CN116068784B - Device for performing laser vibration-resistant focusing by using strong scattering medium and use method - Google Patents
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Abstract
The invention provides a device for carrying out laser anti-vibration focusing by utilizing a strong scattering medium and a use method thereof, wherein the device sequentially comprises a laser, a reflective rotary attenuation sheet, a first adjustable circular diaphragm, a laser beam expander, a second adjustable circular diaphragm, a half wave plate, a cube polarization beam splitter, a spatial light modulator, a first lens, a third adjustable circular diaphragm, a second lens, a first objective lens, a scattering medium, a second objective lens and a COMS photoelectric detector; by the technical scheme, the strong correlation between adjacent incident light channels of the scattering medium can be utilized to offset the focal position change in the phase translation process of the incident light caused by vibration.
Description
Technical Field
The invention relates to the technical field of laser focusing, in particular to a device for performing laser anti-vibration focusing by using a strong scattering medium and a use method thereof.
Background
Laser focusing plays an important role in the fields of process processing, biomedicine, material science, atmospheric optics and the like. Such as laser cutting, laser welding, confocal microscopy imaging, laser star-guiding techniques, etc., all require focusing. The traditional laser focusing light path consists of optical elements such as a laser source, a lens and the like. The light path is very sensitive to vibration, and the light source is often subjected to small displacement or rotation, and the focus is also moved along with the light path or even does not exist, so that the stability and the vibration resistance of the traditional laser focusing light path are poor.
Disclosure of Invention
In view of the above, an object of the present invention is to provide a focusing device and a method for laser anti-vibration using a strong scattering medium, which can counteract the focal position change during the phase shift of the incident light caused by vibration by using the strong correlation between adjacent incident light channels of the scattering medium when the light source or the system vibrates.
In order to achieve the above purpose, the invention adopts the following technical scheme: a device for performing laser vibration-resistant focusing by using a strong scattering medium sequentially comprises a laser, a reflective rotary attenuation sheet, a first adjustable circular diaphragm, a laser beam expander, a second adjustable circular diaphragm, a half-wave plate, a cube polarization beam splitter, a spatial light modulator, a first lens, a third adjustable circular diaphragm, a second lens, a first objective lens, a scattering medium, a second objective lens and a COMS photoelectric detector;
The laser is used as a light source to emit laser, and the laser is shaped into a circular light spot after passing through the reflective rotary attenuation sheet and the first adjustable circular diaphragm; then the collimation and the beam expansion of the incident light are realized through a laser beam expander; the laser after beam expansion passes through a second adjustable circular diaphragm, a half wave plate and a cube polarization beam splitter and then enters a modulation plane of the spatial light modulator in a horizontal polarization state; the modulated incident light passes through a 4f filtering system consisting of a first lens, a third adjustable circular diaphragm and a second lens, and is collected and focused on the front surface of a scattering medium by a first objective lens; the emergent light after the scattering medium is collected by the second objective, and finally is received by the COMS photoelectric detector and the light intensity information is recorded.
The invention also provides a using method of the laser anti-vibration focusing device by using the strong scattering medium, which adopts the device for laser anti-vibration focusing by using the strong scattering medium and comprises the following steps:
step S1: opening a laser and a spatial light modulator, loading a phase correction chart and a blazed grating gray chart on the spatial light modulator, constructing an anti-vibration light path, and taking the light path as a follow-up transmission matrix measuring and focusing light path;
S2, a step of S2; loading phase gray maps corresponding to different input lights on a spatial light modulator, modulating the phases of the input lights by using a four-step phase shift method, and collecting output speckle patterns under each phase shift by using a COMS (complementary metal oxide semiconductor) photoelectric detector;
s3, a step of S3; converting the collected speckle pattern into matrix data, and carrying the matrix data into a calculation formula to obtain a transmission matrix of the tested scattering medium;
S4, a step of S4; and calculating a phase mask by using a phase conjugation method, and loading the phase mask onto a spatial light modulator to realize anti-vibration focusing at a specified position.
