JP3472200B2 - Diffraction type chromatic aberration correction scanning optical system - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning optical system used as an optical system of a scanning optical device such as a laser printer, and more particularly to a scanning optical system with chromatic aberration corrected.

[0002]

2. Description of the Related Art In a scanning optical device such as a laser printer, a light beam emitted from a light source such as a semiconductor laser is deflected by a polygon mirror and is formed on a surface to be scanned such as a photosensitive drum via an fθ lens (scanning lens). By forming an image, a spot for scanning in the main scanning direction is formed on this surface.

The fθ lens is composed of a single lens or a plurality of lenses, and various aberrations are corrected so that the spot on the surface to be scanned linearly scans at a constant speed as the polygon mirror rotates. In a scanning optical system applied to a multi-beam device that forms a plurality of scanning lines at the same time with light beams from a plurality of light sources, a chromatic aberration of an fθ lens is corrected by combining a positive lens and a negative lens with different dispersions. Alternatively, by selecting light sources and using a combination of light emission wavelengths close to each other, it is possible to reduce the influence of variations in light emission wavelengths among the light sources on drawing.

Taking a semiconductor laser as an example, the wavelength error between the light sources has a standard value of ± 15 nm as a specification error (catalog specification). In a multi-beam apparatus using two or more semiconductor lasers as a light source, Up to 30n
A wavelength error of m may occur. Here, if the fθ lens has a lateral chromatic aberration, the writing start position and the writing end position differ depending on the scanning line, and the drawing pattern is affected beyond the allowable range.

[0005]

However, in the configuration in which the chromatic aberration of the fθ lens is corrected by the combination of lens materials (glass materials) having different dispersions as in the above-mentioned conventional example,
The number of lenses is increased compared to the case where chromatic aberration is not corrected, and the lens material must be decided considering not only the refractive index but also the dispersion, so the type of lens material that can be used is limited The degree of freedom of is reduced. On the other hand, when the light sources are selected and used according to the difference in the emission wavelength, there is a problem that the selection work itself is troublesome and the case where the difference in the emission wavelength occurs due to the aging of the light source cannot be dealt with.

The present invention has been made in view of the above-mentioned problems of the prior art, and corrects the chromatic aberration of magnification without increasing the number of scanning lenses and avoids the deterioration of the drawing performance due to the variation of the emission wavelength of the light source. It is an object of the present invention to provide a scanning optical system capable of performing the above.

[0007]

A diffractive chromatic aberration correction scanning optical system according to the present invention has the following object.
The scanning lens includes a light source, a deflector that deflects the light beam emitted from the light source, and a scanning lens that forms an image of the light beam deflected by the deflector on a surface to be scanned. And has a diffractive lens structure on at least one surface, the diffractive lens structure has a function of correcting lateral chromatic aberration generated by the power of the scanning lens as a refraction lens, and a light beam incident at the maximum angle of view. M is the number of periodic structures of the diffractive lens and W is the maximum angle of view.
(Degree), the condition (1) 0.3 ≦ m / W ≦ 4.0 is satisfied (claim 1).

With the above arrangement, it is possible to correct lateral chromatic aberration generated by the refractive lens element without increasing the number of lenses. Particularly, when it is desired to minimize lateral chromatic aberration, it is desirable to narrow the range of the condition (1) so as to satisfy the following condition (2). (2) 0.7 ≦ m / W ≦ 3.0

As the light source, it is possible to use a multi-beam light source which emits a plurality of light beams condensed at mutually different positions on the surface to be scanned, and in this case, the deflector simultaneously deflects the plurality of light beams. When using a plurality of semiconductor lasers to obtain a plurality of light beams, the wavelengths of the respective light beams may be different from each other. Even in such a case, if the lateral chromatic aberration is corrected, the image can be formed with high accuracy.

The diffractive lens structure is preferably formed as concentric ring zones centered on the optical axis of the lens structure. Further, it is desirable that the diffractive lens structure is formed on the rotationally symmetrical surface of the refractive lens element. From the viewpoint of mold processing, if the diffractive lens structure is formed on a lens surface that is rotationally symmetric, processing is easy. Further, the diffractive lens structure is preferably formed on the lens surface closest to the deflector.

