System and method for detecting isotope by using interferometer differential measurement phase shift signal

Abstract

The invention discloses a system and a method for detecting isotopes by using an interferometer differential measurement phase shift signal. The system comprises a high-power laser ablation module, a laser interferometer module, a phase modulation module, a sample pool, a signal detection module and a signal processing unit. The high-power laser ablation module ablates the surface of a to-be-detected sample through high-power laser pulse to prepare plasma to be detected; the laser interferometer module configures detection light and reference light for interference, the phase modulation module is used for performing phase modulation on the detection light, a detector in the signal detection module performs homodyne detection on a detection light electric field, and the signal processing unit obtains a radio frequency signal carrying phase information through demodulation. And finally processing into a phase shift curve of which the phase shift is changed along with the laser frequency. According to the invention, the problem of measurement of two optical phase differences can be converted into the problem of measurement of a radio-frequency signal phase, so that high-precision and high-stability detection of weak atom signals is realized.

Description

Technical Field

[0001] This invention relates to the field of laser plasma emission spectroscopy detection technology, and in particular to a system and method for detecting isotopes using lasers. Background Technology

[0002] Isotopes are different nuclides of the same element with the same number of protons but different numbers of neutrons. The differences in their mass and radioactivity characteristics underpin the important role of isotope analysis in the field of detection and analysis. Isotope analysis is an analytical technique that obtains desired biological, geological, and environmental information by measuring the composition and proportion of isotopes in a sample. It is divided into stable isotope analysis and radioactive isotope analysis. Currently, isotope analysis is widely used in many fields such as geology, biology, medicine, environmental science, and archaeology. For example, the lead isotope ratio in bronze artifacts (e.g., ...) It can trace the source of minerals and reveal ancient trade routes; the carbon-nitrogen ratio in human tissue can infer the geographical origin of the deceased. The urea breath test can detect Helicobacter pylori in the gastric epithelium; Isotopes are primarily used in nutritional research; hydrogen and oxygen, as components of water molecules themselves, are natural tracers for studying the water cycle. Furthermore, radioactive isotopes, as markers, can be used to deduce the migration paths and patterns of research subjects by tracking their dynamic changes in location and concentration. For example, radiocarbon (14C) tracing technology and short-wave infrared spectroscopy (FTIR) technology have been used to study soil organic carbon turnover. Tracing plant phosphorus uptake pathways to optimize fertilization strategies. The isotopic composition of man-made radionuclides tritium and plutonium can be used to trace water masses and biogeochemical processes; the tracing function of radioisotopes in the medical field can be used for in vitro analysis, functional assays, imaging, and targeted therapy.

[0003] Currently, mass spectrometry is the main method for isotope detection, relying on the element's mass-charge ratio. It is widely used for determining isotope ratios, especially with the development of thermal ionization solid-state mass spectrometry and multi-collector inductively coupled plasma mass spectrometry (ICP-MS), which have greatly promoted isotope detection research. They have not only significantly impacted the analysis of widely existing radioactive isotopes in nature but have also revealed the mass fractionation of stable isotopes. However, mass spectrometry also has many drawbacks, such as high cost, long detection time, large sample volume requirements, and interference from the same element. Summary of the Invention

[0004] To address the shortcomings in the aforementioned background technology, this invention proposes a system and method for detecting isotopes using differential measurement of phase shift signals with an interferometer, which solves the problems of long detection time and interference from the same element in the prior art.

[0005] The technical solution of this invention is implemented as follows: A system for detecting isotopes using differential measurement of phase-shift signals using an interferometer includes a sample cell, a high-power laser ablation module, a laser interferometer module, a phase modulation module, a signal detection module, and a signal processing unit. The high-power laser ablation module includes a pulsed laser. The laser pulses from the pulsed laser reach the sample cell via a first optical element and ablate the sample to be tested to obtain the plasma to be tested. The laser interferometer module includes a continuously tunable laser, a first beam splitter, and a second beam splitter. The laser emitted by the continuously tunable laser is split into a probe beam A and a reference beam B by the first beam splitter. Probe beam A reaches the sample cell after passing through the phase modulation module and maintains spatial overlap with the plasma to be tested. Reference beam B enters the second beam splitter through a third reflector with a PZT (Polarization Zone Transformer). Reference beam B and probe beam A overlap and interfere at the second beam splitter. The signal detection module includes a first photodetector and a second detector. The first photodetector and the second detector measure the two output optical paths of the second beam splitter, respectively. The electrical signals from the first photodetector and the second detector are processed by the signal processing unit. Using spectroscopic tools to analyze atoms, ions, or molecules in plasma can support in-situ and isolated analysis, and even support the manufacture of portable instruments for on-site analysis under atmospheric conditions, further expanding the applicability of isotope spectroscopy in various disciplines.

