JPH03197901A - Optical band-pass filter - Google Patents

Abstract

PURPOSE:To obtain the optical band-pass filter which can be used in open constitution and is small in ripple by alternately laminating low-refractive index layers consisting of SiO2 and high-refractive index layers consisting of TiO2 on a glass substrate and setting the thicknesses of the respective layers. CONSTITUTION:This optical band-pass filter is constituted by alternately laminating the low-refractive index layers consisting of the SiO2 and the high-refractive index layers consisting of the TiO2 in this order to 41 layers on a glass substrate 1 in such a manner that the outermost layer is the low-refractive index layer. The design values of the thicknesses of the respective layers are set as follows when the thickness equal to the length of 1/4 the central wavelength of the pass bands is 1.0: The thickness of the 2nd layer is set at 0.9, the 38th layer at 0.22, the 39th layer at 0.33, the 40th layer at 1.65, the 41st layer at 0.93, the 4th, 11th, 27th, and 34th layers at 2.0 and the other layers at 1.0. The optical band-pass filter which can be used in open constitution and is small in ripple is obtd. in this way.

Description

[Detailed Description of the Invention] Summary The present invention relates to an optical band-pass filter that transmits light in a desired band, and aims to provide an optical band-pass filter that can be used in an open configuration and has small ripples, and is made of a glass substrate. A low refractive index layer consisting of Sin and TiO2
An optical bandpass filter consisting of 41 layers laminated in this order alternately with high refractive index layers and high refractive index layers such that the outermost layer is the low refractive index layer, the design value of the thickness of each layer or the design value and tolerance The deviations that can occur are constructed as features.

INDUSTRIAL APPLICATION FIELD The present invention relates to an optical bandpass filter that transmits light in a desired band.

In the field of optical transmission, when performing WDM transmission (wavelength division multiplexing), it is necessary to separate two or more lights with different wavelengths transmitted through the same transmission path into different optical paths, or to separate two or more lights with different wavelengths into different optical paths. An optical bandpass filter is sometimes used to guide the above lights to the same optical transmission path.

This type of optical bandpass filter can be constructed by laminating a plurality of dielectric films having different refractive indexes on a glass substrate. What is required of an optical bandpass filter is (a) small ripple in the transmittance-wavelength characteristic curve, (b) no complexity in the configuration of the device constructed using the optical bandpass filter, and specific requirements. Among them, there is no need for a short configuration (a configuration in which the outermost layer is not directly exposed to the air) and it can be used in an open configuration (a configuration in which the outermost layer is directly exposed to the air).

Conventional technology 37 layers of low refractive index and high refractive index are alternately formed on a glass substrate.
FIG. 5 shows an example of the transmittance (%) vs. wavelength (nm) characteristic curve of a conventional optical bandpass filter formed by laminating layers. A is a characteristic curve when the incident angle is set to Oo and used in an open configuration, and B is a characteristic curve when the incident angle is set to 15° and used in an open configuration. From these characteristic curves, it can be seen that when used in an open configuration, 0.3 to 0.
It is clear that a ripple of 5 dB has occurred. If ripples occur in the characteristic curve, the insertion loss of the optical bandpass filter will vary depending on changes in the wavelength of light from the light source. Therefore, conventionally, optical bandpass filters have been used in a short configuration in order to suppress ripples in the characteristic curve. In Figure 5, C
What is shown is a characteristic curve when the incident angle is set to 0° and the short configuration is used, and it can be seen that the ripple in the transmission band is reduced. To make the optical bandpass filter into a short configuration, for example, another glass substrate that has been subjected to anti-reflection treatment is tightly attached to the dielectric multilayer film laminated surface of the glass substrate using an optical adhesive. .

Problem to be Solved by the Invention If ripples occur in the characteristic curve, the insertion loss of this optical pass filter will change or vary depending on the fluctuation or dispersion of the wavelength of the light from the light source, and the system This is extremely inconvenient for building. On the other hand, if the optical bandpass filter is used in a short configuration, ripples can be suppressed, but there is a drawback that the configuration becomes complicated or larger. In this way, in the conventional technology, it is either possible to simplify the configuration by allowing ripples to occur in the characteristic curve, or to suppress ripples by allowing the configuration to become complicated. I had to choose.

The present invention was created in view of these circumstances, and aims to provide an optical bandpass filter that can be used in an open configuration and has small ripples.

Means and actions to solve the problem The present invention will be explained in detail with reference to FIG.

The optical bandpass filter of the present invention has five
It is constructed by laminating 41 layers of low refractive index layers made of 102 and high refractive index layers made of T i 02 alternately in this order so that the outermost layer is the low refractive index layer. Then, when the thickness equal to a quarter of the length of the center wavelength of the transmission band is 1.0, the design values of the thicknesses of the above-mentioned layers are set as follows.

