JPS63208731A - Measuring instrument for back raman scattered light of optical fiber - Google Patents

Abstract

PURPOSE:To improve measurement sensitivity and to decrease the number of components by employing an optical system which uses the same interference filters as a light splitting means and a wave-length selecting means. CONSTITUTION:Light from a light source such as a laser is made incident on a 1st interference filter 11 and an end surface of an optical fiber 4 is placed on the optical axis of light with center wavelength lambda0 which is transmitted through the filter. Back scattered light generated in the optical fiber 4 with the incident light is made incident on the 1st interference filter 11 in the opposite from the incident light and light with wavelength which is not nearby the Rayleigh scattered light with the same wavelength lambda0 with the incident light to the optical fiber 4 is reflected by the interference filter 11. A 2nd interference filter 12 is installed on the optical axis of this reflected light. Only anti-Stokes' light is transmitted through the 2nd interference filter 12 and is made incident on a photodetector 9. The photodetected light is converted into an electric signal. Its intensity is measured. Only Stocks' light is transmitted through a 3rd interference filter 13, and photodetected by a photodetector 7 and converted into an electric signal so that the intensity of the anti-Stokes' light is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an apparatus for inputting monochromatic light into an optical fiber and measuring its backward Raman scattered light.

[Prior Art] A phenomenon known as the Raman effect is that when monochromatic light is applied to a substance and is scattered, the scattered light is mixed with light whose wavelength has changed by an amount specific to that substance. This phenomenon occurs when part of the photon's energy is absorbed by the material, or when the material's energy is added to the photon. In the former case, the wavelength of the scattered light that has undergone the Umann effect is the same as that of the material. Wavelength λ of monochromatic light. (Stokes light), and in the latter case, the wavelength of the scattered light subjected to the Raman effect is λ. It becomes shorter by λ8 than (anti-Stokes light). λ, λ
8 is m that is specific to a substance, and in the case of a solid, phenomena such as changes in lattice imaging occur due to energy exchange with photons. Therefore, by knowing the scattered light subjected to the Raman effect, it is possible to know the type, physical properties, state, etc. of the substance.

For example, the Stokes light intensity I3 and the anti-Stokes light intensity I are respectively ■ 1 and oc1/( exp (fIω; /kT) −1)
1ac-c1/(1-exp(-fiω, /kT))
is a function of absolute temperature T, and the ratio of Stokes light intensity to anti-Stokes light intensity is also Ia/l3 − ((ω0+ω1)/(ω0−ω, ))48″xp(−
Wω, /kT) and absolute temperature T. however,
ω0 is the number of images of incident light. Therefore, by measuring either or both of the Stokes light intensity and the anti-Stokes light intensity, it is possible to measure the temperature of the substance that is generating these scattered lights.

Applying this principle, the temperature of the optical fiber can be measured by injecting light into the optical fiber and measuring the Raman scattered light (Stokes light, anti-Stokes light) generated within the optical fiber. The length of the optical fiber can be determined by using pulsed light and measuring the time required for the backscattered light (light in the direction opposite to the traveling direction of the incident light) of the Bomann scattered light generated within the optical fiber to be emitted. It has also been devised to measure the temperature distribution in the direction (Patent application 1986).
- No. 11359).

FIG. 2 shows the configuration of a conventional optical fiber input optical system and an output optical system for measuring backscattered light.

The input optical system passes the light emitted from the light source 1 through an optical filter 2 and filters it to a specific wavelength λ. The other light is removed, and after passing through the directional coupler 3, the light is made to enter the optical fiber 4. The output optical system passes the light emitted from the optical fiber 4 through the directional coupler 3, divides the light into two by a half mirror 5, and guides one side to a Stokes light intensity measurement system consisting of an optical filter 6 and a light receiver 7. , and the other is guided to an anti-Stokes light intensity measurement system consisting of an optical filter 8 and a light receiver 9.

[Problem that the invention seeks to solve] If such a measurement system is used, the incident light wavelength λ.

It is possible to measure Stokes light and anti-Stokes light generated in an optical fiber having different wavelengths by λ and -λ. However, the measurement efficiency of the above measurement system is low.

First, consider the intensity of light incident on the optical fiber 4 IN.

