JP2021113896A - Transmission type diffraction grating element - Google Patents

Abstract

To provide a transmission type diffraction grating element having a structure in which a phase control layer is formed of inorganic materials while achieving high form accuracy.SOLUTION: A grating layer 2 formed on a substrate 1 transparent to light of wavelength in the range of 790 nm or more and 890 nm or less has a microstructure in which grids 21 having refractive index difference with respect to a gap 20 are periodically disposed. The grid 21 is formed of inorganic materials such as silicon oxide. When a duty ratio, a width ratio of the grid 21 to a grating period p, is denoted by d, height of the grid h satisfies 2.40dp≤h≤3.70dp and duty ratio d satisfied 0.5≤d≤0.7. Light entering the grating layer 2 at an incident angle α is emitted by diffraction at an emission angle β.SELECTED DRAWING: Figure 1

Description

The invention of this application relates to a technique of a transmission type diffraction grating.

Diffraction gratings are typical dispersion elements and are widely used in spectroscopic and other applications. A typical diffraction grating is an engraved line type reflective diffraction grating. This type of reflective diffraction grating has the advantages that it generally has less loss than the transmissive type and is not affected by internal stray light. However, the transmissive type is superior in terms of miniaturization of the optical system and the device to be incorporated. In the transmission type diffraction grating, a collimator and an imaging optical element can be arranged in the vicinity, which contributes to overall miniaturization.

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The above-mentioned reflection type diffraction grating employs a mechanical manufacturing method, and it is difficult to obtain high shape accuracy. Therefore, there is a limit to the improvement of diffraction efficiency. As for the transmission type diffraction grating, a mechanical manufacturing method such as molding is often adopted, and there is a similar problem.
The conventional general diffraction grating described above should be called an amplitude type, but in recent years, a phase type diffraction grating has been actively studied and a part of it has been put into practical use. A phase-type diffraction grating is an element that has a structure in which portions that change the phase of propagating light are periodically arranged, and suppresses 0th-order light to obtain a diffraction action. The periodic structure includes a structure in which a fine uneven shape is provided on the surface of the substrate constituting the element (relief type) and a structure in which regions having different refractive indexes are formed in the substrate (refractive index modulation). Type) is known.

Further, as a phase type diffraction grating, a phase type holographic diffraction grating produced by utilizing holography is known. There are two types of phase-type holographic gratings, relief type and refractive index modulation type. The refractive index modulation type (volume phase type holographic diffraction grating, Non-Patent Document 1) has higher diffraction efficiency than the relief type. Because of its effectiveness, it is used in applications such as astronomy and hyperspectral imaging.

The Volume Phase Holographic Grating (VPHG) is a bi-light beam interferometry method in which a photosensitive layer is provided with an organic photosensitive material such as gelatin dichromate while being sandwiched between glass substrates. This element is manufactured by projecting interferometric fringes and performing refractive index modulation. Since the VPHG is an element manufactured by such an optical process, it has high shape accuracy and can greatly improve the diffraction efficiency.

However, in a structure in which a phase control layer (a layer that changes the phase periodically) is formed of an organic material such as VPHG, a problem may occur in terms of product reliability. For example, when used in a humid environment, the properties may change due to moisture absorption. In addition, it may deteriorate as a result of being irradiated with strong light for a long period of time. Therefore, it is desirable to have a structure in which the phase control layer is formed of an inorganic material.
The present invention has been made to solve such a problem, and provides a transmission type diffraction grating element having a structure in which a phase control layer is formed of an inorganic material while achieving high shape accuracy. It is an object.

In order to solve the above problems, the transmission type diffraction grating element according to the invention of the present application is a transmission that uses light diffraction to advance light in a direction corresponding to a wavelength for light having a wavelength in the range of 790 nm or more and 890 nm or less. It is a type diffraction grating element, and includes a transparent substrate and a grating layer formed on the substrate. The lattice layer has a fine structure in which grids having a difference in refractive index with respect to the gap are periodically arranged. The grid is made of an inorganic material, and when the lattice period, which is the total width of one grid and one gap, is p, and the duty ratio, which is the ratio of the width of one grid to p, is d, it is a transmission type. In the diffraction grating element, the height h of the grid is 2.40 dp ≦ h ≦ 3.70 dp, and the duty ratio d is 0.5 ≦ d ≦ 0.8. Further, in another transmission type diffraction grating element, the height h of the grid is 2.74 dp ≦ h ≦ 3.70 dp, and the duty ratio d is 0.5 ≦ d ≦ 0.7. In another transmission type diffraction grating element, the height h of the grid is 3.09 dp ≦ h ≦ 3.70 dp, and the duty ratio d is 0.6 ≦ d ≦ 0.7.

