JP2006234956A - Optical scanner - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical scanner capable of controlling a beam spot diameter inexpernsively without sacrificing light quantity. <P>SOLUTION: In order to control a polarizing direction with respect to a part of beams emitted by a laser array 14, a 1/2 wavelength plate 12 is entered between a collimator lens 16 and a first cylinder lens 18 as a controlling element of the polarizing direction. By entering the 1/2 wavelength plate 12 on the position where a beam diameter after the collimator lens 16 is magnified, the polarizing direction of the beams is partially changed. That is, the part of beams emitted by the laser array 14 passes through the 1/2 wavelength plate 12 and, thereby, the polarizing direction is partially changed (Bpol in the figure) and the part of beams are image-formed on the polygon mirror 20 together with a beam (Borg in the figure) which does not pass through the 1/2 wavelength plate 12. That is, like p2 on the polygon mirror 20, a beam spot having a profile made by combining the profile (Ppol) of the beam of which the polarizing direction is partially changed and the profile (Porg) of the beams which do not pass through the 1/2 wavelength plate 12 is obtained. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical scanning device, and more particularly to an optical scanning device that forms an image on a photosensitive surface with laser light.

  2. Description of the Related Art Conventionally, in an optical scanning apparatus that forms a beam spot on a scanned surface of a photosensitive member using a laser array as a light source and performs scanning exposure, a technique for determining the interval and the beam diameter of the scanned beam on the scanned surface is required. Yes.

For example, the interval (r1) in the sub-scanning direction of the laser array cannot be arbitrarily reduced due to limitations of the laser structure and the manufacturing process. On the other hand, the divergence angle θ1 (half angle of 1 / e ² intensity) of the beam emitted from the laser array is determined by the laser structure.

When this laser array is used as a light source and a laser NFP (near field pattern) is projected onto the surface to be scanned through a laser scanning optical system, a beam diameter d3 (1 / e ² diameter) and a spot interval obtained on the surface to be scanned are obtained. r3 is given by the following equations, where m is the projection magnification of the optical system and λ is the laser wavelength.

d3 = m · d1 = m · 2λ / (π · θ1)
r3 = m · r1
That is, the ratio of d3 and r3 is constant for any optical system magnification m and cannot be arbitrarily set. This means that an arbitrary beam diameter cannot be set for an arbitrary scanning line density.

  In order to solve this problem, an optical device having an arrangement in which an aperture is inserted in the beam optical path after laser emission to reduce the incident beam diameter to the imaging lens, and the beam diameter d3 on the scanning surface obtained by the imaging lens is enlarged. A system has been proposed (see, for example, Patent Document 1).

  In the above example, as shown in FIGS. 9 and 10, an aperture is placed in the beam optical path after laser emission to reduce the incident beam diameter to the imaging lens, and the beam on the scanned surface obtained by the imaging lens. The diameter d3 can be enlarged.

  For example, the diameter d2 of the aperture 102 in FIG. 9 has the same size as the diameter of the portion having a half intensity of the highest intensity portion (beam center portion) of the laser beam having the outer diameter D collimated by the collimator lens 106. Diameter (= 1/2 intensity diameter. This 1/2 intensity diameter is D / 1.70 = 0.59D in the case of a beam having a Gaussian intensity distribution), as shown in FIG. The value of d3 that is the diameter of the beam spot that becomes the beam profile p103 on the image plane 118 is d3 = 1.94.

  Similarly, if the inner diameter of the aperture 102 is 1 / e2 intensity diameter (that is, 1 / e2 of the highest intensity portion of the laser beam = 0.135 is the same as the diameter of the portion having the intensity of 0.135), the value of d3 Is d3 = 1.28.

  When the aperture 102 has an inner diameter that is much larger than the diameter D of the collimated laser beam or when the aperture 102 is not used, the value of d3 is d3 = 1 and the beam spot diameter is the smallest.

