Doppler velocity measurement system based on whispering gallery microcavity Brillouin laser

Abstract

The invention discloses a Doppler velocity measurement system based on whispering gallery microcavity Brillouin laser, and belongs to the technical field of laser detection. The system comprises an echo wall microcavity Brillouin laser implementation unit, a light selection and amplification unit and a Doppler velocity measurement unit. Pump light is input into a high-quality factor echo wall microcavity through evanescent coupling, a cascade stimulated Brillouin scattering process is stimulated, and narrow-linewidth cascade Brillouin laser is generated; a tunable filter is used for screening out specific-order laser, and the specific-order laser is amplified by an erbium-doped optical fiber amplifier to serve as a detection light source; and after the probe light is reflected by the moving target, feedback light carrying Doppler frequency shift is re-injected into the microcavity and generates a self-mixing interference beat frequency signal with laser in the cavity, and finally the target speed is demodulated through photoelectric detection and spectral analysis. According to the invention, the microcavity enhancement effect and the cascade Brillouin process are utilized to narrow the line width, and the self-mixing interference structure is combined, so that laser Doppler velocity measurement with high resolution, high signal-to-noise ratio, compact structure and strong robustness is realized.

Description

Technical Field

[0001] This invention belongs to the field of laser detection technology, specifically relating to a Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser. Background Technology

[0002] Laser Doppler velocimetry, as a high-precision, non-contact motion sensing method, has important application value in fields such as fluid mechanics, aerospace, industrial automation, and biomedical imaging.

[0003] This technology calculates velocity by detecting the coherent beat frequency signal between the reflected or scattered light from a moving target and a reference light. Its measurement accuracy and detection limit are primarily constrained by the coherence of the laser source and the system's detection sensitivity. Specifically, the minimum velocity change the system can resolve is directly related to the linewidth of the source; the narrower the linewidth, the better the velocity resolution. The effective detection distance of the system is limited by the coherence length of the laser. Traditional laser Doppler velocimetry systems mainly rely on spatial optical elements to construct interferometric optical paths. While such systems can achieve high-precision measurements, they suffer from inherent limitations such as complex structure, large size, difficulty in optical alignment and adjustment, and sensitivity to environmental vibrations and temperature changes, making it difficult to meet the requirements of modern industry for portability, robustness, and long-term stability. In recent years, all-fiber velocimetry schemes have become a research hotspot due to their compact structure and strong anti-interference capabilities. These schemes use fiber optic devices to construct sensing links, commonly using distributed feedback lasers or distributed Bragg reflector lasers as the source. However, the typical linewidth of such lasers is on the order of megahertz, and the limited coherence length restricts the effective detection distance of the system. The relatively wide spectral linewidth also limits the system's velocity resolution. In addition, existing solutions still rely on external reference optical paths and balanced detectors, which poses a bottleneck to improving system integration and sensitivity. Summary of the Invention

[0004] To address the shortcomings of existing laser Doppler velocimetry systems in terms of integration, sensitivity, and functional completeness, this invention provides a Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser. This aims to solve the problems of complex structure, poor robustness, and low resolution in existing velocimetry systems.

[0005] To achieve the above objectives, the present invention provides a Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser, comprising: The whispering-gallery microcavity Brillouin laser realization unit is used to input pump light into the whispering-gallery microcavity through evanescent coupling to excite stimulated Brillouin scattering and generate cascaded Brillouin lasers. The light selection and amplification unit is connected to the whispering-gallery microcavity Brillouin laser realization unit. It is used to select a Brillouin laser of a specific order from the cascaded Brillouin lasers and amplify its power to obtain a detection light source. The Doppler velocimetry unit, connected to the light selection and amplification unit, is used to emit a detection light source to a moving target and receive feedback light carrying a Doppler frequency shift reflected by the moving target; re-inject the feedback light into the whispering-gallery microcavity so that it generates a beat frequency signal with the Brillouin laser oscillating within the cavity; and detect and demodulate the beat frequency signal to obtain the velocity information of the moving target.

[0006] Preferably, the whispering-gallery microcavity Brillouin laser realization unit includes: a pump source, an evanescent coupling device, an isolator, a polarization controller, and a first circulator; The pump light source is used to emit pump light; An isolator is placed in the light output path of the pump light source to isolate backscattered light; A polarization controller is placed after the isolator to adjust the polarization state of the pump light; The first circulator is positioned after the polarization controller to transmit the pump light to the evanescent coupling device and to receive the Brillouin laser generated by the whispering-gallery microcavity in the opposite direction. Evanescent coupling devices are used to couple pump light from a circulator to a whispering-gallery microcavity via an evanescent field.