As described below, according to the transmissive diffraction grating element of this application, since no organic material is used for the grating layer, it contributes to high reliability of the product. That is, even if it is used in a harsh environment in terms of humidity, temperature, etc., or under conditions where it is exposed to strong light for a long period of time, there is little deterioration and good quality diffracted light can be obtained for a long period of time. Therefore, the reliability of the embedded product can be maintained high.
Further, when the grid height h is 2.40 dp ≦ h ≦ 3.70 dp and the duty ratio d is 0.5 ≦ d ≦ 0.7 in the lattice period p and the duty ratio d, the wavelength is 790 nm to 890 nm. It is preferable in the application of controlling the direction by utilizing the diffraction of the light of. If the height h of the grid is 2.74 dp ≦ h ≦ 3.70 dp and the duty ratio d is 0.5 ≦ d ≦ 0.7, the direction is determined by utilizing the diffraction of light having a wavelength of 790 nm to 890 nm. It becomes more preferable in the application for controlling. Further, if the height h of the grid is 3.09 dp ≦ h ≦ 3.70 dp and the duty ratio d is 0.6 ≦ d ≦ 0.7, the direction is determined by utilizing the diffraction of light having a wavelength of 790 nm to 890 nm. It becomes even more preferable in the application for controlling.

It is sectional drawing of the transmission type diffraction grating element of embodiment. It is a perspective schematic view of the transmission type diffraction grating element of an embodiment. It is the schematic which showed the operation principle of the transmission type diffraction grating element of embodiment. It is a figure which showed the refractive index of silicon oxide which was a precondition in a simulation experiment. It is the schematic which showed the result of the simulation experiment. It is the schematic which showed the result of the simulation experiment. It is the schematic which showed the result of the simulation experiment. It is the schematic which showed the result of the simulation experiment. It is the schematic which showed the manufacturing method of the transmission type diffraction grating element of embodiment.

Next, an embodiment (embodiment) for carrying out the invention of this application will be described.
1 and 2 are schematic views of the transmission type diffraction grating element of the embodiment, FIG. 1 is a schematic cross-sectional view, and FIG. 2 is a schematic perspective view.
The diffraction grating element of the embodiment is a transmission type diffraction grating element that advances light in a direction corresponding to a wavelength by utilizing diffraction of light. This diffraction grating element includes a transparent substrate 1 and a grating layer 2 formed on the substrate 1. The term "diffraction grating element" has the same meaning as the commonly used term "diffraction grating", but since it has a part called a lattice layer, "diffraction grating" is used to avoid confusion. In other words, "element".

The substrate 1 is preferably made of an inorganic material and is typically made of glass such as quartz glass or borosilicate glass. In addition, a sapphire substrate may be used. The term "transparent" may be used as long as it is transparent at a wavelength for which direction control is performed by utilizing diffraction, and may not be transparent at a wavelength region where it is not necessary. The thickness of the substrate 1 is often on the order of mm, and FIGS. 1 and the like do not faithfully represent this thickness.
This diffraction grating element is a kind of phase type, and the lattice layer 2 is a layer for obtaining a diffraction action while suppressing 0th-order light by periodically changing the phase in the in-plane direction of the substrate 1. The lattice layer 2 is formed by forming a fine structure in which grids 21 having a refractive index difference with respect to the gap 20 are periodically arranged.

The gap 20 is not filled with material and has a refractive index of air or vacuum. For the grid 21, a material having a refractive index difference with respect to the gap 20 is appropriately selected. In this embodiment, the material of the grid 21 is silicon oxide. The grid 21 is formed by etching the sedimentary film as described later, and is in an amorphous state.

In this embodiment, the target wavelength (wavelength for controlling the desired direction by diffraction action) is in the wavelength range of 790 to 890 nm. The refractive index of silicon oxide (the real part of the refractive index) is about 1.4, and there is a difference in the refractive index with respect to the gap 20 which is air or vacuum.
As shown in FIG. 2, the lattice layer 2 has a so-called line and space (L & S, Line and Space) structure in a plane parallel to the substrate 1. The grid 21 has a linear shape extending parallel to each other, and the gaps 20 are evenly spaced.

