JP4749789B2 - Transmission type diffractive optical element - Google Patents

Description

The present invention relates to a diffractive optical element for incident light wavelengths of 1350 nm to 1750 nm using a spectroscopic optical element used in a spectroscopic analysis instrument or an optical instrument using a plurality of wavelengths, particularly a transmissive diffraction grating having a diffraction grating having a grating interval of about the wavelength. It relates to an element.

A diffractive optical element represented by a diffraction grating is widely used for analyzing an optical spectrum in a spectroscopic analysis instrument. In spectroscopic analysis, high energy utilization efficiency is required over a wide band. In order to obtain high diffraction efficiency in a wide band, a reflective diffraction grating is suitable. Reflective diffraction gratings are widely used in spectroscopic analyzers because they have a good ratio of change in diffraction angle with respect to wavelength, that is, wavelength angle dispersion characteristics.

However, the reflection type diffraction grating whose diffraction grating interval is about the wavelength has a problem that the wavelength dispersion is large and the wavelength loss characteristic changes remarkably with respect to the polarization state of light. Since such a diffraction grating operates in the resonance region, it exhibits a complicated wavelength loss characteristic and cannot realize a stable operation over a wide band (see, for example, Non-Patent Document 1).

On the other hand, the transmission diffraction grating is characterized by low polarization dependent loss (PDL) and high diffraction efficiency in a specific wavelength region. In the case of a transmissive diffraction grating having a periodic groove structure, the shape of the groove and the refractive index of the material forming the groove have a great influence on the diffraction efficiency. In particular, for a so-called laminar transmission diffraction grating 100 having a rectangular cross section perpendicular to the longitudinal direction of the groove 112 as shown in FIG. 1, the groove depth h should be sufficiently deep with respect to the groove period p. Thus, it is known that high diffraction efficiency can be obtained. The ratio of the groove depth to the groove period is called an “aspect ratio”, and it can be said that a diffraction grating having a groove with a high aspect ratio (h / p) has a high diffraction efficiency.

However, it is difficult to manufacture a groove with a high aspect ratio with high accuracy in terms of processing technology, and it is particularly difficult to manufacture when the groove period is small. Considering the groove shape of the diffraction grating and its optical characteristics normalized by wavelength, it is desirable to use a high refractive index material and to produce a transmission diffraction grating with a small aspect ratio in order to avoid processing problems. I understand that. That is, in order to obtain equivalent optical characteristics, it is more advantageous in terms of manufacturing process to make the groove shallower by using a material having a higher refractive index as much as possible than to make a deep groove in a material having a low refractive index.

FIG. 2 shows an example of the diffraction efficiency of a typical laminar diffraction grating and the wavelength dependence of PDL. The substrate in FIG. 1 is made of quartz glass, and the ridge portion is made of Ta ₂ O ₅ which is a high refractive index material. The groove depth h is 1350 nm, the groove period p is 1111 nm, and the groove width is 555 nm. From the figure, it is understood that the diffraction efficiency is almost 70% or more for both the TE mode and the TM mode over a wide wavelength range of 1350 to 1750 nm, and the PDL is also as small as −0.7 to +0.5 dB.

In a diffraction grating, when light having a wide wavelength band is incident on the diffraction grating, the high-order diffraction angle for short-wavelength incident light and the low-order diffraction angle for long-wavelength incident light overlap, making it impossible to separate the wavelengths. It is well known. The wavelength range in which diffraction gratings can be used under the condition that diffracted light does not overlap due to this phenomenon is called the free spectral range of the diffraction grating, the shortest wavelength is λ ₁ , the longest wavelength is λ ₂ , and the diffraction order is m. Then, it is defined in the range where the following equation holds. λ ₂ −λ ₁ ≦ λ ₁ / m (where λ ₁ <λ ₂ ) The diffraction grating can be used under the condition that the overlapping of diffracted light does not occur within the wavelength range in which this equation is satisfied.
Tatsuta Tatsuo, "Applied Optics 1", Bafukan, 1990

The transmission type diffraction grating has a wide band and low PDL characteristics as compared with the reflection type diffraction grating. However, as can be seen from the above example, the absolute value of the PDL tends to increase in the wavelength range where the diffraction efficiency is high (1500 to 1600 nm). In addition, there is a problem that the wavelength range with high diffraction efficiency and sufficiently low PDL is not so wide.

