JP2007183297A - Method and device for precisely measuring group refractive index of optical material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for easily and accurately measuring a group refractive index without requiring thickness information of a sample to be measured. <P>SOLUTION: An interferometer is constituted by serially connecting a first interferometer 2 of a Michelson type or the like and a second interferometer 3 of the same type by using a low-coherence light source as its light source. Then, an optical material to be measured 13 whose group refractive index is to be measured, and a first compensating plate 14 made of the same material are disposed on the first interferometer 2, and a second compensating plate 15 is disposed on the second interferometer. A mirror in the interferometer is scanned in the direction of its optical axis, while switching optical paths, and positions where low-coherence interference fringes appear are measured in a plurality of times, and the group refractive index is calculated based on the positions. In another method, the optical material to be measured whose group refractive index is to be measured, and one compensating plate made of the same material are employed to a triangular optical path interferometer. While the optical material to be measured is operated together with one compensating plate in the direction of its optical axis, the positions where the low-coherence interference fringes appear are measured, and the group refractive index is calculated. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

  Conventionally, various methods for measuring the refractive index of optical materials have been proposed. D. Hopler et al. , “Interferometric measurement of group and phase reflexive index,” Appl. Opt. 30, 735 (1991), a sample is inserted into a low-coherence interferometer, and the optical thickness (sample thickness × refractive index) is obtained from the displacement of the position where interference fringes are generated. There is a technique (first technique) for calculating the refractive index by obtaining the thickness of the film by another method.

  D. F. Murphy et al. , “Dispersion-insensitive measurement of thickness and group refractive index by low-coherence interferometry,” Appl. Opt. 39, 4607 (2000), a sample is inserted into a low coherence interferometer, a reference plane is introduced at the back of the sample, and thickness and refractive index are measured simultaneously. A method (second method) for analyzing and calculating the wavelength dependence of the refractive index has also been proposed.

  Furthermore, Itaya et al., "Simultaneous measurement of refractive index and thickness by low coherence light interference, measurement method and measurement example using reference plane," Proceedings of the 29th Lightwave Sensing Technology Study Group, 119, (2002 (6), a method (third method) has been 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. Yes.

M.M. D. Hopler et al. , “Interferometric measurement of group and phase reflexive index,” Appl. Opt. 30, 735 (1991) D. F. 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 light interference, measurement method and measurement example using reference plane,” Proceedings of the 29th Lightwave Sensing Technology Research Group, 119, (2002. 6) )

  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.

  Therefore, the present invention can accurately measure the group refractive index even with a thick sample, and can measure the group refractive index without knowing the thickness information of the sample without taking in and out of the sample during the measurement. An object of the present invention is to provide a method for accurately measuring the group refractive index of an optical material, and an apparatus for carrying out the method.

  In order to solve the above-described problem, the method for accurately measuring the group refractive index of an optical material according to the present invention uses a low-coherence light source, reflects the incident light separated by the beam splitter by a mirror, and emits it on the same optical axis. Are connected in series, a sample to be measured whose group refractive index is to be measured is placed in one of the separated optical paths of one interferometer, and a first compensator made of the same material as the sample is placed in the other of the separated optical paths Then, a second compensator is arranged in one of the separation optical paths of the other interferometer, and the position of the low coherence interference fringe is measured by scanning the mirror of the interferometer in the optical axis direction while switching each optical path. Thus, the group refractive index is calculated.

