CN113534503A - Wavefront shaping method based on light intensity dependence - Google Patents
Wavefront shaping method based on light intensity dependence Download PDFInfo
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- CN113534503A CN113534503A CN202110462377.9A CN202110462377A CN113534503A CN 113534503 A CN113534503 A CN 113534503A CN 202110462377 A CN202110462377 A CN 202110462377A CN 113534503 A CN113534503 A CN 113534503A
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
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Abstract
The invention discloses a wave front shaping method based on light intensity dependence, and the central idea of the method is that more modulation phase units are distributed in a region with larger incident light intensity ratio on a spatial light modulator. Pixels on the spatial light modulator are non-uniformly combined and non-uniformly modulated according to the energy distribution of incident light. The output image collected by the CCD camera calculates a feedback signal, and the iterative optimization algorithm is used for controlling the spatial light modulator to modulate the phase of incident light, so that the transmission control of the light passing through the random scattering medium is realized more efficiently. The invention can realize the transmission control of light through the random scattering medium, and obtains higher enhancement factors with fewer iteration times when realizing the focusing of the light through the scattering body.
Description
Technical Field
The invention belongs to the field of laser physics, and relates to a transmission control device for realizing light passing through a random scattering medium and a wave front shaping method based on light intensity dependence.
Background
When laser light propagates in a random scattering medium, multiple scattering of incident light by random particles in the scattering medium can cause information carried by the incident light to be lost, and spatial coherence and temporal coherence of the incident light can be destroyed. In some random scattering media, random spatial fluctuations in mass density or dielectric constant can result in random changes in the direction of propagation of an elastic or electromagnetic wave. From a macroscopic point of view, these variations lead to a diffusion of the scattering phenomenon. In many scientific and engineering applications, it is important to control the transmission process of light in a randomly scattering medium. Researchers from theoretical physics and micro physics to the subjects of applying electromagnetism, applying mathematics, statistics, optics, acoustics, bioengineering, and the like, all attempt to discover and characterize the mechanism of the scattering phenomenon and attempt to control the scattering phenomenon. The realization of the transmission control of light through the random scattering medium can be applied to various engineering fields, including remote sensing, ultrasound, microwave imaging, nondestructive tissue imaging and the like.
There are various methods for controlling the transmission of light through a random scattering medium, and a wavefront shaping method based on feedback iteration is one of them. Its advantages are easy implementation and high effect. The wave front shaping method is to change the phase of incident light to compensate the phase distortion caused by multiple scattering, thereby influencing the emergent light field. The incident light is divided into N sub-wave sources which are respectively subjected to phase modulation by N phase modulation units on the spatial light modulator. The spatial light modulator provides millions of pixel modulation units. To balance time consumption and optimize efficiency, one typically merges pixels on the spatial light modulator. However, the wavefront shaping methods based on feedback iteration reported in the literature at present uniformly merge pixels on the spatial light modulator (Opt. Lett.32 (2007); Opt.express 5 (2012); J.Appl. Phys.8 (2018)). But the phase modulation efficiency of uniform combining is low and the resulting enhancement factor is limited.
Disclosure of Invention
The invention aims to provide a wavefront shaping method based on light intensity dependence, which can realize the focusing of light through a random scattering medium, obtains a higher enhancement factor in a shorter time and has the characteristics of simplicity and effectiveness.
The core idea of the invention is to take the intensity distribution of the incident light into account in the phase modulation process, thereby realizing more efficient transmission control of light through the random scattering medium.
In order to achieve the purpose, the invention adopts the technical scheme that:
a wave front shaping method based on light intensity dependence divides a spatial light modulator into a plurality of sub-regions, and phase modulation unit number distribution of each sub-region is carried out after light intensity ratio is calculated and multiplied by total phase modulation unit number of each sub-region. The feedback signal I is calculated by monitoring the light intensity in the target area collected by the CCD camera with a personal computerfeedbackAnd the iterative optimization algorithm is used for controlling the spatial light modulator to modulate the phase of the incident light, so that the aim of controlling the transmission and control of the laser through the random scattering medium is fulfilled. In the process of controlling the transmission of light through the random scattering medium by the feedback signal,
wherein
For the average intensity of the focus within the particular range selected,
the average intensity of other speckles collected by the CCD camera outside the selected specific range.
The invention has the following advantages: the pixels of the spatial light modulator are non-uniformly combined according to the light intensity spatial distribution of incident light, the incident light is regulated and controlled by combining an iterative optimization algorithm, the operation method is relatively simple, the spatial light modulator is combined and expanded to different degrees only through matrix operation, and the control of the transmission process of light through the random scattering medium can be completed by selecting the specific range of the CCD camera to be monitored on a computer. Meanwhile, compared with the traditional wave front shaping method, the method can obtain the focusing effect with higher enhancement factor.
Drawings
FIG. 1 is a graph of the results of the present invention controlling the focusing of light through a randomly scattering medium.
Fig. 1(a) is a random speckle image of light passing through a random scattering medium when the spatial light modulator is not phase-loaded.
Fig. 1(b) is a diagram of focusing effect of light modulated by the spatial light modulator through a random scattering medium.
Fig. 2 is a phase distribution diagram of the wavefront shaping method based on the light intensity dependence of the present invention.
Fig. 2(a) is a phase diagram of uniform combination of spatial light modulators by a conventional wavefront shaping method.
