JP3711446B2 - Wavelength filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength filter, and more particularly to a wavelength filter having a narrow selection wavelength characteristic.
[0002]
[Prior art]
A plate-like wavelength filter can be created by applying periodic refractive index modulation to a slab type (layer type) optical waveguide or by forming periodic fine irregularities on the upper or lower surface of the waveguide. . This wavelength filter uses a resonance phenomenon in a periodic structure, and has a narrow band pass characteristic that reflects only light in an extremely narrow wavelength range of several nanometers or less.
In 1985, Bulgarian Mashev and Popov prototyped a filter for visible light with a wavelength of 633 nm and confirmed its resonance phenomenon (L. Mashev and E. Popov, “Zero order anomaly of dielectric coated gratings”, Optics Communications, Vol.55, No.6 (1985) pp.377-380). However, in the filter structure proposed by them, the reflectance at the resonance wavelength is high, but the reflectance at the non-resonance wavelength is also several percent, and the wavelength selection efficiency is poor. In order to reduce the reflectivity of non-resonant wavelengths, US R. Magnusson and S. Wang have thin film layers with different refractive indexes above or below a grating layer (a layer whose refractive index or shape changes periodically). (US Pat. No. 5,598,300) (R. Magnusson and SS Wang, “Transmission bandpass guided-mode resonance filters”, Applied Optics, Vol. 34, No. 35 (1995) 8106-8109 page). On the other hand, apart from the above wavelength filter, a method of eliminating reflection of light with a surface structure finer than the wavelength of light is also considered. This principle reduces reflection by gradually changing the average refractive index of the surface with a fine structure, and has long been known as the surface fine structure of the eyelid (Moth Eye structure). The feature of this antireflection structure is that the reflectance is small over a wide wavelength range. The construction of antireflection structures for visible light and near infrared rays began around 1987 (Y. Ono, Y. Kimura, Y. Ohta, and N. Nishida, “Antireflection effect in ultrahigh spatial-frequency holographic relief gratings, Applied Optics. , Vol.26, No.6 (1997) 1142-1146), research and development have been promoted so far in terms of both theoretical analysis and production methods. High-aspect-ratio triangular or conical microstructures can be created by etching using high-density plasma with a pattern as a mask (Japanese Patent Application No. 2000-088524, Takahara, Toyota).
[0003]
[Problems to be solved by the invention]
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 is 0% (all transmission) at other wavelengths. In order to reduce the reflectance of non-resonant wavelengths by the method of Magnusson et al., It is necessary to form a plurality of thin films including a lattice layer. In the case of mass production of filters, the formation of multiple thin films having different refractive indexes raises the problem of increasing production costs.
[0004]
An object of the present invention is to provide a narrow-band reflection type wavelength filter that is easy to manufacture, can reduce production costs, and can obtain high wavelength selectivity.
[0005]
[Means for Solving the Problems]
The object of the present invention is to provide a substrate having fine unevenness on the surface and a dielectric layer covering the fine uneven surface, and the fine unevenness of the substrate is provided with an antireflection effect on the substrate surface. When the atmosphere, dielectric layer, and substrate are used as the medium, the average refractive index at each height in the height direction of the fine irregularities is gradually changing from the ambient atmosphere side to the substrate side, and the dielectric layer, substrate, When the refractive index of the ambient atmosphere is n _d , n _s , and n _air , respectively,
n _d > n _s , n _air
The fine irregularities and the dielectric layer form a waveguide layer of light incident on the irregular surface,
The period of the fine unevenness of the substrate is Λ, the refractive index of the substrate and the surrounding atmosphere is n _s , n _air , and the wavelength and incident angle of the irradiation light are λ and θ, respectively.
_{Λ <λ / (n s +} n air sinθ)
Λ <λ / (n _air + n _air sin θ)
Is a range that satisfies both,
The period Λ of the fine irregularities is Λ> λ / (n _gmax + n _air sin θ) where _{ng max} is the highest average refractive index in the waveguide layer.
Is a range that satisfies
This is achieved by a wavelength filter characterized in that the fine concavo-convex shape has an aspect ratio of 1.5 or more .
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a narrowband reflection type wavelength filter according to an embodiment of the present invention will be described with reference to the drawings.
[0007]
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 unevenness on the surface and a dielectric layer 2 covering the fine uneven surface. In this example, the fine unevenness of the substrate 1 is formed in a triangular wave shape as shown in the figure. This fine unevenness is not limited to this, but the average refractive index of the ambient atmosphere (usually air) A and the dielectric layer 2 is such that the average refractive index of the dielectric layer 2 is such that a sine curve shape or the like can be obtained at the substrate surface The shape can be changed gently 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 so that the convex portion is tapered.