Further, when a diffraction grating is used at a wavelength in the infrared region, visible light may be used as guide light. For example, in the case of light with wavelengths of λ ₁ = 633 nm and λ ₂ = 1550 nm, even if m = 1. It falls outside the range where the above equation holds and is outside the free spectral range. Therefore, when these wavelengths are simultaneously incident on the diffraction grating and measurement is performed, noise due to overlapping of wavelengths is generated. In order to avoid such a problem, means for blocking incident light with an unnecessary wavelength using an optical filter or switching the photodetector is taken, but the optical system becomes complicated and the measurement operation is complicated. There was a problem to become.

The present invention has been made in order to solve such a problem, and provides an additional function without adding new parts, or compensates for the disadvantages of the transmission diffraction grating, and has a high diffraction efficiency. An object of the present invention is to provide a transmission type diffractive optical element for incident light wavelengths of 1350 nm to 1750 nm with a small PDL.

In the present invention, in order to solve the above-mentioned problem, a diffraction grating in which a large number of ridge-shaped convex portions having a substantially rectangular cross-sectional shape perpendicular to the longitudinal direction are periodically provided on the substrate surface at an arrangement period of about the wavelength of incident light The following means were used for a transmissive diffractive optical element having an incident light wavelength of 1350 nm to 1750 nm .

A plurality of parallel striped thin film layers having a uniform composition in a plane parallel to the substrate surface is provided only on the top surface of the ridge-shaped convex portion, and parallel to the substrate surface and in the substrate surface or in the substrate. A thin film layer having an interface parallel to the surface and having a uniform composition in a plane parallel to the substrate surface and having no periodic structure is provided in the same direction as the arrangement period of the ridge-shaped convex portions.

Various functions for diffracted light can be imparted to the thin film layer having no periodic structure in the same direction as the arrangement period of the ridge-shaped protrusions. For example, if an optical filter that removes light other than light having a wavelength using this thin film layer is used, a function of removing incident light outside the free spectral range can be provided without providing an optical filter outside.

On the other hand, the striped thin film layer provided only on the top surface of the ridge-shaped convex portion has an effect of reducing loss when light enters the diffraction grating. Therefore, by using these two types of thin film layers in combination with a so-called laminar diffraction grating, the incident light has a function not found in a conventional transmission diffraction grating, has a high diffraction efficiency in a wider wavelength range, and has a low PDL. A transmission type diffractive optical element for wavelengths of 1350 nm to 1750 nm can be provided.

The striped thin film layer is provided only on the top surface of the ridge-shaped convex portion. The top of the ridge of the laminar diffraction grating is one of the light incident surfaces to the diffraction grating, and is suitable as a surface on which a thin film layer for reducing reflection of incident light is formed. Since the top of the ridge has a striped plane, a striped thin film layer can be obtained by forming a thin film layer here.

When the ridge is made of a material having a refractive index n _{H and} the groove depth at which a desired characteristic is obtained as a transmission diffraction grating is h, the refractive index n _L of the striped thin film layer provided on the top surface of the ridge, The height h _{H of the} ridge made of a material having a film thickness h _L and a refractive index n _H is set so as to satisfy the following formula: n _H × h = n _L × h _L + n _H × h _H

A laminar diffraction grating made of a single material can provide the best optical characteristics with a specific groove shape. Given the groove depth that gives the best optical properties for the refractive index of the ridge material, the thin film layer is refracted so that the groove depth is optically equivalent to this when the thin film layer is provided. By selecting the ratio and film thickness, the optical characteristics of the striped thin film layer are added to the characteristics of the diffraction grating, the diffraction efficiency is high in a wider wavelength range, and the transmission light diffraction for incident light wavelengths of 1350 nm to 1750 nm is low. An optical element can be provided.

