CN113534503B - Wavefront shaping method based on light intensity dependence - Google Patents
Wavefront shaping method based on light intensity dependence Download PDFInfo
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- CN113534503B CN113534503B CN202110462377.9A CN202110462377A CN113534503B CN 113534503 B CN113534503 B CN 113534503B CN 202110462377 A CN202110462377 A CN 202110462377A CN 113534503 B CN113534503 B CN 113534503B
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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
- G02F1/0121—Operation of devices; Circuit arrangements, not otherwise provided for in this subclass
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Abstract
The invention discloses a wavefront shaping method based on light intensity dependence, which has the central idea that more modulation phase units are distributed in a region with larger light intensity ratio of incident light 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 the incident light. The output image acquired 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 through the random scattering medium is more efficiently realized. 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 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 wavefront 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 may result in loss of information carried by the incident light, and spatial and temporal coherence may 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 elastic waves or electromagnetic waves. From a macroscopic point of view, these changes lead to the diffusion of scattering phenomena. In many scientific and engineering applications, it is important to control the transmission process of light in a random scattering medium. Researchers in the disciplines of theory physics and microcosmic physics, such as electromagnetics, mathematics, statistics, optics, acoustics and bioengineering, all attempt to discover and characterize the mechanism of scattering phenomena, and attempt to control scattering phenomena. 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 implementing the transmission control of light through a random scattering medium, and a wave front shaping method based on feedback iteration is one of them. Its advantages are easy implementation, and simple and convenient operation. The wave front shaping method compensates phase distortion caused by multiple scattering by changing the phase of incident light, thereby influencing an emergent light field. The incident light is divided into N sub-wave sources which are phase modulated by N phase modulation units on the spatial light modulator, respectively. Spatial light modulators provide millions of pixel modulation units. To balance time consumption and optimize efficiency, one typically merges pixels on a spatial light modulator. However, the wavefront shaping methods reported in the current literature based on feedback iteration all uniformly combine pixels on spatial light modulators (opt. Lett.32 (2007); opt. Express 5 (2012); j. Appl. Phys.8 (2018)). But the uniformly combined phase modulation is inefficient 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 focusing of light through a random scattering medium, obtain 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 during the phase modulation, so that a more efficient transmission control of the light through the random scattering medium is achieved.
In order to achieve the above purpose, the invention adopts the following technical scheme:
a wavefront shaping method based on light intensity dependence divides a spatial light modulator into a plurality of subareas, and each subarea respectively calculates the light intensity duty ratio and multiplies the total phase modulation unit number and then distributes the phase modulation unit number of each subarea. The feedback signal I is calculated by monitoring the light intensity in the target area collected on the CCD camera by means of a personal computer feedback And the iterative optimization algorithm is used for controlling the spatial light modulator to modulate the phase of the incident light, so that the purpose of controlling the transmission control of laser through the random scattering medium is achieved. During the process of controlling the transmission of light through the random scattering medium,
wherein->
For a focal point within a selected particular rangeAverage intensity,/->
The average intensity of other speckles collected for the CCD camera outside the particular range selected.
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 the 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 the light through the random scattering medium can be completed by selecting a 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 factors.
Drawings
FIG. 1 is a graph of the result of the present invention controlling the focusing of light through a random 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 showing the focusing effect of light modulated by the spatial light modulator by a random scattering medium.
Fig. 2 is a phase distribution diagram of the wavefront shaping method based on light intensity dependence of the present invention.
Fig. 2 (a) is a phase diagram of a conventional wavefront shaping method for uniformly combining spatial light modulators.
Fig. 2 (b) is a phase diagram of a non-uniform combination of spatial light modulators based on a wavefront shaping method that depends on light intensity.
Fig. 2 (c) - (f) are schematic diagrams of phase modulation element divisions for each layer in non-uniform combining.
