Full-depth dispersion compensation method and system for polarization sensitive optical coherence tomography

Abstract

The application discloses a full-depth dispersion compensation method and system for polarization-sensitive optical coherence tomography, which comprises the following steps: step one, imaging a mirror, and the optical path difference of original interference signals is delta z; step two, subtracting background to obtain signals as real parts and performing Hilbert transform to obtain imaginary parts; step three, inverse Fourier transform and filtering to filter out information of polarization artifacts; step four, extracting phase information; step five, moving a reference arm to a symmetric position of zero optical path to obtain original interference signals with an optical path difference of-delta z, and repeating steps two to four to extract phase information; and according to a formula, a dispersion function is obtained to perform dispersion compensation on an image.

Description

Technical Field

[0001] This application relates to a full-depth dispersion compensation method and system for polarization-sensitive optical coherence tomography (PS-OCT), which is applied to PS-OCT to compensate for the dispersion present in the PS-OCT system, and belongs to the field of polarization-sensitive optical coherence tomography technology. Background Technology

[0002] OCT (Optical Coherence Tomography) is an important imaging technology in the field of biomedical optical imaging, possessing advantages such as non-invasiveness, non-damage, and high resolution. Since its invention in 1991, it has become a hot topic in biomedical non-invasive optical imaging research and is currently applied in clinical diagnosis and pathological research in ophthalmology and cardiology. With the development of technology, the functions of OCT have gradually expanded, resulting in various functional OCT types, such as angiography OCT and polarization-sensitive OCT.

[0003] OCT is based on partially coherent optical interferometry, and axial resolution is one of the key factors in OCT imaging quality. To obtain high axial resolution, OCT requires the use of a broadband light source. Therefore, any refractive index mismatch between the sample arm and the reference arm will cause dispersion in the system, thereby broadening the axial point spread function (PSF), resulting in a decrease in resolution and signal-to-noise ratio. For PS-OCT systems, it is necessary to obtain two interference signals with mutually perpendicular polarization directions and calculate the OCT functional image. Therefore, the decrease in axial resolution caused by dispersion will seriously affect image quality.

[0004] Currently, methods for compensating for dispersion mainly include hardware and software methods. Hardware methods compensate for dispersion by adding dispersion compensation elements to the optical path, but this increases cost and system complexity. Software methods mainly process the acquired interference signals and perform dispersion compensation through numerical calculations. A common method is the iterative method, which uses indicators such as image sharpness to evaluate the dispersion compensation results and calculates the second- and third-order dispersion coefficients to eliminate dispersion (reference: M. Wojtkowski, V.S. J. Rinivasan, TH. Ko, J. G. Fujimoto, A. Kowalczyk, and JS. Duker, “Ultra-high-resolution, high-speed, Fourier domain optical coherence tomography and methods for dispersion compensation,” Opt. Express, 12, 2404-2422 (2004)). However, this type of method can only correct up to the third-order dispersion and cannot correct higher-order dispersion. Furthermore, it involves a large amount of computation, reducing the imaging speed of the system. Another method is the symmetrical phase measurement method (Reference: K. Singh, G. Sharma and GJ Tearney, “Estimation and compensation of dispersion for a high-resolution optical coherence tomography system”, J. Opt., 20, 025301 (2018)). This method calculates the dispersion compensation curve by measuring the phase of mirrors at symmetrical positions, thereby eliminating dispersion. However, this method is only applicable to conventional OCT systems. For complex PS-OCT systems, the cross-coupling of orthogonal polarization modes in the polarization-maintaining fibers used in the sample arm and reference arm causes polarization artifacts in the acquired images, making it impossible to accurately calculate the dispersion compensation curve. Summary of the Invention

[0005] The technical problem this application aims to solve is how to effectively compensate for the dispersion present in the PS-OCT system.

[0006] To address the aforementioned technical problems, this application provides a full-depth chromatic dispersion compensation method for PS-OCT systems, comprising the following steps:

[0007] Step 1: Use the PS-OCT system to image the mirror and obtain the original interference signal. At this time, the optical path difference between the two arms is δz.

[0008] Step 2: Subtract the background from the horizontal interference signal in the two mutually perpendicular polarization signals to obtain signal S(k,δz) as the real part. Perform a Hilbert transform on S(k,δz) to obtain the imaginary part, thus obtaining the amplitude and phase information of the signal.

[0009] Step 3, for An inverse Fourier transform is performed, and the result is filtered to remove polarization artifacts. Specifically, the position of the mirror signal is first located, and the signal is windowed around this position to eliminate the influence of polarization artifacts.