2nd layer ・0.9 38th layer; 0.22 39th layer; 0.33 40th layer; 1.65 41st layer; 0.93 4.11.19.27.34 layer; 2.0 Other layers: 1,0 The 1st to 37th layers, indicated by A in Figure 1, constitute a so-called 5-cavity bandpass filter, and the presence of this laminated portion allows only light in a desired band to pass through. functions occur. The presence of the 38th to 41st layers (B) additionally provided in the laminated portion indicated by A makes it possible to suppress ripples without requiring use in a short configuration, and It becomes possible to effectively prevent Fresnel reflection of light in the transmission band and increase the transmittance of light in the transmission band.

By keeping the thickness deviation of each layer within ±1% of the above design value, the ripple is sufficiently small for practical use without any manufacturing difficulties, and there is no need to use it in a short configuration. It becomes possible to provide optical bandpass filters.

Example The present invention will be explained in detail below.

The material of the glass substrate 1 is BK-7 (refractive index 1.5).
can be used, and the shape of the glass substrate 1 can be made into a flat plate shape. Examples of methods for laminating the dielectric multilayer film on the glass substrate 1 include sputtering, electron beam evaporation, and the like. The refractive index of 5to2 is 1.46
The refractive index of TiO□ is 2.3.

FIG. 2 is a diagram showing the relationship between transmittance (%) and wavelength (ns) when the thickness of each layer of the dielectric multilayer film is set as the designed value. A solid line A is a characteristic curve representing the above relationship, and for reference, a dash-dotted line B represents a characteristic when an optical bandpass filter having a conventional configuration is used in a short configuration. Both characteristics were measured with the angle of incidence on the dielectric multilayer film set at 15°. According to this invention, even though the optical bandpass filter is used in an open configuration,
It is clear that a characteristic curve equivalent to that obtained when a conventional product is used in a short configuration can be obtained, and ripples are almost completely suppressed.

However, strictly controlling the thickness of each layer of a dielectric multilayer film involves manufacturing difficulties. Therefore, with the aim of providing an optical bandpass filter that has small ripples and can be used in an open configuration without any manufacturing difficulties, the thickness of each layer must be adjusted to make the ripples sufficiently small for practical use. Consider the deviation.

FIG. 3 is a diagram in which the range that the characteristic curve can take when the thickness deviation of each layer is within the range of ±1% from the design value is shown by diagonal lines. In the wavelength range of 1280 to 1320 (nm), the decrease in transmittance due to ripples is 0. If it is smaller than 45 (dB), it can be said that the ripple is acceptable in practice. Thus, it is clear that when the deviation is within the range of ±1%, the ripple is acceptable in practice.

FIG. 4 is a diagram showing the possible range of the characteristic curve when the thickness deviation of each layer is within the range of ±2% from the design value. In this case, the wavelength is 1280 to 1320 (r+n+)
The decrease in transmittance within the range of 0, 45 (dB
), it cannot be said that the ripple can be suppressed to a practically acceptable range.

Therefore, by keeping the deviation of the thickness of each layer within ±1% of the design value, it is possible to create an optical bandpass filter that has sufficiently small ripples for practical use and can be used in an open configuration. Can be provided without accompaniment.

As described in the detailed description of the invention, according to the present invention, by specifying the design value of the thickness of each layer of a dielectric multilayer film laminated on a glass substrate, it can be used in an open configuration, and , it is possible to provide an optical bandpass filter with small ripples.

In addition, by specifying the design value of the thickness of each layer and the deviation of the thickness of each layer, it is possible to create an optical bandpass filter that can be used in an open configuration and has small ripples. You will be able to provide it without any hassle.

[Brief explanation of drawings]

Fig. 1 is a diagram for explaining the present invention in detail, Fig. 2 is a characteristic diagram when the thickness of each layer is set to the design value, and Fig. 3 is a diagram in which the deviation of the thickness of each layer is ±1 from the design value. Figure 4 shows the range that the characteristic curve can take when the thickness deviation of each layer is within 2% of the design value. The figure shown in FIG. 5 is an explanatory diagram of the prior art and its problems. 1...Glass substrate.

Claims (1)

[Claims] 1. 41 layers of a low refractive index layer made of SiO_2 and a high refractive index layer made of TiO_2 are alternately arranged in this order on a glass substrate (1) so that the outermost layer is the low refractive index layer. In an optical bandpass filter formed by laminating layers, when the thickness is equal to 1/4 of the length of the center wavelength of the transmission band and is 1.0,
The design values for the thickness of each of the above layers are as follows: 2nd layer: 0.9 38th layer: 0.22 39th layer: 0.33 40th layer: 1.65 41st layer: 0.93 4th, 11th, 19th layer , 27, 34 layers: 2.0 Other layers: 1.0. 2. Deviation of the thickness of each layer above by ±1% from the above design value
2. The optical bandpass filter according to claim 1, wherein the optical bandpass filter is within the range of .