In the case where the striped thin film layer is a multilayer film of two or more layers made of two or more kinds of materials, when the average refractive index of the entire multilayer film is n _La and the total film thickness is h _Lt , the n _{L Is} replaced with n _La and h _L is replaced with h _Lt. The effect of reducing reflection is generally better when a multilayer film is used than when a single layer film is used. In this case, by replacing the refractive index and film thickness of the single-layer film by the average refractive index and the total film thickness of the entire multilayer film, the diffraction efficiency is high in a wide wavelength region and the incident light wavelengths for 1350 nm to 1750 nm are low . A transmission type diffractive optical element can be provided.

Further, a transmission characteristic having wavelength selectivity is imparted to a thin film layer having no periodic structure in the same direction as the arrangement period of the ridge-shaped protrusions. For example, the thin film layer has a film configuration of an optical filter that transmits light to be used and reflects or absorbs other light. By providing a thin film layer with such a film structure between the ridge-shaped convex part and the substrate, incident light outside the free spectral range can be blocked, reducing noise during measurement. it can. Accordingly, it is not necessary to provide a separate optical filter or switch the photodetector.

According to the present invention, the thin film layer having no periodic structure in the same direction as the arrangement period of the ridge-shaped convex portions can perform various functions without adding new components to the diffracted light of the transmission diffraction grating. Can be granted. Further, the striped thin film layer provided only on the top surface of the ridge-shaped convex portion has an effect of reducing loss when light enters the diffraction grating. Therefore, by using these two types of thin film layers in combination with a so-called laminar type diffraction grating, it has a function not found in conventional transmission type diffraction gratings, or compensates for defects, and has high diffraction efficiency in a wider wavelength range, and A transmission type diffractive optical element for incident light wavelengths of 1350 nm to 1750 nm with low PDL can be provided.

The transmission type diffractive optical element for incident light wavelengths of 1350 nm to 1750 nm according to the present invention has a ridge-shaped convex portion (hereinafter simply referred to as a ridge) 14 having a rectangular xy section on the substrate 50 as shown in FIG. The thin film layer 34 is provided between the ridge 14 and the substrate 50 based on a laminar type diffraction grating periodically arranged with an arrangement period of about the wavelength of incident light. The thin film layer 34 is uniformly formed on the substrate surface and has no periodicity in at least the direction of the arrangement period of the ridges 14 (x direction). Various functions can be imparted to the thin film layer 34 by designing the film configuration.

The thin film layer can be provided with a wavelength selective transmission characteristic that transmits light of a wavelength to be used and reflects or absorbs light of other wavelengths, that is, an optical filter function. By providing such an optical filter between the ridge 14 and the substrate 50, stray light can be reduced. In particular, an optical filter that cuts off short-wavelength light that generates high-order diffracted light and transmits long-wavelength light that is the wavelength band used is effective in reducing stray light.

For example, when performing spectroscopic measurement in the infrared region, optical path adjustment and sample position adjustment are performed using visible light as guide light, and the higher-order diffracted light of this guide light becomes noise during measurement. In such a case, the thin film layer 34 is an edge filter or a band pass filter, and noise can be reduced by transmitting the wavelength range of the signal and reflecting the wavelength of the guide light. One example will be described below.

In the transmission type diffractive optical element having the structure shown in FIG. 3, a substrate 50 is made of quartz glass, and a ridge 14 made of Ta ₂ O ₅ is formed on the substrate 50. SiO ₂ and Ta ₂ are formed between the substrate and the ridge. A thin film layer 34 made of a laminate of O ₅ is inserted. The thin film layer 34 has a film configuration in which a three-layer structure of Ta ₂ O ₅ / SiO ₂ / Ta ₂ O ₅ (film thickness 38 nm / 108 nm / 38 nm) is repeated 12 times. As the shape of the diffraction grating portion, the groove depth h was 1350 nm, the groove period p was 1111 nm, and the groove width was 555 nm, as in the conventional example of FIG.

