JP2006003479A - Optical element and illumination optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element capable of making a polarized state regarded as being substantially random. <P>SOLUTION: The directions of lines in respective areas shown in Fig. (a) correspond to the directions of slits. In Fig. (a), the area where the line is not formed is the area where the slit is not formed, and the polarized state is not changed in this area. In the polarized state of emitted light obtained, when linearly polarized light, having the axis of polarization in a direction shown by an arrow in (a), is made incident on the optical element having such structure, each area has the action of a wavelength plate, so that the emitted light is changed to the linearly polarized light, elliptically polarized light and circularly polarized light having a different optical axis direction, as shown in Fig. (b). Thus, when light emitted from the respective areas is collected to one, it includes light having various polarized states, and turns into a light in a state near to random polarized light (non-polarized light) as a whole. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  This means relates to an optical element suitable for use for the purpose of converting incident light into non-polarized light (randomly polarized light), and an illumination optical system using them.

  When an optical system including an element having optical anisotropy is used, unpolarized (randomly polarized) illumination light is often used in order to avoid an adverse effect due to anisotropy. In an imaging system close to the resolution limit, the imaging characteristics are greatly affected by the polarization state. That is, in the illumination close to linearly polarized light, directionality occurs in the imaging characteristics, and therefore unpolarized (randomly polarized) illumination is often used.

  When a laser often used in applications requiring monochromaticity or strong light intensity is used as a light source, the polarization state generally obtained is uniform and cannot be regarded as non-polarized light. Therefore, it is necessary to control the polarization state in order to make such polarization state light non-polarized illumination light.

  A birefringent crystal such as quartz is often used for controlling the polarization state. When light passes through the birefringent crystal, the polarization state can be changed because the optical path length varies depending on the polarization direction.

"Light control by a grating structure finer than the wavelength" (Optics, Vol. 27, No. 1, p12-17)

  In order to realize spatially different polarization states using a single birefringent crystal, it is necessary to spatially change the thickness in the optical axis direction. However, in this case, since the incident surface and the exit surface of the element are not parallel, the light beam that has passed through the element is bent by refraction, so that it is necessary to correct the bending of the light beam when incorporated in the apparatus.

  In order to make the incident surface and emission surface of the element parallel, there are cases where complementary elements are used in combination. However, the thickness of the element is more than twice, the transmittance is lowered, and the cost is high. In addition to this, there are limitations on the adhesive used for bonding, assembly accuracy, and the like.

  Furthermore, in order to control both the ellipticity and the polarization axis, it is necessary to use two or more crystals each functioning as a phase shifter and an optical rotator. Furthermore, even in an element made in this way, the spatial distribution of the polarization state can only be changed continuously, and is limited to a certain degree of arbitraryness (randomness).

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an optical element capable of creating a polarization state regarded as almost random, and an illumination optical system using the optical element.

  A first means for solving the above problem is that at least a portion where incident light is incident is divided into regions sufficiently small with respect to the area of the incident light, and each region has an ellipticity and a polarization axis of polarization of incident light. Each of the regions has at least one of the ellipticity and the direction of the polarization axis that is changed by light incident on each region, and the entire emitted light is random. The optical element is characterized in that it is formed as follows.

  In this means, the optical element surface is divided into small regions. As for the size of the division, it suffices that the number of divided small regions is such that the outgoing light can be regarded as having a random ellipticity and a polarization axis direction, depending on the properties required for the outgoing light. It can be decided arbitrarily.

  Each small region has a function as a wave plate, and the light passing through each small region is changed in at least one of its ellipticity and the direction of the polarization axis (because it is random, both of them change) In some cases, such a small area is included in the scope of the present invention. The degree of change in ellipticity and polarization axis direction in each small area is determined so that the degree of change in ellipticity and polarization axis direction of light passing through each small area is random as a whole. Yes. Therefore, no matter what polarization state light is incident on this optical element, the emitted light becomes random polarized light (non-polarized light) as a whole.