  That is, d3 can be changed by making the diameter of the aperture variable. The relationship between the aperture diameter D2 (= the laser beam diameter D) and the beam diameter d3 on the scanning plane is approximately d3 = 60 μm when D2 = 0.9 mm, and d3 = 20 μm when D2 = 2.7 mm.

  However, in order to obtain a large beam diameter d3, it is necessary to reduce the aperture diameter D2, and as a result, the amount of transmitted light of the laser is extremely reduced. In this example, when the aperture diameter D2 = 0.9 mm, the light transmittance at the aperture portion is about 10%, and the light amount loss is extremely large.

  As described above, when the amount of light loss at the aperture portion increases, a higher laser output is required to ensure the same exposure amount on the surface to be scanned. This results in a significant reduction in laser life. For example, the laser of this example has an average life of 1000 hours when the output is 1 mW, whereas the average life is about 140 hours when the output is doubled to 2 mW, which is about one-seventh, which is a serious problem in terms of device performance and cost. .

  Alternatively, as another example, there has been proposed a method of obtaining an arbitrary beam diameter d3 by changing only the beam divergence angle θ1 by arranging a lens for each laser light source (see, for example, Patent Documents 2 and 3). ).

However, in the above example, a lens array having the same interval as the laser interval is required, and since extremely fine processing accuracy is required, it is difficult to manufacture and the cost increases. In addition, the alignment between the laser and the lens array is difficult because the distance is extremely fine.
JP-A-5-53068 JP-A-5-66354 JP-A-5-297326

  In consideration of the above-described facts, the present invention has an object to provide an optical scanning device that can control the beam spot diameter at a low cost without sacrificing the amount of light.

  The optical scanning device according to claim 1, comprising: a laser light source that emits a plurality of beams; and an optical system that forms an image of the plurality of beams on an exposure surface. A polarizing means for polarizing a part of the beam is provided in the optical path.

  In the invention with the above configuration, a sufficiently large beam spot diameter can be obtained while increasing the aperture diameter of the aperture and maintaining the amount of light.

  The optical scanning device according to claim 2 is characterized in that the polarizing means is provided at a position not optically conjugate with the exposure surface.

  In the invention with the above configuration, the beam spot diameter can be controlled by providing the polarizing means on the surface that is not optically conjugate with the surface to be scanned.

  According to a third aspect of the present invention, the polarizing means is a transmissive half-wave plate.

  In the invention having the above-described configuration, since it is not necessary to change the arrangement of each element of the optical system by using the transmission type half-wave plate, it is not necessary to add a mirror or the like, and a simple configuration can be achieved.

  The optical scanning device according to claim 4 is characterized in that the beam is incident on the polarization means as substantially parallel collimated light.

  In the invention with the above configuration, the laser beam incident on the polarization means is made to be substantially parallel collimated light, whereby the beam position on the surface to be scanned can be accurately defined and the spot diameter can be set strictly.

  The optical scanning device according to claim 5 is characterized in that the polarization means is provided at a position where the principal rays of the plurality of beams substantially intersect in the optical path.

  In the invention with the above configuration, it is advantageous that the same beam diameter can be obtained on the scanning plane by rotating the polarization direction of the same portion of the beam profile at a position where the principal rays substantially intersect with a plurality of beams.

  The optical scanning device according to a sixth aspect is characterized in that an aperture for limiting a beam diameter is provided in the optical path.

  In the invention with the above configuration, the beam spot diameter can be easily controlled by making the aperture diameter variable as a parameter that can be arbitrarily set.

  The optical scanning device according to claim 7 is characterized in that the aperture is provided in the vicinity of the polarizing means.

  In the invention with the above configuration, the influence of polarization control and diffraction given to each beam can be made the same by providing the aperture in the vicinity of the polarization means.

  The optical scanning device according to claim 8 is characterized in that the polarization means makes the ratio of polarization in the beam variable.

  In the invention with the above configuration, the beam diameter on the surface to be scanned can be controlled by making the polarization rotation region variable.

