CN114764220B - Method for improving speckle autocorrelation reconstruction effect based on off-axis digital holography - Google Patents
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Abstract
The invention provides a method for improving speckle autocorrelation reconstruction effect based on off-axis digital holography, and belongs to the field of optical scattering medium imaging. In the prior research scheme aiming at the condition that an imaging target is in the range of the optical memory effect of a scattering medium, although the imaging method based on speckle auto-correlation is simple, convenient and easy to implement, due to the existence of imaging environment noise and instrument thermal noise, serious noise phenomenon can occur when the imaging target is reconstructed by utilizing single-frame speckle auto-correlation operation and a phase recovery algorithm, and the reconstruction effect is influenced. The invention provides a speckle autocorrelation imaging method for reducing the influence of static noise in the process of reconstructing an image based on digital off-axis holography. Firstly, utilizing diffraction of off-axis hologram to reproduce and separate out speckle light fields of imaging object light beams, then utilizing a phase shift method to calculate the speckle light fields before and after phase shift by a difference value so as to effectively eliminate noise items of the imaging object light speckle light fields, and finally utilizing speckle autocorrelation and a phase recovery algorithm to reconstruct an imaging target.
Description
Technical Field
The invention belongs to the field of optical scattering medium imaging, and relates to a method for improving speckle autocorrelation reconstruction effect based on off-axis digital holography.
Background
The problem of imaging scattering media has been an important research topic in the fields of biological imaging, astronomical imaging, etc. for decades. With the rapid development of science and technology, various new technologies are applied to solve the problem of scattering medium imaging, and many research results have been achieved. For example: an optical coherence tomography technology for directly measuring ballistic photons, an adaptive optical technology developed from astronomy, a phase conjugation technology, a wave-front shaping technology for feedback optimization, a measurement technology based on an optical transmission matrix, and the like. With further development of computer technology, neural networks and deep learning methods have shown great potential as important approaches to solving scattering medium imaging. The application of holographic optics also provides a new technical scheme for solving the problem of imaging of a scattering medium, for example, goodman et al in 1966 propose a holographic technology for improving the resolution of an image passing through the scattering medium by using a wavefront reconstruction method, and Kogelnik et al in 1968 propose an off-axis holographic technology based on the Goodman technology. In recent years, the development of relevant imaging technology of speckle is promoted by the rise of statistical optics, the spatial invariance of scattering medium point spread function in the optical memory effect range is utilized, in the process of applying speckle autocorrelation, the speckle light field autocorrelation of an imaging target is in direct proportion to the autocorrelation of an object light field, and then the imaging target is effectively reconstructed by a phase recovery algorithm.
In various existing technical schemes aiming at the condition that an imaging target is in the range of the optical memory effect of a scattering medium, the imaging method based on speckle autocorrelation has the characteristics of simplicity, convenience and easiness in implementation, however, in the practical application process, noise cannot be effectively restrained when the imaging target is reconstructed by utilizing single-frame speckle autocorrelation operation and a phase recovery algorithm due to the existence of imaging environment noise and instrument thermal noise, and a better reconstruction effect is obtained.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a speckle autocorrelation imaging method for reducing the influence of static noise in the process of reconstructing an image based on digital off-axis holography aiming at the condition that an imaging target is positioned in the optical memory effect range of a scattering medium. On the basis of digital off-axis holographic imaging, the difference value operation between two digital holographic speckle interference fields after any one phase shift is introduced to eliminate static noise items in the imaging process so as to improve the speckle autocorrelation imaging effect.
The imaging structure schematic diagram of the invention is shown in fig. 1, the basic architecture is a schematic diagram based on digital off-axis holography, and the difference is that a scattering medium is introduced into an optical path of object light imaging to serve as an analog imaging environment of scattering medium imaging. In the actual imaging process, in order to effectively eliminate the influence of static noise items formed by environmental noise, thermal noise and the like on the reconstructed imaging target, any one-time phase shift method in holographic optics is adopted. Firstly, utilizing diffraction of off-axis holography to reproduce and separate out speckle light field of imaging object light beam, then utilizing phase shift method to obtain holographic speckle interference field after any phase shift, calculating difference value to remove static noise item in the imaging object light speckle light field, and finally utilizing speckle autocorrelation and phase recovery algorithm to reconstruct imaging target with better denoising effect.
