A particle velocity measurement and control device

Abstract

The application provides a kind of particle velocity measurement and control device.It is characterized by: it is composed of laser, beam splitter, mirror, lens, hollow waveguide, acoustooptic modulator, drive controller, photodetector and data acquisition card.The application applies hollow waveguide optical tweezers technology, and realizes single and multiple particle instantaneous vector velocity measurement by using Doppler velocimetry.At the same time, the laser intensity is modulated at high speed by using acoustooptic modulator, and the particle movement velocity is controlled by signal feedback mode.The application provides a kind of new device for high-precision measurement and control of particle vector velocity, which can be applied to distributed optical fiber sensing, quantum precision measurement, micro material transport and other fields.

Description

(I) Technical Field

[0001] This invention relates to a particle velocity measurement and control device, primarily concerning optical manipulation, Doppler velocimetry, and feedback control. More specifically, it relates to a novel device for measuring and controlling the instantaneous velocity of single or multiple particles in a hollow waveguide. Based on the Doppler velocimetry principle and acousto-optic modulation devices, it enables high-precision control of the particle's position, oscillation frequency, velocity, and direction within the waveguide. (II) Background Technology

[0002] Optical tweezers (Optics Letters, 18(5):288-290, 1986) is an optical technique that uses the mechanical effects of light to manipulate particles in three-dimensional space. Due to its advantages such as low damage and non-contact precision operation, optical tweezers have become an important tool in many research fields and have been widely used in fundamental physics, biomedicine, and aerosol science. Hollow waveguide optical tweezers are a novel optical manipulation technology developed in recent years. They can overcome the limitations of traditional free-space optical tweezers in terms of optical manipulation distance, manipulation stability, and manipulation dimension, making them an ideal platform for distributed fiber optic sensing, quantum precision measurement, and micro-matter transport.

[0003] For example, patent publication number CN100580491C proposes a capillary fiber optical tweezers and its fabrication method. The hollow capillary in this device provides a storage location for microparticles. Particles can be stored in the capillary through micro-negative pressure adsorption, and simultaneously, micro-positive pressure can be applied to the fiber-end optical tweezers. This approach allows for the continuous assembly and manipulation of a large number of microparticles, but it lacks sufficient control over the particle movement speed and precision. Chinese patent CN102147502B further improves the microparticle transport device and method based on hollow fused-core capillary optical fibers. It proposes that particles generate manipulation and propulsion forces under the action of an evanescent field. Therefore, under the action of optical forces, the particles move along the inner wall of the capillary close to the fiber core, realizing the transport of microparticles by the capillary fiber. This approach solves the problem of particle manipulation speed, but the direction of particle transport is relatively unidirectional, resulting in poor controllability. Meanwhile, the all-fiber adjustable particle stirring device and the micro-nano particle manipulator based on capillary optical fiber proposed by utility model patents CN213339703U and CN213935671U cannot determine the instantaneous speed of the particles and provide real-time feedback.

[0004] Therefore, researchers (Optics Letters, 36(11):2022-2022, 2011) proposed using the Doppler frequency shift method to measure the axial instantaneous velocity of suspended particles in hollow optical fibers. When a particle moves along the fiber axis, a Doppler frequency shift occurs between its backscattered light and the incident light, meaning the Doppler frequency shift value is proportional to the particle's axial velocity. In the experiment, the backscattered light introduced by the suspended particle can interfere with the reflected light from the fiber end face (which does not produce a Doppler frequency shift). By applying a short-time Fourier transform to the interference signal in real time, the particle's axial instantaneous velocity can be calculated. This scheme can overcome the influence of insufficient CCD frame rate on particle velocity measurement in imaging methods and the problem of too small a field of view in velocity measurement. However, since the redshift and blueshift signals have a beat frequency of absolute value (near zero frequency) when there is no relative frequency shift, it is impossible to determine the direction of particle motion.

