This application claims priority under 35 USC 119 from Japanese Patent Application No. 2004-4096, the disclosure of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to: an optical fiber, which guides a light beam; a method of manufacturing the optical fiber; and an image forming apparatus, which forms an image by irradiating a recording medium with the light beam guided by the optical fiber.
2. Description of the Related Art
An image forming apparatus is know in which: an optical fiber is connected to a light source, such as a semiconductor laser; a light beam output from the optical fiber is focused with a focusing lens (focusing optical system); and a recording medium, arranged at the focusing position of the light beam, is irradiated to form (record) an image on a recording medium.
An optical fiber having a core/cladding configuration is usually used in an image forming apparatus. The core/cladding configuration includes a core and a cladding, the core having a circular sectional shape, in a direction orthogonal to an optical axis direction of the light beam, and the core is coated with the cladding. A profile of the output light of the optical fiber has a circular shape, and a shape of the light beam spot scanned on a recording medium also has a circular shape. However, it is considered that a rectangular shape is more desirable than a circular shape for the shape of the light beam spot. This is because light intensity is substantially constant in a width direction (sub-scanning direction), variations in line width and dot density (coverage) caused by increases or decreases in light quantity are small, and a good image can be reliably obtained.
A rectangular beam spot can be obtained by using as the light source a semiconductor laser whose output light profile is rectangular in shape, e.g., a broad area type semiconductor laser. However, since a broad area type semiconductor laser has a high output, the amount of heat generated is large and, particularly when arrayed, there is a problem that cooling becomes difficult. Further, when a broad area type semiconductor laser is incorporated as a light beam output source into an exposure head of an image forming apparatus, there is the problem that replacement of a new semiconductor laser becomes troublesome when a semiconductor laser stops functioning correctly.
In order to obtain a rectangular beam spot without using a broad area type semiconductor laser, an optical fiber in which the core is formed rectangular in shape has been proposed (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2000-310746). A beam spot, based on a laser beam output from such an optical fiber, is formed in a rectangular shape on the recording medium. This means that the above-described effect can be obtained, in which the line width is not changed even if the light quantity is varied, and the like. Further, if the recording medium is a heat mode type photosensitive material, when the beam size (width) in a main scanning direction is made smaller than that in the sub-scanning direction, sensitivity can be greatly improved.
A heat mode type photosensitive material is one in which recording is performed by causing a physical change or a chemical change by photo-thermal conversion after the exposure. When exposure speed becomes slow, a heat mode type photosensitive material has low-intensity reciprocity law failure characteristics in which a greater exposure energy is required due to the heat generated being dissipated (characteristics where by the photosensitivity decreases as the intensity decreases and the exposure time increases). Therefore, when an image is formed (recorded) using a heat mode type photosensitive material, from the viewpoint of decreasing exposure energy using high intensity and short-time exposure, it is desirable that the beam size in the main scanning direction is smaller than that in the sub-scanning direction.
However, in order to manufacture an optical fiber whose core shape is rectangular, as shown in
Further, it is difficult to integrally form the preform 60, including the core 62 whose sectional shape is rectangular, and the cladding 64 which covers the core 62. In reality the cladding 64 is separated into plural parts to cover the rectangular core 62. Therefore, air can contaminate the joint surfaces of the parts of the cladding 64 and, in a finished optical fiber 70, there is the problem that scattering is generated because of air bubbles formed at the interface between the core 62 and the cladding 64. That is, since the interface between the core 62 and the cladding 64 has poor smoothness, there is a problem that propagation loss in the light beam is large and the utilization efficiency of the light beam is decreased greatly.
Therefore, in the optical fiber 70, a process of optically polishing the interface between the core 62 and the cladding 64 is required, in order to solve the above-described problems, and this results in the optical fiber 70 being remarkably expensive. Thus, an optical fiber with a light beam whose spot shape is rectangular can be obtained, but the optical fiber cannot yet be favorably utilized.
The present invention has been made in view of the above circumstances and provides an optical fiber having a rectangular core, in which propagation loss in the light beam is small, and manufacturing can be performed at low cost. It also provides methods of manufacturing the optical fiber, and an image forming apparatus including the optical fiber.
An optical fiber of a first aspect of the invention includes: a core, whose sectional shape in a direction orthogonal to an optical axis direction of a light beam is a rectangle; and a cladding, with which the core is covered, wherein the cladding is structured by multiple hollow capillaries integrated with the core by melt-fusing.
In the optical fiber of the first aspect, since the cladding is formed by multiple hollow capillaries integrated with the core by melt-fusing, the problem of bubbles existing in the interface between the cladding and the core does not occur. Therefore, there is no possibility that scattering is generated. As a result, a process of optically polishing the interface between the cladding and the core is not required, so the optical fiber can be manufactured at low cost.
