JP2006317806A - Multi-layer diffractive optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multi-layer diffractive optical element which has a structural configuration capable of making light-quantity distributions on the central part and outer peripheral part of the multi-layer diffractive optical element uniform, suppressing the degradation of NA and preventing the deterioration of a convergent spot by adjusting incident light uniformly in the multi-layer diffractive optical element. <P>SOLUTION: The multi-layer diffractive optical element 10A is provided with: a first diffractive grating 11 in which a plurality of phase areas RZ are set so as to gradually narrow the pitch toward the outer side from the center part on an optical substrate OBP having a light transmission property and stepwise diffractive grating parts 12a for diffracting diffracted light of specified orders for the incident light into respective phase areas RZ and converging the diffracted light into prescribed converged positions are formed in multi-layer; and a second diffractive grating 12 in which phase modulation area parts 12a having a predetermined retardation for diffracting diffracted light besides the specified orders for the incident light and adjusting the light transmittance for the incident light are respectively superposed into a plurality of blocks formed by dividing the first diffractive grating 11 two-dimensionally into m×n pieces and an occupancy area proportion for the first diffractive grating 11 of the phase modulation area parts 12a is made variable by every block in accordance with the required light transmittance. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  According to the present invention, a first diffraction grating for diffracting a specific order of diffracted light with respect to incident light and condensing it at a predetermined condensing position on an optical substrate having optical transparency is two-dimensionally m. A second diffraction grating that divides into x blocks and diffracts diffracted light other than a specific order with respect to incident light and adjusts light transmittance with respect to incident light in each block. The present invention relates to a multistage diffractive optical element in which the ratio of the occupied area of the second diffraction grating to the first diffraction grating is varied for each block in accordance with the required light transmittance.

  Recently, Fresnel lenses, multi-stage Fresnel lenses, multi-stage cylindrical lenses, multi-stage focus diffractive lenses, etc., which are a kind of diffractive optical element, can be used to set optical aberrations to small values by combining them with aspherical lenses. For this reason, it is frequently used in optical pickups for optical disk devices and optical systems for optical communication devices.

FIGS. 17A and 17B are a plan view, a longitudinal sectional view and a general fresnel lens,
FIG. 18 is a longitudinal sectional view showing a conventional multistage Fresnel lens,
19A to 19J are process diagrams showing a process of manufacturing a conventional multistage Fresnel lens;
FIG. 20 is a diagram schematically showing a state in which a conventional multistage Fresnel lens is irradiated with a laser beam having a high light intensity at the central portion and a low light intensity at the outer peripheral portion.

  As shown in FIGS. 17A and 17B, a general Fresnel lens 100 is formed on a disc-like optical substrate (Optical Base Plate) OBP using a glass substrate or a quartz substrate having light transmittance. In addition, a plurality of ring zone regions (Ring Zone) RZ draw a ring-shaped concentric circle with the center 0 as the center, and the ring zone pitch is gradually narrowed from the center 0 toward the outer peripheral side. In the annular zone RZ, the diffraction grating 101 is formed in a curved shape, and a lens function is provided by utilizing the diffraction effect of each diffraction grating 101 in each annular zone RZ.

  By the way, as described in Non-Patent Document 1 below, when the above-described Fresnel lens 100 is used as, for example, a diffractive optical element of a pickup of an optical disc apparatus, the element is reduced in size and the Fresnel lens 100 is used. In order to effectively use the beam power of the incident laser light, both high diffraction efficiency and high frequency efficiency are required. In order to realize these, the Fresnel lens uses the microfabrication technology of the semiconductor manufacturing process, and FIG. A multistage Fresnel lens 200 as shown is employed.

  That is, in the conventional multi-stage Fresnel lens 200 shown in FIG. 18, a plurality of annular zones RZ are formed from the center 0 toward the outer peripheral side on the disc-shaped optical substrate OBP having light transmittance, as described above. Although the annular pitch is set to be gradually narrowed, unlike the above, the stepped diffraction grating portion 201 gradually reduces the number of steps in the annular zone RZ toward the outer peripheral side, for example, 8 It is formed in multiple stages.

  The structure of the multi-stage Fresnel lens 200 is made to diffract a first-order diffracted light as a specific-order diffracted light at a predetermined condensing position, for example. The depth per step corresponds to an optical length of 1 / 8λ of the wavelength λ of the laser beam used.

  At this time, when the multi-stage Fresnel lens 200 is manufactured, as will be described later, the staircase is formed by performing an etching process using the upper surface of the optical substrate OBP as a reference surface, so that the deepest part of the staircase has seven steps from the upper surface. The depth corresponds to an optical length of 7 / 8λ of the wavelength used. Further, 95.3% of the diffracted light intensity diffracted by the stepped diffraction grating portion 201 formed in the plurality of annular zones RZ is concentrated on the first-order diffracted light. In other words, the first-order diffracted light diffracted by the stepped diffraction grating portion 201 formed in the plurality of annular zones RZ is condensed at a predetermined condensing position. By changing the phase stepwise, high diffraction efficiency can be obtained.

According to the following Non-Patent Document 1, in a multistage Fresnel lens, when the multistage structure of the stepped diffraction grating is set to 2, 4, 8, and 16 stages, the diffraction efficiency is 39.4, respectively. %, 81.0%, 95.3%, and 98.9%, which indicate that the diffraction efficiency increases as the number of steps is increased.
Design and fabrication technology for diffractive optical elements.

  At this time, a method for producing a multistage Fresnel lens is produced by applying a semiconductor process to an optical substrate such as a glass substrate or a quartz substrate.

  More specifically, as shown in FIGS. 19A to 19J, when the step-like diffraction grating has a four-stage structure, a two-stage mask as shown in FIG. 19A. Using pattern M1, as shown in FIGS. 19B to 19E, resist coating, mask alignment, exposure, development, etching, and resist removal processes are performed to produce a two-stage diffraction grating first. To do.

  In this case, in FIG. 19, (a) is the mask pattern M1 for two steps as described above, (b) is the resist coating on the optical substrate OBP, (c) is the exposure and development after exposure and development with the mask pattern mask M1. The resist pattern, (d) shows the substrate shape after etching the substrate with the resist, and (e) shows the substrate shape after removing the resist.

  Thereafter, using a four-step mask pattern M2 as shown in FIG. 19 (f), the resist is applied again in substantially the same process as shown in FIGS. 19 (g) to (j). A mask alignment, exposure, development, etching, resist removal process is performed to produce a four-stage diffraction grating.

In general, an N square multi-stage structure can be formed by N masks and the above procedure.

  By the way, a semiconductor laser used for a light source such as an optical pickup has a light intensity distribution. As shown in FIG. 20, the laser light L emitted from the semiconductor laser is not shown in the conventional multistage Fresnel lens 200. In general, the central portion of the light beam of the laser light L having a substantially circular cross-sectional area when sectioned at an appropriate position has a high light intensity (the light intensity is small). Large), and the light flux at the outer peripheral portion tends to have low light intensity (small amount of light). When the light beam of the laser light L having such a non-uniform light intensity distribution (light quantity distribution) is incident on the conventional multistage Fresnel lens 200 and condensed, the light intensity at the outer periphery of the lens is compared with the central part. Therefore, the phenomenon becomes equivalent to a state in which the aperture of the lens is reduced. For this reason, the numerical aperture (NA) of the lens is lowered, and the phenomenon that the condensing spot becomes larger than that of the original lens design occurs, which causes a problem of deterioration of optical characteristics.

