JPH11183814A - Multi-beam scanner - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To achieve good multi-beam scanning by favorably correcting curvature of field in the main and sub-scanning directions while maintaining good conjugate functions and uniform velocity functions in a scanning imaging lens. I do. A plurality of beams from a plurality of light emitting units are respectively formed into long line images in a main scanning corresponding direction, and a constant angular velocity is obtained by an optical deflector having a deflecting reflection surface in the vicinity of an image forming position of each line image. And converge each deflected beam on the surface 8 to be scanned by the same scanning image forming lens as a light spot separated in the sub-scanning direction, thereby performing uniform scanning of a plurality of scanning lines on the surface 8 to be scanned. The scanning imaging lens is composed of two lenses 6 and 7, and the lens 6 is a positive meniscus lens having a concave surface facing the optical deflector and both surfaces have a coaxial aspherical shape. The lens 7 has at least one surface having a non-arcuate shape in the main scanning section, and a curvature center line connecting the centers of curvature in the sub-scanning section in the main scanning direction in the main scanning direction has a main scanning direction. In the cross section, As the different curves, in which the radius of curvature in the sub-scan section is changed corresponding to the main scanning direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a multi-beam scanning device.

[0002]

2. Description of the Related Art Each of a plurality of beams emitted from a plurality of light sources is formed as a long line image in the main scanning direction, separated from each other in the sub-scanning direction, and formed near the image forming position of each line image. The light is deflected at an equal angular velocity by an optical deflector having a deflecting reflection surface, and each deflected beam is condensed on the surface to be scanned by the same scanning imaging lens as a light spot separated in the sub-scanning direction. A multi-beam scanning device that performs constant-speed optical scanning of a plurality of scanning lines is intended to be realized in connection with an "image forming apparatus" such as an optical printer or a digital copying machine. The “main scanning corresponding direction” refers to a direction corresponding to the main scanning direction on the optical path from the light source to the surface to be scanned, and the direction corresponding to the sub scanning direction on the optical path is referred to as a “sub scanning corresponding direction”. To tell. The scanning imaging lens has a conjugate function that sets the imaging position of each line image and the surface to be scanned to a “geometrically conjugate relationship” with respect to the sub-scanning corresponding direction,
It has a “constant speed function” for equalizing the optical scanning speed. The conjugation function is a function for correcting "surface tilt" of the deflecting reflection surface in the optical deflector.

In order to achieve good multi-beam scanning,
In addition to the good conjugate function and uniform velocity function of the scanning imaging lens, it is necessary that the curvature of field in the main and sub-scanning directions is well corrected. If the curvature of field in the main and sub-scanning directions is not sufficiently corrected, the light spot diameter fluctuates with the image height of the light spot, so that the resolution of an image to be written is remarkably reduced, and the image quality is degraded. In multi-beam scanning, the field curvature in the sub-scanning direction is
It is necessary that each of the plurality of light spots condensed on the surface to be scanned be properly corrected. Otherwise, the diameters of the light spots in the sub-scanning direction of the plurality of light spots are not uniform, and the resolution of an image to be written is "variable" for each light spot. In order to satisfactorily correct the curvature of field in the sub-scanning direction, the radius of curvature in one or more lens surfaces of the scanning imaging lens in a sub-scanning cross section (a flat cross section near the lens surface and orthogonal to the main scanning direction). Is known according to the position in the main scanning corresponding direction (for example, JP-A-6-230308), but the radius of curvature in the sub-scanning section is “in the main scanning corresponding direction”. The radius of curvature greatly differs between the deflection angle of 0 and the maximum deflection angle (corresponding to the end of the effective main scanning area) since the curvature angle monotonically increases as the distance from the optical axis increases. Assembly error "
In this case, the curvature of field in the sub-scanning direction is significantly deteriorated, so that there is a problem that the tolerance of assembly is strict and the workability of assembling the optical system is poor.

Further, the scanning image forming lens can be manufactured by "plastic molding".
In some cases, undulation occurs and it is difficult to obtain a lens having a desired shape. Further, the optical performance of a lens made of plastic is liable to change under the influence of changes in temperature and humidity. In addition, since a rotating polygonal mirror generally used as an optical deflector does not have a rotating axis of the deflecting / reflecting surface within the deflecting / reflecting surface, the positional relationship between each line image formed near the deflecting / reflecting surface and the deflecting / reflective surface is not sufficient. However, there is a problem of so-called "sag" which fluctuates with the rotation of the rotating polygon mirror.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to realize good optical scanning by each light beam in a multi-beam scanning device. Further, it facilitates the assembly of the optical system in the multi-beam scanning device, and when the scanning imaging lens is formed of plastic, the desired lens can be easily obtained with high yield, thereby enabling the cost reduction of the scanning device. Another object is to make scanning less susceptible to temperature, humidity, and the like, and to effectively remove the effect of sag.