[0006] Further preferably, the phase modulation module includes an electro-optic modulator and a first function generator. The first function generator provides an radio frequency signal to the electro-optic modulator, and the electro-optic modulator is placed in the optical path of the probe light A and performs phase modulation.

[0007] Further optimized, the electro-optic modulator performs phase modulation on the probe light A to generate a frequency of ω0±nω. m The sideband, ω0 is the laser carrier frequency, n is an integer, ω m The modulation frequency is used; the probe light A is reflected to the sample cell through the second mirror.

[0008] Further optimized, the signal processing unit includes a differential amplifier, a first-order sideband processing circuit, and a second-order sideband processing circuit; the differential amplifier performs differential processing on the signals from the first photodetector and the second photodetector, and then transmits the signals to the first-order sideband processing circuit and the second-order sideband processing circuit; the second-order sideband processing circuit extracts signals from the 2ω... m Related phase-shifted signals; extraction of ω by first-order sideband processing circuit m Related phase shift signals.

[0009] Further preferred, the first-order sideband processing circuit includes a first bandpass filter and a first mixer. The first bandpass filter receives the signal from the differential amplifier and transmits the processed signal to the first mixer. The local oscillator signal of the first mixer is provided by a first function generator.

[0010] Further preferred, the second-order sideband processing circuit includes a second bandpass filter, a second mixer, and a PID servo module. The second bandpass filter receives the signal from the differential amplifier and transmits the processed signal to the second mixer. The signal processed by the second mixer is processed by the PID servo circuit and fed back to the PZT for phase locking. The local oscillator signal of the second mixer is provided by the second function generator.

[0011] Further preferred, the first optical element includes a first reflector and a first lens, and the pulsed laser generates a laser pulse with energy higher than the ablation threshold of the sample surface and is focused onto the surface of the sample to be tested through the first reflector and the first lens.

[0012] Further optimization involves setting the sample cell on a three-dimensional translation stage, with an air inlet and an air outlet, and a glass window on the sample cell.

[0013] A method for detecting isotopes using differential measurement of phase-shift signals with an interferometer, employing the aforementioned system for detecting isotopes using differential measurement of phase-shift signals with an interferometer; the steps are as follows:

[0014] S1 is configured with a series of standard samples with known concentrations. The system is used to measure their phase shift signals, obtain the magnitude of the phase shift signal corresponding to the concentration, and establish a calibration curve.

[0015] S2 uses a pulsed laser to generate a laser pulse with energy higher than the ablation threshold of the sample surface to ablate the sample, thereby generating the test plasma in the sample cell.

[0016] The S3 continuously tunable laser emits a laser with a wavelength range covering the isotope transition peaks of the sample under test. Its probe light A enters the sample cell and interacts with the plasma. The center frequency of the probe light A of the continuously tunable laser is scanned, and the intensity signal after interference is measured by the first photodetector and the second detector. The intensity signal is then differentially amplified, filtered, and mixed by the signal processing unit to obtain the magnitude of the phase shift signal that varies with the laser center frequency.

[0017] S4 obtains the isotope concentration of the sample to be tested by using the phase shift signal magnitude in S3 and the calibration curve in S1.

[0018] The physical mechanism of this scheme is based on the phase shift generated by the interaction of laser light through plasma and atoms, and the prediction of lead isotope content based on the phase shift signal. This invention utilizes zero-difference detection to extract the magnitude of the phase shift generated by the interaction between laser light and atoms. Phase modulation is performed in the probe optical path of the laser interferometer using an electro-optic modulator to generate a frequency of (ω0±ω...). m ) sideband (ω mIt is the modulation frequency, which is also the detuning of the sideband relative to the atomic transition resonance frequency. The positive and negative sideband light fields interact with the atoms to generate phase shift signals of equal magnitude but opposite sign. The phase shift signal, which is a dispersion-type signal, is obtained by differential detection by a detector, and the spectral lines of the phase shift signal change with the laser frequency. Finally, the isotope content is estimated by the magnitude of the phase shift signal.

[0019] When probe light A passes through the plasma, the interaction between the light and atoms alters the speed of light propagation, causing a phase shift that affects atomic transitions. Caused laser phase shift The relationship between the number of atoms and the detuning of the probe light can be expressed as:

[0020] ……①

[0021] Where λ is the laser wavelength, l is the length of the probed plasma, q is the laser polarization state, and I is the spin quantum number of the atomic nucleus. It is in a state The number of atoms, V is the volume of the plasma being probed. It is the natural linewidth of the detected transition. Ω0 is the detuning of the laser relative to the transition, J' is the total angular momentum quantum number of the upper energy level, J is the total angular momentum quantum number of the lower energy level, F' is the total hyperfine angular momentum quantum number of the upper energy level, F is the total hyperfine angular momentum quantum number of the lower energy level, and m' is the total hyperfine angular momentum quantum number of the lower energy level. F It is the magnetic quantum number of the total hyperfine angular momentum of the transition to the next energy level, m F It is the magnetic quantum number of the total hyperfine angular momentum of the energy level under the transition;

