RU2727783C1 - Transparent samples refraction index distribution measurement device - Google Patents

Abstract

FIELD: measurement.SUBSTANCE: invention relates to measurement equipment, namely to devices for three-dimensional analysis of refraction index of material using interferometry-based optical devices, and can be used for tomographic inspection of samples of optical products: optical fibers and their workpieces, gradient lenses, various optics and microelectronics products, including those obtained by additive techniques from polymer and other transparent materials. Device for measuring distribution of refraction index of transparent samples has a coherent light source, a Mach–Zehnder interferometer, a recording system and a test sample mounting unit. Interferometer is equipped with an input phase-shifting unit and a beam splitter forming the reference and measuring channels. Test sample installation unit is located in the measuring channel and contains a cell with an immersion liquid and a rotary device. Device is also provided with a similar reference sample installation unit located in the support channel and containing a cell with an immersion liquid and a rotary device.EFFECT: invention increases accuracy and range of measurements.4 cl, 3 dwg

Description

The invention relates to measuring technology, namely to devices for three-dimensional analysis of the refractive index of a material using optical means based on interferometry, and can be used for tomographic control of samples of optical products: optical fibers and their blanks, gradient lenses, various products of optics and microelectronics, including those obtained by the method of additive technologies from polymer and other transparent materials. The device makes it possible to detect the presence of internal defects in the investigated products, as a rule, characterized by material inhomogeneity caused by a local change in the refractive index. Such defects affect the image quality of these optical products and the transmitted optical signals. For this reason, it is important to control the inhomogeneity of lens material and other optical elements.

М. (2017). Holographic tomography with object rotation and two-directional off-axis illumination. Optics Express, 25(20), 23920. doi:10.1364/oe.25.023920). Известный метод, в отличие от традиционных одноракурсных, повышает пространственное разрешение измерения показателя преломления и позволяет компенсировать ошибки, связанные с расфокусировкой в связи со смещением оси вращения и оси образца. Таким образом, в данном устройстве сочетаются преимущества двух методов: вращения образца и его двухракурсного освещения.A holographic device for tomography is known from the prior art, in which the sample under study rotates while illuminating it from two angles (see Kostencka, J., Kozacki, T.,

M. (2017). Holographic tomography with object rotation and two-directional off-axis illumination. Optics Express, 25 (20), 23920.doi: 10.1364 / oe.25.023920). The known method, in contrast to traditional single-angle ones, increases the spatial resolution of the refractive index measurement and makes it possible to compensate for errors associated with defocusing due to the displacement of the rotation axis and the sample axis. Thus, this device combines the advantages of two methods: rotation of the sample and its two-angle illumination.

An apparatus is known from the prior art, which uses a scheme of a two-beam Mach-Zehnder interferometer and a device for rotating a sample in a cuvette (see Yunseong Jeon, Yunseong Jeon, Chung-Ki Hong, Chung-Ki Hong, "Rotation error correction by numerical focus adjustment in tomographic phase microscopy, "Optical Engineering 48 (10), 105801 (October 1, 2009). https://doi.org/10.1117/1.3242833).

A device for measuring the wavefront of light beams based on a Mach-Zehnder interferometer using an inclined plate is also known from the prior art (see CN1270159C, class G01B 9/02, publ. 16.08.2006). This setup is used for high-precision determination of the wavelength of semiconductor lasers.

The above-described devices and methods, as a rule, have one measuring channel and implement only a direct measurement scheme. This allows examining samples with a uniform or smooth change in the refractive index. However, for samples such as a gradient lens, in which the refractive index in the cross section has sharp changes (large gradient), these methods are not suitable. For this, a relative measurement method must be used.

The prior art discloses a method for spectrometric investigation of the refractive index of a sample by comparing a difference signal with a predetermined one, by changing the phase between the measuring and reference beams, as well as a corresponding device that has two measuring channels located in different arms of the Mach-Zehnder interferometer (see. publication WO 2019018870 A1, class G01N 21/45, publ. 31.01.2019). However, in the process of measurements, phase reconstruction algorithms are not used and, accordingly, the spatial distribution of the phase is not restored. This scheme is used to determine the integral refractive index of transparent substances and is not suitable for determining the spatial distribution of the refractive index (RFI) in the investigated volume.

The closest in technical essence to the proposed invention is a device for measuring the distribution of the refractive index of transparent samples, containing a source of coherent illumination, a Mach-Zehnder interferometer with an input beam splitter forming the reference and measuring channels, and a phase-shifting unit, a registration system and a test sample installation unit located in measuring channel and containing a cuvette with immersion liquid and a rotary device (see patent JPH08304229A, class G01N 21/41, publ. 22.11.1996). The known device is designed to measure the distribution of the refractive index of optical elements and allows you to minimize the influence of changes in environmental parameters, such as temperature, etc. The cuvette of the described device contains a hollow part for placing the controlled optical element, which is placed an immersion solution having the same refractive index (or close) as the refractive index of the measured sample. When measuring the refractive index distribution, the test sample is rotated around an axis perpendicular to the optical detection axis. Reconstruction of the PRPP is carried out by the known method of computed tomography. The main disadvantage of the known device, like the above-described single-channel devices, is the impossibility of using it for samples with a sharp change in the refractive index.