[0006]

According to a multi-beam scanning apparatus of the present invention, "each of a plurality of beams emitted from a plurality of light-emitting sources is separated from each other in a sub-scanning corresponding direction as a long line image in a main scanning corresponding direction. And deflected at an equal angular velocity by an optical deflector having a deflecting / reflecting surface in the vicinity of the image forming position of each line image, and separates each deflected beam on the surface to be scanned in the sub-scanning direction by the same scanning image forming lens. A multi-beam scanning apparatus that performs light scanning at a constant speed on a plurality of scanning lines on a surface to be scanned by condensing the light spots as a light spot, and is characterized by the following points. That is, a scanning imaging lens for condensing each deflection beam as a light spot on the surface to be scanned is constituted by two lenses. Of the two lenses constituting the scanning imaging lens, the lens on the optical deflector side is a “positive meniscus lens having a concave surface facing the optical deflector side, and both surfaces have a coaxial aspherical shape.” The lens on the side "has at least one surface having a non-arc shape in the main scanning section, and a curvature center line connecting the centers of curvature in the sub-scanning section on the lens surface in the main scanning corresponding direction is the main scanning direction. The radius of curvature in the sub-scanning cross section is changed in the main scanning corresponding direction so that a curve different from the non-circular shape is obtained in the cross section. "
“Main scanning section” refers to a plane section including the lens optical axis and the direction corresponding to main scanning in the vicinity of the lens surface of each lens constituting the scanning image forming lens. The “sub-scan section” refers to a plane section near the lens surface and orthogonal to the main scanning direction. As will be described later, a tilt angle can be given to the two lenses constituting the scanning image forming lens in the main scanning section in order to reduce the influence of sag. The scanning cross section is defined when the tilt angle is set to 0 in each lens, that is, “in a state before the tilt angle is given”. "Non-arc shape"
When coordinates: X are taken in the lens optical axis direction and coordinates: Y are taken in the optical axis orthogonal direction, the paraxial radius of curvature is R, the conic constant is K,
A, B, C, D,. . . As well ^{known, X = (Y 2 / R} ) / [1 + √ {1- (1 + K) (Y / R) 2}] + A · Y 4 + B · Y 6 + C · Y 8 + D · Y 10. . . (1) R, K, A, B, C, D,. . And the curve shape specified by

Since the lens on the side of the optical deflector is a "meniscus lens", the thickness difference between the center and the periphery, particularly, the thickness difference between the center and the periphery in the main scanning direction is effectively reduced. Can be effectively prevented from being deformed such as "sinking or undulation" when it is formed by molding with a resin such as plastic. In addition, since the lens on the optical deflector side is provided with the concave surface facing the optical deflector side, the distance from the center of deflection in the main scanning direction and the peripheral portion to the incident side lens surface from the starting point of deflection. Is small, so that the "difference in the lateral magnification in the sub-scanning corresponding direction" between the central portion and the peripheral portion can be reduced.

As described above, in the scanning image forming lens according to the first aspect, since at least three surfaces in the main scanning section are "non-circular", these non-circular shapes are optimized. This makes it possible to satisfactorily correct the curvature of field and the uniform velocity characteristic in the main scanning direction for each beam.
Further, in a shape in a plane parallel to the optical axis and orthogonal to the main scanning section (in the case where the tilt angle is given, a shape in a state where the tilt angle is set to 0), two surfaces (the lens of the optical deflector side) Both surfaces have a non-arc shape, and at least one surface of the lens on the scanning surface side changes the radius of curvature in the sub-scanning cross section in the main scanning corresponding direction. By optimizing the change in the radius of curvature in accordance with the non-arc shape optimized for use, the curvature of field in the sub-scanning direction can be effectively corrected.

