Image forming apparatus and scanning unit to scan a target surface using light fluxes

Abstract

A scanning unit in an image forming apparatus includes a light source, a coupling lens, an aperture, an image forming lens, and a polygon mirror. The light source includes a plurality of surface-emitting lasers. The coupling lens, the aperture, and the image forming lens are arranged on the optical path of light beams emitted by the light source. The polygon mirror deflects light beams of an image formed by the coupling lens towards a photosensitive drum for scanning. The focal length of the image forming lens in a sub-scanning direction is set to be equal to or smaller than an optical path length between the image forming lens and the aperture.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The values of the conic constant and each coefficient of the incident surface of the first scanning lens 11a are given below in Table 3.

	TABLE 3
	K	0	B1	0
	A4	8.885 × 10-7	B2	0
	A6	−2.629 × 10−10	B3	0
	A8	2.1846 × 10−14	B4	0
	A10	1.368 × 10−17	B5	0
	A12	−3.135 × 10−21	B6	0
			B7	0
			B8	0

The values of the conic constant and each coefficient of the output surface of the first scanning lens 11a are given below in Table 4.

	TABLE 4
	K	0	B1	−1.594 × 10−6
	A4	9.2240 × 10−7	B2	−4.332 × 10−6
	A6	6.7782 × 10−11	B3	4.9819 × 10−9
	A8	−4.1124 × 10−14	B4	−2.8594 × 10−9
	A10	1.3727 × 10−17	B5	−2.677 × 10−12
	A12	2.069 × 10−21	B6	2.8778 × 10−13
			B7	−1.916 × 10−15
			B8	2.0423 × 10−15
			B9	1.0141 × 10−13
			B10	−6.729 × 10−19

The values of the conic constant and each coefficient of the incident surface of the second scanning lens 11b are given below in Table 5.

	TABLE 5
	K	0	B1	0
	A4	3.286 × 10−7	B2	−1.1328 × 10−6
	A6	−7.085 × 10−11	B3	2.60612 × 10−10
	A8	6.269 × 10−15	B4	7.8961 × 10−11
	A10	−2.7316 × 10−19	B5	−5.027 × 10−14
	A12	4.739 × 10−24	B6	1.4051 × 10−14
			B7	4.5538 × 10−18
			B8	−2.0140 × 10−18
			B9	−1.546 × 10−22
			B10	7.4893 × 10−23

The values of the conic constant and each coefficient of the output surface of the second scanning lens 11b are given below in Table 6.

	TABLE 6
	K	0	B1	0
	A4	1.2779 × 10−7	B2	2.311 × 10−7
	A6	−4.629 × 10−11	B3	0
	A8	4.049 × 10−15	B4	0
	A10	−1.659 × 10−19	B5	0
	A12	2.585 × 10−24	B6	0
			B7	0
			B8	0

The optical system arranged on the optical path between the polygon mirror 13 and the photosensitive drum 901 is referred as a scanning optical system that includes the first scanning lens 11a and the second scanning lens 11b.

The photosensitive drum 901 is arranged at a position where the optical path length between the output surface of the second scanning lens 11b and the photosensitive drum 901 is, e.g., 142.5 millimeters (‘d10’ in FIG. 7). A dust-tight glass 22 having refractive index of 1.5112 and thickness of 1.9 millimeters is arranged between the second scanning lens 11b and the photosensitive drum 901 (refer to FIG. 2).

The length of the portion on the surface of the photosensitive drum 901 on which scanning is possible, that is, the width in which image writing in the direction Dir_main is possible, is, e.g., 323 millimeters.

In the coupling optical system and the scanning optical system, the lateral magnification in the direction Dir_main is 5.7 times, while the lateral magnification in the direction Dir_sub is 1.2 times. That is, the absolute value of the lateral magnification in the direction Dir_main is more than the absolute value of the lateral magnification in the direction Dir_sub. As a result, the scanning-line distance narrows thereby improving the image resolution. The scanning-line distance in the scanning unit 900 is, e.g., 5.3 micrometers. Consequently, a resolution of, e.g., 4800 dots per inch (dpi) can be achieved.

