A method for solving light crosstalk in dual-wavelength digital holography

Abstract

In order to solve the light crosstalk problem caused by the low transmittance of general light splitting device in dual-wavelength digital holography, the application provides a method for solving the light crosstalk in dual-wavelength digital holography, which mainly adjusts the spot size of different wavelengths, adjusts the position of reference light while adjusting the size and direction of carrier frequency, and avoids the crosstalk area. The application can skillfully avoid the crosstalk area, is not limited by the transmittance of polarization splitting prism, dichroic mirror, filter and other light splitting devices, and can effectively solve the light wave crosstalk problem caused by the light splitting device with arbitrary transmittance in dual-wavelength digital holography.

Description

Technical Field

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

[0002] Digital holography is an effective means of quantitatively measuring the phase of an object, offering advantages such as full-field, rapid, high-precision, and non-contact measurement. It is widely used in fields such as object surface topography measurement and biological cell imaging. This technology modulates the measurement light by the object under test to form a sample light wave, which then interferes with a reference light wave without sample modulation. A phase extraction algorithm is used to extract phase information from one or more interferograms recorded by a 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 folds between [-π, π]. A numerical unwrapping algorithm is needed to unfold the extracted wrapped phase to obtain the true phase of the object. However, when the object under test has a high aspect ratio structure, a stepped surface topography, or high surface roughness, the numerical unwrapping algorithm may introduce significant errors, leading to unwrapping failure.

[0003] To address this issue, researchers proposed a dual-wavelength digital holography technique. This technique uses two different wavelengths of light to acquire the corresponding wavelength's enveloping phase, and subtracts the two to obtain a phase map of the equivalent wavelength. Since the equivalent wavelength is larger than either of the wavelengths introduced for measurement, and the closer the two wavelengths are, the larger the equivalent wavelength becomes, the measurement range of single-wavelength digital holography can be expanded from a few micrometers to the millimeter level.

[0004] Dual-wavelength digital holography introduces an additional light wave compared to single-wavelength digital holography. Current research primarily utilizes wavelength multiplexing techniques based on Michelson or Mach-Zehnder interferometers, employing a single image sensor to simultaneously record interferograms of different wavelengths, enabling real-time dynamic measurements. These wavelength multiplexing techniques often use polarizing beam splitters, dichroic mirrors, or filters in the reference optical path, combined with plane mirrors, to adjust the magnitude and direction of different wavelength carrier frequencies. However, due to the low transmittance of typical polarizing beam splitters, dichroic mirrors, or filters, some of the light that should be fully transmitted is reflected, causing optical crosstalk. Existing methods use high-transmittance beam splitters to avoid crosstalk, but the fabrication and processing of high-transmittance beam splitters are technically challenging and costly. Therefore, there is an urgent need for a method that can solve the optical 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] To address the optical crosstalk problem caused by the low transmittance of general beam splitters in dual-wavelength digital holography, this invention proposes a method for resolving optical crosstalk in dual-wavelength digital holography. This method mainly involves adjusting the size of different wavelength light spots and adjusting the position of the reference light while adjusting the carrier frequency and direction to avoid crosstalk regions.

[0007] 2. Technical Solution

[0008] Dual-wavelength digital holography typically uses two lasers, emitting a first wavelength laser and a second wavelength laser, respectively. Before multiplexing the first and second wavelength lasers, beam expanders or other optical devices with adjustable spot sizes are used to adjust the spot sizes of the first and second wavelength lasers, respectively. The first beam expander adjusts the spot radius of the first wavelength laser to r1, and the second beam expander adjusts the spot radius of the second wavelength laser to r2, where r1 > r2. Then, the first and second wavelength lasers are coupled (with the spot centers coinciding) and transmitted, and a first beam splitter (such as a polarizing beam splitter or a non-polarizing beam splitter) separates the reference beam (first wavelength reference beam and second wavelength reference beam) and the object beam (first wavelength object beam and second wavelength object beam) into two beams, respectively.