FIG. 4 shows an example of diffraction efficiency and wavelength dependence of PDL over a wide wavelength range of 1350 to 1750 nm of this transmission type diffractive optical element. Compared with the characteristics of FIG. 2, it can be seen that the insertion of the thin film layer 34 improves the diffraction efficiency for the TE mode and the TM mode over a wide wavelength range. Also, no adverse effect on PDL is seen.

The function of the diffractive optical element will be described using an example using a general infrared absorption measurement system shown in FIG. A xenon lamp 121 was used as the light source, and a helium neon laser 111 having a wavelength of 633 nm was used as the guide light 201. The white light 301 emitted from the xenon lamp 121 is converted into parallel light by the lens 122, and the visible light component is removed by the infrared light transmission filter 124. The infrared light 302 is incident on the subject 116, becomes detection light 303 having a spectrum that has been changed according to the properties of the subject, and is incident on the transmission diffractive optical element 100. The detection light 303 passes through the thin film layer 34 and is diffracted by the diffraction grating. The diffracted light 304 is angle-separated by the wavelength component, and is condensed on the incident surface of the photodetector array 115 by the lens 132.

On the other hand, the guide light 201 is combined with the infrared light 302 by the semi-transparent mirror 118 and irradiates the surface of the subject 116. The guide light that has passed through the subject 116 is reflected by the thin film layer 34 of the transmissive diffractive optical element 100 and becomes the light 202 separated from the detection light 303.

The amount of light obtained by adding the 0th-order transmitted light and transmitted diffracted light generated by the diffractive optical element was measured as the transmitted light amount. When the transmittance is the ratio of the amount of transmitted light to the amount of incident light, the transmittance of light having a wavelength of 633 nm is 66% when the thin film layer 34 is not provided, and the transmittance of light having a wavelength of 1550 nm is 83% for TE polarized light and TM polarized light. 94%. On the other hand, when the thin film layer 34 was provided, the transmittance of light having a wavelength of 633 nm was 2% or less, and the transmittance of light having a wavelength of 1550 nm was 83% for TE polarized light and 94% for TM polarized light.

That is, infrared light of 1550 nm does not change depending on the presence or absence of the thin film layer, and maintains high transmittance. However, by attaching the thin film layer 34, the transmittance of light at 633 nm, which causes noise, can be reduced to 1/30 or less, and it is not necessary to provide an optical filter or the like outside.

The above is an example of a function that can be added by inserting a thin film layer having no periodicity in the same direction as the ridge arrangement direction. This additional function is not necessarily limited to an optical function. For example, the thin film layer may be an etching stop layer when a groove is formed by dry etching.

On the other hand, a transmission type diffraction grating using such a high refractive index material tends to have a large reflection loss on the surface of the diffraction grating, which tends to lower the diffraction efficiency. Conventionally, as means for improving the diffraction efficiency of a normal transmissive diffraction grating, means for providing a film having a structure having a dereflection effect on the incident side has been taken (for example, Japanese Patent Laid-Open No. 10-177107 or Shiozaki). Gaku, 1 other, "Polarization-independent design of multilayer diffraction gratings using rigorous coupled wave theory and its prototype", IEICE Technical Report, The Institute of Electronics, Information and Communication Engineers, January 2004, PN2003-60, p. 47-50).

Accordingly, the transmission type diffractive optical element of the present invention is also provided with a thin film layer 32 on the top surface of the ridge 14 of the laminar type diffraction grating as shown in FIG. The thin film layer 32 on the top surface of the ridge 14 has a stripe shape substantially the same shape as the top surface of the ridge, and exists in parallel with the same period in the same x direction as the ridge period of the diffraction grating. Further, it is desirable to apply a dereflection coating 38 on the back surface of the substrate 50 in order to prevent the diffracted light from being reflected at the interface between the substrate and air.