Meanwhile, even if a large number of light-emitting members are arranged in a two-dimensional array, the sub-scanning beam pitch may still deviate from a predetermined beam pitch depending on the positional errors or the shape errors of the light-emitting members. It is necessary to maintain a stable sub-scanning beam pitch to prevent deterioration in the image quality.

To obtain a stable sub-scanning beam pitch, the distance between adjacent light beams, which are incident on a scanning lens having its power lens in the sub-scanning direction, is narrowed such that the incident angle of the light beams with respect to a target surface becomes smaller. As a result, irrespective of positional errors or shape errors in the light-emitting members, the deviation of the sub-scanning beam pitch can be reduced.

FIG. 9 is a graph of an optical path of a light beam (ch1) emitted by the 1st light-emitting member and an optical path of a light beam (ch40) emitted by the 40th light-emitting member. In FIG. 9, ‘P105’ indicates the position of the coupling lens 15, ‘P106’ indicates the position of the aperture plate 16, ‘P107’ indicates the position of the image forming lens 17, ‘P103’ indicates the position of the polygon mirror 13, ‘P101a’ indicates the position of the first scanning lens 11a, ‘P101b’ indicates the position of the second scanning lens 11b, and ‘P910’ indicates the position of the photosensitive drum 901.

In FIG. 9, the focal length of the image forming lens 17 in the direction Dir_sub (fs) is set to be equal to or smaller than the optical path length between the image forming lens 17 and the aperture plate 16 (L1). As a result, mutual widening of adjacent light beams output from the image forming lens 17 in the direction Dir_sub can be prevented, i.e., a stable sub-scanning beam pitch can be obtained. Subsequently, the incident angle of the light beams with respect to the photosensitive drum 901 becomes smaller. Thus, irrespective of positional errors or shape errors in the light-emitting members 101, the deviation of the sub-scanning beam pitch can be reduced. Moreover, because the light beams in the direction Dir_sub pass thorough the proximity of the optical axis of the first scanning lens 11a and the second scanning lens 11b, the overall optical characteristics of the scanning unit 900 also improve.

As compared to the graph shown in FIG. 9, FIG. 10 is a graph of ch1 and ch40 when fs is set to be greater than L1. As shown in FIG. 10, the light beams ch1 and ch40 gradually widen with respect to the direction Dir_sub. As a result, the deviation of the sub-scanning beam pitch varies depending on positional errors or shape errors in the light-emitting members 101.

FIG. 11 is a table depicting the diameters of the light beams (beam diameters) in the direction Dir_main and the direction Dir_sub depending on the image height on the photosensitive drum 901. As shown in FIG. 11, the beam diameter in the direction Dir_sub is smaller than the beam diameter in the direction Dir_main. The beam diameter is 1/e² of the maximum value of the optical intensity.

FIG. 12 is a diagram of a conventional aperture plate 16′ that includes a conventional aperture 16b having a width (Wm) of 5.6 millimeters in the direction Dir_main and a width (Ws) of 0.8 millimeters in the direction Dir_sub. FIG. 13 is a table depicting the beam diameters in the direction Dir_main and the direction Dir_sub depending on the image height on the photosensitive drum 901 when the conventional aperture plate 16′ is used instead of the aperture plate 16. As shown in FIG. 13, the beam diameter in the direction Dir_main is smaller than the beam diameter in the direction Dir_sub.

Thus, it is clear that the beam diameter in the direction Dir_sub is smaller when the aperture plate 16 is used. That leads to improved granularity of an image thereby decreasing its rough feel. Thus, an image with high resolution can be output. Because the value of Ws of the apertured curve 16 is 1.6 times greater than that of the conventional aperture plate 16′, the light use efficiency of the scanning unit 900 improves by about 60%.

Meanwhile, if the center of the two-dimensional array 100 is aligned with the center of the aperture plate 16 in the direction perpendicular to the optical axis, the center of the light beams emitted by the light-emitting members 101 forming the outermost layer of the two-dimensional array 100 (i.e., from 1st to 10th, from 31st to 40th, 11th, 21st, 20th, and 30th with reference to FIG. 4) does not match with the center of the aperture plate 16. Consequently, the light use efficiency of the light beams emitted by the outermost light-emitting members 101 is worse than the light beams of the light-emitting members 101 arranged in the midsection of the two-dimensional array 100. That may cause uneven density in the image. To avoid such a problem, the outermost light-emitting members 101 can be configured to emit more amount of light than the remaining light-emitting members 101.