[0005] To achieve instantaneous axial vector velocity measurement of single and multiple particles, this invention proposes a particle velocity measurement and control device. Based on a hollow-core optical waveguide platform, the manipulated particle and the probe light wave are simultaneously confined within an air fiber core. The optical gradient force provided by the fiber mode field suspends the particle in the center of the core, while the scattering force drives the particle to move along the fiber axis. By adjusting the power difference between the two paths, the light scattering force on both sides of the particle can be controlled, achieving optical propulsion and manipulation of the particle along the fiber core axis. Compared to traditional particle optical manipulation and measurement schemes, this device offers the following technical advantages: it utilizes Doppler velocimetry to achieve instantaneous axial velocity measurement of the particle; it uses a photodetector to receive the beat frequency signal of the particle's scattered light and the diffracted light after frequency shifting by an acousto-optic modulator, thereby determining the particle's motion direction in the hollow-core waveguide; and it uses an acousto-optic modulator to control the intensity of the manipulated beam, achieving real-time feedback control of the manipulated light power. These technical advantages make the device applicable to fields such as distributed fiber optic sensing, quantum precision measurement, and micro-matter transport. (III) Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of low precision in controlling the velocity of particles in hollow waveguides, relatively unidirectional particle transport, poor controllability, inability to measure instantaneous particle vector velocity, and difficulty in real-time feedback in existing technologies. This invention provides a particle velocity measurement and control device. The device utilizes the optical gradient force provided by the fiber mode field to suspend the particle in the center of the fiber core. By adjusting the power difference of the back-propagating dual-beam optical tweezers, optical propulsion and manipulation of the particle along the fiber core axis are achieved. Doppler velocimetry is applied to measure the instantaneous axial velocity of the particle. A photodetector receives the beat frequency signals of the particle's backscattered light and the diffracted light after frequency shifting by the acousto-optic modulator, thereby determining the particle's motion direction in the hollow waveguide. Simultaneously, the acousto-optic modulator allows adjustment of the manipulation beam intensity, achieving real-time feedback control of the manipulation optical power.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] This particle velocity measurement and control device consists of a laser, beam splitter, reflector, beam splitter lens, coupling lens, hollow waveguide, focusing lens, acousto-optic modulator, drive controller, photodetector, and data acquisition card. The laser serves as the manipulation light source. After the beam splitter adjusts the intensity of the two beams, the laser is coupled into a core mode of the hollow waveguide via the coupling lens. Beam splitters are placed at the ends of the hollow waveguide. The photodetector receives the scattered light from the particles, the reflected signal from the ends of the hollow waveguide, and the diffracted light from the acousto-optic modulator. Under the combined action of the two beams split by the beam splitter, the particles move within the core of the hollow waveguide. The velocity direction of one or more particles is controlled by adjusting the power ratio of the two beams. Since the scattered light from the particles exhibits a Doppler frequency shift relative to the incident light during motion, the instantaneous velocity of the particles can be calculated by measuring the instantaneous Doppler frequency shift value in the signal received by the photodetector. The data acquisition card collects the changes in optical power during the movement of particles in real time. The processed electrical signal is then used by the drive controller to act on the acousto-optic modulator, which controls the position, oscillation frequency, speed and direction of the particles by modulating the laser light intensity.

[0009] The following section will elaborate on the basic principles of using the Doppler velocimetry method to measure axial instantaneous velocity and using beat frequency signals to determine the direction of particle motion in a hollow waveguide.

[0010] Laser Doppler velocimetry can measure the instantaneous velocity and position of particles along an optical fiber with high precision. The Doppler effect causes a change in the optical frequency Δυ = v. p The offset of / λ must be considered twice. First, with velocity v p The frequency redshift observed in particles that have left the light source is Δυ = v p / λ; secondly, the light emitted by the moving particles is "observed" by the fixed photodetector (PD), resulting in another redshift Δυ=v. p / λ. In general, for non-relativistic particle velocities, the frequency of the reflected light is υ. D =υ L -2v p / λ. Therefore, the frequency difference is linearly proportional to the particle velocity: Where v p Δυ is the velocity of the particles in the fiber core. D This is the frequency difference between the particle and the light reflected from the fiber end face. Therefore, by measuring the beat frequency signal of the two reflected beams at the fiber incident end, the velocity and position information of the particles in the fiber core can be retrieved at high speed.

