Diamond nv-center based digital phase-locked microwave magnetic field imaging system and method

Abstract

This invention relates to the field of quantum imaging technology and proposes a digital phase-locked microwave magnetic field imaging system and method based on diamond NV centers. This invention images microwave fields based on the principle of power-broadened magnetic resonance. The system consists of multiple subsystems: an excitation optical path, an imaging optical path, an acquisition system, and a bias magnetic field. The excitation light is used to excite the diamond NV centers; the imaging optical path captures the fluorescence changes during microwave exposure; the acquisition system controls the testing system and analyzes data through a programmed host computer; and the bias magnetic field splits the resonance peaks to increase the bandwidth of the measured microwave field. Furthermore, imaging at different resolutions can be achieved by changing the objective lens. This invention enables non-destructive, non-invasive measurement of microstrip lines at room temperature using the NV center ensemble, thereby analyzing their operating status. In practical applications, this method offers advantages such as high sensitivity and rapid imaging.

Description

Technical Field

[0001] This invention relates to the field of quantum imaging technology, and more specifically, to a digital phase-locked microwave magnetic field imaging system and method based on diamond NV color centers. Background Technology

[0002] Microwave magnetic field imaging technology has significant application value in many cutting-edge fields such as electromagnetic field detection and condensed matter physics. However, its traditional imaging methods rely on technologies such as radiation field sensors and superconducting quantum interference devices, which suffer from bottlenecks such as insufficient resolution and sensitivity, poor environmental adaptability, and weak anti-interference ability of analog signal processing. These limitations make it difficult to meet the high-end requirements of modern scientific research and industry for micro-nano-scale, weak signal, and room-temperature stable imaging. Diamond NV color centers, as a novel quantum spin defect, possess advantages such as room-temperature quantum coherence, ultra-high sensitivity and spatial resolution, optical readability, and broad-spectrum response, providing a new path to overcome these bottlenecks. Furthermore, the preparation technology of NV centers is becoming increasingly mature. However, existing imaging methods based on NV centers use photodetector magnetic resonance technology combined with traditional signal processing, which suffers from problems such as low accuracy of high-power microwave imaging, difficulty in extracting weak signals, and poor system integration. Digital phase-locked loop technology can efficiently extract weak signals from strong noise and is highly compatible with the quantum sensing characteristics of NV centers. The combination of the two can play a synergistic role. Therefore, the development of a digital phase-locked loop microwave magnetic field imaging method and system based on diamond NV centers to achieve high-sensitivity, high-resolution, and room-temperature stable imaging of microwave magnetic fields has important theoretical research value and practical application significance. Summary of the Invention

[0003] The purpose of this invention is to address the problem of how to quickly and efficiently perform online testing of microstrip lines or microwave chips under standard conditions. It provides a digital phase-locked microwave magnetic field imaging method based on diamond NV centers and builds a complete system that can perform microwave magnetic field imaging on the sample under test under standard conditions to analyze the internal situation, conduct phenomenological studies, and achieve high-resolution magnetic field imaging.

[0004] This invention provides the following technical solutions: A digital phase-locked microwave magnetic field imaging system based on diamond NV color centers includes: The excitation optical path is used to generate and transmit 532nm excitation light to irradiate the diamond NV color center sample. The excitation optical path includes, in sequence along the optical path propagation direction, a 532nm laser, an aperture, an optical isolator, a half-wave plate, a polarizing beam splitter, a first convex lens, an acousto-optic modulator (AOM), a second convex lens, a third convex lens, a 45° reflector, a beam expander, and a beam shaper. The imaging optical path, used to collect and image the fluorescence generated by the excited NV color center of diamond, includes a dichroic beam splitter, a high-magnification objective lens, a filter, a beam splitter, a CMOS camera, and an APD avalanche tube detector; the dichroic beam splitter is used to reflect the excitation light and transmit the fluorescence, and the high-magnification objective lens is used to focus the excitation light and collect the fluorescence. A microwave system for providing an FM-modulated microwave signal to a sample under test includes a microwave source and a power amplification module. The microwave source supplies microwaves with a frequency range of 600 kHz–6 GHz, which are then FM-modulated and applied to the diamond NV color center. The bias magnetic field module includes a ring magnet mounted above the diamond sample and a 360° adjustment device, used to provide a stable bias magnetic field to split the NV color center resonance peak. The acquisition and control system, including a CMOS scientific camera, an APD avalanche tube detector, and a host computer, is used to synchronously control the timing of the laser, microwave source, camera, and APD, and to perform digital phase-locked demodulation processing on the acquired signals to achieve microwave magnetic field imaging.