A technical problem is the elimination of the above disadvantages and the creation of a universal device that allows examining samples with a large gradient of the refractive index (for example, a gradient optical fiber), controlling transparent products both along the optical axis (cuts) and across (tomographic mode), and also minimizing influence of external factors (for example, temperature) on the results of high-precision measurements. The technical result consists in increasing the accuracy and range of measurements. The problem is solved, and the technical result is achieved by the fact that a device for measuring the distribution of the refractive index of transparent samples containing a source of coherent illumination, a Mach-Zehnder interferometer with an input beam splitter that forms the reference and measuring channels, and a phase-shifting unit, a registration system and a unit for installing a test sample, located in the measuring channel and containing a cuvette with immersion liquid and a rotary device, equipped with a block for setting a reference sample, located in the reference channel and containing a cuvette with immersion liquid and a rotary device. The phase-shifting unit is preferably made in the form of a movable mirror fixed on a piezoelectric element. The test sample installation unit and the reference sample installation unit can be equipped with slice holders with a passage window and equipped with displacement devices that provide the possibility of alternately installing the cuvette or the passage window of the holder on the optical axis of the corresponding channel. The proposed device can be equipped with an additional sliding measuring unit located between the source of coherent illumination and the input beam splitter and containing a cuvette with immersion liquid and a rotary device, and in the reference channel there is a rotary plane-parallel plate, the axis of rotation of which is perpendicular to the optical axis of the reference channel.

FIG. 1 shows the configuration of a device for tomography of a test sample in an immersion liquid cuvette;

in fig. 2 is a configuration for high-precision measurement of the refractive index of a thin section of a sample in a section holder;

in fig. 3 — Configuration for tomography of a test sample in a self-comparison mode (shear mode).

The proposed device for measuring the distribution of the refractive index of transparent samples contains a source 1 of coherent illumination and a Mach-Zehnder interferometer with an input beam splitter 2 forming the reference and measuring channels, a phase-shifting unit in the form of a movable mirror 3 on a piezoelectric element, an output beam splitter 4 and a rotary mirror 5. Interferograms are obtained with using a registration system that includes a focusing lens 6 and a video camera 7. A cube or a plane-parallel plate with a reflective coating can be used as beam splitters 2, 4.

In the measuring channel there is a unit 8 for installing a test sample, containing a rotary device 9 in the form of a stepper motor for installing and rotating an extended sample 10 and a transparent cuvette 11 with an immersion liquid (solution) 12. In the same side 8, a holder 13 of sections with a pass-through window. In this case, the block 8 is equipped with a displacement device, which makes it possible to alternately install the cuvette 11 or the passage window of the holder 13 on the optical axis of the measuring channel.

In the support channel there is a rotary plane-parallel plate 14, the axis of rotation of which is perpendicular to the optical axis of this channel. When installed perpendicular to the optical axis, the plate 14 does not change the geometry of measurements. In the shift mode of comparison with itself, the plate 14 is set at an angle to the optical axis, while using an additional sliding measuring unit 15, located between the source 1 of coherent illumination and the input beam splitter 2. Block 15 also contains a cuvette with immersion liquid and a rotary device.

In addition, in the reference channel there is a unit 16 for installing a reference extended sample 17, which similarly contains a rotary device and a transparent cuvette with immersion liquid. In block 16, a holder 18 of reference slices with a passage window can be installed. In this case, the block 16 is equipped with a displacement device that allows the alternate installation of the cuvette or the passage window of the holder 18 on the optical axis of the measuring channel.

The use of the Mach-Zehnder scheme makes it possible to perform absolute and relative measurements both along the optical axis of the sample and across it. The proposed design allows measurements in the following modes:

I Absolute measurements:

1) Along the optical axis (slices);

2) Across the optical axis (tomographic mode);

II Relative measurements (comparison with the reference):

1) Along the optical axis (slices);

2) Across the optical axis (tomographic mode);

III Relative measurements (comparison with itself, shear mode):

1) Along the optical axis (slices);

2) Across the optical axis (tomographic mode).

The proposed device works as follows.