[0007] Preferably, the evanescent coupling device comprises: micro / nano optical fiber and a whispering-gallery microsphere cavity; Micro- and nano-fibers are drawn from single-mode fibers and are used for evanescent coupling with whispering-gallery microspheres. The whispering-gallery microsphere is made by heating and burning the end of a single-mode optical fiber.

[0008] Preferably, the light selection and amplification unit includes: a tunable filter and an erbium-doped fiber amplifier; A tunable filter is connected to the whispering-gallery microcavity Brillouin laser realization unit to receive cascaded Brillouin lasers and filter out Brillouin lasers of a specific order from the cascaded Brillouin lasers by adjusting its center wavelength and bandwidth. An erbium-doped fiber amplifier is connected to a tunable filter to amplify the power of the filtered Brillouin laser of a specific order.

[0009] Preferably, the transmission wavelength of the tunable filter is controlled by a tuning mechanism, its center wavelength is aligned with the wavelength of the third-order Brillouin laser in the cascaded Brillouin laser, and its filtering bandwidth is less than the wavelength interval between adjacent Brillouin laser orders.

[0010] Preferably, the Doppler velocity measurement unit includes a coupler, a collimating lens, a second circulator, a photodetector, and a spectrum analyzer; The input of the coupler is connected to the optical selection and amplification unit to receive the amplified Brillouin laser and feedback light; one output of the coupler is connected to the second circulator, and the other input is connected to the photodetector. Collimating lenses are used to collimate laser light from the coupler and project it onto a moving target; The second circulator is positioned between the whispering-gallery microcavity and the collimating lens to conduct the probe light into the collimating lens and guide the feedback light from the moving target to the whispering-gallery microcavity. Photodetectors are used to convert received optical signals into electrical signals; The spectrum analyzer is connected to the photodetector to perform spectrum analysis on the electrical signal to obtain the beat frequency signal.

[0011] Preferably, the feedback light is re-injected into the whispering-gallery microcavity, where it undergoes self-mixing interference with the Brillouin laser oscillating within the cavity. The beat frequency signal detected by the photodetector corresponds to the Doppler frequency shift.

[0012] Preferably, the working principle of cascaded Brillouin laser generation by whispering-gallery microcavities includes: When the pump light coupled into the whispering-gallery microcavity resonates with the optical mode of the microcavity, the enhanced optical field inside the cavity generates Stokes photons through the electrostriction effect. The resonance enhancement of the microcavity enables the Stokes photons to find a phase-matched optical mode, thereby forming a coherent Brillouin laser output. As the pump power increases, the first-order Stokes light acts as a new pump source, exciting the second-order stimulated Brillouin process, and so on. If resonant modes resonate in each stimulated gain region, a cascaded Brillouin laser output is formed; in the cascading process, each higher-order Stokes beam is generated by the previous-order Brillouin laser as a pump.

[0013] Preferably, based on the inherent high quality factor (Q value) of the whispering-gallery microcavity, the linewidth of the microcavity laser is continuously narrowed during successive excitation: in, For Brillouin laser linewidth, and It is the Stokes damping of the acoustic and optical modes. and Thermal quantum numbers of mechanical and optical fields For the pump laser linewidth, Qtot This represents the overall quality factor of the microcavity. Qext This represents the external cavity quality factor of the microcavity. Pout Indicates laser power. Denotes Planck's constant. This indicates the laser angular frequency.

[0014] Preferably, the speed information is demodulated through the following relationship: in, nThe refractive index of the medium, v Let be the speed of the laser beam at the point where the laser spot is on the moving object. Let the angle between the velocity and the Brillouin laser propagation direction be denoted as . This refers to the Brillouin laser wavelength.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention utilizes an optical microcavity for both narrow-linewidth laser source generation and high-sensitivity interferometric sensing, eliminating the need for external reference optical paths and balanced detectors required by traditional heterodyne detection. This significantly simplifies the system structure, reduces system size and complexity, and improves system stability and integration potential. Furthermore, the all-fiber optic and micro-optical component-based main optical path provides the system with excellent resistance to electromagnetic interference and mechanical vibration, making it suitable for stable operation in complex industrial environments and on mobile platforms.

