JP2007058100A - Optical element, light source unit, optical scanner, and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element, a light source unit, a optical scanner, and an image forming apparatus, in which a plurality of functions are integrated, and the reduction of the number of components, downsizing, and cost reduction are attained. <P>SOLUTION: The light source unit 1 is provided with a surface emitting laser 10 as a light source and a coupling lens 12. The light beam emitted from the surface emitting laser 10 shows a random polarization (TE polarization and TM polarization are included at random) and the light beam passing through a coupling lens 12 is linearly polarized light(TM polarization) having a predetermined direction. The coupling lens 12 has also a function as a polarizer and passes only the light linearly polarized in a certain direction. This is realized by generating structural double refraction by providing a fine ruggedness structure 2 having a periodic character on the surface 12a of the curved optical surface on the side of the surface emitting laser 10 of the coupling lens 12. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to an optical element having optical anisotropy, a light source unit having the optical element, an optical scanning device having the light source unit, and an image forming apparatus such as a copying machine, a printer, a facsimile, and a plotter having the optical scanning device. .
The present invention can be applied to an optical device using a surface emitting laser.

In recent years, it has been desired to improve the printing speed and the writing density of image forming apparatuses. Therefore, as one of means for achieving high-speed and high-density optical scanning in the optical scanning device constituting the image forming apparatus, there is a method of increasing the deflection speed of the optical deflector, that is, increasing the rotational speed of the polygon mirror.
However, there are problems such as noise and heat associated with high-speed rotation, and there is a limit to improving the rotation speed. On the other hand, as another means for achieving high-speed and high-density optical scanning, there is a method of scanning a plurality of light beams at a time and simultaneously scanning a plurality of lines.
As a multi-beam light source unit capable of scanning a plurality of light beams, one multi-beam light source (laser array light source having a plurality of light emitting points in one package) that generates a plurality of light beams is used. This can be realized by replacing the conventional optical scanning device using one light source.
On the other hand, many methods for achieving a multi-beam light source unit using a plurality of conventional single beam light sources (laser light sources having one light emitting point in one package) have been proposed.

A semiconductor laser is generally used as a light source, and an edge-emitting laser has been the mainstream in the past. In recent years, however, surface emitting lasers called VCSELs have appeared. Since surface emitting lasers are easier to array than edge emitting lasers, the edge emitting lasers have a limit of about 4 to 8 beams, whereas surface emitting lasers have 16 to 32 beams. In addition, further arraying is possible.
Therefore, it is expected as a light source for improving the printing speed and writing density of the image forming apparatus.

Optical element having a structure (SWS = Subwavelength Structure; subwavelength structure) in which two media having different refractive indexes (for example, one is air and the other isotropic medium) has a periodic structure smaller than the wavelength of light. Then, optical anisotropy called structural birefringence appears (see Non-Patent Document 1).
Conventionally, in order to use birefringence, it is necessary to use a birefringent crystal such as quartz or calcite, and it is difficult to change the birefringence because it is a property specific to a substance.
On the other hand, birefringence can be changed depending on the shape of a general medium without using a special crystal in structural birefringence, and can be controlled relatively easily. Thereby, a polarizing beam splitter or the like that does not use a birefringent crystal can be realized.

Since it has the above-mentioned structural birefringence, the phase difference can be changed by controlling the thickness of the TE polarized light and TM polarized light, and the function of λ / 2 plate and λ / 4 plate is generated. Can be made.
If the refractive indices for TE polarized light and TM polarized light are n (TE) and n (TM), the wavelength of light to be used is λ, and the thickness of the structure is d, the generated phase difference φ is
φ = 2π {n (TE) −n (TM)} d / λ
It can be expressed as
Further, by selecting an appropriate thickness d, it is possible to function as a polarizer that transmits only one of the polarized light.
Optical anisotropy also appears in optical elements with a so-called resonance region structure with a period of about the wavelength (Λ to several λ) as well as the sub-wavelength region (Λ <λ), and polarization-dependent diffraction characteristics An optical element having the following has been proposed.

JP 2003-215485 A JP 2004-155083 A JP 2004-361906 A Japanese Patent No. 2555317 JP-A-8-56049 JP-A-8-330661 JP-A-9-288244 JP-A-10-325933 Japanese Utility Model Publication No. 4-121771 Optics Vol. 27, No. 1 (p.12-17)

However, when a light source unit using a surface emitting laser is applied to a conventional optical scanning device, it has the following problems with respect to an edge emitting laser.
(1) In the edge-emitting laser, the polarization direction of the emitted light beam is linearly polarized light having a direction parallel to the active layer of the edge-emitting laser (see FIG. 25). Since it is basically random polarization, polarization control by some means is required.
In an optical scanning device, a light beam is reflected by an optical deflector and transmitted and reflected by a number of optical elements. At this time, since transmission and reflection at the interface have polarization dependence, the polarization direction is in a plane parallel to the incident surface (P-polarized light), and the polarization direction is in the direction perpendicular to the incident surface. (S-polarized light) has different transmittance and reflectance. Therefore, in the multi-beam light source unit, it is desirable that the polarization directions of the light beams emitted from the respective light sources are equal.

In addition, when a light source unit using a surface-emitting laser is replaced with an optical scanning device manufactured assuming a light source unit using an edge-emitting laser, the polarization directions must be matched equally. In this case, the light quantity characteristic in one line (in one scan) is remarkably deteriorated.
Therefore, in an image output by such an image forming apparatus, a density variation due to the polarization dependency of the transmittance or reflectance of the optical deflector or optical element occurs, and a good image cannot be obtained. Cause problems.
Therefore, as a polarization control means when using a surface emitting laser, as shown in Patent Document 5, a structure for controlling the polarization direction can be provided in the device structure itself, but the element structure is complicated. It becomes difficult to produce.
Further, it is speculated that the control structure varies depending on the structure of the surface emitting laser and the manufacturing method, and it is not necessarily applicable to any surface emitting laser.

