JPH02193111A - Fabry-perot type optical filter - Google Patents

Abstract

PURPOSE:To obtain the Fabry-Perot type optical filter with small size and stably operated, capable of enlarging a passing wavelength variable range by constituting a cell of two pieces of mirrors for constituting a resonance circuit, and controlling a temperature of a substance packed in the cell. CONSTITUTION:On the inside surfaces of two pieces of substrates 1a, 1b placed so as to be opposed to each other, mirrors 2a, 2b having a high reflection factor are provided. By these mirrors 2a, 2b and a spacer 3, a cell 4 being a closed space is formed, it is packed with a thermo-optical effect substance 5, and also, a heater 6 being a temperature control means for heating it is provided. In such a state, when a temperature of the substance 5 in the cell is varied by a temperature control means 6, a resonance condition of a light beam which is made incident roughly vertically on one of two pieces of mirrors, and wavelength of a light beam which passes through an optical filter is varied. In such a way, the optical filter becomes smaller in size and stably operated, and its passing wavelength can be varied to a comparatively large range.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a Fabry-Perot type optical filter with variable transmission wavelength.

<Prior art> A conventional example of a Fabry-Perot optical filter with a variable transmission wavelength consists of two mirrors arranged in parallel and facing each other on a common base, and a screw-type or piezo-type fine adjustment device. By changing the distance between the two mirrors via
One example is one in which the passing wavelength is variable.

<Problem to be solved by the invention> However, the conventional fabrication process as described above
In the Perot type optical filter, since the two mirrors are separate parts, their alignment is easily lost due to the effects of temperature, vibration, etc. Also, the fine movement device for moving the mirrors is relatively large, so it can be made smaller. The problem is that it cannot be configured.

Therefore, the present inventors provided two mirrors constituting a resonant circuit on two opposing planes of a dielectric parallel plate having an electro-optical effect, and applied an electric field to the dielectric parallel plate to He first invented a Fabry-Perot optical filter in which the wavelength of transmission can be varied by changing the refractive index (Japanese Patent Application No. 198903/1983).

This Fabry-Perot optical filter does not have a movable part, so it can solve the above problems, but it has the problem that the variable range of the passing wavelength is quite small. for example,
In a Fabry-Perot optical filter using dielectric parallel plates made of lithium niobate with a thickness of about 100 μm and which has a passing wavelength of about 1.3 μm, even when a voltage of about 1 kV is applied, the passed wavelength is 35 pm. The problem is that it only changes in degree.

In view of these circumstances, the present invention is small and stable,
An object of the present invention is to provide a Fabry-Perot optical filter that can change its passing wavelength over a relatively wide range.

<Means for Solving the Problems> The Fabry-Perot optical filter according to the present invention that achieves the above object constitutes a resonant circuit by arranging two high-reflectance mirrors facing each other in parallel, In the Fabry-Perot optical filter, which passes only the light with a wavelength that satisfies the resonance condition among the light that is almost perpendicularly incident on one of the mirrors, and does not pass the remaining light with other wavelengths, the above two filters are used. Temperature control that forms a cell with mirrors, has an optically transparent and isotropic substance within the cell, and changes the refractive index by heating or cooling the substance to change the resonance conditions. It is characterized by comprising means.

Effect> In the above configuration, when the temperature control means changes the temperature of the substance in the cell formed by the two mirrors, the resonance condition of light that is almost perpendicularly incident on either of the two mirrors changes. do.

Therefore, this changes the passing wavelength, that is, the wavelength of the light that passes through the optical filter.

<Embodiments> Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a partially cutaway structural diagram of a Fabry-Perot optical filter according to this embodiment, and FIG. 2 is a longitudinal sectional view thereof. As shown in both figures, two substrates 1a are placed facing each other.

Mirrors 2a and 2b with high reflectivity are provided on the inner surface of 1b. These mirrors 2a, 2b are arranged parallel to each other with a certain distance between them via a spacer 3, and these mirrors 2a, 2b and the spacer 3 form a cell 4, which is a sealed space. The cell 4 is filled with a thermo-optic effect substance 5, and a heater 6 serving as a temperature control means for heating the thermo-optic effect substance 5 is provided.

