JPH0484739A - Refractive-index measuring apparatus - Google Patents

Abstract

PURPOSE:To provide an apparatus for obtaining refractive index readily and highly accurately by using a variable frequency light source, and measuring a vacuum part and an atmospheric part at the same time. CONSTITUTION:The output light from a variable frequency light source 1 is split into two light beams through a beam splitter 2. One light beam is transmitted through the splitter 2, cast into an interferometer 41 and split into two light beams. The light beams are reflected from reflecting surfaces 51a and 51b of an atmospheric part 51 in a reference space part 5, respectively, returned into the interferometer 41 and cast into detector 61. The other light beam is reflected from the beam splitter 2 and a mirror 3, cast into an interferometer 41 and split into two light beams. The light beams are reflected from reflecting surfaces 52a and 52b of a vacuum part 52 in the reference space part 5, respectively, returned into the interferometer 42 and cast into a detector 62. The outputs of the interference signals obtained from the interferometers 41 and 42 are measured in the detectors 61 and 62. The phases are measured with phase measuring devices 71 and 72. The absolute value of the refractive index is obtained in an operator 8 based on the change in phase. Thus, the highly accurate apparatus can be readily obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an apparatus for measuring a refractive index using interference, and particularly to a refractive index measuring apparatus using a variable frequency light source as a light source.

<Prior art> As a prior art of a refractive index measuring device using a variable frequency light source as a light source, Japanese Patent Application No. 0f-1 filed by the applicant of the present application
No. 28135 "Refractive Index Measuring Apparatus" The present invention is an improvement of the "Refractive Index Measuring Apparatus" of this previously filed application.Below, before explaining the present invention, regarding this previously filed application,
An outline will be explained using FIG.

In FIG. 3, reference numeral 11 denotes a variable frequency light source that selectively generates a plurality of coherent lights with different frequencies, and is composed of, for example, a light source 11a with a constant frequency and a frequency shifter 11b that shifts the frequency by an arbitrary amount. The output of the 0-frequency variable light source 11, which sequentially generates light having frequencies of , is branched into two by an interferometer 12. These 2
The two lights are incident on the reference spacing section 13, are reflected by the reflecting surfaces 13a and 13b of the reference spacing section 13, return to the interferometer 12 again, are combined, and enter the detector 14. The output of this detector 14 is input to a calculator 15. Arithmetic unit 1
5, the absolute value of the refractive index can be determined from the change in interference order detected by the detector 14 when the frequency of the variable frequency light source 11 is changed.

Here, the interference order change m measured by the detector 14 when changing the optical frequency of the variable frequency light source 11 is m=2nL (1/λIv-1/λ2v), and from this formula, n=m/2L・ (1/ (1/λ1v-1/λ2v
))==AV-m/2L-(m/2L)(Cv/△f)-・・
■However, n: refractive index λ1■, λ2v: wavelength A before and after change in vacuum■: λ1v
and λ2v combined wavelength Cv.2 Speed of light in vacuum △f: Frequency variable width.

Therefore, by setting the spacing between the reflective surfaces 13a, ]3b of the reference spacing section 13 and the frequency variable width Δf in advance, and measuring the interference order change m, the absolute value of the refractive index can be determined from the above formula 0. I can do it.

<Problems to be Solved by the Invention> However, in the refractive index measuring device shown in the conventional search technique described above, in order to obtain the absolute value of the refractive index with higher accuracy, it is necessary to adjust the interval between the reflective surfaces of the reference interval part and the frequency variable width △. High precision is required for all measurements of f and interference order change fMm, which has hindered high precision of the absolute value of the refractive index.

The present invention has focused on this point and aims to provide a refractive index measuring device using a variable frequency light source that can easily determine the absolute value of the refractive index with higher precision.

Means for Solving the Problems> The configuration of the present invention for solving the above problems includes a variable frequency light source, an optical component that splits the output light of the light source into two, and a light beam split into two by the optical component. It is composed of two interferometers into which two beams of light are respectively incident, a vacuum part whose reflecting surfaces are arranged parallel to each other with predetermined equal length standards, and a vacuum part sealed in a vacuum state and an atmospheric part which is opened to the atmosphere. Due to the optical path length difference between the reference interval part formed so that the incident light from the two interferometers is emitted after being reflected on the reflective surfaces of the vacuum part and the atmospheric part, and the light reflected from this reference interval part, a plurality of detectors that measure the output of the interference signal obtained by the interferometer; a plurality of phase measuring devices that measure the phase of the output interference signal; and when the frequency of the frequency variable light source is continuously swept. The present invention is characterized in that it is provided with a computing unit that computes the absolute value of the refractive index from a change in the phase of the interference signal.

