JP3189922B2 - Diffractive optical element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffractive optical element, and more particularly to a diffractive optical element having good diffraction efficiency even in a region where a grating period is small for obliquely incident light.

[0002]

2. Description of the Related Art A diffractive optical element is an optical element utilizing a light diffraction phenomenon, and is constituted by a plurality of grating patterns (diffraction gratings). Generally, how much diffraction efficiency can be achieved in a diffractive optical element is a very important factor. As a conventional diffractive optical element, a diffractive microlens for normal incidence shown in FIG. 9 is known (J. Jahns
and SJ Walker: "Two-dimensional array of diff
ractive microlenses fabricated by thin film deposi
tion ", Applied Optics Vol. 29, No. 7, pp. 931-936
(1990).). In FIG. 9, (a) is a plan view,
(B) is a side sectional view thereof. As is apparent from FIG. 9, this conventional diffractive optical element is for condensing or collimating vertically incident light, and a concentric grating pattern 7 is provided on a substrate 1. The grating pattern 7 is configured such that the grating period becomes smaller toward the outer periphery. The cross section of each grating pattern is rectangular, and the duty ratio, which is the ratio of the area other than the air layer in one cycle, is 0.5.
Designed with. The reason is that when the duty ratio is 0.5 when the cross section of the grating pattern is rectangular,
This is because it is known that the diffraction efficiency is the highest (41%).

[0003]

Considering the relationship between the diffraction efficiency and the duty ratio of a grating pattern having a rectangular cross section, when the grating pattern of a diffractive optical element has a rectangular cross section, the diffraction efficiency is 0.5 when the duty ratio is 0.5. Is likely to be the highest. In fact, when the incident light becomes obliquely incident light inclined from the vertical direction, in the region where the grating period is large,
As estimated, the diffraction efficiency was highest when the duty ratio of the rectangular section was approximately 0.5. However, in the conventional diffractive optical element in which the duty ratio of the rectangular cross section of the grating pattern is 0.5, there is a problem that the diffraction efficiency is reduced in a region where the grating period is close to the wavelength of the incident light. Were found. The present invention has been made to solve the above problems, and has an object to provide a diffractive optical element having a grating having a rectangular cross section, which has good diffraction efficiency even in a region where the grating period is small with respect to oblique incident light. And

[0004]

[MEANS FOR SOLVING THE PROBLEMS] To achieve the above object
The diffractive optical element of the present invention comprises a substrate and a
Equipped with a grating section whose cross section is rectangular
And the period of the grating portion is such that first-order diffracted light is generated.
In the range,The grating section has a cycle of incident light.
A first region close to the wavelength of
A second region larger than the period in
Area is the duty ratio of the rectangular section of the second area.
(Ratio of area other than air layer in one cycle)Less than
With a low duty ratio,
To enhance the diffraction efficiency of the first region.Characterized by
You. In the above configuration, the grating period is incident
In a region smaller than three to four times the wavelength,
The duty ratio of the rectangular section of the bearing part to less than 0.5
Is preferred. Also, the rectangular section of the grating
The duty ratio of the surface is such that the period of the grating part is small.
It is preferred that it gradually decreases as it becomes. Also,
The depth of the groove in the grating portion is
2. The method according to claim 1, wherein the difference is made in accordance with the period of
Diffractive optical element. In addition, the period of the grating
In an area smaller than 3 to 4 times the length,
It is preferable that the depth of the groove of the ring portion gradually decreases.
The pattern of the grating part is symmetrical about the center, and
The curve is convex in one direction, and the gray curve is in the convex direction.
It is preferable that the ringing cycle gradually decreases. Also,
The pattern of the rating part is a straight line and gradually becomes gray.
It is preferable that the timing cycle changes.

[0005]

In the case where incident light is incident on a grating having a rectangular cross section while being inclined from the vertical direction, there is an optimum duty ratio at which the diffraction efficiency is increased in accordance with the period of the grating section. Therefore, by setting the duty ratio (the ratio of the area other than the air layer in one cycle) of the rectangular section of the grating section to an optimal value according to the cycle of each grating section, the entire area of the diffractive optical element is set. Diffraction efficiency increases.