  Therefore, in the present invention, the multistage diffractive optical element that condenses the laser light has a function of adjusting the light intensity distribution (light quantity distribution) of the laser light source in addition to the condensing function. By uniformly adjusting the incident light, the light intensity distribution at the center and outer periphery of the multi-stage diffractive optical element is made uniform, the above-mentioned NA reduction is suppressed, and the deterioration of the focused spot can be prevented. Multistage diffractive optical elements are desired.

The present invention has been made in view of the above problems, and in the invention described in claim 1, a plurality of phase regions are set on a light-transmitting optical substrate by gradually narrowing the pitch from the center to the outside. In addition, a first diffraction in which a stepped diffraction grating portion for diffracting a specific order of diffracted light with respect to incident light and condensing it at a predetermined condensing position in each phase region is formed in multiple stages. Lattice,
In order to diffract diffracted light of a specific order with respect to the incident light and to adjust light transmittance with respect to the incident light, a phase modulation region portion having a predetermined phase difference serves as the first diffraction grating. Each block is superposed in a plurality of blocks that are two-dimensionally divided into m × n, and the ratio of the area occupied by the phase modulation region portion to the first diffraction grating is required for each block according to the light transmittance. A second diffraction grating made variable to
A multi-stage diffractive optical element.

The invention according to claim 2 is the multistage diffractive optical element according to claim 1,
The phase modulation region portion of the second diffraction grating is formed in a rectangular shape in each block or in a line shape toward one direction in each block. Type diffractive optical element.

  According to the multistage diffractive optical element according to the present invention, the first order for diffracting a specific order of diffracted light with respect to incident light and condensing it at a predetermined condensing position on an optical substrate having optical transparency. The diffraction grating is two-dimensionally divided into m × n blocks, and diffracted light with a specific order other than a specific order is diffracted in each block and the light transmittance is adjusted for the incident light. The second diffraction grating for the first diffraction grating is overlapped, and the occupation area ratio of the second diffraction grating to the first diffraction grating is varied for each block according to the required light transmittance. When the element is applied to an optical pickup, for example, even if the incident light incident on the multistage diffractive optical element has a light amount distribution, the light amount distribution is corrected uniformly with respect to the incident light, and the incident light is Since the light can be condensed at the condensing position, It is possible to obtain a light-collecting spot with no reduction.

  Of course, an arbitrary light transmittance can be set according to the characteristics of the optical system combined with the multistage diffractive optical element, and an arbitrary light transmittance distribution can be obtained.

  Hereinafter, an example of a multistage diffractive optical element according to the present invention will be described in detail in the order of Example 1 to Example 3 with reference to FIGS.

  The multistage diffractive optical element according to the present invention is formed in a lens shape such as a multistage Fresnel lens, a multistage cylindrical lens, or a multistage focus diffractive lens. For example, the optical pickup of an optical disc apparatus or the optical of an optical communication apparatus Applied to the system.

  At this time, when the multistage diffractive optical element according to the present invention is applied to, for example, an optical pickup of an optical disc apparatus, it is used in combination with an aspheric lens, and the nonuniformity emitted from the laser light source in the optical pickup is used. When a light beam having a light intensity distribution (light quantity distribution) is incident on a multistage diffractive optical element, the multistage diffractive optical element is corrected to a light beam having a uniform light intensity distribution (light quantity distribution) and emitted. Has been.

  That is, the multistage diffractive optical element according to the present invention is a first for diffracting a specific order of diffracted light with respect to incident light and condensing it at a predetermined condensing position on an optical substrate having optical transparency. The diffraction grating is divided into m × n blocks in a two-dimensional manner, and diffracted light of a specific order is diffracted with respect to the incident light and the light transmittance is adjusted with respect to the incident light. The second diffraction grating is overlapped, and the ratio of the occupied area of the second diffraction grating to the first diffraction grating is varied for each block according to the required light transmittance. is there.

  In Examples 1 to 3 below, the first diffraction grating is formed as a primary structure that condenses the first-order diffracted light as a specific-order diffracted light at a predetermined condensing position, whereas the second diffraction grating The case where the grating diffracts diffracted light other than the first-order diffracted light will be described. However, depending on the design specifications of the multi-stage diffractive optical element, a second-order structure, a third-order structure,... ... but it is possible.

  FIG. 1 is a top view showing a quarter of the entire multistage Fresnel lens when the multistage diffractive optical element of Example 1 according to the present invention is applied to a multistage Fresnel lens.

  As shown in FIG. 1, the multi-stage diffractive optical element 10A according to the first embodiment of the present invention is applied to a multi-stage Fresnel lens. Hereinafter, the multi-stage Fresnel lens 10A will be referred to as a multi-stage Fresnel lens 10A.

  The multi-stage Fresnel lens 10A described above is a ring-shaped phase region on a disk-shaped optical substrate using a light-transmitting glass substrate or quartz substrate, and the pitch is gradually narrowed from the center toward the outer periphery. A plurality of RZs (hereinafter referred to as annular zones) are set, and for example, a first-order diffracted light is diffracted as a diffracted light of a specific order with respect to incident light respectively incident on the plurality of annular zones RZ. A first diffraction grating 11 having a multistage stepped diffraction grating portion 11a is formed for condensing light at a condensing position (focal position), and in the optical substrate on which the first diffraction grating 11 is formed. 7 is divided into m × n blocks two-dimensionally as shown in FIG. 7 to be described later, the incident light is diffracted into a diffraction order different from that of the first diffraction grating 11 in each block, and the incident light is converted into the incident light. To adjust the light transmittance for A second diffraction grating 12 having a square (rectangular) phase modulation area portion 12a having a constant phase difference is superimposed, and the phase modulation area portion 12a of the second diffraction grating 12 is a first diffraction grating. The occupied area ratio with respect to the grating 11 is varied for each block in accordance with the required transmittance.

  At this time, on the disc-shaped optical substrate, the plurality of annular zones RZ are set so as to be gradually narrowed from the center toward the outer peripheral side, so that they enter the respective annular zones RZ in parallel. Incident light has its incident angle changed according to the stepped diffraction grating portion 11a formed at each pitch width, so that the refraction angle is different according to this, so that it can be condensed at a predetermined condensing position (focal position). it can.

  Further, the multistage Fresnel lens 10A described above has a radius of 775 μm, and in the figure, displays a quarter region of the entire multistage Fresnel lens. Further, when the focal length of the multistage Fresnel lens 10A is set to 20 mm and a laser beam having a light source wavelength λ of 0.4 μm is used, the phase distribution displays 2π corresponding to one wavelength in 8 levels at the gray level. ing.

  Here, before describing the multistage Fresnel lens 10A described above in detail, an optical system when the multistage Fresnel lens 10A is applied will be described with reference to FIGS.

FIG. 2 is an optical path diagram for explaining an optical path of an optical system when the multistage diffractive optical element according to Example 1 of the present invention is applied to a multistage Fresnel lens;
FIG. 3 shows the light intensity distribution and light transmittance of the optical system corresponding to the positions X1 to X3 in FIG. 2 when the multistage diffractive optical element of Example 1 according to the present invention is applied to a multistage Fresnel lens. Figure,
FIGS. 4A to 4C are perspective views showing a three-dimensional distribution of the amount of incident light on the multistage Fresnel lens when the multistage diffractive optical element of Example 1 according to the present invention is applied to the multistage Fresnel lens. ,
FIGS. 5A and 5B three-dimensionally show a case where the light quantity distribution is corrected on the multistage Fresnel lens when the multistage diffractive optical element of Example 1 according to the present invention is applied to the multistage Fresnel lens. FIG.