[0008]
As for the material of the dielectric layer and the substrate, when the refractive indexes of the dielectric layer, the substrate, and the surrounding atmosphere are n _d , n _s , and n _air , respectively, the refractive index n _d of the dielectric is the largest among them. That is, n _d > n _s , n _air
Is selected.
[0009]
With this configuration, a light guide layer is formed in which fine unevenness and a dielectric layer are incident on the uneven surface.
[0010]
The period of the fine irregularities of the substrate is Λ, the refractive index of the substrate and the surrounding atmosphere is n _s , n _air , and the wavelength and incident angle of the irradiation light are λ and θ, respectively.
Λ <λ / (n _s + n _air sinθ)
Λ <λ / (n _air + n _air sinθ)
It is made the range which satisfies both of these. By setting the period of the fine unevenness within this range, generation of diffracted light on the atmosphere side and the substrate side is suppressed.
[0011]
Further, the period Λ of the fine irregularities is Λ> λ / ( _{ng max} + n _air sin θ) where _{ng max} is the highest average refractive index at _ng as the average refractive index of the waveguide layer.
It is made the range which satisfies. Incident light can be propagated into the waveguiding layer by setting the period of the fine irregularities within this range.
[0012]
As the substrate material, glass, plastic, silicon single crystal, or the like can be used. Among these, glass and plastic having a low refractive index are particularly desirable.
[0013]
Further, as a material for forming the dielectric layer, a normal optical thin film forming material such as TiO ₂ , MgF ₂ , and SiO ₂ can be used. Among these, TiO ₂ and MgF ₂ having a high refractive index are particularly desirable. Moreover, when irradiation light is infrared rays, Si, Ge, ZnSe, etc. can be used.
[0014]
The period Λ of the fine unevenness 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 incident on the filter surface perpendicularly, it is preferably about 0.9 times (reference wavelength λ / substrate refractive index n _s ).
[0015]
For example, the fine irregularities are formed by combining interference fringe exposure, electron beam exposure and etching to form minute irregularities on the substrate, and forming a high refractive index film on the surface by vacuum deposition, sputtering, or the like. Can be formed. Moreover, the minute unevenness of the substrate can be mass-produced using a mold.
[0016]
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 action as a resonance mode grating filter. Hereinafter, these actions and their principles will be described.
[0017]
a. Antireflection Action FIG. 2 shows an example of an antireflection structure having a triangle as a basic unit. Let Λ be the period of the triangle, n _{air be} the refractive index of the surrounding atmosphere (air layer), and n _{s be} the refractive index of the substrate 1a. When light having a wavelength of λ in vacuum is incident at an angle q, the condition for not generating a diffracted wave is:
Λ <λ / (n _s + n _air sinθ) (1)
Λ <λ / (n _air + n _air sinθ) (1 ')
It is to satisfy both. The average refractive index in the height direction is schematically shown by a graph on the right side of FIG. In this structure, the refractive index of the upper ambient atmosphere changes from the refractive index of the substrate layer. Since reflection of light occurs due to a sudden change in refractive index, light hardly reflects when the refractive index gradually changes as shown in the figure. Further, the concave-convex cross-sectional shape of the substrate is not necessarily a triangular shape. If the average refractive index is a structure that gradually changes in the height direction, an antireflection effect appears. The optimum change in average refractive index is a constant change from the refractive index of air to the refractive index of the substrate.
The reflectance and transmittance can be obtained using a rigorous analysis calculation method (such as Rigorous Coupled-wave Analysis) of the diffraction grating. 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.
[0018]
b. Resonant Mode Lattice Filter FIG. 3 shows the basic configuration of a lattice filter using a resonance phenomenon. The filter has a waveguide layer 2b having a periodic shape change such as refractive index modulation or rectangular wave on the substrate 1b. Incident light is diffracted by the periodicity of the waveguiding layer. However, the diffracted wave is set so as to be diffracted to 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 ambient 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, the highest average refractive index as _{n gmax Λ> λ / (n} gmax + n air sinθ) in the waveguiding layer (3)
Must meet the requirements of
[0019]
The incident wave generates a diffracted wave in the waveguide layer because of the lattice structure of the waveguide layer. When this diffracted wave satisfies the condition for propagating in the waveguide layer, the diffracted wave is coupled with the periodic structure again to generate a diffracted wave (reflected wave) in the direction of specular reflection with respect to the incident light. 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 waveguide condition is satisfied, a highly efficient filter having a reflection efficiency of almost 100% is obtained. This filter is a band-pass reflection type filter, and the half-value width of the filter can be controlled by adjusting the modulation amount of the refractive index. Therefore, a filter with a half width of several angstroms can be made by appropriately designing the amount of modulation of the refractive index. The propagation condition of the waveguiding layer varies depending on the polarization direction of the electric field. Therefore, even with the same structure, the resonance wavelength varies depending on the polarization direction of incident light. In general, TM waves (waves whose magnetic field is perpendicular to the paper surface) have a narrower half-value width than TE waves (waves whose electric field is perpendicular to the paper surface).