[0005] Furthermore, in the excitation optical path: The first convex lens has a focal length of 100mm and is used to compress the laser beam before it passes through the AOM. The second convex lens has a focal length of 50mm, and the third convex lens has a focal length of 100mm. The combination of the two converts the beam passing through the AOM into parallel light. The beam expander is a variable magnification beam expander used to adjust the beam diameter to fill the imaging objective; the beam shaper is used to convert the Gaussian beam into a flat-top beam.

[0006] Furthermore, in the imaging optical path: The high-power objective lens is a replaceable magnification objective lens used to eliminate stray light and collect fluorescence; The beam splitter is used to direct fluorescence to the CMOS camera and the APD detector, respectively. The CMOS camera is used for high-resolution imaging and real-time monitoring of the position of the sample under test, and the APD is used to connect to the lock-in amplifier to extract weak microwave signals or to connect to an oscilloscope to observe the ODMR spectrum.

[0007] Furthermore, the bias magnetic field module adjusts the center position of the ring magnet through a 360° adjustment device to generate a stable magnetic field that is perpendicular or parallel to the surface of the diamond sample, which is used to split the NV color center resonance peak to improve the measurement bandwidth.

[0008] Furthermore, the acquisition and control system employs digital phase-locked demodulation technology to demodulate the fluorescence image acquired by the CMOS camera pixel by pixel, extracting the signal components synchronized with the FM modulation frequency, and suppressing electromagnetic noise and fluorescence background interference.

[0009] Furthermore, the system supports two operating modes: Weak fluorescence signals can be extracted using an APD avalanche tube detector in conjunction with a lock-in amplifier; or fluorescence images can be acquired using a CMOS camera and digitally demodulated by a host computer to achieve wide-field microwave magnetic field imaging.

[0010] Furthermore, the beam shaper is located after the beam expander and before the dichroic beam splitter, and is used to convert the Gaussian-distributed 532nm green light into flat-top light to illuminate the surface of the diamond NV color center.

[0011] This invention further proposes a digital phase-locked microwave magnetic field imaging method based on diamond NV color centers using the above system, comprising the following steps: Step 1: Place the diamond sample above the sample to be tested. If you need to detect frequencies other than 2.87 GHz, turn on the bias magnetic field module to apply a constant magnetic field. Step 2: Turn on the 532nm laser. The excitation light is transmitted through the excitation optical path, and then passes through the aperture to filter out stray light, the optical isolator to isolate the back light, the half-wave plate and polarizing beam splitter to adjust the power, the AOM to control the pulse, the beam expander to expand the beam and the beam shaper to shape the beam. After being reflected by the dichroic beam splitter and focused by the high-power objective lens, it illuminates the surface of the diamond NV color center. Step 3: The microwave system outputs an FM-modulated scanning microwave signal, which is amplified and applied to the sample under test. When the microwave frequency resonates with the energy level difference of the NV color center, the fluorescence intensity of the NV color center produces a periodic change synchronized with the modulation frequency. Step 4: The imaging optical path collects the fluorescence generated by the NV color center, which is transmitted through a dichroic beam splitter, collected by a high-magnification objective lens, filtered by a filter, and then acquired by a CMOS camera or APD detector. Step 5: The acquisition and control system performs digital phase-locked demodulation on the acquired fluorescence signal, extracts the signal amplitude at the modulation frequency, and combines the power broadening effect of photodetector magnetic resonance to invert and calculate the microwave magnetic field intensity distribution on the surface of the sample to be tested, thereby realizing magnetic field imaging. The method is applicable to optically probed magnetic resonance (ODMR) detection under continuous wave conditions. By monitoring the change of NV center fluorescence with microwave frequency, the quantum state of the NV center and the intensity of the local microwave magnetic field can be determined.

