Single reference arm multi-wavelength off-axis digital holographic imaging device based on three-prism refraction

Abstract

A single reference arm multi-wavelength off-axis digital holographic imaging device based on three-prism refraction comprises three laser light sources arranged side by side, three-in-one optical fibers connected thereto respectively, a first lens for collimation, a beam splitting prism, a transparent object to be measured arranged in sequence on the object light side, a second lens, a second mirror and a third lens, and a first mirror arranged on the reference light side of the first beam splitting prism and a three-prism. The present application adopts multi-wavelength digital holographic technology, multiple wavelengths share one optical path, the optical path structure is an off-axis structure in which object light and reference light are separated, the dispersion characteristics of the three-prism in the reference light path cause different deflection of reference light of different wavelengths, different wavelengths have different object parameter light angles, so that the multi-wavelength digital hologram obtained by single acquisition can be separated from each other in the frequency spectrum space, the amplitude and phase information of multiple wavelengths can be calculated through a multi-wavelength digital hologram obtained by single acquisition, and the present application is suitable for dynamic real-time three-wavelength digital holographic measurement scenes.

Description

Technical Field

[0001] This invention relates to a technology in the field of digital holographic imaging, specifically a single-reference-arm multi-wavelength off-axis digital holographic imaging device based on prism refraction. Background Technology

[0002] Digital holography is a classic phase measurement technique, characterized by non-contact, high precision, and rapid measurement. It is widely used in morphology and deformation measurement in fields such as semiconductors and biological cells. Multi-wavelength digital holography offers a solution for expanding the phase measurement range and color digital holography. However, achieving dynamic measurement in multi-wavelength digital holography is a crucial problem to solve. To achieve dynamic measurement, researchers have proposed multi-reference-arm optical path structures, where different wavelengths of reference light pass through different optical paths. However, this structure is complex and unsuitable for instrumentation. Other researchers have used color cameras to collect data in single-reference-arm multi-wavelength digital holographic optical paths, but the spectra of each channel of the color camera have a certain range, leading to crosstalk between digital holograms from different channels and affecting reconstruction accuracy. Additionally, researchers have added grating elements to the reference optical path based on single-reference-arm optical paths. Different wavelengths of laser light are deflected differently after passing through the grating, achieving spectral separation of different wavelengths. However, the grating is susceptible to stray light, and combining it with an aperture array to filter out stray light makes the system difficult to adjust. Summary of the Invention

[0003] This invention addresses the contradiction between the simplicity of existing multi-wavelength off-axis digital holographic optical paths and dynamic measurement, as well as the problems of existing relatively simple optical paths that can perform dynamic measurements but suffer from crosstalk, complex adjustments, and the inability to simultaneously acquire and process digital holograms of three or more wavelengths. It proposes a single-reference-arm multi-wavelength off-axis digital holographic imaging device based on prism refraction. Employing multi-wavelength digital holography technology, multiple wavelengths share a single optical path. The optical path structure is an off-axis structure with separate object and reference beams. In the reference beam path, the dispersive properties of the prism cause different deflections of the reference beams of different wavelengths, resulting in different object-reference beam angles for different wavelengths. This allows the multi-wavelength digital holograms acquired in a single acquisition to be separated in the spectral space. A single multi-wavelength digital hologram can be used to calculate the amplitude and phase information of multiple wavelengths, making it suitable for dynamic real-time three-wavelength digital holographic measurement scenarios.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to a single-reference-arm, multi-wavelength off-axis digital holographic imaging device based on prism refraction, comprising: three laser sources arranged in parallel and connected to them by a three-in-one optical fiber, a first lens for collimation, a beam-splitting prism, a transparent object to be tested arranged sequentially on the object light side, a second lens, a second reflector, and a third lens, and a first reflector and a prism arranged on the reference light side of the first beam-splitting prism, wherein: the exit ends of the third lens and the prism are respectively facing the two incident surfaces of the beam-combining prism, and a camera is provided on the exit surface of the second beam-combining prism, wherein: the three wavelengths of laser light share a Mach-Zehnder interference optical path, and the object light path and the reference light pass through the same... The path of the three-wavelength digital hologram is simultaneously interfered with on the camera surface, forming a three-wavelength digital hologram. The three-in-one fiber couples three wavelengths of laser light and outputs a divergent laser light, which is then collimated by the first lens. The collimated light is then split into object light and reference light by the beam splitter prism. The object light passes through the transparent test object, is magnified by the second lens, and is reflected by the second mirror. Finally, it is collimated by the third lens and reflected by the beam combiner prism to the camera's photosensitive surface. The reference light is reflected by the first mirror, and after being deflected at different wavelengths by the three-wavelength prism, it passes through the beam combiner prism to the camera's photosensitive surface, where it interferes with the object light of the corresponding wavelength. This achieves the separation of the multi-wavelength digital holograms in their Fourier spectra.

