JPH03215803A - Band-pass filter - Google Patents

Abstract

PURPOSE:To provide the band-pass filter which has no ripples even with a substrate consisting of Ge, Si, etc., used in an IR region and having a high refractive index and has a high transmittance by forming alternate layers which are laminated repeated with high-refractive index layers and low-refractive index layers from the Si or Ge substrate and ends at the low-refractive index layer on the above-mentioned substrate. CONSTITUTION:The band-pass filter is basically constituted by laminating the alternate layers of the high-refractive index layer (H) and the low-refractive index layer (L) on the high-refractive index substrate consisting of the germanium (4.03 refractive index) or the silicon (3.43 refractive index), etc., and further, providing another layer of the low-refractive index layer as the extreme surface layer on the air side on the low-refractive index layer which is the uppermost layer of the alternate layers. The transmittance of the transmission region, for example, (about 3 to 6mum) is improved by 20 to 30% by this constitution. In addition, the ripples are decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a bandpass filter, particularly to a bandpass filter with excellent infrared transmission.

Conventional technology and its challenges Traditionally, long wave bass filters (which cut short wavelengths and transmit long wavelengths) have been used as optical elements.
(Long wave pass filter)
There is.

This generally has a structure in which a high refractive index layer and a low refractive index layer made of a high refractive index material and a low refractive index material are laminated on a glass substrate, and various structures are known. There is.

However, with the conventional layer structure, it was not possible to eliminate ripples in the transmission region when the refractive index of the substrate increased.

On the other hand, long wave bass filters in the infrared region, which are attracting attention for various observation purposes such as detectors and infrared cameras, are desired, and demand has been increasing in recent years. Substrate materials used in this area include G6, Si, ZnSSZ
nSe etc., but due to ease of processing and availability, Ge, S
i is commonly used.

3 However, Ge and Si have a refractive index of 4.03 and 3.43, which is very high compared to glass, and even if a short wavelength cut filter is made on top of this with the conventional configuration, the transmittance will deteriorate significantly in the transmission range. invite.

For example, a bandpass filter using Si or Ge as a substrate is disclosed in Japanese Patent Application Laid-open No. 1-108504, but this is a monochromatic filter that transmits only a desired wavelength, and is completely different in purpose, structure, and effect from the present invention. It is something.

Problems to be Solved by the Invention The present invention has been made in view of the above-mentioned circumstances. The purpose is to provide a high bandpass filter.

Means for Solving the Problems, That is, the present invention is formed on a Si or Ge substrate, has alternating layers of repeating higher refractive index layers and lower refractive index layers than the substrate, and ends with the lower refractive index layer, and further The present invention relates to a bandpass filter characterized in that it has a low refractive index layer laminated on the above-mentioned alternating layers and is the uppermost layer on the air side.

Furthermore, the present invention provides an i-layer (i represents an integer of 8 or more) multilayer film consisting of a high refractive index layer and a low refractive index layer formed on a Si or Ge substrate, the first layer being the first layer from the substrate. is a high refractive index layer, the second layer is a low refractive index layer, and its optical thickness is 0.2λ (n.di + n2d2≦0.5λ0.05
λ< n ld+ [where λ is the design dominant wavelength; n1 is the refractive index of the first layer; n2 is the refractive index of the second layer; d1 is the thickness of the first layer; d2 is the thickness of the second layer; n1d1 is the optical thickness of the first layer; n2d2 is the optical thickness of the second layer], and furthermore, starting from the third layer, which is a high refractive index layer, There are alternating layers of high refractive index layers and low refractive index layers, each of which has an optical thickness of 0.2λ.
Furthermore, the i-layer, which is the uppermost layer on the air side, is a low refractive index layer, and its optical thickness is 0.3λ<n1d1<λ. rate; ton is the film thickness of the i-th layer: n,
di represents the optical thickness of the i-th layer].