[0016] (2) Whispering-gallery microcavity Brillouin lasers have advantages such as low threshold, small size, and relatively mature fabrication technology, which are conducive to the miniaturization, low power consumption, and low cost manufacturing of the system, and promote the practical application and popularization of high-performance velocimetry technology. The Brillouin laser generated by the ultra-high Q value microcavity has an extremely narrow linewidth, which is conducive to further improving the velocity resolution of the system. At the same time, the resonant enhancement effect of the feedback light in the cavity can significantly improve the signal-to-noise ratio of the Doppler signal of the system. Attached Figure Description

[0017] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 Specific implementation device diagrams provided for examples of the present invention; Figure 2 Transmission spectrum for quality factor measurement of a whispering-gallery microcavity provided as an example of the present invention Figure 3 A schematic diagram illustrating the working principle of generating Brillouin lasers in a whispering-gallery microsphere cavity; Figure 4 The spectrum of a cascaded Brillouin laser excited by a whispering-gallery microcavity provided as an example of the present invention, and the spectrum of the third-order Brillouin laser filtered out. Figure 5 The system measurement results of the Doppler signal provided in this embodiment of the invention are shown in the figure. Figure 6 The system resolution test diagram provided for an example of the present invention.

[0019] Explanation of reference numerals in the attached figures: 1. Pump light source; 2. Ephemeral coupling device; 3. Isolator; 4. Polarization controller; 5. First circulator; 6. Tunable filter; 7. Erbium-doped fiber amplifier; 8. Coupler; 9. Collimating lens; 10. Moving object; 11. Second circulator; 12. Photodetector; 13. Spectrum analyzer. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] Example like Figure 1 The diagram shown is a schematic representation of the system structure of an embodiment of the present invention, including: a whispering-gallery microcavity Brillouin laser realization unit, an optical selection and amplification unit, and a self-mixing interferometric velocimetry unit. The whispering-gallery microcavity Brillouin laser realization unit is used to excite stimulated Brillouin scattering in an optical microcavity using pump light, thereby generating a narrow-linewidth, cascaded Brillouin laser. The optical selection and amplification unit is used to select and amplify the Brillouin laser from the cascaded laser as a detection light source. The Doppler velocimetry unit is used to re-inject the feedback light reflected from the target into the same microcavity, directly demodulating the target velocity information using the beat frequency effect between the intracavity laser and the feedback light.

[0023] The following will describe in detail, with reference to this embodiment, how the present invention solves technical problems in real life.

[0024] First, pump light source 1 emits pump light, which enters the microcavity through evanescent coupling device 2. In this embodiment, the pump light source is a tunable laser with an operating wavelength of 1550nm. In this embodiment, the whispering-gallery microcavity Brillouin laser realization unit includes, in addition to the pump light source and evanescent coupling device, an isolator 3, a polarization controller 4, and a first circulator 5. The isolator is used to prevent backscattered light from entering the pump light source, thus protecting the pump light source laser; the polarization controller is used to adjust the polarization state of the input light coupled into the microcavity; the circulator is used to transmit the pump light into the evanescent coupling device and receive the backscattered whispering-gallery microcavity Brillouin laser.

[0025] This experiment utilizes evanescent coupling between micro / nano-fibers and a microcavity to efficiently couple pump light into a whispering-gallery microcavity. This excites stimulated Brillouin scattering and matches the resonant modes within the microcavity, achieving resonance and generating whispering-gallery microcavity Brillouin laser. The micro / nano-fibers are drawn from single-mode fiber with a diameter of approximately 2 μm. The microcavity, with a diameter of approximately 250 μm, is fabricated by heating the ends of the single-mode fiber using a CO2 laser fusion splicer. Its quality factor measurement transmission spectrum is shown below. Figure 2 As shown, the quality factor of this microcavity can reach 1.8 × 10⁻⁶. 8 The working principle and process of generating cascaded Brillouin lasers using whispering-gallery microcavities are as follows: When the frequency is The pump light enters the whispering-gallery microsphere cavity via a micro / nano fiber-microcavity coupling device and evanescent field coupling. When it resonates with the microcavity optical mode, the enhanced optical field within the cavity interacts with the thermal phonons through electrostriction, resulting in a frequency shift of approximately 11 GHz (corresponding to the phonon frequency). The phonons and frequencies of ) are Stokes photons. The high-density mode distribution within the cavity ensures that Stokes photons can find phase-matched optical modes, thus forming a coherent Brillouin laser output. As the pump power increases, the first-order Stokes light can act as a new pump source to excite the second-order stimulated Brillouin process, and so on. If resonant modes resonate in each stimulated gain region, a stable cascaded Brillouin laser output can be formed. Its working principle is as follows: Figure 3 As shown, based on the theory of Brillouin laser linewidth: (1) in, For Brillouin laser linewidth, and It is the Stokes damping of the acoustic and optical modes. and Thermal quantum numbers of mechanical and optical fields For the pump laser linewidth, Qtot This represents the overall quality factor of the microcavity. Qext This represents the external cavity quality factor of the microcavity. Pout Indicates laser power. Denotes Planck's constant. This indicates the laser angular frequency.