From another point of view, in the diffractive chromatic aberration correction scanning optical system according to the present invention, the diffractive lens structure has a surface in which the width of the light beam incident at the maximum angle of view in the main scanning direction on the lens surface is the largest. It is characterized in that it is formed (claim 7).

From another point of view, in the diffractive chromatic aberration correction scanning optical system according to the present invention, the diffractive lens structure is formed on the surface closer to the deflector than the surface having the positive power closest to the deflector. It is characterized (claim 8).

From another point of view, in the diffractive chromatic aberration correction scanning optical system according to the present invention, the diffractive lens structure is formed on the surface where the incident angle of the light beam incident at the maximum angle of view on the lens surface is the largest. It is characterized in that (claim 9).

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a diffractive chromatic aberration correction scanning optical system according to the present invention will be described below with reference to the drawings.

FIG. 1 is an explanatory view of the main scanning direction (indicated by symbol Y in FIG. 1) of the diffractive chromatic aberration correction scanning optical system 100 according to Examples 1 to 3, and FIG. 2 is its sub-scanning direction ( FIG. 3 is an explanatory diagram of (indicated by reference numeral Z in FIG. 2). Scanning optical system 100
Has a light source 1 such as a multi-point light emitting semiconductor laser. The light source 1 emits a plurality of parallel laser beams arranged in the sub-scanning direction. The laser light emitted from the light source 1 and collimated by the collimator lens 2 is converted into a parallel light flux through the cylindrical lens 3 having a power only in the sub-scanning direction and the rotating shaft 4
It is incident on a polygon mirror (deflector) 4 which is rotationally driven around a, is scanned and deflected by the polygon mirror 4, and is scanned through an fθ lens 20 of a three-lens structure, which is a scanning lens, to be scanned 5
Form a plurality of spots of approximately circular shape on the top.

Although a multi-point emitting semiconductor laser is used as the light source 1 here, instead of this, light beams from a plurality of different semiconductor lasers may be combined using a beam combining element. In such a case, the plurality of light emitting units or the plurality of light emitting elements are arranged so that the spots formed on the surface to be scanned are separated by a predetermined distance in the sub-scanning direction.
That is, it is arranged so that a plurality of scanning lines are formed by one scanning. Further, the light source 1 may be a single semiconductor laser that emits a single laser beam.

The cylindrical lens 3 has a positive power in the sub-scanning direction, and images the light beam emitted from the light source 1 into a linear shape extending in the main scanning direction near the mirror surface of the polygon mirror 4. The fθ lens 20 re-images the light beam, which is linearly imaged in the main scanning direction near the mirror surface, on the image surface (scanning target surface) as a substantially circular spot.

The fθ lens 20 is a meniscus-shaped first lens 21 having positive power in both the main scanning direction and the sub-scanning direction in order from the polygon mirror 4 side toward the scanning target surface 5 side.
And a plano-convex second lens 22 having positive power in both main scanning and sub-scanning directions, and a long third lens 23 having positive power only in the sub-scanning direction. .
The first lens 21 and the second lens 22 are arranged near the polygon mirror 4, and the third lens 23 is arranged near the scan target surface 5.

On the surface 21a of the first lens 21 on the polygon mirror 4 side, there is formed a Fresnel lens-like diffractive lens structure composed of a diffractive surface (for example, a plurality of ring zones) having a periodic structure. The diffractive lens structure has a function of correcting lateral chromatic aberration caused by the refractive power of the fθ lens 20. Each ring zone has a wedge-shaped cross section, and the boundary between adjacent ring zones is formed as a step that gives a predetermined optical path difference. The base curve of the surface 21a (shape as a refracting surface excluding the diffractive lens structure) is a rotationally symmetric aspherical surface. If the diffractive lens structure is formed on such a rotationally symmetric surface, it is relatively easy to process the mold. Scanning target surface 5 of the first lens 21
The lens surface 21b on the side is a rotationally symmetric aspherical surface.

