JPS6134403A - Optical interferometer - Google Patents

Abstract

PURPOSE:To improve the freedom of an installed place, by using optical fibers for light paths, utilizing the free bending characteristic, and combining the parts of an optical interferometer. CONSTITUTION:Light, which is emitted from a light source 1 is split into two light beams by a half mirror 2. One light passes a lens 10 and is converged at the end surface of an optical fiber 11. The light is converted into parallel light, totally reflected by a corner cube 4, which is provided in front of an object material 3, and returned. The other light is totally reflected by a mirror 6, shifted by a frequency (f) by an optical frequency converter 7, converged at the end surface of an optical fiber 14 by a lens 13 and collimated by a lens 15. The light is further relfected by a total reflecting mirror 16 and synthesized with the reflected light from the corner cube 4 by a half mirror 17 after transmitted through a half mirror 22 so as to interfere with the reflected light from the corner cube 4. The synthesized light is guided to a light detector 9 through a condenser lens 18, an optical fiber 19 and a lens 20. The amount of change in phase of the output signal is measured with the output signal of a light detector as a reference.

Description

Detailed Description of the Invention ('Technical Field) The present invention relates to an optical interferometer, in which an optical interferometer, an optical interference detector, and a measuring device can be installed separately by using optical fibers. Regarding the meter.

(Prior Art) Conventionally, optical interferometry has been used to measure distances and other physical quantities. Figure 1 shows the distance to a certain object (
FIG. 2 is a block diagram showing a conventional optical interferometer that measures the distance (distance from a total reflection mirror 5 to an object 3). In the figure, the light emitted from light source 1 is divided into two by the second half mirror,
One of them is reflected by the corner cube 4 in front of the object 3 and turned back. (This reflected light is called signal light).

The other light is reflected by the total reflection mirror 6.

Optical frequency modulator (usually an ultrasonic optical modulator)
7, the light is shifted by the frequency f (the shifted light is referred to as a reference light). Two lights that differ from each other by a frequency f are combined by a total reflection mirror 2-5 and a half mirror 8, and are received by a photodetector 9.

The amplitude of the light received by the photodetector 9 is determined by the angular frequency of the light emitted from the light source l by ω (=2π, c: speed of light, λ: light wave number)
Then, the following formula is obtained.

(Signal light) = A6cos (ω is - ψ8th 2π・T) −
(1) (Reference light)=Arcos((ω+2πf)t−ψ
,)...(2)ζHere, 2π・da is the phase amount given by the round trip of the light to the object 3, and φS and ψr are the phase amounts determined by the distance between each optical component. F), As, A
r indicates the amplitude of each light. These two lights are combined and interfered by the bar 7mi 2-8.

The intensity of the interference light can be expressed as follows.

AB +Ar+2ABA1CO8(2πft+2π・
-(ψ8-ψr)) (3) According to equation (3), the AC component of the output signal of the photodetector 9 is a signal with a frequency f, and its phase changes with the distance l. Therefore, by measuring the phase of the output signal of the frequency f of the photodetector 9, it is possible to detect the phase amount 2π·7, and from this phase amount the distance l can be measured. In the optical interferometer, depending on the position of the object 3 (corner keys 7 and 4), the emission direction of the light source 1, the position of the second half mirror, the total reflection mirror 6, and the light including the total reflection mirror 5 and the half mirror 8 are determined. Therefore, the position of the coupling part and the position of the photodetector 9 are determined.
8. If you are trying to assemble an optical interferometer by installing each part of the optical interferometer in a different location from the predetermined setting position due to reasons such as not being able to secure the installation space for the mirror 5.6, the position of each optical system. It becomes very difficult to match. , As explained above, the conventional method has the drawback that the installation location of the optical interferometer cannot be freely selected0 (Object of the invention) The object of the present invention is to eliminate the drawbacks of the conventional method described above The object of the present invention is to provide an optical interferometer in which the degree of freedom in installation location is improved by using an optical fiber in the optical path and utilizing its free bending characteristics to connect each part of the optical interferometer. (Structure of the Invention) According to the present invention, the first, second, third and fourth optical fibers transmit the first and second lights having a predetermined frequency difference f from each other to the first and second optical fibers, respectively. a first optical coupling section leading to an optical fiber, and a signal light whose output from the first optical fiber propagates a predetermined distance and returns and combines the output from the second optical fiber so as to interfere with each other. a second optical coupling portion that guides the fourth light beam to the fourth optical fiber 7 arbor;
a first photodetector that detects the interference light output from the optical fiber 7; and a second detector that detects the interference light output from the fourth optical fiber; An interferometer that measures the physical quantity of the propagation path can be obtained by measuring the phase of the electrical signal of frequency f output from the second photodetector.

