of field on the photoreceptor drum 2 in the sub-scanning direction. In this embodiment, a beam is directed to a polygon mirror 31 by a cylindrical lens 30.
On the other hand, the toroidal surface of the toric lens 35 maintains a conjugate relationship between each reflecting surface of the polygon mirror 31 and the light collecting surface. On the other hand, the spherical surface of the toric lens 35 mainly corrects field curvature in the main scanning direction and also corrects distortion. Next, image plane defocus in convergent light (fθ mirror system) will be analyzed. Regarding the main scanning direction, in FIG. 6, a1: distance from the Fresnel lens to its object point b1: distance from the Fresnel lens to its image point L1: distance from the Fresnel lens to the scanning lens b2: distance from the scanning lens to its image plane When set as the distance to , the defocus amount ΔXH
is expressed by the following equation (2b). [Equation 7] For example, a1=6mm, b1=600mm
, L1=200mm, b2=150mm, ΔX
H becomes 1378.3Δfco. On the other hand, regarding the sub-scanning direction, in FIG. 7, in addition to FIG. 6, L2: distance from the Fresnel lens to the cylindrical lens b3: distance from the cylindrical lens to its image point β
: When setting the horizontal magnification of the scanning optical system in the sub-scanning direction, the defocus amount ΔXV is calculated using the following formula (3b
). [Equation 8] For example, a1=6mm, b1=600mm
, L2=50mm, b3=50mm, β=2,
ΔXV becomes 729ΔfCO. In addition, the main scanning direction beam diameter DH (1/e2 value) and the sub-scanning direction beam diameter DV (1/e2 value) after defocusing are calculated by the following formula (11), formula (
12). [Equation 9] θH: Laser beam main scanning direction spread angle θ
V: Coefficient determined from the relationship between laser beam sub-scanning direction divergence angle ΔDH: θH and the aperture diameter NA of the Fresnel lens ΔDV: Coefficient determined from the relationship between θV and the aperture diameter NA of the Fresnel lens. Similar to the first embodiment, if the problem is the increase in beam diameter in the main scanning direction, the above equation (2b) representing the image plane defocus amount ΔXH in the main scanning direction and the above equation (4) representing the beam diameter. ), the following equation (13) is obtained. ##EQU11## Therefore, in the second embodiment, it is sufficient to select a Fresnel lens whose focal length fCO is larger than the value on the right side of equation (13). Furthermore, as explained in the first embodiment, the practically acceptable beam diameter D1 is about 1.25 times the beam diameter D0 due to the wavelength λ. The amount of wavelength change Δλ of the laser diode can be suppressed to about 1 nm, and if the allowable value of the beam diameter on the image plane is 25%, then
The above equation (13) can be rewritten as follows. [Equation 12] Here, b1=600mm, L1=200
When setting mm, b2=110mm, fCO>3.9
All you have to do is choose a Fresnel lens. [Other Embodiments] The laser light source device according to the present invention is not limited to the embodiments described above, and can be modified in various ways within the scope of the invention. For example, the scanning optical system may be constructed by using a light source unit that emits parallel light as in the first embodiment, and installing an fθ mirror system as in the second embodiment after the polygon mirror. Further, the laser light source device according to the present invention can be widely used not only in image forming apparatuses but also in sensors, optical pickups, and the like. Effects of the Invention As is clear from the above description, according to the present invention, a light source unit that is small and lightweight can be obtained because a condensing lens having a diffraction effect is used in combination with a laser diode. , it can be incorporated into the scanning optical system without any adjustment. In addition, since we used a condensing lens whose focal length fCO satisfies the above formulas (6) and (13), fluctuations in the focal length of the condensing lens caused by changes in wavelength due to temperature rise of the laser diode are suppressed as much as possible. In the end, the defocus of the scanning optical system can be suppressed to a level that does not pose a practical problem.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a printer equipped with a laser beam scanning optical system incorporating a laser light source device according to the present invention.

FIG. 2 is a perspective view of a laser beam scanning optical system incorporating a first embodiment of the laser light source device according to the present invention.

FIG. 3 is a partially cutaway perspective view of the laser light source device shown in FIG. 2;

FIG. 4 is a perspective view showing the light focusing effect of the Fresnel lens shown in FIG. 3;

FIG. 5 is a perspective view of a laser beam scanning optical system incorporating a second embodiment of the laser light source device according to the present invention.

FIG. 6 is an explanatory diagram showing a state of beam convergence in the main scanning direction in the laser beam scanning optical system shown in FIG. 5;

FIG. 7 is an explanatory diagram showing a state of beam convergence in the sub-scanning direction in the laser beam scanning optical system shown in FIG. 5;

[Explanation of symbols]

20, 20'...Laser beam scanning optical system 21, 21'...
Laser light source unit 23...Laser diode 24...Fresnel lens

Claims (1)

[Claims] 1. A laser light source device that transmits a laser beam emitted from a laser diode to the outside through a condensing lens having a diffraction effect, wherein the condensing lens has a focal length substantially equal to The following formula must be satisfied, [Equation 1] λ: Laser diode oscillation wavelength at reference temperature λ': Laser diode oscillation wavelength when temperature rises Δλ: Laser diode wavelength change fCO: Wavelength λ Focal length of the condenser lens D0: Beam diameter on the image plane (1/e2 value) due to the wavelength of λ D1: λ+Δ
Beam diameter on the image plane (1/e2 value) (tolerable value) according to the wavelength of λ fH: A laser light source device characterized by the focal length in the main scanning direction of a scanning optical system using the laser light source device.