JP2007147864A - Optical scanner - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical scanner capable of precisely correcting the irregularity of light quantity of beams on the surface of a photoreceptor and stably obtaining a high-quality image. <P>SOLUTION: Two beams of a beam A0 having a polarizing direction in a main-scanning direction and a beam B0 having a polarizing direction in a sub-scanning direction are made incident to a mirror 1. Reflectance Rp for P-polarization and reflectance Rs for S-polarization at incident angle θ1 to the mirror 1 are respectively set to be 0.8 and 0.9. With respect to a beam A1 obtained by reflecting the beam A0 and a beam B1 obtained by reflecting the beam B0 with the mirror 1, optical intensities A1 and B1 respectively become A0*0.9 and B0*0.8. Subsequently, when beams are made incident to a mirror 2, with respect to a beam A2 obtained by reflecting the beam A1 and a beam B2 obtained by reflecting the beam B1 with the mirror 2, optical intensities A2 and B2 respectively become A0*0.9*0.8 and B1*0.8*0.9, therefore, the overall reflectances after passage of the beams A2, B2 through the mirrors 1, 2 get to 0.72, that is, the same value in both, and the light quantity of the two beams having different polarizing directions after passage through the optical system can be made even. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical scanning device, and more particularly to an optical scanning device that performs scanning exposure by deflecting light beams emitted from a plurality of light sources with a deflecting element.

  A scanning optical system using a 32-beam VCSEL array (surface emitting laser array) has been proposed as a technical means for achieving high speed and high image quality of laser printers and digital copiers (see, for example, Non-Patent Document 1). .

  A surface emitting laser, which is a laser light source, generates linearly polarized light, and its polarization direction rotates depending on the oscillation intensity of the laser. The degree of this rotation differs for each of a plurality of light sources included in the surface emitting laser. The cause is that the oscillation modes of the surface emitting lasers differ among the lasers, but the oscillation modes depend on the temperature distribution and refractive index distribution in the laser cavity. The temperature distribution and refractive index distribution in the laser cavity vary depending on the amount of current applied to the laser, the distribution of oscillation laser light, and the state of heat dissipation, so the polarization directions are aligned among multiple lasers in one surface emitting laser array. It is difficult.

  On the other hand, the optical scanning device is constituted by a plurality of optical elements. When linearly polarized light is incident on the optical element, the reflectance changes depending on the incident angle to the element. When the polarization directions of a plurality of beams from the light source are different, the ratio of the P-polarized component and the S-polarized component with respect to the mirror surface differs between the beams at the incidence on the same mirror surface. A difference occurs in the amount of light. FIG. 16 shows the incident angle / reflectance relationship on the glass surface without coating.

  As shown in FIG. 16 (c), since the reflectance differs between P-polarized light and S-polarized light (Rs, Rp), variation in the amount of light occurs between the beams. Alternatively, even in a transmission type element such as a window, the transmittance varies for the same reason, and thus the light amount variation between beams occurs.

  When the light amount variation as described above occurs, a light amount variation occurs on the image scanning surface (here, the photosensitive member surface), which causes image quality deterioration such as unevenness in the image.

  For example, in the conventional example shown in FIG. 15, the polarization directions of the laser beams from the LD-A and LD-B of the LD array 14 coincide with the main scanning direction for LD-A and the sub-scanning direction for LD-B. ing. At this time, the reflectance transmittance of linearly polarized light in the main scanning direction and the sub-scanning direction is substantially equal to the elements from the LD to the cylinder lens 18 and the f-θ lenses 22 and 23. However, the reflectance of the linearly polarized light in the main scanning direction and the sub-scanning direction with respect to the polygon mirror 20, the cylinder mirror 124, and the cylinder mirror 125 is different in the incident angle of the beam to each optical element. And the beam from LD-B are in different states of S-polarized light or P-polarized light, resulting in a difference in reflectance between the two as shown in FIG. In the example of FIG. 17, a total reflectance difference of 5.5% occurs between the main scanning and the sub-scanning.