Furthermore, the present invention provides a multilayer film of i+1 layers (I represents an integer of 8 or more) consisting of a high refractive index layer and a low refractive index layer formed on a Si or Ge substrate, the first layer being the first layer from the substrate. is a high refractive index layer, the second layer is a low refractive index layer, and its optical thickness is 0.2λ<n, di+n2d2≦0.5λ0.05λ<
n + a + [where λ is the design dominant wavelength; n1 is the refractive index of the first layer; n2
is the refractive index of the second layer; d1 is the thickness of the first layer; d2 is the thickness of the second layer; n1d1 is the optical thickness of the first layer; n2d2 is the optical thickness of the second layer. In addition, there are alternating layers of high refractive index layers and low refractive index layers starting from the third layer, which is a high refractive index layer, and ending with the i-1st layer, which is a high refractive index layer, and the optical thickness thereof is Each layer 0.2λ
Furthermore, the two layers, the air side layer and the i+1 layer, are low refractive index layers, and their refractive index and optical thickness are n + + + < n 7 0.3λ < nld : +n+++d+++ <λ [where n, is the refractive index of the i-th layer; nl+1 is the refractive index of the 1+1st layer; dl is the thickness of the first layer; dial is the thickness of the i+1st layer; nidl is the i-th layer optical film thickness + ni + Idi
+1 represents the optical thickness of the first +1 layer] The present invention relates to a bandpass filter that satisfies the following relationship.

The bandpass filter of the present invention is made of germanium (GeXff refractive index 4.03) or silicon (SiX refractive index 3.43).
A high refractive index layer (H) and a low refractive index layer (
The basic structure is that alternating layers of L) are laminated, and another low refractive index layer is provided as the outermost surface layer on the air side on the low refractive index layer that is the uppermost layer of the alternating layers.

Furthermore, by setting the air-side outermost surface layer to (2L), the filter has a structure in which high refractive index layers and low refractive index layers are simply laminated as alternating layers on the substrate, that is, (substrate) + (HL)''. It is possible to improve the transmittance in the transmission region (about 3 μm to 6 μm) by 20 to 30%, and to reduce ripples.

In the above basic configuration, the high refractive index layer (H) or the low refractive index layer (L) has a film thickness of 0.2 to 0.3λ (λ is the design dominant wavelength), more preferably about 0.25λ. do. If each of these layers is formed to a thickness outside the range of 0.2 to 0.3λ, ripples will occur in the transmission region.

A high refractive index layer (H) and a low refractive index layer (L) formed on the substrate.
) The total number of alternating layers (including the outermost layer), that is, the number of i referred to in this specification, is preferably 8 or more, and the larger i is, the steeper the rise in the transmission range can be obtained.

The thickness of the outermost surface layer (2L) is 0.3 to λ, preferably about 0.5λ. If the film thickness is outside the above range, ripples may become large in the transmission region or ripples may occur in the reflection region.

In addition, the high refractive index and low refractive index mentioned in this specification are considered based on the refractive index of the substrate, and when it is higher than the refractive index of the substrate,
It is called a high refractive index, and if it is lower than the refractive index of the substrate, it is called a low refractive index.

More preferably, the optical thickness of the first layer on the substrate (n+d
+Xn+ is the refractive index of the first layer on the substrate, d. represents the film thickness of the first layer on the substrate) is a thickness of 0.05λ or more, preferably 0.1 to 0.15, more preferably about 0.12
5, and the optical film thickness (nxdz) of the low refractive index layer formed on the high refractive index layer, where Cnv is the refractive index of the second layer on the substrate, and d2 is the film of the second layer on the substrate. 0.2λ<neat+n2d2≦0.5λ, more preferably 14dl+n2dz#0. It is formed so that the relationship 2 5λ is satisfied. By doing so, the transmittance in the transmission region can be further improved and ripples can be reduced.

By laminating the refractive index layer and the low refractive index layer, a structure in which the high refractive index layer and the low refractive index layer are simply laminated as alternating layers on the substrate (substrate + (HL)) has a transmittance several tens of percent higher than that of two filters. This makes it possible to reduce ripples in the transmission region.

If the range of the total optical film thickness (n, di + r + 2d2) is outside the above range, ripples in the transmission region will increase, which is preferable? do not have. Furthermore, if the optical thickness of nidi becomes thinner than 0.05λ, the effects of the present invention cannot be obtained.

A further low refractive index layer may be provided on the low refractive index layer closest to the air. In this case, the first layer from the air side is preferably a low refractive index layer made of a material having a lower refractive index than the second layer from the air side.

In this case, the optical thickness of the second layer from the air side (n1d+)
(n+, d+ are the same as above) and the optical thickness of the first layer from the air side (n+++d+++Xn+++ is the refractive index of the i+1st layer, di+ is the thickness of the i+1st layer) is 0.3λ(n 1 d + + n l+ ■d +
＋． Make sure that the relationship <λ is satisfied. By doing so, the transmittance in the transmission region can be further improved and ripples can be eliminated.