In a preferred embodiment, the step 2 specifically includes opening the spatial light modulator when the optical path is constructed, loading a phase correction chart on the modulation surface, and implementing phase correction to accurately modulate the incident light; meanwhile, a blazed grating gray scale image is loaded on the modulation surface, zero-order diffraction light which is not modulated is separated, and the modulated first-order diffraction light is selected and filtered through a 4f filtering system.
In a preferred embodiment, step 3 specifically includes dividing the spatial light modulator into N super-pixels, each super-pixel consisting of 6*6 small pixels and serving as an input free mode for the scattering medium measurement system; incident light in Hadamard form, for the nth input light waveWhen the phase of the light changes alpha, according to the interference superposition principle of light, the light intensity on the m-th emergent channel is as follows:
In the formula, S m represents the complex amplitude of the light field generated in the m output channel of the COMS photodetector after the reference light is scattered by the medium, For S m conjugate transpose, re () represents the real part; when alpha is 0, pi/2, pi and 3 pi/2 respectively, the light intensity distribution on the mth output channel is/>, respectively Proportional to the real part of the transmission matrix of the scattering medium to be measured,/>Is proportional to the imaginary part of the transmission matrix of the scattering medium to be measured, so that the relation between the intensity of the mth output channel and the matrix element k mn is:
For a particular output channel of the device, Is a constant value, i.e./>Seen as known, but the light intensity at different phase angles alpha/>The transmission matrix element k mn of the scattering medium can be measured by directly measuring by a COMS photoelectric detector and bringing the acquired data into a formula;
Repeating the steps, changing the input free mode from 1 to N, performing four-step phase shift on each input free mode, collecting corresponding output light intensity, and calculating all k mn after the corresponding output light intensity is brought into a formula to obtain a complex transmission matrix of the whole sample.
In a preferred embodiment, step 4 specifically includes calculating a phase mask using a phase conjugation method: assuming that the required output light field isAnd calculating a complex conjugate matrix T H of the complex transmission matrix obtained by measurement, wherein the required input light field is as follows: /(I)And converting the calculation result into a gray phase mask and loading the gray phase mask onto the spatial light modulator, namely realizing focusing at a specified position.
Compared with the prior art, the invention has the following beneficial effects:
(1) The light path is simple and easy to construct, a single-arm measuring system is adopted when the scattering medium transmission matrix is measured, independent reference light is not needed, and the stability is high.
(2) The invention adopts the pure phase space light modulator to generate the blazed grating to modulate the incident light, can adjust the grating parameters according to the light path requirement, does not need to process actual grating elements, and has flexible operation and low cost.
(3) The invention separates zero-order diffraction light without modulation effect generated by the spatial light modulator by using the blazed grating, and simultaneously filters the first-order diffraction light effectively modulated after the spatial light modulator by using the 4f system, thereby effectively reducing the interference of invalid light and high-frequency noise in an optical path and enabling the obtained scattering medium transmission matrix to be more accurate.
(4) The invention utilizes the relativity between adjacent incidence transmission channels of the scattering medium, effectively counteracts the phase change caused by the incident light change, thereby eliminating the influence on the focusing point behind the scattering medium and improving the vibration resistance of the whole system.
Drawings
Fig. 1 is a schematic structural view of a focusing apparatus for laser vibration resistance using a strong scattering medium according to a preferred embodiment of the present invention.
FIG. 2 is a schematic illustration of the correlation of adjacent channels of scattering media in accordance with a preferred embodiment of the present invention;
FIG. 3 (a) is a plot of the focus acquired by the detector without displacement of the phase of the incident light in accordance with a preferred embodiment of the present invention;
fig. 3 (b) is a focus map acquired by the detector after the phase shift of the incident light according to the preferred embodiment of the present invention.
Detailed Description
The invention will be further described with reference to the accompanying drawings and examples.