As described above, the scanning unit 900 includes the light source 14 having a plurality of surface-emitting lasers (i.e., light-emitting members 101), the polygon mirror 13, the coupling optical system, and the scanning optical system. The coupling optical system includes the coupling lens 15, the aperture plate 16, the image forming lens 17, and the reflecting mirror 18 that are arranged between the light source 14 and the polygon mirror 13 on the optical path of the light beams emitted by the light source 14. The polygon mirror 13 receives light beams of an image formed by the image forming lens 17 and deflects the light beams to the photosensitive drum 901 via the scanning optical system. The focal length of the image forming lens 17 in the direction Dir_sub is set to be equal to or smaller than the optical path length between the image forming lens 17 and the aperture plate 16. As a result, a stable sub-scanning beam pitch can be obtained.

Moreover, in the coupling optical system and the scanning optical system, the absolute value of the lateral magnification in the direction Dir_main is set to be more than the absolute value of the lateral magnification in the direction Dir_sub. Furthermore, over the surface of the photosensitive drum 901, the beam diameter in the direction Dir_sub is set to be equal to or smaller than the beam diameter in the direction Dir_main but greater than the scanning-line distance. As a result, the loss in the amount of light is much less in the scanning unit 900 than in a conventional scanning unit. As a result, it is possible to perform beam shaping that in turn improves the light use efficiency of the scanning unit 900.

As shown in FIGS. 14A and 14B, a cross-section perpendicular to the optical axis of the light beam emitted by a VCSEL forms a substantially circular shape. Hence, if the width of an aperture in the direction Dir_main and the width of the aperture in the direction Dir_sub fairly differ from each other, the amount of light falls short of the required amount for handling high-speed imaging.

To solve such a problem, the beam diameter in the direction Dir_main is set to be greater than the beam diameter in the direction Dir_sub over the surface of the photosensitive drum 901. Thus, the difference between the width of the aperture 16a in the direction Dir_main and the width of the aperture 16a in the direction Dir_sub is reduced thereby improving coupling efficiency (i.e., ratio of optical power emitted by a light-emitting member to optical power output from an aperture).

Moreover, the multibeam technology implemented in the light source 14 enables high-resolution and high-speed imaging. In this case, because the scanning-line distance decreases, it is possible to set the beam diameter in the direction Dir_sub to be greater than the scanning-line distance. As a result, no gaps remain in the direction Dir_sub thereby uniformly filling the whole image.

Generally, in an scanning unit implementing the multibeam technology, two methods can be used to improve the image resolution in the direction Dir_sub: (1) to reduce the lateral magnification in the direction Dir_sub; and (2) to reduce the distance between light-emitting members in the Dir_sub. However, in the first method, the amount of light falls short of the required amount because the width of an aperture in the direction Dir_sub needs to be reduced to decrease a beam diameter over a target surface. In the second method, the light beams emitted by the light-emitting members mutually interfere and it is also difficult to secure sufficient space for wiring of each light-emitting member.

As described above, the two-dimensional array 100 in the light source 14 includes four rows of the light-emitting members 101 in the direction Dir_sub, each row including 10 light-emitting members 101 equally spaced along the direction of the tilt angle α, i.e., along the direction T, between the direction Dir_main and the direction Dir_sub. The distance between any two adjacent light-emitting members 101, when orthographically-projected on a virtual line extending in the direction Dir_sub, is equal. In this configuration, the light-emitting members 101 are mutually spread out in the direction Dir_main, which has no effect on the high-resolution of an image in the direction Dir_sub. Thus, even after reducing the distance between the light-emitting members 101 in the Dir_sub, the light beams do not mutually interfere and a sufficient space can be secured for wiring.

Moreover, the focal length of the coupling lens 15 is set to be greater than the optical path length between the coupling lens 15 and the aperture plate 16 thereby reducing the overall optical path length from the light source 14 to the photosensitive drum 901. That is, the aperture plate 16 is arranged between the coupling lens 15 and the back focal position of the coupling lens 15. Such a configuration is different than a conventional configuration in which an aperture plate is arranged at the back focal position of a coupling lens.