As described above, the striped thin film layer 32 provided with a period that matches the period of the grooves of the diffraction grating has a role of reducing loss when light enters the diffraction grating, and is the same as the period of the grooves of the diffraction grating. It can be said that the thin film layer 34 having no periodic structure in the direction plays a role of correcting optical characteristics after being diffracted. Therefore, by adopting such a configuration, it is possible to realize a transmissive diffractive optical element for incident light wavelengths of 1350 nm to 1750 nm that has higher diffraction efficiency and lower polarization dependent loss (PDL) in a wider wavelength range than conventional diffraction gratings. .

A laminar diffraction grating made of a single material can provide the best optical characteristics in a specific groove shape. In the case of a transmissive diffraction grating, the optical characteristics vary depending on the number of grooves, the refractive index of the material, the depth of the grooves, the width of the ridge, and the like. Here, the refractive index of the material when the best optical characteristics can be obtained is n _H , the groove depth at which the best optical characteristics can be obtained is h, and even when a thin film layer is provided in each part, apparently the entire ridge A condition is determined so that the optical length (n _H × h) of the height (groove depth) does not change.

First, the case where the striped thin film layer 32 shown in FIG. 6 is provided only on the top surface of the ridge 14 will be described. As shown in FIG. 7A, a thin film layer 34 is provided between the ridge 14 of the diffraction grating and the substrate 50. However, when the thin film layer is not provided on the top surface of the ridge, the ridge 64 is bent. Let the rate be n _H and the groove depth at which the best optical properties are obtained be h. As shown in FIG. 7B, when the striped thin film layer 32 is provided only on the top surface of the ridge 14 of this diffraction grating, the height of the ridge is h _H and the period coincides with the period of the grooves of the diffraction grating. When the refractive index of the provided striped thin film layer 32 is n _L and the film thickness is h _L , n _H × h = n _H × h _H + n _L × h _L (1) may be satisfied.

The striped thin film layer 32 may be a multilayer film. In the case of a multilayer film, the refractive index of each material constituting the multilayer film is n1, n2, n3,..., _NN , and the respective film thicknesses are h _L1. , H _L2 , h _L3 ,..., H _N , n _L, h _L is n _L = (n1 × h _L1 + n2 × h _L2 + n3 × h _L3 +... + _{N N} × h _N ) / h _L When defined by h _L = h _L1 + h _L2 + h _L3 +... + h _N , the expression (1) or (2) is satisfied.

As described above, the optical length of the ridge height (groove depth) at which the best characteristics can be obtained only with the basic configuration of the diffraction grating is provided with a period that matches the period of the groove of the diffraction grating so that it does not change. By setting the height of the stripe-shaped thin film layer and the ridge and providing each layer, the optical characteristics in the case of the basic configuration of the diffraction grating can be maintained, and the optical characteristics of the stripe-shaped thin film layer can be added.

For example, when the striped thin film layer that matches the groove period of the diffraction grating has a reflection-reducing effect in the used wavelength band, the reflection loss when light enters the diffraction grating can be reduced, so only the basic structure of the diffraction grating It is possible to obtain optical characteristics that exceed the diffraction efficiency in this case.

When the stripe-shaped thin film layer is a reduced reflection film, the film configuration is such that the transmitted light is maximized with respect to light in the working wavelength range incident on the diffraction grating at a specific incident angle. When providing anti-reflection films with different thicknesses on both the ridge top surface and the groove bottom surface, the ridge top surface side is configured to produce a anti-reflective effect on the ridge material, and the groove bottom surface side However, it is desirable that the film has a structure in which the anti-reflection effect is generated with respect to the material at the bottom of the groove. The anti-reflection film may be a single layer film or a multilayer film.