It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the application. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the present application; as used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.
Referring to fig. 1, a focusing device for laser vibration resistance by using a strong scattering medium comprises a laser 1, a reflective rotary attenuation sheet 2, a first adjustable circular diaphragm 3, a laser beam expander 4, a second adjustable circular diaphragm 5, a half-wave sheet 6, a cube polarization beam splitter 7, a spatial light modulator 8, a first lens 9, a third adjustable circular diaphragm 10, a second lens 11, a first objective lens 12, a scattering medium 13, a second objective lens 14 and a COMS photodetector 15; the laser comprises a laser 1, a reflective rotary attenuation sheet 2, a first adjustable circular diaphragm 3, a laser beam expander 4, a second adjustable circular diaphragm 5, a half-wave sheet 6, a cube polarization beam splitter 7, a spatial light modulator 8, a first lens 9, a third adjustable circular diaphragm 10, a second lens 11, a first objective lens 12, a scattering medium 13, a second objective lens 14 and a COMS photodetector 15. The scattering medium 13 is silica frosted glass or zinc oxide scatterer.
The laser 1 is used as a light source to emit laser, and the laser is shaped into a circular light spot after passing through the attenuation sheet 2 and the first adjustable circular diaphragm 3; then the collimation and the beam expansion of the incident light are realized through a laser beam expander 4; the laser after beam expansion passes through a second adjustable circular diaphragm 5, a half-wave plate 6 and a cube polarization beam splitter 7 and then enters a modulation plane of a spatial light modulator 8 in a horizontal polarization state; the modulated incident light passes through a 4f system consisting of a first lens 9, a third adjustable circular diaphragm 10 and a second lens 11, and is collected and focused on the front surface of a scattering medium 13 by a first objective lens 12; the outgoing light after passing through the scattering medium 13 is collected by the second objective lens 14, and finally received by the COMS photodetector 15 and the light intensity information is recorded.
The laser 1 is a continuous single-wavelength laser and emits continuous laser light in the visible light band.
The emergent light of the laser is collimated light, the circular adjustable aperture 2 can shape the emergent laser of the laser into a circle, and meanwhile, the reflective rotary attenuation sheet 3 can continuously adjust the light intensity of the incident light, so that the proper incident light intensity can be conveniently selected.
The laser beam expander 4 is a continuous variable-magnification beam expander, and the maximum beam expansion ratio is 20 times.
The second adjustable circular diaphragm 5 may select a uniform portion of the expanded laser light as the laser light incident to be modulated. The half-wave plate 6 and the cube polarizing beam splitter 7 cause the laser light to be incident on the spatial light modulator in a horizontally polarized state.
The spatial light modulator 8 is a reflective spatial light modulator, and implements phase-only modulation on horizontally polarized incident light energy.
The first lens 9, the third adjustable circular diaphragm 10 and the second lens 11 form a 4f system, so that high-frequency noise in modulated light is filtered, the modulated light is purer, and a more accurate measurement result is obtained. Wherein the 4f system in the device requires the first lens 9 and the second lens 11 to have the same focal length; the spatial light modulator 8 is at a focal length in front of the first lens 9; the third adjustable circular diaphragm 10 is placed at the focus of the first lens 9; the second lens 11 is at a focal length behind the distance diaphragm 10; the receiving surface of the first objective lens 12 is at a focal length behind the second lens 11.
The scattering medium 13 is a 220 mesh ground glass scattering sheet; the example is also applicable to other scattering media such as sand glass scattering sheets, biological tissues, zinc oxide scattering particle coatings or optical fibers with higher mesh numbers.
The first objective lens 12 is a 10-magnification microscope objective lens, and the first objective lens 14 is a 20-magnification microscope objective lens. In setting up the measuring beam path, a scattering medium is placed at the focal plane of the objective lens.
The detector is 15 COMS detector with variable gain, and can adjust the intensity of the collected speckle according to the experimental requirement, and the example is also applicable to CCD detector.