[0020]
In the structure of FIG. 3, reflected light due to the difference in refractive index is generated on the upper and lower surfaces of the waveguiding layer, so the reflectance of the non-resonant wave does not become zero, and a few percent of reflected light remains. FIG. 4 shows a multilayer structure for reducing the reflectance of non-resonant waves proposed by Magnusson et al. In this example, thin film layers 3c and 4c are added to the top and bottom of the modulation waveguide layer 2c one by one and provided on the substrate 1c. In this multilayer structure, the reflectance is reduced by causing incident light to be reflected by the upper and lower layers and causing interference so as to cancel each other. The thickness of the thin film layer is set so that the optical film thickness is half or 1/4 of the wavelength so that such interference occurs. In addition, by replacing one of the upper and lower layers with the interface of the modulation waveguide layer, the other thin film layer can be omitted. Further, 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 the non-resonant wave is the same as that of the well-known dielectric multilayer film.
[0021]
c. Resonance mode grating filter of antireflection structure The present invention is based on the above principle. Hereinafter, the present invention will be described based on a cross-sectional view of the resonance mode grating filter shown in FIG. This filter is obtained by attaching a dielectric thin film having a higher refractive index than that of the substrate to the antireflection structure of FIG. 2 by a method such as vacuum deposition. The surface shape of the thin film desirably preserves the uneven shape of the substrate. The distribution of the average refractive index with respect to the height direction is shown by a graph on the right side of the figure. The average refractive index gradually increases from the value of the ambient atmosphere, and then decreases to coincide with the refractive index of the substrate. Therefore, it performs the same function as the gradient index optical waveguide, and the diffracted wave is confined in this region. The refractive index is modulated in the lateral direction with a triangular lattice period. When this diffracted wave satisfies the conditions for propagating in the waveguiding layer shown in Equations (2) and (3), it couples with the periodic structure again, and the diffracted wave in the direction of specular reflection with respect to the incident light. (Reflected wave) is generated.
[0022]
The reflection characteristics of this filter are strongly influenced by the aspect ratio of the concavo-convex shape (ratio of concavo-convex height and period), the film pressure and refractive index of the dielectric thin film, and the refractive index of the substrate material. In general, to reduce the half-value width (wavelength width of the peak region), increase the aspect ratio (increase the peak), set the refractive index of the thin film low, or decrease the thickness of the thin film. realizable. In order to reduce the reflectance at non-resonant wavelengths, it is necessary to increase the aspect ratio.
[0023]
Specific examples are introduced below using the results of numerical calculations. A calculation algorithm called RCWA (Rigorous Coupled Wave Analysis) was used to obtain the light reflection characteristics from the shape. This calculation algorithm is a strict electromagnetic calculation method for a diffraction grating, and is used throughout the world as a method for accurately obtaining the diffraction efficiency of the resonance region.
[0024]
Fig. 5 (a) shows the longitudinal section and position of a wavelength filter in which a dielectric thin film with a refractive index of 2.25 (TiO ₂ ) is formed on a quartz substrate having a structure with an aspect ratio of 2.5 with a thickness 0.4 times the reference wavelength. FIG. 6A shows the calculated value of the spectral reflectance characteristic in this case. A numerical value with λ indicates a value normalized by the wavelength, and a numerical value with nm indicates a value when the wavelength is specifically 633 nm. The incident light was designed to resonate at the reference wavelength, assuming a TM wave. The grating period is 0.64 times the reference wavelength. The full width at half maximum is very 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 thin film is formed in the antireflection structure functions as a narrow-band wavelength filter, and the reflectance of non-resonant light can be reduced.
[0025]
5 (b) and 5 (c) show 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 1.5 and 0.7 with a thickness 0.4 times the reference wavelength. The average refractive index corresponding to the longitudinal section and its position is shown. FIG. 6B and FIG. 6C are calculated values of spectral reflectance characteristics in each case. In (b), the effect of preventing reflection of non-resonant light appears well, but in (c), the effect is reduced and the reflectance is increased on the long wavelength side. However, the half width of (b) is considerably wider than that of (a).
[0026]
In a structure with irregularities larger than the thickness of the thin film, the refractive index modulation as a waveguide becomes stronger as the aspect ratio is lower. For this reason, the half width increases as the aspect ratio decreases.
[0027]
In order to realize a narrow half width at a low aspect ratio, it is necessary to reduce the film thickness or refractive index of the thin film. However, in such a configuration, the average refractive index of the waveguide layer is small (the difference from the refractive index of the substrate is small), the conditions for generating the first-order diffracted wave of transmission are close to the resonance conditions, and the resonance wavelength In a shorter wavelength region, a diffracted wave is generated. FIG. 7 shows how the transmitted first-order diffracted wave is generated in the filter of FIG. 7 (a), (b), and (c) have the same aspect ratio as that of FIG. 5 (a), (b), and (c). When the aspect ratio is 2.5, the first-order diffracted wave is generated away from the resonance wavelength. When the aspect ratio is 0.7, diffracted light is generated at almost the same wavelength as the resonance wavelength. When the filter is used 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. There is no need to pay attention. 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.