  Because of the above reasons, the reflectance and transmittance of each optical element are different for each laser beam. Therefore, the total reflectance and transmittance of the optical system are different for each beam. Bring. Although the figure shows an example of 32 beams, a variation in the amount of light occurs about 6%, and this variation in the amount of light causes unevenness such as streaks in the image.

  Further, as shown in FIGS. 19A to 19C, the reflectivity with respect to the S-polarized light and the P-polarized light of the polygon mirror 20, the cylinder mirror 124, and the cylinder mirror 125 using the single layer film mirror is as shown in FIG. It is determined by the coating composition of the optical surface and the angle of incidence of the beam on each mirror.

  The reflectivity Rp (the square of the reflection coefficient ρp) with respect to the incident angle θ in each single layer mirror is, as shown in FIGS. 20 to 22, particularly (a) at an incident angle of 42 ° and (c) at 50 °. The difference between the P-polarized component and the S-polarized component is remarkable, and the reflectance difference between the P-polarized component and the S-polarized component when passing through the optical system configured using the single-layer film mirror is the polygon mirror 20, This is the product of the difference in reflectance between the P-polarized component and the S-polarized component of the cylinder mirror 124 and cylinder mirror 125, and the difference in the amount of light when the image is finally formed on the photosensitive member is not negligible.

In order to solve the above problems, an object of the present invention is to provide an optical scanning device in which the total transmittance or total reflectance for the polarization components in the main scanning direction and the sub-scanning direction is substantially the same in all optical elements through which the light beam passes. The purpose is to do.
Junichi Ichikawa, Shuho Ikeda, Nobuaki Ueki, Printer Exposure Equipment Using Multi-Beam Surface Emitting Laser Elements, 29th Optical Symposium, Japan Optical Society, 2004

  In consideration of the above-described facts, an object of the present invention is to provide an optical scanning device that can accurately correct a light amount variation of a beam on the surface of a photoreceptor and stably obtain a high-quality image.

The optical scanning device according to claim 1, a plurality of light sources;
An optical scanning apparatus comprising: an optical system including a plurality of optical elements that guides a plurality of light beams emitted from the plurality of light sources to the surface of the object to be scanned and forms an image on the surface of the object to be scanned; At least one of the configuration of the coating formed on the optical element and the incident angle of the light beam incident on the optical element with respect to the P-polarized component and the S-polarized component that are polarization components of the light beam By controlling the P-polarized light component and the S-polarized light component so that the reflectance or transmittance is substantially equal.

  In the invention with the above-described configuration, by controlling the transmittance or reflectance of the P-polarized component and the S-polarized component for each of the plurality of light beams, the light amount variation of the light beams emitted from the plurality of light sources having uneven rotation angles in the polarization direction. Can be suppressed.

  According to a second aspect of the present invention, the plurality of light beams emitted from the plurality of light sources are linearly polarized light.

  In the invention with the above configuration, the light quantity can be accurately controlled by using linearly polarized light whose vibration direction is constant regardless of time as compared with circularly polarized light / elliptically polarized light.

  According to a third aspect of the present invention, the light source is a surface emitting laser.

  In the invention with the above configuration, even in a surface emitting laser having a plurality of adjacent light emitting points, it is possible to suppress the light amount difference for each beam and make the brightness for each dot uniform.

  Since the present invention has the above-described configuration, it is possible to obtain an optical scanning device that can accurately correct the light amount variation of the beam on the surface of the photosensitive member and stably obtain a high-quality image.

<Basic configuration>
1 and 2 show an optical scanning device according to the present invention.

  As shown in FIGS. 1 and 2, the optical scanning device 10 emits a light beam using the LD array 14 as a light source, the collimator 16 collimates the collimated light beam with the aperture 12, passes through the cylinder lens 18, and then rotates. The light beam is deflected by the reflecting surface of the polygon mirror 20 to be reciprocated and scanned and exposed in the main scanning direction.