[0006] This invention relates to a single-reference-arm, multi-wavelength off-axis digital holographic imaging method based on prism refraction using the aforementioned device, comprising:

[0007] Step 1: The three wavelengths of laser light pass through the same optical path and form different object-parameter angles after passing through the prism;

[0008] Step 2: Acquire a multi-wavelength digital hologram containing information from three wavelengths;

[0009] Step 3: Extract the amplitude and phase maps of three wavelengths from the multi-wavelength digital hologram in Step 2, specifically including:

[0010] 3.1 Perform a Fourier transform on the multi-wavelength digital hologram I to obtain the Fourier spectrum.

[0011] Where u and v are frequencies.

[0012] 3.2 Extract the target spectrum for each wavelength from the Fourier spectrum. Then shift its frequency to the center of the matrix;

[0013] 3.3 Extracted frequency-shifted target spectrum Reconstructing complex wavefronts using numerical diffraction methods

[0014] Among them: wave number It is the inverse Fourier transform;

[0015] 3.4 Take the absolute value of the complex wavefront to obtain amplitude diagram A. i =|O i |; Taking the phase angle of the complex wavefront yields the wrapped phase diagram.

[0016] Step 4: To meet the requirements of color holographic measurement, the amplitude maps of the three wavelengths from Step 3 are combined to form a color result image. Its color matrix distribution Where M is the transformation matrix, A1, A2 and A3 are the amplitude diagrams of the three wavelengths, and R, G and B are the corresponding color components.

[0017] Step 5: Obtain the thickness measurement result using the phase diagram of the three wavelengths from Step 3. in: The phase difference is obtained by subtracting any two phase diagrams from the three wavelengths.

[0018] Technical effect

[0019] Compared with existing technologies, this invention improves measurement efficiency and accuracy by embedding a prism in the reference arm of a single-reference-arm off-axis digital holographic interference optical path. This allows for the separation of the corresponding spectra of different wavelengths in the Fourier spectrum of a single-acquired three-wavelength digital hologram, as well as dynamic real-time measurement of the three-wavelength digital hologram. This invention is applicable not only to transmissive optical paths but also to reflective optical paths and digital holographic optical paths with more than three wavelengths. Attached Figure Description

[0020] Figure 1 This is the optical path diagram for Example 1;

[0021] Figure 2 This is the optical path diagram for Example 2;

[0022] Figure 3 (a) and (b) are respectively the three-wavelength digital holograms and their spectra obtained by optical path multiplexing without embedded prisms in the embodiment;

[0023] Figure 4 (a) and (b) are respectively the three-wavelength digital holograms and their spectra obtained by embedding a prism with a vertex angle of 10 degrees in the optical path multiplexing embodiment;

[0024] Figure 5 (a), (b), and (c) are respectively examples from the embodiments. Figure 3 The phase diagrams of the three wavelengths calculated in the solution. Detailed Implementation