In the cladding integrated with the core, multiple holes aligned in the optical axis direction of the light beam are formed to have a periodic structure and decrease an effective refractive index, so that the light beam can be guided while trapped in the core. Therefore, propagation loss of the light beam (quantity of the light loss) is decreased, and a decrease in utilization efficiency of the light beam is prevented.
An optical fiber manufacturing method of a second aspect of the invention includes: forming a base material of a core whose section is rectangular; inserting the base material of the core into a cylindrical tube and arranging coaxially the base material of the core and the tube; producing a preform by filling the tube into which the base material of the core has been inserted with multiple hollow capillaries; performing wire drawing while the preform is melt-fused; and covering the preform with a cover layer to produce an optical fiber having a rectangular core.
According to the second aspect, the optical fiber is produced by arranging coaxially the base material of the core whose section is rectangular and a cylindrical tube by inserting core into the tube, forming a preform by filling the tube with multiple hollow capillaries, and performing wire drawing while the preform is melt-fused. Therefore, the melt-fused multiple hollow capillaries become the cladding, with which the core is integrally covered, and the rectangular shape of the core is maintained (the second aspect provides excellent reproducibility of the rectangular core).
Since the problem of bubbles existing at the interface between the cladding and the core does not occur, there is no possibility that scattering is generated. As a result, a process of optically polishing the interface between the cladding and the core is not required, so the optical fiber can be manufactured at low cost. In the cladding integrated with the core, multiple holes aligned in the optical axis direction of the light beam are formed to have a periodic structure (at even spacings) and substantially decrease the effective refractive index, so that the light beam can be guided while trapped in the core. Therefore, the propagation loss of the light beam (quantity of light loss) is decreased, and a decrease in utilization efficiency of the light beam does not occur.
An optical fiber manufacturing method of a third aspect of the invention includes: filling a cylindrical tube with multiple hollow capillaries; extracting hollow capillaries, from the multiple hollow capillaries with which the tube is filled, in a rectangular area having a predetermined size in a portion substantial central to the multiple hollow capillaries, and inserting rods having the same diameter as the extracted hollow capillaries, replacing the extracted hollow capillaries to produce a preform; performing wire drawing while the preform is melt-fused; and covering the preform with a cover layer to produce an optical fiber having a rectangular core.
According to the third aspect, the optical fiber is produced by filling a cylindrical tube with multiple hollow capillaries, extracting hollow capillaries, from the multiple hollow capillaries with which the tube is filled, in a rectangular area having a predetermined size in a portion substantial central to the multiple hollow capillaries, and inserting rods having the same diameter as the extracted hollow capillary, replacing the extracted hollow capillaries to produce a preform, and performing wire drawing while the preform is melt-fused. Therefore, the rectangular core is formed by the multiple melt-fused rods, and the multiple melt-fused hollow capillaries become the cladding with which the core is integrally covered, so that the third aspect provides excellent reproducibility of the rectangular core.
Since the problem of bubbles existing in the interface between the cladding and the core does not occur, there is no possibility that scattering is generated. As a result, a process of optically polishing the interface between the cladding and the core is not required, so the optical fiber can be manufactured at low cost. In the cladding integrated with the core, the multiple holes aligned in the optical axis direction of the light beam are formed to have a periodic structure (substantially evenly spaced) and decrease the effective refractive index, so that the light beam can be guided while trapped in the core. Therefore, the propagation loss of the light beam (quantity of light loss) is decreased, and a decrease in utilization efficiency of the light beam is does not occur.
An image forming apparatus of a fourth aspect of the invention includes: a light source which emits a light beam; an optical fiber of the first aspect which is optically connected to the light source; and an optical focusing system which focuses the light beam output from the optical fiber onto a recording medium.
According to the fourth aspect, the recording medium is exposed by focusing the light beam output from the optical fiber of the first aspect with the optical focusing system. Since a profile of the light beam output from the optical fiber has a rectangular shape, a shape of the light beam spot focused on the recording medium also becomes rectangular. Therefore, variations in line width and dot density (coverage) caused by increases or decreases in light quantity are small, and good images can be obtained reliably.
As described in the above, according to the invention are provided: an optical fiber having a rectangular core in which the propagation loss of the light beam is small and which can be manufacturing at low cost; methods of manufacturing the optical fiber; and an image forming apparatus including the optical fiber.
Referring to the accompanying drawings, an embodiment of the present invention will be described in detail below.