  Since the present invention has the above-described configuration, an optical scanning apparatus that can control the beam spot diameter at a low cost without sacrificing the amount of light can be obtained.

  1 and 2 show the configuration of the optical scanning device according to the first embodiment of the present invention.

  As shown in FIG. 1, the optical scanning apparatus 10 converts a laser beam emitted from a laser array 14 having a beam profile such as p1 into parallel light by a collimator lens 16, and a part of the laser beam has a transmittance of about 90%. After passing through the transmissive half-wave plate 12 and partially changing the polarization direction of the beam, an image is formed on the polygon mirror 20 by the first cylinder lens 18.

  The polygon mirror 20 reflects the incident beam while rotating at an equiangular speed toward the fθ lens 22 and uses it as scanning light in the main scanning direction. The fθ lens 22 shapes the incident beam so as to perform constant velocity scanning, and a beam spot is formed on the surface to be scanned which is the surface of the photosensitive member 28 by the second cylinder lens or cylinder mirror 24 and the third cylinder lens or cylinder mirror 26. As an image.

  As shown in FIG. 2, in order to control the polarization direction of a part of the beams emitted from the laser array 14, a half-wave plate 12 is used as a polarization direction control element, a collimator lens 16 and a first cylinder lens 18. In between. By inserting the half-wave plate 12 at a position where the beam diameter after the collimator lens 16 is enlarged, the polarization direction of the beam is partially changed.

  That is, the polarization direction is partially changed by passing a part of the beam emitted from the laser array 14 through the half-wave plate 12 (Bpol in the figure) and passes through the half-wave plate 12. An image is formed on the polygon mirror 20 together with the missing beam (Borg in the figure). That is, on the polygon mirror 20, a profile obtained by synthesizing the profile (Ppol) of the beam whose polarization direction has been partially changed and the profile (Porg) of the beam that has not passed through the half-wave plate 12, as p 2. It becomes a beam spot.

  At this time, the problem is the Babinet's principle in Fraunhofer diffraction, that is, the sum of the amplitude distributions of diffracted light that can be made at an arbitrary point by two openings complementary to each other (the opening and the shielding part are reversed), It is equivalent to the amplitude distribution when nothing is placed.

  FIG. 3 shows the principle of beam spot synthesis in the optical scanning device according to the present invention.

For example, in this embodiment, what corresponds to the two complementary openings described above can be said to be the presence or absence of the half-wave plate 12 in the optical path. According to Babinet's principle, the wavefront amplitude at the image plane formed through the slit A shown in FIG. 3A is UA, and the image plane formed through the slit B shown in FIG. If the amplitude of the wavefront at UB is UB, and the wavefront due to slit A and slit B can interfere with the image plane, the amplitude distribution U0 by combining UA and UB is
U0 = UA + UB
The beam diameter of the amplitude distribution U0 is equal to the beam diameter of the image when neither slit A nor slit B exists in the optical path. In other words, it is not simply the sum of the two amplitude distributions, but the amplitude distribution when no slit originally exists. That is, even if the aperture is reduced by the slit B having a small opening and the beam diameter is increased, if the beam is reduced by the complementary slit A and interferes with the image plane, the profile before being reduced by the slit is restored.

  In the present embodiment, the polarization direction of one of UA and UB is changed by the ½ reflector 12, and specifically, the polarization directions of UA and UB are made substantially orthogonal to each other, so that UA and UB are mutually crossed on the image plane. No interference occurs.

  Therefore, the amplitude distribution of the image U0 is the sum of the scalars of UA and UB. Therefore, the beam diameter on the image plane does not return to the original profile, and the beam diameter can be enlarged.

  Accordingly, in the present embodiment, when an image is formed on the photosensitive member 28 in FIG. 2, a beam that has passed through the half-wave plate 12 (Bpol: broken line in the figure) and a beam that has not passed through (Borg: one-dot chain line in the figure). Do not interfere with each other, that is, without returning to the original profile, an image is formed as a beam spot having a profile p4 that is the sum of the two. For this reason, p4 can be imaged as a spot having a larger diameter than the original profile.