The invention discloses a method for improving speckle autocorrelation reconstruction effect based on off-axis digital holography, which comprises the following steps:
(1) An off-axis holographic imaging process of an imaging target passing through a scattering medium;
(2) Denoising by a single phase shift method;
(3) Reconstructing an undisturbed object light speckle light field by digital holography;
(4) The autocorrelation operation in combination with the phase recovery algorithm reconstructs the imaged object from the speckle.
The specific implementation process of the step (1) is as follows:
in the imaging process of off-axis holography, as shown in fig. 1, considering the influence of static noise in noise items such as environmental noise and thermal noise in the imaging process, a holographic speckle interference field after interference of object light speckle and reference light can be expressed as:
o as above S (x, y) represents a speckle pattern formed by the imaging target O (x, y) passing through the scattering medium, R (x, y) represents a reference beam in off-axis holography, C 1 (x, y) represents the static noise term during imaging. I S (x, y) represents the off-axis holographic field formed by the interference of the object light speckle field and the reference light.
The specific implementation of the step (2) is as follows:
using a spatial light modulator to introduce a known random non-2pi integer multiple constant phase shift into the reference beam
The phase distribution is such that,
I′ S (x, y) represents the holographic interference speckle light field after single phase shift, and the difference value of the holographic interference speckle light fields before and after phase shift is calculated to eliminate the static noise item C 1 The interference of (x, y),
because off-axis holography is adopted, the speckle light field of the reconstructed light and the conjugate field thereof can be effectively separated in the diffraction reconstruction process, and the zero-order diffraction term is completely eliminated in the difference value operation.
The specific implementation of the step (3) is as follows:
the undisturbed object light speckle field is reconstructed using digital diffraction of the original reference beam R (x, y), expressed as follows:
and regarding the obtained speckle interference digital hologram as an initial plane light field, and reconstructing an undisturbed object light speckle light field through diffraction of the original reference light. (4) The first term represents reconstructing the conjugate optical speckle light field
The second term represents the reconstruction of the optical speckle field +.>
The plural items in the brackets do not affect the distribution of the reconstructed image, so that the interference-free optical speckle light field O for removing the noise items can be finally obtained S (x,y)。
The specific implementation of the step (4) is as follows:
when the size of the imaging target is in the optical memory effect range of the scattering medium, the system point spread function S (x, y) is kept unchanged in the optical memory effect imaging range, so that the autocorrelation S (x, y) operation between the point spread functions is an ideal two-dimensional impulse response function delta (x, y), and the interference-free optical speckle light field O obtained in the step three S The (x, y) autocorrelation yields the following formula:
it can be seen that the autocorrelation function of the speckle light field is proportional to the autocorrelation function of the object light field, and is determined from O according to the wiener Xin Qin theorem S The autocorrelation function of (x, y) derives the power spectral density of O (x, y):
O(x,y)∝F -1 {sqrt{F{(O S (x,y)★O S (x,y))}}} (6)
and then, the iterative GS phase recovery algorithm is utilized to realize the complete reconstruction of the imaging target O (x, y).
The invention has the following advantages:
(1) The method can effectively remove static noise items in imaging noise items. Only one non-2 pi integer multiple constant phase shift is required to be introduced
The phase distribution can eliminate the static term.
(2) The method is simple to operate. Only the application of a spatial light modulator is required to introduce an interference speckle field with a once constant phase shift.
Drawings
FIG. 1 is a schematic diagram of the imaging method of the present invention.
Fig. 2 shows an imaged object pattern 1024 x 1024 in an example of the invention.
Fig. 3 shows off-axis digital holographic interference speckle light field patterns 1024 x 1024 prior to phase shifting in an example of the present invention.
Fig. 4 shows the phase shifted off-axis digital holographic interference speckle light field pattern 1024 x 1024 in an example of the present invention.
Fig. 5 shows a 1024 x 1024 interference-free off-axis digital holographic interference speckle light field pattern with static noise term cancellation by difference operation before and after phase shift in the embodiment of the present invention.
FIG. 6 is a diagram of an example of the present invention for reconstructing an undisturbed object light speckle field using digital diffraction of an original reference beam.
Fig. 7 is a flow chart of a phase recovery algorithm used in an example of the present invention.
Fig. 8 illustrates the reconstruction of an imaged target amplitude and phase pattern 1024 x 1024 by a speckle auto-correlation and phase recovery algorithm in an example of the invention.
Fig. 9 shows patterns 1024 x 1024 of the improvement of the autocorrelation imaging effect of other different imaging targets in the present embodiment, and the comparison result of the result similarity (SSIM) of the reconstructed imaging targets before and after the phase shift.
