JP2005292046A - Method and apparatus for measuring refractive index distribution in optical crystal wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure the refractive index distribution or birefringence rate distribution of a plurality of principal axes in a sample by a precise and rapid method, without being affected by the interference by the front and the rear surfaces of a wafer and the distribution of the thickness of the wafer. <P>SOLUTION: In a method for measuring the refractive index distribution and in a sample, having two facing main surfaces in which light can enter, at the first, second, and third inclination angles of a main surface perpendicular direction with respect to the optical axis, a first transmission wavefront when P polarization is allowed to coincide with the first principal axis; and a second transmission wavefront, when P polarization is made to coincide with a second principal axis orthogonally crossing the first principal axis are measured; and the refractive index distribution of the first, second, and third principal axis polarization are measured. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

With the progress of optical communication and IT, single crystal wafers with electro-optic effect have become an important role to play. In particular, in optical fiber communication, a wavelength division multiplexing WDM system that multiplexes and transmits light of many wavelengths on one optical fiber is used in order to cope with an explosive increase in communication volume. Furthermore, in order to cope with the increase in the amount of information, the modulation speed is increasing from 2.5 GHz to 10 GHz, and further to 40 GHz and 80 GHz in the future. In particular, an optical modulator using a lithium niobate (LiNbO ₃ ) crystal that is an inorganic single crystal is used as a modulator used in a long-distance trunk line system. An optical modulator using lithium niobate forms a Mach-Zehnder-type waveguide by titanium diffusion and applies an electric field to the Mach-Zender arm to modulate the refractive index of the arm on one side. Is to modulate.

As the modulation rate increases from 10 GHz to 40 GHz, the required element length increases and a more uniform lithium niobate wafer is desired. Also, if the Li concentration is different, the titanium diffusion speed at the time of waveguide creation is different, so the optical waveguide size changes and the mode diameter of the light changes. It is considered that the manufacturing margin decreases as the design is performed at a higher speed, and even a slight change in waveguide characteristics causes a change in the operating voltage, resulting in a deterioration in yield. For this reason, a LiNbO ₃ substrate for a high-speed modulator of 40 GHz or more is desired to have a uniform composition distribution with a Li concentration fluctuation of 0.01 mol% or less.

On the other hand, LiNbO ₃ crystal is known to Li / Nb composition ratio is changed during crystal growth and Li / Nb composition ratio deviates from congruent composition (Li = 48.5 mole%). Therefore, the composition ratio is usually different between the upper part and the lower part of the grown ingot. Variations in composition are also observed within the wafer. Therefore, it is important to measure the Li / Nb composition ratio distribution with high accuracy in order to evaluate wafers for optical applications.

  Conventionally, as a method for measuring the Li / Nb composition ratio, a method is known in which a wafer is cut and a Curie temperature is measured by thermal analysis such as DTA (Differential Thermal Analysis) and DSC (Differential Scanning Calorimetory). This is based on the fact that the Curie temperature Tc changes linearly with respect to the Li concentration (ΔTc / ΔLi to 1 ° C / mol%), but the measurement accuracy itself is limited (0.6 ° C) The spatial resolution is also poor (3 mm) and cannot be used practically. In addition, it is a destructive inspection and takes a long time to measure (3 hours for one point measurement).

In general, a birefringence measurement system using a photoelastic element has been developed as a birefringence distribution measuring device, and it has been proved that it is effective to accurately measure slight birefringence in isotropic materials such as CaF2. . However, this method has a problem that it cannot be applied to an anisotropic material having a large birefringence. (Non-Patent Document 1)
It can be easily imagined that the refractive index concentration distribution in the wafer is measured and calculated by measuring the change in the transmitted wavefront of the wafer. It is well known that the refractive index of lithium niobate crystals varies with the Li composition. Therefore, it is possible to calculate the Li composition distribution by measuring the refractive index. As a device for measuring such a transmitted wavefront, for example, an interferometer such as a Twiman Green interferometer or a Michelson interferometer has been put into practical use, which has characteristics of non-contact, non-destructive, and non-contaminating. However, this method has problems in the following two points.