In the second lens 22, the lens surface 22a on the polygon mirror 4 side is a flat surface, and the lens surface 22 on the scanning target surface 5 side.
b is formed as a convex spherical surface. Third lens 23
Is a deformed toric surface defined by the lens surface 23a on the polygon mirror 4 side as a locus obtained by rotating a non-arc curve in the main scanning surface about an axis of rotation that is perpendicular to the optical axis and included in the main scanning surface. The lens surface 23b on the surface 5 side is formed as a convex spherical surface.

FIG. 3 is an enlarged sectional view showing the boundary position of the zones (stepped portion between the zones). Assuming that the height of the step between the adjacent ring zones Z1 and Z2 is T, when the light beam enters the boundary position of the ring zones at the incident angle α as shown in FIG. 3, the side surface of the ring zone Z1 (indicated by Z1A in the figure). Since the light flux incident on the surface) passes through the side surface Z1A of the ring zone Z1, a light amount loss occurs. Here, the incident angle α is an angle formed in the main scanning plane by the central axis of the incident light beam and the radius of the base curve of the surface at the incident position of the light beam. Further, due to the limit of the die machining, the stepped portion does not switch to a substantially right angle like the surfaces Z1A and Z2A in FIG. 3, but on the surface Z2A from the position separated from the boundary position of the ring zone by the distance L. It is formed as a curved surface Z2B that gradually switches to the band Z1. For this reason the ring Z
With respect to the light flux entering toward 2, the light amount loss occurs for the distance L from the boundary position. This light amount loss is more remarkable as the incident angle α is larger. Further, since the light amount loss occurs at each step, the light amount loss increases as the number of ring zones in the region where the light beam enters within the scanning range increases.

Assuming that the fθ lens 20 is an optical system in which chromatic aberration is not corrected by selecting glass, the chromatic aberration of magnification increases as the angle of view increases. For this reason, the number of annular zones formed to correct chromatic aberration also increases in the portion corresponding to the light flux having a large angle of view. Therefore, the light amount loss increases as the angle of view increases. Here, the angle of view (scanning angle) means the angle formed by the optical axis of the fθ lens 20 and the central axis of the light beam reflected by the polygon mirror 4 in the main scanning plane.

The present invention is for coping with the above-mentioned loss of light quantity. Once the amount of chromatic aberration correction is determined, the number of ring zones included in a light beam having a certain angle of view is substantially determined regardless of which lens surface is provided with the diffractive lens structure. Further, if the angle of view and the lens surface forming the diffractive lens structure are determined, the angle of incidence on the lens surface on which the ring zone is formed is determined. Then, if the number of ring zones and the incident angle are determined, it is possible to know the total area of the portion where the light amount loss occurs. The relationship between the amount of light loss, the total area of the portion causing the light amount loss, and the area of the light flux can be expressed as follows.

[Equation 1] Here, LOSS is the light amount loss, Ks is a constant, SA is the total area of the portion that causes the light amount loss, and sf is the light beam area on the lens surface on which the light beam is incident.

By using the right side of the equation 1, that is, the lens surface on which SA / sf is small, as the lens surface to which the diffractive lens structure is added, the light quantity loss can be suppressed.

First, a method of reducing SA will be described. As described above, the number of ring zones (the number of periodic structures) of the diffractive lens structure is determined according to the amount of chromatic aberration correction. Therefore, considering the allowable range of the chromatic aberration correction amount, the target of the chromatic aberration correction is set relatively gently, and the maximum angle of view of the light beam incident on the diffractive lens structure is W (degrees), and the maximum angle of view is set to When the number of ring zones (periodic structure) of the diffractive lens structure included is m,

[Equation 2] By forming the diffractive lens structure so as to satisfy the condition, it is possible to suppress the light amount loss while securing the necessary chromatic aberration correction amount (the number m of the periodic structure will be described later).

If a (= m / W) is below the lower limit of the condition shown in the equation 2, sufficient chromatic aberration correction cannot be realized. Further, when a exceeds the upper limit of the condition shown by the equation 2, the light amount loss becomes large and the diffraction efficiency is lowered. Here, when the target of chromatic aberration correction is 0,

[Equation 3] It becomes the conditional expression.

The above has described the case where the right side of the equation (1) is reduced by reducing the SA, but the light amount loss can be suppressed by increasing the sf of the equation (1). That is, assuming that SA is constant, by forming a diffractive lens structure on the lens surface such that sf is as large as possible, the right side of Expression 1 can be reduced.