In the scanning image forming lens, the lens on the surface to be scanned has a non-circular shape in the main scanning section, and the center of curvature in the sub-scanning section on the lens surface is connected to the main scanning corresponding direction. The lens surface whose curvature radius is changed in the main scanning direction in the sub-scanning section so that the center line of curvature becomes a curve different from the non-circular shape in the main scanning section is referred to as an optical deflector. Side lens surface "(claim 2). In this case, the lens surface on the scanned surface side of the lens on the scanned surface side can have “an arc shape in the main scanning section”.

The lens on the surface to be scanned in the scanning imaging lens can have a negative refractive power in the main scanning section. As described above, the lens on the optical deflector side in the scanning imaging lens is a “positive meniscus lens”.
Therefore, if the “refractive power in the main scanning section” of the lens on the scanning surface side is negative, the combination of the refractive power of the scanning imaging lens in the main scanning section is a combination of “positive / negative”. When the two lenses constituting the scanning imaging lens are configured as “both plastic lenses”, the influence of the temperature and humidity changes acts so that each lens cancels out each other. It is less susceptible to humidity changes.

The lens on the surface to be scanned has a non-arcuate shape in the main scanning section, and a center line of curvature of the lens surface in the sub-scanning section in the main scanning corresponding direction. The absolute value of the radius of curvature of the “lens surface whose radius of curvature in the sub-scanning cross section is changed in the main scanning corresponding direction so as to be a curve different from the non-arc shape in the main scanning cross section” is “main scanning”. In the corresponding direction, it increases smoothly and monotonically toward the local maximum value as it leaves the optical axis, and after it exceeds the maximum position, it decreases smoothly and monotonically as it leaves the optical axis. " . When the coordinates in the main scanning corresponding direction are η and the radius of curvature is r (η), if there is a position error in the main scanning direction: Δη at r (η), the error of the radius of curvature at the position: η is
{dr (η) / dη} Δη, and if r (η) is “monotonically increasing as η increases”, {dr (η) / dη} will always have a constant sign, so {dr (η) η) / dη} Δη can be significantly large, but as in the present invention, r
If (η) has a local maximum, the sign of {dr (η) / dη} changes before and after the local maximum, so that {dr (η) / dη} Δη is effectively reduced. Therefore, the "tolerance to the assembly tolerance" of the lens on the scanning surface side to the optical system of the multi-beam scanning device.
Is effectively alleviated.

In the multi-beam scanning apparatus according to any one of the first to fifth aspects, a semiconductor laser array in which a plurality of laser light emitting units are monolithically arranged is preferably used as a light source having a plurality of light emitting sources. Alternatively, a light source in which beams from independent semiconductor lasers are combined by beam combining means can be used.

The beams from each light source are coupled to subsequent optics by "separate or common coupling lenses". Each beam coupled by the coupling lens may be a “weakly converging or diverging beam”, but each is a “parallel beam” by the coupling lens, and each parallel beam is separated or shared. The linear image forming optical system can form a linear image in the vicinity of the deflecting reflective surface in the main scanning corresponding direction (claim 6). In this case, since the beam becomes a parallel beam between the coupling lens and the line image forming optical system, the degree of freedom in arranging the optical system in this portion is large. Further, in the multi-beam scanning apparatus according to any one of claims 1 to 6, a rotary polygon mirror is used as an optical deflector, and a main scanning is performed on each lens of the scanning imaging lens in order to reduce the influence of sag. A tilt angle can be given in the cross section (claim 7). Note that a plurality of rotating polygon mirrors having the same shape may be provided on a common axis as the rotating polygon mirror, and the deflection of a plurality of beams may be distributed to each rotating polygon mirror.

[0014]