Another possibility is to control the polarization of a light beam emitted from a surface emitting laser at a stage where it does not receive polarization dependency of transmission or reflection.
As a method using polarization control means outside the light emitting part of the surface emitting laser, in Patent Document 9, a polarizing beam splitter is arranged on the emission side of the surface emitting laser to transmit only a light beam in a desired polarization direction. .
In these conventional techniques, light quantity control is not performed.

(2) The edge-emitting laser is generally driven by APC (Auto Power Control) control while monitoring the light emitted backward, whereas the surface-emitting laser is rearward because of its structure. Since no outgoing light is generated, it is necessary to control the amount of light by some means.
In an image output by an image forming apparatus using a light source unit to which light amount control cannot be applied, a density variation due to a light output variation of the light source unit occurs, and there is a problem that a good image cannot be obtained.
Therefore, as a light amount control means when using a surface emitting laser, a part of the light beam emitted from the surface emitting laser having a certain predetermined ratio is branched and guided to a photodetector, and the light Depending on the output of the detector, a means for driving the surface emitting laser by controlling the drive current so that the light output of the surface emitting laser becomes a predetermined output in the laser light quantity control device can be considered.

As a method for branching a part of the light beam and guiding it to the photodetector, in Patent Document 6, a part of the light beam is branched using a beam splitter. Further, in Patent Document 1, a part of a light beam is branched using a half mirror as a beam separation optical element.
In these conventional techniques, the polarization control of the surface emitting laser is not performed.
Further, Patent Document 7 discloses an example in which light path control is performed separately using a polarization control means (polarizer) and a half mirror to separate light quantity control.

On the other hand, for the production of the uneven microstructure, it was common to form it on a flat SiO ₂ or resin. Recently, Patent Document 2 and Patent Document 3 show that a fine structure is provided on a curved optical surface, and both are structures for preventing reflection.

  SUMMARY OF THE INVENTION The main object of the present invention is to provide an optical element, a light source unit, an optical scanning device, and an image forming apparatus that have a plurality of functions in an integrated manner and can realize reduction in the number of parts, downsizing, and cost reduction. .

  In order to achieve the above object, according to the first aspect of the present invention, in the optical element having at least one curved optical surface, an uneven microstructure having a structural birefringence function is provided on the surface of the curved optical surface. Is provided.

  According to a second aspect of the present invention, in the optical element according to the first aspect, the fine structure transmits a linearly polarized light in one direction.

  According to a third aspect of the present invention, in the optical element according to the second aspect, the fine structure is a zero-order lattice.

  According to a fourth aspect of the present invention, in the optical element according to the second aspect, the fine structure generates diffracted light with respect to linearly polarized light in directions different by 90 degrees.

  According to a fifth aspect of the present invention, in the optical element according to the first aspect, the fine structure has an N-step shape.

  According to a sixth aspect of the present invention, in the optical element according to the first aspect, at least one of the period, height, and fill factor of the fine structure is changed depending on the position of the surface of the curved optical surface. It is characterized by that.

  According to a seventh aspect of the present invention, in the optical element according to the first aspect, when the fine structure has a period Λ, the period Λ is approximately the same as the wavelength λ of light incident on the optical element (Λ to several λ). It is characterized by being.

  According to an eighth aspect of the present invention, in the optical element according to the first aspect, when the fine structure has a period Λ, the period Λ is smaller than the wavelength λ of light incident on the optical element.

  According to a ninth aspect of the present invention, in the light source unit comprising a surface emitting laser as a light source and a coupling lens for coupling a light beam emitted from the surface emitting laser, the coupling lens is claimed in the invention. 4. The optical element according to 2 or 3, wherein a linearly polarized light component in one direction is transmitted by the optical element.

  According to a tenth aspect of the present invention, there is provided a surface emitting laser as a light source, a coupling lens for coupling a light beam emitted from the surface emitting laser, and a photodetector for detecting a part of the light beam. In the light source unit provided, the coupling lens is an optical element according to claim 4, wherein diffracted light generated by the optical element is detected by the photodetector.

  According to an eleventh aspect of the present invention, in the light source unit according to the ninth or tenth aspect, a surface emitting laser array is used as the light source.

  According to a twelfth aspect of the present invention, in the optical scanning device, the light source unit according to any one of the ninth to eleventh aspects and a first light guide for guiding the light beam guided from the light source unit to the optical deflector. 1 optical system, an optical deflector for deflecting and scanning the light beam guided from the first optical system, and an image of the light beam deflected and scanned by the optical deflector as a light spot on the surface to be scanned And a second optical system.

  According to a thirteenth aspect of the present invention, in the image forming apparatus, the optical scanning device according to the twelfth aspect is used as an optical writing unit.

  In the invention described in claim 14, the optical element according to any one of claims 1 to 8 is manufactured by a photo-curing method that forms an uneven microstructure using a multiphoton absorption process of laser light. It is characterized by.

  According to a fifteenth aspect of the present invention, the optical element according to any one of the first to eighth aspects is manufactured by a laser ablation method in which an uneven fine structure is formed using laser light.

  According to a sixteenth aspect of the present invention, in the optical element according to the fourteenth or fifteenth aspect, an ultrashort pulse laser having a pulse width of 10 picoseconds or less is used as the laser light.

According to the first aspect of the present invention, since a birefringence function can be superimposed on an optical surface without using a birefringent material by providing a fine structure having structural birefringence on a curved surface, it has been conventionally separated. The lens effect and the birefringence effect can be combined into one. Therefore, the number of parts can be reduced, the optical system can be downsized, and the cost can be reduced.
According to the second aspect of the present invention, since light can be transmitted with respect to linearly polarized light in one direction, the functions of the polarizer can be integrated in addition to the focusing and divergence functions. Compared to the conventional combination of a polarizing element and a lens, by integrating it, the polarization separation function can be configured with a lens material. Therefore, the performance deterioration due to the mounting error between the polarizing element and the lens, and the polarizing element (especially The performance deterioration with respect to the environmental change (especially temperature change) of a polarizing film) can be reduced.
According to the third aspect of the present invention, since the grating is 0, it is possible to prevent generation of diffracted light that can be flare light or ghost light.
According to the fourth aspect of the invention, since diffracted light can be generated with respect to linearly polarized light in directions different by 90 degrees, in addition to the focusing and diverging functions, the optical path separation function utilizing the diffraction effect is integrated. be able to.