As described above, the laser printer 500 includes the scanning unit 900 that can achieve a stable sub-scanning beam pitch. Thus, the laser printer 500 can perform a high-quality and high-speed image forming process.

Instead of the aperture plate 16, an aperture plate having much smaller width in the direction Dir_main can also be used. In that case, the focal length of the coupling lens 15 needs to be readjusted corresponding to the smaller width of the aperture plate.

Moreover, the mesa portion in each light-emitting member 101 need not be of a circular shape and can be of an elliptical shape, a square shape, or a rectangular shape.

As described above, the two-dimensional array 100 includes four rows of the light-emitting members 101 and each row includes 10 light-emitting members 101. However, as long as the number of the light-emitting members 101 in a single row is more than the total number of rows, there is no limitation on the number of the light-emitting members 101 and the number of rows.

As described above, the distance between the light-emitting members 101 orthographically-projected on a virtual line extending in the direction Dir_sub (‘c’ in FIG. 4) is set to 4.4 micrometers. However, the distance is not limited to 4.4 micrometers and can be changed.

Moreover, as described above, the distance between adjacent rows of the light-emitting members 101 in the direction Dir_sub (‘d’ in FIG. 4) is set to 44.0 micrometers, while the distance between the light-emitting members 101 in each row of the light-emitting members 101 (‘X’ in FIG. 4) in the direction T is set to 30.0 micrometers. However, the distances are not limited to those values and can be changed.

Furthermore, instead of the two-dimensional array 100, a two-dimensional array 200 can be used as shown in FIGS. 15 to 18. The two-dimensional array 200 includes a plurality of light-emitting members 201, which are VCSELs. Each light-emitting member 201 includes a plurality of semiconductor layers stacked together. Some of the semiconductor layers are made of a different material than that of the semiconductor layers in the two-dimensional array 100. More particularly, instead of the first spacer layer 113, the active layer 114, and the second spacer layer 115 in the two-dimensional array 100, the two-dimensional array 200 includes a first spacer layer 213, an active layer 214, and a second spacer layer 215, respectively.

The first spacer layer 213 is made of (Al_0.7Ga_0.3)_0.5In_0.5P that is a wide-band-gap semiconductor material.

As shown in FIG. 17, the active layer 214 includes a quantum well layer 214a made of GaInPAs and a barrier wall layer 214b made of Ga_0.6In_0.4P. The quantum well layer 214a has a three-layered compressively-strained structure having a band gap wavelength of 780 nanometers, while the barrier wall layer 214b has a four-layered tensile-strained structure.

The second spacer layer 215 is made of (Al_0.7Ga_0.3)_0.5In_0.5P that is a wide-band-gap semiconductor material.

The first spacer layer 213, the active layer 214, and the second spacer layer 215 together are referred as a resonator structure that has an optical thickness equal to one oscillation wavelength (refer to FIG. 17).

Because the layers of the two-dimensional array 200 are made of the AlGaInP material system, it is possible to maintain a much wider band gap between the spacer layers, i.e., the first spacer layer 213 and the second spacer layer 215, and the active layer 214 as compared to the multilayered structure of the two-dimensional array 100.

FIG. 18 is a table depicting different band gaps when VCSELs having multilayered structures of different material composition are used. As shown in FIG. 18, a VCSEL of a 780-nanometer band including a spacer layer and a quantum well layer made of AlGaAs/AlGaAs material system (hereinafter, “VCSEL_A”), a VCSEL of a 780-nanometer band including a spacer layer and a quantum well layer made of AlGaInP/GaInPAs material system (hereinafter, “VCSEL_B”), and a VCSEL of a 850-nanometer band including a spacer layer and a quantum well layer made of AlGaAs/GaAs material system (hereinafter, “VCSEL_C”) are used for comparing the band gaps between the respective spacer layers and the active layer, and between the respective barrier wall layer and the quantum well layer. The VCSEL_A corresponds to the light-emitting member 101, while the VCSEL_B having x=0.7 corresponds to the light-emitting member 201.