The working process of the invention is as follows:
Opening a laser (1) and a spatial light modulator (8), loading a phase correction chart and a blazed grating gray chart on the spatial light modulator (8), and constructing a light path; loading phase gray scale patterns corresponding to different input lights on a spatial light modulator (8), modulating the phases of the input lights by using a four-step phase shift method, and collecting output speckle patterns under each phase shift by using a COMS (complementary metal oxide semiconductor) photoelectric detector (15); and converting the collected speckle pattern into matrix data, and carrying the matrix data into a calculation formula to obtain a transmission matrix of the tested scattering medium (13).
According to the invention, independent reference light is not required to be introduced in measurement of the transmission matrix, and measurement can be completed by using a single-arm optical path system, so that the difficulty in setting up an experimental optical path is reduced, and the stability of the experimental system is improved.
A method for carrying out vibration-resistant focusing by utilizing stable strong scattering medium comprises the following specific steps:
Step 1, setting up an experimental device shown in fig. 1, and shaping laser emitted from a laser 1 into a circular light spot with proper light intensity after passing through an attenuation sheet 2 and a first adjustable circular diaphragm 3; then the collimation and the beam expansion of the incident light are realized through a laser beam expander 4; the laser after beam expansion passes through a second adjustable circular diaphragm 5, and a uniform part is selected as incident light to be modulated; after passing through the half wave plate 6 and the cube polarization beam splitter 7, the incident light irradiates on a modulation plane of the spatial light modulator 8 in a horizontal polarization state; the modulated incident light is filtered after passing through a 4f system consisting of a first lens 9, a third adjustable circular diaphragm 10 and a second lens 11, and is collected by a first objective lens 12 and focused on the front surface of a scattering medium 13; the outgoing light after passing through the scattering medium 13 is collected by the second objective lens 14, and finally received by the COMS photodetector 15 and the light intensity information is recorded.
Step 2, opening the spatial light modulator when the light path is built, loading a phase correction chart on the modulation surface, and realizing phase correction to accurately modulate incident light; meanwhile, due to the structural characteristics of the spatial light modulator, the generated zero-order diffraction light is not modulated and has strong light intensity, the blazed grating gray pattern is loaded on the modulating surface, the zero-order diffraction light which is not modulated is separated, and the modulated first-order diffraction light is selected and filtered through a 4f system.
And 3, dividing the spatial light modulator into N super pixels, wherein each super pixel consists of 6*6 small pixels and is used as an input free mode of a scattering medium measurement system. Incident light in Hadamard form, for the nth input light waveWhen the phase of the light changes alpha, according to the interference superposition principle of light, the light intensity on the m-th emergent channel is as follows:
In the formula, S m represents the complex amplitude of the light field generated in the m-th output channel of the COMS photodetector (15) after the reference light is scattered by the medium, For S m conjugate transpose, re () represents the real part; when alpha is 0, pi/2, pi and 3 pi/2 respectively, the light intensity distribution on the mth output channel is/>, respectivelyProportional to the real part of the transmission matrix of the scattering medium (13) to be measured,/>Is proportional to the imaginary part of the transmission matrix of the scattering medium (13) to be measured, so that the relation between the intensity of the mth output channel and the matrix element k mn is:
For a particular output channel of the device, Is a constant value, i.e./>Can be seen as known, but the light intensity at different phase angles α/>The transmission matrix element k mn of the scattering medium (13) can be measured by directly measuring by the COMS photoelectric detector (15) and bringing the acquired data into a formula;
Repeating the steps, changing the input free mode from 1 to N, performing four-step phase shift on each input free mode, collecting corresponding output light intensity, and calculating all k mn after the corresponding output light intensity is brought into a formula to obtain a complex transmission matrix T of the whole sample.
