JPS59127007A - Dichroic filter - Google Patents

Abstract

PURPOSE:To approximate the transmittivity of the 2nd wavelength to an ideal value by providing plural reflecting surfaces which has a low reflection factor to light of the 1st wavelength, a high reflection factor to light of the 2nd close wavelength, and varies in reflection factor monotonously in an intermediate wavelength range and its circumferential wavelength ranges. CONSTITUTION:Normal etalon, etc., are used for reflecting surfaces 2 on the optical polished surface of a substrate 1; and a multilayered film having about 20 layers for obtaining a high reflection factor is vapor-deposited and a film having about 10 layers for compensation is formed to obtain sharp monotonous reflection factor characteristics. The reflecting surfaces 2 are opposed to each other in parallel by using a spacer 3. This multilayered vapor-deposited film has such spectral characteristics that the film has the low reflection factor R1 to the 1st wavelength lambda1 and the high reflection factor R2 to the 2nd wavelength lambda2 and varies in reflection factor monotonously with wavelength, so the transmittivity is almost 100% in a range of the wavelength lambda1 and a waveform range wherein said condition is satisfied is wide. On the other hand, the transmittivity to the 2nd wavelength lambda2 is extremely low because of extremely small transmittivity condition and the high reflection factor R2 and about (1-R2)<2>/4- 2.5X10<-5>.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical filter, and particularly to a dichroic filter that increases the degree of separation of light having close wavelengths.

Conventionally, dichroic filters that separate two wavelengths of light that are close to each other have been made using a multilayer film deposited mirror with about 30 layers, which has a large difference in reflectance for a 2tfl length, and a dichroic filter that has a high reflectance for the two wavelengths. Two main types are known that utilize etalons with opposing surfaces. However, each of these has drawbacks as described below.

First, when using a multilayer mirror, it is difficult to increase the difference in reflectance for the 2V length, especially when the two wavelengths are close to each other. There was a drawback that the transmittance was 9, which was considerably lower than the ideal value of 100clj, and conversely, the transmittance for light of the second wavelength, which had a high reflectance, was slightly higher than the ideal value of 0%. Therefore, 2. To increase the degree of separation from the light of the boyfriend,
Optically arranging the multilayer film deposits in tandem resulted in a drawback that the transmittance for the light of the first wavelength further decreased.

On the other hand, when the entire circumference of the etalon exists, the interference mirror surface is constituted by a reflecting surface having a similar high reflectance for the first and second wavelengths. At this time, well-known periodic transmission peaks appear in the spectral transmission characteristics of this etalon, so the first and second wavelengths are set to the minimum and maximum wavelengths of the etalon's transmittance, respectively. If made compatible, this etalon will function as a dichroi and a kutsuilter.

In this case, the reflectance Rt of the reflective surface is made as high as possible in order to increase the degree of separation of the two wavelengths of light. In this way, the transmittance for the first wavelength is approximately (1-R)/4, which can be made sufficiently low. On the other hand, the transmittance of the second wavelength is Rt-100
In a high finesse state near %, the surface accuracy and parallelism of the reflecting surface and the insufficient directivity of the incident beam often decrease to half the ideal value of 100 degrees. This is a drawback of the guichroic mirror using an etalon.

The purpose of the present invention is to eliminate the above-mentioned drawbacks of the conventional dichroic filter, combine the features of the conventional 2a type filter, and further increase the transmittance of the second wavelength to an ideal value, which is a feature that the two filters do not have. The object of the present invention is to provide a dichroic and Kutsuilter having characteristics close to those of 100.

According to the present invention, the reflectance is low for light of the first wavelength;
The reflectance is high for light of a second wavelength close to the first wavelength, and the reflectance changes monotonically in a wavelength range between and around the first wavelength and the second wavelength. There is obtained a dichroic filter characterized in that it has a plurality of reflecting surfaces, and the plurality of reflecting surfaces mutually constitute interference surfaces.

Hereinafter, specific embodiments of the present invention will be described in detail using the drawings.

FIG. 1 is a sectional view of an embodiment of the present invention, in which reflective surfaces 2, each having a multilayer film deposited on an optically polished surface of a substrate 1, are arranged to face each other in parallel using a spacer 3. FIG.

The spectral characteristics of this multilayer deposited film are as shown in Figure 2.
The reflectance R1 is low at the first wavelength λ, and the reflectance R1 is low at the first wavelength λ.
, the reflectance R7 is high at the second wavelength λ2 close to , and the reflectance changes monotonically with respect to the wavelength. Specifically, at a wavelength of around 1.3 μm, the difference between λ and λ is approximately 0.1 μm.
In the case of m, it is possible to deposit a multilayer film consisting of about 30 layers having separation characteristics of about R,; 0.2 and R, :; 0.99. The distance d between the two reflective surfaces 20 is the wavelength λ.