In case of the VCSEL_B, it is possible to maintain a wider band gap than in case of the VCSEL_A and the VCSEL_C. More particularly, the band gap between the spacer layers and the quantum well layer in the VCSEL_B is 767.3 milli-electron volts, which is greater than the band gap of 465.9 milli-electron volts in the VCSEL_A and the band gap of 602.6 milli-electron volts in the VCSEL_C. Similarly, the band gap between the barrier wall layer and the quantum well layer in the VCSEL_B is 463.3 milli-electron volts, which is greater than the band gap of 228.8 milli-electron volts in the VCSEL_A and the band gap of 365.5 milli-electron volts in the VCSEL_C. Thus, a better carrier confinement can be achieved in case of the VCSEL_B.

Moreover, because the quantum well layer 214a in each light-emitting member 201 has a compressively-strained structure, a higher gain can be obtained due to band separation between a heavy hole and a light hole. As a result, a high optical output can be achieved at a low threshold. Furthermore, the degree of reflection of the upper reflecting mirror 117 (refer to FIG. 16) can also be reduced to further enhance the optical output. Because of a higher gain, the optical output can be prevented from deteriorating due to rise in temperature. Thus, it is possible to arrange mutually close light-emitting members 201 in the two-dimensional array 200.

Because the quantum well layer 214a and the barrier wall layer 214b do not contain aluminum (Al), the active layer 214 is less oxidized thereby curbing the formation of a non-emitting recombination center. As a result, the durability of the light-emitting members 201 can be improved.

If in an optical writing device, a two-dimensional array of VCSELs having poor durability is used, the optical writing device needs to be disposed after one use. However, because the two-dimensional array 200 includes the durable light-emitting members 201, an optical device can be reused for a number of times thereby conserving resources and decreasing environmental burdens. Any other optical device can achieve such advantages by using the two-dimensional array 200.

As described above, the oscillation wavelength of the light emitted by each light-emitting member 101 or 201 is 780 nanometers. However, the oscillation wavelength is not limited to 780 nanometers and can be adjusted depending on the sensitivity characteristics of the photosensitive drum 901. In that case, the material composition or the multilayered structure of the light-emitting members 101 or 201 needs to be changed depending on the oscillation wavelength.

Meanwhile, the amount of optical output from a VCSEL is less than that from an edge-emitting laser. Thus, it is necessary to improve the light use efficiency of the VCSEL. As shown in FIGS. 19A and 19B, the polarization direction of a light beam in an edge-emitting laser is parallel to the direction of an active layer AL. To improve light use efficiency of an scanning unit using the edge-emitting laser, the main scanning direction is set perpendicular to the active layer AL such that the divergence angle is greater. Consequently, the polarization direction becomes same as the sub-scanning direction. On the other hand, a light beam emitted by a VCSEL forms a substantially circular shape. Hence, in case of a scanning unit using a two-dimensional array of VCSELs, the two dimensional array can be arranged such that the angle between the polarization direction and the main scanning direction is greater than the angle between the polarization direction and the sub-scanning direction. Such an arrangement helps in improving transmissivity through a soundproof glass, a scanning lens, and a dust-tight glass without changing the beam diameter. Such an arrangement is especially effective in an optical scanning system using a soundproof glass and a dust-tight glass. Moreover, in case of using a retroreflector, the reflectivity of the retroreflector can also be improved.

FIG. 20 is table depicting light use efficiency of various optical elements at the near side to a light source (topside with reference to FIG. 2) and with respect to a nearest image height. The amount of light use efficiency is given when the polarization direction is same as the main scanning direction and when the polarization direction is same as the sub-scanning direction. As shown in FIG. 20, except for a polygon mirror, the light use efficiency of the optical elements is better when the polarization direction is same as the main scanning direction.

In the above description, the image forming apparatus was considered to be the laser printer 500. However, any other image forming apparatus including the scanning unit 900 can perform a high-quality and high-speed image forming process.

In case of a color-image forming apparatus, an scanning unit compatible to the color-image forming apparatus can be used to achieve a high-quality and high-speed image forming.