[0010] Step 4: Filter the filtered product Extract the phase information of the mirror

[0011] Step 5: Move the reference arm to a position symmetrical to the imaging position in Step 1 about zero optical path length. This yields the original interference signal with a path difference of - between the two arms at the symmetrical position. Repeat steps 2-4 to obtain the phase information.

[0012] According to the formula: The dispersion function was calculated. Multiply the two interference signals with mutually perpendicular polarization directions by the dispersion function. Dispersion compensation is performed, and finally, the PS-OCT image is obtained by calculating the intensity signals after these two dispersion compensations.

[0013] Specifically, In the formula l c Let λ be the coherence length of the light source, λ0 be the center wavelength of the light source, and Δλ be the bandwidth of the light source; the signal S(k,δz) can be expressed as:

[0014]

[0015] Among them, I s (k) and I r (k) represents the reflected signal intensity of the sample arm and the reference arm, respectively. The dispersion function that needs to be obtained;

[0016] The phase of the signals from two mirrors with zero optical path symmetry can be expressed as:

[0017]

[0018]

[0019] Therefore, the calculated dispersion function for:

[0020]

[0021] The two acquired interference signals with mutually perpendicular polarization directions are multiplied by the dispersion function to perform dispersion compensation, resulting in two dissipated OCT intensity signals. Finally, the PS-OCT image is calculated using these two intensity signals.

[0022] This application also provides a PS-OCT system for implementing the above-mentioned full-depth dispersion compensation method, including a broadband light source. The light output from the broadband light source is linearly polarized in the vertical direction after passing through a polarization controller. The linearly polarized light enters a circulator. The light emitted from the circulator passes through a polarization beam splitter and the a-end of a 50:50 polarization-maintaining coupler, and is then split into a reference beam and a sample beam. The reference beam passes through a collimator, a quarter-wave plate with a fast axis angle of 22.5° to the vertical direction, a dispersion compensator, and an aperture, and is reflected back to the c-end of the polarization-maintaining coupler by a mirror. The sample beam passes through a collimator and a quarter-wave plate with a fast axis angle of 45° to the vertical direction, causing the sample beam to change from a linearly polarized state to a circularly polarized state. After passing through a scanning galvanometer and a focusing mirror, the sample is illuminated. The light scattered from the sample returns to the d-end of the polarization-maintaining coupler along the original path. The reference light entering the c-end of the polarization-maintaining coupler and the sample light entering the d-end of the polarization-maintaining coupler interfere, and the interfered light exits from the polarization-maintaining coupler. After the light emitted from end a of the polarization-maintaining coupler enters the polarization beam splitter, the vertical light passes through the circulator and enters the first balanced detector, while the horizontal light enters the second balanced detector. After the light emitted from end b of the polarization-maintaining coupler enters the polarization beam splitter, the vertical light enters the first balanced detector, while the horizontal light enters the second balanced detector. The first and second balanced detectors receive the light and convert it into electrical signals, which are then transmitted to the signal processing device for calculation. The dispersion-compensated sample polarization image is obtained through full-depth dispersion compensation and PS-OCT data processing.

[0023] This application also provides an electronic device, including a processor and a memory connected to the processor. The memory stores instructions for the processor to execute, and the processor is configured to execute the instructions stored in the memory to perform the steps of the full-depth dispersion compensation method described above.

[0024] The advantages of this application are:

[0025] 1. This invention solves the system dispersion problem caused by the refractive index mismatch between the two arms of the PS-OCT system, without the need to introduce additional hardware equipment, thus saving system costs;

[0026] 2. A filtering method was used to accurately calculate the dispersion compensation curve of the PS-OCT system in the presence of polarization artifacts;

[0027] 3. This method can theoretically correct all levels of dispersion, requiring only one calculation to compensate for system dispersion in real time during subsequent imaging processes;

[0028] 4. Data processing is performed on the interference signal in the horizontal polarization direction, and the results are used for dispersion compensation of the two signals with mutually perpendicular polarization directions to achieve image quality optimization of the PS-OCT system. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the PS-OCT system structure provided in the embodiment;

[0030] Figure 2 This is a flowchart of the full-depth dispersion compensation method provided in the embodiment;

[0031] Figure 3-1 The image before dispersion compensation;

[0032] Figure 3-2 This is the image after dispersion compensation. Detailed Implementation

[0033] To make this application more apparent and understandable, preferred embodiments are described in detail below with reference to the accompanying drawings.