[0009] A second beam splitter (such as a filter, dichroic mirror, or polarizing beam splitter) is used in conjunction with a first plane mirror in the reference optical path to adjust the propagation directions of the first and second wavelength reference lights, thereby regulating the magnitude and direction of the carrier frequencies of different wavelengths. During this adjustment process, the positions of the light spots of the first and second wavelength reference lights 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 propagate simultaneously to the second beam splitter, since the reflectivity of a typical beam splitter can be close to 1, the first wavelength reference light is almost entirely reflected by the second beam splitter. However, the transmittance of a typical beam splitter is not high. Therefore, a portion of the second wavelength reference light (the specific proportion depends on the transmittance of the beam splitter; reflection ratio = 1 - transmission ratio) is transmitted by the second beam splitter, while the other portion is reflected along with the first wavelength reference light. Thus, the central region of the first wavelength reference light spot (radius r1) contains a second wavelength reference light spot (radius r2) due to the low transmittance of the second beam splitter. This region is the crosstalk region of the two light waves; the region outside this is the crosstalk-free region of the first wavelength reference light. At this point, by adjusting the second beam splitter, the propagation direction of the first wavelength reference light reflected by it is adjusted so that the crosstalk-free region of the first wavelength reference light is received by the image sensor (the crosstalk region is outside the image sensor's receiving area), and at the same time, it forms a certain angle with the first wavelength object light in the y direction, producing interference fringes; the portion of the second wavelength reference light transmitted through the first beam splitter is reflected by the first plane mirror and then transmitted through the second beam splitter again. By adjusting the first plane mirror, the propagation direction of the second wavelength reference light is adjusted 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), producing interference fringes.

[0011] 3. Technical Effects

[0012] In this way, the image sensor can simultaneously receive off-axis interference patterns of the first and second wavelength lasers. The interference fringes of the first and second wavelength lasers are orthogonally distributed and free of crosstalk. The phase information of the object at different wavelengths can be easily extracted using Fourier transform.

[0013] The proposed method can cleverly avoid the crosstalk region and is not limited by the transmittance of beam splitting devices such as polarizing beam splitters, dichroic mirrors, and filters. It can effectively solve the problem of optical crosstalk caused by beam splitting devices with arbitrary transmittance in dual-wavelength digital holography.

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

[0015] The present invention also provides a Michelson interferometer that uses the above method to solve the optical crosstalk problem in dual-wavelength digital holography. Attached Figure Description

[0016] To more clearly illustrate and understand the present invention, a detailed description is provided below in conjunction with the accompanying drawings.

[0017] Figure 1 This is a schematic diagram showing the position and size of each light spot at the image receiving surface.

[0018] Figure 2 This is a schematic diagram showing the angles between the propagation directions of the first and second wavelength object beams and the first and second wavelength reference beams as they propagate to the receiving surface of the image sensor.

[0019] Figure 3 This is a schematic diagram of one embodiment of the present invention used in a Mach-Zehnder interferometer;

[0020] Figure 4 This is a schematic diagram of one embodiment of the present invention used in a Michelson interferometer. Detailed Implementation

[0021] Example 1

[0022] like Figure 3 The image shows an embodiment of the device for dual-wavelength digital holography based on a Mach-Zehnder interferometer according to the present invention, comprising: 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 polarizing beam splitter, 108 a long-pass dichroic mirror, 109 a first plane mirror, 110 a second plane mirror, 111 a sample to be tested, 112 an objective lens, 113 a sleeve lens, 114 a non-polarizing beam splitter, 115 a third polarizer, and 116 a monochromatic image sensor.

[0023] The first wavelength laser 101 and the second wavelength laser 102 output a first wavelength laser (wavelength 532nm) and a second wavelength laser (wavelength 632.8nm), respectively. After passing through a polarizer and a beam expander, the two wavelength lasers are split into two beams by a polarizing beam splitter 107: one beam is reflected by the second plane mirror 110, modulated by the sample under test 111 to form an object beam, collected by the infinity imaging objective lens 112 and converged by the sleeve lens 113, and finally the object beam enters the monochromatic image sensor 116 perpendicularly along the -z direction; the other beam serves as a reference beam. By rotating the first polarizer 103 and the second polarizer 104, the distribution ratio of the object beam and the reference beam of the two wavelengths can be adjusted respectively.

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

[0025] In the reference optical path, the first wavelength reference light is reflected by the long-pass dichroic mirror 108, and most of the second wavelength reference light is transmitted through the long-pass dichroic mirror 108. Since the transmittance of a typical long-pass dichroic mirror is not high, a small portion (the specific proportion depends on the transmittance of the long-pass dichroic mirror) of the second wavelength reference light is reflected by the long-pass dichroic mirror 108, creating crosstalk in the overlapping area with the first wavelength reference light. Adjusting the long-pass dichroic mirror 108 adjusts the propagation direction of the first wavelength reference light, so that after the first wavelength reference light is reflected by the non-polarizing beam splitter prism 114, the monochromatic image sensor 116 only receives the crosstalk-free area of ​​the first wavelength reference light. 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, producing interference fringes along the x-direction. Furthermore, the propagation direction of the second-wavelength reference light is adjusted by adjusting the first plane mirror 109. After being reflected by the first plane mirror 109, the second-wavelength reference light is transmitted again through the long-pass dichroic mirror 108, and then reflected by the unpolarized beam splitter prism 114. It is then received by the monochromatic image sensor 116, and forms a certain angle with the second-wavelength object light in the y-direction, producing interference fringes along the y-direction. The transmission axis of the third polarizer 115 is placed at an angle of 45° with the y-direction. The monochromatic image sensor 116 simultaneously acquires off-axis interference patterns of the two orthogonal interference fringes. In this way, the phase information of the object at different wavelengths can be easily extracted using Fourier transform.