FIG. 1 schematically shows an embodiment of a multi-beam scanning apparatus. As shown in FIG. 1A, a plurality of beams (only one beam is shown for simplicity of the drawing) emitted from a light source 1 are coupled by a coupling lens 2 and transmitted through a cylinder lens 3. The light is reflected by the mirror 4, deflected by the optical deflector 5, and condensed as a light spot on the surface 8 to be scanned by the lenses 6 and 7 constituting the scanning image forming lens. Is a "photoconductive photoreceptor"). In this embodiment, the light source 1
As shown in FIG. 1B, it is a “semiconductor laser array in which two laser emitting units are monolithically arranged”.
Laser beams are independently emitted from two adjacent laser emission units. Each of the emitted beams is converted into a “parallel beam” by a common coupling lens 2, “beam-shaped” by an aperture AP, and then sub-scanned by a cylinder lens 3 which is a line image forming optical system. The light is converged in the corresponding direction, is reflected by the mirror 4, and is deflected by the deflecting and reflecting surface 5A of the rotary polygon mirror which is the optical deflector 5.
An image is formed at a nearby position as a “long line image in the main scanning corresponding direction”. Since the two light emitting units in the light source 1 are arranged in the sub-scanning corresponding direction, the two line images are separated from each other in the sub-scanning corresponding direction. The mirror 4 is a semiconductor laser 1
May be omitted depending on the layout of the optical system from the lens to the deflecting / reflecting surface 5. The cylinder lens 3 is a "concave cylinder mirror".
May be substituted. The light deflector 5 is a "rotating polygon mirror" and its rotation axis 5B is separated from the deflecting / reflecting surface 5A. Displacement, so-called "sag" occurs. The two deflecting beams transmitted through the scanning imaging lens are converged toward the surface 8 to be scanned, and are formed on the surface 8 to be scanned by a light spot formed separately in the sub-scanning direction. The scanning lines are scanned at a constant speed. Of the two lenses 6 and 7 constituting the scanning imaging lens, the lens 6 on the optical deflector 5 side is a “positive meniscus lens having a concave surface facing the optical deflector” and both surfaces are coaxial aspherical surfaces. .

The lens 7 provided on the surface 8 to be scanned has a lens surface on the side of the optical deflector 5 which has a non-arc shape in the main scanning section, and has a curvature in the sub scanning section on the lens surface. The radius of curvature in the sub-scanning cross section changes in the main scanning corresponding direction such that the center line of curvature connected to the center in the main scanning corresponding direction is a curve different from the non-circular shape in the main scanning cross section. . The refractive power in the main scanning section of the lens 7 is negative.

The shape of the surface of the lens 7 on the optical deflector side will be described with reference to FIG. As will be described later, the lenses 6 and 7 are given a tilt angle in the main scanning section.
In the following description, it is assumed that the tilt angle is 0. At this time, both the main scanning corresponding direction and the sub scanning corresponding direction are orthogonal to the optical axis of the lens 7. In FIG. 2, the Y axis is taken in the main scanning corresponding direction. The X axis is the direction of the optical axis of the lens 7, and the positive direction (the right direction in the figure) of the X axis is the surface to be scanned. The XY plane is the “main scanning section”. The “sub-scan section” is orthogonal to the XY plane, and
It is a plane section parallel to the axis. In FIG. 2A, X (Y) is the shape of the lens surface of the lens 7 in the main scanning cross section, which is “non-arc shape”. Further, r (η) represents “the radius of curvature in the sub-scan section” at the position coordinate: η in the main scanning corresponding direction (Y direction). The radius of curvature: r (η) changes according to the position coordinates: η, and the curvature center line L connecting the “curvature center in the sub-scanning section” at the position: η in the main scanning corresponding direction is “in the main scanning section. Non-arc shape: Curve different from X (Y) ". FIG. 2B shows the position: η = 0.
Shows how the absolute value of the difference between the radii of curvature in the sub-scan section at the position and any position: η: | r (η) | − | r (0) | I have. The surface of the lens 7 on the side of the surface 8 to be scanned has an arc shape in the main scanning section, and the lens 7 has “negative refractive power in the main scanning section”. Returning to FIG. 1, tilt angles α ₆ and α ₇ are given to the lenses 6 and 7 in the main scanning section in order to effectively reduce the influence of sag.

Therefore, in the embodiment shown in FIG. 1, each of a plurality of beams emitted from a plurality of light-emitting sources is formed as a line image long in the main scanning direction and separated from each other in the sub-scanning direction. A deflecting / reflecting surface 5 near the imaging position of each line image.
A light is deflected at an equal angular velocity by an optical deflector 5 having an A, and each of the deflected beams is condensed on the surface 8 to be scanned as a light spot separated in the sub-scanning direction by the same scanning image forming lenses 6 and 7 to be scanned. In a multi-beam optical scanning device that performs constant-speed optical scanning of a plurality of scanning lines on the surface 8, the scanning image forming lens is
The lens 6 on the optical deflector 5 side is a positive meniscus lens having a concave surface facing the optical deflector 5 side, has a coaxial aspheric surface on both sides, and has a lens on the scanning surface side. Numeral 7 indicates that at least one surface has a non-arc shape (X (Y) in FIG. 2A) in the main scanning section, and the center of curvature of the lens surface in the sub scanning section is in the main scanning direction. The continuous curvature center line (L in FIG. 2A) becomes a curve different from the non-arc shape: X (Y) in the main scanning section.
The radius of curvature in the sub-scan section is changed in the main scanning direction (claim 1).

