JPH03257353A - Apparatus for measuring refractive index of air - Google Patents

Abstract

PURPOSE:To measure the refractive index of air with high accuracy by correcting the fluctuation of a length standard by providing a vacuum part and an air part both of which respectively have a pair of reflecting surfaces arranged thereto to a spacer constituted of a material uniform in the change of length in an optical axis direction. CONSTITUTION:The spacer 61 of a reference interval part 6 is formed from a material, for example, glass or ceramic, uniform in the change of length in an optical axis direction based on expansion/contraction due to temp., pressure or the like and the change with time of the material. When the output beam of a laser beam source 1 is split by a half mirror 2 and the split beam are further respectively split by interferometers 4, 5 to be incident to the interval part 6, they are respectively reflected by a pair of the reflecting surfaces 64, 65 of an air part 6 and a part of the reflecting surfaces 66, 67 of a vacuum part 6b to again return to the interferometers 4, 5 and detected as interference signals by beam detectors 7, 8 and these signals are converted to electric signals to be sent to an operator 9. The operator 9 converts the electric signals to phase signals to operate the absolute value of the refractive index of air. In this case, since the ratio of the length of the air part 6a and that of the vacuum part 6b is always equal, the refractive index of air can be measured with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a device for measuring the refractive index of air using interference, and particularly to improving measurement accuracy.

<Prior Art> FIG. 2(A) is a block diagram showing an example of the principle of an air refractive index measuring device using general interference.

In FIG. 2(A), the output of the laser light source 11 is split into two by the interferometer 12, and these two lights are transmitted to the reference interval 1.
3, and the reflective surfaces 13a and 13b of the reference interval part 13
The beam is reflected by the beam, returns to the interferometer 12, and enters the photodetector 14 as an interference beam. The interference signal is converted into an electrical signal by the photodetector 14 and input to the arithmetic unit 15 . Arithmetic unit 15
The refractive index of the air is calculated from the change in the interference signal due to the change in the optical path length of the light reflected from the reference interval section 13. At this time, the refractive index (n) of air changes, and the optical path length difference (Δn
If L>changes, the optical path length change output of the interference signal (M
) holds the following relationship.

Δ n L=M where L is the distance (length standard) between the reflective surfaces 13a and 13b of the standard spacing section 13. Therefore, the length criterion (L
) is obtained in advance, the refractive index change (Δn>) of the air can be obtained from the optical path length change output (M) of the interference Shingetsu.

However, in the configuration described in item 2, although the change in the refractive index of air can be determined, the absolute value cannot be determined. Therefore, Figure 2 (b)
As shown in Figure 2, by holding the reference spacing part 13 in a vacuum container 16, opening it from the vacuum to the atmosphere, and measuring the optical path length difference between the reflecting surfaces, the absolute refractive index of air can be calculated from the following formula. You can find the value.

n=1+-(Δm/L) (λ/2) where Δm=change in interference order when the atmosphere is changed from vacuum to atmosphere λ: wavelength of the laser light source in vacuum.

<Problem to be Solved by the Invention> However, in the air refractive index measuring apparatus which is not shown in the above-mentioned prior art, the length reference (L), which is the distance between the reflective surfaces 13a and 1 and 3b of the reference interval part 13, is Since the refractive index of the air fluctuates due to the temperature and pressure of the atmosphere, changes in the material of the reference interval over time, etc., there is an error when determining the refractive index of the air, which poses a problem in that the refractive index of the air cannot be measured with high accuracy.

The present invention has been made in view of the above-mentioned problems of the prior art, and is an air refractive index measuring device capable of correcting variations in the length reference of the reference interval part and measuring the absolute value of the refractive index of air with high precision. The purpose is to provide the following.

<Means for Solving the Problems> The configuration of the present invention for solving the above problems includes a laser light source, an interferometer into which the output light of the laser light source is input, and an optical axis in a plane perpendicular to the optical axis. It is equipped with a spacer made of a material whose length changes uniformly in the direction, and two sets of reflective surfaces facing the same direction are arranged on both end faces perpendicular to the optical axis of this spacer, and one A reference spacing part is arranged between the reflecting surfaces of the set in a vacuum part, and a part between the reflecting surfaces of the other set is arranged in an air part having communication holes through which surrounding air can circulate, and from this reference spacing part. two photodetectors that measure respective interference intensity signals in the vacuum section and the air section obtained by the interferometer as the optical path length of the reflected light changes and convert them into electrical signals; and the photodetector The apparatus is characterized in that it is provided with an arithmetic unit that converts the electrical signal output from the air into a phase signal and calculates the absolute value of the refractive index of the air in the air portion from the change in the phase signal.