[0011] Acousto-optic modulators are classified into intensity modulators, frequency modulators, and phase modulators based on the different physical parameters modulated. They can also be classified into analog modulators (also called sine wave modulators) and digital modulators (also called pulse modulators) based on the different modulated signals. A modulated electrical signal drives a transducer, generating ultrasonic waves whose intensity varies with the electrical signal. These ultrasonic waves enter the acousto-optic modulator, and a laser beam, focused by a lens, passes through the modulator, producing acousto-optic diffraction and completing the optical modulation process. The working principle of an acousto-optic modulator can be calculated from the acousto-optic diffraction efficiency of weak acousto-optic interactions. M2 is called the acousto-optic figure of merit, which is only related to the acousto-optic medium and its operating mode. Therefore, it is often used as an evaluation standard for the quality of the acousto-optic medium and its operating mode. L and H are parameters describing the length and width of the transducer, and P... a It is the energy flow of sound waves, i.e., sound power.

[0012] The above equation shows that diffraction efficiency is proportional to acoustic power, meaning the intensity of diffracted light is proportional to the power of the driving electrical signal. This ensures that the signal remains undistorted during acousto-optic modulation. Simultaneously, changing the ultrasonic frequency υ... s This can cause the frequency of the diffracted light to change.

[0013] With frequency υ s An equal-amplitude oscillating electrical signal is applied to the acousto-optic modulator. A laser beam with frequency υ0 emitted from the laser passes through the acousto-optic modulator, and the output light contains zero-order light with frequency still υ0 and light with frequency changed to υ0+υ. s The diffracted light. The light path not modulated by the acousto-optic modulator is reflected by the moving particles, resulting in a Doppler frequency shift; the frequency of the reflected light becomes υ0 + Δυ. D The reflected and diffracted light are simultaneously introduced into the photodetector to generate a beat signal with a frequency equal to υ. s -Δυ D The electrical signal can be used to calculate the velocity of the particle in the hollow waveguide using formula (1). Due to the red-shift and blue-shift phenomena caused by the forward-backward motion of the moving particle, the spectral signal remains at the operating frequency υ of the acousto-optic modulator during the particle's reciprocating motion. s The particles fluctuate in the vicinity. When the difference frequency is below the operating frequency, the particles move towards the light source; conversely, they move away from the light source. Simultaneously, the velocity measurement results are fed back to the acousto-optic modulator, enabling not only instantaneous vector velocity measurement of the particles but also real-time feedback cooling control of the moving particles in the hollow waveguide.

[0014] The particle velocity measurement and control device of the present invention may further include:

[0015] The hollow waveguide described herein can transmit the control light emitted from the laser.

[0016] The microparticle material is a dielectric material, a metallic material, a semiconductor material, a crystalline material, a biological material, a transparent material, a non-transparent material, or a mixture of the above materials.

[0017] The operating frequency of the aforementioned acousto-optic modulator is less than half the sampling frequency of the photodetector and the data acquisition card.

[0018] In summary, the technical solutions conceived by this invention have the following beneficial effects compared with the prior art:

[0019] 1. Compared with traditional particle velocity measurement and control devices, the present invention adopts the Doppler velocity measurement principle and advanced hollow waveguide technology to realize the real-time measurement of the motion state of single and multiple particles, improve the velocity measurement accuracy and range, and solve the problem of accurate measurement and control of particle motion state in waveguide.

[0020] 2. This invention introduces an acousto-optic modulation device, which solves the problem of not being able to determine the particle motion direction when the signal beat frequency is near zero, and realizes the measurement of the instantaneous axial vector velocity of the particles. Combined with the intensity modulation function of the acousto-optic modulation device, real-time feedback control of the manipulated optical power is achieved.

[0021] 3. The hollow waveguide used in this invention can be completely sealed, enabling optical manipulation and measurement of particles in a low-pressure environment. This makes it widely applicable in fields such as distributed fiber optic sensing, quantum precision measurement, and micro-matter transport, and it also has high practical value. (iv) Description of the attached drawings

[0022] Figure 1 It is a particle velocity measurement and control device. It consists of a laser 1, a beam splitter 2, a reflector 3, a beam splitter 4, a coupling lens 5, a hollow waveguide 6, a focusing lens 7, an acousto-optic modulator 8, a drive controller 9, a photodetector 10, and a data acquisition card 11.