And 4, calculating by using a phase conjugation method to obtain a phase mask: assuming that the required output light field isAnd calculating a complex conjugate matrix T H of the complex transmission matrix obtained by measurement, wherein the required input light field is as follows: /(I)And converting the calculation result into a gray phase mask and loading the gray phase mask onto the spatial light modulator, so that focusing at a specified position can be realized.
An apparatus and method for anti-vibration focusing using a stable strong scattering medium, the specific anti-vibration principle is as follows:
When the transmission matrix of the scattering medium is measured, the larger the number of the input and output channels is, the larger the transmission matrix is, the more accurate the description of the medium is, and the better focusing and imaging effects are realized.
According to random matrix theory, open intrinsic channels with high transmission transmittance exist in the scattering medium, and when passing through the channels, the channels transmit through the scattering medium with low loss.
Experimentally we measure the transmission matrix by discretizing the incident light field with spatial light modulator modulation, which discretization is limited, resulting in a limited number of measured input-output channels. The distribution of the actual light field is continuous, so that the transmission matrix measured with limited division is incomplete.
On the premise of incomplete matrix measurement, the optimal phase distribution in the actual light field and the result obtained by phase conjugation calculation have a certain gap. This gap results in that the modulated light cannot be fully and effectively controlled, most of the modulated light enters the open intrinsic channel, but the light field at the super-pixel edge of the spatial light modulator cannot enter the open channel, reducing the focusing or imaging effect.
Meanwhile, due to the continuity of the light field, adjacent input channels are not independent, but have strong correlation. At this time, if the phase mask is displaced, a part of the light field deviates from the optimal phase along with the displacement, so that the light field cannot enter the open channel, and the focusing effect is reduced, but the light field at the edge of the super pixel of the spatial light modulator enters the open intrinsic channel due to the displacement, so that the intensity of the focus is improved. I.e. in case of phase shift, the focus can still be kept unchanged at the target position, while the light intensity can also be maintained at a high level.
The laser displacement, rotation or vibration of the light source can cause the phase to change, under the system, even if the phase changes, the laser can still keep effective focusing after passing through the scattering medium, so that effective vibration resistance can be realized.
Fig. 2 is a schematic diagram of the principle of correlation between adjacent channels of a scattering medium. Blue, green and red represent different phases, respectively. Fig. 2 (a) is a graph of the actual light field being continuous, we using a gradient color to represent this continuity, assuming that the actual phase distribution of the target output channel can be optimized; FIG. 2 (b) is an optimal phase distribution calculated by the measured transmission matrix, and is also the phase of the super-pixel loaded onto the spatial light modulator, since discretization is limited, the loaded optimal phases are independent of each other, with distinct frames as a distinction; FIG. 2 (c) represents the deviation between the actual phase and the loading phase, more pronounced at the edges; fig. 2 (d) shows the phase distribution after the phase mask has been shifted, where although some pixels deviate from the optimal solution, the pixels at the edges can be in the optimal phase, so that this part of the light field enters the open channel.
As shown in fig. 3, fig. 3 (a) shows a focus map collected by the detector when the incident light phase is not shifted, and fig. 3 (b) shows a focus map collected by the detector after the incident light phase is shifted. It can be seen that even if the incident light is shifted, the modulation phase of the incident light is changed, the focal position is unchanged, the light intensity is slightly reduced, and a remarkable focusing effect is still achieved. The whole system has good vibration resistance.
While the foregoing is directed to embodiments of the present invention, it will be appreciated by those skilled in the art that changes and modifications may be made without departing from the principles of the invention, and it is intended that the invention also be limited to the specific embodiments shown.