Effect> According to the present invention, a vacuum part and an air part are provided in the same member,
The structure is such that the ratio of length changes is always equal,
It is possible to reduce the influence of fluctuations in the length standard of the standard interval portion due to temperature, pressure, aging of materials, etc.

<Example> Hereinafter, the present invention will be explained based on the drawings.

FIG. 1A is a block diagram showing an embodiment of an air refractive index measuring device according to the present invention.

In FIG. 1(a), 1 is a laser light source that generates laser light with a stable wavelength, 2 is a half mirror 13 that splits the output light of the laser light source 1 into two, and 4.5 is an interferometer. , a differential interferometer is used to improve measurement accuracy. Reference numeral 6 denotes a standard interval section consisting of an air section 6a and a vacuum section 6b, and this standard interval section 6 is subject to changes in length in the optical axis direction (i.e., expansion due to temperature, contraction due to pressure, aging of materials, etc.). ) consists of a spacer 61 made of a uniform material in a plane perpendicular to the optical axis, and windows 62 and 63 made of transparent glasses placed on both sides of this spacer 61. Here, as shown in FIG. 1 (b), the spacer 6
1, two holes 61a and 61b are made along the optical axis direction in a material whose length in the optical axis direction is uniform in a plane perpendicular to the optical axis, and one hole (for example, 61a) has a side surface. The apertures 6] and c are joined together. Also, both end faces 6
1d.

61e is perpendicular to the optical axis. Figure 1 (a)
Returning to , windows 62 are formed on both end surfaces 61d and 61e of this spacer.
．． 63 are combined to form an air section 6a and a vacuum section 6b. Also, spacers 6 are provided in the air section 6a and the vacuum section 6b.
1 and 2 pairs of reflective surfaces 64 and 65 facing in the same direction on the plane perpendicular to the optical axis (on the window 12.13) at both ends of 1. 66 and 67 have the same length reference and are parallel to each other. is supported by 68 is the opening 61c of the spacer 61, that is, the air portion 6
This is a communication hole provided in a for circulating the surrounding air. 7.8 is a photodetector that converts interference signals from the air section 6a and vacuum section 6b of the reference interval section 6 into electrical signals; 9 is a photodetector that converts the electrical signals obtained from the photodetector 7.8 into phase signals; This is a calculator that calculates the absolute value of the refractive index of air.

In such a configuration, the output light of the laser light source 1 is split into two by the half mirror 2, and one light is split into two by the half mirror 2.
It passes through and enters the interferometer 4.

The incident light is further split into two by the interferometer 4 and then sent to the reference interval part 6.
The signals are reflected by a pair of reflecting surfaces 64 and 65 of the air portion 6a, return to the interferometer 4, and enter the photodetector 7 as an interference signal. The other light is reflected by the Hough mirror 2 and mirror 3 and enters the interferometer 5. The incident light is further split into two by the interferometer 5, reflected by a pair of reflecting surfaces 66 and 67 of the vacuum section 6b of the reference spacing section 6, and returned to the interferometer 5 again as an interference signal to the photodetector 8. is incident on the The interference signals are each converted into electrical signals by photodetectors 7 and 8 and sent to a calculator 9. After converting the electrical signal into a phase signal, the computing unit 9 calculates the absolute value of the refractive index of air.6 The computing operation of the computing unit 9 will be described below.

Length reference La of air section 6a and length reference LV of vacuum section 6b
changes at the same rate, the relationship is expressed by the following equation.