  The first problem is the multiple interference effect. Since the wafer is usually polished in parallel, when light is allowed to pass, some of the light undergoes multiple reflections, and so-called interference patterns also occur at the same time. For this reason, there is a problem that necessary information is buried because the interference pattern is superimposed on the image of the transmitted wavefront itself. In order to avoid such an interference effect, it is necessary to polish the wafer to a substrate having a wedge of about 3 ° instead of a parallel plate. Wedge polishing is technically difficult, especially for large wafers, and requires a lot of labor. Further, since the wafer as a product is a parallel plate, there is a problem that it cannot be used for product inspection. Although non-reflective coating can be performed on both sides, there is a problem that a coating process is required and it cannot be used particularly in product inspection or in-line rapid measurement.

  The second problem is that only the refractive index distribution cannot be detected. When the thickness of the measurement sample is ideally parallel and the thickness distribution is so small that it can be ignored, the refractive index distribution can be calculated by measuring the transmitted wavefront distortion with the sample thickness being constant. However, in general, this assumption does not hold. In particular, since a large thickness unevenness exists for a wafer-shaped large sample, a normal transmitted wavefront phase difference measurement cannot distinguish the refractive index distribution from the wafer thickness unevenness and the refractive index distribution. This will be specifically described as follows.

If the spatial change in refractive index is Δn (x, y), the spatial change in wafer thickness is ΔL (x, y), and the constant values are n ₀ and L ₀ , the refractive index is n (x, y) and the wafer The thickness L (x, y) is expressed as shown in equations (A) and (B).
n (x, y) = n ₀ + Δn (x, y) (A)
L (x, y) = L ₀ + ΔL (x, y) (B)
Accordingly, the optical length OL (Optical Length) is expressed by equation (C).
OL (x, y) = n (x, y) · L (x, y) = (n ₀ + Δn (x, y)) · (L ₀ + ΔL (x, y))
= n ₀ L ₀ + L ₀ · Δn (x, y) + n ₀ · ΔL (x, y) + Δn (x, y) · ΔL (x, y) (C)
If the secondary minute amount (fourth term on the right side) of equation (C) is ignored, equation (C) is approximated as (D).

L (x, y) to n ₀ L ₀ + L ₀ · Δn (x, y) + n ₀ · ΔL (x, y) (D)
Although it is desirable to make the wafer thickness uniform, in reality, the deviation ΔL (x, y) from the parallel wafer thickness distribution is about 1 μm with a 5-inch diameter. Since n ₀ is about 2.2, the size of the third term is a value of 2.2 μm. Further, the refractive index fluctuation Δn (x, y) corresponding to the Li concentration deviation of 0.01 mol% is about 0.0001, and the wafer thickness L ₀ is about 800 μm, so the second term size is about 0.08 μm. is there. Therefore, the size of the second term of the equation (D) to be measured is 2 to 3 orders of magnitude smaller than the size of the third term due to thickness unevenness. For this reason, it is impossible to measure the refractive index distribution by wavefront observation. Because of these two problems, it is difficult to measure the refractive index distribution using an interferometer.
Baoliag Wang, Koji Hosogai: “Measurement of birefringence at 157 nm”, O plus E, 2003 p.1226 November 2003

When measurement is performed at normal incidence, multiple interference occurs between the front and back of the wafer, and the multiple interference pattern is slightly superimposed on the phase distribution observed due to this influence, which affects the extraction of a slight refractive index distribution. I will receive.
In view of the problems of the prior art, the object of the present invention is to avoid the influence of wafer front and back interference, and the influence of the wafer thickness distribution, so that a plurality of spindles of the sample can be obtained in a more accurate and quick manner. In other words, the refractive index distribution or the birefringence distribution can be measured.

