JP2002258034A - Wavelength filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a narrow range reflection type wavelength filter in which formation of a thin film is required to performed only once and high reflectance can be obtained. SOLUTION: The fine recess and projection pattern of a substrate coated with a dielectric layer is controlled in such a manner that when the ambient atmosphere, the dielectric layer and the substrate are regarded as a medium, the average refractive index depending on the height gently changes from the ambient atmosphere side to the substrate side and that the refractive indices nd , ns , nair in the dielectric layer, the substrate and the ambient atmosphere, respectively, satisfy the relation of nd >ns , nair . The fine recesses and projections and the dielectric layer form a waveguide layer for the light incident to the surface having the recesses and projections. The period Λ of the fine recesses and projections is specified to the range satisfying Λ<λ/(ns +nair sinθ) and Λ<λ/(nair +nair sinθ), wherein ns and nair are the refractive indices of the substrate and the ambient atmosphere, respectively, and λ and θ are the wavelength and the incident angle of the irradiating light, respectively. Or the period Λ of the fine recesses and projections is specified to satisfy the relation of Λ>λ/(ngmax +nair sinθ), wherein ngmax is the highest average refractive index in the waveguide layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength filter, and more particularly, to a wavelength filter having a narrow characteristic of a selected wavelength.

[0002]

2. Description of the Related Art A slab type (layer type) optical waveguide is provided with a periodic refractive index modulation, or a periodic fine unevenness is formed on the upper or lower surface of the waveguide to form a plate-like wavelength filter. Can be created. This wavelength filter is
Utilizing a resonance phenomenon in a periodic structure, it has a narrow band-pass characteristic that reflects only light in an extremely narrow wavelength range of several nanometers or less. In 1985, the Bulgarian Ma
Shev and Popov have prototyped a filter for visible light with a wavelength of 633 nm and confirmed the resonance phenomenon (L. Mashev a
nd E. Popov, “Zeroorder anomaly of dielectric coa
ted gratings ”, Optics Communications, Vol. 55, No.
6 (1985) pp.377-380). However, in the filter structure proposed by them, although the reflectance at the resonance wavelength was high, the reflectance at the non-resonance wavelength was several percent, and the efficiency of wavelength selection was poor. To reduce reflectance at non-resonant wavelengths, the US
R. Magnusson and S. Wang propose that a thin film layer of different refractive index be provided above or below a grating layer (a layer whose refractive index or shape changes periodically) (US Pat. No. 5,598,300).
No.) (R. Magnusson and SS Wang, “Transmission
bandpass guided-mode resonance filters ”, Applied
Optics, Vol. 34, No. 35 (1995) 8106-8109). On the other hand, apart from the above-mentioned wavelength filter, a method of eliminating light reflection with a surface structure finer than the wavelength of light has been considered. This principle is to reduce the reflection by gradually changing the average refractive index of the surface by a fine structure, and has long been known as a moth eye surface fine structure (Moth Eye structure). The feature of this antireflection structure is that the reflectance is low over a wide wavelength range. Creation of anti-reflection structures for visible light and near-infrared rays began around 1987 (Y. On
o, Y. Kimura, Y. Ohta, and N. Nishida, “Antirefl
ection effect in ultrahigh spatial-frequency holog
raphic relief gratings, Applied Optics, Vol. 26, N
o.6 (1997) pp. 1142-1146), research and development have been promoted for practical use from both theoretical analysis and production methods. In recent years, etching has been performed using high-density plasma using metal thin lines and dot patterns as masks,
It has also become possible to create triangular or conical microstructures with high aspect ratios (Japanese Patent Application No. 2000-088524, Takahara,
Toyota).

[0003]

As for the performance of a highly efficient wavelength filter, it is desirable that the reflectance at the center wavelength is close to 100% and the reflectance at other wavelengths is 0% (all transmission). In order to reduce the reflectance at non-resonant wavelengths by the method of Magnusson et al., It is necessary to form a plurality of thin films including a lattice layer. When mass-producing a filter, forming multiple thin films having different refractive indices causes a problem of increasing the production cost.

An object of the present invention is to provide a narrow-band reflection type wavelength filter which is simple to manufacture, can reduce the production cost, and can obtain high wavelength selectivity.