The light beams that pass through the fθ lenses 22 and 23 having power in the main scanning direction and are reflected by the cylinder mirrors 24 and 25 are imaged as beam spots on the scanning surface 28. The imaged beam spot scans the scanning surface 28 in the main scanning direction as the polygon mirror 20 rotates. As a result, an image by a beam spot is formed on the scanning surface 28.
<Basic concept>
FIG. 3 shows an optical system according to the present invention.

  As shown in FIG. 3, the reflectance / transmittance for each polarization component in the main scanning direction and the sub-scanning direction is adjusted by adjusting the configuration of the coating disposed on the reflecting surfaces of the polygon mirror 20, the cylinder mirror 24, and the cylinder mirror 25. To control. As a result, the total reflectance / transmittance for each polarization component in the main scanning direction and the sub-scanning direction through the entire optical system can be made substantially the same, and the light amount difference for each beam during imaging can be suppressed.

  A beam having a polarization direction in the main scanning direction is A0, and a beam having a polarization direction in the sub-scanning direction is B0. These two beams are incident on the mirror 1. At this time, the mirror 1 is assumed to have a surface coating layer having an arbitrary configuration.

The normal of the mirror 1 is inclined by θ1 in the sub-scanning direction, the beam A0 is S-polarized with respect to the mirror 1, and the beam B0 is P-polarized with respect to the mirror 1. The reflectance for P-polarized light at the incident angle θ1 to the mirror 1 is Rp (= 0.8), and the reflectance for S-polarized light is Rs (= 0.9). At this time, the light intensity of the beam A1 reflected by A0 and the beam B1 reflected by B0 is
A1 = A0 * Rs = A0 * 0.9
B1 = B0 * Rp = B0 * 0.8
It becomes.

  Next, the beams A1 and B1 enter the mirror 2. The mirror 2 has a surface coating layer having the same configuration as that of the mirror 1.

The normal line of the mirror 2 is inclined by θ2 in the main scanning direction. The beam A1 is P-polarized with respect to the mirror 2, and the beam B1 is S-polarized with respect to the mirror 2. If the incident angle θ2 to the mirror 2 is equal to the incident angle θ1 of the mirror 1, the light intensity of the beam A2 reflected by the mirror 2 and the beam B2 reflected by the B1 is
A2 = A1 * Rp = A0 * 0.9 * 0.8
B2 = B1 * Rs = B0 * 0.8 * 0.9
It becomes.

Therefore, the total reflectance of the beam A2 and the beam B2 after passing through both the mirror 1 and the mirror 2 is Rp = Rs = 0.72, and both of them have the same value, and the two after passing through the mirror 1 and the mirror 2 are the same. It becomes possible to make the light quantity of the beam uniform.
<Application to optical scanning device>
FIG. 4 shows an optical system of the optical scanning device according to the first embodiment of the present invention.

  FIG. 6 shows the configuration of the coating disposed on the reflecting surfaces of the polygon mirror 20, the cylinder mirror 24, and the cylinder mirror 25 shown in FIG. 4, and the incident angle dependence of the reflectance with respect to S-polarized light and P-polarized light at each mirror. Is shown in FIGS.

  By combining the configuration of the coating formed on the reflection surface of each mirror and the incident angle with respect to the laser beam to an appropriate value, the total transmittance for the polarization components in the main scanning direction and the sub-scanning direction is almost as shown in FIG. It is possible to be the same.

  As shown in FIG. 4, a beam having a polarization direction in the main scanning direction is A0, and a beam having a polarization direction in the sub-scanning direction is B0. These two beams are incident on the polygon mirror 20.