[0006]

FIG. 1 shows a first embodiment of the diffractive optical element of the present invention.
This will be described in detail with reference to FIGS. In FIG.
(A) is a sectional view showing a basic configuration of a first embodiment of the diffractive optical element of the present invention, and (b) is a plan view thereof. FIG. 2 shows an incident angle θ of incident light of 20 ° in the first embodiment.
FIG. 9 is a diagram showing a relationship between the normalized grating period and the diffraction efficiency of the first-order diffracted light in the case of FIG. FIG. 3 is a diagram showing the relationship between the normalized grating period and the diffraction efficiency of the first-order diffracted light when the incident angle θ of the incident light is 30 ° in the first embodiment. FIG. 4 is a diagram showing the relationship between the normalized grating period and the duty ratio in the first embodiment. FIG.
FIG. 6 is a diagram showing a state of light collection in the first embodiment. As shown in FIG. 1, the diffractive optical element according to the first embodiment includes:
Substrate 1 and grating portion 2 formed on substrate 1
Is provided. The grating section 2 has a rectangular cross section, and is configured such that the duty ratio represented by d / Λ shown in FIG. Here, Λ is the grating period, and d is the size of the area other than the air layer in that period. As will be described later in detail, in a region where the grating period of the grating unit 2 is large, the duty ratio is substantially 0.5, but the duty ratio is set to decrease as the grating period decreases. Further, the depth h of the groove is constant regardless of the grating period. The diffractive optical element of the first embodiment has, for example, an aperture of 1 mm (circular aperture), a wavelength λ of incident light = 0.6328 μm, an incident angle θ = 20 °,
The grating section 2 has a period of, for example, from 0.633 μm to 6.3 μm, and the maximum groove depth is, for example, h = 0.63 μm.

When the incident light is obliquely incident light inclined from the vertical direction, in a region where the grating period of the grating section 2 is large, the diffraction efficiency is highest when the rectangular duty ratio is approximately 0.5. However, the present inventors have discovered that in a region where the grating period approaches the wavelength of the incident light, the smaller the duty ratio, the higher the diffraction efficiency. The details are described below. FIG. 2 shows the relationship between the diffraction efficiency and the normalized grating period Λ / λ (λ: wavelength) when the incident angle of the incident light is θ = 20 ° and the refractive index of the grating unit 2 is n = 1.5. .
As shown by the solid line in FIG.
In the case of 5, the diffraction efficiency is about 40% in a region where the grating period is large, but the diffraction efficiency decreases when the grating period becomes small and becomes about three times the wavelength. On the other hand, when the duty ratio is 0.4 and 0.3, the diffraction efficiency is smaller than the case where the duty ratio is 0.5, but the grating period is small in the region where the grating period is large as shown by the one-dot chain line and the dotted line, respectively. It can be seen that the diffraction efficiency is more improved when the duty ratio is 0.5. Similarly, the diffraction efficiency and the normalized grating period Λ / λ (λ: wavelength) when the incident angle is θ = 30 °
Is shown in FIG. Again, it can be seen that there is a duty ratio at which the diffraction efficiency increases for each grating period. Therefore, it is understood that the diffraction efficiency is increased over the entire area of the diffractive optical element if the duty ratio is set to increase the diffraction efficiency according to the grating period of the grating section 2.

In the first embodiment, the duty ratio is set, for example, as shown in FIG. That is, in a region where the grating period is large, the duty ratio is set to 0.45 to 0.5.
5, and the duty ratio was approximately 0.5. On the other hand, in the region where the grating period becomes small, the duty ratio was changed according to the curve shown in FIG. Although the optimum change curve of the duty ratio varies depending on various conditions, the duty ratio may be smaller than 0.5 in a region where the period of the grating portion is smaller than 3 to 4 times the incident wavelength. As a general tendency, it can be said that the diffraction efficiency is improved by gradually reducing the duty ratio as the period of the grating section is reduced.