  The second means for solving the problem is the first means, wherein each region has a grating structure finer than the wavelength of light to be used, thereby having a function as a wave plate. It is what is characterized by the above-mentioned (Claim 2).

  According to “Optical control by a grating structure finer than the wavelength” (Optics, Vol. 27, No. 1, p12-17) (Non-Patent Document 1), irregularities with a pitch smaller than the wavelength of light used on the surface of the optical substrate By forming a grating structure having, a phase difference can be generated between light having a polarization axis in the length direction of the grating and light having a polarization axis in the width direction, and a wave plate can be formed. I know. Therefore, by controlling the pitch and height of the grating, it is possible to control at least one of the ellipticity and polarization axis of the light passing through the phase plate.

  Such a lattice structure can be easily formed by photolithography, and a region having a predetermined lattice structure can be made into a fine region. Therefore, a small region having a different lattice structure can be randomly formed by a photolithography process. By forming, the optical element as the first means can be easily formed.

  The third means for solving the above-mentioned problem is the first means, and functions as an antireflection structure on the surface opposite to the surface on which the grating structure providing a function as a wave plate is formed. A lattice structure having the following is formed (claim 3).

  According to Patent Document 1, by providing a triangular or pyramidal uneven structure with a pitch smaller than the wavelength of light used on the surface of the optical substrate, it is possible to provide a function as an antireflection structure. it can. Therefore, by forming a concavo-convex structure for providing a function as a wave plate on one side of the optical substrate, and a concavo-convex structure for providing a function as an antireflection structure on the other surface, the light transmittance is high. And it can be set as the optical element which forms random polarization | polarized-light.

  A fourth means for solving the above-described problem is any one of the first to third means, and the entrance surface and the exit surface viewed macroscopically are parallel to each other. (Claim 4).

  “The entrance surface and the exit surface viewed macroscopically are parallel” means that, when viewed microscopically, the surface of the optical element has a concavo-convex structure. The incident surface and the exit surface can be considered to be parallel, which means that the incident light travels straight. In this means, unlike the optical element of the prior art, since the incident light goes straight, no special optical system for aligning the optical axis is required.

  A fifth means for solving the above-mentioned problem is an illumination optical system (Claim 5) including any one of the optical elements of the first to fourth means.

  In this means, illumination light of random polarization (non-polarization) can be formed.

  ADVANTAGE OF THE INVENTION According to this invention, the optical element which can produce the polarization state regarded as substantially random, and an illumination optical system using the same can be provided.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing the shape of a small region in an optical element according to an embodiment of the present invention. A slit-like unevenness (lattice) is formed on one surface of an optical substrate 1 that is transparent to light to be used, such as quartz or glass, and the sum of the width a of the convex portion 2 and the width b of the concave portion 3 ( a + b) is shorter than the wavelength of the light used.

  In an optical element having such a structure, as described in Non-Patent Document 1, birefringence occurs with the length direction of the slit being the slow axis and the width direction of the slit being the fast axis, and as a phase plate Can be used.

  FIG. 2 is a diagram schematically showing the operation of the optical element in which the light incident area is divided into 4 × 4 areas and slit-like gratings in different directions are formed in the respective areas. The direction of the line in each region shown in (a) corresponds to the direction of the slit. In (a), a region where no line is formed is a region where a slit is not formed, and the polarization state does not change in this region.

  The polarization state of outgoing light obtained when linearly polarized light having a polarization axis in the direction indicated by the arrow in (a) is incident on the optical element having such a structure is schematically shown in (b). The incident linearly polarized light is converted into linearly polarized light, elliptically polarized light, and circularly polarized light having different polarization axis directions as shown in FIG. Therefore, when the light emitted from each region is collected into one, the light includes light in various polarization states, and as a whole is in a state close to random polarization (non-polarization).

  In FIG. 2, the region is divided into 4 × 4 regions for the sake of explanation. However, by increasing the number of regions and the direction of the slits, a light beam that is considered to be more randomly polarized (non-polarized) can be obtained. Can be formed.

