JP3214964B2 - 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,
The present invention relates to a diffractive optical element having good diffraction efficiency even in a region where the grating period is small with respect to 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 vertical incidence diffractive microlens having a step-like cross section as shown in FIG. 9 is known. (J. Jahns and SJ Walke
r: "Two-dimensional array of diffractive microle
nses fabricated by thin film deposition ", Applied
Optics Vol. 29, No. 7, pp. 931-936 (1990).). FIG.
2A is a plan view, and FIGS. 2B and 2C are side sectional views. As is apparent from FIG. 9, this conventional diffractive optical element is for condensing or collimating vertically incident light, and concentric grating patterns 8, 8 'are provided on a substrate 1. The grating patterns 8, 8 'are configured such that the grating period becomes smaller as going to the outer periphery. The cross section of each grating pattern is step-shaped, and the number of steps (the number of steps) is two-step as shown in FIG.
There are four stages to approximately sixteen stages as shown in FIG. In such a conventional diffractive optical element, even if the grating periods are different, the same element has the same number of steps in the stepped shape of the grating and the same maximum film thickness throughout the element. In the two-stage diffractive optical element shown in (b), the diffraction efficiency, which is the rate at which incident light can be diffracted, is 41%.
With the four stages shown in FIG. 3C, the diffraction efficiency is improved to 81%. In addition, 95% in 8 stages, 9% in 16 stages
The diffraction efficiency increases as the number of steps in the stairs increases, such as 9%.

[0003]

Considering the relationship between the diffraction efficiency and the number of steps of a grating pattern having a step-shaped cross section, no matter what kind of diffractive optical element is used, the diffraction pattern of the step-shaped grating pattern can be used. It is easily presumed that the greater the number of steps, the higher the diffraction efficiency. In fact, when the incident light becomes obliquely incident light inclined from the vertical direction, in a region where the grating period is large, the diffraction efficiency is still higher as the number of steps is larger. However, the present inventors have found that in a region where the grating period is close to the wavelength of the incident light, an element having a large number of steps of the grating pattern has a problem that the diffraction efficiency sharply decreases. further,
In a region where the grating period is close to the wavelength of the incident light, it is difficult to perform fine processing due to the small grating period, and an element with a large number of steps in the grating pattern cannot achieve the designed step shape, resulting in a decrease in optical characteristics. There were also problems. The present invention has been made to solve the above problems, and provides a diffractive optical element that has a high diffraction efficiency over the entire area of a diffractive optical element with respect to oblique incident light, and that can be easily manufactured even in a region where a grating period is small. It is intended to be.

[0004]

In order to achieve the above object, a diffractive optical element according to the present invention is a diffractive optical element having a substrate and a grating formed on the substrate and having a stepped cross section. The grating section is
First a predetermined number of times smaller than the first region of the cycle the incident wavelength, the period comprising a first predetermined number of times greater than the second region of the incident wavelength, the first region number of steps of the staircase shape is smaller than the number of steps of the stepped shape of the second region, against oblique Me incident light, the period is the
The diffraction efficiency of the first region is substantially equal to the diffraction efficiency of the second region when the wavelength is near the first predetermined number times the incident wavelength . In the above configuration,
It is preferable that the grating section includes an area other than the first area and the second area, and that the number of steps in the staircase shape gradually decreases as the period of the grating section decreases. The number of steps is 3 or more in a region where the period of the grating portion is larger than the first predetermined number times the incident wavelength, and the number of steps is 2 in a region where the period of the grating portion is smaller than the first predetermined number times the incident wavelength. It is preferable that the first predetermined number is any value between 1.5 and 3. Further, in the region where the number of steps is 2, the duty ratio of the grating portion (the ratio of the region other than the air layer in one cycle) is preferably any value between 0.15 and 0.5. In a region where the period of the grating section is larger than the second predetermined number times the incident wavelength, the number of steps is 4 or more, the period of the grating section is smaller than the second predetermined number times the incident wavelength, and In an area larger than the predetermined number, the number of steps is 3, and the second predetermined number is any value between 2 and 5 (however, the first predetermined number is smaller than the second predetermined number). ) Is preferable. Further, the number of steps is 5 or more in a region where the period of the grating portion is larger than a third predetermined number times the incident wavelength, and the step number is smaller than the third predetermined number times and larger than the second predetermined number times. Is preferably 4, and the third predetermined number is any value between 4 and 7 (however, the second predetermined number is smaller than the third predetermined number). Further, it is preferable that the maximum film thickness of the grating portion is different depending on the number of steps. Further, it is preferable that 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 period is gradually reduced in the convex direction. Further, it is preferable that the pattern of the grating portion is a straight line, and the period gradually changes.