<Function> According to the present invention, since the vacuum area and the atmospheric area are measured simultaneously, and the refractive index (n-1) is measured, high accuracy is achieved in the interval between the reflective surfaces of the reference interval part, the frequency variable width, etc. The absolute value of the refractive index can be determined with higher precision even if the refractive index is not required.

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

FIG. 1 is a block diagram showing an embodiment of the refractive index measuring device of the present invention. In the first diagram, 1 is a frequency variable light source, 2
is a beam splitter that splits the output light of the frequency variable light source 1 into two; 3 is a mirror; 41° and 42 are interferometers; 5 is a reference interval formed from the same spacer;
1a and 51b, and 52a and 52b are arranged in parallel on the basis of equal length, and one forms an atmospheric part 51 that is open to the atmosphere, and the other forms a vacuum part 52 that is sealed in a vacuum state.

61.62 is a detector that measures the output of the interference signal obtained from the interferometer 41.42 as the optical path length of the light reflected from the reference interval part 5 changes, and 71.72 is the detector 61.6.
2 is a phase measuring device that measures the phase of the interference signal outputted, and 8 is a computing unit that calculates the absolute value of the refractive index from the change in the phase of the interference signal outputted from the phase measuring devices 71 and 72.

In such a configuration, the output light of the frequency variable light source 1 is split into two by the beam splitter 2.

One of the lights passes through the beam splitter 2 and passes through the interferometer 41.
is incident on the The incident light is further split into two by the interferometer 41, and reflected by the reflective surface 51a of the atmospheric part 51 of the reference interval part 5.
, 51b and return to the interferometer 41 again,
The light is incident on the detector 61.

The other light is reflected by beam splitter 2 and mirror 3,
The light is incident on the interferometer 42. The incident light passes through an interferometer 42
The light is further branched into two parts, reflected by the reflecting surfaces 52 a and 52 b of the vacuum part 52 of the reference interval part 5 , returns to the interferometer 42 again, and enters the detector 62 . Detector 61
．． At 62, the output of the interference signal obtained from the interferometers 41 and 42 is measured, and after the phase of the interference signal is measured at the phase measuring device 71 and 72, the refractive index is calculated from the phase change of the interference signal at the arithmetic unit 8. Absolute value is required.

Here, as shown in FIG. 2(a), the frequency variable light source 1
The outputs of the interference signals obtained from the detectors 61 and 62 when the frequency of is continuously changed (in the figure, ν → ν1) are shown in Figure 2 (B) and (C), respectively. . The output of this interference signal is θvl=<2 v o / c
) L ・=(1)θv2= (2v + /
c) L...(2) θa1= (2vo
/ c ) n L ... (3) θa2=(2ν
+/c) nL -C4) where C is the speed of light in vacuum.

Here, if S1-S0=Δν, the above (1) to (4
) formula is (2/c) ΔJ/L−θv2−θvt =(
5) (2/c) ΔvnL=θa2-θa 1
-(6) From these equations (5) and (6), n=
(θa2-θal)/(θv2-θv1)...(7)
It is expressed as Refractive index (n) obtained from this equation (7)
The accuracy of is determined by the accuracy of the output θ of the interference signal, and is generally not highly accurate.

Next, from the above equations (2)- and (4), 2v1 /c ・ (n 1 )L=θa2-θ
v2=M+α...(8) However, M: is a positive integer α; it is a decimal number. In this equation (8), the decimal number α is the detector 61, 62
The output θa2. It can be found from θv2, but M cannot be found.However, from the above equation (7), an approximate value of the refractive index n can be found, and from the above equation (8), M is a positive integer. Therefore, M can be determined without error.

Therefore, the refractive index n can be determined with high precision from n-1=(c/2ν+L)(M+cz)-(9), which is a modification of the equation (8).