According to the fifth aspect of the present invention, it is possible to improve the diffraction efficiency and to make the distribution of the + 1st order and −1st order diffraction efficiency asymmetric by using an N-stage step-like microstructure. Thus, the degree of freedom in design can be improved.
According to the sixth aspect of the present invention, it is possible to change the parameters of the fine structure of the unevenness according to the position of the curved surface, and obtain good birefringence characteristics and diffraction characteristics according to the inclination of the curved surface and the incident angle of light. Therefore, the degree of design freedom can be improved.
According to the seventh aspect of the present invention, by using a diffractive element in the resonance region, a polarization-dependent diffractive element can be realized, and optical path separation becomes possible.
According to the eighth aspect of the present invention, a polarization-dependent diffraction element can be realized by using a diffraction element in the sub-wavelength region, and a diffraction angle can be increased by reducing the period, thereby separating the optical path. In addition, it is possible to improve the degree of freedom of the layout of the light source unit.

According to the ninth aspect of the present invention, it is possible to control the polarization of the surface emitting laser without adding an element for controlling the polarization. Compared to the conventional combination of a polarizing element and a lens, by integrating it, the polarization separation function can be configured with a lens material. Therefore, the performance deterioration due to the mounting error between the polarizing element and the lens, and the polarizing element (especially The performance deterioration with respect to the environmental change (especially temperature change) of a polarizing film) can be reduced.
According to the tenth aspect of the present invention, the polarization control of the surface emitting laser and the light amount control of the light source unit can be performed without adding an element for polarization control and an element for optical path separation. Therefore, the light output fluctuation of the light source unit can be suppressed.
According to the eleventh aspect of the present invention, by using the surface emitting laser array, it is possible to realize a light source unit for improving the printing speed and the writing density.
According to the twelfth aspect of the present invention, it is possible to realize an optical scanning device in which the polarization dependency of the transmittance and reflectance of the optical deflector and the optical element does not occur. Further, it is possible to realize an optical scanning device for achieving an improvement in printing speed and an improvement in writing density.

According to the thirteenth aspect of the invention, by controlling the light amount of the surface emitting laser, the density fluctuation caused by it does not occur on the image. Further, by controlling the polarization of the surface emitting laser, the polarization dependency of the transmittance and the reflectance of the optical deflector and the optical element does not occur, and the density fluctuation caused by it does not occur on the image. By suppressing these density fluctuations, it is possible to realize an image forming apparatus capable of obtaining a good image even when a surface emitting laser is used.
Further, by applying the surface emitting laser, it is possible to improve the printing speed and the writing density of the image forming apparatus. On the other hand, in configuring an optical scanning device having the same printing speed and the same scanning density as when a surface emitting laser is used, the rotational speed of the optical deflector can be reduced. It can contribute to the reduction of noise and heat generation associated with exercise.

According to the fourteenth aspect of the present invention, the optical element can be directly processed by being produced by a photocuring method that forms an uneven microstructure using a multiphoton absorption process of laser light. In addition, the use of a galvanometer mirror or a piezo stage makes it possible to produce a fine structure with high speed and high accuracy.
According to the fifteenth aspect of the present invention, since the laser ablation method is used to form an uneven fine structure using a laser beam, the optical element can be directly processed. In addition, the use of an XYZ stage makes it possible to produce a fine structure. Furthermore, the use of a galvanometer mirror can cope with high speed.
According to the sixteenth aspect of the present invention, since the pulse width of the laser beam used for the production is an extremely short pulse laser having a pulse width of 10 picoseconds or less, the influence of thermal melting is small, so that a highly accurate fine structure is obtained. Fabrication is possible. Further, processing with lower energy can be realized.

Hereinafter, a first embodiment of the present invention will be described with reference to FIGS.
As shown in FIG. 1, the light source unit 1 in this embodiment includes a surface emitting laser 10 as a light source and a coupling lens 12. The light beam emitted from the surface emitting laser 10 is coupled by the coupling lens 12, converted into a parallel or converging or diverging light beam, and emitted from the light source unit 1.
Here, the cover glass for dust-proofing the surface emitting laser, the housing of the light source unit 1, and the like are not shown.

  FIG. 2 shows an example of the configuration of the surface emitting laser 10. The active layer 10c is sandwiched between two clad layers 10a and 10b, and has a structure in which reflecting surfaces 10d and 10e having high reflectivity are formed on the upper and lower surfaces of the clad layer so as to face each other. . The region sandwiched between the two reflecting surfaces is a so-called Fabry-Perot resonator perpendicular to the substrate. Laser oscillation occurs in the oscillation region 10f of the active layer 10c, and the direction perpendicular to the substrate, that is, shown in the figure. A light beam is oscillated in the direction of arrow A. The surface emitting laser 10 basically has random polarization due to this structure.

As shown in FIG. 1, the light beam emitted from the surface emitting laser 10 shows random polarization (TE polarization and TM polarization are included at random), but the light beam transmitted through the coupling lens 12 is predetermined. It is linearly polarized light (TM polarized light) having a direction.
Here, TE polarized light indicates a direction perpendicular to the paper surface, and TM polarized light indicates a direction in the paper surface perpendicular to the TE polarized light.
That is, the coupling lens 12 also has a function as a polarizer, and transmits only one direction of linearly polarized light.
This is because structural birefringence is provided by providing an irregular microstructure 2 having periodicity on one curved surface of the coupling lens 12, that is, on the surface 12a of the curved optical surface on the surface emitting laser 10 side. It is realized because it generates.
In FIG. 1, the fine structure 2 is shown enlarged. The actual size is generally a microstructure 2 having a wavelength λ (on the order of 400 to 800 nm) on the coupling lens 12 on the order of millimeters.

