JPH0338555B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical propagation time difference measurement method using interference.

Conventionally, a Rayleigh interference refractometer as shown in FIG. 1 has been used to measure the optical propagation time difference. In the figure, light incident from a slit 11 is first made parallel by a collimator lens 12. Next, the light passes through the multiple slits 13 and 14 and is combined again by the lens 19, and interference fringes are formed on the screen 20.
made on top. On the optical paths 1a and 2a, there are placed tubes 15 and 16 whose end surfaces are parallel to each other and which are capable of filling gas, and the reference light passes through one of them.

When measuring the optical propagation time difference, first,
Both tubes 15 and 16 are evacuated and the inclinations of the parallel plane plates are changed to adjust the Ziyaman compensation plates 17 and 18, which change the optical path length, to optimize the brightness and darkness of the interference fringes caused by both. Next, fill one tube with the gas you want to measure. The movement of the interference fringes caused by this is compensated by the compensating plate 17 or 18, and the difference in light propagation time is measured by the zero method.

This method uses n ₁ −1 as the refractive index of gas (n ₁ ).
It is suitable for cases where the refractive index is quite small, around 10 ^-4 , but when n ₂ -1 is small, such as the refractive index (n ₂ ) of a solid,
When the interference fringes are as large as 0.5, the number of moving interference fringes becomes several hundred, making it impossible to accurately count the number of moving fringes and making it difficult to accurately measure the difference in light propagation time.

An object of the present invention is to accurately measure the difference in light propagation time of a substance having anisotropy, and to accurately measure the refractive index from the difference in light propagation time.

The present invention uses coherent light to determine the peak visibility by measuring fluctuations in the optical power input to the photoreceiver, and calculates the amount of movement of the variable optical delay path, that is, the optical path length difference, by measuring the fluctuation of the optical power input to the photoreceiver. It is unique in that it measures the difference in light propagation time.

The measurement of the optical propagation time difference and the measurement of the refractive index will be explained in detail below using the drawings.

There is a word called coherence that describes the property of light. This expresses the degree of coherence, and includes temporal coherence and spatial coherence. Temporal coherence indicates how long light oscillates at the same frequency with no phase fluctuation.This time multiplied by the speed of light c is commonly called the coherence length and is the length of time. It represents the degree of coherence.

As shown in FIG. 2, the light emitted from the light source 21 is split into two by a beam splitter 22, one of which is reflected by a fixed mirror 23, and the other by a mirror 24 placed on a variable optical delay path 25. Return. If these two beams are adjusted so that they overlap at the receiver 26, if the optical path lengths of the two beams are within the coherence length, the interference between the two beams will cause the level to fluctuate due to subtle movements of the variable optical delay path 25. cause The maximum value Imax and minimum value Imin of this level fluctuation
If the two optical powers split into two by the beam splitter 22 are I ₁ and I ₂ respectively, then Imax=I ₁ +I ₂ +2γ ₁₂ √ ₁・₂ Imin=I ₁ +I ₂ −2γ ₁₂ √ ₁・₂ Become. Further, the visibility (visibility) V is V=Imax−Imin/Imax+Imin, and when I ₁ =I ₂ , V=γ ₁₂ . Here, γ ₁₂ is called the degree of coherence, which indicates the coherence of the light source, and is a coefficient related to the type of light source and the difference in optical path length between two light beams.

FIG. 3 shows the relationship between visibility and optical path length difference using coherence length as a parameter, and the coherence length becomes shorter from curve a to curves b and c. If a light source with a short coherence length is used, the range of the variable optical delay path exhibiting interference becomes narrower, as shown in FIG. In other words, if a light source with a short coherence length is used, the point A at which the visibility curve shown in FIG. This means that the light passing through the optical path 2b can be made to coincide with the light passing through the light receiving surface 26.

