JP2005156207A - Method and instrument for precisely measuring group refractive index of optical material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a precise measuring method of the group refractive index of an optical material, capable of easily and accurately measuring the group refractive index, without requiring the thickness data of a sample to be measured and capable of simplifying and miniaturizing a measuring instrument, and a precise measuring instrument therefor. <P>SOLUTION: The incident light from a low-coherence light source is separated into a first optical path 5 and a second optical path 6 by a beam splitter 2, and the reflected light from the first mirror 11 of the first optical path 5 and the reflected light from the second mirror 9 of the second optical path 6 are guided to a retroreflector 3 from the beam splitter 2; while the optical axes of both reflected lights are made to coincide with each other. The reflected light from the retroreflector 3 is separated into a third optical path 8 and a fourth optical path 7 by the beam splitter 2, and the optical axis of the reflected light from the third mirror 10 of the third optical path 8 is allowed to coincide with that of the reflected light from the second mirror 9 of the fourth optical path 7 to detect the reflected light from the beam splitter 2 by a photodetector 4. An interferometer, constituted so that the same functions as those of an interferometer consisting of the series connection of two interferometers are performed by one interferometer, is used. The sample 13 to be measured is arranged to the first optical path 5 and compensation plates 14 and 15 comprising the same material as that of the sample 13 to be measured are arranged to the second and fourth optical paths 6 and 7. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention is used for a refractive index evaluation apparatus and dispersion evaluation apparatus in optical material development, optical components, optical device maintenance management, etc., and an optical material capable of precisely measuring a group refractive index without thickness information of the optical material The present invention relates to a group refractive index precise measurement method and an apparatus for performing the method.

  Various methods for measuring the refractive index of optical materials have been proposed, for example, in MD Hopler et al., “Interferometric measurement of group and phase refractive index,” Appl. Opt. 30, 735 (1991). Insert the sample into a low-coherence interferometer as shown, determine the optical thickness (sample thickness x refractive index) from the misalignment where the interference fringes occur, and determine the sample thickness by another method. There is a method (first method) for calculating the refractive index obtained in step (1).

  In addition, a sample was applied to a low coherence interferometer as shown in DF Murphy et al., “Dispersion-insensitive measurement of thickness and group refractive index by low-coherence interferometry,” Appl. Opt. 39, 4607 (2000). Is inserted, a reference plane is introduced behind the sample, and the thickness and refractive index are measured simultaneously. At that time, the interference fringes are Fourier analyzed to determine the wavelength dependence of the refractive index (second method). Has been.

  Furthermore, Itaya et al., “Simultaneous measurement of refractive index and thickness by low coherence interference, two-dimensional profile measurement, measurement method and example using reference plane,” Proceedings of the 29th Lightwave Sensing Technology Study Group, 119, (2002.6 ), A method (third method) is proposed in which a sample is inserted into a low coherence interferometer, a reference plane is introduced behind the sample, and thickness and refractive index are measured simultaneously.

  On the other hand, the refractive index of an optical material has a phase refractive index and a group refractive index, and there is a relational expression that connects the two. A widely used refractive index measurement method measures a phase refractive index, and measures a refraction angle by processing a sample to be measured into a prism shape. Since it is necessary to process the sample, this method cannot be used when non-destructive properties are required.

  Recently, group refractive index is often required for broadband optical communication, application of ultrashort pulse laser, optical medical measurement, and the like. There is a method using a low coherence interferometer to measure the group refractive index accurately and nondestructively. Here, the optical thickness (sample thickness × group refractive index) is measured, and there are many methods for obtaining the group refractive index with the sample thickness known as in the first method. However, it is difficult to accurately measure the thickness of the sample and correctly place it in the optical system. Further, when the sample is thick, the group refractive index is different for each wavelength, so the optical thickness is different depending on the wavelength. For this reason, the generation position cannot be accurately determined due to the spread of interference fringes and asymmetricality.