The polarization separation property (TM polarized light is transmitted and TE polarized light is reflected) will be described with reference to FIG. FIG. 3 shows a polarization separation element formed on a flat substrate at the time of parallel light incidence shown by Tyan et al. (J. Opt. Soc. Am. A, Vol. 14 (1997), p. 1627-1636). It is an example.
Seven layers of Si layers and SiO ₂ layers are alternately formed on the SiO ₂ substrate. At this time, the period is 600 nm, the width of the fine structure is 300 nm (fill factor 0.5), and the height is 1.35 μm. The fill factor represents the ratio of the width of the fine structure to the period.
At this time, when TE polarized light and TM polarized light are incident, TM polarized light is transmitted and TE polarized light is reflected with an efficiency close to 100% with respect to the wavelength λ = 1.523 μm. That is, only one direction of linearly polarized light can be transmitted.
For light having a different wavelength, for example, λ = 0.78 μm, the polarization separation structure can be similarly designed by changing the period, structure width (fill factor), height, and material.

The feature of this embodiment is that such a polarization separation structure is formed on a curved surface, not on a flat plate. Thus, the ability to superimpose the birefringence function on the curved surface means that the lens effect (focusing and divergence) and the birefringence effect that have been conventionally separated can be integrated into one, reducing the number of parts and downsizing the optical system. As a result, cost reduction can be realized.
There is also a method of forming an optical multilayer film with birefringence on a curved surface, but there are also problems such as difficulty in film formation on resin materials and durability of the multilayer film, so make a structure directly on the curved surface The benefits are great.

As shown in FIG. 4, an uneven microstructure 2 having a structural birefringence function is provided on a curved surface 12a. Here, the shape of the fine structure shown in FIG. 3 is added on the curved surface as a reference.
Since both the curved surface shape and the fine structure represent the phase distribution with respect to light, the phase distribution can be added by adding the fine structure on the curved surface.
Since divergent light is incident in this embodiment, it is necessary to consider the effect of the incident angle (not parallel light incident) depending on the radius of curvature of the curved surface 12a. Of course, if the curvature is loose, it can be ignored.
FIG. 5 shows an example in which an uneven microstructure having a structural birefringence function is processed on a curved surface. First, as shown in FIG. 5A, the multilayer film 4 is disposed on the lens 3 having a curved surface, and the resist material 5 is disposed thereon by a technique such as coating and dipping (FIG. 5B). Thereafter, the resist material 5 is exposed by irradiation with a laser or an electron beam (FIG. 5C). At this time, it is possible to form a pattern on the spherical surface by scanning the laser or electron beam condensing position.

Then, by developing the resist material 5, an uneven fine structure can be formed on the multilayer film 4 (FIG. 5D). Further, a part of the multilayer film 4 is removed by a method such as dry etching using the resist material 5. If necessary, the remaining resist 5 is removed, and as shown in FIG. 5 (e), it is possible to produce the multilayer microstructure 6 on the spherical surface.
Strictly speaking, the microstructure 6 produced by this processing method is slightly different from that shown in FIG. 4, and the layer of the multilayer film 4 has a structure along the spherical surface. Of course, the microstructure 2 can also be formed by this method.

The microstructure 2 to be formed can be a zero-order lattice. This is because the 0th-order grating does not generate diffracted light. That is, since there is no diffracted light that can be ghost light or flare light, there is no loss of light quantity of the light beam, and there is no need to consider removal of unnecessary light.
Here, the zero-order lattice will be briefly described. When normal incidence to the fine structure 2 is considered, when the period Λ satisfies the following relationship with respect to the refractive index n and the wavelength λ of the fine structure, a zero-order grating is formed.
Λ <λ / n
The fine structure in FIG. 3 is n≈2.1 and Λ = 0.6 μm. For λ = 1.523 μm, the above formula is satisfied, and the condition of the zero-order lattice is satisfied.

FIG. 6 shows the light source unit 1 and the subsequent optical system in the present embodiment. The light beam emitted from the surface emitting laser 10 is converted into a substantially parallel light beam by the coupling lens 12 having a structural birefringence function, the light beam width is limited by the aperture 13, and a part of the light beam is left as it is by the half mirror 14. The light is transmitted and incident on the cylindrical lens 15.
At this time, the light beam in the randomly polarized state of the surface emitting laser 10 is transmitted only through one direction of linearly polarized light by the coupling lens 12 and propagates through the subsequent optical system.
On the other hand, the light beam reflected by the half mirror 14 enters the photodetector 16.

In contrast, an example of an optical system using a conventional light source unit is shown in FIG. Here, a coupling lens 18 having no structural birefringence function is used, and a polarizer 17 is disposed between the aperture 13 and the half mirror 14. In the conventional example, the polarization of the light beam emitted from the surface emitting laser 10 is controlled by the polarizer 17.
Therefore, in this embodiment, the polarizer 17 is not required, the number of parts is reduced, and the function is superimposed on the coupling lens 12 and integrated.

A second embodiment will be described with reference to FIGS. Note that the same parts as those in the above embodiment are denoted by the same reference numerals, and unless otherwise specified, description of the configuration and functions already described is omitted, and only the main part will be described (the same applies to other embodiments below).
As shown in FIG. 8, the light source unit 20 in this embodiment includes a surface emitting laser 10, a coupling lens 12, a photodetector 16, and a laser light control device 22. Reference numeral 23 denotes a holder, and 24 denotes an L-shaped base.
The coupling lens 12 has a polarizer function as described above, but also has a function as a diffractive element. For the linearly polarized light in a direction different from the linearly polarized light to be transmitted by 90 degrees. Diffracted light is generated.
This is achieved by providing a concavo-convex fine structure 2 with periodicity on one curved surface of the coupling lens 12 to realize a polarization-dependent diffraction element (polarization separation diffraction element) that generates diffracted light. Has been.

