JPS61247901A - Detecting method for phase difference of interferometer - Google Patents

Abstract

PURPOSE:To decide whether the polarity is positive or negative even when the range of a phase difference is expanded greatly by obtaining an envelope signal on which an interference signal of interference light consisting of plural known wavelength components is superposed. CONSTITUTION:The intensity of output interference light when coherent light beams which have wavelength lambda and are mutually phi out of phase is expressed as 2(1+cosphii). When this interference light output is superposed as to i=1, 2 and spectrum component of lambda1 and lambda2 are detected at the same time, the sum IT is as shown by an equation 1 and the difference is as shown by an equation 2; when phi1 and phi2 vary uniformly, the IT and ID draw what is called 'beat waveform'. Then, envelope outputs IHC and IHS of the equations 1 and 2 are as shown by an expression 3. A two-wavelength system using the equation 1 is judged within a range 0<=¦B¦<=pi on condition that envelope informa tion is obtained at the same time. Namely, the direction of variation is judged from ¦B¦, so the direction of variation is judged over a wide range without reference to the values phi1 and phi2 when phi1-phi2 is set small.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for measuring physical quantities using an optical interferometer, and in particular, a method for measuring physical quantities such as length, displacement, temperature, and pressure using an optical interferometer. It is related to the measurement method.

[Conventional technology]

A physical quantity measuring device using an optical interferometer divides the light beam emitted from one coherent light source into two or more light beams, and when each light beam propagates in a medium, it splits into two light beams depending on the physical quantity to be measured. The idea is to measure physical quantities by interfering with and observing this interference.

Measurement technology using this interference phenomenon has features such as non-contact property and high sensitivity, and is used for measurement of minute displacements, distance measurements, photoelasticity measurements, etc. Furthermore, with the recent development of optical fibers and related technologies, optical fibers have come to be applied to this type of measurement technology. This increases the flexibility of the optical system and provides environmental resistance such as explosion resistance, and has been used in a variety of applications.

Next, we will discuss the principle of measurement using interference. The intensity I(φ) of the interference light of two coherent beams having the same intensity, wavelength, and polarization plane components and a phase difference of φ is I(φ) −2Io(
1+cosφ)-...-(1). However, 10
represents the intensity of one luminous flux.

If φ has a known functional relationship with the measured physical quantity S, then ■
It can be written as (φ)=I(S), and since ■ and S have a certain relationship, if you want to know ΔS, you can measure ΔI.

There are two methods for measuring Δ■: two beams of light are emitted onto a screen, spatially creating interference fringes, and the movement of the interference fringes is observed, and the other is focusing on only one point on the screen. Alternatively, there is a method in which two lights are concentrated at one point and changes in light intensity at this point are observed. The latter method has the characteristic that it does not require spatial expansion, and is applied to optical fibers and the like.

However, when putting this measurement technique into practical use, the following restrictions were imposed because the response of the interferometer output intensity ■ to the phase difference φ between the two interfering beams follows a cosine function.

Determination of whether the signal change of the a6 phase difference φ is positive or negative cannot be determined only by the interferometer output intensity ■.

The response to minute changes in the b1 phase difference φ signal is poor.

Therefore, it is actually used as follows.

i. The relationship between the measured quantity and the phase difference φ is uniquely determined, and when the measured quantity changes, the sign of the rate of change is known.

ii. It is used in a range where the range of change in φ is 0≦1φ1≦π or π≦Iφ1≦2π.

iii, π/ to any one of the light beams to be interfered with
Applying a phase bias of 2 (rad), cos (φ+
π/2)=sin(φ), that is, a sine function, is obtained, and the above problems a and b are solved and used from this and the above equation (1).

iv, when φ, which depends on the measured quantity, has wavelength dependence,
φ at one wavelength. A light wave that produces a phase difference of φ. +
Using a light wave of another wavelength that produces a phase difference of π/2, the former light wave has cosφ.

, in the latter light wave cos (φo+π/2) = 3 i
nφ. The two light waves are switched and used to obtain the two signals.

When φ has wavelength dependence like v and iv, the phase difference is φ. From φ. +2π, the wavelength is temporally swept with a sawtooth wave having an angular frequency ω, and the phase difference φ of a pseudo continuous signal cos (ωt+φ.) with the sweeping period ω.