More specifically, the incident light is separated into two optical paths by a beam splitter, and the same configuration as that of the first interferometer that emits light with the optical axes of the reflected light from the mirrors arranged in the respective optical paths aligned. A first interferometer connected in series, an interferometer that receives low-coherence light into the first interferometer and receives light emitted from the second interferometer with a photodetector, and The optical path difference of the second interferometer is made zero, and a sample and a compensation plate made of the same material as the sample are separately disposed on the two optical paths separated by the beam splitter in the first interferometer, and the optical path difference between the two optical paths is The distance d ₁ is measured by moving the mirror of the optical path in which the compensation plate is arranged so as to be zero, the optical path in which the sample to be measured of the first interferometer is arranged is closed, and the compensation plate of the interferometer is Close the mirror of the placed optical path and separate it with the beam splitter in the second interferometer A compensation plate disposed in one of the optical paths, the light reflected by the surface of the compensation plate in the first interferometer and passing through the optical path in which the compensation plate of the second interferometer is disposed, and the compensation plate in the first interferometer The amount of movement d is determined by moving the mirror of the optical path in which the compensation plate of the second interferometer is arranged so that the optical path difference with the light reflected by the back surface of the light and passing through the optical path in which the compensation plate of the second interferometer is not arranged is zero. ₂ was measured, the optical path on which the compensation plate of the first interferometer was placed was closed, the mirror of the optical path on which the sample was placed was closed, and the compensation plate of the second interferometer was placed on the surface of the sample to be measured. The compensation plate of the second interferometer is arranged so that the optical path difference between the light passing through the optical path and the light reflected by the back surface of the sample to be measured and passing through the optical path where the compensation plate of the second interferometer is not arranged is zero. move the mirror of the optical path to measure the amount of movement _{d 3,} Gun屈by equation group index _{n g = (d 2 -d 3} ) / (d 2 -d 3 + d 1) It is obtained so as to determine the rate.

  In addition, the group refractive index precision measuring apparatus for other optical materials according to the present invention uses a low-coherence light source, and in series two interferometers that reflect the incident light separated by the beam splitter by a mirror and emit it on the same optical axis. And a sample to be measured whose group refractive index is to be measured is arranged in one of the separated optical paths of one interferometer, and a first compensator made of the same material as the sample is arranged on the other of the separated optical paths, A second compensator is arranged in one of the separation optical paths of the other interferometer, and includes a shutter that can be opened and closed in each optical path, and a means for moving the mirror of the interferometer in the optical axis direction to measure the amount of movement. The group refractive index is calculated by scanning the mirror of the interferometer in the optical axis direction while switching the optical path by the shutter, and measuring the position where the low coherence interference fringes are generated.

  In the conventional group refractive index measurement method, it is necessary to know the thickness of the sample in advance, but this is not necessary in the method of the present invention, and the group refractive index can be measured easily and accurately. In addition, while the technique for measuring the thickness and refractive index simultaneously proposed in the past does not require the thickness to be known in advance, the measurement becomes inaccurate because the interference fringes are distorted when the sample is thick. This is not the case in the present invention.

  Furthermore, although the thickness limit depends on the spectral width of the light source, there are few methods that can accurately measure a 10 mm thick sample in the prior art. Since the method according to the present invention only affects the difference in thickness by using the compensation plate, the thickness of the sample itself may be any thickness. In addition, it is not necessary to know the thickness difference value. In addition, in the conventional technique for obtaining the group refractive index for each wavelength by Fourier analysis, it is necessary to put in and out of the sample and reproducibility of the position and inclination, but according to the method of the present invention, once the entire device is set After that, since the measurement can be performed only by opening and closing the optical path by the shutter, it is not necessary to put in and out of the sample, and the reproducibility can be improved and the measurement can be performed in a short time.

  The present invention aims to provide a method for easily and accurately measuring a group refractive index without requiring thickness information of a sample to be measured. The present invention aims to provide an incident light separated by a beam splitter using a low coherence light source. Two interferometers that are reflected by a mirror and output on the same optical axis are connected in series, and a sample to be measured whose refractive index is to be measured is placed in one of the separated optical paths of one interferometer. A first compensation plate made of the same material as that of the sample is arranged on the other side, a second compensation plate is arranged on one of the separation optical paths of the other interferometer, and the mirror of the interferometer is placed on the optical axis while switching the optical paths. It was realized by scanning in the direction and calculating the group refractive index by measuring the position where the low coherence interference fringes occur.