If a single-layer film, a material having a refractive index lower than the material of the ridge, the thickness is to use wavelength lambda, and the refractive index of the antireflection film and n _L, and typically lambda / 4n _L about Become. However, depending on the size of the incident angle, polarization dependence occurs in the characteristics of the antireflection film portion. If the thickness of the anti-reflection film is shifted from λ / 4n _L by an appropriate value so that the polarization-dependent characteristic of the anti-reflection film and the polarization-dependent characteristic of the diffraction grating cancel each other, not only the diffraction efficiency is high, A transmission type diffraction grating having a small polarization dependency can be obtained.

Further, by correcting the wavelength characteristic of the diffraction efficiency by using the ripple generated in the wavelength characteristic of the transmittance due to the thin film layer, the PDL is reduced and the transmittance in the use band is increased at the same time. An effect can be acquired and the maximum value of diffraction efficiency can be raised.

Although the striped thin film layer has been described as an example on the surface of the diffraction grating, it may be provided inside the ridge. Similarly, a thin film layer having no periodicity in the direction of the ridge arrangement period may be provided not only between the substrate and the ridge but also in the substrate.

Hereinafter, an embodiment of a transmission diffraction grating for incident light wavelengths of 1350 nm to 1750 nm in the present invention will be described. First, the manufacturing method will be described.

(Manufacturing method) First, the manufacturing method of the transmission type diffraction grating 10 of this invention which has the striped thin film layer 32 of the period which corresponds to the arrangement period of a ridge only on the top surface of the ridge 14 as shown in FIG. First, a thin film layer having no periodic structure is formed on a glass substrate by a sputtering method or a vapor deposition method in the same direction as the arrangement period (x direction) of the ridges of the diffraction grating. Next, the material for forming the ridge 14 is formed by sputtering or vapor deposition until a predetermined designed film thickness is obtained. Furthermore, a thin film layer provided on the top surface of the ridge of the diffraction grating is formed by sputtering or vapor deposition so as to have a predetermined film configuration. Thereafter, a Cr film serving as a mask for forming the groove is formed by sputtering, and a mask pattern is formed by photolithography and etching so as to have a predetermined period and groove width. The material film that becomes the ridge portion through this mask is dry-etched by an inductive coaxial plasma type reactive ion etching (ICP-RIE) apparatus until the groove depth according to a predetermined design is obtained, and the Cr film is removed. Then, the transmissive diffraction grating 10 can be manufactured.

[Examples] Examples of the present invention will be described. The diffractive optical element for incident light wavelengths of 1350 nm to 1750 nm of this example has a basic configuration as shown in FIG. A transmission type diffraction grating in which a ridge made of Ta ₂ O ₅ is formed on a quartz glass substrate is provided, and a multilayer film made of a laminate of SiO ₂ and Ta ₂ O ₅ is inserted as a thin film layer between the substrate and the ridge. Yes. Further, a SiO ₂ film is formed on the top surface of the ridge. The multilayer film has a film structure in which a three-layer structure of Ta ₂ O ₅ / SiO ₂ / Ta ₂ O ₅ (film thickness 38 nm / 108 nm / 38 nm) is repeated 12 times.

The transmission type diffractive optical element of this example is manufactured by the manufacturing method described above. The multilayer film is formed on a quartz glass substrate, this becomes a ridge of the diffraction grating the Ta ₂ O ₅ film is formed on the multilayer film, and a SiO ₂ film formed thereon. Thereafter, a Cr film pattern mask was prepared so that the number of grooves was about 900 / mm and the width of the grooves was 555 nm thereon. Vapor phase etching was performed directly above the multilayer structure through this mask, and a diffractive optical element having a striped SiO ₂ film layer provided on the top surface of the ridge was fabricated. The back surface of the quartz glass substrate was provided with a anti-reflection coating to prevent diffracted light from being reflected at the interface between the substrate and air.