The lens 7 has a non-arcuate shape in the main scanning section, and a center line of curvature of the lens surface in the sub-scanning section in the main scanning direction corresponds to the main scanning section. The lens surface in which the radius of curvature in the sub-scanning section is changed in the direction corresponding to the main scanning so as to have a curve different from the non-circular shape in FIG. 2 is the lens surface on the optical deflector 5 side. ), The lens surface on the scanned surface 8 side is “arc-shaped in the main scanning section” (claim 3), and the lens 7 is “negative in refractive power in the main scanning section” (claim 4). ). Further, the absolute value of the radius of curvature: | r (η) | of the radius of curvature in the sub-scan section on the surface of the lens 7 on the side of the optical deflector 5 is shown in FIG.
As shown in (b), as the optical axis (X-axis) moves away from the optical axis (X-axis) in the main scanning corresponding direction (Y-direction), it increases smoothly and monotonically from | r (0) | After exceeding, it is determined to decrease smoothly and monotonously as it leaves the optical axis (claim 5).

Each beam from the light source 1 is converted into a parallel beam by a coupling lens 2, and each parallel beam is converted by a line image forming optical system 3 into a vicinity of a deflecting reflection surface 5 A and a line image long in a main scanning corresponding direction. (Claim 6). The light deflector 5 is a “rotating polygon mirror”, and each of the lenses 6 and 7 of the scanning image forming lens is used to reduce the effect of sag.
Are provided with tilt angles: α ₆ and α ₇ in the main scanning section (claim 7).

[0020]

FIG. 1 shows an embodiment of the embodiment shown in FIG. The semiconductor laser array as the light source 1 has an emission wavelength of 670 nm.
The optical deflector 5 is a rotating polygon mirror having 6 deflecting and reflecting surfaces and an inscribed circle radius of the deflecting and reflecting surface of 25 mm. The incident direction of the light beam incident on the optical deflector 5 from the side of the optical path bending mirror 4 (tilt angle:
The angle formed by the optical axes of the lenses 6 and 7 of the scanning image forming optical system (when α ₆ = α ₇ ≡0) is 60 degrees.

The optical system on the optical path from the light source 1 to the optical deflector 5 is referred to as a “first group”, and the optical system on the optical path from the optical deflector 5 to the surface 8 to be scanned is referred to as a “second group”. The radius of curvature of the cover glass of the semiconductor laser array as the light source 1, the deflecting / reflecting surface, and the lens surface of each lens (the paraxial radius of curvature for those not having an arc shape) is Rm for the main scanning corresponding direction, and Rs for the sub-scanning corresponding direction. Where D is the distance on the optical axis and N is the refractive index of the material. An amount having a "length dimension" is in "mm" units.

First group data: Surface number Rm Rs DN 0 0.50 Laser emitting unit 1 ∞ ∞ 0.30 1.514 Cover glass 2 ∞ ∞ 10.00 3 ∞ ∞ 2.80 1.681 Coupling lens 4 -8.414 -8.414 20.00 5 ∞ 48.00 3.00 1.514 Cylindrical Lens 6∞ ∞ 91.42.

As shown in FIG.
The two laser emitting units 11 and 12 are “10 in the sub-scanning corresponding direction”.
μm apart ”, and these two laser emitters are
One of the laser light emitting units (the first light emitting unit 1) is located “one side in the sub-scanning corresponding direction” of the optical axis of the coupling lens 2.
1) is located at a distance of 5 μm from the optical axis, and the other laser emitting unit (second emitting unit 12) is located at a distance of 15 μm from the optical axis. D = 91.42 is the distance from the exit side surface of the cylindrical lens 3 to the deflecting / reflecting surface of the optical deflector (the line image forming position). The exit side surface (the surface number: 4) of the coupling lens 2 is a “coaxial aspherical surface”, and the coupled light beam is a “substantial parallel light beam”.
In the formula (1), the coaxial aspheric surface has a paraxial radius of curvature: R (= Rm = Rs) and a conic constant: K, Y = 4.
Next, sixth, eighth, and tenth order aspherical coefficients: A, B, C, D
Has the following values: R = −8.414, K = −0.021, A = 1.23
E-4, B = 1.36E-6, C = 1.24E-
8, D = 1.54E-10 Note that "E-4" and the like indicate "power". For example, "E
-4 "means" ^10-4 ", this value is applied to the immediately preceding value.