  1 to 4 show the principle of the first embodiment of the present invention. As shown in FIG. 1, two interferometers are connected in series. That is, the light emitted from the first interferometer 2 is made incident light from the second interferometer 3. The low-coherence light 1 interferes on the photodetector 4 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 9 and 10 and the mirrors 11 and 12 are adjusted so that the optical paths 5 and 6 and the optical paths 7 and 8 have the same length.

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

Next, the optical path is controlled as shown in FIG. That is, the optical path of the first interferometer 2 including the sample 13 to be measured is closed by the shutter 17. The optical path difference between the optical path 62 reflected on the front surface of the compensation plate 14 and the optical path 63 reflected on the back surface is 2 _ng L _C1 . At this time, since the light transmitted through the compensation plate 14 and reflected by the mirror 10 is unnecessary, the shutter 18 is closed to remove it. In the second interferometer 3, if the mirror 11 has moved d ₂ from the initial position, the optical path difference between the optical path 8 and the optical path 71 is 2 {( _ng −1) L _C2 −d ₂ }. When these two optical path differences match, the optical path difference between the light passing through the optical path 71 through the optical path 62 and the light passing through the optical path 63 through the optical path 63 becomes zero, and an interference signal is generated on the photodetector 4. Arise. At this time,
_{d 2 = (n g -1)} L C2 -n g L C1 (2)
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 14 is closed by the shutter 20. Then, the light that has passed through the optical path 71 of the second interferometer through the optical path 52 reflected from the surface of the sample 13 to be measured, and the optical path 8 of the second interferometer through the optical path 53 reflected from the back surface of the sample to be measured 13. The position of the mirror 11 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 11 when the interference fringe is generated and d _{3,
d 3 = (n g -1)} L C2 -n g L S (3)
It is. At this time, the shutter 19 is closed in order to remove light transmitted through the sample 13 to be measured and reflected by the mirror 9.
Subtracting equation (3) from equation (2),
_{d 2 -d 3 = n g (} L S -L C1) (4)
Then, when equation (1) is added to equation (4),
d ₂ −d ₃ + d ₁ = L _S −L _C1 (5)
It becomes. Dividing equation (4) by equation (5) gives
(D ₂ −d ₃ ) / (d ₂ −d ₃ + d ₁ ) = _ng (6)
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 calculating _ng in Equation (6), and the value of the geometric thickness of the sample or the like is not necessary. In the formulas (1) to (3), since the coefficient of the group refractive index ng is a difference in geometric thickness, if this is reduced, the interference fringes are distorted even if the sample itself is thick. Absent. The value of the geometric thickness difference is also not necessary for the calculation.

  Furthermore, if the sample 13 and the compensation plate 14 in the first interferometer have the same group refractive index, the calculated value is not affected even if the group refractive index of the compensation plate 15 in the second interferometer is slightly different. There are also advantages. This is proved below.

_{N g1} the group refractive index of the compensating plate 14 and the sample to be measured 13 in the first interferometer, when the group refractive index of the compensator 15 in the second interferometer and _{n g2,} equation (1) to (3) ,
_{d 1 = (n g1 -1)} (L C1 -L S) (7)
d ₂ = (n _g2 −1) L _C2 −n _g1 L _C1 (8)
d ₃ = (n _g2 −1) L _C2 −n _g1 L _s (9)
And subtracting equation (9) from equation (8),
_{d 2 -d 3 = n g1 (} L S -L C1) (10)
Then, when equation (7) is added to equation (10),
d ₂ −d ₃ + d ₁ = L _S −L _C1 (11)
It becomes. When equation (10) is divided by equation (11),
_{(D 2 -d 3) / (} d 2 -d 3 + d 1) = n g1 (12)
Thus, the group refractive index _{ng1 of the} sample to be measured is obtained. The value of the group refractive index may be slightly different depending on the history and temperature, but in this method, the group refractive index of the sample to be measured and the compensation plate in the first interferometer may be the same.