[0005]

SUMMARY OF THE INVENTION The object of the present invention is to provide a substrate having fine irregularities on the surface and a dielectric layer covering the fine irregularities, wherein the fine irregularities of the substrate are reflected on the surface of the substrate. The average refractive index at each height in the fine unevenness height direction when the surrounding atmosphere, the dielectric layer and the substrate are used as a medium changes gradually from the surrounding atmosphere side to the substrate side so that the prevention action can be obtained. When the refractive indices of the dielectric layer, the substrate, and the surrounding atmosphere are n _d , n _s , and n _air , respectively, _nd > _ns , n _air is satisfied. forms a waveguide layer of the light incident on the surface, the period of the fine uneven the substrate, the phase peripheral lambda, respectively n _s, n _air, the wavelength of the irradiated light and the incident the substrate and the refractive index of the ambient atmosphere each corner lambda, When θ, Λ <λ / (n s + n air sinθ) Λ <λ / (n air + n air sinθ) Is a range satisfying both the period of the fine irregularities Λ
It is achieved by the wavelength filter characterized in that it is a range satisfying the highest average refractive index of the waveguide layer and _{n gmax Λ> λ / (n} gmax + n air sinθ).

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A narrow band reflection type wavelength filter according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 schematically shows a cross section of a wavelength filter according to an embodiment of the present invention. This wavelength filter includes a substrate 1 having fine irregularities on the surface, and a dielectric layer 2 covering the fine irregularities. The fine irregularities on the substrate 1
In this example, the section is formed in a triangular wave shape as shown. The fine irregularities are not limited to this, and the average refractive index between the ambient atmosphere (usually air) A and the dielectric layer 2 is set to be lower than the ambient From the substrate side to the substrate side. For this reason, it is desirable that the cross-sectional shape of the fine unevenness is formed such that the convex portion is tapered.

[0008] dielectric layer and the material of the substrate, a dielectric layer, the substrate, each n _d the refractive index of the surrounding atmosphere, n _s, when the n _air, the refractive index n _d of the dielectric Among these It is chosen to be the largest, ie, n _d > n _s , n _air .

According to this configuration, a waveguide layer for light having the fine unevenness and the dielectric layer incident on the uneven surface is formed.

The period of the fine irregularities on the substrate is as follows: 周期 <λ, where 屈折 is the period, n _s and n _air are the refractive indices of the substrate and the surrounding atmosphere, and λ and θ are the wavelength and incident angle of the irradiation light, respectively. / (N _s + n _air sin θ) Λ <λ / (n _air + n _air sin θ). By setting the period of the fine irregularities in this range, generation of diffracted light on the atmosphere side and the substrate side is suppressed.

Further, the period 微細 of the fine irregularities corresponds to the highest average refractive index of the waveguide layer at _ng of _{ng max.}
It is to the _{Λ> λ / (n gmax +} n air sinθ) range satisfying the a. By setting the period of the fine unevenness in this range, the incident light can be propagated in the waveguide layer.

As a material of the substrate, glass, plastic, silicon single crystal, or the like can be used. Among these, glass or plastic having a low refractive index is particularly desirable.

Further, as a material for forming the dielectric layer,
Ordinary optical thin film forming materials such as TiO ₂ , MgF ₂ , and SiO ₂ can be used. Among these, Ti, which has a high refractive index,
O ₂ and MgF ₂ are particularly desirable. When the irradiation light is infrared light, Si, Ge, ZnSe, or the like can be used.

The period の of the fine irregularities is appropriately determined according to conditions such as the refractive index of the material used and the wavelength of the irradiation light. When the irradiation light is perpendicularly incident on the filter surface, it is desirable to be about 0.9 times (reference wavelength λ / substrate refractive index n _s ).

The fine irregularities are formed by, for example, forming a fine irregularity on the substrate by combining interference fringe exposure or electron beam exposure with etching, and forming a high refractive index film on the surface by vacuum evaporation, sputtering, or the like. It can be formed as follows. Further, minute irregularities on the substrate can be mass-produced using a molding die.

The present invention has a function of selectively reflecting light of a specific wavelength in incident light. This contributes to an antireflection effect on incident light and an operation as a resonance mode grating filter. Hereinafter, these actions and the principle thereof will be described.