  The normal line of the polygon mirror 20 is inclined by 42 ° in the main scanning direction, the beam A0 is P-polarized with respect to the polygon mirror 20, and the beam B0 is S-polarized. The polygon mirror 20 has a coating having the configuration shown in the left diagram of FIG. 6, and as shown in FIG. 7, the reflectances Rs and Rp of the S-polarized light and the P-polarized light when the incident angle is 42 ° are Rs = 0.90 and Rp = 0.83. At this time, the thickness (nm) of the coating is represented by λ (nm) × δ × 1/2 × (1 / n) × (1 / cos θ (j)) and is 300.9 nm. Here, λ is the laser wavelength, δ is the phase shift amount of the coating layer, n is the refractive index of the coating layer, and θ (j) is the incident angle of the light beam in the j layer to the j + 1 layer.

The light intensity of the beam A1 reflected by the polygon mirror 20 and the beam B1 reflected by the B0 is:
A1 = A0 * Rp = A0 * 0.83
B1 = B0 * Rs = B0 * 0.90
It becomes.

  Next, the beams A 1 and B 1 are incident on the cylinder mirror 24. The cylinder mirror 24 has a surface coating layer having the structure shown in FIG.

  The normal line of the cylinder mirror 24 is inclined by 15 ° in the sub-scanning direction. The beam A1 is S-polarized with respect to the cylinder mirror 24, and the beam B1 is P-polarized with respect to the cylinder mirror 24. As shown in FIG. 8, the reflectances Rs and Rp when the incident angle of the S-polarized light and the P-polarized light of the cylinder mirror 24 is 15 ° are Rs = 0.94 and Rp = 0.94, respectively. At this time, the coating thickness (nm) is 93.6 nm for the first layer and 128.5 nm for the second layer.

Therefore, the light intensity of the beam A2 reflected by the cylinder mirror 24 and the beam B2 reflected by B1 is
A2 = A1 * Rs = A0 * 0.83 * 0.94
B2 = B1 * Rp = B0 * 0.90 * 0.94
It becomes.

  Next, the beams A 2 and B 2 are incident on the cylinder mirror 25. The cylinder mirror 25 is assumed to have a surface coating layer configured as shown in the right side of FIG.

  The normal line of the cylinder mirror 25 is inclined by 50 ° in the sub-scanning direction. The beam A2 is S-polarized with respect to the cylinder mirror 25, and the beam B2 is P-polarized with respect to the cylinder mirror 25. As shown in FIG. 9, the reflectances Rs and Rp when the incident angles of the S-polarized light and the P-polarized light of the cylinder mirror 25 are 50 ° are Rs = 0.97 and Rp = 0.89, respectively. At this time, the coating thickness (nm) is 99.7 nm for the first layer and 151.3 nm for the second layer.

Therefore, the light intensity of the beam A3 reflected by the cylinder mirror 25 and the beam B3 reflected by B2 is
A3 = A2 * Rs = A0 * 0.83 * 0.94 * 0.97 = A0 * 0.756
B3 = B2 * Rp = B0 * 0.90 * 0.94 * 0.89 = B0 * 0.753
Thus, the total reflectivities of the three mirrors for the beam A0 and the beam B0, that is, the polygon mirror 20, the cylinder mirror 24, and the cylinder mirror 25 are 0.756 and 0.753, respectively, which are substantially equal.

Due to the above effects, the amount of beam emitted from the scanning optical system can have a substantially uniform distribution with little variation in the amount of light for each beam as shown in FIG.
<Control by incident angle>
As shown in FIG. 9, when the incident angle to the cylinder mirror 25 is changed from 50 ° to 45 °, the reflectances Rs and Rp of the S-polarized light and the P-polarized light are Rs = 0.969 and Rp = 0.899, respectively. Rs = 0.965 and Rp = 0.90.

Therefore, the total reflectivity of the three mirrors is A3 = A2 * Rs = A0 * 0.83 * 0.94 * 0.965 = A0 * 0.753
B3 = B2 * Rp = B0 * 0.90 * 0.94 * 0.90 = B0 * 0.761
It becomes.

As described above, by changing the incident angle to the mirror, it is possible to change the total reflectance for each polarization component of the three mirrors for the beam A0 and the beam B0. As a result, for example, even when the reflectance of another mirror fluctuates for some reason, the reflectance for each polarization component can be controlled by changing the incident angle instead of replacing the mirror. .
<Application to optical scanning device>
FIG. 11 shows an optical system of an optical scanning device according to the second embodiment of the present invention.