  As shown in FIG. 2, when using the multistage Fresnel lens 10A of Example 1, the position X1 is a position corresponding to the incident light, the position X2 is a position corresponding to the multistage Fresnel lens 10A, When the position X3 is a position corresponding to the emitted light and the position X4 is a condensed position of the emitted light, the laser light L emitted from the laser light source (not shown) is paralleled by a collimator lens (not shown). The parallel light of the laser light L having a substantially circular cross-section when converted into light and cross-sectioned at an appropriate position is incident from one surface of the multistage Fresnel lens 10A, and the first and first light beams are incident on the other surface. When the light is diffracted by the two diffraction gratings 11 and 12 and then emitted, in particular, the first-order diffracted light diffracted by the first diffraction grating 11 is on the optical axis K at a predetermined condensing position corresponding to the focal length f1. Spot It is light, and, first diffraction light other than order diffracted light diffracted by the second diffraction grating 12 is adapted to be diffracted in different diffraction position and the first diffraction grating 11.

  Here, as indicated by the dotted line in FIG. 3, when the parallel light of the laser light L having a substantially circular cross section is incident on the multistage Fresnel lens 10A, the incident light flux at the position X1 shown in FIG. Assuming that the light intensity distribution is symmetric about the optical axis K, the light intensity at the lens radii of 500 μm, 1000 μm, and 1500 μm, respectively, is 100% due to the characteristics of the semiconductor laser. 97.5%, 90.0%, and 77.5%, which are convex light intensity distribution characteristics with the central portion protruding in a convex shape.

  In this way, when the incident light beam having a convex light intensity distribution at the center is condensed by the conventional multistage Fresnel lens 200 as described above with reference to FIG. Since it is smaller than the lens portion, it becomes a phenomenon equivalent to a state in which the aperture of the lens is reduced, the numerical aperture (NA) of the lens is lowered, and the focused spot becomes larger than the original lens design. Such a phenomenon occurs, and the optical characteristics deteriorate.

  In order to solve this problem, in the light transmittance distribution of the light beam on the multistage Fresnel lens 10A at the position X2 in FIG. 2, the lens radii are 0 μm and 500 μm as shown by the solid line in FIG. , 1000 μm, 1500 μm, the ratio of the occupied area of the second diffraction grating 12 to the first diffraction grating 11 so that the light transmittance is 77.5%, 79.5%, 86.1%, 100%, respectively. , I.e., inverted with respect to the convex light intensity distribution characteristic at the position X1, and reversely corrected so as to have a concave light transmittance distribution characteristic in which the central portion is concavely recessed at the position X2.

  With this setting, when the incident light is transmitted through the multistage Fresnel lens 10A and emitted, the light intensity distribution of the emitted light at the position X3 shown in FIG. 2 is shown by using a one-dot chain line in FIG. The product of the amount of incident light and the light transmittance of the multistage Fresnel lens results in flatness.

  As a result, the amount of light at the outer periphery of the multi-stage Fresnel lens 10A is substantially equal to that at the center, which is equivalent to a state in which the aperture of the lens does not change, the numerical aperture (NA) of the lens does not change, and the condensing spot. However, the phenomenon that becomes larger than the original lens design does not occur.

  However, when the light intensity distribution of the emitted light becomes flat, the total amount of light decreases. However, if a light source with a larger amount of light such as a high-intensity laser is used, the decrease in the amount of light can be prevented.

  Here, when the light quantity distribution of the incident light beam to the multistage Fresnel lens 10A is three-dimensionally illustrated using FIGS. 4A to 4C, the X axis and Y axis in FIG. The distance from FIG. 2) indicates the light intensity.

  At this time, the state shown in FIG. 4A is an example in which the light amount distribution of the incident light beam to the multistage Fresnel lens 10A is assumed to be symmetrical with respect to the optical axis K (FIG. 2), that is, the Z axis. Although it is rotationally symmetric, in an actual optical system, depending on the beam collimation, beam shaping conditions, or the characteristics of the optical components to be combined, the light quantity distribution is not Z-axis rotationally symmetric and is shown in FIGS. 4B and 4C. In some cases, the distribution is asymmetric in the X-axis direction and the Y-axis direction. FIG. 4C shows the intensity distribution of FIG. 4B rotated by 90 degrees.

  Then, as shown in FIG. 4B, when the light quantity distribution of the incident light beam is Z-axis rotationally asymmetric, the light transmittance distribution on the multistage Fresnel lens 10A is shown in FIG. Correction is made to the inverted version of 4 (b). With this setting, the light quantity distribution of the incident light is the product of the incident light quantity distribution and the multistage Fresnel lens light transmittance distribution on the multistage Fresnel lens 10A. As a result, as shown in FIG. The emitted light quantity distribution from the mold Fresnel lens 10A is flat.

  Next, in the multi-stage Fresnel lens 10A, the first diffraction grating 11 and the second diffraction grating 12 superimposed on the first diffraction grating 11 will be described in detail with reference to FIGS.

6A and 6B are a plan view, a cross-sectional view, and a cross-sectional view showing the first diffraction grating in the multistage Fresnel lens,
FIG. 7 is a plan view showing a state in which the first diffraction grating shown in FIG. 6 is divided into m × n blocks;
8A and 8B are views for explaining a second diffraction grating formed in a block obtained by dividing the first diffraction grating,
FIG. 9 is a diagram showing the relationship between the occupied area ratio when the second diffraction grating is superimposed in the block of the first diffraction grating and the light transmittance of the 0th-order light by the second diffraction grating;
FIG. 10 is an enlarged cross-sectional view of the first diffraction grating,
FIG. 11 is an enlarged sectional view of the second diffraction grating,
FIG. 12 is an enlarged cross-sectional view showing a state in which the second diffraction grating is optically superimposed on the first diffraction grating.

As shown in FIGS. 6A and 6B, the first diffraction grating 11 in the multi-stage Fresnel lens 10A (FIG. 1) is a disk-like shape using a light-transmitting glass substrate or quartz substrate. On the optical substrate (Optical Base Plate) OBP, the ring zone region RZ draws a ring-shaped concentric circle with the center 0 as the center, and the ring zone pitch Rp _n from the center 0 toward the outer peripheral side (where n = 0 or more) A plurality of positive integers are gradually narrowed.

  Further, in the plurality of annular zones RZ formed in the first diffraction grating 11, for example, the first-order diffracted light is diffracted as a specific order diffracted light with respect to the incident light, and passes through the center 0 of the optical substrate OBP. A stair-like diffraction grating portion 11a for condensing light at a predetermined condensing position (focal position) on the optical axis K is formed in multiple stages by gradually reducing the number of steps in the staircase toward the outer peripheral side.

  At this time, the stepped diffraction grating portion 11a formed in the first diffraction grating 11 is set to a diffraction grating having eight steps of the first order diffraction structure by setting, for example, eight steps.

In the first diffraction grating 11, when the plurality of annular zones RZ are set as RZ ₀ , RZ ₁ , RZ ₂ ,..., RZ _{n in} order from the center 0 of the optical substrate OBP to the outer peripheral side. firstly, n (where, n: 0 or a positive integer) around the center 0 of the optical substrate OBP a-th lens corresponding to ring zones RZ _n radius (zonal radius) R _n from the number 1 of the following Can be sought.