Next, FIG. 5 and FIG. 6 are principle diagrams of the second embodiment of the present invention. When neither the sample 104 to be measured nor the compensation plate 105 is arranged, the optical path length that goes around the triangular optical path interferometer is D. Further, the place where the clockwise and counterclockwise optical paths from the beam splitter 103 are equal is set as the origin of the interferometer. As shown in FIG. 5, the sample 104 to be measured and the compensation plate 105 are arranged in a triangular optical path interferometer. FIG. 6 shows the optical path in detail. The geometric thickness of the sample 104 to be measured is L _S , the geometric thickness of the compensation plate 105 is L _C , and the group refractive index of the sample to be measured and the compensation plate is _ng . The sample to be measured and the compensation plate are arranged in parallel, and the distance between them is Δ. When the center position of the sample to be measured moves by d along the optical axis from the origin of the triangular optical path interferometer, the optical path lengths of the optical paths 106 to 111 are as follows:
Optical path 106: D-L _S + 2d (13)
Optical path _{107: D-L S + 2n} g L s + 2d (14)
Optical path 108: D + L _S + 2Δ + 2d (15)
Optical path _{109: D-L S -2d (} 16)
Optical path 110: D-L _S -2Δ-2L _C +2 _ng L _C -2d (17)
Optical path 111: D-L _S -2Δ-2L _C -2d (18)
It becomes. When the center position of the sample to be measured when the light passing through the optical path 106 interferes with the light passing through the optical path 111 is d ₁ , the expressions (13) and (18) are equal,
d ₁ = (− Δ−L _C ) / 2 (19)
It becomes. When the center position of the sample to be measured when the light passing through the optical path 107 interferes with the light passing through the optical path 110 is d ₂ , the expression (14) and the expression (17) are equal.
_{d 2 = {- Δ-L} C -n g (L S -L C)} / 2 (20)
It becomes. When the center position of the sample to be measured when the light passing through the optical path 108 interferes with the light passing through the optical path 109 is d ₃ , the expression (15) and the expression (16) are equal.
d ₃ = (− Δ−L _S ) / 2 (21)
It becomes. Subtracting equation (20) from equation (19),
_{d 1 -d 2 = n g (} L S -L C) / 2 (22)
It becomes. Subtracting equation (21) from equation (19)
d ₁ −d ₃ = (L _S −L _C ) / 2 (23)
It becomes. Dividing equation (22) by equation (23) gives
(D ₁ -d ₂ ) / (d ₁ -d ₃ ) = _ng (24)
Thus, the group refractive index ng is obtained.

Here, the calculation of _ng in equation (24) requires only the movement amounts d ₁ , d ₂ , and d _{3 of the} sample to be measured and the compensation plate, and the value of the geometric thickness of the sample and the like is not necessary. is there. Further, since the coefficient of the group refractive index _{ng in} the equation (22) is a difference in thickness, if this is reduced, the interference fringes are not distorted even if the thickness of the sample itself is thick. The value of the geometric thickness difference is also not necessary for the calculation.

  The group refractive index measurement method according to the present invention as described above can be proved analytically by the mathematical formula as described above, but the principle was confirmed also by computer simulation. The light source had a center wavelength of 680 nm and a spectral width of 10 nm. The material was BK7, and the approximate expression of the wavelength dependence of the phase refractive index described in the manufacturer's catalog was used. The group refractive index was calculated from the value of the phase refractive index using the group refractive index definition formula.

  With the above technique, several combinations were calculated from the sample thickness of 1 mm and the compensation plate thickness of 0.9 mm and 0.8 mm to the combination of the sample thickness of 11 mm and the compensation plate thickness of 6.1 mm. All of these agreed with the theoretical value within the accuracy range of the original approximate expression of refractive index, and it was found that the method according to the present invention was correct.