  As shown in FIG. 11, the configuration in which incident light is deflected by the polygon mirror 20, reflected by the reflecting surfaces of the cylinder mirror 24 and the cylinder mirror 25, and guided to the surface to be scanned is the same as in the first embodiment.

  In the present embodiment, for example, a window 30 that is a transmissive optical element is provided on the incident side of the polygon mirror 20, and transmission of the entire optical system with respect to P-polarized light and S-polarized light using the difference in transmittance with respect to P-polarized light and S-polarized light. The goal is to match rates.

  The window 30 is an optical glass having a refractive index of 1.5, and is provided in a state inclined by 25 ° in the sub-scanning direction with respect to the incident laser beam. That is, since the direction is inclined with respect to B0, A0 is S-polarized light and B0 is P-polarized light (see FIG. 11).

As shown in FIG. 12, the transmittance of the window 30 for S-polarized light is 0.944 per surface at an incident angle of 25 °, and the transmittance for P-polarized light is 0.968 per surface, and the transmission surface of the window 30 is the incident surface. Therefore, the transmittance Ts for S-polarized light is Ts = 0.944 * 0.944 = 0.899.
The transmittance Tp for P-polarized light is Tp = 0.968 * 0.968 = 0.937
It becomes. Therefore, the light intensity of the beam transmitted through the window 30 is A1 = A0 * Tp = A0 * 0.89.
B1 = B0 * Ts = B0 * 0.94
It becomes.

Subsequently, the beam transmitted through the window 30 is scanned and deflected by the polygon mirror 20. At this time, the light intensity of the beam A1 reflected by the polygon mirror 20 and the beam B1 reflected by B0 is:
A2 = A1 * Rp = A0 * 0.89 * 0.83
B2 = B1 * Rs = B0 * 0.94 * 0.90
This is the same as in the first embodiment.

  Next, the A2 and B2 beams are incident on the cylinder mirror 24. The cylinder mirror 24 has a surface coating layer having the structure shown in FIG.

The light intensity of the beam A3 reflected by the cylinder mirror 24 and the beam B3 reflected by B2 is
A3 = A2 * Rs = A0 * 0.89 * 0.83 * 0.875
B3 = B2 * Rp = B0 * 0.94 * 0.90 * 0.866
It becomes.

  Next, the beams A 2 and B 2 are incident on the cylinder mirror 25. The cylinder mirror 25 is assumed to have a surface coating layer configured as shown in the right side of FIG.

The reflectances Rs and Rp when the incident angles of the S-polarized light and the P-polarized light of the cylinder mirror 25 are 50 ° are Rs = 0.92 and Rp = 0.81, respectively. Therefore, the light intensity of the beam A4 reflected by the cylinder mirror 25 and the beam B4 reflected by B3 is
A4 = A3 * Rs
= A0 * 0.89 * 0.83 * 0.875 * 0.92
= A0 * 0.595
B4 = B3 * Rp
= B0 * 0.94 * 0.90 * 0.866 * 0.81
= B0 * 0.593
The total transmission / reflectance of one window and three mirrors for beam A0 and beam B0, that is, window 30, polygon mirror 20, cylinder mirror 24, and cylinder mirror 25 is 0.595 and 0.593, respectively. Almost equal. FIG. 13 shows the transmission / reflectance in the main scanning direction / sub-scanning direction in the entire optical system after the LD array 14. As is apparent from this table, the transmittance difference in the main / sub direction is 1.004, which is less than 1%.

Due to the above effects, the amount of light emitted from the scanning optical system can have a substantially uniform distribution with little unevenness in the amount of light for each beam.
<Summary>
As described above, even if the polarization directions of laser beams emitted from a plurality of light sources are different, it is possible to align the transmittance / reflectance for each beam of the scanning optical system.

  Even when the laser oscillation mode in the laser cavity changes during laser modulation and the polarization direction changes instantaneously, the beam transmittance / reflectance does not change depending on the polarization direction. Can also be avoided. As a result, it is possible to accurately correct the variation in the light amount of the beam on the surface of the photoreceptor, and a high-quality image can be stably obtained.

  In addition, the optical system transmittance of a beam having an arbitrary polarization direction can be adjusted without depending on the individual polarization direction of the laser array, so that high image quality can be obtained and the beam spacing generated during assembly of the scanning optical system can be adjusted. The yield of the scanning optical system with respect to variations in the amount of light could be significantly improved.

In addition, the above-described configuration can be dealt with only by adjusting the surface coating of each mirror, and other special components are not required, so that the entire scanning optical system can be manufactured at low cost.
<Others>
As mentioned above, although the Example of this invention was described, it cannot be overemphasized that this invention is not limited to said Example at all, and can implement in a various aspect in the range which does not deviate from the summary of this invention.

  For example, the present invention can be used for various optical devices that include a plurality of light sources and optical elements and in which a difference in the amount of light for each light source is a problem.

1 is a perspective view showing an optical scanning device according to the present invention. 1 is a schematic diagram showing an optical scanning device according to the present invention. It is a figure which shows the deflection | deviation control of the mirror which concerns on this invention. It is a figure which shows the deflection | deviation control of the mirror which concerns on 1st Embodiment of this invention. It is a table | surface which shows the deflection | deviation control of the mirror which concerns on 1st Embodiment of this invention. It is a figure which shows the surface coating of the mirror which concerns on 1st Embodiment of this invention. It is a figure which shows the incident angle and reflection coefficient of the mirror which concern on 1st Embodiment of this invention. It is a figure which shows the incident angle and reflection coefficient of the mirror which concern on 1st Embodiment of this invention. It is a figure which shows the incident angle and reflection coefficient of the mirror which concern on 1st Embodiment of this invention. It is a figure which shows the light quantity nonuniformity of the beam spot which concerns on this invention. It is a figure which shows the incident angle and reflection coefficient of the mirror which concern on 2nd Embodiment of this invention. It is a schematic diagram which shows the optical scanning device which concerns on 2nd Embodiment of this invention. It is a figure which shows the influence on the deflection | deviation of the window which concerns on 2nd Embodiment of this invention. It is a perspective view which shows the conventional optical scanning device. It is a schematic diagram which shows the conventional optical scanning device. It is a figure which shows the relationship between the incident angle by the conventional polarization method, and a reflectance. It is a figure which shows the influence on the deflection | deviation of the conventional mirror. It is a figure which shows the light quantity nonuniformity of the conventional beam spot. It is a figure which shows the surface coating of the conventional mirror. It is a figure which shows the incident angle and reflection coefficient of the conventional mirror. It is a figure which shows the incident angle and reflection coefficient of the conventional mirror. It is a figure which shows the incident angle and reflection coefficient of the conventional mirror.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical scanning device 12 Aperture 14 LD array 16 Collimator 18 Cylinder lens 20 Polygon mirror 22 f (theta) lens 23 f (theta) lens 24 Cylinder mirror 25 Cylinder mirror 28 Scanning surface 30 Window

Claims (3)

Multiple light sources;
An optical scanning apparatus comprising: an optical system including a plurality of optical elements that guides a plurality of light beams emitted from the plurality of light sources to the surface of the object to be scanned and forms an image on the surface of the object to be scanned;
For the P-polarized component and the S-polarized component that are the polarized components of the light beam,
A coating configuration formed on the optical element;
The reflectance or transmittance of the P-polarized component and the S-polarized component is controlled to be substantially equal by controlling at least one of the incident angle of the light beam incident on the optical element. Optical scanning device. The optical scanning device according to claim 1, wherein the plurality of light beams emitted from the plurality of light sources are linearly polarized light. 3. The optical scanning device according to claim 1, wherein the light source is a surface emitting laser.

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