A part of the light beam diffracted at an angle θ from the coupling lens 12 is incident on the photodetector 21. The light intensity signal from the light detector 16 is sent to the laser light quantity control device 22, and here the drive current is controlled so that the light output of the surface emitting laser 10 becomes a predetermined output.
The current signal from the laser light quantity control device 22 is fed back to the surface emitting laser 10, and the surface emitting laser 10 is driven with a predetermined light output.
Here, the diffraction angle θ depends on the period Λ of the fine structure 2. That is, when the period Λ is reduced, that is, by making the diffraction element from the resonance region to the sub-wavelength region (Λ <λ), the diffraction angle (separation angle) θ can be increased, and the layout flexibility of the light source unit 20 is increased. growing. Further, there is an advantage that high-order diffracted light can be suppressed and unnecessary light that can be flare light or ghost light can be reduced.

In a region called a so-called resonance region or sub-wavelength region in which the period Λ is about the wavelength λ emitted from the surface emitting laser (Λ to several λ) or the period Λ is smaller than the wavelength λ (Λ <λ) Has polarization dependence. In such a diffractive surface, the behavior varies depending on the polarization direction of the light beam incident on the diffractive surface (for example, TE polarized light or TM polarized light).
The polarization-dependent diffraction element (TM polarized light is diffracted and TE polarized light is transmitted) will be described with reference to FIG. FIG. 9 is an example of a polarization-dependent diffractive element formed on a flat substrate at the time of parallel light incidence shown by Schmitz et al. (Optics Letter, Vol. 20 (1995), p. 1830-1831). .
A substrate having a dielectric constant of 3.95 (refractive index of 1.99) has a binary structure with a period of 2λ, a fine structure width of 0.26λ (fill factor of 0.13), and a height of 1.3λ.
When TE-polarized light and TM-polarized light are incident, TM-polarized light is diffracted in the ± first-order direction, and TE-polarized light is transmitted. That is, linearly polarized light in one direction is transmitted, and linearly polarized light in a direction different by 90 degrees is diffracted. At this time, since Λ = 2λ, this is a polarization-dependent diffraction element in the resonance region.

The feature of this embodiment is that such a polarization separation structure is formed on a curved surface, not on a flat plate. As described above, in the configuration in which the birefringence function and the diffraction function are superimposed on the curved surface, the conventionally separated lens effect (focusing and divergence), the birefringence effect, and the optical path separation effect by diffraction can be integrated into one. Therefore, the number of parts can be reduced, the optical system can be downsized, and the cost can be reduced.
There is also a method of forming an optical multilayer film with birefringence on a curved surface, but there are also problems such as difficulty in film formation on resin materials and durability of the multilayer film, so make a structure directly on the curved surface The benefits are great. Moreover, the merit which can reduce the optical element for optical path separation is very big.

As shown in FIG. 10, an uneven microstructure 2 having a structural birefringence function is provided on a curved surface 12a. Here, the shape of the fine structure shown in FIG. 9 is added on the curved surface as a reference. Since both the curved surface shape and the fine structure represent the phase distribution with respect to light, the phase distribution can be added by adding the fine structure on the curved surface.
Since divergent light is incident in this embodiment, it is necessary to consider the effect of the incident angle (not parallel light incidence) depending on the radius of curvature of the curved surface. Of course, if the curvature is loose, it can be ignored.

FIG. 11 shows an example in which an uneven microstructure having a structural birefringence function is processed on a curved surface. First, as shown in FIG. 11A, a resist material 8 is disposed on the curved surface 7 by a technique such as spin coating or dipping. An ultraviolet laser is divided with respect to the resist material 8, and a lattice pattern having a wavelength smaller than or about the wavelength is formed by two-beam interferometry.
At this time, the pitch of the fine structure can be reduced to 1/2 of the laser wavelength to be irradiated by the two-beam interference. At this time, the irradiation area is defined so that sufficient interference is possible even on the spherical surface (curved surface 7). If necessary, interference exposure is repeated by changing the irradiation position (FIG. 11B).
Thereafter, the resist material 8 is developed to form an uneven fine structure on the curved surface 7 (FIG. 11C). Further, by performing dry etching of the substrate using this resist, it is possible to produce an uneven microstructure 9 in the substrate material (FIG. 11D). The fine structure 2 can be formed by this method. This technique can be applied to many materials such as glass, quartz, polymer, and silicon as substrate materials.

FIG. 12 shows the light source unit 20 of this embodiment and an optical system subsequent thereto.
The light beam emitted from the surface emitting laser 10 is converted into a substantially parallel light beam through the coupling lens 12 having structural birefringence and diffraction effect, and is converted into a substantially parallel light beam. Is limited and is incident on the cylindrical lens 15.
On the other hand, a part of the light beam diffracted by the coupling lens 12 is incident on the photodetector 16 and the characteristics of the light beam are detected.

A third embodiment (a modification of the structure of the polarization-dependent diffraction element) will be described with reference to FIG.
In the present embodiment, a diffraction grating having a period Λ for branching an optical path by generating diffraction and a diffraction grating having a period Λ ′ for generating structural birefringence are integrated into two fine periods. A polarization-dependent diffraction element having a composite structure is formed. When compared by period, Λ> Λ ′.
In the case of showing structural birefringence, the periodic structure portion having the period Λ ′ can be approximated as a uniform isotropic medium by using the concept of effective refractive index as shown by the following equation.
When the refractive indexes for TE polarized light and TM polarized light are n (TE) and n (TM), the refractive index of the substrate 25 is n, the wavelength of light is λ, the period is Λ ′, and the fill factor is f ′.
n (TE) = √ {fn ² + (1−f)}
n (TM) = √ [n ² / {f + (1−f) n ² }]

Therefore, the periodic structure portion having the period Λ ′ can be handled so as to have different effective refractive indexes for the TE polarized light and the TM polarized light. That is, as shown in FIG. 13C, it looks like a diffraction grating having a refractive index n (TE) with a period Λ for TE polarized light, and for TM polarized light as shown in FIG. 13B. It looks like a diffraction grating consisting of a refractive index n (TM) of period Λ.
Therefore, it is possible to configure a polarization separation mirror that can exhibit different behavior with respect to TE polarized light and TM polarized light, and transmits one linearly polarized light but reflects linearly polarized light orthogonal thereto. Thereby, polarization control is possible.
Furthermore, in order to generate the diffracted light and branch the optical path, by selecting the period Λ, for example, ± 1st-order diffraction can be generated, so that a part of the light beam is sent to the photodetector 16. It can be branched, and can act as an optical path branching means.
Here, reference is not made to the fine shape of the fine structure of irregularities having structural birefringence, but the fine structure is applied to a curved surface. Regarding the fine shape of the fine structure, the above-described examples and the above-described techniques can be referred to.