In the second lens group, “α” is the aforementioned “tilt angle (clockwise is“ positive ”and the unit is“ degree ”)”
Represents

Second group data: Surface number Rm Rs DN α 0 ∞ ∞ 52.71 Deflection reflection surface 1 -312.6 -312.6 31.40 1.527 -0.04 Lens 6 2 -82.95 -82.95 78.00 3 -500.00 -47.85 3.50 1.527 +0.26 Lens 7 4 -1000.00 -23.38 D = 52.71 is the distance from the deflecting reflecting surface to the incident side surface of the lens 6.

Both surfaces (surface numbers 1 and 2) of the lens 6 are “coaxial aspherical surfaces”, and the exit side surface of the lens 7 (surface number:
4) is a "normal toroidal surface". Incident side surface of lens 6: In the above equation (1), paraxial radius of curvature: R (= Rm = Rs), conical constant: K, Y, 4
Next, sixth, eighth, and tenth order aspherical coefficients: A, B, C, D
Has the following values: R = -312.6, K = 2.667, A = 1.79
E-7, B = -1.08E-12, C = -3.18E-
14, D = 3.74E-18 The exit side surface of the lens 6: In the above equation (1), the paraxial radius of curvature: R (= Rm = Rs), and the conic constants: K, Y = 4
Next, sixth, eighth, and tenth order aspherical coefficients: A, B, C, D
Has the following values: R = -82.95, K = 0.02, A = 2.50E
-7, B = 9.61E-12, C = 4.54E-15,
D = -3.03E-18.

The incident side surface (surface number 3) of the lens 7 has a "non-arc shape" in the main scanning section, and in the state where the tilt angle is α ₇ = 0, the curvature in the sub scanning section is Cs ( Y)
Is, Cs (Y) = {1 / Rs (0)} + Σb j · Y ** j (j
= 1, 2, 3,. . ) (Rs in 2) (0) is specified by giving the b _j. The radius of curvature in the sub-scan section at the position in the main scanning corresponding direction: Y is “1 / Cs (Y)”. "Y
** j "represents the j-th power of Y.

The non-circular arc shape is expressed by the above equation (1), and paraxial radius of curvature: R (= Rm), conical constant: K, Y, fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients : A,
B, C, and D have the following values. R = -500.00, K = -71.73, A = 4.3
3E-8, B = -5.97E-13, C = -1.28E
-16, D = 5.73E-21 Further, in the above (2), Rs (0), b _j has the following values. Rs (0) (= R) = − 47.85, b ₂ = 1.59E−
_{3, b 4 = -2.32E-7} , b 6 = 1.60E-1
_{1, b 8 = -5.61E-16} , b 10 = 2.18E-
20, b ₁₂ = −1.25E−24 The coefficients related to the odd order of Y are all 0, and therefore, equation (2) relating to the incident side surface of the lens 7 is optical axis symmetric with respect to the Y direction.

When the absolute value of the radius of curvature is obtained based on “Cs (Y)” determined as described above, the absolute value is as shown in FIG.
As shown in (b), "the distance increases gradually and monotonically toward the maximum value as the optical axis is separated in the main scanning corresponding direction, and then decreases smoothly and monotonously as the distance from the optical axis increases after exceeding the maximum position. " Since the exit side surface of the lens 7 is the “normal toroidal surface” as described above, the arrangement of the optical system including the scanning image forming lens is determined as described above.