A. Anti-Reflection Function FIG. 2 shows an example of an anti-reflection structure using a triangle as a basic unit. Triangular period is Λ, ambient atmosphere (air layer)
The refractive index of the to n _air, the refractive index of the substrate 1a and the n _s. If light having a wavelength size of lambda in vacuum is incident at an angle q, the condition for not generating a diffracted _{wave, Λ <λ / (n s} + n air sinθ) (1) Λ <λ / ( n _air + n _air sin θ) (1 ′). The average refractive index in the height direction is schematically shown in the graph on the right side of FIG. In this structure, the refractive index of the upper ambient atmosphere changes to the refractive index of the substrate layer. Since the reflection of light is caused by a sharp change in the refractive index, when the refractive index changes gradually as shown in the figure, the light hardly reflects. Further, the uneven cross-sectional shape of the substrate is not necessarily required to be triangular. With a structure in which the average refractive index gradually changes in the height direction, an antireflection effect appears. The optimum change in the average refractive index is a constant change from the refractive index of air to the refractive index of the substrate. The reflectance and transmittance are calculated by the exact analysis method of the diffraction grating (Rigorous Coupled-
wave analysis). The fine unevenness has an antireflection effect including a longer wavelength as the height of the unevenness is higher (as the aspect ratio is higher). Desirably, the height (depth) of the fine irregularities is equal to or greater than the wavelength of the irradiation light.

B. Resonant mode grating filter FIG. 3 shows a basic structure of a grating filter utilizing a resonance phenomenon. The filter has a waveguide layer 2b having a periodic shape change such as a refractive index modulation or a rectangular wave shape on a substrate 1b. The incident light is diffracted by the periodicity of the waveguide layer. However, the diffracted wave is set to diffract at an angle at which total reflection occurs at the boundary between the waveguide layer and the substrate layer and between the waveguide layer and the surrounding atmosphere.
For this purpose, waveguide layers higher refractive index than the substrate layer and the ambient atmosphere _{n g> n air, n s} (n g is the average refractive index of the waveguiding layer) without must have a (2), also, guide the modulation period of the wave layer lambda, must satisfy the highest average refractive index as _{n gmax Λ> λ / (n} gmax + n air sinθ) condition (3) in the waveguide layer.

The incident wave generates a diffracted wave in the waveguide layer due to the lattice structure of the waveguide layer. When this diffracted wave satisfies the condition for propagating in the waveguide layer, it again couples with the periodic structure, and the diffracted wave (reflected wave) in the direction of specular reflection with respect to the incident light
Occurs. On the other hand, a diffracted wave that does not satisfy the waveguide condition cannot enter the waveguide layer and becomes a transmitted wave with respect to incident light. When the above waveguide condition is satisfied, the reflection efficiency is approximately 10%.
It becomes a highly efficient filter of 0%. This filter is a band-pass reflection type filter, and the half width of the filter can be controlled by adjusting the modulation amount of the refractive index. Therefore, by appropriately designing the amount of modulation of the refractive index,
A filter with a half-width of several angstroms can be made. The propagation conditions of the waveguide layer differ depending on the polarization direction of the electric field. Therefore, even with the same structure, the resonance wavelength differs depending on the polarization direction of the incident light. In general, the half-width of a TM wave (a wave whose magnetic field is perpendicular to the paper) is narrower than that of a TE wave (a wave whose electric field is perpendicular to the paper).

In the structure shown in FIG. 3, since the reflected light is generated on the upper and lower surfaces of the waveguide layer due to the difference in the refractive index, the reflectance of the non-resonant wave does not become zero, and several percent of the reflected light remains. Figure 4 shows Magn
This is a multilayer structure proposed by usson et al. for reducing the reflectance of non-resonant waves. In this example, the thin film layers 3c and 4c are added one by one on the upper and lower sides of the modulation waveguide layer 2c and provided on the substrate 1c. In this multilayer structure, the reflectivity is reduced by causing incident light to be reflected by upper and lower layers and interfere so as to cancel each other. The thickness of the thin film layer is set such that the optical film thickness is half or quarter of the wavelength so that such interference occurs. Further, by substituting one of the upper and lower layers at the interface of the modulation waveguide layer, the other thin film layer can be omitted. In order to further reduce the reflectance, it is necessary to increase the number of thin film layers. The principle of reducing the reflectance of non-resonant waves is the same as the well-known reduction of reflectance by a dielectric multilayer film.

C. Anti-reflection structure type resonance mode grating filter The present invention is based on the above principle. Hereinafter, the present invention will be described based on the cross-sectional view of the resonance mode grating filter shown in FIG. This filter has the anti-reflection structure of FIG.
A dielectric thin film having a higher refractive index than the substrate is attached by a method such as vacuum evaporation. The surface shape of the thin film desirably preserves the concavo-convex shape of the substrate. The distribution of the average refractive index in the height direction is graphically shown on the right side of the figure. The average refractive index gradually increases from the value of the surrounding atmosphere and then decreases to match the refractive index of the substrate. Therefore, it performs the same function as the refractive index distribution type optical waveguide, and the diffracted wave is confined in this region. Then, the refractive index is modulated in the horizontal direction by the triangular lattice period. When this diffracted wave satisfies the conditions for propagating in the waveguide layer shown in Equations (2) and (3), it again couples with the periodic structure, causing the diffracted wave to enter the direction of specular reflection with respect to the incident light. (Reflected waves).