A so-called pseudo-heterogeneous gain method is used to measure .

[Problem that the invention seeks to solve]

The conventional methods described above each have the following problems.

In method (a), i, the change in φ is obtained in the form of cosφ, so it cannot deal with the uncertainty of the change in φ, that is, when the sign of the rate of change in φ is unknown.

If the range of change of φ in (b) and ii is limited, the measurement range of the measured quantity is extremely narrowed.

In methods (c) and iii, a phase element of π/2 (r a d) is inserted into one of the light beams, but in cases where the optical paths of the two light beams are spatially the same and only the plane of polarization differs. That is, in the case of two light beams propagating inside a birefringent fiber, a complicated process such as separating the lights outside the fiber must be taken.

(When using the dl, iv, and v methods, a complicated signal processing device is required.

[Means for solving problems]

In order to solve such problems, the present invention emits light consisting of a plurality of known wavelength components, causes this light to interfere with each other using an interferometer, generates interference light, and converts the interference signal of this interference light into the above-mentioned interference signal. The interference signals are separated into known wavelength components, and signals of at least two wavelengths among these interference signals are superimposed to obtain an envelope signal of the superimposed signal.

[Effect]

In the present invention, it is possible to determine whether the polarity is positive or negative even if the range of the phase difference is greatly expanded.

〔Example〕

First, the principle according to the present invention will be explained. When coherent light beams with a wavelength λi and phases different from each other by φi are caused to interfere with one interferometer, the output interference light intensity is 2 (1+c
o3φi). For i=1.2, this interference light output is optically or electrically superimposed, and λ1. λ2
When the spectral components of
+cosA-cosB) ...(2) However, A-
(φ1+φ2)/2 B=(φ1-φ2)/2, and the difference ID is: ID=2 (cosφ1-cosφ2)=-4s 1n
A-s inB...13).

Assuming that φ1 and φ2 change uniformly, since φ1 and φ2 have wavelength dependence, IT and ID become so-called "beat waveforms." Therefore, if we take the envelope line of this beat waveform, we get equation (2).

Respective envelope outputs IHC and IH for equation (3)
3 becomes IHCycosB, IH3ys inB. (1)
In order to uniquely determine the direction of phase change using a conventional interferometer as in the equation, it can only be determined within the range of, for example, 0≦1φ1≦π. However, in the two-wavelength method using equation (2), if envelope information is also obtained at the same time, 0≦I
It can be determined within the range of BI≦π. In other words, when looking at the direction of change in phase when the same degree of phase difference is measured by the conventional interferometer and the interferometer according to the present invention, in the case of the present invention, IB
Since the direction of change can be determined by +, if φ1-φ2 is set small, φ1. The direction of change can be determined over a wide range regardless of the size of φ2.

Furthermore, if equations (2) and (3) are used simultaneously, the intensities for the B and A components have a relationship between sine and cosine, so
By monitoring the intensity shown by equations (2) and (3), φ1. The direction of change in φ2 can be seen.

Moreover, if the envelope intensities of equations (2) and (3) are simultaneously monitored, the detection range of phase changes can be doubled. That is, the direction of phase change can be determined within the range of 0≦B≦2π.

Next, FIG. 1 shows a system for explaining an embodiment of a phase difference detection method in an interferometer according to the present invention. In FIG. 1, ■ is a light source, 2 is a two-beam separation means, 3 is a transmission means, 4 is an optical combiner stage, 5 is an optical/electrical conversion means, and 6 is a control/converter.
It is a signal conversion means.

The light sources 1 have mutually different wavelengths λ1. Two laser light sources 11.12 that oscillate at λ2 and a combiner 13.2 that combines their light beams; the beam separation means 2 includes a half mirror 21, a mirror 22, and a lens 23°24; the transmission means 3 includes a sensing fiber 31 and a reference fiber. 32.2
It has a symmetrical structure with the light beam separating means 2, and the light combining stage 4 for combining two light beams is a half mirror 41, a mirror 42 and lenses 43, 44, and the optical/electrical converting means 5 is a lens 5.
1. Diffraction grating 52 as a wavelength separation element. Photodetector 53
．． 54 and low-pass filters 55, 56, the control/signal conversion means 6 is composed of a control means 6a and a signal conversion means 6b, the control means 6a is composed of integrating circuits 61, 62, the signal conversion means 6b is composed of analog arithmetic units 63, 64, . It consists of clip amplifiers 65 and 66. The intensity of the laser light sources 11 and 12 can be changed by external control signals m and n, respectively.

