CN112526761B

CN112526761B - Imaging method based on optical phase control speckle field

Info

Publication number: CN112526761B
Application number: CN202011599786.5A
Authority: CN
Inventors: 袁园; 陈辉; 王高
Original assignee: Individual
Current assignee: Suzhou Guangyin Technology Co ltd; Yuan Yuan
Priority date: 2020-12-29
Filing date: 2020-12-29
Publication date: 2022-04-22
Anticipated expiration: 2040-12-29
Also published as: CN112526761A

Abstract

The invention relates to the technical field of optical phased array imaging, and discloses an imaging method based on an optical phased speckle field, which comprises the following steps: s1: dividing light beams emitted by a coherent light source into multiple paths of sub-light beams, and modulating the phase of each sub-light beam respectively to enable the modulated multiple paths of sub-light beams to interfere with each other in the transmission process to form a speckle field; s2: dividing the speckle field into an illumination speckle field and an imaging speckle field, wherein the illumination speckle field is used for illuminating a target surface where a target object is located, and the imaging speckle field is projected onto an imaging surface; s3: detecting the light intensity of the illumination speckle field reflected from the target surface in real time; s4: and modulating the coherent light source in real time according to the detected light intensity to enable the real-time output light intensity change curve and the detected light intensity change curve to meet a preset relation, so that the photosensitive device on the imaging surface displays the image of the target surface in real time. The invention fully utilizes the characteristic that the optical phased array light source can output ultra-high speed variable speckles, and realizes ultra-high speed imaging.

Description

Imaging method based on optical phase control speckle field

Technical Field

The invention relates to the technical field of optical phased array imaging, in particular to an imaging method based on an optical phased speckle field.

Background

Currently, ultra-high speed imaging can be divided into two modes:

1) the detection speed of an array detector (such as a CCD) is improved, the current fastest CCD camera can reach the imaging rate of 1MHz, but the resolution is low and the light intensity is required to be strong.

2) An object is illuminated with high-speed varying speckle (spatial light intensity distribution has a certain structure) and then detected with a single-point detector (for example: barrel detector) to detect the intensity of the echo at high speed, and the image of the object is obtained through calculation.

The second common technique is a single-pixel method such as laser radar and ghost imaging. The lidar employs a point-by-point scanning (i.e., each speckle is a moving pixel), while the ghost employs random, Hadamard, etc. speckles. To achieve ultra-high speed imaging, not only is the speed at which the speckles change required to be fast, but also the exact pattern of each speckle needs to be known in real time in order to compute the image of the object.

Techniques for generating rapidly varying speckle, commonly referred to as spatial light modulation techniques, can be divided into modulation of incoherent light and coherent light. The incoherent light spatial modulator directly modulates the light intensity of each point in space, such as a DMD, an LED array and the like. The number of the lattice and the modulation rate of the incoherent light spatial modulator are two mutually restricted relations. For example, the DMD has a large number of lattices, but its modulation rate is slow, typically around 20 kHz; the modulation rate of the LED array can reach hundreds of MHz, but when the dot matrix is increased, the modulation rate is linearly reduced, and the imaging with high rate and high resolution is difficult to realize. The coherent light space modulator modulates the phase of each point light field in space, and the light fields of each point interfere with each other in propagation and form an interference pattern on a target surface. The coherent light spatial modulator can form a complex interference pattern through the light field interference of a few dot matrixes, thereby realizing the spatial light intensity modulation with large equivalent dot matrix quantity and high modulation rate. Therefore, coherent light spatial modulation techniques are the subject of intense research in high-speed, high-resolution imaging techniques.

Optical phased arrays are an important direction of coherent optical spatial modulation techniques, which derive from conventional microwave phased array techniques. The optical phased array is mainly used for generating beam deflection, has wide application prospect in the aspect of military and civil beam scanning, and is the most important application in the military application fields of laser phased array radar, space laser communication and the like except in the aspects of laser display, laser communication, laser phototypesetting and the like. Currently, there are several methods for implementing optical phased arrays for beam deflection: based on liquid crystals; based on PLZT; based on an optical waveguide; based on MEMS devices. Each of these methods has advantages and disadvantages. Although the phased array adopting the liquid crystal material has low working voltage and can realize large-angle deflection, the response of the liquid crystal is very slow, and the modulation rate is low. The speed of the PLZT-based optical phased array reaches sub-microsecond, and the PLZT-based optical phased array has the capability of rapid continuous scanning, but the modulation voltage required by the PLZT material is too high, and the scanning angle is small. The optical waveguide phased array has high response speed, but the two-dimensional large-angle scanning is difficult to realize, and the modulation rate and the phase stability are difficult to realize, so that the further development of the optical waveguide phased array is limited. Although the scanning speed of the MEMS optical phased array is high, the scanning angle is limited (the modulation rate of the traditional phased array is inversely proportional to the scanning field angle, the field angle is generally not more than 3 degrees at the rate of MHz or more, and is generally not more than 30 degrees at the low rate of 100 Hz). Therefore, the current phased array technology cannot realize the performance of large-angle and high-speed two-dimensional light beam scanning, and the development of point scanning imaging (laser radar) is restricted.