In order to solve these problems, the present invention focuses on the fact that the interference effect between the front and back sides can be reduced if only P-polarized light is used, and the present invention focuses on the difference in the composition ratio dependence of the refractive index due to the difference in polarization. It was. That is, according to the present invention, in a refractive index distribution measurement method by transmitted wavefront measurement, in a sample having two opposing main surfaces into which light can enter, the vertical direction of the main surface is the first, second, and third relative to the optical axis. When the P-polarized light is aligned with the first principal axis of the measurement sample and the P-polarized light is aligned with the second principal axis perpendicular to the first principal axis This is a method of measuring a refractive index distribution by measuring a second transmitted wavefront and measuring a refractive index distribution of the first, second, and third principal axis polarizations.
Accordingly, the refractive index distribution of a plurality of main axes of the sample can be obtained with a high accuracy and a quick method without being affected by the interference between the front and back of the wafer and without being influenced by the distribution of the wafer thickness. The birefringence distribution can also be obtained by measuring the refractive index distribution.

Here, the first, second, and third inclination angles are preferably within a Brewster angle of ± 5 °. As measured in the present invention, LiNbO ₃ major surface vertically matches the x or y _axis, or, is preferred LiNbO ₃ main surface perpendicular direction is coincident with the z-axis.

  Further, the present invention provides means for separating the light generated from the light source unit into reference light and measurement light and allowing the measurement light to pass through the test object, and interfering the measurement light passing through the test object with the reference light to generate an interference image. In the refractive index distribution measurement apparatus including an image acquisition unit to be captured and a calculation unit for analyzing the acquired image, the refractive index distribution measurement apparatus calculates a refractive index distribution of the test object using the measurement method.

The present invention will be described in detail below. As shown in FIG. 1, consider a case where the principal surface normal direction of a LiNbO ₃ wafer is inclined with respect to the optical axis so that the incident angle is θ. If the plane including the normal direction of the main surface of the wafer and the optical axis is defined as the incident plane, the polarized light in the incident plane is referred to as P-polarized light, and the polarized light in the direction perpendicular to the incident plane is referred to as S-polarized light. FIG. 2 shows the calculated reflectance per one wafer surface when the incident angle θ is changed.
In the case of vertical incidence of θ = 0 degree, reflection of about 15% occurs for both S wave and P polarization. When tilted, the reflectivity decreases with respect to P-polarized light, and there is a point at which no reflection occurs at about θ = 65 degrees. This is known to exist only in P-polarized light at an angle called the Brewster angle. (Optical and electromagnetic theory, Miyoshi Tanroku, Baifukan, 1987) On the other hand, for S waves, the reflectance increases monotonically with the angle, and there is no angle at which no reflection occurs.

  The present invention pays attention to this point and uses only P-polarized light by tilting the normal direction of the main surface of the wafer with respect to the optical axis to reduce or eliminate the effect of interference and to set the transmitted wavefront at at least three different angles. By measuring, it came to come up with measuring only refractive index distribution. Ideally, the reflectivity can be completely reduced to 0 by inclining it so as to coincide with the Brewster angle. However, since the tilt angle is large and the image of the transmitted wavefront is flattened, the spatial resolution is deteriorated. Moreover, the refractive index distribution cannot be calculated only by tilting the wafer. Since the wafer main surface normal direction is used obliquely, further detailed analysis is required.

The principle of operation will be described below using an X-cut LiNbO ₃ wafer as an example. Consider a case where the wafer is tilted so that the incident angle with respect to the normal direction of the wafer main surface is θ ₁ . First, as shown in FIG. 3A, the wafer is set in a direction in which the polarization axis of the P wave coincides with the z axis of the crystal. The angle of incidence within the crystal at this time is θ _1z . Assuming that the wafer thickness has a distribution in the plane (x, y), the thickness L ₀ of a constant value regardless of the position and the component ΔL (x, y) depending on the two-dimensional position of the wafer. Expressed in the sum. If the constant refractive index component n _ez (θ _1z ) and the spatially dependent component are Δn _ez (x, y, θ _1z ), and the phase difference distribution observed at this time is ΔZ ₁ (x, y), the phase difference The distribution is expressed as:

Here, φ ₁ is a constant component of the phase difference.