[0005]

In the case where incident light is incident on a grating having a step-shaped cross section at an angle from the vertical direction, the diffraction efficiency does not increase as the number of step-like steps increases in all regions. There is a number of steps at which the diffraction efficiency increases. Therefore, by setting the number of steps of the stair-shaped cross section of the grating section to an optimum value according to the period of each grating section, the diffraction efficiency is increased over the entire area of the diffractive optical element. In addition, since the optimal number of steps tends to be smaller as the grating period becomes shorter, the fabrication becomes easier even in a region where the grating period is shorter.

[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 is a diagram showing the relationship between the grating period and the diffraction efficiency of the first-order diffracted light in the first embodiment. FIG. 3 is a diagram showing the relationship between the grating period and the diffraction efficiency of the first-order diffracted light using the duty ratio of the grating section 2A as a parameter in the first embodiment. FIG. 4 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 a substrate 1 and a grating section 2 formed on the substrate 1. The grating portion 2 includes a region of the grating portion 2A having a short grating period and a region of the grating portion 2B having a long grating period. Each of the grating sections 2A and 2B has a stepped cross section. Grating part 2 with large grating period
In the region B, the number of steps in the staircase shape is three. Also, the grating portion 2A having a small grating period
In the region, the number of steps in the staircase shape is two. As shown in FIG. 2, a region where the grating period is smaller than, for example, 1.6 times the wavelength of the incident light is the grating portion 2A having two steps, and a region larger than that is the grating portion 2B having three steps.

In the case where the incident light is obliquely incident light inclined from the vertical direction, in the area of the grating portion 2B having a large grating period, the diffractive optical element having a larger number of steps in the step shape has a higher diffraction efficiency and a step in the step shape. The diffraction efficiency was low with a small number of diffractive optical elements. However, in the area of the grating portion 2A where the grating period is close to the wavelength of the incident light, the diffraction efficiency of the element having a large number of steps is sharply reduced, whereas the diffraction period of the diffraction optical element having a small number of steps is small. The present inventors have discovered that even in a region approaching the wavelength of light, the rate of decrease in diffraction efficiency is small, or rather the diffraction efficiency tends to increase. The details are described below. FIG. 2 shows the relationship between the diffraction efficiency and the grating period when the incident angle of the incident light is θ = 20 ° and the refractive indices of the grating portions 2A and 2B are n = 1.5. As shown by the solid line in FIG. 2, the region 2B where the grating period is large
Then, an element having three steps has a diffraction efficiency of 50% or more. However, when the grating period is reduced and the grating period is in a region 2A about twice the wavelength of the incident light, the grating period is sharply reduced. On the other hand, as shown by the dotted line, in the region 2B where the grating period is large, the number of steps is two.
Has a diffraction efficiency of 30 to 40%. However, the grating period becomes smaller, and the diffraction efficiency increases from a region where the grating period is about two to three times the wavelength of the incident light. For example, the grating period is 1.
It can be seen that the diffraction efficiency of the element having two steps and the diffraction efficiency of the element having three steps are the same at six times.
Therefore, for example, in a region where the grating period is greater than 1.6 times the wavelength of the incident light, the number of steps in the staircase shape of the grating unit 2 is set to 3, and in a region where the grating period is smaller than this, the number of steps in the staircase shape is set to 2. Thereby, the diffraction efficiency can be increased over the entire area of the diffractive optical element.