[0026] Example 2

[0027] like Figure 4 The image shows an embodiment of the device for dual-wavelength digital holography based on a Michelson interferometer according to the present invention, comprising: 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 unpolarized beam splitter, 206 a second unpolarized beam splitter, 207 a first plane mirror, 208 a long-pass filter, 209 a second plane mirror, and 210 a monochrome image sensor.

[0028] First-wavelength laser 201 and second-wavelength laser 202 output first-wavelength laser (wavelength 532nm) and second-wavelength laser (wavelength 632.8nm), respectively. A first collimator 203 with a magnification of 10x amplifies the spot of the first-wavelength laser by 10x, and a second collimator 204 with a magnification of 3x amplifies the spot of the second-wavelength laser by 3x. The two wavelengths of light, after collimation and amplification, are combined and coupled by a first unpolarized beam splitter 205, and then split into two paths by a second unpolarized beam splitter 206 (transmission-to-reflection ratio of 1:1). One path is the object beam path. Both the first and second wavelength object beams are reflected by a first plane mirror 207, then transmitted through the second unpolarized beam splitter 206, and enter the monochromatic image sensor 210 perpendicularly along the negative z-axis. The other path is the reference light path. When the first and second wavelength reference lights are transmitted to the long-pass filter 208, the first wavelength reference light is completely reflected, and a portion of the second wavelength reference light is reflected. The reflected portion overlaps with the central region of the first wavelength reference light, generating crosstalk. At this time, by adjusting the long-pass filter 208, the first wavelength reference light is reflected by the second unpolarized beam splitter 206, and the monochrome image sensor 210 only receives the crosstalk-free region of the first wavelength reference light. The crosstalk region 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. By adjusting the second plane mirror 209, the second wavelength reference light is received by the monochrome image sensor 210, and it 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 using Fourier transform.

[0029] The above embodiments are merely illustrative examples of several implementations of the present invention, but the scope of protection of the present invention is not limited thereto. Within the technical scope protected by the present invention, other individuals skilled in the art may make local adjustments in different ways without departing from the principles and spirit of the present invention, and these adjustments also fall within the scope of protection of the present invention.

Claims

1. A method for solving optical crosstalk in dual-wavelength digital holography, characterized in that, The process includes the following steps: Before multiplexing the first wavelength laser and the second wavelength laser, the spot sizes of the first wavelength laser and the second wavelength laser are adjusted so that the spot radius of the first wavelength laser is larger than that of the second wavelength laser; the adjusted first wavelength laser and the second wavelength laser are coupled and transmitted, and the reference light and the object light are separated by the first beam splitter. The reference light includes the first wavelength reference light and the second wavelength reference light, and the object light includes the first wavelength object light and the second wavelength object light. In the reference optical path, a combination of a second beam splitter and a first planar reflector is used 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 propagate to the second beam splitter at the same time, the propagation direction of the first wavelength reference light reflected by the second beam splitter is adjusted so that the crosstalk-free area of ​​the first wavelength reference light is received by the image sensor, and the crosstalk area is located outside the image sensor receiving area. At the same time, it forms a certain angle with the first wavelength object light in the y direction, generating interference fringes. The portion of the second-wavelength reference light transmitted through the first beam splitter is reflected by the first plane mirror and then transmitted again through the second beam splitter. The propagation direction of the second-wavelength reference light is adjusted by adjusting the first plane mirror, so that the light spot of the second-wavelength reference light is received by the image sensor. At the same time, it forms a certain angle with the second-wavelength object light in the x-direction, producing interference fringes. The image sensor simultaneously receives the off-axis interference patterns of the first-wavelength laser and the second-wavelength laser. The interference fringes of the first-wavelength laser and the second-wavelength laser are orthogonally distributed and without crosstalk. Then, Fourier transform is used to extract the phase information of the object at different wavelengths.

2. The method for solving optical crosstalk in dual-wavelength digital holography according to claim 1, characterized in that, Use a beam expander or other optical devices that can adjust the spot size of the first and second wavelength lasers respectively.

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 polarizing beam splitter or a non-polarizing beam splitter; the second beam splitter is a filter, a dichroic mirror, or a polarizing beam splitter.

4. A Mach-Zehnder interferometer, characterized in that, The optical crosstalk problem in dual-wavelength digital holography can be solved by any of the methods described in claims 1-3.

5. A Michelson interferometer, characterized in that, The optical crosstalk problem in dual-wavelength digital holography can be solved by any of the methods described in claims 1-3.

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