Next, as shown in FIG. 3B, the sample is rotated by 90 ° so that the polarization direction coincides with the y-axis direction of the crystal. The phase difference distribution ΔY ₁ (x, y) observed when the incident angle in the crystal is θ _1y is given by equation (2).

Here, since n _ey (θ _1y ) >> Δn _ey (x, y, θ _1y ), L ₀ >> ΔL (x, y), Δn _ey (x, y, θ _1y ). The (x, y) term can be ignored. Φ _Z = n _eZ (θ _1Z ).
L ₀ , φ _Y = _{ne y} (θ _2Y ). Assuming L _0, from Equation (1) and Equation (2)

Using equation 3, equation 1 is transformed into equation 4.

here

The variable differential coefficient in Equation 6 is calculated using Equation 5.

Calculated as in Equation 7. Therefore, the first term on the left side of Equation 4 is

It becomes. On the other hand, to calculate the second term on the left side of Equation 4

Using

Therefore, using Equation 10, the second term on the left side of Equation 4 is

It is expressed by Formula 11 as follows. Therefore, Expression 4 can be finally expressed as Expression 12.

Here we define the following functions:

Using the function of Equation 13 and the normalized variables X, Y, and Z of Equation 14, Equation 12 can be expressed as a simple linear equation of Equation 15.

Next, as shown in FIG. 4A, the tilt angle is set to θ ₂ and the polarization axis of the P wave and the z axis of the crystal are set to coincide with each other. Let the phase difference distribution observed at this time be ΔZ ₂ (x, y). Further, as shown in FIG. 4B, the sample is rotated by 90 ° so that the polarization direction coincides with the y-axis direction of the crystal. Assuming that the phase difference distribution ΔY ₂ (x, y) observed at this time is obtained, Expression 16 is obtained in the same manner as Expression 15.

As further shown in FIG. 5 (a), set in the direction of z-axis of the polarization axis and the crystal matches the set by the inclination angle theta ₃ P-wave. The phase difference distribution observed at this time is represented by ΔZ ₃ (x, y). Further, as shown in FIG. 5B, the sample is rotated by 90 ° so that the polarization direction coincides with the y-axis direction of the crystal. Assuming that the phase difference distribution ΔY ₃ (x, y) observed at this time is obtained, Expression 17 is obtained in the same manner as Expression 15.

It can be seen that Equations 15, 16, and 17 are non-said linear simultaneous equations with respect to variables X, Y, and Z. Therefore, it can be solved uniquely with a simple determinant. Now, if you define a matrix like equation 18,

The simultaneous equations of Expression 15 to Expression 17 are expressed by one determinant of Expression 19.

The variable Ω can be solved by applying an inverse matrix M ⁻¹ of the matrix M from the left.

The unknown Ω is determined by Equation 20. That is, all values of the variables X = Δn _X / n _X , Y = Δn _Y / n _Y, and Z = Δn _Z / n _Z can be obtained independently. Further, an arbitrary birefringence distribution Δn _X −Δn _Y, Δn _Y −Δn _Z, Δn _Z −Δn _X, etc. can be obtained from these values.

  Although the measurement principle has been described above, an actual measurement apparatus and calculation procedure will be described using an X-cut LN as an example. As shown in FIG. 6, a so-called interferometer 1 is configured to measure the sample transmission phase. The interferometer includes a laser light source unit 30, a partial reflection mirror 20, and an interference unit that causes light reflected by the reflection mirror 3 to return to the interferometer through the sample 10 disposed in the sample unit. The phase of the sample can be calculated from the movement of the interference ring by slightly changing the optical length to the reference mirror with a piezo element or the like attached to the reference mirror.

As shown in FIG. 3 (a), the sample portion is tilted by θ ₁ degree and arranged so that the P-polarized light coincides with the z-axis direction of the crystal. The transmitted wavefront image ΔZ ₁ (x, y) obtained at this time has an elliptical shape shortened in the z-axis direction as shown in FIG.