[0012] Furthermore, the FM modulation in step 3 is continuous wave frequency modulation. The modulated microwave causes the NV color center fluorescence to jitter at a fixed modulation frequency, and the weak signal is extracted from the strong noise background through subsequent digital phase-locked demodulation.

[0013] Furthermore, the digital phase-locked demodulation in step 5 includes: The temporal fluorescence signal of each pixel acquired by the CMOS camera is cross-correlated or Fourier transformed with the reference modulation signal to extract the amplitude and phase information at the modulation frequency; the microwave magnetic field distribution image in the entire field of view is obtained by pixel-by-pixel reconstruction.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention deeply integrates diamond NV center quantum sensing with digital phase-locked loop (PLL) technology, offering several significant advantages. In terms of detection performance, it overcomes bottlenecks such as insufficient sensitivity of traditional probes, the need for extremely low temperatures for superconducting devices, and limited resolution of fiber optic sensors, achieving nanotesla-level high sensitivity and submicron-level high-resolution imaging at room temperature and atmospheric conditions. Regarding signal processing, digital PLL technology accurately extracts weak signals, significantly suppressing electromagnetic noise and fluorescence background interference, substantially improving the imaging signal-to-noise ratio and measurement stability, and effectively addressing the weak anti-interference capabilities of traditional methods. For high-power microwave scenarios, this invention alleviates spin response saturation and temperature frequency shift issues, ensuring imaging accuracy across a wide power range. Simultaneously, the system employs a digital architecture and optimized optical path design, resulting in higher integration, smaller size, and simpler operation. It overcomes the limitations of traditional equipment, such as complexity, stringent environmental requirements, and difficulty in engineering applications, enabling the study of the working state of microstrip line samples under standard conditions and achieving magneto-optical detection of microstrip lines.

[0015] Overall, this invention offers comprehensive improvements in sensitivity, resolution, environmental adaptability, system stability, and practicality, increases imaging time by about ten times, and enables all components to be controlled by a host computer, making it more suitable for high-precision near-field imaging of weak and high-power microwave magnetic fields. Attached Figure Description

[0016] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. In the drawings: Figure 1 This is a schematic diagram illustrating the principle of illuminating NV color centers with 532nm green light in this invention; Figure 2 This is a schematic diagram of the digital phase-locked microwave magnetic field imaging system based on diamond NV color centers according to the present invention; Figure 3 This is a schematic diagram of the physical structure of the digital phase-locked microwave magnetic field imaging system based on diamond NV color centers according to the present invention; Figure 4 This is a schematic diagram illustrating the digital phase-locked loop demodulation principle of the present invention; Figure 5 A simulation image of a microstrip line sample using the system of this invention; Figure 6The image shows a measured image of a microstrip line sample using the system of this invention. Detailed Implementation

[0017] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the disclosure to those skilled in the art. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0018] This embodiment proposes a digital phase-locked microwave magnetic field imaging method based on diamond NV centers and constructs a complete system. It proposes a digital phase-locked imaging method based on the working principle of a lock-in amplifier. Specifically, microwaves are FM modulated, and a signal with a fixed modulation frequency is applied to the NV centers. This signal is captured by a camera and demodulated to obtain the demodulated curve. Finally, the power broadening effect of photodetector magnetic resonance is utilized to perform micrometer-level magnetic field imaging.