[0025] Example 1

[0026] like Figure 1 As shown in this embodiment, a single-reference-arm multi-wavelength off-axis digital holographic imaging device for thickness measurement of a transparent object O using a three-wavelength digital holographic method is provided. The device includes: three laser light sources arranged in parallel, a three-in-one fiber TF connected to each source, a first lens L1 for collimation, a beam splitter prism BS1, and a transparent object O, a second lens L2, a second reflector M2, and a third lens L3 sequentially arranged on the object light side. A first reflector M1 and a prism P are also arranged on the reference light side of the first beam splitter prism BS1. The exit ends of the third lens L3 and the prism P are respectively aligned with the two incident surfaces of the beam combiner prism BS2. A camera CCD is mounted on the exit surface of the second beam combiner prism BS2. The three wavelengths of laser light share a common optical path. In the Mach-Zehnder interferometer optical path, the object beam and the reference beam follow the same path and interfere simultaneously on the camera surface, forming a three-wavelength digital hologram. The three-in-one fiber TF couples three wavelengths of laser light and outputs a divergent laser. The laser light is then collimated by the first lens L1, and then split into object light and reference light by the beam splitter prism BS1. The object light passes through the transparent test object O, is magnified by the second lens L2, and then reflected by the second mirror M2. Finally, it is collimated by the third lens L3 and reflected by the beam combiner prism BS2 to the camera's photosensitive surface. The reference light is reflected by the first mirror M1, and after being deflected by different wavelengths by the three-prism P, it passes through the beam combiner prism BS2 to the camera's photosensitive surface, where it interferes with the object light of the corresponding wavelength to form a three-wavelength digital hologram.

[0027] The wavelengths of the three laser sources are λ1 = 633 nm, λ2 = 532 nm, and λ3 = 473 nm, respectively.

[0028] The camera CCD transmits the acquired three-wavelength digital holograms to the data processing module for further processing, specifically including:

[0029] Step 1: Perform a Fourier transform on the acquired multi-wavelength digital hologram I to obtain...

[0030] Step 2: Extract the target spectrum for each wavelength from the Fourier spectrum obtained in Step 1 using spectral filtering. The target spectrum is then transplanted to the center of the matrix.

[0031] Step 3: Using the target spectra of each wavelength extracted in Step 2, the complex wavefronts O1, O2 and O3 of each wavelength are reconstructed using the angular spectral diffraction method.

[0032] Step 4: Take the absolute values ​​of the reconstructed complex wavefront from Step 3 to obtain amplitude diagrams A1, A2, and A3, and take the phase to obtain the phase diagram. and

[0033] Step 5: Combine the three phase maps from Step 4 in pairs to obtain a result with a larger measurement range.

[0034] Step 6: Color restoration of the amplitude diagram in Step 4 yields a color digital holographic result.

[0035] Through specific practical experiments, the principle was verified using three laser light sources with wavelengths of 633nm, 532nm, and 473nm. The laser power was adjusted to ensure that the power of each wavelength was basically consistent and moderate; the position and angle of the optical elements were adjusted to ensure that the object light and reference light formed clear interference fringes on the camera's photosensitive surface; and three-wavelength digital holograms were acquired using the camera. A Fourier transform was performed on the acquired single three-wavelength digital hologram to obtain the spatial spectrum; the spectrum of the corresponding wavelength was extracted from the spatial spectrum using spectral filtering.

[0036] Example 2

[0037] like Figure 2 As shown, this embodiment is a transmission-type holographic imaging device. Compared with embodiment 1, this embodiment adds a microscope objective MO between the transparent test object O and the second lens L2 to demonstrate the effect of prism spectrum separation. During the experiment, experiments were conducted with and without a prism and with a prism inserted at a 10-degree apex angle.

[0038] like Figure 3 The image shows the spectrum without the prism inserted; the three wavelengths are very close in spectral distance and difficult to separate. Figure 4 The spectrum is the result of inserting a prism with a 10-degree apex angle, showing a clear spectral separation effect.

[0039] like Figure 5 As shown, to utilize Figure 3 The phase map extracted after the separation of the spectrum and reconstruction yielded a good phase quality.

[0040] Compared with existing technologies, this invention achieves the separation of three wavelengths in the spectral space by inserting a prism P into the reference optical path of a multi-wavelength digital holographic optical path in a single reference arm, thus avoiding spectral aliasing between different wavelengths. This method enables simultaneous acquisition and real-time processing of digital holograms of multiple wavelengths, optimizing the system and improving measurement efficiency.