Moreover, as shown in FIG. 21, the image forming apparatus can also be a tandem-type color-image forming apparatus that includes a plurality of photosensitive drums, each photosensitive drum forming a toner image corresponding to a single color, and a set of components corresponding to each photosensitive drum. More particularly, the tandem-type color-image forming apparatus shown in FIG. 21 includes a photosensitive drum K1 for forming black toner images, a photosensitive drum C1 for forming cyan toner images, a photosensitive drum M1 for forming magenta toner images, and a photosensitive drum Y1 for forming yellow toner images. The set of components corresponding to the photosensitive drum K1 includes a charger K2, a developer K4, a cleaning unit K5, and a charging unit for transfer K6. Similarly, the set of components corresponding to the photosensitive drum C1 includes a charger C2, a developer C4, a cleaning unit C5, and a charging unit for transfer C6. The set of components corresponding to the photosensitive drum M1 includes a charger M2, a developer M4, a cleaning unit M5, and a charging unit for transfer M6. The set of components corresponding to the photosensitive drum Y1 includes a charger Y2, a developer Y4, a cleaning unit Y5, and a charging unit for transfer Y6. Apart from that, the tandem-type color-image forming apparatus includes the scanning unit 900, a transfer belt 80, and a fixing unit 30.

In this case, the light-emitting members 101 in the light source 14 are classified into four groups, each group emitting light to one of the photosensitive drums K1, C1, M1, and Y1. Alternatively, four units of the two-dimensional array 100 (or the two-dimensional array 200) can be arranged corresponding to each of the photosensitive drums K1, C1, M1, and Y1. Moreover, four units of the scanning unit 900 can also be arranged corresponding to each of the photosensitive drums K1, C1, M1, and Y1.

As shown in FIG. 21, each of the photosensitive drums K1, C1, M1, and Y1 rotates in the clockwise direction. The charger (K2, C2, M2, and Y2), the developer (K4, C4, M4, and Y4), the cleaning unit (K5, C5, M5, and Y5), and the charging unit for transfer (K6, C6, M6, and Y6) are sequentially arranged along the rotational direction of the corresponding photosensitive drum (K1, C1, M1, and Y1). The charger (K2, C2, M2, and Y2) uniformly charges the surface of the corresponding photosensitive drum (K1, C1, M1, and Y1). The charged surface of the photosensitive drum (K1, C1, M1, and Y1) is exposed to light emitted from the scanning unit 900 such that a latent image is formed on the surface of the photosensitive drum (K1, C1, M1, and Y1). The developer (K4, C4, M4, and Y4) develops the corresponding latent image to form a toner image in the corresponding color (black, cyan, magenta, and yellow). The charging unit for transfer (K6, C6, M6, and Y6) transfers the corresponding single-color toner image onto a recording paper such that all four toner images are superimposed to form a full-color toner image. Finally, the fixing unit 30 fixes the full-color toner image on the recording paper.

Sometimes, color drift occurs in the images formed by a tandem-type color-image forming apparatus. However, by using the scanning unit 900 that includes the two-dimensional array 100 of the high-density light-emitting members 101 (VCSELs), it is possible to selectively switch ON the light-emitting members 101 to precisely correct the color drift.

Meanwhile, an image forming apparatus that includes a silver halide film as an image carrier can also be used. In that case, a latent image is formed on the silver halide film by optical scanning. The latent image can be developed by a usual developing process performed in silver halide photography. Such an image forming apparatus can be implemented as an optical plate-making apparatus or an optical lithography device to plot a computed tomography (CT) scan image.

Moreover, an image forming apparatus including a color-developing medium (e.g., a positive photographic paper), which develops colors due to heat energy of a beam spot, as an image carrier can also be used. In that case, the image can be developed directly on the image carrier by optical scanning.

Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.

Claims

1. A scanning unit that scans a scanning target surface by using light fluxes, the scanning unit comprising:

a light source having a plurality of surface-emitting lasers each emitting a light beam;

a coupling lens that receives the light beams from the light source and renders the light beams as substantially parallel light;

an aperture that receives the parallel light and defines a diameter of the parallel light thereby obtaining a diameter-defined parallel light;

an image forming lens that receives the diameter-defined parallel light and forms an image in a sub-scanning direction; and

an optical deflector that is arranged close to a focal point of the image forming lens, and that receives light beams of the image and deflects the light beams for scanning a target surface, wherein

a focal length of the image forming lens in the sub-scanning direction is smaller than an optical path length between the image forming lens and the aperture, and

a beam diameter of the diameter-defined parallel light in the sub-scanning direction is equal to or smaller than a beam diameter of the diameter-defined parallel light in the main scanning direction but greater than a distance between adjacent scanning lines, for all image heights.