[0019] like Figure 1 The diagram shows the energy level structure of the NV color center. At the start of detection, the NV color center is first excited to the excited state using 532nm green light. (The diagram is incomplete and requires further context.) Figure 2 As shown, the digital phase-locked microwave magnetic field imaging system based on diamond NV color centers includes a 532nm laser, an aperture, an optical isolator, a half-wave plate, a polarizing beam splitter, a 100 mm focal length convex lens, an AOM, a 50 mm focal length convex lens, a 100 mm focal length convex lens, a 45° reflector, a variable magnification beam expander and a beam shaper, a dichroic beam splitter, a high-magnification objective lens, a CMOS camera, an APD avalanche tube detector, a plano-convex lens, and a ring magnet. Specifically, the system in this embodiment includes the following structure: The excitation optical path is used to provide a stable, high-quality laser for the excitation of diamond samples. The imaging optical path uses a high-resolution, wide-field-of-view CMOS camera, which can quickly locate the position of the sample to be tested, and monitor and record the fluorescence changes of the diamond on the microstrip line in real time and image it during the detection process. At the same time, the sample imaging area can be adjusted by adjusting optical devices such as fluorescent reflectors. The acquisition system systematically collects the weak optical signal changes generated by the diamond on the sample under test when stimulated, for further imaging analysis. A bias magnetic field is used to provide a stable magnetic field, splitting the resonance peaks to increase the measurement bandwidth.

[0020] The microwave system is used to provide microwave signals to the sample, and there is also a power amplification module to amplify the microwave power.

[0021] The physical image of the system provided in this embodiment is as follows: Figure 3 As shown, specifically, the excitation optical path includes a 532nm laser, an aperture, an optical isolator, a half-wave plate, a polarizing beam splitter, a 100mm convex lens, an aperture stop (AOM), a 50mm convex lens, a 100mm convex lens, a 45° reflecting mirror, a variable magnification beam expander, and a beam shaper. These components together form the excitation optical path. The laser emits 532nm visible green light. The aperture is used to filter out stray light and prevent light returning from the rear mirror from damaging the laser. The half-wave plate and polarizing beam splitter are used to control the laser power. The 100mm convex lens narrows the light beam through the AOM for pulse control. The 50mm and 100mm convex lenses convert the narrowed beam into parallel light. The beam expander expands the emitted light to fill the imaging objective lens. The dichroic beam splitter reflects the excitation light and transmits the fluorescence emitted by the diamond.

[0022] Specifically, the imaging optical path includes a dichroic beam splitter prism, replaceable magnification objective lenses, filters, a camera, and an APD. These constitute the imaging optical path, where the dichroic beam splitter prism transmits the fluorescence signal emitted by the diamond, the objective lens eliminates stray light and collects fluorescence information to prevent light pollution, the camera performs high-resolution imaging of the detection position, and the APD is used to transmit the fluorescence signal. It can also be fed into a microwave circuit and connected to an oscilloscope to observe the ODMR and ODMR eight-peak splitting conditions, performing the sensing tests required for quantum imaging. Passing the FM-modulated signal into the microwave circuit, and then passing the APD signal into a lock-in amplifier, further extracts weak microwave signals. If the camera is used to capture the fluorescence signal, digital phase-locked loop processing technology is used for pixel-by-pixel demodulation imaging.

[0023] Specifically, the acquisition system includes a CMOS scientific camera, an APD avalanche tube detector, a plano-convex lens, a beam splitter, and a high-magnification objective lens. The CMOS scientific camera provides high-resolution imaging of the detection location. The acquisition system acquires magnetic imaging data by controlling the laser and microwave sources, as well as the synchronous control of the camera and APD. The data is then processed on a host computer.

[0024] Specifically, the bias magnetic field is positioned above the diamond to place the diamond sample in a stable magnetic field. The detection diamond is placed on the sample to be tested. When the excitation light signal passes through the dichroic beam splitter and the high-power objective lens to the diamond surface, the microwave source transmits microwaves with added modulation signals to the NV color center, enabling the NV color center to generate modulated optical magnetic resonance (ODMR) signals. The resulting fluorescence changes are transmitted to the CMOS camera through the dichroic beam splitter and then acquired by the camera. The data is then transmitted to the host computer for processing to achieve high-resolution detection and imaging.