[0034] The full-depth dispersion compensation method for the PS-OCT system provided in this embodiment proposes a solution to the dispersion problem existing in the deep-depth PS-OCT system. It uses the symmetrical phase measurement method to calculate the dispersion magnitude of the system and determine the dispersion compensation curve to compensate for the dispersion existing in the system.

[0035] PS-OCT system structure as follows: Figure 1As shown, the system includes a broadband light source 101. The light output from the broadband light source 101 is linearly polarized in the vertical direction after passing through a polarization controller 102. This linearly polarized light enters a circulator 103. The light emitted from the circulator 103 passes through a polarization beam splitter 104 and a 50:50 polarization-maintaining coupler 105a, and is then evenly split into a reference beam and a sample beam. The reference beam passes through a collimator 106, a quarter-wave plate 107 with a fast axis angle of 22.5° to the vertical direction, a dispersion compensator 108, and an aperture 109, and then returns to its original path via a mirror 110. The light beam is reflected back to the polarization-maintaining coupler 105c. The sample beam then passes through the collimator 111 and a quarter-wave plate 112 with a fast axis at a 45° angle to the vertical direction, causing the sample beam to change from linear polarization to circular polarization. After passing through the scanning galvanometer 113 and the focusing lens 114, the sample beam is illuminated. The light scattered from the sample beam 115 returns to the polarization-maintaining coupler 105d along the original path. The reference light entering the polarization-maintaining coupler 105c and the sample light entering the polarization-maintaining coupler 105d interfere with each other, and the interfered light exits from the polarization-maintaining coupler 105. Light emitted from the polarization-maintaining coupler 105a enters the polarization beam splitter 104. Vertical light passes through the circulator 103 and enters the first balanced detector 117, while horizontal light enters the second balanced detector 118. Light emitted from the polarization-maintaining coupler 105b enters the polarization beam splitter 116. Vertical light enters the first balanced detector 117, while horizontal light enters the second balanced detector 118. The first and second balanced detectors 117 and 118 receive the light and convert it into electrical signals, which are then transmitted to the signal processing device 119 for calculation. The dispersion-compensated sample polarization image is obtained through full-depth dispersion compensation and PS-OCT data processing.

[0036] The full-depth dispersion compensation method provided in this embodiment is implemented by the aforementioned PS-OCT system.

[0037] The axial resolution of a PS-OCT system depends on the center wavelength and bandwidth of the light source, and its expression is as follows:

[0038]

[0039] In the formula l c λ is the coherence length of the light source, λ0 is the center wavelength of the light source, Δλ is the bandwidth of the light source, and δz is the optical path difference between the sample and the reference arm mirror. In this embodiment, the center wavelength of the light source is 1310 nm, the bandwidth is 100 nm, and the theoretical axial resolution in air is 7.6 μm.

[0040] The signal S(k,δz) obtained by subtracting the background from the interference spectrum recorded by the detector can be expressed as:

[0041]

[0042] Among them I s (k),I r (k) represents the reflected signal intensity of the sample arm and the reference arm, respectively. Let be the dispersion function we need to obtain. Therefore, we only need to measure the two mirrors that are symmetric about zero optical path and calculate their phases to obtain the dispersion function. The phase of the signals from two mirrors with zero optical path symmetry can be expressed as:

[0043]

[0044]

[0045] Therefore, the calculated dispersion function for:

[0046]

[0047] The two acquired interference signals with mutually perpendicular polarization directions are multiplied by the dispersion function to perform dispersion compensation, resulting in two dissipated OCT intensity signals. Finally, the PS-OCT image is calculated using these two intensity signals.

[0048] See Figure 2 This embodiment provides a full-depth dispersion compensation method for the PS-OCT system, and the specific steps are as follows:

[0049] Step 1: Use the PS-OCT system to image the mirror and obtain the original interference signal. At this time, the optical path difference between the two arms is δz.

[0050] Step 2: Subtract the background from the horizontal interference signal in the two mutually perpendicular polarization signals to obtain signal S(k,δz) as the real part. Perform a Hilbert transform on S(k,δz) to obtain the imaginary part, thus obtaining the amplitude and phase information of the signal.

[0051] Step 3, for An inverse Fourier transform is performed, and the result is filtered to remove polarization artifacts. Specifically, the position of the mirror signal is first located, and the signal is windowed around this position to eliminate the influence of polarization artifacts.

[0052] Step 4: Filter the filtered product Extract the phase information of the mirror

[0053] Step 5: Move the reference arm to a position symmetrical about zero optical path from the imaging position in Step 1. This yields the original interference signal with a path difference of -δz between the two arms at the symmetrical position. Repeat steps 2-4 to obtain the phase information.