An exposure head 14 is arranged near the drum 12 and opposing the outer peripheral surface of the drum 12. The exposure head 14 is supported while being movable in the direction of a rotating axis of the drum 12. The opposite direction (arrow M direction) to the rotating direction (arrow X direction) of the drum 12 is the main scanning direction, and the direction of the rotating axis of the drum 12 (arrow S direction) is the sub-scanning direction, in which the exposure head 14 is moved. The two-dimensional image is formed (recorded) in the heat mode type photosensitive material 50 by sequentially scanning a laser beam output from the exposure head 14 in the main scanning direction and the sub-scanning direction.
An output portion 30B side of an optical fiber 30 is connected to the exposure head 14 through a holding member 18. An incident portion 30A side of the optical fiber 30 is connected to an optical unit 20 which includes a semiconductor laser 24 as the light source. A focusing lens 16 acting as an optical focusing system is arranged in the exposure head 14. The focusing lens 16 focuses the laser beam output from the optical fiber 30 onto an exposure surface (recording surface) of the heat mode type photosensitive material 50.
As shown in
As shown in
In the optical fiber 30, when the preform 31 is heated and wire-drawn, the hollow capillaries 34 are melt-fused, and the core 32, the hollow capillaries 34, and the tube 36 are integrated together as shown in
Between the core 32 and the tube 36, multiple holes 40 aligned in the optical axis direction of the laser beam are formed to have a periodic structure by the hollow capillaries 34. Particularly, the holes 40 near the core 32 are regularly arrayed by the forming of a hexagonal close packed configuration. Therefore, an effective refractive index is decreased by the multiple holes 40, so that the refractive index of the tube 36 becomes lower than that of the core 32, and the light (laser beam) can be guided while trapped in the core 32 (total reflection). Accordingly, the propagation loss of the light (quantity of the light loss) is decreased and a decrease in utilization efficiency of the laser beam is prevented.
For example, the core 32 and the hollow capillaries 34 can be made of anhydrous quartz, having a high refractive index, and the tube 36 can be made of quartz, glass or the like, having a refractive index lower than that of the anhydrous quartz. The cover layer 38 is made, for example, of ultraviolet curing resin having a refractive index lower than that of the tube 36. For example, in the optical fiber 30, the size (Wm by Ws) of the core 32 can be about 30 μm by 60 μm, an outer diameter of the first cladding which is formed by melt-fusing the hollow capillaries 34 can be about 90 μm, and the outer diameter of the second cladding (tube 36) can be about 125 μm. Although a small number of hollow capillaries 34 in contact with the inner surface of the tube 36 are drawn in
It is also possible to manufacture the optical fiber 30 in the following way. As shown in
After forming the preform 31, which includes the quartz rods 33, the hollow capillaries 34, and the tube 36, a similar wire drawing process is performed at a predetermined tension while the preform 31 is heated to a predetermined temperature. Then the cover layer 38 is coated onto the outer periphery. That is, the optical fiber 30 is manufactured by integrating the tube 36, the quartz rods 33 and the hollow capillaries 34 by heating and melt-fusing, and the cover layer 38 is coated onto the outer periphery surface of the tube 36.
Accordingly in this method of manufacturing the optical fiber 30, since the rectangular core 32 having a predetermined size is formed by melt-fusing multiple quartz rods 33, when compared with the optical fiber 30 shown in
In the configurations in which the refractive indexes and the sectional shapes of the core 32 and the first cladding are formed in the above-described ways, when the intensity distribution is determined by integrating the light energy of the laser beam with respect to the main scanning direction orthogonal to the optical axis direction, the laser beam output from the optical fiber 30 exhibits a single mode intensity distribution (shape having a narrow single peak with respect to the main scanning direction) as shown in
On the other hand, the light intensity distribution in the sub-scanning direction does not have a Gaussian distribution, and the light intensity distribution in the sub-scanning direction becomes substantially a broad rectangular shape. Therefore, even if the intensity of the laser beam is slightly changed, i.e., even if the light quantity is slightly changed, an extent of image formation (transverse width) determined by a threshold value for image formation of the heat mode type photosensitive material 50 is not changed, and the writing line width of the image with respect to the sub-scanning direction is held constant. When an area of gradation is constructed by using dots, the line width is held constant and so the dot density (coverage) is also held constant.
The operation of the optical fiber 30 and the image forming apparatus 10 having the above described configurations will be described below. In the optical unit 20, the laser beam output from the semiconductor laser 24, modulated according to image information, is condensed onto the incident portion 30A of the optical fiber 30, which acts as the optical transmission path, by the cylindrical lens 26, and the laser beam is transmitted through the optical fiber 30. Since the optical fiber 30 has the above-described configuration, the optical fiber 30 can sufficiently transmit a light quantity necessary for the formation (recording) of an image with little propagation loss.