In other words, in the scanning optical system, the diffractive lens structure is formed on the surface of the lens surface where the width of the light beam incident at the maximum angle of view in the main scanning direction is the largest, and It is possible to suppress a decrease in the diffraction efficiency due to the loss of the amount of light in the step portion of the structure).

From another point of view, since the light beam deflected by the polygon mirror 4 forms an image on the surface to be scanned via the fθ lens, the diameter of the light beam becomes larger as the system is closer to the polygon mirror 4. it is conceivable that. From this,
By selecting the lens surface closer to the polygon mirror 4 than the lens surface having the positive power closest to the polygon mirror 4 as the surface forming the diffractive lens structure, it is possible to increase the sf of Expression 1.

From another point of view, the larger the incident angle of the light beam is, the larger the width of the light beam on the surface is. Therefore, on the lens surface where the incident angle of the light beam incident at the maximum angle of view is the largest. Even if the diffractive lens structure is formed on the sf, the sf of the equation 1 can be increased.

Next, an embodiment configured to satisfy the above conditions will be described. First, prior to the description of the embodiments, a method of defining the shapes of a rotationally symmetric aspherical surface, a diffractive lens structure, and a modified toric surface will be described. The rotationally symmetric aspherical surface is represented by the following equation 4 as the sag amount X (h) from the tangent plane at the height h from the optical axis.

[Equation 4] Where r is the radius of curvature on the optical axis, K is the conic coefficient, and A4, A
6 and A8 are aspherical coefficients of the 4th, 6th and 8th orders, respectively.

The diffractive lens structure is represented by the optical path length addition amount Δφ (h) defined by the following equation (5).

[Equation 5] Here, P2, P4, P6, P8, and P10 are secondary, quaternary, and 6 respectively.
Next, 8th and 10th coefficients, h is the height from the optical axis.

The actual shape of the diffractive lens structure is represented by the following equation (6).

[Equation 6] In Equation 6, the function MOD (x, y) represents the remainder when x is divided by y, C is a constant (0 ≦ C <1) for setting the phase of the boundary position of the annular zone, and n is the refraction. The ratio λ is the wavelength used. The point of h at which (MOD (Δφ (h) + C, −1) = 0 is the boundary of the ring zone. In each of the following examples, C = 0.5, and E is the diffractive lens structure of the light beam. This is a coefficient for correcting a change in the amount of phase addition that occurs when the light is obliquely incident on.

When the diffractive lens structure is formed on a rotationally symmetric aspherical surface, the final shape is expressed as the sag amount TS (h) below.

[Equation 7] If the diffractive lens structure is formed on a plane, the final shape TS (h) will be equal to S (h).

The number m of the periodic structure of the diffractive lens included in the light beam incident at the maximum angle of view is expressed by the following equation 8 to be exact.

[Equation 8] Here, hmax is the height of the point at which the distance from the optical axis is the maximum when the light beam that is incident on the scanning lens at the maximum angle of view is incident on the diffractive lens structure, and hmin is the maximum image angle on the scanning lens. It is the height at the point where the distance from the optical axis becomes the minimum when a light beam incident at an angle enters the diffractive lens structure.

The deformed toric surface is defined as a locus obtained by rotating a non-circular curve around the rotation axis included in the main scanning surface and perpendicular to the optical axis. The non-circular curve is defined by the following equation 9.

[Equation 9] Here, X (Y) is the sag amount, that is, the distance from the tangent line at the point of the distance Y in the main scanning direction from the optical axis, and other symbols are the same as in the equation 4.

[0037]

Example 1 Table 1 shows a scanning optical system 10 according to Example 1.
The configuration on the scanning target surface 5 side of the cylindrical lens 3 of 0 is shown. In the table, symbol f is the focal length of the fθ lens 20 in the main scanning direction, SW is the scanning width, W is the maximum angle of view, λ is the wavelength used, ry is the radius of curvature in the main scanning direction, and rz is the radius of curvature in the sub-scanning direction. (Omitted in the case of a rotationally symmetrical surface), d is the distance between the surfaces on the optical axis, and n is the refractive index at the design wavelength of 780 nm.