[0022] If the laser is phase-modulated, when the laser carrier frequency resonates with the transition frequency, the phase shifts of the positive and negative sidebands are equal in magnitude and opposite in sign. Using a laser interferometer with modulated probe light, the positive and negative first-order sidebands ω0±ω in the probe light are differentially measured. m The phase-shifted signal, and the first-order radio frequency component in the resulting photocurrent signal that is related to the modulation frequency. for:

[0023] ……②

[0024] Atomic phase shift Reference phase difference ;

[0025] in, o It is the phase of the local oscillator signal. g It is the global phase; It is the phase shift of the positive first-order sideband. It is the phase shift of the negative first-order sideband. It is positive first-order sideband laser noise. It is negative first-order sideband laser noise. m J1 is the modulation phase, J1 is the first-order Bessel function, and a is the modulation depth.

[0026] After mixing and filtering, pure atomic phase shifts are extracted. As can be seen from formula ①, the atomic phase shift is directly proportional to the number of atoms, thus yielding the number of atoms.

[0027] The beneficial effects of this invention are as follows: By utilizing optical interference and radio frequency mixing, this invention transforms the problem of measuring the phase difference between two optical signals into the problem of measuring the phase of a radio frequency signal, thereby achieving high-precision and high-stability detection of weak atomic signals. The differential phase shift measurement provided by this invention precisely eliminates interference such as mechanical vibration and temperature drift that equally affect the carrier and all sidebands, making the system extremely insensitive to low-frequency environmental noise. In this invention, the reference light amplifies the signal without reducing its signal-to-noise ratio, and even under saturated probe light intensity, the phase shift magnitude is not limited by classical optical noise sources such as laser light intensity jitter. Attached Figure Description

[0028] To more clearly illustrate the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying 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.

[0029] Figure 1 This is a schematic diagram of the system structure of the present invention.

[0030] Figure 2 A schematic diagram of the phase shift curve obtained by simulating the interaction of 10E+11 lead atoms in the probed region of plasma.

[0031] Figure 3 A schematic diagram of the phase shift curve obtained to simulate the interaction of 10E+5 lead atoms in the probed region of plasma. Detailed Implementation

[0032] 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.

[0033] Example 1, such as Figure 1As shown, a system for detecting isotopes using differential interferometry to measure phase shift signals includes a sample cell 8, a high-power laser ablation module, a laser interferometer module, a phase modulation module, a signal detection module, and a signal processing unit. The sample cell 8 is used for high-power pulsed laser ablation of the sample and provides a space for tunable laser-plasma interaction, offering different types or pressures of gas environments. The high-power laser ablation module uses a high-power pulsed laser to ablate the sample to obtain the plasma to be tested; the laser interferometer module is used to configure the reference light and the probe light and make them interfere; the phase modulation module is used to modulate the phase of the probe light to generate a series of positive and negative modulation sidebands; the signal detection module uses a photodetector to convert the optical signal of the interferometric light field into an electrical signal; the signal processing unit is used to perform differential, filtering and mixing processing on the two electrical signals to obtain the phase shift signals related to the first-order sideband and the second-order sideband, and the processed second-order sideband phase shift signal is fed back to the reflector 10 carrying PZT to lock the phase of the interferometer, on the one hand reducing the phase jitter caused by the external environment, and on the other hand increasing the intensity of the first-order sideband phase shift signal, which is used to determine the isotope content.

[0034] The high-power laser ablation module includes a pulsed laser 1. The laser pulse from the pulsed laser 1 reaches the sample cell 8 via a first optical element and ablates the sample to be tested to obtain the plasma to be tested. The first optical element includes a first reflector 2 and a first lens 3. The pulsed laser 1 generates a laser pulse with energy higher than the ablation threshold of the sample surface, which is then focused onto the surface of the sample to be tested through the first reflector 2 and the first lens 3. The movable lens 3 is used to adjust the laser focal point position, adjust the spot size, and thus adjust the laser energy density used for ablation.

[0035] The laser interferometer module includes a continuously tunable laser 5, a first beam splitter 6, and a second beam splitter 11. The laser emitted by the continuously tunable laser 5 is split into a probe beam A and a reference beam B by the first beam splitter 6. The probe beam A, after passing through a phase modulation module, reaches the sample cell 8 and interacts with the plasma, maintaining spatial overlap with the plasma to be measured. The reference beam B enters the second beam splitter 11 through a third reflector 10 equipped with a PZT; the reference beam B and the probe beam A overlap and interfere at the second beam splitter 11. The continuously tunable laser 5 emits lasers with a wavelength range covering the transitions of the isotopes to be measured, and its linewidth is smaller than the isotopic shifts between different isotopes of the element to be measured. The PZT in the third reflector 10 is used to receive feedback signals, thereby fine-tuning the reference optical path for phase locking.