[0025] Specifically, this embodiment uses a ring magnet to generate a constant magnetic field, and by adjusting the center position of different ring magnets through a 360° adjustment device, a stable magnetic field in the vertical or parallel direction can be generated.

[0026] Specifically, the microwave system can provide microwave signals to this magnetic imaging system; the microwave source supplies microwaves ranging from 600 kHz to 6 GHz; and the power amplifier module amplifies the microwave power to meet the conditions required for different scenarios. In specific testing, if frequencies other than 2.87 GHz need to be measured, a ring magnet needs to be installed and fixed; if the frequency is 2.87 GHz, a ring magnet is not required. A 3mm × 3mm × 0.5mm diamond sample is placed above the sample to be tested. The laser is turned on for preheating, and the excitation light emitted from the laser passes through an aperture to filter out stray light interference, then through an optical isolator, through a combination of a half-wave plate and a polarizing beam splitter, and after being beam-constricted by a lens, it passes through an AOM (Optical Optical Array) to control the presence or absence of light. After exiting the AOM (Optical Oscillator), the light is collimated by a combination of two lenses. It then passes through a variable-magnification beam expander to precisely adjust the excitation beam size. Next, it enters a beam shaper to convert the Gaussian light into a flat-top beam. This beam then reaches a dichroic beam splitter, is reflected by the dichroic beam splitter, and is focused by a high-magnification objective lens, ultimately reaching the diamond surface. At this point, a camera captures the fluorescence changes in the diamond caused by the microwave field changes in the sample. This signal is collected by the high-magnification objective lens, passes through the dichroic beam splitter, and is focused into the camera via the imaging optical path. The CMOS camera connects to a data acquisition module for data acquisition, and the data is processed by a host computer to observe the microwave magnetic field image of the sample, thus achieving magnetic field imaging and completing the sample detection.

[0027] Figure 4This diagram illustrates the principle of digital phase-locked demodulation in this embodiment. It explains that this embodiment deeply embeds digital phase-locked technology into the quantum sensing process of diamond NV centers, overcoming the limitations of traditional optically detected magnetic resonance (ODMR) which relies on analog signal processing and struggles to extract weak microwave magnetic field signals from strong noise. It employs frequency modulation (FM) to modulate the microwaves, enabling the fluorescence response of the NV centers to carry specific modulation frequency information. Subsequently, the camera acquires fluorescence signals pixel-by-pixel, and the digital phase-locked algorithm in the host computer demodulates this modulation frequency, thereby achieving precise extraction of weak signals simultaneously in both the spatial and temporal domains. Compared to traditional methods, this digital demodulation approach not only significantly suppresses electromagnetic noise and fluorescence background interference, dramatically improving the imaging signal-to-noise ratio, but also achieves parallel processing of magnetic field signals at micrometer-level spatial resolution.

[0028] Figure 5 and Figure 6 Simulation and experimental results of microwave magnetic field imaging of microstrip line samples using the system of this invention are presented respectively. The two results show a high degree of agreement in magnetic field distribution characteristics, fully verifying the feasibility and reliability of the system in achieving high-precision, high-resolution microwave magnetic field imaging under room temperature and atmospheric conditions. Figure 5 The typical characteristics of the magnetic field distribution around the microstrip line are clearly presented. Figure 6 Based on diamond NV color centers and digital phase-locked loop demodulation technology, a high signal-to-noise ratio magnetic field image consistent with simulation trends was successfully acquired, achieving sub-micron spatial resolution and accurately resolving local magnetic field details in microstrip line structures. This strongly demonstrates the outstanding advantages of this invention in weak microwave magnetic field signal extraction, background noise suppression, and imaging fidelity, breaking through the bottlenecks of traditional methods in sensitivity and resolution, and providing reliable technical support for online non-destructive testing of devices such as microstrip circuits and microwave chips.