In this number 1, R _n is the n-th lens radius, lambda is the wavelength of the incident light, the wavelength lambda is using a laser beam of 0.4μm as the incident light. Further, f1 is a focal length of the first-order diffracted light by the first diffraction grating 11, and in this Example 1, f1 = 20 mm is set. Further, d is the diffraction order, and d = 1 because the first-order diffracted light from the first diffraction grating 11 is targeted in the first embodiment.

Therefore, lens radii R ₀ , R ₁ , R ₂ ,..., R _n are obtained corresponding to RZ ₀ , RZ ₁ , RZ ₂ ,..., RZ _n with the center 0 of the optical substrate OBP as the center.

Next, in the case of n = 0 described above, the annular zone pitch Rp ₀ of the annular zone RZ ₀ including the center 0 of the optical substrate OBP can be obtained from the following formula 2.

Then, to the exclusion of the case of n = 0 as described above, n-th ring zone pitch Rp _n can be determined from the number 3 below.

Therefore, although only the central annular zone RZ ₀ is set to be circular with the annular zone pitch Rp ₀ as the radius from the _equations 2 and 3, the annular zones RZ ₁ , RZ ₂ ,..., RZ _n band pitch _{Rp 1, Rp 2, ......,} is set in a ring shape at the pitch of Rp _n.

In the first diffraction grating 11, when the maximum value of n corresponding to the maximum value of the lens radius is set to 140, for example, λ = 0.4 μm, f1 while sequentially changing n in the range of 0 to 140. = 20 mm, d = 1 is substituted into Equation 1, and the lens radius R _n is calculated. Thereafter, according to Equations 2 and 3, for example, the lens radius R _n is near 0 μm including the center of the optical substrate OBP, 500 μm. , 1000 .mu.m, when obtaining each ring-shaped zone pitch Rp _n in _{1500μm, 126.5μm (Rp 0),} 16.1μm (Rp 15), 8.0μm (Rp 62), 5.4μm (Rp 140) is obtained.

In the case of designing the first diffraction grating 11 may be determined for all lenses corresponding to the ring zones _RZ n (n = _0~140) radius _{R n} and annular pitch Rp _n.

Next, in the first diffraction grating 11, the depth s per step of the stepped diffraction grating portion 11a formed in multiple stages can be generally obtained from the following equation (4).

  In Equation 4, s is the depth per step in the stepped diffraction grating portion 11a, λ is the wavelength of the light source, d is the diffraction order, an integer of 1 or more, k is the refractive index of the optical substrate OBP, v Is the number of steps.

  In the first embodiment, the multistage step-like diffraction grating portion 11a formed on the first diffraction grating 11 uses a diffraction grating having an eight-order first-order diffraction structure, so that λ is 0.40 μm, d is 1, When k is 1.46 and v is 8, the depth s per step is 0.109 μm.

  As described above, the method for forming the eight-step staircase diffraction grating portion 11a uses a semiconductor process, and uses at least three mask patterns for an optical substrate OBP such as a glass substrate or a quartz substrate. This can be realized by repeatedly performing resist coating, mask alignment, exposure, development, etching, and resist removal processes.

  Then, as shown in FIG. 7, the annular zone RZ of the first diffraction grating 11 formed as described above is two-dimensionally divided into m × n blocks BL. In the first embodiment, since the first diffraction grating 11 adopts an eight-value first-order diffraction structure, the phase distribution displays 2π corresponding to one wavelength in eight levels at the gray level, and Since the outer shape of one diffraction grating 11 is formed in a circular shape, each block BL is divided into a square shape if m = n when substantially dividing into m × n blocks. Become.

  Then, in each block BL obtained by dividing the first diffraction grating 11 into m × n, as shown in FIGS. 8A and 8B, the second diffraction grating 12 corresponds to the use wavelength λ. For example, it is superimposed in the shape of a square phase modulation region 12a having a predetermined phase difference of λ / 4 (0.5π) or the second phase as shown in FIGS. 8 (c) and 8 (d). The diffraction grating 12 is superimposed in the shape of a line-shaped phase modulation region portion 12b having a predetermined phase difference of λ / 4 (0.5π), for example, with respect to the use wavelength λ, but is not limited thereto. In addition, the phase modulation region portion of the second diffraction grating 12 may have an arbitrary shape, or may be set to an arbitrary phase difference.

  Here, when designing the second diffraction grating 12 to be superimposed in each block BL of the first diffraction grating 11, the incident light distribution is measured for each divided block BL or obtained by simulation. When the maximum value in the entire incident light distribution is set to 1, matrix data of the incident light distribution I (m, n) is generated for the block BL in the m-th row and the n-th column. In this case, m and n are natural numbers, and the distribution uniformity improves as the number of m and n increases.

Then, from the minimum value I (min) in the entire incident light distribution with respect to the incident light distribution I (m, n) of m rows and n columns, the required light transmittance T (m, n) for each block BL is expressed by the following equation (5). Seeking more.

  At this time, the required light transmittance T (m, n) in each block BL of the second diffraction grating 12 is determined by the area ratio of each phase modulation region portion 12a (or 12b) to the first diffraction grating 11. This does not depend on the shape of each phase modulation region 12a (or 12b).

  It is generally known that the intensity and diffraction efficiency in the diffraction order of incident light can be calculated by Fourier transforming the phase distribution of the second diffraction grating 12.

  Specifically, as shown in FIG. 9, when the second diffraction grating 12 is superimposed on the first diffraction grating block, the light transmittance is calculated by Fourier transform, and the first axis is plotted on the horizontal axis. The ratio of the area occupied by the second diffraction grating relative to the diffraction grating is shown, and the vertical axis shows the light transmittance of the 0th-order light by the second diffraction grating.

  From FIG. 9, when the ratio of the area occupied by the second diffraction grating 12 with respect to the first diffraction grating 11 increases, the light transmittance of the 0th-order light by the second diffraction grating 12 decreases. Therefore, by using the numerical table shown in FIG. 9, the required light transmittance to the second diffraction grating 12 and the occupied area ratio of the phase modulation region 12a (or 12b) can be calculated.

  At this time, in the second diffraction grating 12 in the first embodiment, as shown in FIG. 1, the square phase modulation region portion 12 a is optically superimposed in the block of the first diffraction grating 11.

  Further, the light quantity distribution of the incident light beam to the multi-stage Fresnel lens 10A is not symmetrical with respect to the Z-axis rotation, but becomes an asymmetric distribution in the X-axis direction and the Y-axis direction as previously shown in FIG. Assuming that the required light transmittance of the square phase modulation region 12a is obtained with respect to the second diffraction grating 12, the occupation area ratio of the phase modulation region 12a is varied for each block. As a result, the light transmittance distribution on the multistage Fresnel lens 10A is corrected to the inverted light amount distribution as shown in FIG. By this correction, the light quantity distribution of the incident light becomes the product of the incident light quantity distribution and the multistage cylindrical lens light transmittance distribution on the multistage Fresnel lens 10A, and as a result, as shown in FIG. The distribution of the quantity of light emitted from the multistage Fresnel lens 10A is flat.

  More specifically, FIGS. 10 to 12 show the first diffraction grating 11, the second diffraction grating 12, and the second diffraction grating 12 superimposed in the first diffraction grating 12, respectively. In these drawings, the horizontal axis indicates the lens radial direction of the multistage Fresnel lens, and the vertical axis indicates the phase difference (λ) as 0 on the upper surface side of the optical substrate and toward the lower surface side. 1.0.