On the other hand, the sample is rotated by 90 ° so that the P-polarized light direction coincides with the y-axis of the sample (FIG. 3B). The transmitted wavefront image ΔY ′ ₁ (x, y) (FIG. 7B) obtained at this time has an elliptical shape shortened in the y-axis direction of the crystal. It is necessary to devise to calculate the two images having different short axis directions by using Expression 20. Both images are enlarged in the minor axis direction and converted into circular images having the same ratio in the y and z axis directions (FIGS. 8A and 8B). In this state, since the axial directions of both images are orthogonal, one of the images (ΔY ′ ₁ (x, y) image in the explanation) is reversely rotated by 90 ° so that the coordinate axis directions of both images coincide (FIG. 8). (C)). The images ΔZ ₁ (x, y) (FIG. 8 (a)) and ΔY ₁ (x, y) (FIG. 8 (c)) thus converted are converted into new ΔZ ₁ (x, y), ΔY ₁ (x, y ) Image.

Next, the sample portion tilts the sample by θ ₂ degrees as shown in FIG. ΔZ ₂ (x, y) and ΔY ₂ (x, y) images are obtained by the same procedure as that for θ ₁ . Further, as shown in FIG. 5A, the sample portion tilts the sample to θ ₃ degrees. ΔZ ₃ (x, y) and ΔY ₃ (x, y) images are obtained by the same procedure as in the case of θ ₁ .
The images ΔZ ₁ (x, y), ΔY ₁ (x, y), ΔZ ₂ (x, y), ΔY ₂ (x, y), ΔZ ₃ (x, y), ΔY ₃ thus obtained. (x, y) in Equation 13

By substituting in the right side of Equation 19

Is determined. Solving the determinant 19 at all the coordinate positions x and y, it is possible to derive unknowns X = Δn _X / n _X , Y = Δn _Y / n _Y ,, _Z = Δn _Z / n _Z at each point. I can do it.

As described above, in the case of an X-cut LiNbO ₃ wafer, the sample is placed at an angle, and the sample is rotated 90 ° with P polarization, and the optical path length ΔZ (x, It was found that the refractive index distribution Δn _z (x, y) can be measured from Equation 20 by measuring y) and ΔY (x, y) with an optical interferometer or the like.

By performing such an operation, it was possible to accurately calculate a slight refractive index distribution of about 1/10000 that was hidden behind the wavefront distortion due to the thickness distribution ΔL (x, y) and was not visible.
In addition, since only P-polarized light is used, it is possible to measure accurately without the influence of interference patterns due to multiple interference on the front and back of the wafer. The angles θ ₁ , θ ₂ , and θ ₃ are desirably set near the Brewster angle θ _B. This is because if the angle deviates greatly from the Brewster angle, the reflectivity increases and the influence of multiple interference cannot be ignored. Here, the inclination angle θ ₁ = 64 degrees, θ ₂ = 66 degrees, and θ ₃ = 68 degrees with respect to the Brewster angle θ _B = 66 degrees.

  In order to further improve accuracy, a method for solving the following problem is desirable. First, when there is distortion or inclination (tilt component) in the transmitted wavefront of the interferometer body when there is no sample, this bias component is superimposed on the transmitted wavefront when the sample is passed. Therefore, the same slope component is also superimposed on FIGS. 8 (a) and 8 (b). As shown in FIG. 8 (c), this tilt component is also rotated by 90 ° in order to rotate one image by 90 degrees. Therefore, even if the bias component is superimposed by the same amount, the refractive index of Observed as a slope component.

  Second, in order to measure by rotating the sample by 90 °, even if calculation is performed using the observed phase on the same y and z coordinates, the optical path is not the same and the y and z coordinates are the same. The phase difference passing through the optical path rotated by 90 ° is used. For this reason, when the sample thickness has a tilt or large thickness unevenness, the effect of subtracting the sample thickness unevenness is weak, and as a result, the calculated refractive index distribution has a tilt component and apparent nonuniformity.