Furthermore, according to the present invention, the first and second lights having a predetermined frequency difference f from each other are guided to the first and second optical fibers, respectively. <The first optical coupling part, the signal light which the output from the first optical fiber has propagated a predetermined distance and returned, and the second
a second optical coupling unit which combines the output from the optical fibers of the optical fibers 7 and 7 by interfering with the output from the optical fibers, and guides the third light 7 to the IPA; and a photodetector which detects the interference light output from the third optical fibers. An interferometer is obtained which measures the physical quantity of the propagation path by measuring the phase of the electrical signal of frequency f output from the photodetector.

(Example acute IgJ) Next, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a diagram showing a first embodiment of the present invention.

This is a diagram. In the figures below, concentrated lines with arrows indicate light, and the content of the arrow indicates the traveling direction of the light. The light emitted from the light source 1 is divided into two parts by the second half mirror, and one of the parts is focused on the end face of the optical fiber 11 (first optical fiber) through the lens 10.

The light propagated through the optical fiber 11 is converted into parallel light by the lens 12, and is totally reflected at the corner Φ 4 in front of the object 3 (installed) and returns. The other light is totally reflected by the mirror 6, and shifted by the frequency f by the optical frequency converter (for example, an ultrasonic light converter) 7.The shifted light is transferred to the optical fiber 14 (second optical fiber) by the lens 13. ) is focused on the end face of the beam 7, propagates through the eyeglass 14, and is collimated by the lens 15.Furthermore, it is reflected by the total reflection mirror 2-16, and after passing through the bar 7 mirror 22, it is reflected by the half mirror 17 into the corner beam 4. 0 Combined light #i is combined so as to interfere with the reflected light from the condenser lens 18. Optical fiber 19 (third
optical fiber), passes through the lens 20 to the photodetector 9
The intensity of the combined light is detected.

On the other hand, a part of the light propagating through the optical fibers 11 and 14 is separated by the half mirror 5 No. 21 and 22, and the half mirror 2
It is synthesized in 3. The combined light passes through the condensing lens 24, the optical fiber 25 (fourth original fiber), and the lens 26.
The light is incident on the second photodetector 27.

The output line of the photodetector 9.27 is a signal with a frequency f as shown in equation (4).

For example, the output of the photodetector 9 is the distance from the total reflection mirror 17 to the object 3! 1, it can be expressed as follows.

As, +As, +2As, As, cos(2gft+
〉'7'-φS, →rρ)...(4) However, All,: Corner key, -amplitude of signal light reflected at -p 4, A@,: Reference reflected by total reflection mirror 16 Light amplitude, ψ111eψr1: phase amount determined by the distance between each optical component in the optical path of the signal light and reference light.

λ: Wavelength of the light emitted from the light source 1. The phase of the output signal of the photodetector 9 is determined by the distance 11 between the object 3 and the half mirror 17.
However, the phase of the output signal of the photodetector 27 is always constant. Therefore, by measuring the amount of phase change of the output signal of the detector 9 using the output signal of the photodetector 27 as a reference, the distance to the object 3 can be measured with high accuracy.

In this embodiment, four optical fibers are used, but if they are combined into one optical fiber cable 28, wiring will be easier. Mirror 6° half mirror = 1 A first optical coupling section consisting of an optical frequency modulator 7 and a lens 10.13, a photodetecting section consisting of a photodetector 9.27 and a lens 20.26, and a lens 12.15.1B , 24. The second optical coupling section consisting of the half mirrors 17, 21, 22, 23 and the mirror 16 can be installed separately, and the optical coupling section that does not use active elements can be downsized. The freedom of interferometer installation is dramatically improved.

Since the length and refractive index of optical fibers made of glass generally change depending on temperature and external pressure, the phase of reflected light changes even if the object 3 does not move.This ratio is 1. In other words, the temperature change in the refractive index of silica glass is 10-
5/"O, so if the length of the optical fiber is 100 m, a wavelength error of 1 μm will occur for 1 degree temperature change. Optical fiber 1 is installed in the optical path of the signal light and the optical path of the reference light.
In this embodiment in which 1.14 is arranged, it is possible to compensate for variations in the phase amount due to variations in the characteristics of each optical fiber.