-Dan 1422 and 1- Hereinafter, a detailed explanation will be given using a specific example.

Frequency variable width Δν = 3TH2, length reference L = 100am
, when the phase interpolation accuracy is 11500, the above (5), (
From formula 6), (θv2-θv1), (θa2-θa1)
The approximate value of is approximately 2X10''. Also, the above equation (7) is: ldn/nl; 1-0
0, and there is no error in the integer part, so d(θa2
-θal)l=ld(θv2-θvl)=11500,', ldn/nl=2xlO'Therefore, the accuracy of the refractive index n found from the above equation (7) is 2X1
It is about 0'.

Next, I/l = 3.8x 10”H2 (A+ = 78
0nll)n-1+2.7X10-, the above (8
) From the formula, M + α'', 68 Also, the formula [8] is: d (M + α) / (M + α) = ld shi 1 / shi 1 d (n-1) / (n-1) + ldL / Ll ...It becomes OD.This 0
In the equation 0, (dsi1/shi1)ku104 can be easily obtained. l dn/n 1=2x1. o', d (n-1)/(n-1) = 2xlO'/2.7xlO' = 7.4xlO°2. Furthermore, if the measurement accuracy of L is 10 μm, dL/L
=10',°, ld(M+α)l=0.51<1 Therefore, the integer part M can be determined without error.

From the above formula (■), I d (n-1)/ (n-1> = ldshi1/shi, l+ldL/L 10'ld(M+α)/JM10') = 10'+10'10(11500)/68 =1.4X1
Since 0' n-1 = 2.7X104, dn/n l =3.8X10', which can be determined with higher precision than the absolute value of the refractive index determined from equation (7) above.

In the above embodiment, by using a double-pass interferometer as the interferometer, it is possible to improve the accuracy of matching the length standards of the atmospheric part and the vacuum part of the reference interval part, so that the absolute refractive index can be determined with higher precision. You can find the value.

In addition, by once setting the atmospheric part of the standard interval part in a vacuum state and calibrating the ratio of the two standard intervals, it is possible to compensate for the discrepancy between the length standards of the atmospheric part and the vacuum part of the standard interval part. The absolute value of the refractive index can be determined with high precision.

Further, as a phase detection method, a heterodyne method, a polarization interference method, or the like may be used.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, the vacuum region and the atmospheric region are simultaneously measured using a variable frequency light source, and the refractive index (n- 1), it is possible to easily obtain the absolute value of the refractive index with higher precision without requiring high accuracy in the interval (length standard) of the reflective surfaces in the reference interval section or the frequency variable width. It is possible to realize a refractive index measuring device using a variable frequency light source.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram showing an embodiment of the refractive index measuring device of the present invention, Fig. 2 is a diagram showing the detector output when the variable frequency light source of the device shown in Fig. 1 is changed, and Fig. 3 is a diagram showing the main unit. FIG. 1 is a configuration diagram showing a prior example of a refractive index measuring device of the invention. 1... Frequency variable light source, 2... Beam splitter,
3...Mirror 5...Reference interval part, 8...Calculator, 41°42...Interferometer, 51...Atmospheric part, 52...
...Vacuum section, 51a, 51b, 52a, 52b-Reflection surface of reference interval section, 61.62...Detector, 71.72.
...Phase measuring device. Figure 1 Figure 2 (-gn

Claims (1)

[Claims] A variable frequency light source, an optical component that splits the output light of the light source into two, two interferometers into which the two lights split by the optical component are respectively incident, and a reflecting surface. It consists of a vacuum section that is arranged in parallel with predetermined equal length standards and is sealed in a vacuum state and an atmospheric section that is open to the atmosphere, and the incident light from the two interferometers is reflected from the vacuum section and the atmospheric section. a reference interval portion formed to be emitted after being reflected by a surface; and a plurality of detection units for measuring the output of an interference signal obtained by the interferometer according to the optical path length difference of the light reflected from the reference interval portion. a plurality of phase measuring devices for measuring the phase of the output interference signal; and calculating the absolute value of the refractive index from the change in the phase of the interference signal when the frequency of the variable frequency light source is continuously swept. 1. A refractive index measuring device characterized by having a configuration including a computing unit.