[0028]
In the above example, the condition that light enters perpendicularly to the grating structure is set. However, the same operation is performed even when light is incident obliquely within the drawing sheet.
[0029]
【The invention's effect】
The wavelength filter according to the present invention comprises a substrate having fine unevenness on the surface and a dielectric layer covering the fine uneven surface, so that the fine unevenness of the substrate has an antireflection effect on the substrate surface, When the ambient atmosphere, the dielectric layer, and the substrate are used as the 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. Thereby, the antireflection effect with respect to incident light is acquired. Further, when the refractive index of the dielectric layer, the substrate, and the surrounding atmosphere is n _d , n _s , and n _air , respectively,
n _d > n _s , n _air
Each refractive index is determined such that the fine unevenness and the dielectric layer form a waveguide layer of light incident on the uneven surface.
[0030]
The period of the fine irregularities of the substrate is Λ, the refractive index of the substrate and the surrounding atmosphere is n _s , n _air , and the wavelength and incident angle of the irradiation light are λ and θ, respectively.
Λ <λ / (n _s + n _air sinθ)
Λ <λ / (n _air + n _air sinθ)
Thus, the generation of diffracted light on the atmosphere side and the substrate side is suppressed.
[0031]
Further, the period Λ of the fine irregularities is Λ> λ / ( _{ng max} + n _air sinθ) where _ng is the average refractive index of the waveguide layer and _{ng max} is the highest average refractive index in the waveguide layer .
Thus, incident light can be propagated into the waveguide layer.
[0032]
In this way, the light incident on the wavelength filter propagates in the waveguide layer, recombines with the fine uneven periodic structure, and exits 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 other wavelengths passes through the substrate. As a result, reflection at non-resonant wavelengths is suppressed and high wavelength selectivity is obtained.
[0033]
In order to manufacture this filter, it is only necessary to provide the substrate with an antireflection structure by fine irregularities and to form a single layer of dielectric thin film thereon. Therefore, unlike conventional slab-type grating filters that have multiple layers of optical thin films to reduce the reflectivity of non-resonant waves over a wide wavelength range, a complex dielectric deposition process over multiple layers is not required. The production is simple and the production cost can be reduced.
[0034]
Applications of this filter include a cavity mirror for laser oscillation, a wavelength selection element for spectroscopy, a wavelength division element for wavelength multiplexing optical communication, a polarization separation element, and the like, as in a normal resonance mode grating filter.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of the operating 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 its position.
FIG. 2 is an explanatory diagram of the principle underlying the present invention, showing a longitudinal section of an antireflection structure and an average refractive index corresponding to its position.
FIG. 3 is an explanatory diagram of the principle underlying the present invention, and shows a longitudinal section of a resonant mode grating filter.
FIG. 4 is an explanatory diagram of a structure related to the present invention, and shows a longitudinal section of a multilayer structure type filter.
FIG. 5 shows a longitudinal section of a wavelength filter having a structure with three types of aspect ratios and an average refractive index corresponding to the position.
6 is a graph of spectral reflectance characteristics of each of the wavelength filters shown in FIG.
7 is a graph showing how a transmission first-order diffracted wave is generated 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

Claims (1)

A substrate having fine unevenness on the surface and a dielectric layer covering the fine uneven surface, and the fine unevenness of the substrate has an ambient atmosphere, a dielectric layer, and a substrate so as to obtain an antireflection effect on the substrate surface. The average refractive index at each height in the height direction of the fine irregularities when using the medium as the medium gradually changes from the ambient atmosphere side to the substrate side, and the refractive index of the dielectric layer, the substrate, and the ambient atmosphere is changed. When n _d , n _s , and n _air are set,
n _d > n _s , n _air
The fine irregularities and the dielectric layer form a waveguide layer of light incident on the irregular surface,
The period of the fine unevenness of the substrate is Λ, the refractive index of the substrate and the surrounding atmosphere is n _s , n _air , and the wavelength and incident angle of the irradiation light are λ and θ, respectively.
_{Λ <λ / (n s +} n air sinθ)
Λ <λ / (n _air + n _air sin θ)
Is a range that satisfies both,
The period Λ of the fine irregularities is Λ> λ / (n _gmax + n _air sin θ) where _{ng max} is the highest average refractive index in the waveguide layer.
Is the range that satisfies,
The wavelength filter, wherein the fine concavo-convex shape has an aspect ratio of 1.5 or more .

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