[0054] According to the formula: The dispersion function was calculated. Multiply the two interference signals with mutually perpendicular polarization directions by the dispersion function. Dispersion compensation is performed, and finally, the PS-OCT image is obtained by calculating the intensity signals after these two dispersion compensations.

[0055] For example, imaging was used to verify the dispersion compensation effect of 10 layers of adhesive tape. Figure 3-1 and Figure 3-2 For the comparison before and after dispersion compensation, Figure 3-1 The image quality of the tape before compensation. Figure 3-2 This is the imaging effect after dispersion compensation.

[0056] The full-depth chromatic dispersion compensation method of the PS-OCT system provided in this embodiment can be implemented not only in the PS-OCT system provided in this embodiment, but also in an electronic device after an image is generated by a similar imaging system. The electronic device includes a processor and a memory connected to the processor. The memory stores instructions for the processor to execute. The processor is configured to execute the instructions stored in the memory to perform the steps of the full-depth chromatic dispersion compensation method of the PS-OCT system provided in this embodiment.

Claims

1. A full-depth dispersion compensation method for polarization-sensitive optical coherence tomography (PS-OCT), characterized in that, Includes the following steps: Step 1: Use a PS-OCT system to image the mirror and obtain the original interference signal. At this point, the optical path difference between the two arms is... ; Step 2: Subtract background from the horizontal interference signal in the two mutually perpendicular polarization signals to obtain the signal. As the real part, for By performing a Hilbert transform on the imaginary part, we can obtain the amplitude and phase information of the signal. ; Step 3, for Perform an inverse Fourier transform and filter the result to remove polarization artifacts. Step 4: Filter the filtered product Extract the phase information of the mirror ; Step 5: Move the reference arm to a position symmetrical to the imaging position in Step 1 about zero optical path length. The optical path difference between the two arms at this symmetrical position is... The original interference signal is used to obtain phase information by repeating steps two through four. ; According to the formula: The dispersion function was calculated. Multiply the two interference signals with mutually perpendicular polarization directions by the dispersion function. Dispersion compensation is performed, and finally the PS-OCT image is obtained by calculating the intensity signals after these two dispersion compensations. Signal It can be represented as: ； in, , These represent the reflected signal intensities of the sample arm and the reference arm, respectively. The dispersion function to be obtained; The phase of the signals from two mirrors with zero optical path symmetry can be expressed as: ； ； Therefore, the calculated dispersion function for: ； The two acquired interference signals with mutually perpendicular polarization directions are multiplied by the dispersion function to perform dispersion compensation, resulting in two dissipated OCT intensity signals. Finally, the PS-OCT image is calculated using these two intensity signals.

2. A PS-OCT system implementing the full-depth dispersion compensation method of claim 1, characterized in that, Including a broadband light source, the light output from the broadband light source is linearly polarized in the vertical direction after passing through a polarization controller. The linearly polarized light enters a circulator, and the light emitted from the circulator is split into a reference beam and a sample beam after passing through a polarization beam splitter and the a end of a 50:50 polarization-maintaining coupler. The reference beam passes through a collimator, a quarter-wave plate with a fast axis at an angle of 22.5° to the vertical, a dispersion compensator, and an aperture, and is reflected back to the polarization-maintaining coupler (end c) by a mirror. The sample beam passes through a collimator and a quarter-wave plate with a fast axis at an angle of 45° to the vertical, converting the sample beam from linear polarization to circular polarization. After passing through a scanning galvanometer and a focusing mirror, the sample is illuminated, and the light scattered from the sample returns to the polarization-maintaining coupler (end d) along the original path. The reference beam entering end c and the sample beam entering end d interfere, and the interfered light exits the polarization-maintaining coupler. The light exiting end a enters the polarization beam splitter; the vertical light passes through a circulator and enters the first balanced detector, while the horizontal light enters the second balanced detector. After passing through the polarization beam splitter, the light emitted from end b of the polarization-maintaining coupler enters the first balanced detector along the vertical direction and the light along the horizontal direction enters the second balanced detector. After receiving light, the first and second balanced detectors convert it into electrical signals, which are then transmitted to the signal processing device for calculation. The dispersion-compensated sample polarization image is obtained through full-depth dispersion compensation and PS-OCT data processing.

3. An electronic device, characterized in that, It includes a processor and a memory connected to the processor, the memory storing instructions for the processor to execute, and the processor being configured to execute the instructions stored in the memory to perform the steps of the full-depth dispersion compensation method of claim 1.

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