[0026] Equation (1) shows that a narrower pump linewidth and a higher quality factor (Q value) are beneficial for generating narrow-linewidth Brillouin lasers. In the cascading process, each higher-order Stokes beam is generated by the previous-order Brillouin laser as a pump, while benefiting from the inherently high Q value of the whispering-gallery microcavity, thereby achieving continuous narrowing of the microcavity laser linewidth in the stepwise excitation.

[0027] The precise filtering and amplification of a third-order Brillouin laser is achieved through an optical selection and amplification unit. Specifically, the desired order of the Brillouin laser is selected by appropriately setting the center wavelength and bandwidth parameters of the tunable filter 6. Specifically, the transmission wavelength of this filter... Controlled by the tuning mechanism, satisfying: (2) in, For the center wavelength of the filter, This represents the filter bandwidth.

[0028] By aligning the center wavelength of the filter with the wavelength of the third-order Brillouin laser, the settings are... A wavelength interval smaller than that between adjacent laser orders (approximately 0.09 nm) is sufficient to precisely filter out third-order Brillouin lasers. The backscattered light received by the circulator and the spectrum after selectively filtering out the third-order Brillouin laser are shown below. Figure 4 As shown. Optical amplification is achieved through erbium-doped fiber amplifier 7. The filtered Brillouin laser power is approximately 10 μW, which is then amplified to 10 mW by the erbium-doped fiber amplifier for velocity detection.

[0029] Finally, the velocity detection and demodulation of the moving external object are achieved through a Doppler velocimetry unit. The Brillouin laser power is amplified to about 10 mW, and then a portion of the light is split off by coupler 8 for velocity measurement, while the other portion is used for demodulation. The generated Brillouin laser is incident on the surface of the moving external object 10 through collimating lens 9. Because the object's motion direction has a velocity component parallel to the laser propagation direction... This will cause a corresponding Doppler frequency shift in the returned feedback light. : (3) in, n The refractive index of the medium, v Let be the speed of the laser beam at the point where the laser spot is on the moving object. Let the angle between the velocity and the Brillouin laser propagation direction be denoted as . This refers to the Brillouin laser wavelength.

[0030] The feedback light carrying velocity information returns to the microsphere cavity via the second circulator 11, where it coherently interferes with the Brillouin laser oscillating within the cavity, thereby modulating the laser's output intensity. Finally, the Doppler signal is converted from an optical signal to an electrical signal by a photodetector 12, and then observed and analyzed in real time using a spectrum analyzer 13. A beat frequency signal peak that varies with velocity can be observed in the spectrum. This beat frequency signal peak is the Doppler signal peak caused by the motion of an external object. The frequency of this signal peak is read as... Substituting into Formula 3, the target's speed information can be demodulated. The speed measurement signal is as follows: Figure 5 As shown. Its Doppler signal signal-to-noise ratio reaches 40 dB, which is an order of magnitude higher than other reported Doppler velocimetry systems. The minimum resolution of the test system was determined by fine-tuning the radial position of the laser spot on the mechanical turntable to change the linear velocity of the motion. The minimum velocity change in which the spectral peaks could be clearly distinguished was defined as the resolution index. Experimental results are shown below. Figure 6 As shown, the Doppler signal frequency shifts significantly with minute changes in the target velocity. Measurements were taken by gradually changing the spot position in 30 μm increments, and the average of eight measurements yielded a minimum system resolution of 1.17 mm / s.

[0031] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser, characterized in that, include: The whispering-gallery microcavity Brillouin laser realization unit is used to input pump light into the whispering-gallery microcavity through evanescent coupling to excite stimulated Brillouin scattering and generate cascaded Brillouin lasers. The light selection and amplification unit is connected to the whispering-gallery microcavity Brillouin laser realization unit. It is used to select a Brillouin laser of a specific order from the cascaded Brillouin lasers and amplify its power to obtain a detection light source. The Doppler velocity measurement unit, connected to the light selection and amplification unit, is used to emit the detection light source to the moving target and receive the feedback light carrying the Doppler frequency shift reflected by the moving target; The feedback light is re-injected into the whispering-gallery microcavity to generate a beat frequency signal with the Brillouin laser oscillating within the cavity; and the beat frequency signal is detected and demodulated to obtain the velocity information of the moving target.