  Here, in the multistage Fresnel lens 10 </ b> A (FIG. 1) of Example 1, as shown in FIG. 10, the first diffraction grating 11 disposed over the entire zone region RZ (FIG. 1) is , Only the first-order diffracted light is diffracted with respect to the incident light, and the first-order diffracted light has a function of condensing at a predetermined condensing position on the optical axis K (FIG. 1). The number of steps of the stepped diffraction grating portion 11a formed in multiple steps by gradually decreasing the number of steps toward the staircase is set to eight, and the total height of the stepped diffraction grating portion 11a is the incident light. Since the height is set to one wavelength λ (λ = 0.4 μm), the height of the steps per step corresponds to an optical length of 1 / 8λ.

  On the other hand, as shown in FIG. 11, the second diffraction grating 12 superimposed in each block BL (FIGS. 7 and 8) obtained by dividing the first diffraction grating 11 into m × n is A function of diffracting diffracted light other than the first-order diffracted light, and a function of adjusting the light transmittance with respect to incident light, and using the upper surface of the optical substrate OBP (FIG. 6) as a reference surface When etched, the flatly etched square phase modulation region 12a has a binary structure in which the phase difference is set to ¼ half wavelength (λ / 4). The phase modulation area unit 12 a performs phase modulation on the first diffraction grating 11.

  Then, as shown in FIG. 12, with respect to the phase distribution by the stepped diffraction grating 11a of the first diffraction grating 11 shown in FIG. 10 within each block BL (FIGS. 7 and 8) of the first diffraction grating 11. The final structural form is obtained by optically superimposing the phase distribution by the square phase modulation region portion 12a formed on the second diffraction grating 12 shown in FIG. That is, the superposition of the phase distribution is specifically the sum of the superposition of the phase distribution of FIG. 10 and the phase distribution of FIG.

  At this time, since the optical substrate OBP (FIG. 6) has a thickness, the second diffraction grating 12 optically superimposed on the stepped diffraction grating portion 11a of the first diffraction grating 11 has the first The stepped diffraction grating portion 11a of the diffraction grating 11 can be arranged at an appropriate position in the lens radial direction, and even if the second diffraction grating 12 is arranged at an appropriate position as described above, the operational effect does not change. .

  Further, in the production of the multistage Fresnel lens 10A described later, the square phase modulation region portion 12a formed on the second diffraction grating 12 is etched at a depth of λ / 4 from the production of the first diffraction grating 11. In view of the processing, the phase modulation area 12a in FIG. 12 is not completely flat unlike the single case shown in FIG.

  At this time, in Example 1, as shown in FIG. 10, the first diffraction grating 11 has an eight-order diffraction grating of the first order diffraction structure, and as shown in FIG. Although a value structure example is shown, the first and second diffraction gratings 11 and 12 are not limited to this diffraction structure. Further, the first and second diffraction gratings 11 and 12 can have the same function even in higher-order diffraction structures such as second-order and third-order.

  Further, in Example 1, in order to make the multistage Fresnel lens 10A function as a convex lens, a plurality of annular zones RZ (in which the pitch is gradually narrowed from the central portion of the first diffraction grating 11 toward the outer peripheral side) In FIG. 1), the stepped diffraction grating 11a is lowered toward the outer peripheral side to increase the phase difference, but the present invention is not limited to this. In order to make the multistage Fresnel lens 10A function as a concave lens, The stair-like diffraction grating 11a is lowered toward the inner peripheral side in a plurality of annular zones RZ (FIG. 1) set with the pitch gradually narrowed from the center of one diffraction grating 11 toward the outer peripheral side. In some cases, the phase difference is increased.

  Further, the multi-stage Fresnel lens 10A is used not only for correcting the light intensity distribution of the emitted light flat at the X3 position in FIG. 3 as in the case of application to an optical pickup, but also for the multi-stage Fresnel lens 10A. On the other hand, an arbitrary light transmittance can be set in accordance with the characteristics of the optical system to be combined, and an arbitrary light transmittance distribution can be obtained.

  From the above, in FIG. 1 described above, since the radius of the first diffraction grating 11 in the multistage Fresnel lens 10A is set to 775 μm as described above, the entire first diffraction grating 11 is, for example, 31 × The block is divided into 31 square blocks. In other words, after dividing one block into an area of 50 μm square, the second diffraction grating 12 is formed in the central portion of each block of the square phase modulation region portion 12a. The phase modulation region portion 12a is made variable for each block in accordance with the required light transmittance.

  Further, as described above, the light amount distribution of the incident light beam on the multi-stage Fresnel lens 10A is not Z-axis rotationally symmetric, but is asymmetric in the X-axis direction and Y-axis direction as previously shown in FIG. When the distribution is set to be a uniform distribution, as shown in FIG. 1, the area occupied by the second diffraction grating 12 is large in the central part of the Fresnel lens and the outer peripheral area around the X axis designed to have a small light transmittance. On the other hand, the area occupied by the second diffraction grating 12 is small in the Y-axis peripheral region of the Fresnel lens outer peripheral portion designed to have a large light transmittance.

  As a result, when the multistage Fresnel lens 10A of Example 1 is applied to, for example, an optical pickup, even if the incident light incident on the multistage Fresnel lens 10A has a light quantity distribution, the light quantity distribution is made uniform with respect to the incident light. Since the incident light can be condensed at a predetermined condensing position by correcting, a condensing spot without deterioration can be obtained.

  Next, a method for producing the multistage Fresnel lens 10A of Example 1 according to the present invention will be described with reference to FIG.

  FIGS. 13A to 13J are process diagrams showing a process for manufacturing the multistage Fresnel lens of Example 1 according to the present invention.

  Since the multistage Fresnel lens 10A of Example 1 has an eight-stage first diffraction grating, the multistage Fresnel lens having the four-stage structure described above with reference to FIGS. On the other hand, using an eight-step mask pattern M3 as shown in FIG. 13A, resist application, mask alignment, exposure, development, and etching are performed as shown in FIGS. 13B to 13E. First, a first diffraction grating having an eight-stage structure is manufactured by performing a resist removal process.

  At this time, in FIG. 13, (a) is the mask pattern M3 for eight steps as described above, (b) is the resist coating on the optical substrate OBP, (c) is the exposure and development after the mask pattern mask M3. The resist pattern, (d) shows the substrate shape after etching the substrate with the resist, and (e) shows the substrate shape after removing the resist.

  After that, as shown in FIG. 13F, using the mask pattern M4 corresponding to the phase modulation region portion of the second diffraction grating, as shown in FIGS. Then, a resist coating, mask alignment, exposure, development, etching, and resist removal processes are performed to produce a multistage Fresnel lens in which the second diffraction grating is optically superimposed on the first diffraction grating.

  More specifically, FIG. 13F shows a mask pattern M4 corresponding to the phase modulation area portion of the second diffraction grating. FIG. 13G shows the first diffraction grating produced in the above-described process coated with a resist. The resist-coated substrate is exposed to the phase modulation region of the second diffraction grating by the mask pattern M4. And develop the resist. FIG. 13 (h) shows the resist shape remaining after development with respect to FIG. 13 (g). Etching is performed using this resist as a mask to obtain the etched shape as shown in FIG. Further, as shown in FIG. 13J, the resist is peeled off to obtain a multistage Fresnel lens in which the phase of the second diffraction grating is optically superimposed on the first diffraction grating.