In Table 1, the surface numbers # 1 and # 2 are the cylindrical lenses 3, the surface number # 3 is the mirror surface of the polygon mirror 4, and the surface numbers # 4 and # 5 are the first lens 2 of the fθ lens 20.
1, surface numbers # 6 and # 7 are the second lens 22, surface number # 8,
# 9 indicates the third lens 23. The diffractive lens structure is formed on the surface 21a (surface number # 4) on the polygon mirror side of the first lens 21. This lens surface 21a has no positive power, that is, has a negative power, and is located closer to the polygon mirror 4 than the lens surface 21b located closest to the polygon mirror 4 side among the positive lens surfaces. There is.

[0039]

[Table 1]

The radius of curvature of each aspherical surface and deformed toric surface in Table 1 is the radius of curvature on the optical axis of each optical element (in the case of the surface on which the diffractive lens structure is formed, the macroscopic shape of the surface). Radius of curvature on the optical axis of), and the conical coefficient, aspherical surface coefficient, and non-circular arc coefficient are shown in Table 2.

[0041]

[Table 2]

Also, the coefficient E of the equation 6 and the coefficient P of the equation 5
The values of 2, P4, P6, P8 and P10 are shown in Table 3.

[Table 3]

Design wavelength 780n with diffractive lens structure alone
The focal length at m is 2662.2 mm. In Example 1, a = 2.7, which satisfies the condition represented by Expression 1, and the light amount loss at the maximum image height is 3.6%.

FIG. 4 shows (A) linearity error, (B) field curvature (broken line: main scanning direction, solid line: sub-scanning direction) of the scanning optical system having the above-described configuration of the first embodiment, and (C). The chromatic aberration of magnification is shown respectively. The vertical axis of each graph is the image height (height from the intersection of the scanning spot and the optical axis on the surface to be scanned), the horizontal axis is the amount of occurrence of each aberration, and the unit is both the vertical axis and the horizontal axis. Is.

[0045]

Second Embodiment Table 4 shows the scanning optical system 100 according to the second embodiment.
The numerical configuration of is shown. The correspondence between each surface number and the optical element is the same as that in the first embodiment. The conical coefficient, aspherical coefficient, and non-circular coefficient are shown in Table 5. The diffractive lens structure includes the first lens 2
It is provided on the lens surface 21a of the first polygon mirror.

[0046]

[Table 4]

[0047]

[Table 5]

Table 6 shows the values of E of the equation 6 and the coefficients P2, P4, P6, P8 and P10 of the equation 5 in the second embodiment.
Shown in.

[0049]

[Table 6]

Design wavelength 780n with diffractive lens structure alone
The focal length at m is 3576.9 mm. Also, a =
In 2.9, the condition of Formula 1 is satisfied, and the light amount loss at the maximum image height is 3.8%.

FIG. 5 shows (A) linearity error, (B) field curvature (broken line: main scanning direction, solid line: sub-scanning direction) of the scanning optical system having the configuration of the above-described second embodiment, and (C). The chromatic aberration of magnification is shown respectively.

[0052]

Third Embodiment Table 7 shows a scanning optical system 100 according to the third embodiment.
The numerical configuration of is shown. The correspondence between each surface number and the optical element is the same as that in the first embodiment. The conical coefficient, aspherical coefficient, and non-circular coefficient are shown in Table 8. The diffractive lens structure includes the first lens 2
It is provided on the lens surface 21a of the first polygon mirror.

[0053]

[Table 7]

[0054]

[Table 8]

Also, the coefficient E of the equation 6 and the coefficient P of the equation 5
The values of 2, P4, P6, P8 and P10 are shown in Table 9.

[0056]

[Table 9]

The focal length of the diffractive lens structure alone in Example 3 at a design wavelength of 780 nm is 4854.8 mm. In Example 3, a = 1.9, the condition represented by the formula 1 was satisfied, and the light amount loss at the maximum image height was 2.
5%.

FIG. 6 shows (A) linearity error, (B) field curvature (broken line: main scanning direction, solid line: sub-scanning direction) of the scanning optical system according to the configuration of Example 3 described above, and (C). The chromatic aberration of magnification is shown respectively.