一方、回折格子素子において、以下に式２として示す回折格子方程式が成立する。式２において、βは出射回折光の角度、ｍは回折光の次数（整数）である。

式１と式２から、出射回折光の角度βについて、以下の式３が成り立つ。

|sinβ|≦１であるから、式３より、次数ｍは以下の式４の条件を満たす整数となる。

The operating principle of the transmission type diffraction grating element of such an embodiment will be described with reference to FIG. FIG. 3 is a schematic view showing the operating principle of the transmission type diffraction grating element of the embodiment.
As shown in FIG. 3, light L1 is incident on the diffraction grating element at an incident angle α. At this time, in order to obtain the emitted diffracted light with high efficiency, it is necessary to satisfy the following equation 1 with respect to the incident angle α. In Equation 1, n is the refractive index of the atmosphere, p is the lattice period, and λ is the wavelength of light.

On the other hand, in the diffraction grating element, the diffraction grating equation shown in Equation 2 below is established. In Equation 2, β is the angle of the emitted diffracted light, and m is the order (integer) of the diffracted light.

From Equations 1 and 2, the following Equation 3 holds for the angle β of the emitted diffracted light.

Since | sinβ | ≦ 1, from Equation 3, the degree m is an integer satisfying the condition of Equation 4 below.

In Equation 4, α may not exist under certain conditions. For example, if n = 1 (air) and p = 0.1λ, α must satisfy sinα = 5, but such α does not exist. Further, if n = 1 and p = 0.5λ, the solution is α = 90 °, but this represents a state in which light is horizontally incident and cannot be realized as an optical arrangement. Conversely, if α for which Equation 4 holds exists, it functions as a diffraction grating. For example, if n = 1 and p = λ, α may satisfy sin α = 1/2. That is, α = 30 ° is a condition for incident light.

When the incident light is incident at an angle α within a certain range in this way, the diffracted light L2 is emitted at an angle β that satisfies the relationship of Equation 2. That is, the transmitted light L2 travels in the direction (emission angle β) according to the wavelength λ. At this time, if the lattice period p is reduced to about the wavelength of light, m = 0.1, and if the light with m = 0 (0th-order light) is suppressed, the first-order diffracted light can be efficiently extracted. ..
The difference in refractive index with respect to the gap is preferably about 0.3 or more, and when the gap is air, the refractive index of the grid 21 is 1.3 or more.

In order to suppress the 0th order light and increase the diffraction efficiency, the transmission type diffraction grating element of the embodiment optimizes the cross-sectional shape of the lattice layer 2. More specifically, the height of the grid 21 is optimized with respect to the ratio of the width of the grid 21 to the total width of the grid 21 and the gap 20 (lattice period p) (referred to as the duty ratio in this specification) d. There is. The inventor thought that in order to improve the diffraction efficiency, the aspect ratio of the grid 21 (the ratio of the height of the grid to the width of the grid) should be increased, and various studies including simulation experiments were conducted. rice field. As a result, it has been found that when the aspect ratio is increased, the diffraction efficiency may decrease, and there is a certain optimum aspect ratio.

4 to 8 are diagrams showing a simulation experiment regarding the aspect ratio of the grid 21. Of these, FIG. 4 is a diagram showing the refractive index of silicon oxide as a precondition in the simulation experiment, and FIGS. 5 to 8 are schematic views showing the results of the simulation experiment.

In this simulation experiment, a diffraction grating element used for light having a wavelength in the range of 790 to 890 nm was assumed as described above. The “light having a wavelength in the range of 790 to 890 nm” may be all wavelengths in the range of 790 to 890 nm, or may target only any wavelength in the range. The lattice period p is preferably equal to or close to the target wavelength, and is 833 nm in this example. The material of the substrate 1 and the grid 21 is assumed to be silicon oxide. The refractive index for silicon oxide is shown in FIG. In FIG. 4, n is the real part of the refractive index and k is the imaginary part (extinction coefficient).

In the simulation experiment, the diffraction efficiency of each wavelength was calculated by the RCWA method while changing the dimensions of the grid 21 on the premise of the refractive index shown in FIG. Specifically, the duty ratio d was changed by 0.1 between 0.4 and 0.8, and the height of the grid 21 was changed by 100 nm between 1200 and 2600 nm. The results are shown in FIGS. 5-8.