As shown in FIG. 5, the diffractive optical element of the first embodiment is a transmissive off-axis lens that emits obliquely incident light 5 vertically (emitted light 6). An off-axis lens is a lens in which the optical axis of incident light and the optical axis of outgoing light are different. In FIG. 5, reflective layers 4A and 4B are deposited on the front surface and the back surface of the substrate 1 in the region where the light 5 propagates, and the light is repeatedly reflected in the substrate 1 and propagates in a zigzag manner. Is done. In order to convert the obliquely incident light 5 into vertically condensed light, the pattern shape of the grating part 2 is a centrally symmetrical and convex curve in one direction as shown in FIG. The grating period is gradually reduced, and at the same time, the curvature of the pattern is increased. More specifically, assuming that the wavelength of the incident light is λ, the refractive index of the substrate 1 is n, and the incident angle is θ in the coordinate systems shown in FIGS. 1 and 5, the phase shift of the diffractive optical element of the first embodiment is described. the function is,
Φ (x, y) = k ((x ² + y ² + f ² ) ^1/2 + nysin
θ−f) −2mπ. Where k = λ / 2π,
m is an integer satisfying 0 ≦ Φ ≦ 2π, and represents the order of the grating pattern. From this phase shift function, the curve shape of the grating section 2 of order m is centered at (0, −nsin θ (mλ + f) / (1−n ² sin).
² theta)) and is, minor axis (the length of the ^{_{x-axis) d x = 2 (m 2}} λ 2 +
2mλf + n ² f ² sin ² θ) ^1/2 / (1-n ² sin
² theta) ^1/2, the long axis of the (y-axis) length _{d y = d x / (1} -n 2 s
in ² θ) ^1/2 is the upper part of the elliptic curve.

The substrate 1 and the grating section 2 need only be transparent to the wavelength used, and are formed of a material such as glass or synthetic resin. When the light used is infrared light, the material of the substrate 1 and the grating part 2 may be Si or GaAs.
It is also possible to use a semiconductor such as s. As a method for manufacturing the diffractive optical element of the first embodiment, an electron beam drawing method was used. That is, for example, a synthetic resin sensitive to an electron beam such as an electron beam resist such as PMMA or CMS is coated on the substrate 1, and the synthetic resin coating layer is irradiated with the electron beam and developed to form a diffractive optical element. did. In addition, as the specifications of the diffractive optical element, any other than the above can be manufactured according to the purpose.

In mass production, for example, a mold is manufactured by a nickel electroforming method, and the same lens element as that of the master is manufactured at a low cost by, for example, duplicating the mold using an ultraviolet (UV) curable resin. It is possible to In particular, when the diffractive optical elements are arranged in an array, by using this method, diffractive optical elements having the same characteristics can be simultaneously formed with high accuracy. Further, by transferring the shape of the grating portion 2 formed of a synthetic resin (electron beam resist) to, for example, the glass substrate 1 by, for example, ion beam etching, the temperature is very stable.

FIG. 6 shows the basic structure of a second embodiment of the diffractive optical element according to the present invention. FIG. 6 is a plan view of the diffractive optical element of the second embodiment. The description of the same parts as in the first embodiment is omitted, and different parts will be described. As shown in FIG. 6, the diffractive optical element of the second embodiment is a cylindrical off-axis lens in which the pattern of the grating section 2 'is a straight line and the grating period is gradually changed. That is, the diffractive optical element of the second embodiment focuses obliquely incident light only in one axial direction (y direction). The cross section is substantially the same as the configuration shown in FIG. Therefore, even in such a uniaxial cylindrical lens, the diffraction efficiency is improved by changing the duty ratio of the rectangular cross section, and the same effect as that of the diffractive optical element of the first embodiment can be obtained.

Next, a third embodiment of the diffractive optical element according to the present invention will be described with reference to FIGS. FIG. 7 is a sectional view showing a basic configuration of the diffractive optical element according to the third example.
FIG. 8 is a diagram showing the relationship between the normalized grating period and the diffraction efficiency of the first-order diffracted light in the diffractive optical element of the third embodiment. The description of the same parts as in the first embodiment is omitted, and different parts will be described.
As shown in FIG. 7, the diffractive optical element according to the third example has a structure in which not only the duty ratio but also the depth of the groove is changed according to the period of the grating section 2 ″. As shown in FIG. 8, the depth of the groove of the grating portion 2 is made constant in a region where the normalized grating period is 3 or more, and the depth of the groove of the grating portion 2 ″ is diffracted in a region where the normalized grating period is 3 or less. The value of the efficiency was changed so as to be almost constant. Although the change curve of the groove depth varies depending on various conditions, the groove depth may be changed in a region in which the period of the grating portion 2 ″ is smaller than 3 to 4 times the incident wavelength. It can be said that the depth of the groove should be gradually reduced as the period of the grating portion becomes smaller.In the third embodiment, the duty ratio and the depth of the groove are set in accordance with the period of the grating portion 2 ". Since the diffraction efficiency is changed so as to prevent the diffraction efficiency from lowering even in the area where the grating period is small, the value of the diffraction efficiency can be made substantially constant over the entire area of the grating section 2 ″, and the converging spot of the diffractive optical element can This has the effect of making the light distribution uniform.