Lv = k −La ---(1) However, k
: It is a constant of proportionality. Now, if the optical system is kept in vacuum,
Since the air portion 6a also becomes a vacuum, the refractive index of the reference interval portion 6 is 1.
becomes. At this time, if the phase signals obtained from the photodetector 7.8 are respectively θaO1θ■0, then θao=(2π/λO) La0・1 ・(2)
θvo=(2π/λO) LVo·1 −(3) where λ: wavelength of the laser light source 1 in vacuum 0: the optical system is in a vacuum. Therefore, the difference in phase signals is θaO−θv
o=(2yr/λO) (LaO−LvO)=(2g
LaO/λO)(1-k), and the proportionality constant is expressed by the following formula.

k=1-[((θaO-θvO)/2 yr L
ao)λ0l-(4) On the other hand, when the optical system is in the air, the phase signal obtained from the photodetector 7°8 is
1. If θ■1, θa1=(2π/λ1)La1−Na−(5)θ
v1 = (2g/λ1) LVl ・1 - (6)
However, Na: the refractive index of air is 1: the optical system is in the air. Therefore, the difference in phase signals is θa1−θ■
1 (2π/λ1) (La1- Na1-Lvl-
1)(2πLa1/λ1) (Na −k), and the refractive index Na of air is expressed by the following equation.

Na=((θa1−θvl)/2 x )(λ1/L
a1) + - (7) From equations (4) and (7), Na -1 = ((θa1-θv1) / 2 yr ) (λ1/L
a1)((θaO−θvo)/2yr)(λ0
/ L ao) ・(8) Here, λ0−λ1 (1
+α〉 La0=Lal (1+ε), equation (8) becomes Na-1 (1/2g) (λ1 /La1) [((θa1-θV1
)(θaO−θvo)((1+α)/(1+ε))J・
... (9) Also, I, VO = Lvl (1 + ε), and the above (3), (6) formula % formula %) [Therefore, from formula (9) and formula 00, Na −1 = (1/ 2π)(λ1/La1)((θa1-θv1)
(θaO−θvO) θv1/θVO) ...0
1). Therefore, the length reference La1 of the air portion 6a
, the wavelength λ1 of the laser light source 1 in vacuum, the phase signal measurement values θal, θ■1 in the air part 6a and the vacuum part 6b, the phase in the air part 6a and the vacuum part 6b with the optical system in a vacuum state By determining the signal measurement value θaO1θ■0, the refractive index Na of air can be calculated.

In addition, the length reference LaLv of the air portion 6a and the vacuum portion 6b is lengthened, and a reference interval portion for only the air portion having a short length reference of a fraction of the length reference is provided separately, and the interpolation is performed. By using this to estimate the phase signal measurement value θa in the air portion 6a, it is possible to measure the absolute value of the refractive index of the air with higher accuracy.

Furthermore, as the material for the spacer 61, Illusion II! By using glass ceramic with a small coefficient, it is possible to determine the refractive index of air using only the interference signal without performing temperature correction or the like.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, the phase of the vacuum part and the air part is adjusted using a reference interval part such that the length ratio of the vacuum part and the air part are always equal. It is possible to measure the absolute value of the refractive index of air with high accuracy because it can compensate for the influence of changes in the length standard of the standard interval, such as temperature, pressure, and aging of materials. A refractive index measuring device can be realized.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of an air refractive index measuring device according to the present invention, and FIG. 2 is a conventional example. J...Laser light source, 2...Half mirror 13...
Mira, 4, 5... Interferometer, 7, 8... Photodetector, 9
...Arithmetic unit, 6...Reference interval part, 6a...Air part, 6b...Vacuum part, 61...Spacer, 62.63
...Window, 64-67...Reflection surface, 68...Communication hole.

Claims (1)

[Claims] A laser light source, an interferometer into which the output light of the laser light source is input, and an interferometer made of a material whose length in the optical axis direction is uniform in a plane perpendicular to the optical axis. A spacer is provided, and two sets of reflective surfaces facing the same direction are arranged on both end faces perpendicular to the optical axis of the spacer, and the space between the reflective surfaces of one set is a vacuum area, and the reflective surface of the other set is Between the surfaces, there is a reference interval section each arranged in an air section having a communication hole through which surrounding air can circulate, and the interferometer obtains the information obtained by the interferometer as the optical path length of the light reflected from this reference interval section changes. two photodetectors that measure the interference intensity signals in the vacuum section and the air section and convert them into electrical signals, and convert the electrical signals output from these photodetectors into phase signals, 1. A refractive index measuring device for air, comprising: a computing unit that computes the absolute value of the refractive index of the air in the air portion from the change.