FIGS. 5 to 7 show the results of evaluating the results of the simulation experiment according to different criteria. In FIGS. 5 to 7, the horizontal axis of the graph in the frame of each square is the wavelength and the vertical axis is the diffraction efficiency, and the diffraction efficiency of the TE wave, the diffraction efficiency of the TM wave, and the average diffraction efficiency of both are shown. There is. FIG. 8 shows each aspect ratio in each graph of FIGS. 5 to 7 for reference. The calculated diffraction efficiency is the so-called absolute diffraction efficiency, which is the ratio of the diffracted light intensity to the incident light intensity. In addition, the diffraction efficiency was calculated for the two polarized lights, the TE wave and the TM wave, and the average value was calculated.

In FIG. 5, preferable conditions in which a diffraction efficiency of 60% or more is obtained are shown in a thick frame. In FIG. 6, more preferable conditions in which a diffraction efficiency of 70% or more is obtained are shown in a thick frame. In FIG. 7, more preferable conditions in which a diffraction efficiency of 80% or more is obtained are shown in a thick frame. 60% or more, 70% or more, and 80% or more are average values for wavelengths and average values for TE waves and TM waves. Further, "obtaining diffracted light" means that the light is advanced in a direction corresponding to a wavelength by a diffracting action. In the above example, for example, in the case of light of 833 nm that matches the lattice period, in principle, it is emitted at an angle of 30 ° (an angle with respect to the direction perpendicular to the substrate 1). In FIGS. 5 to 8, for convenience of illustration, the results when the grid height exceeds 2000 nm are not shown.

The conditions shown in thick frames in FIG. 5 are conditions with a duty ratio of 0.5 to 0.7 and an aspect ratio of 2.40 to 3.60. Under this condition, an average diffraction efficiency of about 60% or more can be obtained in the wavelength range of 790 to 890 nm, the difference between the diffraction efficiencies of the TE wave and the TM wave is 0.22 or less, and the wavelength of 790 to 890 nm. The variation (difference between the maximum value and the minimum value) of the diffraction efficiency of the TE / TM average in the range is 0.1 or less. Diffraction efficiency of 60% or more is achieved on average other than the thick frame, but the difference in diffraction efficiency between TE wave and TM wave exceeds 0.22, and the variation between wavelengths of diffraction efficiency is 0. It may exceed 1. That is, when the duty ratio is 0.5 to 0.7 and the aspect ratio is 2.40 to 3.60, the difference between the diffraction efficiencies of the TE wave and the TM wave is 0.22 or less, and the variation between the wavelengths of the diffraction efficiency is 0. This means that an average diffraction efficiency of 60% or more can be obtained while keeping the value at 1.1 or less.

The conditions shown in thick frames in FIG. 6 are conditions with a duty ratio of 0.5 to 0.7 and an aspect ratio of 2.74 to 3.60. Under this condition, an average diffraction efficiency of about 70% or more can be obtained in the wavelength range of 790 to 890 nm, the difference between the diffraction efficiencies of the TE wave and the TM wave is 0.22 or less, and the TE / TM average. The variation in diffraction efficiency between wavelengths was 0.1 or less. Diffraction efficiency of 70% or more is achieved on average other than the thick frame, but the difference in diffraction efficiency between TE wave and TM wave exceeds 0.22, and the variation between wavelengths exceeds 0.1. Or something. That is, when the duty ratio is 0.5 to 0.7 and the aspect ratio is 2.74 to 3.60, the difference between the diffraction efficiencies of the TE wave and the TM wave is 0.22 or less, and the variation between the wavelengths of the diffraction efficiency is 0. This means that an average diffraction efficiency of 70% or more can be obtained while keeping the value at 1.1 or less.

The conditions shown in thick frames in FIG. 7 are conditions with a duty ratio of 0.6 to 0.7 and an aspect ratio of 3.09 to 3.60. Under this condition, an average diffraction efficiency of about 80% or more can be obtained in the wavelength range of 790 to 890 nm, the difference in diffraction efficiency between the TE wave and the TM wave is 0.12 or less, and the TE / TM average. The variation in diffraction efficiency between wavelengths was 0.1 or less. That is, when the duty ratio is 0.6 to 0.7 and the aspect ratio is 3.09 to 3.60, the difference between the diffraction efficiencies of the TE wave and the TM wave is 0.12 or less, and the variation between the wavelengths of the diffraction efficiency is 0. It means that the diffraction efficiency of 80% or more can be obtained on average while keeping the diffraction efficiency at .2 or less.