In the exposure head 14, the laser beam transmitted through the optical fiber 30 is output from the output portion 30B of the optical fiber 30, held by the holding member 18. That is, the laser beam is output from the rectangular core 32, which is elongated in the sub-scanning direction. The laser beam output from the core 32 is focused with the focusing lens 16, the exposure surface of the heat mode type photosensitive material 50 is irradiated with the laser beam having a beam spot in which the beam size (width) in the main scanning direction is narrower than that in the sub-scanning direction (the beam spot being elongated, and stretched out, in the sub-scanning direction), and an image based on the image information is formed (recorded) on the exposure surface.
The laser beam output from the core 32 of the optical fiber 30 has a profile in which the rectangular spot is elongated extending out in the sub-scanning direction, so that the spot size in the main scanning direction is easily focused. The light intensity of the laser beam is substantially constant in the transverse width direction. Even if the light quantity, the sensitivity of the heat mode type photosensitive material 50 or the like is varied, neither the line width nor the dot density varies substantially. Therefore, a good image can reliably be formed (recorded) on the exposure surface of the heat mode type photosensitive material 50.
The heat mode type photosensitive material 50 is one in which recording is performed by causing a physical or chemical change by photo-thermal conversion after exposure. When exposure speed becomes slow, the heat mode type photosensitive material 50 has low-intensity reciprocity law failure characteristics in which a greater exposure energy is required due to the heat generated being dissipated. When the image is formed (recorded) on the heat mode type photosensitive material 50 by using the optical fiber 30, having the core 32 whose sectional shape is rectangular elongated in the sub-scanning direction, the main scanning speed during the process (rotating speed of the drum 12) becomes substantially constant, so that short-time, high-intensity exposure can be realized saving exposure energy. That is, for a given exposure energy, the exposure speed can be increased (high-speed exposure can be performed).
In this case, a high-output broad area type semiconductor laser 24 is used as the light source. However, the light source of the invention is not limited to this, and it is also possible to use a general semiconductor laser having a circular spot shape. However, when a broad area type semiconductor laser 24 is used, the profile of the output light beam becomes rectangular, so that incident efficiency onto the optical fiber 30 having the rectangular core 32 is improved. It is also possible that exposure is performed with multiple laser beams by arranging multiple semiconductor lasers 24 (optical units 20). In this case, the processing speed of the image forming apparatus 10 can be increased.
It is possible that the optical fiber 30 is formed in such a way that the length of the optical fiber can be lengthened or adjusted. If this is done, because the semiconductor laser 24 (optical unit 20) and the exposure head 14 can be arranged so as to be separated from each other, the degree of freedom of layout is enhanced such that configurations in which the cooling efficiency is improved, in both the semiconductor laser 24 and the exposure head 14, can be easily realized. In the drawings, one optical fiber 30 is used. However, it is also possible that two or more optical fibers 30 are used by connecting optical fibers 30 with optical joints such as fitting connectors. When an optical joint is used, it is easy to separate the optical unit 20 (semiconductor laser 24) from the exposure head 14, so that semiconductor lasers 24 can be easily replaced if they malfunction.
Although a heat mode type photosensitive material 50 using the evaporation film ablation method is used as the recording medium in the embodiment, the recording medium has no particular limitation as long as scanning exposure can be performed with a laser beam onto the recording medium. Diazo photosensitive materials, silver halide photosensitive materials, photopolymerization photosensitive materials, and the like can be used in the invention. Further, photosensitive materials which exhibit low-intensity reciprocity law failure characteristics in a time period corresponding to a pixel exposure time can also be used with no particular limitation.
In the image forming apparatus of the invention, it is preferable that the width of the core of the optical fiber in the main scanning direction is shorter than that in the sub-scanning direction.
In the optical fiber, since the width of the core of the optical fiber in the main scanning direction is shorter than that in the sub-scanning direction, the beam size in the main scanning direction is reduced to smaller than that in the sub-scanning direction. Therefore, high-intensity short-time exposure can be performed, and the exposure can be performed particularly effectively when the recording medium is a heat mode type photosensitive material.
That is, when the exposure speed becomes slow, a heat mode type photosensitive material has low-intensity reciprocity law failure characteristics in which heat generated is dissipated, requiring a larger amount of exposure energy (characteristics such that the sensitivity of a photosensitive material decreases as the intensity decreases and the exposure time becomes longer). Therefore, when the beam size in the main scanning direction is formed smaller than that in the sub-scanning direction, the scanning time can be shortened (instantaneous exposure can be performed) and the exposure energy can be decreased. This means that a good image can be obtained with a heat mode type photosensitive material.
In the image forming apparatus, it is preferable that the light source is a broad area type semiconductor laser.
A high-output light beam can be obtained by forming the light source with such a broad area type semiconductor laser. Since the profile of the output light becomes rectangular, the incident efficiency onto an optical fiber having a rectangular core is improved.
Number | Date | Country | Kind |
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2004-4096 | Jan 2004 | JP | national |