In order to improve these two points, the sample is rotated in 90 ° steps as shown in FIG. 9, and four transmitted wavefronts are measured (here, + Z image (FIG. 9 (a), + Y image (FIG. 9 9 (b), -Z image (referred to as Fig. 9 (c), -Y image (Fig. 9 (d))) -Z image is rotated 180 ° and added to + Z image. Rotate the image 180 ° and add it to the + Y image, where the added images are 2Z images (Fig. 9 (e)) and 2Y images (Fig. 9 (f)). The transmitted wavefront distribution of 2Z is ΔZ ₁ (x, y) (FIG. 9C), and the transmitted wavefront distribution of 2Y is ΔY ₁ (x, y) (FIG. 9 ( f)), the refractive index profile can be calculated from Equation 20. It should be noted that the sample thickness at this time is doubled, and by using 2Z and 2Y images, the wafer thickness direction can be calculated. The effect of rotating the optical path by 90 ° was spatially averaged and could be mitigated by calculating using the four transmitted wavefronts of the festival that rotated the sample by 90 ° as described above. As a result, the spatial resolution is slightly deteriorated, but only the refractive index distribution can be measured accurately without being affected by the bias component of the interferometer.

As described above, according to the present invention, it is possible to provide a measurement method for quickly measuring a refractive index distribution depending on a composition ratio in a crystal such as a LiNbO ₃ crystal whose refractive index changes depending on the composition ratio. it can.

As a first embodiment, FIG. 10 shows a case where a refractive index profile is measured using an X-cut 5 inch LiNbO ₃ wafer (thickness: 1000 μm). 10A is a front view of the measurement system, and FIG. 10B is a side view. In order to measure the transmitted wavefront distortion (optical path length), an interferometer (GPI-XP) manufactured by ZYGO was used. The reflection mirror 3 is mounted on a mirror mount (not shown) with tilt adjustment, and the tilt can be adjusted with a tilt adjustment screw. These optical systems are arranged on a classic anti-vibration optical system, and the whole is arranged in a clean bench controlled at a temperature of 0.1 ° C.

  First, a 6-inch diameter He—Ne laser beam is emitted from the interferometer body 1. The emitted light propagates on the optical axis 12, is reflected by the reflecting mirror 3, and returns to the interferometer body through the same optical axis 12. Adjust the tilt of the reflecting mirror without a sample.

Next, as shown in FIG. 10A, the X-cut LiNbO ₃ wafer crystal is set so that the x-axis of the crystal is 60 ° with respect to the optical axis 12. In this state, the transmission wavefront distortion ΔY ₁ (x, y) is measured with the P-wave polarization direction and the crystal y-axis aligned. The acquired data is once recorded in a storage device of a personal computer (not shown).

Next, as shown in FIG. 10 (b), the wafer 10 is rotated 90 degrees around the sample x axis on the sample stage 8, and the interferometer body P wave polarization direction and the crystal z axis are aligned and transmitted. The wavefront distortion ΔZ ₁ (x, y) is measured. The acquired data is once recorded in a storage device on a personal computer (not shown).

Next, the crystal is tilted and set so that the x-axis is at an angle of 64 degrees with respect to the optical axis 12. In this state, the transmission wavefront distortion ΔY ₂ (x, y) is measured with the P-wave polarization direction and the crystal y-axis aligned. The acquired data is once recorded in a storage device of a personal computer (not shown).

Further, the wafer 10 is rotated 90 degrees on the sample stage 8 around the sample x axis, and is arranged so that the P wave polarization direction of the interferometer body and the crystal z axis coincide with each other, and the transmitted wavefront distortion ΔZ ₂ (x, y) is set. measure. The acquired data is once recorded in a storage device on a personal computer (not shown).

Next, the crystal is set so that the x-axis is inclined at an angle of 68 degrees with respect to the optical axis 12. In this state, the transmission wavefront distortion ΔY ₃ (x, y) is measured with the P wave polarization direction and the crystal y axis aligned. The acquired data is once recorded in a storage device of a personal computer (not shown).

Further, the wafer 10 is rotated 90 degrees around the sample x axis on the sample stage 8 and arranged so that the P wave polarization direction of the interferometer body and the crystal z axis coincide with each other, and the transmitted wavefront distortion ΔZ ₃ (x, y) is set. measure. The acquired data is once recorded in a storage device on a personal computer (not shown).