Claims (3)
1. The application method of the laser anti-vibration focusing device by using the strong scattering medium is characterized by adopting the laser anti-vibration focusing device by using the strong scattering medium, and sequentially comprising a laser, a reflective rotary attenuation sheet, a first adjustable circular diaphragm, a laser beam expander, a second adjustable circular diaphragm, a half wave plate, a cube polarization beam splitter, a spatial light modulator, a first lens, a third adjustable circular diaphragm, a second lens, a first objective lens, a scattering medium, a second objective lens and a COMS photoelectric detector;
The laser is used as a light source to emit laser, and the laser is shaped into a circular light spot after passing through the reflective rotary attenuation sheet and the first adjustable circular diaphragm; then the collimation and the beam expansion of the incident light are realized through a laser beam expander; the laser after beam expansion passes through a second adjustable circular diaphragm, a half wave plate and a cube polarization beam splitter and then enters a modulation plane of the spatial light modulator in a horizontal polarization state; the modulated incident light passes through a 4f filtering system consisting of a first lens, a third adjustable circular diaphragm and a second lens, and is collected and focused on the front surface of a scattering medium by a first objective lens; the emergent light after the scattering medium acts is collected by a second objective, and finally is received by a COMS photoelectric detector and light intensity information is recorded;
the method comprises the following steps:
step S1: opening a laser and a spatial light modulator, loading a phase correction chart and a blazed grating gray chart on the spatial light modulator, constructing an anti-vibration light path, and taking the light path as a follow-up transmission matrix measuring and focusing light path;
S2, a step of S2; loading phase gray maps corresponding to different input lights on a spatial light modulator, modulating the phases of the input lights by using a four-step phase shift method, and collecting output speckle patterns under each phase shift by using a COMS (complementary metal oxide semiconductor) photoelectric detector;
s3, a step of S3; converting the collected speckle pattern into matrix data, and carrying the matrix data into a calculation formula to obtain a transmission matrix of the tested scattering medium;
S4, a step of S4; calculating to obtain a phase mask by using a phase conjugation method, and loading the phase mask onto a spatial light modulator to realize anti-vibration focusing at a specified position;
Step 3, dividing the spatial light modulator into N super pixels, wherein each super pixel consists of 6*6 small pixels and is used as an input free mode of a scattering medium measurement system; incident light in Hadamard form, for the nth input light wave When the phase of the light changes alpha, according to the interference superposition principle of light, the light intensity on the m-th emergent channel is as follows:
In the formula, S m represents the complex amplitude of the light field generated in the m output channel of the COMS photodetector after the reference light is scattered by the medium, For S m conjugate transpose, re () represents the real part; when alpha is 0, pi/2, pi and 3 pi/2 respectively, the light intensity distribution on the mth output channel is/>, respectively Proportional to the real part of the transmission matrix of the scattering medium to be measured,/>Is proportional to the imaginary part of the transmission matrix of the scattering medium to be measured, so that the relation between the intensity of the mth output channel and the matrix element k mn is:
For a particular output channel of the device, Is a constant value, i.e./>Seen as known, but the light intensity at different phase angles alpha/>The transmission matrix element k mn of the scattering medium can be measured by directly measuring by a COMS photoelectric detector and bringing the acquired data into a formula;
Repeating the steps, changing the input free mode from 1 to N, performing four-step phase shift on each input free mode, collecting corresponding output light intensity, and calculating all k mn after the corresponding output light intensity is brought into a formula to obtain a complex transmission matrix of the whole sample.
2. The method for using the strong scattering medium to perform the laser anti-vibration focusing device according to claim 1, wherein the step 2 specifically includes opening the spatial light modulator when the optical path is constructed, loading a phase correction chart on the modulation surface, and implementing phase correction to accurately modulate the incident light; meanwhile, a blazed grating gray scale image is loaded on the modulation surface, zero-order diffraction light which is not modulated is separated, and the modulated first-order diffraction light is selected and filtered through a 4f filtering system.
3. The method of claim 1, wherein step 4 specifically comprises calculating a phase mask using a phase conjugation method: assuming that the required output light field isAnd calculating a complex conjugate matrix T H of the complex transmission matrix obtained by measurement, wherein the required input light field is as follows: /(I)And converting the calculation result into a gray phase mask and loading the gray phase mask onto the spatial light modulator, namely realizing focusing at a specified position.
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