[0029] The detection method provided by this invention is based on the optically detected magnetic resonance (ODMR) method under continuous wave conditions with added modulation signals. It addresses the phenomenon of fluorescence decrease when the applied continuous FM modulated scanning microwave frequency resonates with the energy level difference of the NV color center. The quantum state of the NV color center can be determined based on the microwave frequency and fluorescence intensity. At this point, the fluorescence exhibits frequency jitter. The relationship between the optical and magnetic signals is then obtained through microwave magnetic field calculation. This system is simple to operate and easy to install, enabling the study of the working state of microstrip line samples under standard conditions, and realizing magneto-optical detection of microstrip lines.

[0030] Compared to traditional microwave field detection methods, this invention deeply integrates diamond NV center quantum sensing with digital phase-locked loop (PLL) technology, offering several significant advantages. In terms of detection performance, it overcomes bottlenecks such as insufficient sensitivity of traditional probes, the need for extremely low temperatures for superconducting devices, and limited resolution of fiber optic sensors, achieving nanotesla-level high sensitivity and submicron-level high-resolution imaging at room temperature and atmospheric conditions. In signal processing, digital PLL technology accurately extracts weak signals, significantly suppressing electromagnetic noise and fluorescence background interference, substantially improving the imaging signal-to-noise ratio and measurement stability, and effectively addressing the weak anti-interference capability of traditional methods. For high-power microwave scenarios, this invention alleviates spin response saturation and temperature frequency shift issues, ensuring imaging accuracy across a wide power range. Simultaneously, the system employs a digital architecture and optimized optical path design, resulting in higher integration, smaller size, and simpler operation, overcoming the limitations of traditional equipment's complexity, stringent environmental requirements, and difficulty in engineering applications. Overall, this invention comprehensively improves sensitivity, resolution, environmental adaptability, system stability, and practicality, making it more suitable for high-precision near-field imaging of weak and high-power microwave magnetic fields.

[0031] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A digital phase-locked microwave magnetic field imaging system based on diamond NV color centers, characterized in that, include: The excitation optical path is used to generate and transmit 532nm excitation light to irradiate the diamond NV color center sample. The excitation optical path includes, in sequence along the optical path propagation direction, a 532nm laser, an aperture, an optical isolator, a half-wave plate, a polarizing beam splitter, a first convex lens, an acousto-optic modulator (AOM), a second convex lens, a third convex lens, a 45° reflector, a beam expander, and a beam shaper. The imaging optical path, used to collect and image the fluorescence generated by the excited NV color center of diamond, includes a dichroic beam splitter, a high-magnification objective lens, a filter, a beam splitter, a CMOS camera, and an APD avalanche tube detector; the dichroic beam splitter is used to reflect the excitation light and transmit the fluorescence, and the high-magnification objective lens is used to focus the excitation light and collect the fluorescence. A microwave system for providing an FM-modulated microwave signal to a sample under test includes a microwave source and a power amplification module. The microwave source supplies microwaves with a frequency range of 600 kHz–6 GHz, which are then FM-modulated and applied to the diamond NV color center. The bias magnetic field module includes a ring magnet mounted above the diamond sample and a 360° adjustment device, used to provide a stable bias magnetic field to split the NV color center resonance peak. The acquisition and control system, including a CMOS scientific camera, an APD avalanche tube detector, and a host computer, is used to synchronously control the timing of the laser, microwave source, camera, and APD, and to perform digital phase-locked demodulation processing on the acquired signals to achieve microwave magnetic field imaging.

2. The system according to claim 1, characterized in that, In the excitation optical path: The first convex lens has a focal length of 100mm and is used to compress the laser beam before it passes through the AOM. The second convex lens has a focal length of 50mm, and the third convex lens has a focal length of 100mm. The combination of the two converts the beam passing through the AOM into parallel light. The beam expander is a variable magnification beam expander used to adjust the beam diameter to fill the imaging objective; the beam shaper is used to convert the Gaussian beam into a flat-top beam.