By the way, in the first embodiment, the difference in the characteristics of the optical fibers 11 and 14 appears as a phase change in the output signal of the photodetector 9. However, since the photodetector 27 also combines the output lights of the optical fibers 11 and 14 and detects the interfered light, the photodetector 27
If the phase change of the photodetector 90 is measured using the phase of the output signal of the optical fiber 11 and 14 as a reference, the phase change due to the optical fiber passing through the optical fiber 11 and 14 will be completely compensated for. The difference in phase changes appearing in the output signals of the photodetectors 9.27 due to the arrangement of the fibers 19.25 is so small that it can be ignored compared to the difference in phase changes caused by passing through the optical fibers 11.14.

FIG. 3 is a diagram showing a second embodiment of the present invention.

In this embodiment, the first optical fiber 11. Two lights having an optical frequency difference f passing through the second optical fiber 14 are combined by a half mirror 17, which is a second optical coupling part, and received by the photodetector 9 via a third optical fiber 19. This photodetector 9
The effect of using the second optical fiber 11.14 is the same as in the first embodiment.In this embodiment, the optical If the difference in the characteristics of the fibers 11 and 14 can be ignored, the photodetector 27 is the optical frequency modulator 7.
This is used to compensate for fluctuations in the output signal phase of the photodetector 9 caused by the temperature characteristics of the photodetector 9. That is, the half mirrors 31, 32° 34 and the total reflection mirror 33 take out a part of the two optical signals with a frequency difference f0, combine them, interfere with each other, and the combined light is received by the photodetector 27. Since the output signal phase amount includes changes due to characteristic fluctuations of the optical frequency modulator 7, if the output signal phase change of the photodetector 9 is measured using the phase of the output signal of the photodetector 27 as a reference,
The phase change caused by passing through the optical frequency modulator 7 is completely compensated for. If the characteristics of the optical frequency modulator 7 are stable, the photodetector 27 is not necessary.

The first point mentioned above. Optical fiber 11.14 of the second embodiment
However, if a single mode optical fiber with good phase characteristics is used, the characteristics of the signal at the photodetector can be improved.

Note that the frequency difference f provided by the optical frequency modulator 7 must not be too large in order to cause interference between the signal light and the reference light in the second coupling section. For example, the light source 1 is a He-Ne laser (
When the frequency of the emitted light is approximately 400 terahertz (400 terahertz), f is 40 MHz.

Furthermore, although the light source 11d used in the first coupling section generates light with a wavelength of -Jl, the light source 1 has a predetermined frequency difference f. If the second light is generated, the optical frequency modulator 7 is not necessary.

In the above embodiment, the case where the distance to the object is measured by making it correspond to the output signal phase ii of the photodetector 9 has been explained. If you place one, you can measure the salinity, and if you put one that changes the phase of light depending on the external pressure, you can measure the pressure.

(Effects of the Invention) As detailed above, according to the present invention, there is provided a light generating section that sends out two types of light having a certain frequency difference, and an optical coupling that combines a reference light and a signal light so as to interfere with each other. and a light detection section for detecting the combined light, respectively. Since it is separated by the third optical fiber, the degree of freedom in installing the optical interferometer increases. Furthermore, changes in the characteristics of the light 7 and the eyeglass used have almost no effect on the measurement results, ensuring highly accurate measurement.

[Brief explanation of drawings]

Figure 1 is a diagram showing a conventional optical interferometer, Figure 2 is a diagram showing a first embodiment of the present invention, and Figure 3 is a block diagram showing a second embodiment of the present invention. It is a diagram.
・

Claims (2)

[Claims] (1) first, second, third, and fourth optical fibers, and a first light beam that guides first and second lights having a predetermined frequency difference f to the first and second optical fibers, respectively; and an optical coupling unit that combines the output from the first optical fiber after propagating a predetermined distance and returning the signal light and the output from the second optical fiber so as to interfere with each other, and connects the third optical fiber to the third optical fiber. a second optical coupling section that combines the output from the first optical fiber and the output from the second optical fiber so as to interfere with each other and guides the output to the fourth optical fiber; a first photodetector that detects interference light output from the fourth optical fiber; and a second detector that detects interference light output from the fourth optical fiber; An interferometer that measures physical quantities in a propagation path by measuring the phase of an electrical signal of frequency f output from a detector. (2) a first optical coupling with a first, second, and third optical fiber and guiding first and second lights having a predetermined frequency difference f to the first and second optical fibers, respectively; a third optical fiber that combines the signal light from the first optical fiber that has propagated a predetermined distance and returned and the output from the second optical fiber so as to interfere with each other, and guides the signal light to the third optical fiber; 2, and a photodetector for detecting the interference light output from the third optical fiber, and detects the propagation path by measuring the phase of the electrical signal of frequency f output from the photodetector. An interferometer that measures physical quantities.