Conventionally, in a region where the grating period is close to the wavelength of the incident light, it is difficult to perform fine processing because the grating period is small, and an element having a large number of steps cannot realize a step shape as designed, resulting in poor optical characteristics. However, the present inventors have discovered that the smaller the grating period, the smaller the optimum number of steps tends to be. Fabrication is easy. That is, conventionally, when a diffractive optical element having a step number of 3 is to be manufactured over the entire area of the element, processing of a step shape having a step number of 3 has been successfully performed in a region where the grating period is large, but the grating period is small. In the area, the edge portion of the staircase shape was dull and could not be processed well. However, in the diffractive optical element of the present embodiment, in a region where the grating period is smaller than 1.6 times the wavelength of the incident light, a rectangular shape having two steps may be used. It is possible to do.
In this embodiment, the case where the normalized grating period Λ / λ (Λ: grating period) for switching the number of steps between 2 and 3 is 1.6 is described. Although the optimum value changes depending on the like, it has been confirmed that substantially the same effect can be obtained if Λ / λ = 1.5 to 3.

The diffractive optical element of the first embodiment has, for example, an aperture of 1 mm (circular aperture) and a wavelength of incident light of λ = 0.632.
8 μm, the incident angle θ = 20 °, and the focal length is 2.5 mm. The grating period of the grating section 2B having three steps is, for example, in the range of 1.0 μm to 2.0 μm, and the maximum film thickness is, for example, h _B = 0.84 μm.
It is. Also, the grating section 2A having two steps
The grating period is, for example, in the range of 0.89 μm to 1.0 μm, and the maximum film thickness is, for example, h _A
= 0.63 μm. Thus, the maximum film thickness of the grating portions 2A and 2B is changed according to the number of steps. By changing the maximum film thickness according to the number of steps, the diffraction efficiency can be optimized. In the grating section 2A, for example, the duty ratio is set to 0.3. Here, the duty ratio refers to (a) in FIG.
As shown in the above, d ₁ / Λ _A , that is, the ratio of substances other than the air layer occupying in one grating cycle. FIG. 3 illustrates a case where the duty ratio of the grating unit 2A in the present embodiment is a parameter.
4 shows the relationship between the grating period and the diffraction efficiency of the first-order diffracted light. As is apparent from FIG. 3, particularly at oblique incidence, if the duty ratio of the grating section 2A having the number of steps of 2 is set to any value within the range of 0.15 to 0.5, the grating period becomes larger than that of the incident light. It can be seen that the diffraction efficiency is improved in the region not more than twice the wavelength.

As shown in FIG. 4, 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. 4, reflection layers 4A and 4B are deposited on the front and back surfaces of the substrate 1 in a region where light 5 propagates. Light is repeatedly reflected in the substrate 1, propagates in a zigzag manner, and is vertically emitted from the grating unit 2. 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 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 4, the phase shift of the diffractive optical element of the first embodiment is 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.
Semiconductors such as s can also be used. 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 applied to the substrate 1.
Then, the synthetic resin coating layer is irradiated with an electron beam. At this time, the irradiation amount of the electron beam is controlled in accordance with the cross-sectional shape of the diffractive optical element to be manufactured. (In the case of a positive resist, the irradiation amount of the electron beam is reduced where the film thickness of the element is large. And the development process was performed to form a diffractive optical element. In addition, as the specifications of the diffractive optical element, any other than the above can be manufactured according to the purpose.

In the case of mass production, for example, a mold is manufactured by a nickel electroforming method, and the mold is duplicated from the mold using, for example, an ultraviolet (UV) curable resin. It can be made. 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.