[0023] Figure 2 A schematic diagram of the Doppler velocimeter is given. It consists of a beam splitter 4, a hollow waveguide 6, and a photodetector 10.

[0024] Figure 3 A schematic diagram of the working principle of the acousto-optic modulator is given. It consists of a transducer 801, an acousto-optic medium 802, a sound-absorbing material 803, zero-order diffracted light 804, first-order diffracted light 805, and a drive controller 9.

[0025] Figure 4(a) shows schematic cross-sectional views of three types of hollow waveguides: capillary fiber, hollow photonic bandgap fiber, and hollow anti-resonant fiber. (b) shows schematic views of the air core shapes of hollow waveguides: circular, triangular, and quadrilateral. (V) Detailed Implementation

[0026] The invention will be further illustrated below with reference to the accompanying drawings.

[0027] Combination Figure 1 In this embodiment of the invention, a laser (1) is used as the manipulating light source. After the beam splitter (2) adjusts the intensity of the two beams, the laser is coupled into the core mode of a hollow waveguide (6) through a coupling lens (5). Beam splitters (4) are placed on the ends of the hollow waveguide (6). A photodetector (10) is used to receive the scattered light from the particles, the reflected signal from the ends of the hollow waveguide (6), and the diffracted light from the acousto-optic modulator (8). Under the combined action of the two beams split by the beam splitter (2), the particles move in the core of the hollow waveguide (6). The speed and direction of movement of one or more particles are manipulated by adjusting the power ratio of the two beams. Since the backscattered light of the particles during the movement has a Doppler frequency shift relative to the incident light, the instantaneous axial velocity of the particles can be calculated by measuring the instantaneous Doppler frequency shift value in the signal received by the photodetector (10). The data acquisition card (11) is used to collect the change in optical power during the movement of the particles in real time. The processed electrical signal is applied to the acousto-optic modulator (8) through the drive controller (9). The position, oscillation frequency, movement speed and direction of the particles are controlled by modulating the laser light intensity.

[0028] The construction of a particle velocity measurement and control device can be divided into the following three steps (see Figure 1 ):

[0029] Step 1, Beam Collimation and Laser Coupling: After aligning all components in the device, select a coupling lens with a suitable focal length to perform hollow waveguide fundamental mode coupling;

[0030] Step 2: Doppler velocity measurement device debugging ( Figure 2 ): By introducing polarizers and aperture stops, photodetectors and data acquisition cards are used to collect the reflected light from the particles and the diffracted light from the acousto-optic modulator. The beat frequency signal represents the velocity of the particles in the hollow waveguide.

[0031] Step 3, Adjusting the Acousto-Optical Modulator ( Figure 3 ): By utilizing the intensity modulation and frequency modulation functions of the acousto-optic modulator, the motion state of particles is tested when the light intensity changes, and whether the power spectrum ridge of the beat frequency signal is near the operating frequency of the acousto-optic modulator.

[0032] Optionally, the transmission-to-reflection ratio of the beam splitter is one of 90:10, 70:30, or 50:50.

[0033] Optionally, the hollow waveguide can transmit the manipulation light emitted from the laser. The hollow waveguide can be one of the following: glass capillary, hollow photonic bandgap fiber, capillary fiber, hollow anti-resonant fiber, Kagome fiber, nested anti-resonant fiber, or porous fiber. Figure 4(a) shows the microscopic images of capillary fiber, hollow photonic bandgap fiber, and hollow anti-resonant fiber. The air core shape of the hollow waveguide can be one of the following: circular, triangular, quadrilateral, or other polygonal shapes, as shown in Figure 4(b).

[0034] Optionally, the microparticle material is a dielectric material, a metallic material, a semiconductor material, a crystalline material, a biological material, a transparent material, a non-transparent material, or a mixture of the above materials.

[0035] Optionally, the operating frequency of the acousto-optic modulator is 40MHz, 80MHz, or another frequency, and is less than half the sampling frequency of the photodetector and the data acquisition card.

[0036] To facilitate understanding of the present invention, specific embodiments are described below to further illustrate the invention. The present invention can be implemented in many different forms and is not limited to the embodiments described in this specification.