It is chosen to be an integer multiple of a half wavelength of the wavelength λ, and close to a half integer multiple of a half wavelength of the wavelength λ. d//i that satisfies these conditions
There are an infinite number of values, but here we choose the smallest value. In this case, the spectral transmission characteristics according to this example are shown in FIG.

For the first wavelength λ, the transmittance of this example is extremely high because the maximum transmittance condition for the interferometer in this example is satisfied and the reflectance R8 of the reflective surface is low. 1
00%, and the wavelength range in which this condition is satisfied is wide. On the other hand, the second wavelength λ.

On the other hand, due to the minimum transmittance condition and the high reflectance R7, the transmittance becomes extremely low, and its value is approximately (1-1).
R,)''/4~2.5X10-'.The narrow transmission peak on the longer wavelength side than λ is the peak that appears under the extremely thick transmission condition, which is one interference order smaller than the wavelength λ1. Because the reflectance is extremely high, the maximum transmittance is much lower than 100%, just like the etalon, and the transmission band is also extremely narrow.The existence of these four narrow transmission peaks is due to the dichroic filter. Although it is unfavorable in terms of its characteristics, it is acceptable in practice.

In the present invention, as shown in FIG. 2, wavelengths λ1 and λ
Since a reflecting surface having a reflectance that monotonically changes between and around the wavelength λ is used, the reflectance is low over a wide wavelength range, especially around the wavelength λ. Therefore, the transmittance of the dichroic filter of the present invention near the wavelength λ1 is high, close to 100% over a wide range as described above. This excellent property is used as a reflective surface for ordinary etalons, etc. to obtain high reflectance.
It is obtained by depositing about 0 multilayer films and then applying about 10 compensation films to obtain the steep and monotonous reflectance characteristics described above. In the multilayer film deposition of about 20 layers described above, the wavelength range where the reflectance decreases is 61 at both ends of the wavelength range where the reflectance is high, and if this range is utilized, the effect similar to that of the present invention can be achieved even with a conventional etalon. However, the change in the reflectance of the multilayer film deposition described above is slow, and in the wavelength range where the reflectance decreases, large fluctuations in the reflectance with respect to wavelength are unavoidable.

As a reflection of these factors, conventional etalon-based dichroic filters that use gold in the wavelength range where the reflectance of the reflective surface decreases cannot achieve the high degree of separation for adjacent wavelengths as the present invention, and also The vibration of the transmittance in the high transmittance region O occurs due to the vibration of the reflectance with respect to the wavelength at , and the wide wavelength region of high transmittance, which is a feature of the present invention, cannot be obtained.

As described above, the present invention eliminates the drawbacks of the conventional filter, and provides an unprecedented practical dichroic filter that has a high degree of separation for two adjacent wavelengths, and in particular has an extremely high transmittance for light of nine wavelengths.

Note that several modifications are possible without departing from the spirit of the invention. .

The number of interference surfaces does not need to be limited to two; for example, it is possible to increase the degree of separation for two wavelengths by a composite resonance effect consisting of three interference surfaces. In that case, one of the substrates in FIG. A configuration in which both of the two optical surfaces are total reflection surfaces can be considered.

If the number of interference surfaces is two, it is naturally possible to use one substrate and make both sides parallel to serve as the interference surface, but in that case, it is necessary to reduce the thickness of the substrate. Because of the limitations, extra transmission peaks appear in the spectral transmission characteristics shown in FIG.

Moreover, contrary to Fig. 2, it is easy to make the reflectance of the reflective surface 2 high for λ1 and low for λ, and accordingly, the transmittance for the long wavelength λ2 light is extremely high. Naturally, a high quality dichroic filter can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a complete cross-sectional view of an embodiment of the present invention, in which 1 represents a substrate, 2 represents a reflective surface, and 3 represents a spacer. FIG. 2 shows the spectral reflection characteristics of the reflective surface 2, and FIG. 3 shows the spectral reflection characteristics of the reflective surface 2.
3 shows the spectral transmission characteristics of an example of the present invention. Patent attorney on your behalf - Susumu

Claims (1)

[Claims] The reflectance is low for light of a first wavelength, the reflectance is high for light of a second wavelength close to the first wavelength, and the reflectance is intermediate between the first wavelength and the second wavelength. A dichroic filter having a plurality of reflecting surfaces whose reflectance monotonically changes in wavelength ranges around the wavelength range, the plurality of reflecting surfaces mutually forming interference surfaces.