Besides being used for beam deflection, optical phased arrays can also be used for generating speckle (especially random speckle), and imaging is realized by means of ghost imaging, which is also an important research direction at present. The adoption of a volume type electro-optical modulator can generate an accurate control phase, but the modulation voltage is hundreds to thousands of volts, and the modulation of a multi-lattice exceeding high speed (generally not exceeding 1MHz) is difficult to realize, so the modulation rate and the diversity of speckles are difficult to meet the requirement of high-speed imaging. The waveguide type electro-optic modulator has low working voltage (generally not more than 5V) and high modulation rate of 100GHz, but the waveguide type modulator has serious phase shift, and can shift one wavelength in 1 second. If the environment is vibrated and the temperature changes slightly greatly, the drift of the environment is quicker and more serious. Thus, the speckle produced by high-speed phased arrays made with waveguide-type electro-optic modulators is essentially unpredictable. Since accurate information of speckles is required to be known in real time in ghost imaging, the high-speed phased array light source is difficult to be used for imaging.

In summary, there is no imaging technique based on spatial light modulation technique with high imaging rate, large field angle and high resolution.

Disclosure of Invention

The invention provides an imaging method based on an optical phased speckle field, which solves the problem that in the prior art, a phased array is difficult to realize high-speed beam scanning, so that high-speed imaging cannot be realized.

The invention discloses an imaging method based on an optical phase control speckle field, which comprises the following steps:

s1: dividing light beams emitted by a coherent light source into multiple paths of sub-light beams, and modulating the phase of each sub-light beam respectively to enable the modulated multiple paths of sub-light beams to interfere with each other in the transmission process to form a speckle field;

s2: the speckle field is divided into an illumination speckle field and an imaging speckle field, the illumination speckle field is used for illuminating a target surface where a target object is located, the imaging speckle field is projected onto an imaging surface, and light intensity distribution formed by the illumination speckle field and the imaging speckle field on the imaging surface and the target surface is conjugated respectively;

s3: detecting the light intensity of the illumination speckle field reflected from the target surface in real time;

s4: and modulating the coherent light source in real time according to the detected light intensity to enable the output light intensity change curve and the real-time detected light intensity change curve to meet a preset relation so as to display the image of the target surface on the photosensitive device on the imaging surface in real time.

In step S1, the coherent light source emits a coherent light beam, and the coherent light beam is divided into multiple sub-light beams.

In step S1, the two coherent light sources respectively emit a first coherent light beam and a second coherent light beam, the first coherent light beam and the second coherent light beam are combined into one beam and then divided into multiple sub-light beams, in step S2, a beam splitter is used to separate light from the first coherent light beam and light from the second coherent light beam in the speckle field, the light from the first coherent light beam becomes an illumination speckle field, the light from the second coherent light beam becomes an imaging speckle field, and in step S4, the second coherent light source is modulated.

The beam splitter is a beam splitter with a waveband selection function.

Wherein, in step S1, the method specifically includes:

phase modulation is carried out on the multi-path sub-beams by adopting a random modulation mode, and the following conditions are met after modulation:

the additional phases of the sub-beams are: phi (p)_jT), where ρ_jThe light intensity distribution formed by the multiple sub-beams on the target surface is:

wherein z is the distance between the emission surface of the sub-beam and the target surface, ξ is the coordinate of the emission surface, r is the coordinate of the target surface, λ is the wavelength, δ is the wavelength_aFor delta-like functions, the subscript a characterizes the size of the sub-beams,

the intensity distribution on the imaging plane is conjugate on the target plane and is expressed as:

M(ζ，t)＝α·S(m·ζ，t)，

where, ζ is the coordinate of the imaging surface, m is the amplification factor, t is the time, α is a constant, and represents the difference of the overall brightness of the two surfaces.