  Further, since the second method and the third method measure the thickness and optical thickness of the sample at the same time, the group refractive index can be obtained. However, the third method is the same as the first method in that the interference fringes are distorted and the generation position is not accurately determined when the sample is thick. In addition, since the second method obtains the generation position for each wavelength by performing Fourier analysis of the interference fringes, the influence of the distortion of the interference fringes can be ignored, but in principle, the sample needs to be taken in and out during the measurement. This causes a problem that high reproducibility such as adjustment of the position and inclination of the sample is required.

  In order to solve the conventional techniques as described above, the present inventors previously disclosed in Japanese Patent Application No. 2003-078612 that the group refractive index can be accurately measured even with a thick sample, and the sample is taken in and out during the measurement. And a method for accurately measuring the group refractive index of an optical material capable of measuring the group refractive index without knowing the thickness information of the sample, and an apparatus for carrying out the method are provided (No. 4). Method).

  6 to 9 show the principle diagrams. As shown in FIG. 6, two interferometers are connected in series. That is, the emitted light from the first interferometer 2a is made to be the incident light from the second interferometer 3a. The low-coherence light 1a interferes on the photodetector 4a when the optical path difference of the interferometer is near zero, and an interference signal is observed. First, the optical path difference between the two interferometers is set to zero. That is, the positions of the mirrors 9a and 10a and the mirrors 11a and 12a are adjusted so that the optical paths 5a and 6a and the optical paths 7a and 8a have the same length.

Next, as shown in FIG. 7, the sample 13a to be measured and the compensation plate 14a made of the same material as the sample are inserted into the first interferometer 2a, and the compensation plate 15a is inserted into the second interferometer 3a. The group refractive index of the sample to be measured 13a and the two compensators 14a and 15a is _ng , and the geometric thicknesses are L _s , L _C1 , and L _C2 , respectively. When the optical path difference between the optical path 51a and optical path 61a of the first interferometer 2a as shown in FIG. 7 by adjusting the position of the mirror 10a to be zero, the amount of movement of the mirror 10a at this time is d _{1,
d 1 = (n g -1)} (L C1 -L S) (1a)
It is. At this time, the optical path including the compensation plate 15a is closed by the shutter 16a so that excess light does not reach the photodetector 4a.

Next, the optical path is controlled as shown in FIG. That is, the optical path of the first interferometer 2a containing the sample 13a to be measured is closed by the shutter 17a. Optical path difference between the optical path 63a reflected by the optical path 62a and the back reflected by the surface of the compensation plate 14a is _2n g _{L C1.} At this time, since the light transmitted through the compensation plate 14a and reflected by the mirror 10a is unnecessary, the shutter 18a is closed to remove it. In the second interferometer 3a, when the mirror 11a is _{d 2} moves from the initial position, the optical path difference between the optical path 8a and the optical path 71a is _{2 {(n g -1) L} C2 -d 2}. When these two optical path differences match, the optical path difference between the light passing through the optical path 71a through the optical path 62a and the light passing through the optical path 8a through the optical path 63a becomes zero, and an interference signal is generated on the photodetector 4a. Arise. At this time,
_{d 2 = (n g -1)} L C2 -n g L C1 (2a)
It is.

Finally, the optical path of the first interferometer is switched as shown in FIG. That is, the optical path containing the compensation plate 14a is closed by the shutter 20a. Then, the light that has passed through the optical path 71a of the second interferometer through the optical path 52a reflected by the surface of the sample 13a to be measured and the optical path 8a of the second interferometer through the optical path 53a that is reflected by the back surface of the sample 13a to be measured. The position of the mirror 11a is adjusted so that the optical path difference of the light passing through becomes zero and interference fringes are generated. When the amount of movement of the mirror 11a when the interference fringes occur, and d _{3,
d 3 = (n g -1)} L C2 -n g L S (3a)
It is. At this time, the shutter 19a is closed to remove light transmitted through the sample 13a to be measured and reflected by the mirror 9a.
Subtracting equation (3a) from equation (2a)
_{d 2 -d 3 = n g (} L S -L C1) (4a)
Then, when formula (1a) is added to formula (4a),
_{d 2 -d 3 + d 1 =} L S -L C1 (5a)
It becomes. When equation (4a) is divided by equation (5a),
(D ₂ −d ₃ ) / (d ₂ −d ₃ + d ₁ ) = _ng (6a)
Thus, the group refractive index _ng is obtained.
Since the group refractive index _ng is obtained by obtaining d ₁ , d ₂ , and d ₃ as described above, the measurement order can be arbitrarily performed.