  FIG. 4 shows polarization means of the optical scanning device according to the first embodiment of the present invention.

  As shown in FIG. 4A, by inserting a half-wave plate 12 into a portion of the beam focused by the aperture 15, the polarization direction of the beam in the portion where the half-wave plate 12 enters is changed to the other beam. It is rotated by approximately 90 ° with respect to the portion (the portion not including the half-wave plate). That is, the polarization direction that was originally in the left-right direction (front / back in the figure) can be changed to the up-down direction in the figure by the half-wave plate 12. The fast axis of the half-wave plate 12 at this time (arrows in (b) and (c)) is installed with an inclination of about 45 ° with respect to the polarization direction of the incident beam.

  That is, as shown in FIGS. 4B and 4C, a one-dimensional half-wave plate (b) or a circular half-wave plate (c) provided on the base plate 17 and covering a part of the optical path, As shown in FIG. 4A, the polarization direction of only the beam that has passed through the half-wave plate 12 is rotated by 90 ° and the other portions are not changed. It will be.

  By the way, factors that determine the beam diameter when the imaging magnification of the optical system is fixed include the laser wavelength λ, the aperture diameter of the aperture 15, the diameter of the half-wave plate 12, and the polarization rotation angle of the beam. Of these, the laser wavelength λ is determined by the constituent material of the laser and cannot be arbitrarily set. The polarization rotation angle of the beam is determined by the direction of the fast axis of the half-wave plate 12 with respect to the linear polarization direction of the incident light, and the beam diameter can be controlled, but the polarization directions of the incident light and the emitted light are not orthogonal. Therefore, calculation of the obtained beam diameter becomes complicated.

  Therefore, it is desirable to control the beam diameter d3 on the surface to be scanned by the aperture diameter of the aperture 15 and the diameter of the half-wave plate 12 which are the remaining changeable parameters. In order to change, it is necessary to prepare a plurality of half-wave plates 12 having different diameters, which causes an increase in cost. Therefore, in this embodiment, the diameter of the half-wave plate 12 is fixed, and the beam diameter d3 on the surface to be scanned is adjusted by changing the aperture diameter of the aperture 15.

  FIG. 5 shows polarization means of an optical scanning device according to the second embodiment of the present invention.

  As shown in FIG. 5 (a), by inserting a half-wave plate 12 into a part of the beam focused by the aperture 15, the polarization direction of the beam in the portion where the half-wave plate 12 enters is changed to the other direction of the beam. It is rotated by approximately 90 ° with respect to the portion (the portion not including the half-wave plate). That is, the polarization direction that was originally in the left-right direction (front / back in the figure) can be changed to the up-down direction in the figure by the half-wave plate 12. The fast axis of the half-wave plate 12 at this time (arrows in (b) and (c)) is installed with an inclination of about 45 ° with respect to the polarization direction of the incident beam. In the first embodiment, the half-wave plate 12 is provided at the center of the beam. However, it may be provided at the periphery of the beam as in this embodiment.

  That is, as shown in FIGS. 5B and 5C, a one-dimensional half-wave plate (b) or circular half-wavelength provided on the upper and lower ends or the peripheral portion on the base plate 17 and covering a part of the optical path. As shown in FIG. 5A, the polarization direction of the beam that has passed through the half-wave plate 12 is rotated by 90 ° by the plate (c) and is not changed in the other portions. The angle is shifted and substantially orthogonal.

  FIG. 6 shows the relationship between the presence / absence of the polarizing means according to the present invention and the beam diameter.

  FIGS. 6A and 6B show how the beam diameter d3 on the surface to be scanned changes before and after the half-wave plate 12 is inserted.

  As shown in FIG. 6A, the beam diameter is 60 μm before the half-wave plate 12 is inserted, and the beam diameter after the half-wave plate 12 is inserted without changing the diameter of the aperture 15 is as shown in FIG. It is enlarged to 180 μm as shown in (b).