  Here, since the first diffraction grating uses a first-order diffraction structure eight-stage diffraction grating, the step depth per step corresponds to an optical length of 1 / 8λ. In the second diffraction grating, the phase modulation amount of the phase modulation region is λ / 4, and the diffraction grating has a binary structure.

  As described above, the first diffraction grating having a condensing function as a lens is manufactured in advance, and then the second diffraction grating is manufactured. For example, the light source is changed and different illumination lights are used. When having an intensity distribution, the second diffraction grating is designed corresponding to the intensity distribution, and a mask pattern only for this phase modulation region is prepared. In the manufacturing steps (f) to (j) described above, A multistage Fresnel lens can be obtained.

  In Example 1, when etching the eight steps, the three mask patterns are used to perform the exposure and etching processes three times. However, the present invention is not limited to this. For example, Eight steps can be obtained by preparing seven masks and performing etching seven times.

  Next, a modified multi-stage Fresnel lens obtained by partially modifying the multi-stage Fresnel lens of Example 1 according to the present invention will be briefly described with reference to FIG.

  FIG. 14 is a top view showing a quarter of the entire multi-stage Fresnel lens according to a modification obtained by partially deforming the multi-stage Fresnel lens according to the first embodiment of the present invention.

  As shown in FIG. 14, a modified multi-stage Fresnel lens 10A ′ obtained by partially modifying the multi-stage Fresnel lens of Example 1 according to the present invention is a multi-stage Fresnel lens 10A of Example 1 described above. On the other hand, only the shape of the second diffraction grating 12 is different, and different points from the first embodiment will be mainly described.

  The multi-stage Fresnel lens 10A ′ according to the above-described modified example is formed on a disk-shaped optical substrate using a light-transmitting glass substrate or quartz substrate, with the pitch gradually narrowed from the center toward the outer periphery. A plurality of band-shaped phase regions are set, and for example, a first-order diffracted light is diffracted as a diffracted light of a specific order with respect to incident light respectively incident into the plurality of annular regions, and a predetermined condensing position (focal position) A first diffraction grating 11 having a multi-step staircase diffraction grating portion 11a is formed in order to collect light on the optical substrate, and the inside of the optical substrate on which the first diffraction grating 11 is formed is shown in FIG. As described above, when divided into m × n blocks two-dimensionally, incident light is diffracted into a diffraction order different from that of the first diffraction grating 11 in each block, and light transmittance is increased with respect to the incident light. A laser with a predetermined phase difference to adjust The second diffraction grating 12 having the N-shaped phase modulation region portion 12 b is superimposed, and the phase modulation region portion 12 b of the second diffraction grating 12 has a transmission area that requires an occupied area ratio with respect to the first diffraction grating 11. It is variable for each block according to the rate.

  That is, the multi-stage Fresnel lens 10A ′ of this modification also uses laser light having a radius of 775 μm, a focal length of 20 mm, and a light source wavelength λ of 0.4 μm, as in the first embodiment. In some cases, the phase distribution displays 2π corresponding to one wavelength in 8 levels on a gray level.

  Then, the entire first diffraction grating 11 is divided into, for example, 31 × 31 square blocks in the multistage Fresnel lens 10A ′. In other words, after dividing one block into an area of 50 μm square, In the second diffraction grating 12, a line-shaped phase modulation region portion 12 b is superposed in a central direction of each block in one direction (Y-axis direction) and connected to each block, and this phase modulation region portion 12 b is necessary. The light transmittance is varied for each block according to the light transmittance.

  In this modification as well, the light quantity distribution of the incident light beam on the multi-stage Fresnel lens 10A ′ is not Z-axis rotationally symmetric, but as previously shown in FIG. 4B, the X-axis direction and the Y-axis direction. , The area occupied by the second diffraction grating 12 with respect to the central part of the Fresnel lens and the outer peripheral area around the X axis designed to have a small light transmittance as shown in FIG. However, in the area around the Y-axis periphery of the Fresnel lens designed to have a large light transmittance, the area occupied by the second diffraction grating 12 is small, so the line width is small.

  As a result, when the modified multi-stage Fresnel lens 10A ′ is applied to, for example, an optical pickup, even if the incident light incident on the multi-stage Fresnel lens 10A ′ has a light quantity distribution, On the other hand, since the light quantity distribution can be uniformly corrected and the incident light can be condensed at a predetermined condensing position, it is possible to obtain a condensing spot without deterioration.

  FIG. 15 is a top view showing the entire multi-stage cylindrical lens when the multi-stage diffractive optical element according to Example 2 of the present invention is applied to the multi-stage cylindrical lens.

  As shown in FIG. 15, the multistage diffractive optical element 10B according to the second embodiment of the present invention is applied to a multistage cylindrical lens, and will be hereinafter referred to as a multistage cylindrical lens 10B.

  This multi-stage cylindrical lens 10B is a so-called kamaboko type lens that has a condensing function only in the X-axis direction of the light incident surface and does not have a condensing function in the Y-axis direction orthogonal to the X-axis.

  Accordingly, the multistage cylindrical lens 10B of Example 2 is gradually formed on the rectangular optical substrate using a light-transmitting glass substrate or quartz substrate from the center to the outside in the X-axis direction. A plurality of line-shaped phase regions (hereinafter referred to as “line-shaped regions”) LZ are set with the pitch narrowed, and the incident light incident on each of the plurality of line-shaped regions LZ is diffracted light of a specific order. For example, a first diffraction grating 11 having a multistage stepped diffraction grating portion 11a is formed in order to diffract the first-order diffracted light and collect it at a predetermined condensing position (focal position). When the inside of the optical substrate on which the diffraction grating 11 is formed is divided into m × n blocks two-dimensionally as shown in FIG. 7, a different diffraction order from that of the first diffraction grating 11 is obtained in each block. Diffracts incident light In addition, a second diffraction grating 12 having a square phase modulation region portion 12a having a predetermined phase difference for adjusting light transmittance with respect to incident light is superimposed, and the second diffraction grating 12 is also superimposed. In the phase modulation region portion 12a, the occupied area ratio with respect to the first diffraction grating 11 is varied for each block according to the required transmittance.

  Further, the multistage cylindrical lens 10B described above is formed to have a vertical and horizontal dimension of 1550 μm, respectively, and the entire multistage cylindrical lens is shown in the drawing.

  The multistage cylindrical lens 10B has a focal length in the X direction of 20 mm, an infinite focal length in the Y direction, and a phase distribution of one wavelength when a laser beam having a light source wavelength λ of 0.4 μm is used. 2π corresponding to the minute is displayed in 8 levels at the gray level.

  Here, in the multistage cylindrical lens 10B, with respect to the X-axis direction having a condensing function, Formulas 1 to 3 that are the mathematical formulas of the multistage cylindrical lens 10A described in the first embodiment and the depth of the multistage structure are described. By using the equation (4), the phase distribution of the first diffraction grating 11 having a light condensing function can be calculated.

  On the other hand, the phase distribution in the Y-axis direction that does not have a condensing function is the phase distribution in phase with the X-axis direction.

  Also in the second embodiment, the incident light distribution is measured or calculated by simulation for each block obtained by dividing the first diffraction grating 11 into m × n pieces, and the maximum value in the entire incident light distribution is obtained. Is set to 1, matrix data of the incident light distribution I (m, n) is generated for the block BL in the m-th row and the n-th column. In this case, m and n are natural numbers, and the distribution uniformity improves as the number of m and n increases.