The letters shown in the above figure 1 are:
laser is a Laser, C is a Laser collimator, A1 and A2 are aperture diaphragms, L1-L3 are positive lenses, BS1-BS3 are beam splitters, M is a reflector, P is a polarizer, SLM is a reflective spatial light modulator, object is an imaging target, D is a scattering medium, and CCD represents a speckle digital hologram receiving camera.
The upper left corner of the light field diagram of the split diffraction speckle pattern is shown in the above-described square box of fig. 6.
The two images of fig. 7 show the amplitude (a) and the phase (b) of the reconstructed object after denoising.
Detailed Description
The invention will be further described with reference to the accompanying drawings, in which a specific embodiment of the invention is described.
In this example, the imaging schematic diagram is shown in fig. 1, the imaging target used is a gray image (1024×1024 pixels), as shown in fig. 2, the off-axis digital holographic interference speckle light field pattern before denoising is shown in fig. 3, the off-axis digital holographic interference speckle light field pattern after denoising is shown in fig. 4, the interference-free off-axis digital holographic interference speckle light field pattern for eliminating static noise items by difference operation before and after phase shifting is shown in fig. 5, and the interference-free object light speckle light field is reconstructed by using the original reference beam digital diffraction as shown in the upper left corner block intercepted in fig. 6.
The phase recovery algorithm used in this example is an iterative GS phase recovery algorithm, and a specific algorithm flow chart is shown in fig. 7. The imaged target amplitude and phase pattern is reconstructed using a speckle auto-correlation and phase recovery algorithm as shown in fig. 8 (a) and 8 (b). The whole process is simulated and realized in the MATLAB2018a environment.
Claims (1)
1. A method for improving speckle autocorrelation reconstruction effect based on off-axis digital holography is characterized by comprising the following steps: in the imaging process of a scattering medium, by introducing a phase shift method in off-axis holography and combining an autocorrelation operation and a phase recovery algorithm, eliminating static noise in a reconstructed image obtained by carrying out the autocorrelation operation on a speckle image, and obtaining a target image with better effect; the method comprises the following steps:
step 1 introduces a single phase shift:
the off-axis holographic imaging process after the imaged object passes through the scattering medium is represented by,
wherein O is S (x, y) represents a speckle pattern formed by the imaging target O (x, y) passing through the scattering medium, R (x, y) represents a reference beam in off-axis holography, C 1 (x, y) represents the static noise term in the imaging process, I S (x, y) represents an off-axis holographic light field formed by interference of the object light speckle light field and the reference light;
by means of a spatial light modulator, the reference beam is introduced into a known fixed random phase shift distribution which is not an integer multiple of 2 pi
Obtaining
Wherein I' s (x, y) represents the holographic interference speckle light field after single phase shift, and the difference value of the holographic interference speckle light fields before and after phase shift is calculated to eliminate the static noise term C 1 ) x, y) interference, yielding
Step 2, reconstructing an undisturbed object light speckle light field through digital holography:
taking the speckle interference digital hologram in the step (3) as an initial plane light field, carrying out digital diffraction operation by using reference light, reconstructing an undisturbed object light speckle light field,
wherein the first term represents reconstructing the conjugate optical speckle light field
The second term represents the reconstruction of the optical speckle field +.>
The plural items in the brackets do not affect the light intensity distribution of the reconstructed image, so that the speckle light field represented by the first item and the second item is spatially separated by utilizing the characteristic of off-axis holography, and finally the interference-free optical speckle light field O with noise removed is obtained S (x,y)；
Step 3, reconstructing an imaging target from the speckle using an autocorrelation operation in combination with a phase recovery algorithm:
on the premise that the imaging target is located in the optical memory effect range of the scattering medium, the interference-free optical speckle light field O obtained in the step 2 S The autocorrelation function of (x, y) has the following relationship with the autocorrelation function of the object light field:
O S (x,y)★O S (x,y)＝O(x,y)★O(x,y) (5)
wherein ∈represents the correlation operation, according to wiener Xin Qin theorem, from O S The (x, y) autocorrelation derives the power spectral density of O (x, y) as:
O(x,y)∝F -1 {sqrt{F{(O S (x,y)★O S (x,y))}}} (6)
and then, obtaining the phase distribution of O (x, y) by using an iterative GS phase recovery algorithm, and realizing the complete reconstruction of the imaging target O (x, y).
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