The reflection characteristics of this filter are strongly influenced by the aspect ratio of the uneven shape (ratio of height and period of the unevenness), the film pressure and refractive index of the dielectric thin film, and the refractive index of the substrate material. Generally, in order to narrow the half width (wavelength width of the peak region), the aspect ratio is increased (the peak is increased), the refractive index of the thin film is set lower, and the thickness of the thin film is reduced. realizable. To lower the reflectance at non-resonant wavelengths,
It is necessary to increase the aspect ratio.

Hereinafter, specific examples will be introduced using the results of numerical calculations. RCWA (Ri
gorous Coupled Wave Analysis). This calculation algorithm is an exact electromagnetic calculation method for the diffraction grating, and is used in various parts of the world as a method for accurately obtaining the diffraction efficiency in the resonance region.

FIG. 5A shows a longitudinal section of a wavelength filter in which a dielectric thin film having a refractive index of 2.25 (TiO ₂ ) is formed on a quartz substrate having an aspect ratio of 2.5 at a thickness of 0.4 times the reference wavelength, and its vertical section. FIG. 6A shows the average refractive index corresponding to the position.
This is a calculated value of the spectral reflectance characteristic in this case. The numerical value with λ indicates a value normalized by wavelength, and the numerical value with nm indicates a value when the wavelength is specifically set to 633 nm. Assuming a TM wave as the incident light, the design was made so that it would resonate at the reference wavelength. The grating period is 0.64 times the reference wavelength. The half width is extremely narrow, 3'10 ^-4 times the wavelength. At other wavelengths, the reflectivity is very small, about 0.3%. From this result, it can be seen that the structure in which only one layer of the thin film is formed on the antireflection structure functions as a narrow-band wavelength filter and that the reflectance of non-resonant light can be reduced.

FIGS. 5B and 5C show that the aspect ratio is 1.5.
The vertical cross section of a wavelength filter formed by forming a dielectric thin film having a refractive index of 2.25 (TiO ₂ ) with a thickness of 0.4 times the reference wavelength on a quartz substrate having a structure of 0.7, and the average refractive index corresponding to the position are shown. FIGS. 6B and 6C show calculated values of the spectral reflectance characteristics in each case. In (b), the effect of preventing reflection of non-resonant light is well exhibited, but in (c), the effect is weakened, and the reflectance increases on the long wavelength side. However, the half width of (b) is much wider than that of (a).

In a structure having irregularities larger than the thickness of the thin film, the lower the aspect ratio, the stronger the refractive index modulation as a waveguide. Therefore, as the aspect ratio decreases, the half width increases.

In order to realize a narrow half width at a low aspect ratio, it is necessary to reduce the thickness or the refractive index of the thin film. However, in such a configuration, the average refractive index of the waveguide layer becomes small (the difference from the refractive index of the substrate becomes small), and the condition for generating the first-order diffracted wave of transmission becomes close to the resonance condition.
In a wavelength region shorter than the resonance wavelength, a diffracted wave is generated. FIG. 7 shows how the transmitted first-order diffracted wave is generated in the filter of FIG. (A), (b) and (c) in FIG. 7 are (a), (b) and (c) in FIG.
And the aspect ratio is the same. Aspect ratio is
In the case of 2.5, the first-order diffracted wave is generated at a distance from the resonance wavelength. At an aspect ratio of 0.7, diffracted light is generated at almost the same wavelength as the resonance wavelength. When using a filter as an “element that extracts only one specific wavelength”,
It is sufficient to treat the resonance wave (reflected wave) as an output, which is not affected by the first-order diffracted wave, so that it is not necessary to pay attention to such characteristics. However, in the case of using transmitted light, energy is lost due to diffraction in a wavelength region shorter than the resonance wavelength, resulting in low utilization efficiency.

In the above example, the condition is set that light is incident perpendicularly to the grating structure. However, the same operation is performed even when light is obliquely incident on the plane of each drawing.

[0029]

The wavelength filter according to the present invention includes a substrate having fine irregularities on the surface and a dielectric layer covering the fine irregularities, and the fine irregularities on the substrate have an antireflection effect on the substrate surface. As can be obtained, the average refractive index at each height in the fine unevenness height direction when the ambient atmosphere, the dielectric layer, and the substrate are used as a medium gradually changes from the ambient atmosphere side to the substrate side. Thereby, an anti-reflection effect on incident light can be obtained. Further, the dielectric layer, the substrate, each n _d the refractive index of the surrounding atmosphere, n _s, when the n _air, n _d> n _s, each refractive index is determined such that n _air, fine irregularities And the dielectric layer forms a waveguide layer for light incident on the uneven surface.