In step S4, the first step,

after the illumination speckle field irradiates the target surface, the return light returned from the target surface is detected by the detector, and the total light intensity is as follows:

B(t)＝∫S(r，t)·O(r)dr (1)

wherein, o (r) is the response function of the object to the illumination speckle field, the detected light intensity is used for modulating the light intensity of the coherent light source generating the imaging speckle field in real time, the photosensitive device on the imaging surface performs time integration on the changed speckle field, and the result can be expressed as:

wherein T { } is a specific modulation function, Δ T is the integration time of the photosensitive device, and the photosensitive device on the imaging surface shows the image of the object.

Wherein the modulation function T { } is a linear modulation:

wherein mu is a modulation coefficient, and after modulation, an image displayed on the photosensitive device is,

wherein, delta_D(m ζ -r) is a delta-like function, the subscript D characterizes the peak width, i.e., the resolution, G (ζ, t) is the object and δ_DI.e. an image of the target object with a resolution D.

In step S1, a multi-path phase modulator or an integrated phased array is used to perform phase modulation on the multi-path sub-beams.

In the imaging method based on the optical phase control speckle field, the speckle field which changes along with time is generated by multi-path phase modulation, and is divided into an illumination speckle field and an imaging speckle field, wherein the illumination speckle field is projected to a target surface, the imaging speckle field is projected to an imaging surface, two speckle fields are conjugated on the imaging surface and the target surface, the total light intensity reflected by the illumination speckle after illuminating an object is used for modulating in real time to generate the output power of a coherent light source of the imaging speckle, so that the output light intensity change curve and the light intensity change curve detected in real time meet a preset relation, the correlation of a barrel detection signal (echo signal) and the speckle is realized by the time integration carried out by a photosensitive device of the imaging surface, and the correlation can display the image of the object in real time according to a ghost imaging principle (as shown in the formula (4). This way of correlating the echo signal with the conjugate speckle in real time has a very important advantage: the speckle pattern need not be known throughout the imaging process. Therefore, although the phase drift of the optical phased array built by the waveguide type electro-optic modulator cannot predict the speckle pattern formed by the optical phased array, the method can still accurately image. Moreover, the phased array can realize the modulation rate of 100GHz, so that the imaging rate of MHz level can be realized, even the imaging rate can reach GHz. The invention solves the problem of high-speed imaging caused by the requirement that the speckle pattern needs to be known in real time in the traditional ghost imaging, fully utilizes the characteristic that the optical phased array light source can output ultra-high-speed variable speckles, and realizes ultra-high-speed imaging. The imaging field angle of the invention is not limited by the modulation rate, and the field angle is not limited theoretically, thereby realizing large-field-angle imaging.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

FIG. 1 is a flow chart of an imaging method based on an optical phase-controlled speckle field according to the present invention;

FIG. 2 is a schematic diagram of an optical path for carrying out the method of FIG. 1;

FIG. 3 is a schematic diagram of another optical path for carrying out the method of FIG. 1;

FIG. 4 is a schematic diagram of another optical path for carrying out the method of FIG. 1.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The imaging method based on the optical phased speckle field of the embodiment is shown in fig. 1, and comprises the following steps:

step S1: the light beam emitted by the coherent light source is divided into multiple sub-beams, and the phases of the sub-beams are modulated respectively, so that the modulated multiple sub-beams are mutually interfered to form a speckle field in the transmission process.

Step S2: the speckle field is divided into an illumination speckle field and an imaging speckle field, the illumination speckle field is used for illuminating a target surface where a target object is located, the imaging speckle field is projected onto an imaging surface, and light intensity distribution formed by the illumination speckle field and the imaging speckle field on the imaging surface and the target surface is conjugated respectively; since conjugation generally refers to the conjugation of two planes, it is assumed that the target object is a two-dimensional object, and the plane on which the target object is located is called a target plane or an object plane, and if the target object is a three-dimensional object, the target plane is the plane on which the target object performs two-dimensional mapping.

Step S3: detecting the light intensity of the illumination speckle field reflected from the target surface in real time;

step S4: and modulating the coherent light source in real time according to the detected light intensity to enable the light intensity change curve output by the coherent light source to meet a preset relation with the detected light intensity change curve in real time, so that the photosensitive device on the imaging surface can display the image of the target surface in real time. The predetermined relationship may be a linearly varying relationship in phase or in anti-phase, or a non-linear relationship with image enhancement effects. The sub-beams interfere with each other to form speckles on the imaging surface, and the modulation means modulating the light intensity of the beam emitted by the coherent light source, so that the speckle is totally darkened or lightened through the modulation, but the speckle pattern is not changed. The phases of all the sub-beams are modulated at intervals (for example, 10ns, corresponding to a modulation rate of 100 MHz), then a new speckle pattern is generated, only one speckle pattern irradiates an object, the image of the object cannot be displayed on an imaging surface, the image can be generated after a plurality of different speckles are superposed, and the image is closer to a real object when the number of the speckle patterns is larger, but is only infinitely close. Such as: if the required resolution is 100 x 100, 10 tens of thousands of different speckles can well display one image, if the phase modulation rate is 100MHz, 1ms completes one frame of image, and the imaging frame frequency is 1 kHz.