[0059]

Fourth Embodiment FIG. 7 shows a scanning optical system 200 according to the fourth embodiment.
In the main scanning direction, and FIG. 8 is an explanatory view in the sub scanning direction. In the fourth embodiment, the fθ lens 30 is used instead of the fθ lens 20 of the first embodiment.

The fθ lens 30 has a meniscus-shaped first lens 31 having positive power in both the main scanning direction and the sub scanning direction in order from the polygon mirror 4 side toward the scanning target surface 5 side.
And a meniscus-shaped second lens 32 having a positive power in both the main scanning direction and the sub-scanning direction, and a long third lens 33 having a positive power only in the sub-scanning direction. .

Lens surfaces 31a, 31b, 31a, 31b
Are rotationally symmetric aspherical surfaces. The surface 33a of the third lens 33 on the polygon mirror side is an aspherical surface having no rotation axis whose radius of curvature in the sub-scanning plane at a position away from the optical axis is set regardless of the shape in the main scanning plane. The shape in the main scanning plane is defined by the non-circular curve expressed by the above-mentioned Numerical Expression 9, and the radius of curvature in the sub-scanning direction is defined by the following Numerical Expression 10.

[0062]

[Equation 10] Note that rz (Y) of the equation 10 is the radius of curvature in the sub-scanning direction at the position of the distance Y in the main scanning direction from the optical axis, rz0 is the radius of curvature in the sub-scanning direction on the optical axis, and B1 to B8 are coefficients. Is.

The surface 33 of the third lens 33 on the scanning target surface 5 side
b is a convex spherical surface. Table 10 below shows the numerical configuration of the scanning optical system 200 according to the fourth embodiment. The correspondence between each surface number and the optical element is the same as that in the first embodiment. Surface number # 4, #
Table 11 shows the conical coefficient, the aspherical surface coefficient, and the non-circular arc coefficient of Nos. 5, # 6, and # 7. The diffractive lens structure is provided on the surface 31b (surface number # 5) of the first lens 31 on the scanning target surface 5 side. The lens surface 31b is the surface closest to the polygon mirror 4 among the lens surfaces having positive power.

[0064]

[Table 10]

[0065]

[Table 11]

Cone coefficient for defining the shape of the surface 33a of the third lens 33 on the polygon mirror 4 side in the main scanning plane,
The non-circular arc coefficient is shown in Table 12, and the coefficient defining the radius of curvature in the sub-scanning direction is shown in Table 13.

[0067]

[Table 12]

[0068]

[Table 13]

Further, the coefficient E of the equation 6 and the coefficient P of the equation 5
The values of 2, P4, P6, P8 and P10 are shown in Table 14.

[0070]

[Table 14]

The focal length of the diffractive lens structure alone in Example 4 at a design wavelength of 780 nm is 6468.7 mm. In Example 4, a = 1.0, which satisfies the condition represented by Formula 1, and the light amount loss at the maximum image height is 1.
6%.

FIG. 9 shows (A) linearity error, (B) field curvature (broken line: main scanning direction, solid line: sub-scanning direction) of the scanning optical system according to the configuration of Example 4 described above, and (C). The chromatic aberration of magnification is shown respectively.

[0073]

Fifth Embodiment FIG. 10 shows a scanning optical system 30 according to the fifth embodiment.
0 is an explanatory diagram in the main scanning direction, and FIG. 11 is an explanatory diagram in the sub scanning direction. In Example 5, the fθ lens 20 of Example 1 is used.
Instead, the fθ lens 40 is used.

The fθ lens 40 is a meniscus-shaped first lens 41 having positive power in both the main scanning direction and the sub-scanning direction in order from the polygon mirror 4 side toward the scanning target surface 5 side.
And a plano-convex second lens 42 having a positive power in both the main scanning direction and the sub-scanning direction, and a long third lens 43 having a positive power only in the sub-scanning direction. .

The lens surfaces 41a and 41b are both rotationally symmetric aspherical surfaces. Polygon mirror 4 of second lens 42
The surface 42a on the side is a plane, and the surface 42b on the surface 5 to be scanned is
Is a convex sphere. The surface 43a of the third lens 43 on the polygon mirror side is a deformed toric surface, and is the surface 5 to be scanned.
The side surface 42b is a convex spherical surface.