FIG. 14 shows a fourth embodiment.
This is an example in which diffracted light is reflected by reflection by the coupling lens 12. At this time, since the diffracted light returns to the surface emitting laser 10 side, the photodetector 16 can be arranged in the vicinity of the surface emitting laser 10. Reference numeral 27 denotes a base.
As a result, for example, the circuit board of the surface emitting laser 10 and the photodetector 16 can be shared. Of course, the light source unit 26 can be greatly reduced in size.
In this way, options for the position of the photodetector 16 are widened, and a light source unit with a high degree of freedom in layout can be configured.

FIG. 15 shows a fifth embodiment (a modified example of an uneven microstructure).
The uneven microstructure 2 can have a multi-level N-step shape (here, three levels of a1, a2, and a3). In particular, when producing the diffraction effect, the blazed effect appears by the multi-level, and it is possible to improve the diffraction efficiency or make the distribution of the + 1st order and −1st order diffraction efficiency asymmetrical. The degree of freedom can be improved.

FIG. 16 shows a sixth embodiment (a modified example of the uneven microstructure).
The uneven microstructure 2 can change at least one of the period, height, and fill factor depending on the position of the curved surface. If the curved surface 12a is a spherical surface, the slope of the curved surface is always constant. However, in the case of an aspherical surface (not a spherical surface, including a free-form surface), the slope of the curved surface varies depending on the position on the curved surface.
The structural birefringence and diffraction effects change depending on the slope of the curved surface and the incident angle of light. Therefore, if at least one of the period, height, and fill factor can be changed in order to obtain better characteristics, the design As the degree of freedom increases, the function can be improved.

FIG. 16A shows an example in which the period increases from the center to the periphery, and FIG. 16B shows an example in which the height increases as it goes to the periphery (here, the height of the vertex). FIG. 16C shows an example in which the fill factor increases toward the periphery. Of course, a structure in which these are combined can also be used.
16 (a) and 16 (c) are formed by the laser beam or electron beam irradiation position (that is, the period is changed) and the irradiation intensity (that is, the fill factor is changed) in the processing method shown in FIGS. This can be realized by modulating the above.

FIG. 17 shows an example of a processing method for producing the fine structure of FIG.
A curved surface 28 for arranging a structure having a uniform height on the surface is immersed in a photocurable resin and arranged (FIG. 17A).
Condensed and irradiated with an ultraviolet laser, photocured from the substrate to the liquid bottom. By sequentially performing photocuring while scanning with laser light, it is possible to form a photocurable resin structure having a uniform height on the substrate. At this time, the height can be adjusted by the height of the liquid level (FIG. 17B). Thereafter, by removing the uncured resin, an uneven microstructure 29 can be formed on the curved surface 28 (FIG. 17C).

Furthermore, the optical element having an uneven microstructure according to the present invention can be mass-produced at low cost using a replication technique.
FIG. 18 shows one embodiment of a processing method for replicating a fine structure. Here, an element having an N-step step-like microstructure shown in FIG. 15 is shown, but the same method can be applied to other structures.
A resist material 31 is applied on the substrate 30 (FIG. 18A), and the resist material fine structure 32 formed by electron beam exposure, laser exposure, or the like (FIG. 18B) is used. A conductive film 33 is added (FIG. 18C).
Thereafter, an inverted shape of a metal such as Ni is created on the substrate 30 by electroforming using the conductive film 33 (FIG. 18D) (FIG. 18E). Using this inverted Ni material as the mold 34, the shape is transferred to a transparent resin material or the like using a replication method such as an injection molding method (FIG. 18F). Thereby, the optical element 35 having a fine structure can be manufactured (FIG. 18G).
At this time, the initial structure body need not be an optical material that is transparent to the used wavelength, and may be any material that has an uneven microstructure formed on a curved surface.

As shown in FIG. 8 and the like, the uneven microstructure 2 can be provided on the surface emitting laser 10 side of the coupling lens 12. When the surface emitting laser 10 side is packaged by the holder 23 or the like, the protection of the surface of the fine structure 2 can be omitted, and can be protected from foreign matter or dust. Of course, the uneven microstructure 2 can be provided on the side opposite to the surface emitting laser 10.
In FIG. 8 and the like, as a component constituting the light source unit, a base 24 and a holder 23 are illustrated in addition to the surface emitting laser 10, the coupling lens 12, and the photodetector 16, but a holder in which these are integrated. May be used. In addition, a third holding member may be provided separately. These do not affect the present invention.
Further, the method for supporting the holder and the cover glass is not described in detail, and a known method can be used.
Optical path branching means for branching the light beam from the surface emitting laser 10 into a light beam emitted from the light source unit and a light beam guided to the photodetector 16 in the light source unit, and linearly polarized light beam emitted from the light source unit The polarization control means is integrated in the coupling lens 12 and integrated, and there is a feature of the present invention, and other configurations are not limited at all.