Here, only the mirror movement amounts d ₁ , d ₂ , and d ₃ are necessary for the calculation of _ng in the equation (6a), and the value of the geometric thickness of the sample or the like is not necessary. Further, in the formulas (1a) to (3a), the coefficient of the group refractive index _ng is a difference in geometric thickness. Therefore, if this is reduced, the interference fringes will not occur even if the thickness of the sample itself is thick. Does not distort. Also, the value of the geometric thickness difference is not necessary for the calculation.
Japanese Patent Application No. 2003-078612 MD Hopler et al., "Interferometric measurement of group and phase refractive index," Appl. Opt. 30, 735 (1991) DF Murphy et al., "Dispersion-insensitive measurement of thickness and group refractive index by low-coherence interferometry," Appl. Opt. 39, 4607 (2000) Itaya et al., "Simultaneous measurement of refractive index and thickness by low coherence interference, measurement method and example using reference plane," Proc. 29th Lightwave Sensing Technology Research Group, 119, (2002.6)

  The group refractive index can be obtained without thickness information by the above-described fourth method proposed by the present inventors. In the technique presented here, two interferometers are used. Since they are actually connected in series, there is a problem that the entire apparatus becomes large, and the number of optical elements constituting each interferometer is large, which is complicated and expensive. In addition, there are problems such as the difference in temperature and sample temperature between the first interferometer and the second interferometer.

  Therefore, according to the present invention, a compact, inexpensive, stable and precise apparatus can be configured by corresponding to two interferometers connected in series with one interferometer, and the thickness of the sample can be determined from the measurement result. It is an object of the present invention to provide a method for accurately measuring a group refractive index and a thickness that do not require information on the thickness of an optical material, and an apparatus for carrying out the method, in which information can be obtained.

  In order to solve the above problems, the method for accurately measuring the group refractive index of an optical material according to the present invention uses a low-coherence light source as a light source and connects two interferometers in series to a sample to be measured. Using a single compensator, the mirror of the interferometer is scanned in the optical axis direction while switching the optical path, the position where the low coherence interference fringes are measured, and the group refractive index is calculated. By passing light twice, the one interferometer is made to correspond to the two interferometers connected in series.