[0041] The above-described specific implementations can be partially adjusted by those skilled in the art in different ways without departing from the principles and purpose of the present invention. The scope of protection of the present invention is defined by the claims and is not limited to the above-described specific implementations. All implementation schemes within the scope of the claims are bound by the present invention.

Claims

1. A single-reference-arm, multi-wavelength off-axis digital holographic imaging device based on prism refraction, characterized in that, include: The system comprises three parallel laser sources, connected to a three-in-one optical fiber, a first collimating lens, a beam splitter prism, a transparent object to be tested on the object side, a second lens, a second reflecting mirror, a third lens, and a first reflecting mirror and a prism on the reference light side of the first beam splitter prism. The exit ends of the third lens and the prism are respectively aligned with the two incident surfaces of the beam combiner prism. A camera is mounted on the exit surface of the second beam combiner prism. The three wavelengths of laser light share a single Mach-Zehnder interference path. The object light and the reference light follow the same path and interfere simultaneously at the camera surface, forming... A three-wavelength digital hologram uses a three-in-one fiber optic cable to couple three wavelengths of laser light, which then outputs a divergent laser beam. This beam is collimated by a first lens, and then split into an object beam and a reference beam by a beam-splitting prism. The object beam passes through a transparent object, is magnified by a second lens, reflected by a second mirror, and finally collimated by a third lens. It is then reflected by a beam-combining prism to the camera's photosensitive surface. The reference beam, after being reflected by the first mirror, is deflected at different wavelengths by the three-in-one prism, and then reaches the camera's photosensitive surface through the beam-combining prism, where it interferes with the object beam of the corresponding wavelength. This process achieves the separation of the multi-wavelength digital hologram within its Fourier spectrum.

2. The single-reference-arm, multi-wavelength off-axis digital holographic imaging device based on prism refraction according to claim 1, characterized in that, A microscope objective lens is added between the transparent object to be tested and the second lens.

3. A single-reference-arm multi-wavelength off-axis digital holographic imaging method based on prism refraction using the device described in claim 1 or 2, characterized in that, include: Step 1: The three wavelengths of laser light pass through the same optical path and form different object-parameter angles after passing through the prism; Step 2: Acquire a multi-wavelength digital hologram containing information from three wavelengths; Step 3: Extract the amplitude and phase images of three wavelengths from the multi-wavelength digital hologram in Step 2; Step 4: To meet the requirements of color holographic measurement, the amplitude maps of the three wavelengths from Step 3 are combined to form a color result image. Its color matrix distribution Where: M is the transformation matrix, A1, A2 and A3 are the amplitude diagrams of the three wavelengths respectively, and R, G and B are the corresponding color components; Step 5: Obtain the thickness measurement result using the phase diagram of the three wavelengths from Step 3. in: The phase difference is obtained by subtracting any two phase diagrams from the three wavelengths.

4. The single-reference-arm multi-wavelength off-axis digital holographic imaging method based on prism refraction according to claim 3, characterized in that, Step 3 specifically includes: 3.1 Perform Fourier transforms on the multi-wavelength digital hologram I to obtain the Fourier spectrum. Where u and v are frequencies; 3.2 Extract the target spectrum from the Fourier spectrum at each wavelength. Then shift its frequency to the center of the matrix; 3.3 Extracted frequency-shifted target spectrum Reconstructing complex wavefronts using numerical diffraction methods Among them: wave number It is the inverse Fourier transform; 3.4 Take the absolute value of the complex wavefront to obtain amplitude diagram A. i =|O i |; Taking the phase angle of the complex wavefront yields the wrapped phase diagram.

5. The single-reference-arm multi-wavelength off-axis digital holographic imaging method based on prism refraction according to claim 3, characterized in that, The three wavelengths of laser light are λ1 = 633 nm, λ2 = 532 nm, and λ3 = 473 nm, respectively.

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