  Thereafter, as shown in FIG. 6C, the aperture of the aperture 15 is expanded to 12 mm, which is three times larger, the amount of light transmitted through the aperture 15 is tripled, and the beam diameter on the surface to be scanned is increased even if the beam convergence angle is increased. It was not 1/3 of 20 μm, and almost the same beam diameter of 65 μm could be obtained. Since the transmittance of the half-wave plate 12 itself is about 90%, it is possible to obtain a light amount substantially 2.7 times (= 3 times × 0.9) on the scanned surface without changing the beam diameter. Became possible.

  By the way, it is understood that the same effect as described above can be obtained even if the half-wave plate 12 is installed on the surface position of the polygon mirror 20 or the exit surface of the laser array 14 that is optically conjugate with the surface to be scanned. ing.

  However, since the beam is condensed on the conjugate plane, a very small half-wave plate 12 is required, and the half-wave plate 12 is required to have very high processing accuracy. Problems such as difficulty.

  Since the beam spreads at a position that is not optically conjugate with the surface to be scanned, it is convenient that a desired result can be obtained without requiring processing accuracy and installation accuracy of the half-wave plate 12. Therefore, in this embodiment, the half-wave plate 12 is installed at a position that is not optically conjugate with the surface to be scanned.

  Further, by using a transmissive half-wave plate for the half-wave plate 12, the incident light travels straight. For this reason, it is not necessary to change the position of the optical element after the half-wave plate 12, and it is convenient to suppress an increase in cost because a design change and an increase in the number of parts can be minimized. On the other hand, when a reflective polarizing plate is used, another optical element such as a mirror is required to return the beam to the same optical path, and the optical system becomes complicated.

Further, the equiphase wavefront of the light collimated by the collimator lens 16 is substantially flat,
The wavefront is incident on the half-wave plate 12 at a right angle. Therefore, since the polarization direction of the beam can be accurately set with respect to the fast axis of the half-wave plate 12, the polarization direction of the beam transmitted through the half-wave plate 12 can be accurately rotated by a desired angle. .

  FIG. 7 shows an optical path of the optical scanning device according to the first embodiment of the present invention.

  As shown in FIG. 7, when the polarization direction of the same part of the beam profile is rotated with respect to a plurality of beams, the same beam diameter can be obtained conveniently on the surface to be scanned.

  That is, as shown in FIG. 7, in this embodiment, each beam emitted from the laser array 14 is set in parallel with the collimator optical axis, so that the principal rays of the beams intersect at the image side focal point of the collimator. Therefore, by installing the half-wave plate 12 in the vicinity of the image-side focal point of the collimator (= in the vicinity of the aperture 15 in the figure), the same polarization control can be performed on each beam.

  For the same reason, in order to obtain the same beam diameter on the scanned surface for a plurality of beams, the contribution of the aperture 15 to the plurality of beams also needs to be the same. In this embodiment, since each beam emitted from the laser array 14 is set in parallel with the collimator optical axis, the principal rays of the beams intersect at the image-side focal point of the collimator. Therefore, by installing the aperture 15 and the half-wave plate 12 in the vicinity of the image-side focal point of the collimator, the influence of polarization control and diffraction given to each beam becomes the same.

  FIG. 8 shows polarization means of an optical scanning device according to the third embodiment of the present invention.

  As shown in FIG. 8 (a), by inserting a half-wave plate 12 into a part of the beam focused by the aperture 15, the polarization direction of the beam in the portion where the half-wave plate 12 enters is changed to the other direction of the beam. It is rotated by approximately 90 ° with respect to the portion (the portion not including the half-wave plate). That is, the polarization direction that was originally in the left-right direction (front / back in the figure) can be changed to the up-down direction in the figure by the half-wave plate 12. The fast axis of the half-wave plate 12 at this time (arrows in (b) and (c)) is installed with an inclination of about 45 ° with respect to the polarization direction of the incident beam. The above points are the same as those in the first and second embodiments.