3. The system according to claim 1, characterized in that, In the imaging optical path: The high-power objective lens is a replaceable magnification objective lens used to eliminate stray light and collect fluorescence; The beam splitter is used to direct fluorescence to the CMOS camera and the APD detector, respectively. The CMOS camera is used for high-resolution imaging and real-time monitoring of the position of the sample under test, and the APD is used to connect to the lock-in amplifier to extract weak microwave signals or to connect to an oscilloscope to observe the ODMR spectrum.

4. The system according to claim 1, characterized in that, The bias magnetic field module adjusts the center position of the ring magnet through a 360° adjustment device to generate a stable magnetic field that is perpendicular or parallel to the surface of the diamond sample, which is used to split the NV color center resonance peak to improve the measurement bandwidth.

5. The system according to claim 1, characterized in that, The acquisition and control system employs digital phase-locked demodulation technology to demodulate the fluorescence image acquired by the CMOS camera pixel by pixel, extracting the signal components synchronized with the FM modulation frequency, and suppressing electromagnetic noise and fluorescence background interference.

6. The system according to claim 1, characterized in that, The system supports two operating modes: Weak fluorescence signals can be extracted using an APD avalanche tube detector in conjunction with a lock-in amplifier; or fluorescence images can be acquired using a CMOS camera and digitally demodulated by a host computer to achieve wide-field microwave magnetic field imaging.

7. The system according to claim 1, characterized in that, The beam shaper is located after the beam expander and before the dichroic beam splitter, and is used to convert the Gaussian-distributed 532nm green light into flat-top light to illuminate the surface of the diamond NV color center.

8. A digital phase-locked microwave magnetic field imaging method based on diamond NV color centers, employing the system described in any one of claims 1-7, characterized in that, Includes the following steps: Step 1: Place the diamond sample above the sample to be tested. If you need to detect frequencies other than 2.87 GHz, turn on the bias magnetic field module to apply a constant magnetic field. Step 2: Turn on the 532nm laser. The excitation light is transmitted through the excitation optical path, and then passes through the aperture to filter out stray light, the optical isolator to isolate the back light, the half-wave plate and polarizing beam splitter to adjust the power, the AOM to control the pulse, the beam expander to expand the beam and the beam shaper to shape the beam. After being reflected by the dichroic beam splitter and focused by the high-power objective lens, it illuminates the surface of the diamond NV color center. Step 3: The microwave system outputs an FM-modulated scanning microwave signal, which is amplified and applied to the sample under test. When the microwave frequency resonates with the energy level difference of the NV color center, the fluorescence intensity of the NV color center produces a periodic change synchronized with the modulation frequency. Step 4: The imaging optical path collects the fluorescence generated by the NV color center, which is transmitted through a dichroic beam splitter, collected by a high-magnification objective lens, filtered by a filter, and then acquired by a CMOS camera or APD detector. Step 5: The acquisition and control system performs digital phase-locked demodulation on the acquired fluorescence signal, extracts the signal amplitude at the modulation frequency, and combines the power broadening effect of photodetector magnetic resonance to invert and calculate the microwave magnetic field intensity distribution on the surface of the sample to be tested, thereby realizing magnetic field imaging. The method is applicable to optically probed magnetic resonance (ODMR) detection under continuous wave conditions. By monitoring the change of NV center fluorescence with microwave frequency, the quantum state of the NV center and the intensity of the local microwave magnetic field can be determined.

9. The method according to claim 8, characterized in that, The FM modulation in step 3 is continuous wave frequency modulation. The modulated microwave causes the NV color center fluorescence to jitter at a fixed modulation frequency. The weak signal is then extracted from the strong noise background through subsequent digital phase-locked demodulation.

10. The method according to claim 8, characterized in that, The digital phase-locked demodulation in step 5 includes: The temporal fluorescence signal of each pixel acquired by the CMOS camera is cross-correlated or Fourier transformed with the reference modulation signal to extract the amplitude and phase information at the modulation frequency; the microwave magnetic field distribution image in the entire field of view is obtained by pixel-by-pixel reconstruction.

Images