Further, in the method for accurately measuring the group refractive index of another optical material according to the present invention, the incident light is separated into two optical paths of the first optical path 5 and the second optical path 6 by the beam splitter 2, and the first optical path 5 is arranged in the first optical path 5. The reflected light from one mirror 11 and the optical axis of the reflected light from the second mirror 9 arranged in the second optical path 6 are made to coincide with each other and guided from the beam splitter 2 to the retroreflector 3, and the reflected light from the retroreflector 3 is The beam splitter 2 separates the optical path from the third mirror 10 disposed in the third optical path 8 into the two optical paths 8 and 7, and the reflected light from the third mirror 10 disposed in the third optical path 8 and the second mirror 9 disposed in the fourth optical path 7. By making the optical axis of the reflected light coincide with each other and receiving light from the beam splitter 2 with the photodetector 4, the same function as an interferometer in which two interferometers are connected in series with one interferometer is performed. Using an interferometer, the first optical path 5 and the second optical path 6, The position of each mirror is adjusted so that the three optical paths 8 and the fourth optical path 7 have the same length, so that the optical path difference between the two interferometers becomes zero, and the sample 13 to be measured is placed on the beam splitter 2 and the first mirror. The first compensator 14 made of the same material as the sample to be measured 13 is arranged in the second optical path 6 between the beam splitter 2 and the second mirror 9, and The optical path difference between the optical path 51 including the sample 13 to be measured corresponding to the first optical path 5 and the optical path 61 including the first compensator 14 corresponding to the second optical path 6 is zero. The position of the second mirror 9 is adjusted so that the amount of movement d ₁ of the second mirror 9 at this time is measured, and then the second compensator 15 made of the same material as the sample 13 to be measured is attached to the beam splitter 2 Arranged in the fourth optical path between the second mirror 9 and open the shutter of the fourth optical path 7 The optical path of the sample 13 to be measured is closed with a shutter, the optical path between the first compensator 14 and the second mirror 9 is closed with a shutter, passes through an optical path 62 that is reflected by the surface of the first compensator 14, and the first The optical path difference between the optical path consisting of the optical path 71 transmitted through the second compensation plate 15 and reflected by the second mirror 9 and the optical path 63 reflected by the back surface of the first compensation plate 14 and the optical path consisting of the third optical path 8 is as follows. as becomes zero by adjusting the position of the second mirror 9, a second mirror 9 the movement amount d ₂ is measured, and then the second optical path closed by the shutter, the measurement sample 13 first mirror at this time 11 is closed by a shutter, passes through an optical path 52 that is reflected by the surface of the sample 13 to be measured, and passes through the second compensator 15 and is reflected by the second mirror 9; It passes through an optical path 53 that is reflected from the back surface of the measurement sample 13. Both by adjusting the position of the second mirror so that the optical path difference between the optical path consisting of the third optical path 8 becomes zero, and measuring a moving amount d ₃ of the second lens at this time,
Group index _{n g = (d 2 -d 3} ) / (d 2 -d 3 + d 1)
The group refractive index is obtained by the following formula.

  In the method for accurately measuring the group refractive index of another optical material according to the present invention, the first compensator and the second compensator are used as a single compensator.

  The optical material group refractive index precise measurement apparatus according to the present invention uses a low-coherence light source as a light source, and passes light through one interferometer twice, thereby causing interference corresponding to two interferometers connected in series. A sample to be measured and two compensators are built into the interferometer, and the mirror of the interferometer is scanned in the direction of the optical axis while switching the optical path to generate low coherence interference fringes. The position is measured and the group refractive index is calculated.

  In addition, in the group refractive index precision measuring apparatus for another optical material according to the present invention, the first compensation plate and the second compensation plate are formed as a single compensation plate.

  Since the present invention can perform substantially the same operation as two interferometers connected in series by one interferometer, two interferences in the fourth method proposed by the present inventors as described above. When the meters are placed in series, the two interferometers are configured as separate interferometers, and the cost increases due to the increase in the size of the device and the number of optical elements. This eliminates the problems such as the influence of the difference in ambient temperature of the second interferometer and the large difference from the sample temperature, and is advantageous in that it is compact, inexpensive, and can construct a precise device that operates stably. is there.

  In an interferometer in which two interferometers are connected in series and a low-coherence light source is used as a light source, a sample to be measured whose group refractive index is to be measured and two compensators are used. By scanning the mirror of the interferometer in the optical axis direction while switching the optical path, measuring the position where the low coherence interference fringe occurs, and calculating the group refractive index, by passing light through one interferometer twice This corresponds to the two interferometers connected in series.

  1 to 4 show the principle of the first embodiment of the present invention. FIG. 1 shows only the optical equipment configuration of the interferometer itself of the first embodiment. The low-coherence light 1 incident from the light source is divided into a first optical path 5 and a second optical path 6 by a beam splitter 2, and the first The light in the optical path 5 is reflected by the first mirror 11, and the light in the second optical path 6 is reflected by the second mirror 9. The light that has returned to the beam splitter 2 from the first optical path 5 and the second optical path 6 is both folded back by the retroreflector 3 and divided into a third optical path 8 and a fourth optical path 7 in the beam splitter 2. The light in the fourth optical path 7 is reflected by the second mirror 9, which is the same mirror that reflects the light in the second optical path 6, and the light in the third optical path 8 is reflected by the third mirror 10. Both the light beams returned from the third optical path 8 and the fourth optical path 7 to the beam splitter 2 are received by the photodetector 4.