2. The scanning unit according to claim 1, wherein a focal length of the coupling lens is greater than an optical path length between the coupling lens and the aperture.

3. The scanning unit according to claim 1, wherein

an absolute value of lateral magnification in a main scanning direction is more than an absolute value of lateral magnification in the sub-scanning direction.

4. The scanning unit according to claim 1, wherein

each of the surface-emitting lasers emits a linearly-polarized light beam,

an absolute value of lateral magnification in the main scanning direction is more than an absolute value of lateral magnification in the sub-scanning direction, and

an angle between a polarization direction of the linearly-polarized light beam and the main scanning direction is greater than an angle between the polarization direction and the sub-scanning direction.

5. The scanning unit according to claim 1, wherein

the surface-emitting lasers are arranged in a two-dimensional array such that M number of the surface-emitting lasers (M≧2) are arranged along the sub-scanning direction and N number of the surface-emitting lasers (N>M) are arranged along a direction of a tilt angle α between the main scanning direction and the sub-scanning direction, and

a distance between adjacent light-emitting members, when orthographically-projected on a virtual line extending in the sub-scanning direction, is equal.

6. The scanning unit according to claim 5, wherein at least one of outermost light-emitting members in the two-dimensional array emits a greater amount of light than remaining light-emitting members.

7. The scanning unit according to claim 1, wherein the plurality of surface-emitting lasers include a quantum well layer having a compressively strained structure such that a high optical output can be achieved at a low threshold.

8. An image forming apparatus comprising:

at least one unit of an image carrier; and

at least one scanning unit including:

a light source having a plurality of surface-emitting lasers each emitting a light beam;

a coupling lens that receives the light beams from the light source and renders the light beams as substantially parallel light;

an aperture that receives the parallel light and defines a diameter of the parallel light thereby obtaining a diameter-defined parallel light;

an image forming lens that receives the diameter-defined parallel light and forms an image in a sub-scanning direction; and

an optical deflector that is arranged close to a focal point of the image forming lens, and that receives light beams of the image and deflects the light beams for scanning a target surface, wherein

wherein a focal length of the image forming lens in the sub-scanning direction is smaller than an optical path length between the image forming lens and the aperture, and

wherein the scanning unit scans the image carrier with a plurality of light beams having image information, and

a beam diameter of the diameter-defined parallel light in the sub-scanning direction is equal to or smaller than a beam diameter of the diameter-defined parallel light in the main scanning direction but greater than a distance between adjacent scanning lines, for all image heights.

9. The image forming apparatus according to claim 8, wherein the image information is color-image information for forming a color image.

10. The scanning unit according to claim 1, wherein

the beam diameter in the sub-scanning direction is smaller than the beam diameter in the main scanning direction such that the beam diameter in the main scanning direction is at least 1.2 times the beam diameter in the sub-scanning direction.

11. The image forming apparatus according to claim 8, wherein

the beam diameter in the sub-scanning direction is smaller than the beam diameter in the main scanning direction such that the beam diameter in the main scanning direction is at least 1.2 times the beam diameter in the sub-scanning direction.

12. A scanning unit comprising:

a light source having a plurality of surface-emitting lasers each emitting a light beam;

an optical deflector that receives light beams from the light source through a pre-deflection optical system and deflects the light beams for scanning a target surface; and

a scanning optical system that focuses the deflected light beams, wherein

an absolute value of lateral magnification in a main scanning direction is more than an absolute value of lateral magnification in a sub-scanning direction, and

the pre-deflection optical system and the scanning optical system are configured to satisfy the following condition that a beam diameter of the light received by the optical deflector in the sub-scanning direction is smaller than a beam diameter of the light received by the optical deflector in the main scanning direction but greater than a distance between adjacent scanning lines, for all image heights.