The period of the fine irregularities on the substrate is as follows: 周期 <λ, where 周期 is the period, n _s and n _air are the refractive indices of the substrate and the surrounding atmosphere, and λ and θ are the irradiation light wavelength and incident angle, respectively. / (N _s + n _air sin θ) Λ <λ / (n _air + n _air sin θ), whereby the generation of diffracted light on the atmosphere side and the substrate side is suppressed.

Further, the period 微細 of the fine irregularities is determined by setting the highest average refractive index of the waveguide layer at _ng to _ngmax.
Then, the range is set so as to satisfy Λ> λ / ( _ngmax + n _air sin θ), whereby the incident light can be propagated in the waveguide layer.

In this manner, the light incident on the wavelength filter propagates through the waveguide layer, is recombined with the periodic structure of fine irregularities, and is emitted in the direction of specular reflection with respect to the incident light. Thereby, light of a specific wavelength is obtained as reflected light, and light of another wavelength passes through the substrate. As a result, high wavelength selectivity can be obtained by suppressing the reflection of non-resonant wavelengths.

In order to manufacture the present filter, it is only necessary to provide a substrate with an antireflection structure based on fine irregularities, and to form a dielectric thin film thereon. Therefore, unlike the conventional slab-type lattice filter in which a plurality of optical thin films are provided to reduce the reflectance of non-resonant waves in a wide wavelength range, a complicated dielectric deposition process over a plurality of layers is not required. The production is simple and the production cost can be reduced.

Applications of the present filter include a laser oscillation cavity mirror, a spectral wavelength selecting element, a wavelength division multiplexing optical communication wavelength division element, a polarization separation element, and the like, like the ordinary resonance mode grating filter.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an operation principle of a wavelength filter according to an embodiment of the present invention, showing a longitudinal section of the filter and an average refractive index corresponding to a position thereof.

FIG. 2 is an explanatory view of the principle underlying the present invention, showing a longitudinal section of the antireflection structure and an average refractive index corresponding to the position thereof.

FIG. 3 is an explanatory view of the basic principle of the present invention, showing a longitudinal section of a resonance mode grating filter.

FIG. 4 is an explanatory view of a structure related to the present invention, showing a longitudinal section of a multilayer structure type filter.

FIG. 5 shows a vertical section of a wavelength filter having three types of aspect ratio structures and an average refractive index corresponding to the position.

FIG. 6 is a graph of spectral reflectance characteristics of each of the wavelength filters shown in FIG.

FIG. 7 is a graph showing a state of generation of a transmitted first-order diffracted wave in the filter shown in FIG.

[Explanation of symbols]

 1, 1a, 1b, 1c Substrate 2, 2b, 2c Dielectric layer 3c Thin film layer 4c Thin film layer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Toyoda 247 5-floor, Kuwabaracho, Izumi-shi, Osaka Izumi 102 F-term (reference) 2H047 LA01 TA43 2H048 FA05 FA12 FA15 FA21 GA04 GA09 GA13 GA24 2H049 AA03 AA51 AA59 AA64

Claims (1)

[The claims] 1. A substrate having fine irregularities on its surface, and a dielectric layer covering the fine irregularities, wherein the fine irregularities on the substrate are exposed to an ambient atmosphere so as to obtain an antireflection effect on the substrate surface. When the dielectric layer and the substrate are used as a medium, the average refractive index at each height in the height direction of the fine unevenness gradually changes from the ambient atmosphere side to the substrate side, and the dielectric layer, the substrate, the ambient atmosphere Where n _d , n _s , and n _air , respectively, n _d > n _s and n _air , and the fine unevenness and the dielectric layer form a waveguide layer for light incident on the uneven surface. and is the period of the fine unevenness of the substrate, the phase peripheral lambda, respectively n _s of the substrate and the refractive index of the ambient atmosphere, n _air, lambda, respectively the wavelength and incident angle of the illumination light, when theta, lambda < It is a range satisfying both _{λ / (n s + n air} sinθ) Λ <λ / (n air + n air sinθ), the fine The period of the unevenness lambda, wavelength filter characterized in that it is a range satisfying the highest average refractive index of the waveguide layer and _{n gmax Λ> λ / (n} gmax + n air sinθ).