# US 4,400,058 A · Tunable Fabry-Perot filter

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- Number: US 4,400,058 A
- Document ID: US4400058A
- Status: Granted
- Kind: A
- Application number: 06/384,842
- Filed: 1982-06-04
- Granted: 1983-08-23
- Earliest claimed: 1980-06-16
- Assignees: Honeywell Inc.
- Inventors: William W. Durand; Ronald E. Peterson
- Web page: https://patentgrep.com/patent/US4400058A

## CPC

- G01J 3/26: using multiple reflection, e.g. Fabry-Perot interferometer, variable interference filters
- G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N)

## Abstract

A tunable Fabry-Perot filter for transmitting light in the wavelength range from 3-40 microns is disclosed wherein a pair of low index substrates of refractive index less than 2.4 and positioned between an adjustable spacer means have, on their facing sides, a non-metallic coating of refractive index of at least 4.0 and of optical thickness less than one-half the minimum wavelength to be transmitted. Preferred substrates are potassium halides, cesium halides, and zinc selenide, and preferred high index coatings are from the group of lead telluride, bismuth telluride, germanium telluride and germanium.

## Claims

### Claim 1

A tunable Fabry-Perot filter device, comprising:
a pair of low index substrates having a refractive index of less than 2.4 and mounted in parallel relationship so as to present facing sides to each other in an optical path;
a high index, non-metallic coating having a refractive index of at least 4.0 and an optical thickness less than one half of the minimum wavelength to be transmitted, said high index coating being on said facing sides of each said low index substrates; and
spacer means for mounting said low index substrates and for adjusting the distance between said facing sides.

### Claim 2 (depends on 1)

The device of claim 1, wherein said low index substrate is selected from the group of low index materials consisting of potassium halide, cesium halide and zinc selenide.

### Claim 3 (depends on 2)

The device of claim 2, wherein said potassium halide is potassium chloride.

### Claim 4 (depends on 1)

The device of claim 1, wherein said high index non-metallic coating is selected from the group consisting of lead telluride, bismuth telluride, germanium telluride and germanium.

### Claim 5 (depends on 1)

The device of claim 1 wherein said spacer means is adapted to vary the distance between said facing sides to pass light of a wavelength ranging from 3 microns to 40 microns.

### Claim 6

A pair of low index substrates having a refractive index of less than 2.4 and mounted in parallel relationship so as to present facing sides to each other in an optical path;
a high index non-metallic coating which is less than ten percent absorbing in the 3 to 40 micron wavelength range and has a refractive index of at least 4.0 and an optical thickness less than one half of the minimum wavelength to be transmitted, said high index coating being on said facing sides of each said low index substrates; and
spacer means for mounting said low index substrates and for adjusting the distance between said facing sides.