Wavelength λ emitted from light source 1. The optical intensity of the optical filter 2 is Io, the transmittance of the optical filter 2 is ρ2, the theoretical efficiency of the directional coupler 3 is ρ3(-0,5>, the excess loss of the directional coupler 3 is ρ)
31. If the spatial coupling efficiency ρ is 1 considering the spread of the light source 1 and the incident aperture of the optical fiber 4, then the optical fiber 4
The intensity of the incident light on the receiver 7 is IIN”ρ2×ρ3×ρ31Xρ,
The light intensity 'sa is the intensity of the measured light (Stokes light with a center wavelength λ.+λ) emitted from the optical fiber 4.
, the theoretical efficiency of the directional coupler 3 is ρ3, the excess loss of the directional coupler 3 is ρ,
The theoretical efficiency of half mirror 5 is ρ52a (= 0.5), and the excess loss of half mirror 5 is ρ5a.
, the transmission rate of the optical filter 6 is ρ6a1, and the spatial coupling efficiency due to the spatial spread of the optical fiber output light, the light receiving area of the light receiver 7, etc. is ρ, then 2a 'sa''''ρ3xρ32axρ5xρ5axρ6a×ρ
s2a ×l0UTa ”' (2:
J, and considering that the theoretical efficiencies of the directional coupler 3 and half mirror 5 are both 0.5, equation (1),
+21 formulas are respectively 1, N-0, 5xρ2Xρ31XρslxIO-(:1
)ISa=0.25Xρ32. ×ρ5. ×ρ6a×
ρs2a ×l0UTa...' ”
becomes. IIN and 'sa are not only the optical transmission efficiency of the optical filter 6.8 and the directional coupler 3, but also the spatial coupling efficiency between the light source 1 and the light receiver 7.9, as well as the directional coupler 3 and the half mirror 5. These theoretical efficiencies were very small, totaling 0.125.

When an optical switch is used in place of the directional coupler 3 in Fig. 2, the theoretical efficiency of the optical switch is 1, and the total theoretical efficiency is 0.5. Low efficiency is desirable, and when an optical switch is used, a high-speed electrical circuit is required to drive the optical switch, and the excess loss is greater than that of a directional coupler.

An object of the present invention is to provide a novel optical fiber back Raman scattering light measurement device that eliminates the drawbacks of the prior art described above, has high theoretical efficiency, and does not require a high-speed electric circuit for driving an optical switch. There is a particular thing.

[Means for Solving the Problems] The present invention allows light emitted from a light source to enter a first interference filter that selectively transmits or reflects light with a center wavelength λ0. or the reflected center wavelength λ. An end face of an optical fiber is arranged to receive the light of A second interference filter is provided that receives the transmitted light and selectively transmits or reflects the light with the center wavelength λ1, and the second interference filter transmits or reflects the light with the center wavelength λ1 on the transmission side or the reflection side of the second interference filter. A light receiver is provided to receive reflected light having a center wavelength λ1.

[Function] The light emitted from the light source passes through the first interference filter (
or reflection) and the center wavelength λ. It is sorted into monochromatic light and enters an optical fiber. The center wavelength λ of the backscattered light emitted from the optical fiber. Rayleigh scattered light is transmitted (or reflected) by the first interference filter, and light in other wavelength ranges is reflected (or transmitted) by the first interference filter and enters the second interference filter. Of the backscattered light incident on the second interference filter, light with a center wavelength λ1 is transmitted (or reflected) through the second interference filter and enters the light receiver. By setting the transmission (or reflection) center wavelength λ1 of the second interference filter to be in the Stokes light or anti-Stokes light wavelength range, the intensity of the Stokes light or anti-Stokes light is measured by the light receiver.

[Example] An example of the present invention will be described below with reference to FIG. As shown in the figure, in this example, the transmission center wavelength λ. the first of
interference filter 11 and a transmission center wavelength λ. - second of λ8
interference filter 12 and a transmission center wavelength λ. +λ, the third
The optical system is characterized by an optical system consisting of three interference filters, including the interference filter 13.

The center wavelength λ of light emitted from a light source 1 such as a laser is made incident on the first interference filter 11 and transmitted through the first interference filter 11. The end face of the optical fiber 4 is placed on the optical axis of the light, and the optical fiber 4 has this center wavelength λ. so that the light is incident. Backscattered light generated within the optical fiber 4 by this incident light is emitted from the end face of the optical fiber 4 in the opposite direction to the incident light, and enters the first interference filter 11 in the opposite direction to the light from the light a1.