Next, FIG. 5 shows the basic configuration of a second embodiment of the diffractive optical element of the present invention. FIG. 5 is a plan view showing the basic configuration 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. 5, the pattern of the grating portion 2 ′ is a straight line, and is a cylindrical off-axis lens in which the grating period changes gradually. 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, by setting the number of steps to 3 in a region where the grating period is large and by setting the number of steps to 2 in a region where the grating period is small, the same as in the diffractive optical element of the first embodiment. Has the effect of

Next, a third embodiment of the diffractive optical element of the present invention will be described with reference to FIGS. 6, 7 and 8. FIG. 6 shows the third
It is sectional drawing which shows the basic structure of the diffractive optical element of Example of 1st. FIG. 7 is a diagram showing the relationship between the grating period and the diffraction efficiency of the first-order diffracted light when the incident angle of the incident light in the diffractive optical element of the third embodiment is 20 °. FIG. 8 shows that the incident angle of the incident light in the diffractive optical element of the third embodiment is 30 °.
FIG. 9 is a diagram showing a relationship between the grating period and the diffraction efficiency of the first-order diffracted light in the case of FIG. The description of the same parts as in the first embodiment is omitted, and different parts will be described. As shown in FIG. 6, in the diffractive optical element according to the third example, in the region where the grating period is the largest (grating portion 2D ″), the number of steps in the staircase shape is 5, and as the grating period becomes smaller, The number of steps is 4 (grating unit 2
C "), 3 (grating section 2B"), and 2 (grating section 2A "). These off-axis lenses are configured to be as small as can be seen from FIG. / Λ is, for example, in the range of 1.2 to 1.6 in the grating section 2A ″.
The range from 1.6 to 3.1 corresponds to the area of the grating section 2B ", the range from 3.1 to 4.7 corresponds to the area of the grating section 2C",
The range from 4.7 to 5.5 is the grating section 2D ".
Area. The maximum film thickness of the grating portion in each region is, for example, 0.633 μm (grating portion 2A ″), 0.84 μm (grating portion 2B ″), 0.95 μm (grating portion 2C ″), respectively.
1.01 μm (grating part 2D ″). The diffractive optical element of this embodiment has, for example, an aperture of 1 mm (circular aperture), an incident light wavelength of λ = 0.6328 μm, and an incident angle θ.
= 20 °, focal length is 1.4 mm. This is an off-axis lens having a shorter focal length than the off-axis lens of the first embodiment. In such an off-axis lens having a short focal length, the rate of change in the grating period increases. As shown in FIG. 7, the diffraction efficiency of the diffractive optical element having the number of steps of 5 becomes smaller than the diffraction efficiency of the element having the number of steps of 4 when the normalized cycle is 4.7.
Further, the diffraction efficiency of the element having the number of steps of 4 becomes smaller than the diffraction efficiency of the element having the number of steps of 3 when the normalized cycle is 3.1. Similarly, the diffraction efficiency of the element having three steps becomes smaller than the diffraction efficiency of the element having two steps when the normalized period is 1.6. Therefore, by setting the optimum number of steps in the area of each of the grating sections 2A ", 2B", 2C ", and 2D", the diffraction efficiency can be increased over the entire area of the diffractive optical element.

FIG. 8 shows the relationship between the grating period and the diffraction efficiency of the first-order diffracted light in the diffractive optical element of the third embodiment when the incident angle of the incident light is 30 °. As shown in FIG. 8, when the number of steps is changed based on the incident angle of incident light or the like, the boundary value of the grating period at which the number of steps should be switched changes. However, the grating period at which the number of steps should be changed from 5 to 4 is in the range of about 4 to 7 times the wavelength of the incident light, and the grating period at which the number of steps should be changed from 4 to 3 is the wavelength of the incident light. If the grating period for changing the number of steps from 3 to 2 is in the range of about 1.5 to 3 times the wavelength of the incident light, the same effect can be obtained. confirmed.