  The necessary light transmittance T (m, n) for each block BL is described above from the minimum value I (min) in the entire incident light distribution with respect to the m × n incident light distribution I (m, n). By obtaining from Equation 5 and using the numerical table shown in FIG. 9, the necessary light transmittance to the second diffraction grating 12 and the occupied area ratio of the phase modulation region portion 12a can be calculated. . At this time, the light transmittance of the multistage cylindrical lens 10B is determined by the area ratio occupied by the phase modulation area 12a in one block, and the area occupied by the phase modulation area 12a increases as shown in FIG. As a result, the light transmittance tends to decrease and does not depend on the shape of the phase modulation region 12a.

  Specifically, in the multistage cylindrical lens 10B, the entire first diffraction grating 11 is divided into, for example, 31 × 31 square blocks, in other words, after dividing one block into an area of 50 μm square. The second diffraction grating 12 is superimposed on the central portion of each block in the shape of a square phase modulation area 12a, and the phase modulation area 12a is varied for each block according to the required light transmittance. ing.

  Also in the second embodiment, the phase difference of the square phase modulation region portion 12a formed in the second diffraction grating 12 is set to λ / 4 (0.5π) of the use wavelength λ, but is not limited thereto. In addition, the phase modulation region portion of the second diffraction grating 12 may have an arbitrary shape, or may be set to an arbitrary phase difference.

  From the above, also in the second embodiment, as in the first embodiment, the light quantity distribution of the incident light beam to the multistage cylindrical lens 10B is not Z-axis rotationally symmetric, as previously shown in FIG. 4B. When it is assumed that the distribution is asymmetric in the X-axis direction and the Y-axis direction, the required light transmittance of the square phase modulation region portion 12a is obtained with respect to the second diffraction grating 12, and multistage The light transmittance distribution on the type cylindrical lens 10B is corrected to an inverted light amount distribution as shown in FIG. 5A. By this correction, the light quantity distribution of the incident light becomes the product of the incident light quantity distribution and the multistage cylindrical lens light transmittance distribution on the multistage cylindrical lens 10B. As a result, as shown in FIG. In addition, the distribution of the amount of light emitted from the multistage cylindrical lens 10B is flat.

  In addition, as described above, the light quantity distribution of the incident light beam on the multistage cylindrical lens 10B is not Z-axis rotationally symmetric, but is asymmetric in the X-axis direction and the Y-axis direction as previously shown in FIG. 4B. When the cylindrical lens is designed to have a small distribution, the occupying ratio of the second diffraction grating 12 is increased in the central part of the cylindrical lens and the outer peripheral area around the X axis designed to have a small light transmittance, while the light transmittance is increased. In the designed cylindrical lens outer peripheral portion Y-axis peripheral region, the occupation ratio of the second diffraction grating 12 is small.

  And when producing the multistage cylindrical lens 10B, like Example 1, after producing the 1st diffraction grating 11 with respect to a glass substrate or a quartz substrate using the above semiconductor processes, Similarly, the second diffraction grating 12 may be overlaid on the first diffraction grating 11 by a semiconductor process.

  FIG. 16 is a top view showing the entire multistage multifocal diffractive lens when the multistage diffractive optical element according to Example 3 of the present invention is applied to the multistage multifocal diffractive lens.

  As shown in FIG. 16, the multistage diffractive optical element 10C according to the third embodiment of the present invention is applied to a multistage multifocal diffractive lens. Hereinafter, the multistage diffractive optical element 10C will be referred to as a multistage multifocal diffractive lens 10C. .

  The multistage multifocal diffractive lens 10C has a function of condensing incident light to a plurality of different focal points, and generally has n points of focus. In FIG. It shows the case with focus.

  Along with the above, the multistage multifocal diffractive lens 10C of Example 3 is gradually pitched outward from the central portion on a rectangular optical substrate using a light-transmitting glass substrate or quartz substrate. The first and second phase regions RZ1 and RZ2 having a ring-shaped shape and narrowed in the center portion are set to overlap each other in the center portion. In order to diffract the first-order diffracted light, for example, as a diffracted light of a specific order with respect to the incident light respectively incident on the annular regions RZ1 and RZ2, a multi-step staircase shape is used for condensing the light at a predetermined condensing position (focal position). The first diffraction grating 11 having the diffraction grating portion 11a is formed, and the inside of the optical substrate on which the first diffraction grating 11 is formed is two-dimensionally m × n as shown in FIG. When divided into blocks, the first diffraction in each block Second diffraction grating having a line-shaped phase modulation region portion 12b having a predetermined phase difference for diffracting incident light to a diffraction order different from that of the element 11 and adjusting light transmittance with respect to the incident light 12 is superposed, and the phase modulation region portion 12b of the second diffraction grating 12 has its occupation area ratio with respect to the first diffraction grating 11 varied for each block according to the required transmittance.

  Further, the multistage multifocal diffractive lens 10C described above is formed to have a vertical and horizontal dimension of 1550 μm, respectively, and the entire multistage multifocal diffractive lens is shown in the drawing.

  The multistage multifocal diffractive lens 10C has a focal length on the first annular zone RZ1 side set to 20 mm and a focal length on the second annular zone RZ2 side set to 40 mm. The focal point is focused at positions separated from each other by 200 μm with respect to the optical axis passing through the center of the multifocal diffractive lens, and phase distribution is obtained when laser light having a light source wavelength λ of 0.4 μm is used. Displays 2π corresponding to one wavelength in 8 levels on a gray level.

  Here, in the multistage multifocal diffractive lens 10 </ b> C, the phase distribution corresponding to the focal point in the first annular zone RZ <b> 1 is expressed by the numerical formulas 1 to 1 of the multistage cylindrical lens 10 </ b> A described in the first embodiment. The phase distribution of the first diffraction grating 11 having a light condensing function can be calculated by using Equation 3 and Equation 4 that is an expression of the depth of the multistage structure.

  In addition, regarding the phase distribution corresponding to the focal point in the second annular zone RZ2, similarly to the above, by using the mathematical expression 1 to the mathematical expression 3 and the mathematical expression of the depth of the multistage structure, The phase distribution of the first diffraction grating 11 having a condensing function can be calculated, and thereafter, the phase distributions of the first and second annular zones RZ1 and RZ2 are added to each other in the multistage multifocal diffractive lens 10C. The phase distribution of one diffraction grating 11 is assumed.

  Of course, when there are n focal points, the phase distribution is similarly calculated n times to obtain n phase distributions, and these phase distributions are added to obtain the phase distribution of the first diffraction grating 11. Just do it.

  Also in the third embodiment, the incident light distribution is measured or calculated by simulation for each block obtained by dividing the first diffraction grating 11 into m × n pieces, and the maximum value in the entire incident light distribution is obtained. Is set to 1, matrix data of the incident light distribution I (m, n) is generated for the block BL in the m-th row and the n-th column. In this case, m and n are natural numbers, and the distribution uniformity improves as the number of m and n increases.

  The necessary light transmittance T (m, n) for each block BL is described above from the minimum value I (min) in the entire incident light distribution with respect to the m × n incident light distribution I (m, n). By obtaining from Equation 5 and using the numerical table shown in FIG. 9, the necessary light transmittance to the second diffraction grating 12 and the occupied area ratio of the phase modulation region portion 12a can be calculated. . At this time, the light transmittance of the multistage multifocal diffractive lens 10C is determined by the area ratio occupied by the phase modulation area 12b in one block, and the area occupied by the phase modulation area 12b as shown in FIG. As the value increases, the light transmittance tends to decrease and does not depend on the shape of the phase modulation region 12b.