In each of the embodiments described above, the case where the present invention is applied to an off-axis lens that condenses obliquely incident light vertically is described. However, the present invention is applied not only to an off-axis type lens but also to other types of diffractive optical elements. The same effect can be obtained even when the incident light is oblique.

[0015]

As described above, according to the present invention, the duty ratio of the rectangular cross section of the grating section is set to an optimum value in accordance with the period of each grating section of the diffractive optical element. Therefore, it is possible to realize a diffractive optical element having a rectangular section with good diffraction efficiency even in a region where the grating period is small particularly for obliquely incident light. Further, by changing the duty ratio and the depth of the groove in accordance with the period of the grating section, the value of the diffraction efficiency can be made substantially constant over the entire area of the grating section, and the light of the condensing spot of the diffractive optical element can be obtained. The distribution can be made uniform.

[Brief description of the drawings]

FIG. 1A is a sectional view showing a basic configuration of a first embodiment of a diffractive optical element of the present invention, and FIG. 1B is a plan view thereof.

FIG. 2 shows the incident angle θ of incident light in the first embodiment = 20.
Diagram showing the relationship between the normalized grating period and the diffraction efficiency of the first-order diffracted light in the case of °

FIG. 3 is an incident angle θ of incident light in the first embodiment = 30.
Diagram showing the relationship between the normalized grating period and the diffraction efficiency of the first-order diffracted light in the case of °

FIG. 4 is a diagram showing a relationship between a normalized grating period and a duty ratio in the first embodiment.

FIG. 5 is a diagram showing a state of light collection in the first embodiment.

FIG. 6 is a plan view showing a basic configuration of a second embodiment of the diffractive optical element of the present invention.

FIG. 7 is a sectional view showing a basic configuration of a third embodiment of the diffractive optical element of the present invention.

FIG. 8 is a diagram showing the relationship between the normalized grating period and the diffraction efficiency of the first-order diffracted light in the third embodiment.

9A is a plan view showing a configuration of a conventional diffractive optical element, and FIG. 9B is a cross-sectional view thereof.

[Explanation of symbols]

 1: substrate 2: grating part

Claims (7)

(57) [Claims] 1. A substrate, comprising: a grating portion having a rectangular cross section formed on the substrate, wherein a period of the grating portion is in a range where first-order diffracted light is generated;
The grating section has a period close to the wavelength of the incident light.
1 region and the period is the period in the first region.
A second region that is larger than the first region,
Duty ratio smaller than the duty ratio of the rectangular section of the second region (the ratio of the region other than the air layer in one cycle)
The first region for obliquely incident light.
A diffractive optical element characterized by having increased diffraction efficiency . 2. The period of the grating part is 3 times the incident wavelength.
2. The diffractive optical element according to claim 1, wherein a duty ratio of a rectangular cross section of the grating section is smaller than 0.5 in an area smaller than 4 times. 3. The diffractive optical element according to claim 2, wherein the duty ratio of the rectangular section of the grating section gradually decreases as the period of the grating section decreases. 4. The diffractive optical element according to claim 1, wherein the depth of the groove of the grating portion varies depending on the period of the grating portion. 5. The period of the grating part is 3 times the incident wavelength.
5. The diffractive optical element according to claim 4, wherein the depth of the groove of the grating portion gradually decreases in a region smaller than 4 times. 6. The diffractive optical element according to claim 1, wherein the pattern of the grating portion is symmetric with respect to the center and is a curve that is convex in one direction, and the grating period gradually decreases in the convex direction. . 7. The diffractive optical element according to claim 1, wherein the pattern of the grating section is a straight line, and the grating period changes gradually.