Next, the operation of the apparatus configured as described above will be explained. The electric field intensity Ea of the light a emitted from the light source 1 is expressed as in equation (4).

Ea=E1・exp (j・ω1・t)+E2・e
xp (j ・ω2 ・t) ・ ・ ・ ・(4)
However, El and E2 are the respective electric field strengths of the laser light sources 11 and 12, and ω1. ω2 is the oscillation angular frequency of the laser.

The electric field strength of the light beam c emitted from the two-beam separation means 2 is Eb=Ec=Ea/2...151.

The sensing fiber 31 receives a physical quantity as the measured quantity as an external stress, and at this time the sensing fiber 3
1 is the distorted optical effect and electro-optic effect.

Due to the magneto-optical effect, thermal expansion effect, etc., the refractive index changes, that is, the propagation constant changes, giving a phase change to the light propagating through it in accordance with the amount to be measured.

The electric field strength Ed of the light propagated through the sensing fiber 31,
The electric field intensity Ee of the light propagated through the reference fiber 32 is expressed by equation (6).

・ ・ ・ ・(6) However, φ1j=ni・2π・l/λj ni: Effective refractive index of the fiber l: Fiber length 2 The light beams d and e are combined by the optical law means 4 to become the output light f, and optical-to-electrical conversion Enter method 5. Next, the light beam is separated into two light beams of λl and λ2 components by a diffraction grating 52, and then converted into electric signals by photodetectors 53 and 54, respectively. The output value Vl of the λ1 component signal g, which is the output signal of the low-pass filter 55, and the output value 2 of the λ2 component signal, which is the output signal of the low-pass filter 56, are expressed by equation (7).

y1=1+cos (φ ■ −φ El)V2=
1+cos (φI2−φ2□) (7) After the signal g and the double signal are added by the analog addition calculator 63, they become a beat signal as shown in equation (8).

V63=2+cosΔB−cosΔA (8) where ΔA= (Δφ1+Δφ2)/2ΔB=(Δφ1−
Δφ2)/2 Δφ1=φth −φ21 Δφ2=φ, 2−φ2□ The beat signal output from the analog addition calculator 63 is
Only the amplitude of the cosine signal is extracted by the clip amplifiers 65 and 66, and is outputted from the analog addition calculator 64 as a signal l expressed as l cosΔB−CO3ΔA1 .

Integrating circuits 61 and 62 are for monitoring the light intensities of the λ1 component and the λ2 component, and feed back to the light source 1 so that the intensities of both components are always equal.

Next, the operation of the system shown in FIG. 1 will be described with reference to FIGS. 1 and 2 regarding measurement when the physical quantity is temperature change. When temperature is applied to a single mode fiber, the sensitivity of the phase difference of the two beams for each wavelength component to temperature changes is as follows: Δ(φ, −φ!l)/ΔT, Δ(φIz−φ2t)/Δ
It is represented by T and is approximately constant.

Figure 2 (a) is the waveform of temperature change applied as external stress to the interferometer, and Figure 2 (b) is the interference signal g due to the λ1 component.
FIG. 2(C) is the waveform of the interference signal due to the λ2 component, FIG. ) is the waveform of the converted signal l for taking the envelope, and FIG. 2(f) is the plot of the envelope and interpolation between them.

In this way, for any temperature or rate of temperature change in the temperature range between TA and TB in Figure 2(a), the polarity of the rate of change can be known from the slope of the envelope in Figure 2(f). I can do it. Furthermore, by observing the signals g and h, it is also possible to detect changes in φ shown in FIG. 2 (along) with the same accuracy as before.