After the measurement, the P wave Z axis transmitted wavefront distortions ΔZ ₁ (x, y), ΔZ ₂ (x, y), ΔZ ₃ (x, y) and the P wave Y axis transmitted wave front ΔY measured at different inclination angles. _{1 (x, y), ΔY} 2 (x, y), ΔY 3 (x, y) read from the storage area of the personal computer, the distribution Δn _z (x, y) of the refractive index n _z in accordance with formula 20 calculations went. FIG. 11 shows the refractive index distribution Δnz at the center of the 4-inch x-cut LN. The marking area is 4 cm square. The maximum Δn _z (x, y) amount is about 0.00002, and it can be seen that the refractive index distribution with the fifth decimal point can be extracted. As can be seen from this, it was found that the refractive index distribution of the Δnz component can be accurately measured without being affected by the thickness of the cloth wafer by eliminating the thickness distribution.

In the above embodiment, the case where a LiNbO ₃ crystal is used as a uniaxial crystal has been described. However, the refractive index of other biaxial crystals such as LiTaO ₃ and KLN, which are not limited to this, is not limited thereto. It is clear that it may be used for measurement.

  In the present invention, the refractive index distribution can be measured at the time of development or manufacture of an optical crystal having various functions, and the quality can be easily controlled.

Optical axis diagram at oblique incidence Incident angle dependence of LN reflectivity (A) Measurement layout when the crystal's z-axis and the polarization direction are parallel and incident angle θ ₁ (b) Measurement layout when the crystal's y-axis and the polarization direction are parallel and incident angle θ ₁ (A) Measurement layout when the crystal's z-axis is parallel to the polarization direction and the incident angle θ ₂ · Measurement layout when the crystal's y-axis is parallel to the polarization direction and the incident angle θ ₂ (A) Measurement layout when the crystal's z-axis and the polarization direction are parallel and incident angle θ ₃ (b) Measurement layout when the crystal's y-axis and the polarization direction are parallel and incident angle θ ₃ Configuration diagram of an interferometer for measuring the sample transmission phase (A) Measured ΔZ (x, y) image (b) Measured ΔY (x, y) image (A) ΔZ (x, y) image corrected to a circle (b) ΔY ′ (x, y) image corrected to a circle (c) ΔY (x, y) image rotated 90 ° (A) + Z image (b) + Y image (c) −Z image (d) −Y image (e) —2Z image obtained by rotating the Z image by 180 ° and adding it to the + Z image (f) —180 Y image ° 2Y image added to + Y image after rotating (A) Front view of measuring device (b) Side view of measuring device Refractive index distribution of measured X-cut LN Δnz (x, y)

Explanation of symbols

1 ... Interferometer 3 ... Reflector 7 ... Rotary stage 8 ... Sample stage 10 ... Sample 12 ... Optical axis 20 ... Partial reflector 30 ... Light source

Claims (3)

In a refractive index distribution measurement method by transmitted wavefront measurement, in a sample having two main surfaces facing each other where light can enter, the vertical direction of the main surface is at the first, second, and third inclination angles with respect to the optical axis. The first transmitted wavefront when the P-polarized light is aligned with the first main axis of the measurement sample, and the second transmitted wavefront when the P-polarized light is aligned with the second main axis orthogonal to the first main axis And measuring the refractive index distribution of the first, second, and third principal axis polarizations based on these measurements. 2. The refractive index distribution measuring method according to claim 1, wherein the first, second, and third inclination angles are within a Brewster angle of ± 5 °. Means for separating light generated from the light source unit into reference light and measurement light and allowing the measurement light to pass through the test object; and an image acquisition unit for interfering the measurement that has passed through the test object with the reference light and capturing an interference image; A refractive index distribution measuring apparatus including an arithmetic unit for analyzing an acquired image, wherein the refractive index distribution of the test object is calculated using the measurement method according to claim 1 or 2. Rate distribution measuring device.