  FIG. 3 shows an outline of the illumination optical system according to the embodiment of the present invention. In the illumination optical system shown in (a), the light emitted from the light source 5 is converted into parallel light by the collimator lens 6 and condensed by the condenser lens 7. The optical element 8 of the present invention is arranged at this condensing position to form non-polarized light. That is, the position of the optical element 8 becomes a secondary light source, and the light emitted from the secondary light source is regarded as non-polarized light. This light is converted into parallel light by the projection lens 9 and used as illumination light. In this optical system, since the light beam is focused at the position of the optical element 8, the area of the incident light beam is reduced, but the light source 5 has a finite size and the image 8 also has a finite size. Since the size of the small area of the optical element is extremely small as formed by photolithography, this does not cause a problem.

  In the illumination optical system shown in (b), the light emitted from the light source 5 is converted into parallel light by the collimator lens 6, and the optical element 8 of the present invention is disposed at the position of the parallel light. Therefore, the area of the light beam incident on the optical element 8 can be increased. The light transmitted through the optical element 8 is condensed by the condenser lens 7, but a diffusion plate 10 is provided at the condensing position, and light with different incident angles is mixed and emitted. . The light that has passed through the diffusion plate 10 is converted into parallel light by the projection lens 9 and used as illumination light. The reason why the diffusing plate 10 is required in this optical system is that if this is not provided, the light distribution for each region of the optical element 8 becomes the light distribution of the illumination light as it is. Therefore, when the illumination light is condensed and used at a certain point, the diffusion plate 10 is not necessary.

  FIG. 4 shows an outline of a small area in an optical element according to another embodiment of the present invention. In this optical element, the slit-like convex part 2 and concave part 3 formed on the light emission surface of the optical substrate 1 give the optical element the characteristics as a phase plate as described above. Triangular irregularities 11 are provided on the light incident surface, and this serves as an antireflection structure. The pitch of the triangular irregularities 11 is also made shorter than the wavelength of light to be used, and as a result, as shown in Non-Patent Document 1, an antireflection structure is formed. By using such an optical element, it is possible to obtain characteristics as a wave plate while preventing reflection of light on the surface.

  The method for manufacturing the concavo-convex structure as shown in FIG. 1 and FIG. 4 by photolithography is well known and also described in Non-Patent Document 1, and will not be described.

It is a figure which shows the shape of the small area | region in the optical element which is embodiment of this invention. It is the figure which showed typically the effect | action of the optical element which divided the area | region into which light injects into a 4x4 area | region, and formed the slit-shaped grating | lattice of a different direction in each area | region. It is a figure which shows the outline | summary of the illumination optical system which is embodiment of this invention. It is a figure which shows the outline | summary of the small area | region in the optical element which is another embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Optical base material, 2 ... Convex part, 3 ... Concave part, 5 ... Light source, 6 ... Collimator lens, 7 ... Condensing lens, 8 ... Optical element, 9 ... Projection lens, 10 ... Diffusing plate, 11 ... Concavity and convexity

Claims (5)

At least a portion where incident light is incident is divided into regions that are sufficiently small with respect to the area of the incident light, and each region functions as a wavelength plate that changes at least one of the polarization ellipticity and the direction of the polarization axis of the incident light. And each region is formed such that at least one of the ellipticity changed by the light incident on each region and the direction of the polarization axis is random as the entire emitted light. Optical element. 2. The optical element according to claim 1, wherein each of the regions has a grating structure finer than a wavelength of light to be used, and thereby has a function as a wavelength plate. 3. The optical element according to claim 2, wherein a grating structure having a function as an antireflection structure is formed on a surface opposite to the surface on which the grating structure providing a function as a wave plate is formed. . The optical element according to any one of claims 1 to 3, wherein the entrance surface and the exit surface viewed macroscopically are parallel to each other. An illumination optical system comprising the optical element according to any one of claims 1 to 4.

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