Radiation from source 1 hits the beam splitter 2, which divides it into two beams. A beam expander can be used in the optical design. One beam is reflected from the movable mirror 3, passes through the unit 8 for installing the test sample and hits the second beam splitter 4. The second beam passes through the plane-parallel plate 14, unit 16 for installing the reference sample, is reflected from the mirror 5 and hits the beam splitter 4. After the beam splitter 4, both the beam pass through the lens 6 and fall on the matrix of the video camera 7, which registers their interference pattern. Lens 6 builds an image of the object on the matrix of the video camera 7, therefore blocks 8 and 16 are spaced at the same distance from the beam splitter 4, so that L1 = L2. Each of the two blocks 8, 16 is a movable stage that can move in the direction across the light beam. The table allows you to attach both the samples 10, 17 themselves, and their sections 13, 18.

The operation of the installation is based on the automatic interpretation of interferograms by the method of phase steps, which is implemented using a set of interferograms with different phase shifts. The device allows you to measure the spatial distribution of the refractive index (RFI) of the investigated products along and across the optical axis. In the first case, the radiation passes along the axis of the products, the thickness of which is known in advance. Using the method of phase steps, the optical path difference (OPD) of the radiation is measured, which is then normalized to the thickness of the product and the RPDD along the axis is restored. In this mode, sections of fibers, their blanks and gradient lenses are examined. In the second case, the radiation travels across the axis, and using the phase step method, the OPX is measured at a certain angle. In tomography, this is called a projection. Then the product is rotated at a given angle and the measurement is repeated. Based on a set of projections obtained in the angular range from 0 to 180 degrees using the tomographic reconstruction algorithm, the PRPP is reconstructed across the axis (in section). In this case, there is no need to destroy the investigated product in order to determine the RPP in its section.

The device allows both absolute and relative measurements. In the first case, the wavefront of radiation transmitted through the test sample is compared with a flat reference wavefront. In this mode, samples with a constant or smooth change in the refractive index are examined. In the second case, the wavefront of radiation that has passed through the test sample is compared with a reference wavefront formed using a reference sample located in the reference channel. This allows you to reduce the wavefront gradient due to the refractive index of the test sample by subtracting the reference wavefront from it. The device also allows making relative measurements in the mode of comparison of the wavefront passed through the test sample with a shifted copy of itself - the shift mode. In this mode, the sample is in front of the interferometer, which divides the original wavefront into two, one of which also has a transverse shift. In this case, it is not the OPH that is restored, but the derivative of it. Relative measurements make it possible to study samples with a large refractive index gradient.

The options for operating the unit in the mode of relative measurements are shown below.

FIG. 1 shows the configuration of the device for tomography of the test sample 10. In this configuration, the wavefront passing through the sample 10 is compared with the reference wavefront formed by the reference sample 17. In this case, the plane-parallel plate 14 is installed perpendicular to the incident beam, and an additional measuring unit 15 pushed out of the light propagation zone.

FIG. 2 shows the configuration of a device for high-precision measurement of the refractive index of a thin section 13 of a test sample. In this configuration, the distribution of the refractive index of the slice 13 of the test sample is compared with the slice 18 of the reference sample. In this case, the plane-parallel plate 14 is installed perpendicular to the beam incident on it. An additional measuring unit 15 is extended from the light propagation zone.

FIG. 3 depicts the configuration of an apparatus for tomography of a test sample in a self-comparison mode (shear mode). In this case, to create a shift of one wavefront relative to the other, the plane-parallel plate 14 is rotated by a small angle relative to the vertical axis. An additional measuring unit 15 is shifted into the light propagation zone in front of the beam splitter 2.

Thus, the proposed design makes it possible to carry out high-precision measurements in a wide range and to study various samples, including those with a large gradient of the refractive index along and across their optical axis, as well as to minimize the influence of external factors.

Claims (4)

1. A device for measuring the distribution of the refractive index of transparent samples, containing a source of coherent illumination, a Mach-Zehnder interferometer with an input beam splitter forming the reference and measuring channels, and a phase-shifting unit, a registration system and a test sample installation unit located in the measuring channel and containing a cuvette with an immersion liquid and a rotary device, characterized in that it is equipped with a block for setting the reference sample, located in the reference channel and containing a cuvette with immersion liquid and a rotary device. 2. The device according to claim 1, characterized in that the phase-shifting unit is made in the form of a movable mirror fixed on the piezoelectric element. 3. The device according to claim 1, characterized in that the test sample installation unit and the reference sample installation unit are equipped with section holders with a passage window and are equipped with displacement devices that provide the possibility of alternately installing the cuvette or the passage window of the holder on the optical axis of the corresponding channel. 4. The device according to claim 1, characterized in that it is equipped with an additional movable measuring unit located between the source of coherent illumination and the input beam splitter and containing a cuvette with immersion liquid and a rotary device, and in the reference channel is a rotary plane-parallel plate, the axis of rotation of which is perpendicular to the optical the axis of the reference channel.

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