In the imaging method based on the optical phase control speckle field, the speckle field which changes along with time is generated by multi-path phase modulation, and the speckle field is divided into an illumination speckle field and an imaging speckle field, wherein the illumination speckle field is projected to a target surface, the imaging speckle field is projected to an imaging surface, two speckle fields are conjugated on the imaging surface and the target surface, the total light intensity reflected by the illumination speckle after illuminating an object is used for real-time modulation to generate the total output power of a coherent light source of the imaging speckle, the output light intensity change curve and the real-time detected light intensity change curve meet the preset relation, the correlation of a barrel detection signal (echo signal) and the speckle is realized by the time integration carried out by a photosensitive device of the imaging surface, and the correlation can display the image of the object in real time according to the ghost imaging principle (as shown in the formula (4). This way of correlating the echo signal with the conjugate speckle in real time has a very important advantage: the speckle pattern need not be known throughout the imaging process. Therefore, the optical phased array built by the waveguide type electro-optic modulator can still accurately form images although the speckle patterns formed by the optical phased array cannot be predicted due to phase drift. Moreover, the phased array can realize the modulation rate of 100GHz, so that the imaging rate of MHz level can be realized, even the imaging rate can reach GHz. The invention solves the problem of high-speed imaging caused by the requirement that the speckle pattern needs to be known in real time in the traditional ghost imaging, fully utilizes the characteristic that the optical phased array light source can output ultra-high-speed variable speckles, and realizes ultra-high-speed imaging. The imaging field angle of the invention is not limited by the modulation rate, and the field angle is not limited theoretically, thereby realizing large-field-angle imaging.

The method can be implemented by adopting the following two main types of light path structures:

the first type of optical path configuration is shown in fig. 2, where the beam splitter is a band-free selective beam splitter 50. Coherent light emitted by a coherent light source 100 is divided into a plurality of sub-beams by a beam splitter 200, the plurality of sub-beams are input into a multi-path phase modulator 300, the multi-path sub-beams are subjected to high-speed phase modulation by a phase modulation circuit 11, the output modulated sub-beams are projected by a projector 4, a non-band selection beam splitter 50 divides an emergent light field into an illumination speckle field I and an imaging speckle field II, the illumination speckle field I forms a speckle light field on a target surface and irradiates on a target object 8, the imaging speckle field II is projected onto a photosensitive device 7 (usually a CCD) by a screen projector 6 (usually consisting of one or more lenses), and the speckle field of the imaging surface is conjugated with the speckle field of the target surface. After the illumination speckle field I is reflected by the target object 8, the light intensity of the echo is detected by the detector 9, the output power of the coherent light source 100 is adjusted in real time through the light intensity modulation circuit 10 according to the intensity signal of the light intensity, and the light source enables the real-time output light intensity change curve and the detected light intensity change curve to meet a predetermined relationship (for example, the two change curves are kept consistent in real time, namely, the linear change relationship of the same phase). Because the light intensity distribution of the imaging surface and the target surface is synchronous, the real-time adjustment of the output power of the coherent light source 100 forms the correlation calculation of the echo light intensity and the illumination speckle field, and under the time integration of the photosensitive device 7, the photosensitive device 7 can present the image of the target object 8 in real time according to the imaging mechanism of the ghost imaging. Since the modulation frequency of the multi-channel phase modulator 300 is usually quite high (up to 100GHz), there is currently no photosensitive device with such a speed that the imaging result can be recorded, and therefore the frame frequency of the final imaging usually depends on the refresh rate of the photosensitive device (such as the acquisition frame rate of the CCD). Compared with the traditional CCD imaging, the imaging method has the advantages that high-speed imaging can be well carried out under the conditions of long distance and weak light, the requirement of the CCD on light intensity is too high, and imaging is difficult in the traditional mode under the same conditions.

In the optical path structure, the optical splitter 200 may be a one-to-many optical splitter or a splitter with a closely-arranged optical fiber structure. The multi-path phase modulator 300 may be composed of a plurality of waveguide-type electro-optical modulators, each responsible for adjusting the phase of one sub-beam; if the total modulation rate of the multi-path phase modulator 300 and the phase modulation circuit 11 is 10GHz and the resolution of the illumination speckle field is 100 × 100, the imaging frame frequency can reach 1 MHz. The phase modulation circuit 11 is composed of an FPGA and a plurality of 3PD5651E digital-to-analog conversion chips, and is configured to drive each phase modulator of the multi-path phase modulator 300 to perform phase modulation on a plurality of sub-beams simultaneously.