Of the backscattered light incident on the first interference filter 11,
The wavelength λ is the same as that of the light incident on the optical fiber 4.

The Rayleigh scattered light of λ is transmitted through the first interference filter 11. Light with wavelengths other than those in the vicinity is filtered through the first interference filter 11.
reflected. A second interference filter 12 is installed on the optical axis of this reflected light, and the reflected light is made to enter the second interference filter 12. The center wavelength of the light transmitted through the second interference filter 12 is λ, which is the same as the center wavelength of the anti-Stokes light. -λ, of the backscattered light generated within the optical fiber 4, only the anti-Stokes light is transmitted to the second interference filter 1.
2 and enters a light receiver 9 placed on the optical axis of the transmitted light. The anti-Stokes light received by the light receiver 9 is converted into an electrical signal, and its intensity is measured.

The light reflected without passing through the second interference filter 12 enters the third interference filter 13 placed on the optical axis of the reflected light. Since the center wavelength of the transmitted light of the third interference filter 13 is selected to be λ0+λ, which is the same as the center wavelength of the Stokes light, only the Stokes light of the backscattered light generated within the optical fiber 4 passes through the third interference filter. 13, it is converted into an electrical signal by a light receiver 7 placed on the optical axis of the transmitted light, and the anti-Stokes light intensity is measured.

Note that in the above embodiment, the spatial coupling efficiency determined by the spread of the light source, the dimensions of the entrance aperture of the optical fiber, etc. is not very good, but in order to improve this, it is possible to use a lens. For example, a lens that transforms the light emitted from the light source 1 into parallel rays is inserted between the light source 1 and the first interference filter 11, and the parallel rays are focused between the optical fiber 4 and the first interference filter 11. In addition, a lens for condensing light may be placed in front of the light receivers 7 and 9.

Further, in the above embodiment, the interference filter 11°12.1
By appropriately changing the center wavelength of the transmitted light of 3, the light source 1 and the light receiver 9
Even if the setting positions of the light ff11 and the light receiver 7 are exchanged, the same measurement as described above can be performed.

Furthermore, by changing the center wavelength of the transmitted light of the interference filters 12 and 13, it is also possible to measure the anti-Stokes light with the light receiver 7 and the Stokes light with the light receiver. Furthermore, the interference filters 11, 12, and 13 in the above embodiments were transmission type interference filters that selectively transmitted only light in a specific wavelength range, but reflective type interference filters that selectively reflected only light in a specific wavelength range were used. The optical system may be configured using an interference filter.

[Effects of the Invention] To summarize, according to the present invention, the following effects can be achieved.

(1) In the present invention, by using an optical system that uses interference filters instead of conventionally used directional couplers and half mirrors, it is possible to increase the theoretical efficiency to 1, which could not be achieved with conventional methods. It is possible to improve measurement sensitivity.

(2) Since the same interference filter is used as the optical separation means and the wavelength selection means, the number of component parts can be reduced and the number of optical axis adjustments required between each part can be reduced, reducing costs and reducing costs. It can be made more compact.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an apparatus for measuring backward Raman scattered light of an optical fiber according to the present invention, and FIG. 2 is a block diagram showing a conventional backward Raman scattered light measuring apparatus. In the figure, 1 is a light source, 2, 6, 8 is an optical filter, 3 is a directional coupler, 4 is an optical fiber, 5 is a half mirror 17, 9
11 is a first interference filter, 12 is a second interference filter, and 13 is a third interference filter.

Claims (2)

[Claims] (1) The light emitted from the light source is made incident on the first interference filter that selectively transmits or reflects the light with the center wavelength λ_0, and the light with the center wavelength λ_0 that is transmitted or reflected from the first interference filter is received. The backscattered light from the optical fiber that is emitted from this end face is made to enter the first interference filter in the opposite direction, and the light reflected or transmitted by the first interference filter is A second interference filter is provided that receives the light and selectively transmits or reflects the light with the center wavelength λ_1. 1. An optical fiber back Raman scattering light measuring device, characterized in that it is provided with a light receiver that receives light of wavelength λ_1. (2) A third interference filter that receives the light reflected or transmitted by the second interference filter on the reflection side or the transmission side of the second interference filter and selectively transmits or reflects the light with the center wavelength λ_2. Back Raman scattering of an optical fiber according to claim 1, further comprising a light receiver for receiving the light having the center wavelength λ_2 transmitted or reflected by the third interference filter. Light measurement device.