  The present invention uses the retroreflector 3 as described above to cause the outgoing light of the interferometer to enter the same interferometer again, and uses one interferometer as two interferometers. Causes interference on the photodetector 4 when the optical path difference of the interferometer is near zero, and an interference signal is observed.

  When measuring the thickness of an optical material using such an interferometer, first, the optical path difference between the two interferometers is set to zero. That is, the first mirror 11 and the second mirror 9, and the first mirror 11 and the third mirror 10 are set so that the first optical path 5 and the second optical path 6, the third optical path 8 and the fourth optical path 7 have the same length. Adjust the position.

Next, as shown in FIG. 2, the sample 13 to be measured, which is an optical material, is disposed in the optical path 51 corresponding to the first optical path 5 between the beam splitter 2 and the first mirror 11. Similarly, the first compensator 14 made of the same material as the sample 13 to be measured is disposed in the optical path 61 corresponding to the position of the second optical path 6 between the beam splitter 2 and the first mirror 9. A second compensation plate 15 made of the same material as the sample 13 to be measured is disposed in the fourth optical path 7 between the beam splitter 2 and the first mirror 9. Here, the optical path is closed by the shutter 16. .

In the above interferometer, when the group refractive index of the sample 13 to be measured, the first compensation plate 14 and the second compensation plate 15 is _ng and the geometric thicknesses are L _s , L _c1 and L _c2 , respectively, FIG. The position of the second mirror 9 is adjusted by observing the interference signal so that the optical path difference between the optical path 51 corresponding to the first optical path 5 and the optical path 61 corresponding to the second optical path 6 becomes zero as shown in FIG. When the movement amount of the second mirror 9 is d ₁ ,
_{d 1 = (n g -1)} (L C1 -L s) (1)
It is. At this time, the optical path 71 corresponding to the fourth optical path 7 including the second compensation plate 15 is closed by the shutter 16 so that excess light does not reach the photodetector 4.

Next, as shown in FIG. 3, the optical path 51 corresponding to the first optical path 5 containing the sample 13 to be measured is closed by the shutter 17. In this case, the optical path 62 corresponding to the second optical path 6 reflected by the surface of the first compensator 14, the optical path difference between the optical path 63 corresponding to the second optical path 6 reflected by the back surface is 2n _g L _c1 is there. At this time, since the light transmitted through the first compensation plate 14 and reflected by the second mirror 9 is unnecessary, it is removed, so that it corresponds to the second optical path 6 between the first compensation plate 14 and the second mirror 9. The shutter 18 disposed on the optical path 61 is closed.

As described above, in the second interferometer as the interferometer for the light passing through the second time, which is reflected by the mirror or the compensation plate on each optical path, the first mirror 9 has moved d ₂ from the first position. Then, the optical path difference between the optical path 71 corresponding to the third optical path 8 to the fourth optical path is _{2 {(n g -1) L} c2 -d 2}. When these two optical path differences coincide with each other, the optical path 8 passes through the optical path 71 corresponding to the fourth optical path 7 through the optical path 62 corresponding to the second optical path 6 and the optical path 63 corresponding to the second optical path 6. The optical path difference of the light passing through becomes zero, and an interference signal is generated on the photodetector 4. At this time,
_{d 2 = (n g -1)} L C2 -n g L c1 (2)
It is.

Finally, as shown in FIG. 4, the optical path of the first interferometer is switched. That is, the optical path containing the first compensation plate 14 is closed by the shutter 20, and the optical path between the sample 13 to be measured and the third mirror 11 is removed in order to remove the light that passes through the sample 13 to be measured and is reflected by the mirror 11. Close with shutter 19. Next, the light that has passed through the optical path 71 corresponding to the fourth optical path of the second interferometer through the optical path 52 corresponding to the first optical path reflected on the surface of the sample 13 to be measured and the back surface of the sample 13 to be measured are reflected. The position of the first mirror 9 is adjusted so that the optical path difference of the light passing through the optical path 53 of the second interferometer through the optical path 53 corresponding to the first optical path becomes zero and interference fringes are generated. If the amount of movement of the first mirror 9 when the interference fringes occur is d ₃ ,
_{d 3 = (n g -1)} L C2 -n g L s (3)
It is.