Each layer can be formed by known methods such as vacuum deposition, sputtering,
It can be produced by ion plating or the like.

The high refractive index layer of the present invention can be formed using Ge. The material for the low refractive index layer is MgF, BaF, .
YF. , SrFz, AlF. , L I F s L a
F 2, Sin, , Sin, Mg○, Al203, Z
Although fluorides, oxides, sulfides, etc. such as nS, ZnSe, and ThF 3 can be used, ZnS is preferable in terms of film stress and ease of handling. Use of ZnS can offset film stress, thereby preventing the occurrence of cracks.

The present invention will be further explained below using Examples.

Bandpass filters having the configurations shown in Tables 1 to 6 were fabricated on Si or Ge substrates.

The transmission range characteristics of the obtained band 7 filter (wavelength 2 to 6 μm)
) are shown in FIGS. 1 to 6 in correspondence with the table numbers.

(Hereinafter, blank space) 11 Example 1 Configuration (substrate XHXL)...(HXL)...(HX2
L) (Incidence angle 0° Layer Material name I ZnS 2 'Ge 3 ZnS 4 Ge 5 ZnS 6 Ge 7 ZnS 8 Ge 9 ZnS 10 Ge 12 ZnS 12 Ge 13 ZnS 14 Ge Substrate Si Table 1 Design dominant wavelength Refractive index 2. 25 4.3 2.25 4.3 2.25 4.3 2,25 4.3 2.25 4.3 2.25 4.3 2.25 4.3 3.43 λ=2.3μm) Optics Target film thickness 0.500λ 0.250λ 0.250λ 0.2 5 0λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ ~12 Implementation Example 2 Table 2 (Incidence angle 06 Layer Material name l ZnS 2 Ge 3 ZnS 4 Ge 5 ZnS 6 Ge 7 ZnS 8 Ge 9 ZnS 10 Ge If ZnS 12 Ge 13 ZnS 14 Ge Substrate Si Design dominant wavelength refraction Rate 2,25 4 .3 2.25 4.3 2,25 4.3 2,25 4.3 2,25 4.3 2.25 4.3 2,25 4.3 3.43 λ=2.3μm) Optical film Thickness 0.500λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.2 5 0λ 0.250λ 0.125λ 0.125λ Example 3 Table 3 (Incidence angle 0° Layer Material name Design dominant wavelength refractive index λ - 2.3 μm) Optical film thickness ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge 2.25 4,3 2.25 4.3 2 .25 4.3 2.25 4.3 2.25 4.3 2.25 4.3 2.25 4.3 4.03 0.500λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.25OA O. 250λ 0.250λ 0.250λ 0.125λ 0.125λ 15 Example 4 Table 4 (Incidence angle 0° Layer Material name Design dominant wavelength refractive index λ = 2.3 μm) Optical film thickness ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge 2.25 4.3 2.25 4,3 2,25 4,3 2.25 4.3 2.25 4.3 2,25 4,3 2,25 4.3 3 .43 0.5QOλ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.150λ 0.100λ 16 Example 5 ( Incident angle 0° Layer Material name I ZnS 2 Ge 3 ZnS 4 Ge 5 ZnS 6 Ge 7 ZnS 8 Ge 9 ZnS 10 Ge 11 ZnS 12 Ge 13 ZnS Table 5 Design dominant wavelength refractive index 2.25 4.3 2.2 5 4 .3 2.25 4.3 2.25 4.3 2,25 4.3 2,25 4.3 2.25 λ-2, 3μm) Optical film thickness 0.500λ 0.250λ 0.250λ 0. 250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.150λ I7 Example 6 (Incidence angle 0° Layer Material name table 6 Design dominant wavelength refractive index λ=2. 3 μm) Optical thickness MgF 2 ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge Si l. 35 2.25 4.3 2.25 4.3 2,25 4.3 2.25 4.3 2,25 4.3 2.25 4.3 2.25 4.3 3.43 0.125λ 0 .500λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.125λ 0.125λ ? 18 A band 7 filter having the configuration shown in Table 7 below was fabricated on a flat silicon substrate.

The transmission band characteristics of the obtained bandpass filter (wavelength 2 to 6 μm
) is shown in Figure 7.

Table 7 (Incidence angle 0° Design dominant wavelength λ-2.3μm) Material name Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge ZnS Ge S1 Refractive index 4.3 2.25 4.3 2.25 4.3 2 .25 4.3 2.25 4.3 2.25 4.3 2.25 4.3 3.43 Optical thickness 0.125λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0. 250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.250λ 0.125λ As is clear from Figure 7, the short wavelength cut filter fabricated on the Si substrate has low transmittance in the transmission range. .

