RU2552127C1 - Multilayer optical bandpass filter - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: filter comprises a dielectric substrate coated with thin-film layers of dielectric materials with alternating high refraction index n_H and low refraction index n_L. Part of the layers of the material with refraction index n_L is resonators having electrical thickness π at the centre frequency of the transmission band. All of the other layers form multilayer dielectric mirrors which separate each resonator from the adjacent resonator, the dielectric substrate or outer space. In each i-th mirror, the electrical thickness θ_Li of layers with refraction index n_L is less than π/2, and the electrical thickness θ_Hi of layers with refraction index n_H is greater than π/2 by Δθ_Hi and 2Δθ_Hi for outer and inner layers, respectively, where the values θ_Li and Δθ_Hi satisfy the equation

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EFFECT: providing virtually any required bandwidth with virtually any non-uniformity of the light transmission coefficient in said bandwidth.

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Description

The invention relates to fiber optic technology and can be used in optical communication devices and Raman spectrometers.

Known optical three-resonator multilayer filter [Analog: Goncharov F.N., Lapshin B.A., Petrakov V.A., Politykin R.V., Schmidt A.A. Optical multilayer filter. RF patent No. 2316029, 01/27/2008, IPC G02B 5/28]. The three-cavity filter contains alternating dielectric layers of materials with high and low refractive indices. In it, all the layers with a high refractive index (n _H ) are made of one material, and all the layers with a low refractive index (n _L ) are made of a second material. The three dielectric layers of the filter have an electric (i.e., phase) thickness π at the center frequency of the passband. They are filter resonators. The remaining dielectric layers have an electric thickness of π / 2. They form multilayer dielectric mirrors that separate resonators from each other (internal mirrors) and from external space (external mirrors). The number of layers in the outer and inner mirrors is determined by the proposed mathematical formulas that describe the dependence on only two quantities — the ratio of the refractive indices of the two materials used and the relative filter bandwidth.

A disadvantage of the known three-cavity filter is its low selective properties, which are reflected in a weak attenuation of transmitted light outside the passband and the low slope of the passband itself. This disadvantage is due to the small number of resonators equal to three. Moreover, the increase in the number of resonators in the analogue is not provided according to the claims. Another disadvantage of the analogue is the fundamental impossibility of implementing a filter with a precisely specified bandwidth of light transmission and a precisely specified unevenness of the transmission coefficient in this band, since the number of layers in the outer and inner mirrors of the filter defining these characteristics cannot be fractional.

The closest analogue of the claimed invention is a multi-cavity filter [Prototype: P. Baumeister. Application of microwave technology to design an optical multilayer bandpass filter // Applied Optics. 2003. Vol. 42, No. 13, p. 2407-2414, Fig. 7]. The filter contains alternating dielectric layers of materials with high and low refractive indices, deposited on one side of the glass substrate. In it, all the layers with a high refractive index (n _H ) are made of one material, and all the layers with a low refractive index (n _L ) are made of a second material. Some layers with a low refractive index (n _L ) have an electric thickness π at the center frequency of the passband. They are filter resonators. The remaining layers form multilayer dielectric mirrors that separate each resonator from an adjacent resonator, glass substrate, or external space. In each mirror, the electric thickness of all layers, with the exception of three layers located one after another, is π / 2.

The disadvantage of a multi-cavity filter is that it can be implemented not for any required values of the bandwidth and not for any required values of the unevenness of the transmittance of light in this band. This drawback is due to the fact that the deviation of the electric thickness from π / 2 for only three layers in each multilayer mirror is not always sufficient for the filter to have the required bandwidth parameters.

The technical result of the claimed invention is the ability to implement almost any desired bandwidth with almost any unevenness of the transmittance of light in this band, which expands the scope of the multilayer band-pass filter.

.The technical result is achieved in that in an optical multilayer band-pass filter containing a dielectric substrate with thin-film layers of dielectrics deposited on it with alternating high refractive index n _H and low refractive index n _L , in which part of the layers with low refractive index n _L are resonators having an electric thickness π at the center frequency of the passband, and all other layers form multilayer dielectric mirrors that separate each resonator from adjacent resonator, dielectric substrate or external space. In each ith mirror, the electric (i.e., phase) thickness θ _{Li of} layers with a refractive index n _{L is} less than π / 2, and the electric thickness θ _{Hi of} layers with a refractive index n _H exceeds π / 2 by Δθ _Hi and 2Δθ _Hi for outer and inner layers, respectively, where θ _Li and Δθ _Hi satisfy the equation

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The inventive optical multilayer bandpass filter differs from the prototype in that in each i-th mirror, the electric thickness θ _{Li of the} layers with a refractive index n _{L is} less than π / 2, and the electric thickness θ _{Hi of the} layers with a refractive index n _H exceeds π / 2 by Δθ _Hi and 2Δθ _Hi for the outer and inner layers, respectively, where the quantities θ _Li and Δθ _Hi satisfy the equation

.

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These differences allow us to conclude that the proposed technical solution meets the criterion of "novelty." The features distinguishing the claimed technical solution from the prototype are not identified in other technical solutions when studying this and related areas of technology and, therefore, provide the claimed solution with the criterion of "inventive step".

The invention is illustrated by drawings and tables.

- многослойные зеркала, А - воздушная окружающая среда, G - стеклянная подложка.Figure 1 shows the design of the filter. Here R is single-layer resonators, M ₁ , M ₂ , M ₃ , $$M_{\mathrm{one}}^{\text{'}\text{'}}$$

- multilayer mirrors, A - air environment, G - glass substrate.

Figure 2 presents the amplitude-frequency characteristics (AFC) for the example filter. The solid line shows the dependence of the light transmission coefficient S ₂₁ , and the dashed line shows the dependence of the reflection coefficient S ₁₁ (S _ij are the elements of the scattering matrix). The values of both coefficients are expressed in decibels, the current frequency f is normalized to the center frequency of the passband f ₀ .

Table I shows the electrical thicknesses of thin-film layers in the filter mirrors.

An example embodiment of the invention.

. Крайние зеркала М₁ и $$M_1^{\text{'}}$$

содержат по 5 слоев. Остальные зеркала, то есть М₂ и М₃, содержат по 13 слоев. Значения электрических толщин слоев в зеркалах фильтра с относительной шириной полосы пропускания 2% и величиной неравномерности частотной характеристики в полосе пропускания, характеризуемой уровнем максимального отражения S_{11 max}=-15 дБ, приведены в Таблице I. Все эти значения удовлетворяют единому для всех зеркал условиюAn optical multilayer band-pass filter is made of two dielectric materials: rutile (TiO ₂ ) with a refractive index n _H = 2.39 and quartz (SiO ₂ ) with a refractive index n _L = 1.46. Thin film dielectric layers are deposited on a glass substrate (BK7) with a refractive index of n _G = 1.517. The design of the filter is shown in figure 1. The filter contains a total of 67 thin film layers. Of these, 5 layers are resonators. They are made of material with a refractive index n _L and have an electric thickness π. 1, the resonators are indicated by beech R. The media surrounding the filter are air (left) and a glass substrate (right). They are indicated by the letters A and G, respectively. The remaining 62 thin-film layers form 6 multilayer dielectric mirrors, designated as M ₁ , M ₂ , M ₃ and $$M_{\mathrm{one}}^{\text{'}\text{'}}$$

. Extreme mirrors M ₁ and $$M_{\mathrm{one}}^{\text{'}\text{'}}$$

contain 5 layers. The remaining mirrors, that is, M ₂ and M ₃ , contain 13 layers. The values of the electric thicknesses of the layers in the filter mirrors with a relative bandwidth of 2% and the magnitude of the frequency response in the passband characterized by the maximum reflection level S _{11 max} = -15 dB are shown in Table I. All these values satisfy the condition that is common for all mirrors

The achievement of the technical result, namely the possibility of realizing almost any required bandwidth (in this example, 2%) with almost any unevenness of the transmittance of light in this band (in this example, S _{11 max} = -15 dB), is confirmed by the amplitude-frequency characteristic figure 2, calculated for the parameters of the layered structure presented in Table I.

The given embodiment of the invention differs from the known filter designs in that in each i-th mirror the electric thickness θ _{Li of the} layers with a refractive index n _{L is} less than π / 2, and the electric thickness θ _{Hi of the} layers with a refractive index n _H exceeds π / 2 by Δθ _Hi and 2Δθ _Hi for the outer and inner layers, respectively, where the quantities θ _Li and Δθ _Hi satisfy equation (1).

The filter works as follows. At the center frequency of the passband f _0, each multilayer mirror M _{i is} equivalent to another multilayer mirror with quarter-wave layers. The layers of this equivalent mirror have refractive indices n _H and n _i (n _H > n _i > n _L ). Such multilayer mirrors do not disturb the resonant frequencies of the resonators in the filter, which greatly simplifies the design of multilayer filters. Therefore, the design of the filter is reduced only to finding the optimal values of n _i .

The equivalence of the matched multilayer mirrors means the equality of the corresponding ABCD matrices (i.e., characteristic matrices). In particular, the equality of the diagonal elements of ABCD matrices gives equation (1). It expresses the requirement that all layers of an equivalent mirror have an electric thickness of π / 2.

Thus, the advantage of the inventive optical multilayer filter is that in it, using only two materials with unequal refractive indices, it is possible to smoothly change both the width of its passband and the unevenness of the light transmission coefficient in this band.

Claims (1)

An optical multilayer band-pass filter containing a dielectric substrate coated with thin-film layers of dielectrics with alternating high refractive index n _H and low refractive index n _L , in which some of the layers of material with refractive index n _L are resonators having an electric thickness of π per the central frequency of the passband, and all other layers form multilayer dielectric mirrors that separate each resonator from the neighboring resonator, the dielectric surface or outer space, characterized in that in each i-th mirror the electric thickness θ _{Li of} layers with a refractive index n _{L is} less than π / 2, and the electric thickness θ _{Hi of} layers with a refractive index n _H exceeds π / 2 by Δθ _Hi and 2Δθ _Hi for the outer and inner layers, respectively, where θ _Li and Δθ _Hi satisfy the equation

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