In this way, the lengths of the optical paths 4a and 4b in FIG. If the refractive index of the substance 46 to be measured is n and the length is L, then
This means that it has grown equivalently by 2L (n-1). Note that the interferometer is formed by an optical path from separating light from a light source to recombining it. Therefore, the position of maximum visibility obtained by the light passing through the optical paths 4a and 4b is shifted from the initial position.

If the visibility is determined by moving the variable optical delay path 44, the result will be as shown in FIG. From FIG. 5, the optical path length l ₁ of the optical path 4b that provides the maximum visibility before the substance to be measured 46 enters the optical path 4b, and the optical path length I of the optical path 4b that provides the maximum visibility after the substance to be measured 46 enters the optical path 4b. ₂ (l ₁ − l ₂ ) is calculated. This optical path length difference (l ₁
-l ₂ ) divided by the speed of light C, the light propagation time difference (l ₁ -
l ₂ )/C is found. Furthermore, since the optical path length difference (l ₁ −l ₂ ) is equal to the equivalent elongation of the optical path caused by introducing the substance to be measured 46, the refractive index n of the substance to be measured 46 is n=1+l ₁ −l ₂ /2L, and the refractive index of the substance to be measured 46 can be determined. l ₁ and l ₂ are /μ
The length of the material to be measured can be measured with an accuracy of 10 m.
In cm, the refractive index can be measured with an accuracy of 10 ^-5 . In this example, the optical path length l ₁ was determined by selecting a vacuum as the reference material that serves as a standard, but the reference material that serves as a standard is not limited to vacuum.

FIG. 6 is a schematic diagram of an embodiment of the present invention. This embodiment is characterized in that polarizers 63 and 68 are inserted into the optical path constituting the interferometer, and an analyzer 67 is inserted into the output of the interferometer, as shown in FIG. It is possible to measure the optical propagation time difference and refractive index depending on the polarization direction of the substance to be measured 64, which is an anisotropic substance. In the figure, 61 is a light source, 62 and 66 are beam splitters, 63 and 68 are polarizers, 65 and 69 are mirrors, and 6 are
7 is an analyzer, 610 is a variable optical delay path, 611 is a light receiver, and 612 is a prism.

Another embodiment of the invention is shown in FIG. Measurement of the difference in propagation time and the difference in propagation velocity of light depending on the direction of polarization of an optical guideline such as an optical fiber that exhibits anisotropy depending on the direction of polarization can be performed as shown in FIG. That is, first, polarizers 71 and 73 are placed in the optical path of the interferometer, and their planes of polarization are made orthogonal to each other. Next, the output is transferred to the optical fiber 79.
The λ/2 plate 77 is adjusted so as to match the polarization axis of the light beam, and the light beam enters the optical fiber 79. Then the optical fiber 7
An analyzer 711 is placed on the output side of 9, and 2 orthogonal
The two components are adjusted so that they enter the light receiver 712 with the same polarization.

After adjusting in this manner, the variable optical delay path 76 is operated to determine the optical path length l ₁ of the optical path 7b that provides the highest visibility. Next, a portion of the optical fiber is cut to shorten the length of the optical fiber, and the variable delay path 76 is operated in the same manner as described above to provide the optical path 7 for maximum visibility.
Find the optical path length l ₂ of b. Once the optical path lengths l ₁ and l ₂ are found,
As explained in the first embodiment, the optical path length difference (l ₁ −l ₂ )
By dividing C by the speed of light, the difference in propagation time of light depending on the polarization direction of a long optical fiber and a short optical fiber can be obtained. Next, by dividing the length of the cut optical fiber by this light propagation time difference, the light propagation speed difference depending on the polarization direction of the optical fiber can be determined.

In addition, in FIG. 7, 72 and 74 are mirrors,
75 is a beam splitter, 76 is a variable optical delay path,
78 and 710 are lenses, and 713 is a light source. Further, 714 is a variable optical delay path, and both 76 and 714 may be provided as the variable optical delay path, or only one of them may be provided.