  Specifically, the entire first diffraction grating 11 is divided into, for example, 31 × 31 square blocks in the multistage multifocal diffractive lens 10C, in other words, one block is divided into an area of 50 μm square. Above, the second diffraction grating 12 has a line-shaped phase modulation area portion 12b superimposed on one block (in the Y-axis direction) in a central portion of each block so as to be connected to each block, and this phase modulation area The part 12b is varied for each block according to the required light transmittance.

  Also in the third embodiment, the phase difference of the square phase modulation region portion 12a formed in the second diffraction grating 12 is set to λ / 4 (0.5π) of the use wavelength λ, but is not limited thereto. In addition, the phase modulation region of the second diffraction grating 12 may have an arbitrary shape, or may be set to an arbitrary phase difference.

  From the above, also in the third embodiment, similarly to the first and second embodiments, the light quantity distribution of the incident light beam to the multistage multifocal diffractive lens 10C is not Z-axis rotationally symmetric, and FIG. 4B is used first. When it is assumed that the distribution is asymmetric in the X-axis direction and the Y-axis direction as shown, the required light transmittance of the line-shaped phase modulation region portion 12b with respect to the second diffraction grating 12 is shown. Thus, the light transmittance distribution on the multistage multifocal diffractive lens 10C is corrected to the inverted light quantity distribution as shown in FIG. By this correction, the light quantity distribution of the incident light becomes a product of the incident light quantity distribution and the multistage multifocal diffractive lens light transmittance distribution on the multistage multifocal diffractive lens 10C. As a result, FIG. As shown in b), the quantity of light emitted from the multistage multifocal diffractive lens 10C is flat.

  In addition, as described above, the light quantity distribution of the incident light beam to the multistage multifocal diffractive lens 10C is not Z-axis rotationally symmetric, and as previously shown in FIG. 4B, the X-axis direction and the Y-axis direction. In the central portion of the multistage multifocal diffractive lens and the outer peripheral region around the X axis designed to have a low light transmittance, the occupation ratio of the second diffraction grating 12 is increased. The occupation ratio of the second diffraction grating 12 is small in the Y-axis peripheral region of the outer periphery of the multistage multifocal diffractive lens designed to have a large light transmittance.

  When the multi-stage multifocal diffractive lens 10C is manufactured, the first diffraction grating 11 is applied to the glass substrate or the quartz substrate using the semiconductor process as described above, as in the first and second embodiments. After the production, the second diffraction grating 12 may be similarly laminated on the first diffraction grating 11 by a semiconductor process.

FIG. 5 is a top view showing a quarter of the entire multistage Fresnel lens when the multistage diffractive optical element of Example 1 according to the present invention is applied to a multistage Fresnel lens. It is an optical path diagram for demonstrating the optical path of an optical system, when the multistage type | mold diffractive optical element of Example 1 which concerns on this invention is applied to a multistage type | mold Fresnel lens. When the multistage diffractive optical element of Example 1 according to the present invention is applied to a multistage Fresnel lens, the light intensity distribution and light transmittance of the optical system are shown corresponding to positions X1 to X3 in FIG. is there. (A)-(c) is the perspective view which showed in three dimensions the incident light quantity distribution to a multistage type Fresnel lens, when the multistage type diffractive optical element of Example 1 which concerns on this invention is applied to a multistage type Fresnel lens. . (A), (b) is a three-dimensional perspective view showing a case where the light quantity distribution is corrected on the multistage Fresnel lens when the multistage diffractive optical element of Example 1 according to the present invention is applied to the multistage Fresnel lens. FIG. (A), (b) is the top view and sectional drawing which showed the 1st diffraction grating in a multistage type | mold Fresnel lens. FIG. 7 is a plan view showing a state in which the first diffraction grating shown in FIG. 6 is divided into m × n blocks. (A), (b) is a figure for demonstrating the 2nd diffraction grating formed in the block which divided | segmented the 1st diffraction grating. It is the figure which showed the relationship between the occupation area ratio at the time of superimposing a 2nd diffraction grating in the block of a 1st diffraction grating, and the light transmittance of the 0th-order light by a 2nd diffraction grating. It is sectional drawing which expanded and showed the 1st diffraction grating. It is sectional drawing which expanded and showed the 2nd diffraction grating. It is sectional drawing which expanded and showed the state on which the 2nd diffraction grating was optically superimposed with respect to the 1st diffraction grating. (A)-(j) is process drawing which showed the process of producing the multistage type | mold Fresnel lens of Example 1 which concerns on this invention. It is the top view which showed 1/4 of the whole multistage type | mold Fresnel lens of the modification which partially deformed the multistage type | mold Fresnel lens of Example 1 which concerns on this invention. FIG. 6 is a top view showing the entire multistage cylindrical lens when the multistage diffractive optical element according to Example 2 of the present invention is applied to a multistage cylindrical lens. It is the top view which showed the whole multistage type multifocal diffractive lens when the multistage diffractive optical element of Example 3 which concerns on this invention is applied to a multistage multifocal diffractive lens. (A), (b) is the top view and longitudinal cross-sectional view which showed the general Fresnel lens. It is the longitudinal cross-sectional view which showed the conventional multistage type | mold Fresnel lens. (A)-(j) is process drawing which showed the process of producing the conventional multistage type | mold Fresnel lens. It is the figure which showed typically the state which made the conventional multistage type | mold Fresnel lens the laser beam into which the light beam of a center part has strong light intensity, and the light beam of an outer peripheral part has weak light intensity.

Explanation of symbols

10A: Multistage Fresnel lens of Example 1,
10A ′: a multistage Fresnel lens of a modification obtained by partially deforming the first embodiment,
10B: Multistage cylindrical lens of Example 2,
10C: the multistage multifocal diffractive lens of Example 3,
11 ... 1st diffraction grating, 11a ... Stepped diffraction grating part,
12 ... second diffraction grating,
12a ... Square phase modulation region, 12b ... Line-shaped phase modulation region,
OBP ... optical substrate, 0 ... center of optical substrate,
K: Optical axis,
RZ (RZ ₀ , RZ ₁ , RZ ₂ ,..., RZ _n ) ... Annular phase region (annular region),
R _n (R ₀ , R ₁ , R ₂ ,..., R _n ) ... lens radius,
_{Rp n (Rp 0, Rp 1} , Rp 2, ......, Rp n) ... ring-shaped zone pitch,
LZ: Line-shaped phase region (line region).

Claims (2)

A plurality of phase regions are set on the optical substrate having optical transparency with the pitch gradually narrowed from the center toward the outside, and diffracted light of a specific order is diffracted with respect to incident light within each phase region. A first diffraction grating in which stepped diffraction grating parts for condensing light at a predetermined light collection position are formed in multiple stages,
In order to diffract diffracted light of a specific order with respect to the incident light and to adjust light transmittance with respect to the incident light, a phase modulation region portion having a predetermined phase difference serves as the first diffraction grating. Each block is superposed in a plurality of blocks that are two-dimensionally divided into m × n, and the ratio of the area occupied by the phase modulation region portion to the first diffraction grating is required for each block according to the light transmittance. A second diffraction grating made variable to
A multi-stage diffractive optical element comprising: The phase modulation region portion of the second diffraction grating is formed in a rectangular shape in each block or in a line shape in one direction in each block. Item 4. The multistage diffractive optical element according to Item 1.