From the above formulas (2)-(3), d ₂ -d ₃ = _ng (L _s -L _C1 )
Also, equation (1) + (2) - Since the (3) from _{d 1 + d 2 -d 3 =} (L s -L C1), after all, the equation (3) from the above equation (1),
(D ₂ -d ₃ ) / (d ₂ -d ₃ + d ₁ ) = _ng (4)
Thus, similarly to the fourth method proposed by the present inventors, the group refractive index _ng is obtained without the thickness information of the sample to be measured and the compensation plate. Strictly speaking, the group refractive index of air must be taken into account, but the group refractive index of air can be obtained from empirical formulas based on the temperature, atmospheric pressure, humidity, and carbon dioxide concentration. It can be easily obtained by measuring the environmental conditions of the above and using the empirical formula.

  As described above, in the present invention, as in the fourth method proposed by the present inventors, it is not necessary to measure the value of the geometric thickness of the sample, and the thickness of the sample itself is Even if it is thick, there is an advantage that the interference fringes are not distorted. If the group refractive index of the sample 13 to be measured and the first compensation plate 14 are the same, the calculated value is obtained even if the group refractive index of the second compensation plate 15 is slightly different. Advantages such as no effect are also obtained. In addition, the effect is that it is compact and inexpensive, operates stably, and can perform precise measurements by making it substantially equivalent to two interferometers connected in series with one interferometer structure. Can be played.

FIG. 5 shows the principle of the second embodiment of the present invention. Here, the first compensation plate 14 and the second compensation plate 15 of the first embodiment are replaced with a single compensation plate 140. Assuming that the group refractive index and the geometric thickness of the compensation plate 140 are n _g and L _c1 , respectively, when the position where the interference fringes are generated is measured in the same procedure as in the first embodiment, the equations (1) to (3 ) And L _c2 = L _c1 ,
_{d 1 = (n g -1)} (L C1 -L s) (5)
d ₂ = −L _c1 (6)
_{d 3 = (n g -1)} L C1 -n g L s (7)
It becomes. Since the equation (4) is similarly established for these d ₁ , d ₂ , and d ₃ , the group refractive index is obtained by measuring the position where the low coherence interference fringes are generated.

At this time, from the equations (5) and (7),
d ₁ −d ₃ = L _s (8)
Thus, the thickness of the sample can be obtained at the same time.

  The present invention can be effectively applied to a refractive index evaluation device and a dispersion evaluation device in optical material development, optical component, optical device maintenance and management.

In the interferometer of the first embodiment of the present invention, it is an optical path diagram showing a first state of only the interferometer. It is an optical path figure which shows a 2nd state in the interferometer of the Example. In the interferometer of the same Example, it is an optical path diagram which shows a 3rd state. In the interferometer of the same Example, it is an optical path diagram which shows a 4th state. It is an optical path diagram of the interferometer of the second embodiment of the present invention. In the interferometer which the present inventors proposed previously, it is an optical path diagram which shows the 1st state only of an interferometer. FIG. 6 is an optical path diagram showing a second state in the interferometer. FIG. 6 is an optical path diagram showing a third state in the interferometer. FIG. 6 is an optical path diagram showing a fourth state in the interferometer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Low coherence light 2 Beam splitter 3 Retroreflector 4 Photo detector 5 1st optical path 6 2nd optical path 7 4th optical path 8 3rd optical path 9 2nd mirror 10 3rd mirror 11 1st mirror 13 Test sample 14 1st compensation Plate 15 Second compensator 16 Shutter 17 Shutter 18 Shutter 19 Shutter 20 Shutter 51 Optical path 52 Optical path 53 Optical path 61 Optical path 62 Optical path 63 Optical path 71 Optical path 140 Compensation plate

Claims (5)