FIG. 8 shows yet another embodiment of the invention. In the figure, 81 is a light source, 82 and 86 are beam splitters, 83 and 88 are polarizers, 84 is a measured substance, 85 and 89 are mirrors, 87 is a reference substance, and 8
10 is a prism, 811 is an analyzer, 812 is a light receiver, and 813 is a variable optical delay path. As shown in the figure, in this embodiment, a reference substance 87 and a substance to be measured 84 are respectively placed in two optical paths of an interferometer, and the substance to be measured 84 is heated, cooled, stretched or compressed, By applying electricity, magnetism, etc. to the substance 84, it is possible to measure differences in light propagation time, elongation, and changes in refractive index due to temperature changes, tension, compression, electrical influence, magnetic influence, etc. of the substance 84 to be measured. can.

In each of the above embodiments, it is preferable to use a light source with a short coherence length to improve measurement accuracy, but if high measurement accuracy is not required, a light source with a long coherence length may be used.

As described above, according to the present invention, it is possible to measure minute differences in light propagation time and minute differences in light propagation speed in a substance to be measured that has anisotropy with high temporal resolution. Furthermore, the refractive index of the substance to be measured can be measured with high precision.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of Rayleigh's interference refractometer, Figure 2
The figure is a principle diagram of the optical propagation time difference measurement device used in the present invention, Figure 3 is a diagram showing the relationship between visibility and optical path length difference when coherence length is used as a parameter, and Figures 4 and 6 to 8 are FIG. 5, a schematic diagram of an apparatus used in an embodiment of the present invention, is a diagram showing the relationship between visibility and optical path length. 21, 41, 61, 713, 81... light source, 2
2, 43, 62, 75, 82...beam splitter, 25, 44, 610, 72, 76, 813...
...Variable optical delay path, 46, 64, 84...Measurement substance, 79...Optical fiber, 87...Reference material,
26, 47, 611, 712, 812... Light receiver, 2a, 2b, 4a, 4b, 7a, 7b... Optical path.

Claims (1)

[Claims] 1. A polarizer that divides output light with a short coherence length emitted from a light source into two optical paths, arranges a variable optical delay path in at least one of the optical paths, and controls the plane of polarization in each optical path. It has an optical system consisting of an interferometer which combines the light and guides it to an analyzer, and a light receiver to which the light that has passed through the analyzer is guided, and from the polarizer of the optical system. A substance to be measured that has anisotropy depending on the polarization direction is inserted into one of the optical paths up to the analyzer, and the plane of polarization of the light is aligned with the polarization axis of the substance to be measured using the polarizer. The visibility of the optical system including the substance to be measured is determined from the amount of variation in the electrical signal output from the light receiver with respect to the movement of the variable optical delay path, and the visibility of the optical system including the substance to be measured is determined when the substance to be measured is inserted and when the substance to be measured is not inserted. An optical propagation time difference measuring method, characterized in that the optical path length difference that provides the highest visibility between the two is read from the amount of movement of the variable optical delay path, and the absolute value of the optical propagation time difference is determined from the optical path length difference. 2. Divide the output light with a short coherence length emitted from the light source into two optical paths, arrange a variable optical delay path in at least one of the optical paths, and make the polarization planes perpendicular to each other by polarizers arranged in each optical path,
The light is guided to a λ/2 plate that combines the light again and adjusts the plane of polarization, and then leads to an analyzer that makes the output light the same plane of polarization, and the light that passes through the analyzer is guided. an optical system consisting of a light receiver, and in the optical path between the λ/2 plate and the analyzer,
Insert a substance to be measured that has anisotropy depending on the polarization direction, operate the variable optical delay path to find the optical path length that gives the highest visibility, and then insert the substance to be measured that has anisotropy depending on the polarization direction. /
Insert into the optical path between the two plates and the analyzer, operate the variable optical delay path to determine the optical path length that provides the highest visibility, and calculate the absolute value of the optical propagation time difference from the optical path length difference. A light propagation time difference measurement method characterized by: