Method for solving optical crosstalk in dual-wavelength digital holography

Abstract

In order to solve the problem of optical crosstalk caused by low transmissivity of a common light splitting device in dual-wavelength digital holography, the invention provides a method for solving the optical crosstalk in the dual-wavelength digital holography, which is mainly characterized in that the sizes of light spots with different wavelengths are adjusted, and the position of reference light is adjusted while the size and the direction of carrier frequency are adjusted, so that a crosstalk area is avoided. According to the invention, a crosstalk area can be ingeniously avoided, the device is not limited by the transmissivity of light splitting devices such as a polarization splitting prism, a dichroscope and a filter, and the problem of light wave crosstalk caused by the light splitting device with any transmissivity in dual-wavelength digital holography can be effectively solved.

Description

Technical Field

[0001] The invention relates to a method for solving optical crosstalk in dual-wavelength digital holography, and is mainly applied to optical systems involving dual-wavelength multiplexing, such as dual-wavelength digital holography. Background Art

[0002] Digital holography is an effective means to achieve quantitative phase measurement of objects. It has the advantages of full-field, fast, high-precision, and non-contact. It is widely used in the fields of surface morphology measurement and biological cell imaging. This technology forms an object light wave by modulating the measurement light of the object to be measured, and interferes with another reference light wave without sample modulation. Through the phase extraction algorithm, the phase information is extracted from one or more interference patterns recorded by the digital image sensor. In single-wavelength digital holography, when the optical path difference generated by the measurement light passing through the sample exceeds the wavelength of the measurement light, the reconstructed object phase distribution is folded between [-π, π]. The extracted wrapped phase needs to be unfolded using a numerical unwrapping algorithm to obtain the true phase of the object. However, when the object to be measured has a high aspect ratio structure, the surface morphology contains steps, and the surface roughness is high, the numerical unwrapping algorithm may cause large errors, resulting in unwrapping failure.

[0003] To solve this problem, researchers proposed dual-wavelength digital holography technology, which uses two different wavelengths of light to obtain the corresponding wavelength's wrapped phase, and subtracts the two to obtain the equivalent wavelength phase diagram. Since the equivalent wavelength is greater than any wavelength introduced for measurement, and the closer the two wavelengths are, the larger their equivalent wavelength is. Therefore, the measurement range of single-wavelength digital holography technology can be expanded from a few microns to millimeters.

[0004] Dual-wavelength digital holography introduces one more light wave than single-wavelength digital holography. Currently, researchers mostly use wavelength multiplexing technology based on the structure of Michelson interferometer or Mach-Zehnder interferometer, and use a single image sensor to simultaneously record interference patterns of different wavelengths, which can realize real-time dynamic measurement. This type of wavelength multiplexing technology often uses polarization beam splitters, dichroic mirrors or filters and other beam splitters in the reference light path with plane reflectors to adjust the size and direction of the carrier frequency of different wavelengths. However, due to the low transmittance of general polarization beam splitters, dichroic mirrors or filters, part of the light that originally needs to be fully transmitted is reflected, resulting in optical crosstalk problems. Existing methods use high-transmittance beam splitters to avoid optical crosstalk problems, but the production and processing technology of high-transmittance beam splitters is difficult and costly. Therefore, there is an urgent need for a method that can solve the optical wave crosstalk problem caused by beam splitters with arbitrary transmittance in dual-wavelength digital holography. Summary of the invention

[0005] 1. Purpose of the invention

[0006] In order to solve the optical crosstalk problem caused by the low transmittance of general splitter devices in dual-wavelength digital holography, the present invention proposes a method for solving the optical crosstalk in dual-wavelength digital holography, which is mainly achieved by adjusting the size of light spots of different wavelengths and adjusting the reference light position while adjusting the size and direction of the carrier frequency to avoid the crosstalk area.

[0007] 2. Technical solution

[0008] Dual-wavelength digital holography technology usually uses two lasers to emit a first wavelength laser and a second wavelength laser respectively. Before the first wavelength laser and the second wavelength laser are multiplexed, a beam expander or other optical device capable of adjusting the spot size is used to adjust the spot size of the first wavelength laser and the second wavelength laser respectively. The first beam expander is used to adjust the spot radius of the first wavelength laser to r1, and the second beam expander is used to adjust the spot radius of the second wavelength laser to r2, where r1>r2. The first wavelength laser and the second wavelength laser are then coupled (the centers of the spots coincide) for transmission, and the reference light (the first wavelength reference light and the second wavelength reference light) and the object light (the first wavelength object light and the second wavelength object light) are separated by a first beam splitter (such as a polarization beam splitter prism, a non-polarization beam splitter prism, etc.).

[0009] In the reference light path, a second beam splitter (such as a filter, a dichroic mirror, a polarization beam splitter prism, etc.) is used in combination with a first plane reflector to adjust the propagation directions of the first wavelength reference light and the second wavelength reference light respectively, so as to adjust the magnitude and direction of the carrier frequency of light waves of different wavelengths. In this adjustment process, the spot positions of the first wavelength reference light and the second wavelength reference light at the image receiving surface also change accordingly.

[0010] The specific adjustment process is as follows: When the first wavelength reference light and the second wavelength reference light are transmitted to the second spectrometer at the same time, since the reflectivity of the general spectrometer can be close to 1, the first wavelength reference light is basically reflected by the second spectrometer. However, the transmittance of the general spectrometer is not high, so a part of the second wavelength reference light (the specific proportion depends on the transmittance of the spectrometer, reflection ratio = 1-transmission ratio) is transmitted by the second spectrometer, and the other part is reflected together with the first wavelength reference light. Therefore, the center area of ​​the spot of the first wavelength reference light (radius r1) carries the spot of the second wavelength reference light (radius r2) caused by the low transmittance of the second spectrometer. This area is the overlapping crosstalk area of ​​the two light waves, and the area outside this area is the crosstalk-free area of ​​the first wavelength reference light. At this time, the propagation direction of the first wavelength reference light reflected by the second beam splitter is adjusted by adjusting the second beam splitter, so that the crosstalk-free area of ​​the first wavelength reference light is received by the image sensor (the crosstalk area is outside the receiving area of ​​the image sensor), and at the same time, it forms a certain angle with the first wavelength object light in the y direction, thereby generating interference fringes; the part of the second wavelength reference light transmitted through the first beam splitter is reflected by the first plane reflector and then transmitted through the second beam splitter again, and the propagation direction of the second wavelength reference light is adjusted by adjusting the first plane reflector, so that the light spot of the second wavelength reference light is received by the image sensor, and at the same time, it forms a certain angle with the second wavelength object light in the x direction (the x direction is orthogonal to the y direction), thereby generating interference fringes.

[0011] 3. Technical Effect

[0012] In this way, the image sensor can simultaneously receive the off-axis interference pattern of the first wavelength laser and the second wavelength laser, and the interference fringes of the first wavelength laser and the second wavelength laser are orthogonally distributed and have no crosstalk. The Fourier transform can easily extract the phase information of the object at different wavelengths.

[0013] The proposed method can cleverly avoid the crosstalk area and is not limited by the transmittance of splitter devices such as polarization splitter prisms, dichroic mirrors, and filters. It can effectively solve the light wave crosstalk problem caused by splitter devices with arbitrary transmittance in dual-wavelength digital holography.

[0014] The present invention also provides a Mach-Zehnder interferometer, which adopts the above method to solve the optical crosstalk problem in dual-wavelength digital holography.

[0015] The present invention also provides a Michelson interferometer, which adopts the above method to solve the optical crosstalk problem in dual-wavelength digital holography. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly embody and understand the present invention, a detailed description is given below with reference to the accompanying drawings.

[0017] Figure 1 Schematic diagram of the position and size of each light spot on the image receiving surface;

[0018] Figure 2 A schematic diagram of the angles between the propagation directions of the object light of the first wavelength and the second wavelength and the reference light of the first wavelength and the second wavelength when propagating to the receiving surface of the image sensor;

[0019] Figure 3 A schematic structural diagram of an embodiment of the present invention for a Mach-Zehnder interferometer;

[0020] Figure 4 It is a schematic structural diagram of an embodiment of the present invention used for Michelson interferometer. DETAILED DESCRIPTION

[0021] Example 1

[0022] like Figure 3 As shown, an embodiment device of the present invention for dual-wavelength digital holography based on Mach-Zehnder interferometer includes: 101 a first wavelength laser, 102 a second wavelength laser, 103 a first polarizer, 104 a second polarizer, 105 a first beam expander, 106 a second beam expander, 107 a polarization beam splitter prism, 108 a long-wavelength pass dichroic mirror, 109 a first plane reflector, 110 a second plane reflector, 111 a sample to be measured, 112 an objective lens, 113 a tube lens, 114 a non-polarization beam splitter prism, 115 a third polarizer, and 116 a monochrome image sensor.

[0023] The first wavelength laser 101 and the second wavelength laser 102 output the first wavelength laser (wavelength of 532nm) and the second wavelength laser (wavelength of 632.8nm) respectively. After the two wavelengths of laser light pass through the polarizer and the beam expander, the polarization beam splitter prism 107 separates two paths of light: one path of light is reflected by the second plane reflector 110, modulated by the sample to be measured 111 to form an object light wave, collected by the infinite imaging objective lens 112 and converged by the sleeve lens 113, and finally the object light wave enters the monochrome image sensor 116 vertically along the -z direction; the other path of light is used as reference light. By rotating the first polarizer 103 and the second polarizer 104, the distribution ratio of the object light and the reference light of the two wavelengths can be adjusted respectively.

[0024] In actual use, a first beam expander 105 with a magnification of 10 times can be used to magnify the light spot of the first wavelength laser by 10 times, and a second beam expander 106 with a magnification of 3 times can be used to magnify the light spot of the second wavelength laser by 3 times. The reflection cutoff / transmission starting wavelength of the long-wave pass dichroic mirror 108 used is 605nm.

[0025] In the reference light path, the first wavelength reference light is reflected by the long-wave pass dichroic mirror 108, and most of the second wavelength reference light is transmitted by the long-wave pass dichroic mirror 108. At this time, since the transmittance of a general long-wave pass dichroic mirror is not high, a small portion (the specific proportion depends on the transmittance of the long-wave pass dichroic mirror) of the second wavelength reference light is reflected by the long-wave pass dichroic mirror 108, and forms crosstalk with the overlapping area of ​​the first wavelength reference light. At this time, the long-wave pass dichroic mirror 108 is adjusted to adjust the propagation direction of the first wavelength reference light, so that after the first wavelength reference light is reflected by the non-polarized beam splitter prism 114, the monochromatic image sensor 116 only receives the non-crosstalk area of ​​the first wavelength reference light, and the crosstalk area is outside the receiving surface of the monochromatic image sensor 116, and the first wavelength reference light and the first wavelength object light form a certain angle in the x direction, generating interference fringes along the x direction. In addition, the propagation direction of the second wavelength reference light is adjusted by adjusting the first plane reflector 109. After being reflected by the first plane reflector 109, the second wavelength reference light is transmitted through the long-wavelength dichroic mirror 108 again, and then reflected by the non-polarized beam splitter prism 114, and received by the monochrome image sensor 116, and forms a certain angle with the second wavelength object light in the y direction, generating interference fringes along the y direction. The light transmission axis of the third polarizer 115 is placed at an angle of 45° to the y direction, and the monochrome image sensor 116 simultaneously collects off-axis interference patterns of two orthogonal interference fringes. In this way, the Fourier transform can easily extract the phase information of objects at different wavelengths.

[0026] Example 2

[0027] like Figure 4 As shown, an embodiment device of the present invention for dual-wavelength digital holography based on Michelson interferometer includes: 201 a first wavelength laser, 202 a second wavelength laser, 203 a first collimating beam expander, 204 a second collimating beam expander, 205 a first non-polarizing beam splitter prism, 206 a second non-polarizing beam splitter prism, 207 a first plane reflector, 208 a long-wave pass filter, 209 a second plane reflector, and 210 a monochrome image sensor.

[0028] The first wavelength laser 201 and the second wavelength laser 202 output the first wavelength laser (wavelength of 532nm) and the second wavelength laser (wavelength of 632.8nm) respectively. The first collimating beam expander 203 with a magnification of 10 times is used to magnify the light spot of the first wavelength laser by 10 times, and the second collimating beam expander 204 with a magnification of 3 times is used to magnify the light spot of the second wavelength laser by 3 times. The two wavelength lights after collimation and amplification are combined and coupled by the first non-polarizing beam splitter prism 205, and then separated into two paths of light by the second non-polarizing beam splitter prism 206 (transmission and reflection ratio is 1:1), one of which is the object light path. The first and second wavelength object lights are both reflected by the first plane reflector 207, and then transmitted by the second non-polarizing beam splitter prism 206, and enter the monochrome image sensor 210 vertically along the negative direction of the z-axis. The other path is the reference light path. When the first and second wavelength reference lights are transmitted to the long-wave pass filter 208, the first wavelength reference light is completely reflected, and a part of the second wavelength reference light is reflected. The reflected part overlaps with the central area of ​​the first wavelength reference light to generate crosstalk. At this time, the long-wave pass filter 208 is adjusted so that after the first wavelength reference light is reflected by the second non-polarized beam splitter prism 206, the monochrome image sensor 210 only receives the non-crosstalk area of ​​the first wavelength reference light. The crosstalk area is outside the receiving surface of the monochrome image sensor 210, and the first wavelength reference light and the first wavelength object light form a certain angle in the x direction, generating interference fringes along the x direction; the second plane reflector 209 is adjusted so that the second wavelength reference light is received by the monochrome image sensor 210, and forms a certain angle with the second wavelength object light in the y direction, generating interference fringes along the y direction. Similarly, the spectrum of different wavelengths can be obtained by Fourier transform.

[0029] The above embodiments are only to illustrate several implementation modes of the present invention in detail, but the protection scope of the present invention is not limited thereto. Within the technical scope of the present invention, other technicians familiar with the art can make local adjustments in different ways without departing from the principle and purpose of the present invention, and these all belong to the protection scope of the present invention.

Claims

1. A method for solving optical crosstalk in dual-wavelength digital holography, characterized in that: The steps include: Before the first wavelength laser and the second wavelength laser are multiplexed, the spot sizes of the first wavelength laser and the second wavelength laser are adjusted respectively, so that the spot radius of the first wavelength laser is larger than the spot radius of the second wavelength laser; The adjusted first wavelength laser and second wavelength laser are coupled and transmitted, and reference light and object light are respectively separated by a first beam splitter, wherein the reference light includes a first wavelength reference light and a second wavelength reference light, and the object light includes a first wavelength object light and a second wavelength object light; Using a second beam splitter and a first plane reflector in the reference light path to adjust the propagation directions of the first wavelength reference light and the second wavelength reference light respectively; When the first wavelength reference light and the second wavelength reference light are simultaneously transmitted to the second beam splitter, the propagation direction of the first wavelength reference light reflected by the second beam splitter is adjusted by adjusting the second beam splitter, so that the crosstalk-free area of ​​the first wavelength reference light is received by the image sensor and forms a certain angle with the first wavelength object light in the y direction, thereby generating interference fringes; the part of the second wavelength reference light transmitted through the first beam splitter is reflected by the first plane reflector and then transmitted through the second beam splitter again, and the propagation direction of the second wavelength reference light is adjusted by adjusting the first plane reflector, so that the light spot of the second wavelength reference light is received by the image sensor and forms a certain angle with the second wavelength object light in the x direction, thereby generating interference fringes; An image sensor is used to simultaneously receive off-axis interference patterns of a first wavelength laser and a second wavelength laser. The interference fringes of the first wavelength laser and the second wavelength laser are orthogonally distributed and have no crosstalk. Then, Fourier transform is used to extract phase information of the object at different wavelengths.

2. A method for solving optical crosstalk in dual-wavelength digital holography according to claim 1, characterized in that: The spot sizes of the first wavelength laser and the second wavelength laser are adjusted respectively using a beam expander or other optical devices capable of adjusting the spot size.

3. The method for solving optical crosstalk in dual-wavelength digital holography according to claim 1, characterized in that: The first beam splitter is a polarization beam splitter prism or a non-polarization beam splitter prism; the second beam splitter is a filter, a dichroic mirror or a polarization beam splitter prism.

4. A Mach-Zehnder interferometer, characterized in that: The method described in any one of claims 1 to 3 is used to solve the optical crosstalk problem in dual-wavelength digital holography.

5. A Michelson interferometer, characterized in that: The method described in any one of claims 1 to 3 is used to solve the optical crosstalk problem in dual-wavelength digital holography.

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