Table 15 below shows the numerical configuration of the scanning optical system 300 according to the fifth embodiment. The correspondence between each surface number and the optical element is the same as that in the first embodiment. Table 16 shows the conical coefficient, the aspherical coefficient, and the non-circular coefficient. The diffractive lens structure is provided on the surface 42a (surface number # 6) of the second lens 42 on the polygon mirror 4 side.

[0077]

[Table 15]

[0078]

[Table 16]

Also, the coefficient E of the equation 6 and the coefficient P of the equation 5
The values of 2, P4, P6, P8 and P10 are shown in Table 17.

[Table 17]

The focal length of the diffractive lens structure alone in Example 5 at a design wavelength of 780 nm is 3986.2 mm. In Example 5, a = 2.7, which satisfies the condition represented by Formula 1, and the light amount loss at the maximum image height is 3.
1%.

FIG. 12 shows (A) linearity error, (B) field curvature (broken line: main scanning direction, solid line: sub-scanning direction), and (C) of the scanning optical system according to the configuration of Example 5 described above. The chromatic aberration of magnification is shown respectively.

In the fθ lens 40 of the fifth embodiment, the diffractive lens structure is formed on the surface where the width WS of the light beam incident at the maximum angle of view in the main scanning direction on the lens surface is the largest. Table 18 shows the width WS of the light beam on each lens surface in the main scanning direction. The width WS has a maximum value of 6.07 on the surface 42a of the second lens 42 on the polygon mirror 4 side, and the diffractive lens structure is formed on this surface.

[0083]

[Table 18]

Further, the polygon mirror 4 of the second lens 42
The side surface 42a is a surface on which the incident angle α of the light beam incident at the maximum angle of view on the lens surface is the largest. Table 19 shows the incident angle α of the light flux that is incident at the maximum angle of view on each lens surface.
The incident angle α has a maximum value of 39.72 ° on the surface 42a of the second lens 42 on the polygon mirror 4 side, and as described above, the diffractive lens structure is formed on this surface.

[0085]

[Table 19]

Since the chromatic aberration is a problem not only in the writing optical system as described above but also in the scanning type reading optical system using white light as the illumination light, the scanning lens of the embodiment has such a structure. It can also be applied to a reading optical system. When applied to a reading optical system, the light from the object to be read is incident on a polarizer such as a polygon mirror via a scanning lens, and the light reflected by this polarizer is received at a single point by a light receiving element such as a photodiode. It is configured to receive light. In this configuration, by using a lens having the same diffractive lens structure as that of the above-described embodiment as the scanning lens, it is possible to correct lateral chromatic aberration generated by the power of the scanning lens as the refraction lens.

[0087]

As described above, according to the present invention, since the diffractive lens structure is added to the scanning lens which constitutes the scanning optical system to correct the chromatic aberration of magnification of the refraction lens portion, the existing single lens is used. It is possible to avoid the deterioration of the drawing performance due to the variation of the emission wavelength of the light source without increasing the number of scanning lenses for the beam and suppressing the decrease of the diffraction efficiency due to the light amount loss at the boundary of the ring zone.

[Brief description of drawings]

FIG. 1 is an explanatory diagram in a main scanning direction showing a configuration of a diffractive chromatic aberration correction scanning optical system according to Examples 1 to 3.

2 is an explanatory view of the optical system of FIG. 1 in a sub-scanning direction.

FIG. 3 is a diagram showing a cause of light amount loss at a boundary of an annular zone.

4A and 4B are aberration diagrams of the optical system according to Example 1, in which FIG. 4A shows linearity error, FIG. 4B shows field curvature, and FIG. 4C shows lateral chromatic aberration.

5A and 5B are aberration diagrams of the optical system according to Example 2, where FIG. 5A shows linearity error, FIG. 5B shows field curvature, and FIG. 5C shows lateral chromatic aberration.

6A and 6B are aberration diagrams of the optical system according to Example 3, where FIG. 6A shows linearity error, FIG. 6B shows field curvature, and FIG. 6C shows lateral chromatic aberration.