FIGS. 3 and 4 show a diagram of the field curvature and an equalizing characteristic according to the embodiment. In these figures of the field curvature shown in FIGS. 3 and 4, the broken line indicates “field curvature in the main scanning direction”, and the solid line indicates “field curvature in the sub-scanning direction”. Is indicated by a broken line, and the linearity is indicated by a solid line. FIG. 3 shows the first light emitting unit 1 located at a position 5 μm away from the optical axis of the coupling lens 2 in the sub-scanning corresponding direction.
FIG. 4 is a diagram related to a beam emitted from the first light emitting unit 1 and 15 μm from the optical axis of the coupling lens 2 to the first light emitting unit side.
FIG. 9 is a diagram relating to a beam emitted from a second light emitting unit 12 located at a distance of m. First, the velocity uniformity characteristic indicated by the fθ characteristic and the linearity is corrected very well with respect to the beam from each light emitting unit. Therefore, two light spots (separated in the sub-scanning direction) by each beam are
Very good constant speed scanning is realized. Further, the curvature of field in the main and sub-scanning directions is 1 mm or less in absolute value with respect to the beam from each light emitting unit, which is extremely good. In addition, since the curvature of field in the main and sub scanning directions is “substantially the same” with respect to the beam from each light emitting unit, the spot diameters of the light spots that scan two scanning lines at the same time are substantially the same. Are identical. In FIGS. 3 and 4, “Y” in the ordinate indicates “image height of light spot” instead of coordinates in the main scanning corresponding direction. The curvature of field and the uniformity characteristic are asymmetric on the plus side and the minus side of the image height of the light spot, but this is due to the above-mentioned "sag". When the tilt angle: α ₆ and α ₇ are both 0,
Asymmetry of the curvature of field and the equalization characteristics appear more remarkably, and the curvature of field and the equalization characteristics deteriorate on the plus side or the minus side of the image height of the light spot. As a result, both the curvature of field and the constant speed characteristics are favorably corrected on the plus side and the minus side of the image height of the light spot. In order to remove the influence of the sag, instead of or together with the tilt, a "shift" for shifting the optical axis of the lens 6 and / or the lens 7 by translation in the main scanning corresponding direction may be given. .

Although the embodiment in which the number of light emitting units is two has been described above, it is needless to say that the number of light emitting units can be three or more.

[0032]

As described above, according to the present invention, a novel multi-beam scanning device can be realized. In the multi-beam scanning device according to the present invention, since the lens on the optical deflector side is a meniscus lens among the two lenses constituting the scanning imaging lens, the center and the peripheral portion, particularly the central portion in the main scanning corresponding direction, It is possible to effectively reduce the difference in wall thickness with the peripheral part, and it is possible to effectively prevent deformation such as sink marks and undulations when manufacturing this by molding with a resin such as plastic. It is easy to manufacture, has good yield, and greatly contributes to cost reduction of the multi-beam scanning device. Further, since the lens on the optical deflector side is provided with the concave surface facing the optical deflector side, a change in the distance from the starting point of deflection to the incident side lens surface in the central portion and the peripheral portion in the main scanning corresponding direction. Therefore, the "difference in the lateral magnification in the sub-scanning corresponding direction" can be reduced, the tolerance of the assembly error to the optical system is large, and the assembly of the optical system can be facilitated.

Also, since at least three surfaces of the scanning image forming lens have a "non-arc shape" in the main scanning section, optimization of the non-arc shape allows the beam for each beam from the plurality of light-emitting portions to be adjusted. It is possible to satisfactorily correct the field curvature in the main scanning direction and the velocity uniformity characteristic. In the shape in the sub-scanning cross section, two surfaces are non-circular, and at least one surface of the lens on the side to be scanned has Since the radius of curvature in the sub-scanning section is changed in the direction corresponding to the main scanning, the change in the radius of curvature is optimized in accordance with the non-circular shape optimized for the field curvature in the main scanning direction and the uniform velocity characteristics. Thus, the curvature of field in the sub-scanning direction can be effectively corrected for each beam.

According to the fourth aspect of the present invention, the lens on the scanning surface has a negative refractive power in the main scanning section and the lens on the optical deflector is a positive meniscus lens. The combination of the refractive power of the image lens is
When the two lenses constituting the scanning image forming lens are both plastic lenses,
The effects of temperature and humidity changes act to cancel each other out with each lens. As a scanning imaging lens, a multi-beam scanning device that is hardly affected by temperature and humidity changes and multi-beam scanning is hardly affected by environmental changes can be realized. .

In the multi-beam scanning device according to the fifth aspect, the lens on the surface to be scanned has a non-arcuate shape in the main scanning section and a center of curvature in the sub-scanning section on the lens surface in the main scanning direction. The curvature radius of the lens surface whose curvature radius in the sub-scanning cross section is changed in the main scanning corresponding direction so that the curvature center line connected in the corresponding direction becomes a curve different from the non-circular shape in the main scanning cross section. Absolute value "smoothly and monotonically increases toward the local maximum as the optical axis is separated in the main scanning direction, and exceeds the local maximum,
As the distance from the optical axis increases, the value decreases smoothly and monotonously. Therefore, the "tolerance of assembly tolerance" of the lens on the scanning surface side to the multi-beam scanning device is effectively relaxed, and the multi-beam scanning device is reduced. Is easy to incorporate into an optical system.