Fig. 3 is a diagram showing the comparison of the optimization process of the wavefront shaping method based on the light intensity dependence with the conventional wavefront shaping method under the 1 subarea, the 2 subarea and the 4 subareas.
Detailed Description
The invention is further illustrated in the following examples and figures, which should not be taken to limit the scope of the invention.
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 the reflecting mirror and irradiated on the spatial light modulator. The incident light is focused on a random scattering medium through a 10X microscope objective after being subjected to phase modulation of a spatial light modulator, the random scattering medium scatters the focus, the scattered light is imaged on a CCD camera through a 20X microscope objective, and meanwhile, a personal computer monitors the light intensity of a target area collected on the CCD camera to calculate a feedback signal I feedback Genetic algorithms are used to control the spatial light modulator to alter the phase of incident light to effect control of the transmission of light through the random scattering medium.
The spatial light modulator is a pure phase type reflective spatial light modulator.
The feedback signal
Wherein->
For the average intensity of the focused region within the selected specific range,/>
The average intensity of the speckle for other areas collected by the CCD camera outside the particular range selected.
The genetic algorithm is realized by Matlab software programming.
Example 1: the present invention controls the post-focusing of light through a random scattering medium. The light intensity of the target focusing region collected by the CCD camera is monitored by a computer, and a feedback signal is calculated
Simultaneously using genetic algorithm for feedback signal I feedback Monitoring and continuously optimizing the phase of incident light to make I feedback Gradually increasing in value, eventually forming a uniform and tight focus. As a result, referring to FIG. 1, (a) is a spatial light modulator withoutA speckle image formed by light passing through a random scattering medium when carrying phases, and (b) a focusing point formed by modulating the incident light by a spatial light modulator. Wherein the phase of the incident light is optimized. The number of the subareas is set to be 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. The number of phase modulation units to be allocated to each region is calculated by multiplying the light intensity ratio of each portion by the total number of phase modulation units. Referring to fig. 2, (a) is a phase map generated by uniform combination of the conventional method, (b) is a phase map generated by a wavefront shaping method based on light intensity dependence, and (c) - (f) are phase distribution diagrams of each sub-region 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 the conventional wavefront shaping method.
Claims (3)
1. A wavefront shaping method based on light intensity dependence is characterized in that: setting the number of subareas, carrying out area division on pixels on the spatial light modulator according to the number of subareas, calculating the light intensity duty ratio of each subarea by utilizing the intensity spatial distribution of incident light, combining the pixels on the spatial light modulator to different degrees, and monitoring the intensity of an output image target area collected on a CCD camera to calculate a feedback signal I feedback And controlling the spatial light modulator to modulate the phase of the incident light by using an iterative optimization algorithm to enable the feedback signal I to be feedback Gradually rising values of the sub-regions, finally realizing transmission control of light through the random scattering medium, calculating the light intensity ratio of the sub-regions, wherein the product of the light intensity ratio of each sub-region and the total modulation unit number is the number of phase modulation units which should be divided in each sub-region, combining the space light modulator to different degrees, and uniformly combining pixels in each sub-region of the space light modulator according to the modulation unit number which should be divided in each sub-region until the light intensity ratio is equal to the modulation unit numberFeedback iteration method for controlling transmission control process of light through random scattering medium, feedback signal I during focusing process of light through random scattering medium feedback ＝I focus /I background Wherein I focus For the average intensity of the selected target focus range, I background The average intensity of other pixels collected by the CCD camera outside the selected target focus range.
2. A wavefront shaping method based on light intensity dependence as claimed in claim 1, wherein: setting the number of subareas, randomly setting the number of subareas, wherein the upper limit of the number of subareas is in principle the number of pixels contained in the radius of the narrow side of the spatial light modulator.
3. A wavefront shaping method based on light intensity dependence as claimed in claim 1, wherein: and dividing the light spot coverage area on the spatial light modulator according to the number of the subareas, and dividing pixels on the spatial light modulator according to the beam waist radius of the incident light source.
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