In FIG. 1, optical/electrical conversion means 5. Although the interference waves of the two wavelengths are electrically superimposed by the control/signal conversion means 6, they may be superimposed optically as shown in FIG. In FIG. 3, 5a is a light-to-electricity conversion means;
7 is a photodetector, and 58 is a low-pass filter. In FIG. 3, the same or equivalent parts as in FIG. 1 are given the same reference numerals. Here, optical superposition of light waves is performed using transmitted light. The signal p output from the low-pass filter 58 is the same as the signal shown in FIG. 2(d).

FIG. 4 shows another embodiment, and shows an example of a polarization mode interference type sensor using a polarization constant fiber using a birefringence effect. In Figure 4, 7 is the first
The light source is the same as the light source 1 in the figure, 8 is an optical coupling means, 9 is a transmission means, and 10 is an analyzer. In FIG. 4, the same parts or corresponding bones as in FIG. 1 are given the same reference numerals.

In the case of FIG. 4, the beat signal is expressed by equation (8).

That is, VCX:2+CO5ΔB CO3ΔA ΔB=(Δφ1−Δφ2)/2 ΔA=(Δφ1+Δφ2)/2 Here, Δφ1 is the phase difference between the light waves along the two principal axes of the transmission means 9 with respect to λ1, that is, the orthogonal first The principal axis polarization plane component and the second principal axis polarization plane component are 4 on each axis.
This is the phase difference between the first principal axis component and the second principal axis component when they are caused to interfere as the same polarized light component after passing through an analyzer oriented at 5 degrees. Similarly, Δφ2 is the transmission means 90 for λ2.
It is the phase difference between light waves along two principal axes.

In the conventional measurement system, the temperature vs. phase sensitivity was 3 (rad/-m) when measured at λ=830 (nm). If we try to detect the polarity of the rate of change of the phase change φ using 1/4 period of cosφ, the detection range of φ becomes O≦1φ1≦π, and the polarity detection range of the temperature change rate is at most , T, , = πCr ad) / 3
(ra d/−m)=1.04 (k−m).

On the other hand, to describe an example of the case according to the present invention, Δφ1/ΔT = 3.0. Δφ2/ΔT=3.3
Here, λ1-1-780(n, λ2=830(n
m]. Therefore, 1 (Δφ1−Δφ2)/21<π/2ΔTkuπ/ (
3,3-3,0)=π10.3=10.4, and it can be seen that the temperature measurement range has been expanded about 10 times.

〔Effect of the invention〕

As explained above, the present invention emits light consisting of a plurality of known wavelength components, causes the light to interfere with each other using an interferometer to generate interference light, and generates an interference signal of the interference light for each of the known wavelength components. Conventionally, by superimposing signals for at least two wavelengths of these interference signals and obtaining an envelope signal of this superimposed signal, it is possible to Previously, the polarity could only be determined by measuring within 1/4 period, but now the range of phase difference can be greatly expanded, and polarity information can also be measured without requiring a large-scale configuration or reducing accuracy. It has the effect of being possible.

[Brief explanation of the drawing]

FIG. 1 is a system diagram for explaining one embodiment of the phase difference detection method in an interferometer according to the present invention, FIG. 2 is a signal waveform diagram in that system, and FIG. 3 is a system diagram showing another embodiment. 4 are configuration diagrams showing still another embodiment. 1... Light source, 2... 2 beam separation means, 3...
- Transmission means, 4... Optical combiner stage, 5... Optical/electric conversion means, 6... Control/signal conversion means, 6a...
...Control means, 6b...Signal conversion means, 11.12
... Laser light source, 13 ... Combiner, 21.41
...Half mirror-122,42...Mirror, 2
3,24.43,44.51...lens, 31...
...Sensing fiber, 32...Reference fiber, 52...Diffraction grating, 53.54...Photodetector, 55. 56...Low pass filter, 61
．． 62...Integrator circuit, 63°64...Analog addition calculator, 65.66...Clip amplifier.

Claims (1)

[Claims] In a phase difference detection method using an interferometer, light consisting of a plurality of known wavelength components is emitted, this light is interfered by an interferometer to generate interference light, and an interference signal of this interference light is converted into an interference signal of the above-mentioned known wavelengths. A method for detecting a phase difference in an interferometer, which comprises separating interference signals into components, superimposing signals of at least two wavelengths among these interference signals, and obtaining an envelope signal of the superimposed signal.