FIG. 19 shows a seventh embodiment.
The surface emitting laser 10 is relatively easy to array compared to the edge emitting laser, and a surface emitting laser array having several tens of light emitting points is realized. These elements can also be applied to the light source unit of the present invention.
FIG. 19 shows an example using a surface emitting laser array. The surface-emitting laser array 36 and a plurality of light beams are emitted from the surface-emitting laser array 36 (in the figure, three light beams are shown for simplicity).
Other configurations are the same as those in FIG. Each light beam diffracted by the coupling lens 12 may be detected collectively by the photodetector 16 if the light amount of each light beam of the surface emitting laser array 36 is collectively controlled. You can also. In this case, a photodetector having a size that can be detected collectively may be prepared, or a photodetector having a size that can detect only a few light beams may be prepared as a representative.
On the other hand, if the light amount control is to be performed separately for each light beam of the surface emitting laser array 36, it is necessary to prepare a photodetector for each light beam branched by the optical path branching means. It is also possible to arrange an optical system for guiding to the photodetector.

The eighth embodiment (optical scanning device) will be described with reference to FIGS. FIG. 20 is a cross-sectional view in the main scanning direction in which the light beam is deflected and scanned, and FIG. 21 is a developed cross-sectional view in the sub-scanning direction orthogonal thereto.
The light source unit 1 includes a surface emitting laser 10 and a coupling lens 12. The light beam emitted from the surface emitting laser 10 is shaped into a substantially parallel light beam by the coupling lens 12. Then, the light is guided to the first optical system 42. The first optical system 42 includes the aperture 13 and the cylindrical lens 15.
The beam width of the light beam is limited by the aperture 13, the light beam is converged in one direction so as to be a linear light beam by the cylindrical lens 15, and is formed as a line image on the deflection reflection surface of the optical deflector 37.
Thereafter, the light is guided to the second optical system 43. The second optical system 43 includes, for example, two scanning imaging lenses 38 and 39, and the light beam deflected and scanned by the optical deflector 37 forms an image on a desired light spot by the scanning imaging lenses 38 and 39. . The formed light spot is scanned on the scanning surface 40 at a predetermined interval. In FIG. 20, reference numeral 41 denotes a laser beam detector.

The same applies to the case where a plurality of light beams are used by using a surface emitting laser array. In this case, a plurality of light spots on the surface to be scanned 40 are maintained at predetermined intervals in the main scanning direction and the sub-scanning direction. The optical elements 10 to 39 are arranged so as to obtain this predetermined interval.
If there is an unnecessary diffracted light beam emitted from the light source unit 1, it should be shielded from light so as not to form a light spot on the surface to be scanned 40 inside or outside the optical scanning device as necessary. good.
Although not shown, a part of the light beam branched by the optical path separating means such as a half mirror provided between the aperture 13 and the cylindrical lens 15 or a part of the light beam 1 diffracted by the coupling lens 12. Can be controlled by the photodetector.

A ninth embodiment (image forming apparatus) will be described with reference to FIG.
One of image forming processes for forming an image in an image forming apparatus is an electrophotographic process. An outline of the electrophotographic process will be described below.
A latent image is formed by applying a potential to the image carrier 50 (for example, a photoreceptor) by the charging means 51 (charging process) and irradiating the image carrier 50 with a light spot from the optical writing unit 52 (exposure means). (Exposure process), toner is attached to the latent image by developing means 53 to form a toner image (developing process), the toner image is transferred to recording paper 54 by transfer means 55 (transfer process), and pressure or This is a process in which heat is applied and fused to the recording paper 54 (fixing process).
The toner remaining on the image carrier 50 is cleaned by the cleaner means 57, and the charged portion is discharged by the discharging unit 58.
The optical writing unit 52 in the present embodiment can also be applied to a tandem type image forming apparatus that is advantageous for high-speed color image output.

An optical element processing method will be described.
For a wavelength region of about 1.5 μm used for optical communication from visible light, the uneven microstructure has a period of about sub μm to several μm. There are not so many processing methods for producing such a fine structure, and in general, a laser (interference) exposure method and an electron beam exposure method are widely used.
In the laser interference exposure method, a laser beam having a wavelength in the normal blue to ultraviolet region is interfered and irradiated to a photosensitive polymer material (resist material), and an uneven pattern is produced using the interference pattern. .
In this case, a structure is formed in the resist material formed on the substrate. On the other hand, in the electron beam exposure method, an electron beam is condensed and scanned on a resist material to produce an uneven microstructure. In this case as well, a structure is formed on the resist material formed on the substrate.
In these methods, when producing an uneven microstructure other than a resist material (such as an optical element material such as quartz glass), processing methods such as reactive dry etching and lift-off are further used based on this resist structure. It is necessary to use it.
As shown in FIGS. 5 and 11, the above-described general processing method can be used for the optical element of the present invention.

Further, processing can be performed directly on the optical element material instead of the resist material. These processing methods will be described with reference to FIGS.
FIG. 23 shows an example of processing using a photocuring method. Here, as a photo-curing method, laser light is condensed in a photo-curable material, the material is cured in the vicinity of the condensing point by a multiphoton absorption process, and the laser light is scanned three-dimensionally. A method for producing the microstructure will be described (tenth embodiment).
The laser light emitted from the laser device 60 such as an Nd: YAG laser or Ti: Sapphire laser is adjusted in light intensity by the half-wave plate 61 and the Glan-Thompson prism 62, and then the laser light has a waveform by the spatial filter 63. It is shaped. Then, the light path is changed by the galvanometer mirror 64 and is condensed in a photocurable material (photoreactive material) 69 on the substrate 68 via the relay lens 65, the imaging lens 66, and the objective lens 67.

  The condensing point can also be moved in the direction perpendicular to the substrate by the piezo stage 70. In this way, a structure can be three-dimensionally produced by combining the galvanometer mirror 64 and the piezo stage 70. At this time, the photocurable material 69 includes a photocurable resin or a photocurable organic / inorganic hybrid material, and the photocurable material 69 can function as an optical element as it is. The configuration of this processing apparatus is merely an example, and the present invention is not limited to this. In FIG. 23, reference numeral 71 denotes a stage.