13. The scanning unit according to claim 12, wherein the pre-deflection optical system includes an aperture.

14. The scanning unit according to claim 13, wherein

a focal length of the coupling lens is greater than an optical path length between the coupling lens and the aperture, and

a focal length of the image forming lens in the sub-scanning direction is smaller than an optical path length between the image forming lens and the aperture.

15. The scanning unit according to claim 12, wherein

each of the surface-emitting lasers emits a linearly polarized light beam, and

an angle between a polarization direction of the linearly-polarized light beam and the main scanning direction is greater than an angle between the polarization direction and the sub-scanning direction.

16. The scanning unit according to claim 12, wherein

the beam diameter in the sub-scanning direction is smaller than the beam diameter in the main scanning direction such that the beam diameter in the main scanning direction is at least 1.2 times the beam diameter in the sub-scanning direction.

17. An image forming apparatus comprising:

at least one unit of an image carrier; and

at least one scanning unit including:

a light source having a plurality of surface-emitting lasers each emitting a light beam;

an optical deflector that receives light beams from the light source through a pre-deflection optical system and deflects the light beams for scanning a target surface; and

a scanning optical system that focuses the deflected light beams, wherein

an absolute value of lateral magnification in a main scanning direction is more than an absolute value of lateral magnification in a sub-scanning direction, and

the pre-deflection optical system and the scanning optical system are configured to satisfy the following condition that a beam diameter in the sub-scanning direction of the light received by the optical deflector is smaller than a beam diameter in the main scanning direction of the light received by the optical deflector but greater than a distance between adjacent scanning lines, for all image heights.

18. The image forming apparatus according to claim 17, wherein

the beam diameter in the sub-scanning direction is smaller than the beam diameter in the main scanning direction such that the beam diameter in the main scanning direction is at least 1.2 times the beam diameter in the sub-scanning direction.

19. A scanning unit comprising:

a light source having a plurality of surface-emitting lasers each emitting a light beam;

an optical deflector that receives light beams from the light source and deflects the light beams for scanning a target surface, wherein

a beam diameter in a sub-scanning direction of the light received by the optical deflector is smaller than a beam diameter in a main scanning direction of the light received by the optical deflector but greater than a distance between adjacent scanning lines, for all image heights, and

at least one of outermost light-emitting members in the two-dimensional array emits a greater amount of light than remaining light-emitting members.

20. The scanning unit according to claim 19, wherein a focal length of the coupling lens is different from an optical path length between the coupling lens and the aperture.

21. The scanning unit according to claim 19, wherein

an absolute value of lateral magnification in a main scanning direction is more than an absolute value of lateral magnification in the sub-scanning direction, and

a focal length of the image forming lens in the sub-scanning direction is smaller than an optical path length between the image forming lens and the aperture.

22. The scanning unit according to claim 19, wherein

each of the surface-emitting lasers emits a linearly-polarized light beam,

an absolute value of lateral magnification in the main scanning direction is more than an absolute value of lateral magnification in the sub-scanning direction, and

an angle between a polarization direction of the linearly-polarized light beam and the main scanning direction is greater than an angle between the polarization direction and the sub-scanning direction.

23. The scanning unit according to claim 19, wherein

the beam diameter in the sub-scanning direction is smaller than the beam diameter in the main scanning direction such that the beam diameter in the main scanning direction is at least 1.2 times the beam diameter in the sub-scanning direction.

24. An image forming apparatus comprising:

at least one unit of an image carrier; and

at least one scanning unit including:

a light source having a plurality of surface-emitting lasers each emitting a light beam;

an optical deflector that receives light beams from the light source and deflects the light beams for scanning a target surface, wherein

a beam diameter in a sub-scanning direction of the light received by the optical deflector is smaller than a beam diameter in a main scanning direction of the light received by the optical deflector but greater than a distance between adjacent scanning lines, for all image heights, and

at least one of outermost light-emitting members in the two-dimensional array emits a greater amount of light than remaining light-emitting members.

25. The image forming apparatus according to claim 24, wherein

the beam diameter in the sub-scanning direction is smaller than the beam diameter in the main scanning direction such that the beam diameter in the main scanning direction is at least 1.2 times the beam diameter in the sub-scanning direction.