When a ridge is made of Ta ₂ O ₅ having a refractive index of 2.1, the optimum groove depth h is 1350 nm. On the other hand, when the SiO ₂ film is formed on the top of the ridge, if the thickness of the SiO ₂ film is determined so as to satisfy the expression (1), h _H and h _L in the expression (1) are as follows. Example: h _H = 1140 nm, h _L = 306 nm (groove depth: 1446 nm) For comparison, a comparative example of the following combination that does not satisfy the relationship of formula (1) was prepared. Comparative example: h _H = 1350 nm, h _L = 306 nm (groove depth: 1656 nm)

For these diffraction gratings, the diffraction efficiency was measured when light having a wavelength range of 1350 nm to 1750 nm was incident on the diffraction grating surface side at an incident angle of 45 °. The measurement results are shown in FIG. FIG. 4A shows an example, and FIG. 4B shows a comparative example. When both are compared, the PDL is smaller in the example and in a wider range. For example, the wavelength range in which the PDL is in the range of −0.5 to 0.5 dB is in the range of 1350 to 1630 nm in the example, whereas it is in the range of 1410 to 1590 nm in the comparative example. In the embodiment, the low PDL region is wider especially on the short wavelength side. The wavelength at which the diffraction efficiency of both the TE mode and the TM mode is 85% or more is in the range of 1350 to 1660 nm in the example, and is in the range of 1470 to 1760 nm in the comparative example. Efficiency is high. Of course, when visible light is incident on the diffractive optical element at the same time, it is reflected by the multilayer film and is not mixed into the transmitted diffracted light.

It is a cross-sectional schematic diagram of a conventional laminar diffraction grating. It is a figure which shows the optical characteristic of the conventional laminar type | mold diffraction grating. It is a cross-sectional schematic diagram of a laminar diffraction grating provided with a thin film layer. It is a figure which shows the optical characteristic of the laminar type | mold diffraction grating which provided the thin film layer. It is a figure which shows the measuring system of the infrared absorption measurement which uses a transmission diffraction grating. It is a cross-sectional schematic diagram which shows embodiment of the transmission type diffractive optical element of this invention. It is explanatory drawing which shows the design concept of the transmission type diffractive optical element of embodiment of this invention. It is a figure which shows the optical characteristic of the transmission type diffractive optical element of embodiment of this invention.

10, 100 transmission type diffractive optical element 12 grooves 14,24,64 ridge 32 down striped thin layer 34 thin layer 38 reflective layer 50 group plate

Claims (3)

Transmission for incident light wavelengths of 1350 nm to 1750 nm including a diffraction grating in which a large number of ridge-shaped convex portions having a substantially rectangular cross-sectional shape perpendicular to the longitudinal direction are provided in parallel with an array period of the order of the wavelength of incident light. In the type diffractive optical element, a plurality of parallel striped thin film layers having a uniform composition in a plane parallel to the substrate surface is provided only on the top surface of the ridge-shaped convex portion, and parallel to the substrate surface. A periodic structure in the same direction as the array period of the ridge-shaped protrusions having a uniform composition in a plane parallel to the substrate surface or in the plane parallel to the substrate surface. The ridge-shaped convex portion is formed of a material having a refractive index n _{H and} the groove depth at which a desired characteristic is obtained as a transmission type diffraction grating is h. Top surface of Refractive index of the formed striped thin layer n _L, the thickness h _L, the height h _H of the ridge-shaped convex portions made of a material having a refractive index n _H is, the following expression _{n H × h = n L ×} h L A transmission type diffractive optical element for incident light wavelengths of 1350 nm to 1750 nm, which satisfies + n _H × h _H. The striped thin film layer is a multilayer film of two or more layers made of two or more materials, and when the average refractive index of the entire multilayer film is n _La and the total film thickness is h _Lt , the n _L is n _La The transmissive diffractive optical element for incident light wavelengths of 1350 nm to 1750 nm according to claim 1, wherein the h _L is replaced with h _Lt. 3. The thin film layer having no periodic structure in the same direction as the arrangement period of the ridge-shaped protrusions has a transmission characteristic having wavelength selectivity. 4. Transmission diffractive optical element for incident light wavelengths of 1350 nm to 1750 nm .

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