In the interferometer of 1st Example by this invention, it is an optical path figure which shows a 1st state. 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 figure of the interferometer of 2nd Example by this invention. In the interferometer of the same Example, it is a detailed optical path diagram of a to-be-measured sample and a compensation board part.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Low coherence light 2 1st interferometer 3 2nd interferometer 4 Optical detector 5 Optical path 6 Optical path 7 Optical path 8 Optical path 9 Mirror 10 Mirror 11 Mirror 12 Mirror 13 Sample to be measured 14 Compensation plate 15 Compensation plate 16 Shutter 17 Shutter 18 Shutter DESCRIPTION OF SYMBOLS 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 101 Low coherence light 102 Photo detector 103 Beam splitter 104 Sample to be measured 105 Compensation plate 106 Optical path 107 Optical path 108 Optical path 109 Optical path 110 Optical path 111 Optical path

Claims (3)

Using a low coherence light source, two interferometers that reflect the incident light separated by the beam splitter with a mirror and emit it with the same optical axis are connected in series.
A sample to be measured whose refractive index is to be measured is arranged in one of the separated optical paths of one interferometer, and a first compensator made of the same material as the sample is arranged in the other of the separated optical paths,
Placing a second compensator in one of the separation optical paths of the other interferometer;
A method for accurately measuring the group refractive index of an optical material, wherein the optical axis direction of the interferometer is scanned while switching the optical paths, and the group refractive index is calculated by measuring a position where a low coherence interference fringe is generated. . A first interferometer that divides incident light into two optical paths by a beam splitter and emits the optical axes of reflected light from mirrors arranged in the respective optical paths and a second interferometer having the same configuration are connected in series. A low coherence light is incident on the first interferometer, and the light emitted from the second interferometer is received by a photodetector.
The optical path difference between the first interferometer and the second interferometer is set to zero,
A sample and a compensation plate made of the same material as the sample are separately arranged on the two optical paths separated by the beam splitter in the first interferometer, and the compensation plate is arranged so that the optical path difference between the two optical paths becomes zero. a movement amount d ₁ measures the optical path of lens moves,
Close the optical path where the sample to be measured of the first interferometer is placed, close the mirror of the optical path where the compensation plate of the interferometer is placed, and place the compensation plate in one of the optical paths separated by the beam splitter in the second interferometer The light reflected by the surface of the compensation plate in the first interferometer and passed through the optical path on which the compensation plate of the second interferometer is arranged, and reflected by the back surface of the compensation plate in the first interferometer and reflected by the second interferometer Moving the mirror of the optical path in which the compensation plate of the second interferometer is arranged so that the optical path difference with the light passing through the optical path in which no compensation plate is arranged is zero, and measuring the movement amount d ₂ ;
With the optical path where the compensation plate of the first interferometer is placed closed and the mirror of the optical path where the sample is placed is closed, the mirror of the optical path where the compensation plate of the second interferometer is placed and reflected from the surface of the sample to be measured. The light reflected by the surface of the sample to be measured and passed through the optical path where the compensation plate of the second interferometer is arranged, and the light reflected by the back surface of the sample to be measured and passed through the optical path where the compensation plate of the second interferometer is not arranged move the mirror optical path difference is arranged compensator second interferometer such that the zero and measuring a moving amount d _3,
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: Using a low coherence light source, two interferometers that reflect the incident light separated by the beam splitter with a mirror and emit it with the same optical axis are connected in series.
A sample to be measured whose refractive index is to be measured is arranged in one of the separated optical paths of one interferometer, and a first compensator made of the same material as the sample is arranged in the other of the separated optical paths,
Placing a second compensator in one of the separation optical paths of the other interferometer;
A shutter that can be opened and closed in each optical path, and means for moving the mirror of the interferometer in the direction of the optical axis and measuring the amount of movement;
Precise measurement of the group index of refraction of an optical material by scanning the mirror of the interferometer in the optical axis direction while switching the optical path by the shutter, and measuring the position where the low coherence interference fringe occurs and calculating the group index of refraction apparatus.

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