On the other hand, by setting the optical thickness of the low refractive index layer of the air side final layer prepared in Example 1 to about 1/2 of the designed dominant wavelength, the transmittance was reduced as shown in FIG. is improved.

As produced in Examples 2, 3, 4, and 5, the optical thickness of the first layer (high refractive index layer) on the substrate and the second layer (
As shown in FIGS. 3, 4, and 5, the total optical thickness of the low refractive index layer (low refractive index layer) is approximately 100%, and it can be seen that the transmittance of the filter is further improved than that of the filter of Example 1.

In addition, as shown in Example 6, when a low refractive index layer having a refractive index lower than that of the final layer on the air side (low refractive index layer) is provided, Even more remarkable effects can be obtained.

Effects of the Invention According to the present invention, a bandpass filter with no ripple and excellent transmittance in the mid-infrared region (wavelength 3 to 5 μm) can be obtained on a substrate having a high refractive index such as Si or Ge.

[Brief explanation of drawings]

Figures 1 to 6 show wavelengths 2 to 6 of the bandpass filter of the present invention.
It represents the transmittance for infrared light of 6 μm. FIG. 7 shows the transmittance of a conventional bandpass filter for infrared light having a wavelength of 2 to 6 μm.

Claims (1)

[Claims] 1. It is formed on a Si or Ge substrate, and has alternating layers consisting of repeating higher refractive index layers and lower refractive index layers than the substrate and ending with the low refractive index layer; A bandpass filter characterized in that it is laminated and has a low refractive index layer that is the uppermost layer on the air side. 2. i layer consisting of a high refractive index layer and a low refractive index layer formed on a Si or Ge substrate (i represents an integer of 8 or more)
It is a multilayer film, in which the first layer is a high refractive index layer, the second layer is a low refractive index layer, and the optical thickness thereof is 0.2λ<n_1d_1+n_2d_2≦0.5λ0.
05λ<n_1d_1 [where λ is the design dominant wavelength; n_1 is the refractive index of the first layer; n_
2 is the refractive index of the second layer; d_1 is the thickness of the first layer; d_2 is the thickness of the second layer; n_1 d_1 is the optical thickness of the first layer; n
_2d_2 represents the optical thickness of the second layer] and further includes a high refractive index layer and a low refractive index layer starting from the third layer, which is a high refractive index layer, and ending with the i-1th layer, which is a high refractive index layer. Each layer has an optical thickness of 0.2λ.
Furthermore, the uppermost i layer on the air side is a low refractive index layer, and its optical thickness is in the range of 0.3λ<
A bandpass filter characterized in that n_id_i<λ [where n_i represents the refractive index of the i-th layer; d_i represents the film thickness of the i-th layer; and n_id_i represents the optical film thickness of the i-th layer]. 3. A multilayer film of i+1 layers (i represents an integer of 8 or more) consisting of a high refractive index layer and a low refractive index layer formed on a Si or Ge substrate, where the first layer has a higher refractive index than the substrate. The second layer is a low refractive index layer, and its optical thickness is 0.2λ<n_1d_1+n_2d_2≦0.5λ0.
05λ<n_1d_2 [where λ is the design dominant wavelength; n_1 is the refractive index of the first layer; n
_2 is the refractive index of the second layer; d_1 is the thickness of the first layer; d_2
is the thickness of the second layer; n_1d_1 is the optical thickness of the first layer;
n_2d_2 represents the optical thickness of the second layer. Each layer has an optical thickness of 0.2λ.
0.3λ from
1<λ [where n_i is the refractive index of the i-th layer; n_i_+_
1 is the refractive index of the i+1th layer; d_i is the thickness of the i-th layer; d_
i_+_1 is the thickness of the i+1th layer; n_id_i is the optical thickness of the i-th layer; n_i_+_1d_i_+_1 is the i+th layer
represents the optical thickness of one layer] A bandpass filter characterized by satisfying the following relationship. 4. The high refractive index layer is made of Ge, and the low refractive index layer is made of ZnS.
, ZnSe, MgF_2, AlF_3, BaF_2, L
iF, SrF_2, YF_3, CAF_2, Mg0, S
iO_2, TiO_2, CeO_2, SiO, Al_2
A bandpass filter made of a material selected from the group consisting of O_3.