  Compared to an interferometer that uses a low-coherence light source as the light source and two interferometers connected in series, the sample to be measured and two compensators are used, and the mirror of the interferometer is scanned in the optical axis direction while switching the optical path. In the method of measuring the position where the low coherence interference fringes occur and calculating the group refractive index, the one interferometer is connected in series by passing light through one interferometer twice. A method for accurately measuring a group refractive index of an optical material, characterized in that it corresponds to two interferometers. The incident light is separated by the beam splitter 2 into two optical paths, a first optical path 5 and a second optical path 6, and the reflected light from the first mirror 11 disposed in the first optical path 5 and the second mirror disposed in the second optical path 6. The optical axis of the reflected light from 9 is made coincident and guided from the beam splitter 2 to the retro-reflector 3, and the reflected light from the retro-reflector 3 is passed through the beam splitter 2 to two optical paths of the third optical path 8 and the fourth optical path 7. The optical beam of the reflected light from the third mirror 10 arranged in the third optical path 8 and the reflected light from the second mirror 9 arranged in the fourth optical path 7 are made to coincide with each other so that the beam splitter 2 to the photodetector 4 By using an interferometer that performs the same function as an interferometer formed by connecting two interferometers in series with one interferometer,
Adjust the position of each mirror so that the first optical path 5 and the second optical path 6, the third optical path 8 and the fourth optical path 7 are the same length, respectively, the optical path difference of the two interferometers, respectively,
A sample to be measured 13 is arranged in the first optical path 5 between the beam splitter 2 and the first mirror 11, and a first compensation plate 14 made of the same material as the sample to be measured 13 is placed between the beam splitter 2 and the second mirror 9. Between the second optical path 6 and the fourth optical path 7 with a shutter,
The second mirror 9 is designed such that the optical path difference between the optical path 51 including the measured sample 13 corresponding to the first optical path 5 and the optical path 61 including the first compensator 14 corresponding to the second optical path 6 is zero. Adjust the position, measure the amount of movement d ₁ of the second mirror 9 at this time,
Next, the second compensator 15 made of the same material as the sample to be measured 13 is arranged in the fourth optical path between the beam splitter 2 and the second mirror 9, and the shutter of the fourth optical path 7 is opened. The optical path is closed with a shutter, and the optical path between the first compensator 14 and the second mirror 9 is closed with a shutter,
An optical path consisting of an optical path 71 passing through the second compensation plate 15 and reflected by the second mirror 9 while passing through an optical path 62 reflected by the surface of the first compensation plate 14 and an optical path reflected by the back surface of the first compensation plate 14 together through 63, the optical path difference between the optical path including the third optical path 8 by adjusting the position of the second mirror 9 so that the zero, and measuring a moving amount d ₂ of the second mirror 9 in this case,
Next, the second optical path is closed with a shutter, the optical path between the sample 13 to be measured and the first mirror 11 is closed with a shutter,
An optical path consisting of an optical path 71 passing through the second compensating plate 15 and reflected by the second mirror 9 and an optical path 53 reflecting from the back surface of the measured sample 13 are passed along the optical path 52 reflected by the surface of the measured sample 13. And adjusting the position of the second mirror so that the optical path difference from the optical path consisting of the third optical path 8 is zero, and measuring the movement amount d ₃ of the second mirror at this time,
Group index _{n g = (d 2 -d 3} ) / (d 2 -d 3 + d 1)
A method for precisely measuring a group refractive index of an optical material, wherein the group refractive index is obtained by the formula:   3. The method for accurately measuring the group refractive index of an optical material according to claim 1, wherein the first compensator and the second compensator are a single compensator. Using a low-coherence light source as the light source and passing light through one interferometer twice, an interferometer corresponding to two interferometers connected in series is formed,
A sample to be measured whose group refractive index is to be measured and two compensators are incorporated into the interferometer,
An apparatus for accurately measuring the group refractive index of an optical material, which scans a mirror of an interferometer in the optical axis direction while switching an optical path, measures a position where a low coherence interference fringe is generated, and calculates a group refractive index.   5. The group refractive index precise measurement apparatus for an optical material according to claim 4, wherein the first compensation plate and the second compensation plate are formed as a single compensation plate.

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