The second type of optical path structure is shown in fig. 3, two coherent light sources with different wavebands, i.e., a first coherent light source 101 and a first coherent light source 102, are adopted, and in step S1, light emitted by the two coherent light sources is combined by a beam combiner 110 and input into a subsequent optical path. In step S2, the beam splitter is the beam splitter 51 with wavelength selection, the light field from the first coherent light source 101 can be transmitted to the target surface through the beam splitter 51 and form the illumination speckle field I, and the light field from the second coherent light source 102 is refracted by the beam splitter 51 to the light sensing device 7 and form the imaging speckle field II, in step S4, the light intensity modulation circuit 10 modulates the second coherent light source 102.

Both types of optical path structures may employ a multi-path phase modulator 300 composed of multiple phase modulators, or an integrated phased array 301 as shown in fig. 4. The integrated phased array 301 may be a monolithic electro-optical crystal, and after a beam of relevant light enters, the light is divided into multiple paths and propagates forward inside the crystal, and the phase modulation circuit 11 modulates the multiple paths of light with different phases in real time during propagation in the crystal, so as to form multiple paths of outgoing sub-beams with different phases.

In this embodiment, a random modulation method is adopted to perform phase modulation on multiple sub-beams, and the following conditions are satisfied after modulation:

the additional phases of the sub-beams are: phi (p)_jT), where ρ_jIs the coordinate of the jth sub-beam. The light intensity distribution of the multi-path sub-beams on the target surface is as follows:

in the above formula, z is the distance from the emission surface of the sub-beam (the exit surface of the projector 4) to the target surface, ξ is the coordinate of the emission surface, r is the coordinate of the target surface, λ is the wavelength, δ_aFor delta-like functions, the subscript a characterizes the size of the sub-beams.

M(ζ，t)＝α·S(m·ζ，t)，

in the above formula, ζ is the coordinate of the imaging surface, m is the amplification factor, t is the time, α is a constant, and represents the difference in overall brightness between the two surfaces.

B(t)＝∫S(r，t)·O(r)dr (1)

in the above formula, O (r) is the response function of the object to the illumination speckle field, the detected light intensity is used for modulating the light intensity of the coherent light source generating the imaging speckle field in real time, the photosensitive device on the imaging surface performs time integration on the changed speckle field, and the result can be expressed as

In the above formula, T { } is a modulation function. Δ T is the integration time of the photosensitive device. The photosensitive device on the imaging plane develops an image of the object.

The modulation function T { } is preferably linear modulation:

where μ is the modulation factor. After modulation, the image displayed on the photosensitive device is,

in the above formula, δ_D(m ζ -r) is a delta-like function, and the subscript D characterizes the width of the peak, i.e., the size of the resolution. G (ζ, t) is an object and δ_DI.e. an image of an object with a resolution D.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims

1. An imaging method based on an optically phased speckle field, comprising the steps of:

2. The method for imaging based on optically phased speckle field according to claim 1, wherein in step S1, the coherent light source emits a coherent light beam, and the coherent light beam is divided into multiple sub-beams.

3. The method according to claim 1, wherein in step S1, the two coherent light sources respectively emit a first coherent light beam and a second coherent light beam, the first coherent light beam and the second coherent light beam are combined into a beam and divided into multiple sub-beams, in step S2, a beam splitter is used to separate light from the first coherent light beam and light from the second coherent light beam in the speckle field, the light from the first coherent light beam becomes the illumination speckle field, the light from the second coherent light beam becomes the imaging speckle field, and in step S4, the second coherent light source is modulated.

4. The method for imaging based on optically phased speckle fields of claim 3, wherein the beam splitter is a band selective beam splitter.

5. The optical phased speckle field-based imaging method according to claim 1, wherein the step S1 specifically comprises:

M(ζ，t)＝α·S(m·ζ，t)，

6. The method for imaging based on optically phased speckle field according to claim 5, wherein, in step S4,

B(t)＝∫S(r，t)·O(r)dr (1)

7. The optical phased speckle field-based imaging method of claim 6, wherein the modulation function T { } is a linear modulation:

8. The method for imaging based on optically phased speckle fields as claimed in any one of claims 1 to 7, wherein the step S1 employs a multi-path phase modulator or an integrated phased array to perform phase modulation on the multi-path sub-beams.