Here, it is desirable that the substrates 1a and Ib be formed of a material with low transmission loss of light, but usually optical glass may be used. Further, as the mirrors 2a and 2b, metal mirrors, dielectric multilayer mirrors formed by laminating a plurality of dielectric films with different refractive indexes, etc. can be used, but from the viewpoint of reflectance and light absorption characteristics, A dielectric multilayer mirror is more advantageous in practice.

On the other hand, the material of the spacer 3 is not particularly limited, but it is preferably a material with a small coefficient of thermal expansion, such as Invar or glass with a low coefficient of thermal expansion. In addition, the mirror 2a
, 2b and the spacer 3 may be bonded, for example, by melting low melting point glass powder.

The resonance condition of such a Fabry-Perot optical filter is that the amount of phase change when light makes one round trip between the mirror 2a and the mirror 2b is equal to an integer times 2π.

That is, the refractive index of the thermo-optic effect material 5 is n, and the refractive index of the thermo-optic effect substance 5 is
If the distance between the mirror 2b and the mirror 2b is d, then λ. = 2 n d / m is the passing wavelength λ of the Fabry-Perot optical filter. is determined. Therefore, in the present invention, by changing the refractive index n of the thermo-optic effect material 5, the transmission wavelength λ can be adjusted. is variable. That is, the thermo-optic effect material 5 is heated by the heater 6.
By changing the temperature of the refractive index n, the transmitted wavelength λ is changed. is designed to be controlled.

In other words, when the temperature of the thermo-optic effect material 5 is To, its refractive index is n. And the wavelength passed by the Fabry-Perot optical filter is λ. . , when the temperature of the thermo-optic effect material 5 is T1, its refractive index is nl, and the passing wavelength of the Fabry-Perot optical filter is λ, then input = 2nd/m λ = 2nd/m n, = no(1+k (T, −To)), and λ. 1/λ. . -n, /n.

=1+k (T1-To). However, k is the temperature coefficient of the refractive index n. Then, if 8λ=λ ΔT=T, then 1+Δλ/λ. . = 10 to 8T Δλ/λ. . Second, the formula for ΔT is derived. Therefore, in order to efficiently cause a change in the transmission wavelength Δλ due to a temperature change in the thermo-optic effect material 5, the thermo-optic effect material 5 must have a large temperature coefficient of refractive index n.
It is necessary to use

Therefore, for this reason, as the thermo-optic effect substance 5, a substance having a large temperature coefficient k such as 1,3-cyclobutetetpane, 1,2-dibromopropane, aniline, nidobenzene, and 0-toluidine is used. is preferable.

All of these exemplified materials have a temperature coefficient k of refractive index on the order of -5 x 10''-'/°C.

When these substances are used as thermo-optic effect material 5, and ΔT-±206, Δλ/^. 0 to 0.01. In order to compare this with the case of using a dielectric flat plate made of lithium niobate mentioned above, λ. . =1.3μ
Assuming m, it becomes △λヱ±i3nm, and it is recognized that the passing wavelength range of this example is several hundred times larger than that of an example of a Fabry-Bay type optical filter using a dielectric flat plate. It will be done.

<Effects of the Invention> As explained above, in the present invention, a cell is formed by two mirrors forming a resonant circuit, and the refractive index of the substance is changed by controlling the temperature of the substance filled in the cell. , the passing wavelength is made variable. Therefore, it is small and stable because there are no moving parts.
Moreover, since the refractive index change due to the thermo-optic effect is larger than the refractive index change due to the electro-optic effect, a Fabry-Perot type optical filter with a comparatively large wavelength variable range can be realized.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway structural diagram of a Fabry-Perot optical filter according to an embodiment of the present invention, and FIG. 2 is a longitudinal sectional view thereof. In one drawing, la and lb are substrates, 2a and 2b are mirrors 3 are spacers, 4 is a cell, 5 is a thermo-optic effect material, and 6 is a heater.

Claims (1)

[Claims] A resonant circuit is constructed by arranging two high-reflectance mirrors in parallel and facing each other, and only light of a wavelength that satisfies the resonance conditions among the light that is almost perpendicularly incident on either of the mirrors passes through. In a Fabry-Perot optical filter that does not pass the remaining light having other wavelengths, the two mirrors form a cell, and the cell has an optically transparent isotropic material, A Fabry-Perot optical filter characterized by comprising temperature control means for heating or cooling the substance to change its refractive index and thereby change resonance conditions.