Fig. 2(b) is a phase diagram of non-uniform combination of spatial light modulators based on an intensity-dependent wavefront shaping method.
Fig. 2(c) - (f) are schematic diagrams of phase modulation unit division of each layer in non-uniform combination.
Fig. 3 is a comparison graph of the optimization process of the wave front shaping method based on the light intensity dependence under the sub-regions 1, 2 and 4 compared with the traditional wave front shaping method.
Detailed Description
The present invention will be further described with reference to the following examples and drawings, but the scope of the present invention should not be limited thereto.
The output laser of the micro solid laser passes through the variable continuous attenuator and the polaroid and then passes through the beam expander. The expanded light is reflected by a mirror and irradiated on a spatial light modulator. Incident light is focused on a random scattering medium through a 10 multiplied by microscope objective after being phase modulated by a spatial light modulator, the random scattering medium generates scattering to a focus, the scattered light is imaged on a CCD camera through a 20 multiplied by microscope objective, and simultaneously, a feedback signal I is calculated by monitoring the light intensity of a target area collected on the CCD camera through a personal computerfeedbackThe transmission process of light through the random scattering medium is controlled by using a genetic algorithm to control the spatial light modulator to change the phase of incident light.
The spatial light modulator is a pure phase type reflective spatial light modulator.
Said feedback signal
Wherein
The average intensity of the focus area in the selected specific range,
the average intensity of speckles collected by the CCD camera in other areas outside the selected specific range is adopted.
The genetic algorithm is realized by Matlab software programming.
Example 1: the invention controls the light to pass through the random scattering medium and then focus. Monitoring the light intensity of the target focusing region collected by the CCD camera by using a computer, and calculating a feedback signal
Using genetic algorithm to feedback signal I simultaneouslyfeedbackMonitoring and continuously optimizing the phase of incident light to make IfeedbackGradually increases to finally form a uniform and compact focus point. As a result, referring to fig. 1, (a) is a speckle image formed by light passing through a random scattering medium when the spatial light modulator is not loaded with a phase, and (b) is a focusing point formed by the spatial light modulator after performing phase modulation on incident light. Wherein the phase of the incident light is optimized before it. The number of subregions is set to 4. The spot coverage area on the spatial light modulator is divided into 4 parts according to the beam waist radius of the light beam. And respectively calculating the light intensity ratio of each part, and then multiplying the light intensity ratio by the total number of the phase modulation units to obtain the number of the phase modulation units to be allocated to each area. Phase diagram referring to fig. 2, (a) is a phase diagram generated by uniform combination of the conventional method, (b) is a phase diagram generated by the wavefront shaping method based on the light intensity dependence, and (c) - (f) are schematic diagrams of phase distribution of each subregion from inside to outside, respectively.
Referring to fig. 3, experimental results show that the phase of incident light is modulated by using a spatial light modulator using a wavefront shaping method based on light intensity dependence, the transmission process of light through a random scattering medium can be effectively controlled under different numbers of sub-regions, and a higher enhancement factor is obtained than that of a conventional wavefront shaping method.
Claims (6)
1. A method of wavefront shaping based on intensity dependence, characterized by: setting the number of subregions, carrying out region division on pixels on the spatial light modulator according to the number of the subregions, calculating the light intensity ratio of each subregion by utilizing the intensity spatial distribution of incident light, merging the pixels on the spatial light modulator in different degrees, monitoring the intensity of an output image target region collected on a CCD camera to calculate a feedback signal IfeedbackAnd controlling the spatial light modulator to modulate the phase of the incident light by using an iterative optimization algorithm so as to enable the feedback signal IfeedbackGradually rises, and finally the transmission control of light through the random scattering medium is realized.
2. An optical intensity dependence-based wavefront shaping method as defined in claim 1, further comprising: the number of the sub-regions is set, the number of the sub-regions can be randomly set, and in principle, the upper limit of the number of the sub-regions is the number of pixels contained in the narrow-side radius of the spatial light modulator.
3. An optical intensity dependence-based wavefront shaping method as defined in claim 1, further comprising: and dividing the area covered by the light spot on the spatial light modulator according to the number of the sub-areas, and dividing the pixels on the spatial light modulator according to the beam waist radius of the incident light source.
4. An optical intensity dependence-based wavefront shaping method as defined in claim 1, further comprising: and calculating the light intensity ratio of the sub-regions, wherein the product of the light intensity ratio of each sub-region and the total number of the modulation units is the number of the phase modulation units which each sub-region should be divided.
5. An optical intensity dependence-based wavefront shaping method as defined in claim 1, further comprising: and combining the spatial light modulators to different degrees, and uniformly combining the pixels in each subarea of the spatial light modulator according to the number of the modulation units which each subarea should be divided into until the number of the modulation units is equal to that of the pixels in each subarea.
6. An optical intensity dependence-based wavefront shaping method as defined in claim 1, further comprising: feedback iteration method for controlling transmission control process of light through random scattering medium, and feedback signal of control light in process of focusing through random scattering medium
Wherein
For the average intensity of the selected target focus range,
the average intensity of other pixels collected by the CCD camera (10) outside the selected target focusing range is calculated.
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| CN115696041A (en) * | 2022-10-26 | 2023-02-03 | 清华大学 | Non-invasive scattering medium internal focusing imaging method based on wave-front modulation iteration |
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