[0037] Step 1: Beam Collimation, Laser Coupling, and Fiber Preparation: Select a hollow-core anti-resonant fiber with a cladding diameter of 116 μm and an air core diameter of 28 μm. The focal lengths of the coupling lenses on both sides of the fiber are 75 mm and 80 mm, respectively. After stripping the coating layer from the fiber, cut it flat for later use. Perform beam collimation and fiber fundamental mode coupling as described in the implementation method.

[0038] Step 2: Doppler velocity measurement device debugging ( Figure 2 ): Adjust the polarizer and aperture stop device as described in the implementation method to accurately collect the beat frequency signal;

[0039] Step 3, Adjusting the Acousto-Optical Modulator ( Figure 3 ): Adjust the diffraction efficiency (proportion of zero-order diffracted light) of the acousto-optic modulator as described in the implementation method. The criterion for judging the completion of the adjustment is whether the power spectrum ridge of the beat frequency signal is near the operating frequency of the acousto-optic modulator.

[0040] Step 4: Working principle of the particle velocity measurement and control device (see...) Figure 1 A 1064nm laser is used as the manipulating light source. After the beam splitter adjusts the intensity of the two beams, the laser light is coupled into the core guided mode (such as the fundamental mode HE) of a hollow anti-resonant fiber through a coupling lens. 11A beam splitter with a transmission-to-reflection ratio of 90:10 is placed at each end of the optical fiber. A photodetector receives the backscattered light from the particles, the reflected signal from the fiber end, and the diffracted light after frequency modulation by an acousto-optic modulator. The particles move within the core of the hollow anti-resonant fiber under the combined action of the two beams split by the beam splitter. The speed and direction of movement of one or more particles are controlled by adjusting the power ratio of the two beams. Since the backscattered light of the particles exhibits a Doppler frequency shift relative to the incident light during movement, the instantaneous axial velocity of the particles can be calculated by measuring the instantaneous Doppler frequency shift value in the signal received by the photodetector. A data acquisition card is used to collect the changes in optical power during particle movement in real time. The processed electrical signal is then used to drive the controller and act on the acousto-optic modulator, thereby achieving high-speed modulation of the laser intensity. This enables real-time feedback cooling control of the moving particles in the hollow anti-resonant fiber, thus controlling the particle position, oscillation frequency, speed, and direction.

Claims

1. A particle velocity measurement and control device, characterized in that: It consists of a laser (1), a beam splitter (2), a reflector (3), a beam splitter (4), a coupling lens (5), a hollow waveguide (6), a focusing lens (7), an acousto-optic modulator (8), a drive controller (9), a photodetector (10), and a data acquisition card (11). The laser (1) serves as the manipulating light source. After the beam splitter (2) adjusts the intensity of the two beams, the laser is coupled into the core guided mode of the hollow waveguide (6) through the coupling lens (5). Beam splitters (4) are placed in front of the ends of the hollow waveguide (6). The photodetector (10) receives the scattered light from the particles and the reflected light from the ends of the hollow waveguide (6). The diffracted light from the signal and the acousto-optic modulator (8) causes the particles to move in the core of the hollow waveguide (6) under the combined action of the two beams split by the beam splitter (2). The speed and direction of movement of one or more particles can be manipulated by adjusting the power ratio of the two beams. The instantaneous axial velocity of the particles can be calculated by measuring the instantaneous Doppler frequency shift value in the signal received by the photodetector (10). The change in optical power during the movement of the particles is collected in real time by the data acquisition card (11). The processed electrical signal is applied to the acousto-optic modulator (8) through the drive controller (9). The position, oscillation frequency, speed and direction of the particles are controlled by modulating the laser light intensity.

2. The particle velocity measurement and control device according to claim 1, characterized in that: The hollow waveguide described herein can transmit the control light emitted from the laser.

3. The particle velocity measurement and control device according to claim 1, characterized in that: The microparticle material is a dielectric material, a metallic material, a semiconductor material, a crystalline material, a biological material, a transparent material, a non-transparent material, or a mixture of the above materials.

4. The particle velocity measurement and control device according to claim 1, characterized in that: The operating frequency of the aforementioned acousto-optic modulator is less than half the sampling frequency of the photodetector and the data acquisition card.

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