In each of the above embodiments, the case where the present invention is applied to an off-axis lens which 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.

[0017]

As described above, according to the present invention, since the number of steps in the staircase shape of the grating portion is changed in accordance with the period of each grating portion of the diffractive optical element, the invention is particularly suitable for obliquely incident light. It becomes possible to increase the diffraction efficiency over the entire optical element. Further, by reducing the optimal number of steps as the grating period becomes smaller, it becomes possible to realize a diffractive optical element that can be easily manufactured even in a region where the grating period is small.

[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 grating period and 1 in the first embodiment.
Diagram showing the relationship between the diffraction efficiency of second-order diffracted light

FIG. 3 shows a grating section 2A in the first embodiment.
Showing the relationship between the grating period and the diffraction efficiency of the first-order diffracted light with the duty ratio of the parameter as a parameter

FIG. 4 is a diagram showing a state of light collection in the first embodiment.

FIG. 5 is a plan view showing a basic configuration of a second embodiment of the diffractive optical element of the present invention.

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

FIG. 7 shows an incident angle of incident light of 20 ° in the third embodiment.
Figure showing the relationship between the grating period and the diffraction efficiency of the first-order diffracted light in the case of

FIG. 8 shows an incident angle of incident light of 30 ° in the third embodiment.
Figure showing the relationship between the grating period and the diffraction efficiency of the first-order diffracted light in the case of

9A is a plan view showing the configuration of a conventional diffractive optical element, and FIGS. 9B and 9C are sectional views thereof.

[Explanation of symbols]

 1: substrate 2: grating part

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Claims (9)

(57) [Claims] 1. A diffractive optical element comprising: a substrate; and a grating portion formed on the substrate and having a stepped cross section, wherein the grating portion has a period greater than a first predetermined multiple of an incident wavelength. A small first region and the period
A second region that is larger than the first predetermined number times the incident wavelength, wherein the number of steps in the first region is smaller than the number of steps in the second region. For incident light, when the period is in the vicinity of the first predetermined number times the incident wavelength, the diffraction efficiency of the first region is equal to the diffraction efficiency of the second region. A diffractive optical element, which is substantially equal to: 2. The method according to claim 1, wherein the grating portion includes a region other than the first region and the second region, and the number of steps in the step shape gradually decreases as the period of the grating portion decreases. The diffractive optical element according to claim 1, wherein 3. The number of steps is 3 or more in a region where the period of the grating section is larger than a first predetermined number times the incident wavelength, and in a region where the period of the grating section is smaller than the first predetermined number times the incident wavelength. 3. The diffractive optical element according to claim 2, wherein the number is 2, and the first predetermined number is any value between 1.5 and 3. 4. In a region where the number of steps is 2, the duty ratio of the grating portion (the ratio of the region other than the air layer in one cycle) is any value between 0.15 and 0.5. The diffractive optical element according to claim 3, wherein: 5. The step number is 4 or more in a region where the period of the grating section is larger than a second predetermined number times the incident wavelength, the period of the grating section is smaller than the second predetermined number times the incident wavelength, and In an area larger than the first predetermined number, the number of steps is 3, and the second predetermined number is 2 to 5.
4. The diffractive optical element according to claim 3, wherein the first predetermined number is smaller than the second predetermined number. 6. An area in which the period of the grating section is larger than a third predetermined number times the incident wavelength, the number of steps is 5 or more, and an area smaller than the third predetermined number times and larger than the second predetermined number times. Then, the number of steps is 4, and the third
5. The diffractive optical element according to claim 4, wherein the predetermined number is a value between 4 and 7 (where the second predetermined number is smaller than the third predetermined number). 6. 7. The diffractive optical element according to claim 1, wherein the maximum film thickness of the grating portion varies according to the number of steps. 8. 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 period gradually decreases in the convex direction. 9. The diffractive optical element according to claim 1, wherein the pattern of the grating portion is a straight line, and the period gradually changes.