In each of the above preferable results, the upper limit of the aspect ratio was 3.60, but as a result of a more detailed simulation experiment, it was confirmed that the same result can be obtained if the aspect ratio is up to 3.70. There is. That is, based on the above results, a duty ratio of 0.5 to 0.7 and an aspect ratio of 2.40 to 3.70 are preferable, and a duty ratio of 0.5 to 0.7 and an aspect ratio of 2.74 to 3.70. Is more preferable, and it is concluded that a duty ratio of 0.6 to 0.7 and an aspect ratio of 3.09 to 3.70 are even more preferable.

It should be noted that the higher the aspect ratio, the more difficult it is to obtain shape accuracy, and the higher the aspect ratio, the higher the grid height h must be (this forms a narrower and deeper gap). Therefore, the aspect ratio is preferably 3.70 or less.
Further, when the duty ratio d exceeds 0.7, a narrower groove (gap 20) must be formed, which makes processing difficult. Therefore, it becomes difficult to secure the required shape accuracy. This problem becomes more pronounced at higher aspect ratios. Therefore, in this sense as well, the duty ratio d is preferably 0.7 or less.
As an example of preferable dimensions, the duty ratio can be 0.6 and the aspect ratio can be 3.60 (grid height 1800 nm).

In the transmission type diffraction grating element of such an embodiment, consideration is also given to mechanical durability. That is, as shown in FIG. 1, a protective layer 3 is provided on the side of the lattice layer 2 opposite to the substrate 1 (not shown in FIG. 2).
In this embodiment, the protective layer 3 is made of the same material as the substrate 1 and the grid 21, and is made of silicon oxide. The protective layer 3 is in a state of closing the gap 20 and covers the entire lattice layer 2 including the upper surface of each grid 21. However, although a part of the protective layer 3 can enter the gap 20, the protective layer 3 is not in a state of being filled in the gap 20. That is, a gap 20 is formed as a cavity, and this portion has a refractive index of air or vacuum. The protective layer 3 has an effect of protecting each grid 21 from collapsing or chipping.

A method for manufacturing the transmission type diffraction grating element of such an embodiment will be described below. FIG. 8 is a schematic view showing a method of manufacturing the transmission type diffraction grating element of the embodiment.
As shown in FIG. 8 (1), the lattice layer film 4 is formed on the substrate 1 with the material of the grid 21. Any method can be adopted as the production method, such as sputtering, ALD (atomic layer deposition method), sol-gel method, CVD, and plating. Alternatively, a method of laminating another substrate and polishing it to a predetermined thickness may be used. In any case, the thickness of the lattice layer film 4 is equal to or slightly thicker than the height of the grid 21 of the transmission type diffraction grating element to be produced.

Next, a resist is applied and patterned by photolithography to form a resist pattern layer 5 as shown in FIG. 8 (2). The resist pattern layer 5 has a striped shape similar to the line and space of the lattice layer 2 to be formed. In photolithography, various exposure methods can be adopted as long as the required fine structure can be obtained. For example, g-line, i-line, excimer light exposure, direct drawing exposure with an electron beam or laser, two-luminous flux interference exposure, Talbot interference exposure and the like can be adopted.

Next, dry etching is performed using the resist pattern layer 5 as a mask to form the lattice layer 2. Dry etching is anisotropic etching such as the reactive ion beam method. Then, as shown in FIG. 8 (3), the resist pattern layer 5 is removed by ashing or the like. In addition to the resist, any other material can be used for the mask as long as a sufficient difference in etching rate with respect to the material of the lattice layer film 4 can be obtained.
Then, as shown in FIG. 8 (4), the transmission type diffraction grating element of the embodiment is completed by forming the protective layer 3 on the lattice layer 2. The protective layer 3 is often formed by film formation such as sputtering or CVD, but it is desirable to form an isotropic film formation so that the material is not filled in the gap 20.

The transmissive diffraction grating element of the embodiment manufactured in this manner is arranged in a posture in which light is incident at a predetermined angle. Then, the diffracted light emitted at a predetermined angle is used. The transmission type diffraction grating element of the embodiment can be used for various purposes and devices, and can be used, for example, for an OCT (optical coherence tomography) device. Specifically, in the FD (Fourier Domain) -OCT, a transmission diffraction grating element is placed at a position where the interference light between the reference light and the light from the object is incident, and a line is placed at the position where the emitted diffracted light is incident. Place the sensor. Since the diffracted light travels in different directions depending on the wavelength, light for each wavelength is incident on each pixel of the line sensor, and a spectrum of interference light is obtained.