As another method, FIG. 24 shows a processing example using a laser ablation method.
As the laser ablation method, a laser beam used for processing is made to interfere, and the optical element material is directly processed by the interference pattern, or as shown in FIG. 24, the laser element is directly irradiated to the optical element material to be processed. Thus, there is a method of selectively removing (the eleventh embodiment).
A part of the laser light emitted from the laser device 72 is transmitted through the photomask 73, and the laser light pattern is reduced and projected by the condenser lens 75 via the mirror 74, and irradiated to the optical element material 76. Is done.
The optical element material 76 is installed on the XYZ stage 77 and can be moved three-dimensionally. Of course, a galvanometer mirror or the like may be provided instead of the mirror 74 to scan the laser beam.
In addition, a plurality of patterns (two types in the figure) are prepared for the photomask 73 and can be exchanged according to the pattern to be processed, the optical element material 76 to be processed, or the like. Movement of the XYZ stage 77 and replacement of the photomask 73 are controlled by the PC 78. Of course, the structure of this processing element is an example to the last, and is not limited to this.

In any of the above processing methods, an ultrashort pulse laser having a laser beam pulse width of 10 picoseconds or less can be used (a twelfth embodiment).
The use of an ultrashort pulse laser has advantages such as melting by heat propagation, reduction of damaged layer, and increase of multiphoton absorption cross section, and it has been used in recent years. In addition, since the multiphoton absorption cross section can be increased, the photocuring method can be carried out more effectively and with low energy.

It is a figure which shows the light source unit in the 1st Embodiment of this invention. It is a figure which shows the structure of a surface emitting laser. It is a model figure for demonstrating polarization separation. It is detail drawing of the uneven | corrugated fine structure which has the structure birefringence function provided in the surface of the curved surface. It is a figure which shows the manufacturing process of the uneven | corrugated microstructure. It is a figure which shows a light source unit and an optical system following it. It is a figure which shows the light source unit in the past, and an optical system following it. It is a figure which shows the light source unit in 2nd Embodiment. It is a model figure for demonstrating the polarization-dependent diffraction element. It is detail drawing of the uneven | corrugated fine structure which has the structure birefringence function provided in the surface of the curved surface. It is a figure which shows the manufacturing process of the uneven | corrugated microstructure. It is a figure which shows a light source unit and an optical system following it. It is a figure which shows the structure of the polarization-dependent diffraction element in 3rd Embodiment. It is a figure which shows the light source unit in 4th Embodiment. It is a figure which shows the step-like fine structure in 5th Embodiment. It is a figure which shows the modification of the fine structure in 6th Embodiment. It is a figure which shows the processing method which produces the uneven | corrugated microstructure. It is a figure which shows the processing method which replicates a fine structure. It is a figure which shows the light source unit in 7th Embodiment. It is main scanning sectional drawing which shows the optical scanning device in 8th Embodiment. It is an expanded sectional view in the sub-scanning direction of the optical scanning device. It is a figure which shows the image forming apparatus in 9th Embodiment. It is a figure which shows the processing unit of the optical element in 10th Embodiment. It is a figure which shows the processing unit of the optical element in 11th Embodiment. It is a figure which shows the polarization characteristic of the light beam in an edge emitting laser.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 20 Light source unit 2 Irregular fine structure 10 Surface emitting laser 12 Coupling lens 12a as optical element 12a Curved optical surface 16 Photo detector 36 Surface emitting laser array 37 Optical deflector 40 Scanned surface 42 First optical system 43 Second optical system 52 Optical writing unit

Claims (16)

In an optical element having at least one curved optical surface,
An optical element characterized in that an uneven microstructure having a structural birefringence function is provided on the surface of the curved optical surface. The optical element according to claim 1,
An optical element characterized in that the fine structure transmits a linearly polarized light in one direction. The optical element according to claim 2,
The optical element is characterized in that the fine structure is a zero-order lattice. The optical element according to claim 2,
The optical element is characterized in that the fine structure generates diffracted light with respect to linearly polarized light in directions different by 90 degrees. The optical element according to claim 1,
2. The optical element according to claim 1, wherein the microstructure has an N-stage step shape. The optical element according to claim 1,
An optical element, wherein at least one of a period, a height, and a fill factor of the fine structure is changed depending on a position of a surface of the curved optical surface. The optical element according to claim 1,
When the fine structure has a period Λ, the period Λ is approximately the same as the wavelength λ of light incident on the optical element (Λ to several λ). The optical element according to claim 1,
An optical element, wherein when the fine structure has a period Λ, the period Λ is smaller than a wavelength λ of light incident on the optical element. In a light source unit comprising a surface emitting laser as a light source and a coupling lens for coupling a light beam emitted from the surface emitting laser,
4. The light source unit according to claim 2, wherein the coupling lens is an optical element according to claim 2 or 3, wherein a linearly polarized component in one direction is transmitted by the optical element. In a light source unit comprising a surface emitting laser as a light source, a coupling lens for coupling a light beam emitted from the surface emitting laser, and a photodetector for detecting a part of the light beam,
The light source unit according to claim 4, wherein the coupling lens is an optical element according to claim 4, and diffracted light generated by the optical element is detected by the photodetector. The light source unit according to claim 9 or 10,
A light source unit using a surface emitting laser array as a light source. A light source unit according to any one of claims 9 to 11,
A first optical system for guiding a light beam guided from the light source unit to an optical deflector;
An optical deflector for deflecting and scanning the light beam guided from the first optical system;
And a second optical system for forming an image of the light beam deflected and scanned by the optical deflector as a light spot on the surface to be scanned.   An image forming apparatus using the optical scanning device according to claim 12 as an optical writing unit. The optical element according to any one of claims 1 to 8,
An optical element manufactured by a photo-curing method that forms an uneven microstructure using a multiphoton absorption process of laser light. The optical element according to any one of claims 1 to 8,
An optical element manufactured by a laser ablation method in which an uneven microstructure is formed using laser light. The optical element according to claim 14 or 15,
An optical element using an ultrashort pulse laser having a pulse width of 10 picoseconds or less as laser light.

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