2. The Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser according to claim 1, characterized in that, The whispering-gallery microcavity Brillouin laser realization unit includes: a pump source, an evanescent coupling device, an isolator, a polarization controller, and a first circulator; The pump light source is used to emit pump light; An isolator is placed in the light output path of the pump light source to isolate backscattered light; A polarization controller is placed after the isolator to adjust the polarization state of the pump light; The first circulator is positioned after the polarization controller to transmit the pump light to the evanescent coupling device and to receive the Brillouin laser generated by the whispering-gallery microcavity in the opposite direction. Evanescent coupling devices are used to couple pump light from a circulator to a whispering-gallery microcavity via an evanescent field.

3. The Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser according to claim 2, characterized in that, The evanescent coupling device includes: micro / nano optical fibers and whispering-gallery microspheres; Micro- and nano-fibers are drawn from single-mode fibers and are used for evanescent coupling with whispering-gallery microspheres. The whispering-gallery microsphere is made by heating and burning the end of a single-mode optical fiber.

4. The Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser according to claim 1, characterized in that, The optical selection and amplification unit includes: a tunable filter and an erbium-doped fiber amplifier; A tunable filter is connected to the whispering-gallery microcavity Brillouin laser realization unit to receive cascaded Brillouin lasers and filter out Brillouin lasers of a specific order from the cascaded Brillouin lasers by adjusting its center wavelength and bandwidth. An erbium-doped fiber amplifier is connected to a tunable filter to amplify the power of the filtered Brillouin laser of a specific order.

5. The Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser according to claim 4, characterized in that, The transmission wavelength of the tunable filter is controlled by the tuning mechanism. Its center wavelength is aligned with the wavelength of the third-order Brillouin laser in the cascaded Brillouin laser, and its filtering bandwidth is smaller than the wavelength interval between adjacent Brillouin laser orders.

6. The Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser according to claim 1, characterized in that, The Doppler velocity measurement unit includes a coupler, a collimating lens, a second circulator, a photodetector, and a spectrum analyzer. The input of the coupler is connected to the optical selection and amplification unit to receive the amplified Brillouin laser and feedback light; one output of the coupler is connected to the second circulator, and the other input is connected to the photodetector. Collimating lenses are used to collimate laser light from the coupler and project it onto a moving target; The second circulator is positioned between the whispering-gallery microcavity and the collimating lens to conduct the probe light into the collimating lens and guide the feedback light from the moving target to the whispering-gallery microcavity. Photodetectors are used to convert received optical signals into electrical signals; The spectrum analyzer is connected to the photodetector to perform spectrum analysis on the electrical signal to obtain the beat frequency signal.

7. The Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser according to claim 6, characterized in that, The feedback light is re-injected into the whispering-gallery microcavity, where it undergoes self-mixing interference with the Brillouin laser oscillating within the cavity. The beat frequency signal detected by the photodetector corresponds to the Doppler frequency shift.

8. The Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser according to claim 7, characterized in that, The working principle of cascaded Brillouin lasers generated by whispering-gallery microcavities includes: When the pump light coupled into the whispering-gallery microcavity resonates with the optical mode of the microcavity, the enhanced optical field inside the cavity generates Stokes photons through the electrostriction effect. The resonance enhancement of the microcavity enables the Stokes photons to find a phase-matched optical mode, thereby forming a coherent Brillouin laser output. As the pump power increases, the first-order Stokes light acts as a new pump source, exciting the second-order stimulated Brillouin process, and so on. If resonant modes resonate in each stimulated gain region, a cascaded Brillouin laser output is formed; in the cascading process, each higher-order Stokes beam is generated by the previous-order Brillouin laser as a pump.

9. The Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser according to claim 8, characterized in that, Based on the inherent high quality factor of whispering-gallery microcavities, continuous narrowing of the microcavity laser linewidth is achieved during stepwise excitation: in, For Brillouin laser linewidth, and It is the Stokes damping of the acoustic and optical modes. and Thermal quantum numbers of mechanical and optical fields For the pump laser linewidth, Qtot This represents the overall quality factor of the microcavity. Qext This represents the external cavity quality factor of the microcavity. Pout Indicates laser power. Denotes Planck's constant. This indicates the laser angular frequency.

10. The Doppler velocimetry system based on a whispering-gallery microcavity Brillouin laser according to claim 8, characterized in that, Speed ​​information is demodulated through the following relationship: in, n The refractive index of the medium, v Let be the speed of the laser beam at the point where the laser spot is on the moving object. Let the angle between the velocity and the Brillouin laser propagation direction be denoted as . This refers to the Brillouin laser wavelength.

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