  In the present embodiment, the beam diameter d3 on the surface to be scanned can be controlled by making the area of the region where the polarization is rotated by the half-wave plate 12 variable.

  That is, as shown in FIGS. 8B and 8C, the half-wave plate 12 is inserted from the outside of the beam, and the amount of insertion is adjusted by the fine movement device 19, so that the region where the polarization is rotated can be adjusted. Yes. By the fine movement device 19 to which the half-wave plate 12 is attached, the half-wave plate is slid in the vertical direction in the figure and fixed at an arbitrary position, so that the region where the polarization is rotated can be made variable.

  In the present invention, the beam diameter on the scanned surface is not reduced to 1/3 and the substantially same beam diameter can be obtained even if the aperture opening diameter is tripled and the amount of transmitted light is tripled. I was able to. Since the transmittance of the half-wave plate itself is 90%, it is possible to obtain a light amount of 2.70 times (= 3 times × 0.9) on the surface to be scanned.

  In other words, while maintaining the transmission efficiency of the optical system high, it is possible to freely set the ratio between the beam interval and the beam diameter on the surface to be scanned, and the beam caused by the shallow depth of focus generated when the beam diameter is set small. The defocus problem can be avoided, and high-quality images can be obtained stably.

  Furthermore, by adjusting either the aperture width of the half-wave plate 12 or the aperture width of the aperture 15, the beam diameter on the surface to be scanned can be freely adjusted, and the adjustment man-hours of the scanning optical system can be reduced. It became.

  In this embodiment, it is not necessary to install a lens array corresponding to each light source of the laser array 14, and the adjustment man-hours can be greatly simplified. Moreover, since expensive parts such as a precise lens array are not necessary, the part cost can be reduced.

1 is a perspective view showing an optical scanning device according to a first embodiment of the present invention. It is an optical path figure of the optical scanning device concerning the 1st form of the present invention. It is a figure which shows the principle of the beam synthesis | combination of the optical scanning device based on this invention. It is a figure which shows the polarizing plate of the optical scanning device which concerns on the 1st form of this invention. It is a figure which shows the polarizing plate of the optical scanning device which concerns on the 2nd form of this invention. It is a figure which shows the effect of the polarizing plate which concerns on the 1st form of this invention. It is an optical path figure of the optical scanning device concerning the 1st form of the present invention. It is a figure which shows the polarizing plate of the optical scanning device which concerns on the 3rd form of this invention. It is an optical path figure of the conventional optical scanning device. It is a figure which shows the relationship between the aperture_diaphragm | restriction and beam diameter in the conventional optical scanning device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical scanning device 12 1/2 wavelength plate 14 Laser array 15 Aperture 16 Collimator lens 18 1st cylinder lens 19 Fine movement device 20 Polygon mirror 22 f (theta) lens 24 2nd cylinder lens or cylinder mirror 26 3rd cylinder lens or cylinder mirror 28 Photosensitive body

Claims (8)

A laser light source that emits a plurality of beams;
An optical system for imaging the plurality of beams on an exposure surface;
An optical scanning device comprising:
An optical scanning device characterized in that polarizing means for polarizing a part of the beam is provided in the optical path of the optical system.
The optical scanning apparatus according to claim 1, wherein the polarization unit is provided at a position that is not optically conjugate with the exposure surface.
3. The optical scanning device according to claim 1, wherein the polarizing means is a transmissive half-wave plate.
4. The optical scanning device according to claim 1, wherein the beam is incident on the polarization unit as substantially parallel collimated light.
5. The optical scanning device according to claim 1, wherein the polarization unit is provided at a position where the principal rays of the plurality of beams substantially intersect in the optical path.
6. The optical scanning device according to claim 1, wherein an aperture for limiting a beam diameter is provided in the optical path.
The optical scanning apparatus according to claim 6, wherein the aperture is provided in the vicinity of the polarization unit.
The optical scanning device according to claim 1, wherein the polarization unit is configured to change a ratio of polarization in the beam.

Images