FIG. 7 is an explanatory diagram in the main scanning direction showing the configuration of an fθ lens of the diffractive chromatic aberration correction scanning optical system according to Example 4.

8 is an explanatory view of the fθ lens shown in FIG. 7 in a sub-scanning direction.

9A and 9B are aberration diagrams of the optical system according to Example 4, where FIG. 9A shows linearity error, FIG. 9B shows field curvature, and FIG. 9C shows lateral chromatic aberration.

FIG. 10 is an explanatory diagram in the main scanning direction showing the configuration of an fθ lens of the diffractive chromatic aberration correction scanning optical system according to Example 5;

FIG. 11 is an explanatory diagram of the fθ lens shown in FIG. 10 in the sub-scanning direction.

FIG. 12 is an aberration diagram of an optical system according to Example 5, (A)
Represents linearity error, (B) represents field curvature, and (C) represents lateral chromatic aberration.

[Explanation of symbols]

1 Semiconductor laser 4 polygon mirror 20 fθ lens 21 First lens 22 Second lens 23 Third lens 5 Scanning surface 30 fθ lens 31 First lens 32 Second lens 33 Third lens 40 fθ lens 41 First lens 42 Second lens 43 Third lens

Claims (9)

(57) [Claims] 1. A light source, a deflector for deflecting a light beam emitted from the light source, and a scanning lens for forming an image of the light beam deflected by the deflector on a surface to be scanned. In addition to having a positive power as a lens, a diffractive lens structure is provided on at least one surface, and the diffractive lens structure has a function of correcting chromatic aberration of magnification generated by the power of the scanning lens as a refractive lens, and has a maximum image When the number of periodic structures of the diffractive lens structure included in the light beam incident at an angle is m and the maximum angle of view is W (degree), the condition (1) 0.3 ≦ m / W ≦ 4.0 is satisfied. Characteristic Diffraction-type chromatic aberration correction scanning optical system. 2. The diffractive chromatic aberration correction scanning optical system according to claim 1, wherein the following condition (2) 0.7 ≦ m / W ≦ 3.0 is satisfied. 3. The diffraction according to claim 1, wherein the light source emits a plurality of light fluxes that are condensed at different positions on the scan target surface, and the deflector deflects the plurality of light fluxes. Type chromatic aberration correction scanning optical system. 4. The diffractive chromatic aberration correction scanning optical system according to claim 1, wherein the diffractive lens structure is composed of a plurality of concentric ring zones centered on the optical axis of the diffractive lens structure. system. 5. The diffractive chromatic aberration correction scanning optical system according to claim 1, wherein the diffractive lens structure is formed on a rotationally symmetric lens surface. 6. The diffractive chromatic aberration correction scanning optical system according to claim 1, wherein the diffractive lens structure is formed on a lens surface closest to the deflector. 7. A light source, a deflector for deflecting a light beam emitted from the light source, and a scanning lens for forming an image of the light beam deflected by the deflector on a scan target surface. A diffractive lens structure having a positive power as a lens and having a function of correcting lateral chromatic aberration generated by the power as a refraction lens of the scanning lens has a maximum field angle of a light beam incident on the lens surface in the main scanning direction. A diffractive chromatic aberration correction optical system characterized by being formed on a surface having the largest width. 8. A light source, a deflector for deflecting a light beam emitted from the light source, and a scanning lens for forming an image of the light beam deflected by the deflector on a surface to be scanned. A diffractive lens structure having a positive power as a lens and having a function of correcting lateral chromatic aberration generated by the power as a refraction lens of the scanning lens,
A diffractive chromatic aberration correction optical system, which is formed on a surface closer to the deflector than a surface having a positive power closest to the deflector. 9. A light source, a deflector for deflecting a light beam emitted from the light source, and a scanning lens for forming an image of the light beam deflected by the deflector on a surface to be scanned. The diffractive lens structure having a positive power as a lens and having a function of correcting the lateral chromatic aberration generated by the power as the refraction lens of the scanning lens has the largest incident angle on the lens surface of the light flux incident at the maximum angle of view. Diffractive chromatic aberration correction optical system characterized by being formed on the surface.