Further, as another application, the transmission type diffraction grating element of the embodiment can be used for the purpose of making the wavelength of the laser variable (tunable laser) or stabilizing (wavelength stabilizing laser). For example, the transmissive diffraction grating element of the embodiment is incorporated in an external resonator to resonate the emitted diffracted light, and the emission wavelength can be made variable by changing the posture of the diffraction grating element.

In any of the application examples, since the transmission type diffraction grating element of the embodiment does not use an organic material for the lattice layer 2, it contributes to high reliability of the product. That is, even when it is used in a harsh environment in terms of humidity, temperature, etc., or when it is used under conditions where it is exposed to strong light for a long period of time, there is little deterioration and good quality diffracted light can be obtained for a long period of time. Therefore, the reliability of the embedded product can be maintained high. Further, in the transmission type diffraction grating element of the embodiment, since the lattice layer 2 is patterned by the optical process as described above, the shape accuracy is high and the diffraction efficiency is also high in this respect as well.

In the transmission type diffraction grating element, a layer having an arbitrary function may be formed on the incident side or the outgoing side. For example, an antireflection film may be provided on the entrance surface (upper surface of the protective layer 3 or the lower surface of the substrate 1), or a polarizing beam splitter film may be provided on the entrance surface or the exit surface.

In the above embodiment, the material of the grid 21 is silicon oxide, but any material can be used as long as it is an inorganic material that can be microfabricated and has a solid phase at room temperature. This is because they have a sufficient refractive index difference with respect to air or vacuum.
However, the above-mentioned materials such as silicon oxide are often used as materials for microfabrication in semiconductor processes and the like, and various processing techniques have been developed, which are suitable. Further, it is particularly preferable to use the same material for the substrate 1 and the grid 21 because peeling fracture at the interface is unlikely to occur.
Further, as the protective layer 3, a material other than silicon oxide can be used as long as it is translucent at the wavelength of the light to be diffracted. At this time, it is preferable to use the same material as the grid 21 because peeling fracture at the interface is unlikely to occur.

1 Substrate 2 Lattice layer 20 Gap 21 Grid 3 Protective layer 4 Lattice layer film 5 Resist pattern layer p Lattice period d Duty ratio

Claims (3)

A transmissive diffraction grating element that uses light diffraction to direct light in a direction corresponding to a wavelength for light having a wavelength in the range of 790 nm or more and 890 nm, and is a transparent substrate and a grating formed on the substrate. It has layers and
The lattice layer has a fine structure in which grids having a difference in refractive index with respect to the gap are periodically arranged.
The grid is made of inorganic material and
When the lattice period, which is the total width of one grid and one gap, is p, and the duty ratio, which is the ratio of the width of one grid to p, is d, the height h of the grid is 2.40 dp ≦ h ≦ 3. A transmission type diffraction grating element having a duty ratio of .70 dp and a duty ratio d of 0.5 ≦ d ≦ 0.7. A transmissive diffraction grating element that uses light diffraction to direct light in a direction corresponding to a wavelength for light having a wavelength in the range of 790 nm or more and 890 nm, and is a transparent substrate and a grating formed on the substrate. It has layers and
The lattice layer has a fine structure in which grids having a difference in refractive index with respect to the gap are periodically arranged.
The grid is made of inorganic material and
When the lattice period, which is the total width of one grid and one gap, is p, and the duty ratio, which is the ratio of the width of one grid to p, is d, the height h of the grid is 2.74 dp ≦ h ≦ 3. A transmission type diffraction grating element having a duty ratio of .70 dp and a duty ratio d of 0.5 ≦ d ≦ 0.7. A transmissive diffraction grating element that uses light diffraction to direct light in a direction corresponding to a wavelength for light having a wavelength in the range of 790 nm or more and 890 nm, and is a transparent substrate and a grating formed on the substrate. It has layers and
The lattice layer has a fine structure in which grids having a difference in refractive index with respect to the gap are periodically arranged.
The grid is made of inorganic material and
When the lattice period, which is the total width of one grid and one gap, is p, and the duty ratio, which is the ratio of the width of one grid to p, is d, the height h of the grid is 3.09 dp ≦ h ≦ 3. A transmission type diffraction grating element having a duty ratio of .70 dp and a duty ratio d of 0.6 ≦ d ≦ 0.7.

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