In the multi-beam scanning device according to the present invention, the effect of sag when a rotary polygon mirror is used as an optical deflector can be effectively reduced.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an embodiment of a multi-beam scanning device according to the present invention.

FIG. 2 is a diagram for explaining a lens surface shape of a scanning surface side lens of a scanning image forming lens used in a multi-beam scanning device.

FIG. 3 is a diagram illustrating a field curvature and a constant velocity characteristic of a beam from a first light emitting unit in the example.

FIG. 4 is a diagram illustrating a field curvature and a uniform velocity characteristic of a beam from a second light emitting unit in the example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source (semiconductor laser array) 2 Coupling lens 3 Cylindrical lens 5 Optical deflector 6, 7 Scanning imaging lens 8 Scanning surface

Claims (7)

[Claims] 1. A plurality of beams emitted from a plurality of light-emitting sources are separately formed in a direction corresponding to a sub-scanning direction as a long line image in a direction corresponding to a main scanning, and are formed in the vicinity of an image forming position of each line image. The light is deflected at an equal angular velocity by an optical deflector having a deflecting and reflecting surface, and each deflected beam is condensed as a light spot separated in the sub-scanning direction on the surface to be scanned by the same scanning image forming lens. In the multi-beam optical scanning device for performing constant-speed optical scanning of a plurality of scanning lines, the scanning imaging lens is constituted by two lenses, and the lens on the optical deflector side has a concave surface on the optical deflector side. A positive meniscus lens, the both surfaces of which are coaxially aspheric, and at least one surface of the lens on the surface to be scanned has a non-circular shape in the main scanning section, and the sub-scanning on the lens surface The center of curvature in the cross section The radius of curvature in the sub-scanning cross section is changed in the main scanning corresponding direction so that the center line of curvature connected to the main scanning corresponding direction becomes a curve different from the non-arc shape in the main scanning cross section. A multi-beam scanning device characterized by the above-mentioned. 2. A multi-beam scanning apparatus according to claim 1, wherein the scanning image forming lens has a non-circular shape in a main scanning section of a lens on a surface to be scanned and a sub-scanning on the lens surface. The radius of curvature in the sub-scanning cross section is changed in the main scanning corresponding direction so that the center of curvature connecting the centers of curvature in the cross section in the main scanning corresponding direction is a curve different from the non-circular shape in the main scanning cross section. A multi-beam scanning device, wherein the changed lens surface is a lens surface on the optical deflector side. 3. The multi-beam scanning apparatus according to claim 2, wherein the lens on the scanning surface side of the scanning image forming lens has a circular arc shape in the main scanning section. A scanning imaging lens that is a feature. 4. A multi-beam scanning apparatus according to claim 1, wherein the lens on the scanning surface side of the scanning image forming lens has a negative refractive power in the main scanning section. Scanning imaging lens. 5. The multi-beam scanning apparatus according to claim 1, wherein the scanning image forming lens has a non-circular shape in a main scanning section of a lens on a surface to be scanned, and The radius of curvature in the sub-scanning section is such that the center line of curvature of the lens surface in the sub-scanning section in the sub-scanning section in the main scanning direction is a curve different from the non-arc shape in the main scanning section. The absolute value of the radius of curvature of the lens surface, which is changed in the main scanning corresponding direction, smoothly and monotonically increases toward the maximum value as the optical axis is separated in the main scanning corresponding direction, and after exceeding the maximum position, A multi-beam scanning apparatus characterized in that it is determined so as to decrease smoothly and monotonously as the distance from the optical axis increases. 6. A multi-beam scanning apparatus according to claim 1, wherein a plurality of beams from a plurality of light-emitting sources are respectively converted into parallel beams by a coupling lens, and each parallel beam is converted into a line. A multi-beam scanning apparatus, wherein an image is formed as a long line image in a main scanning corresponding direction near a deflection reflecting surface by an image forming optical system. 7. A multi-beam scanning apparatus according to claim 1, wherein the optical deflector is a rotary polygon mirror, and each of the scanning image forming lenses is provided to reduce the influence of sag. A multi-beam scanning device, wherein a tilt angle is given in a main scanning section.