[0036] The signal detection module includes a first photodetector 12 and a second detector 13, which correspond to the two output optical paths of the second beam splitter 11, respectively. The first photodetector 12 and the second detector 13 undergo signal processing by a signal processing unit. The first photodetector 12 and the second photodetector 13 are used to collect the light intensity of each output arm of the second beam splitter 11 and convert it into an electrical signal.

[0037] The phase modulation module includes an electro-optic modulator 7 and a first function generator 20. The first function generator 20 provides a radio frequency signal to the electro-optic modulator 7. The electro-optic modulator 7 is placed in the optical path of the probe light A and performs phase modulation. The electro-optic modulator 7 generates a sideband with a frequency of ω0±nωm by performing phase modulation on the probe light A, where ω0 is the laser carrier frequency, n is an integer, and ω... m The modulation frequency is used; the probe light A is reflected to the sample cell 8 through the second reflecting mirror 4.

[0038] The signal processing unit includes a differential amplifier 14, a first-order sideband processing circuit, and a second-order sideband processing circuit. The differential amplifier 14 performs differential processing on the signals from the first photodetector 12 and the second detector 13, and then transmits the signals to the first-order sideband processing circuit and the second-order sideband processing circuit. The second-order sideband processing circuit extracts signals from the 2ω... m Related phase-shifted signals; extraction of ω by first-order sideband processing circuit m The relevant phase-shifted signal. Differential amplifier 14 is used to subtract the electrical signals converted by the first photodetector 12 and the second photodetector 13 to cancel the DC component in the signal and eliminate classical light intensity noise caused by local oscillations; then the amplified electrical signal is split into two paths, one of which is filtered out by a second bandpass filter to remove 2ω. m Other signals outside the frequency range are then extracted and mixed with 2ω. m The relevant phase shift signal is finally processed by the PID servo circuit and fed back to the PZT for phase locking; another path filters out ω through the first bandpass filter. m Other signals outside the frequency range are then extracted and compared with ω through mixing. m The relevant phase shift signal is then used to estimate the isotope content.

[0039] The first-order sideband processing circuit includes a first bandpass filter 16 and a first mixer 15. The first bandpass filter 16 receives the signal from the differential amplifier 14 and transmits the processed signal to the first mixer 15. A first function generator 20 corresponds to the first mixer 15, and the local oscillator signal of the first mixer 15 is provided by the first function generator 20. Specifically, the first-order sideband processing circuit is used for signal filtering and mixing. First, the signal is filtered out by the filter 16. m Signals outside the frequency range retain ω, which carries phase information.m The radio frequency component is then extracted and compared with ω by mixer 15. m The relevant phase shift signal is then used to estimate the isotope content.

[0040] The second-order sideband processing circuit includes a second bandpass filter 19, a second mixer 18, and a PID servo module 17. The appropriate PID servo module is selected based on the signal response speed during specific application. The second bandpass filter 19 receives the signal from the differential amplifier 14 and transmits the processed signal to the second mixer 18. The signal processed by the second mixer 18 is then processed by the PID servo circuit and fed back to the PZT for phase locking. The local oscillator signal of the second mixer is provided by the second function generator. Specifically, the second-order sideband processing circuit is used for filtering, mixing, and proportional-integral-derivative feedback processing of electrical signals. First, the 2ω signal is filtered out by the filter 19. m Signals outside the frequency range retain 2ω carrying phase information. m The radio frequency component is then extracted by a mixer and combined with 2ω. m The relevant phase shift signal is then processed by a PID servo circuit and fed back to the PZT for phase locking.

[0041] The sample cell 8 is mounted on a three-dimensional translation stage 9. The sample cell 8 has an air inlet and an air outlet, and a glass window. The three-dimensional translation stage 9 is used to adjust the horizontal and vertical position of the sample cell, thereby changing the position of the ablation point and the relative position of the sample and the laser. The air inlet and outlet are used to fill the sample cell with ambient gas, and a pressure gauge is connected to the outlet to monitor the pressure in the sample cell; the glass window allows the laser beam to pass through.

[0042] Example 2: A method for detecting isotopes using differential interferometry to measure phase shift signals, employing the system described in Example 1 for detecting isotopes using differential interferometry to measure phase shift signals; the steps are as follows:

[0043] S1 involves configuring a series of standard samples with known concentrations, using the system to measure their phase shift signals, obtaining the magnitude of the phase shift signal corresponding to the concentration, and establishing a calibration curve. This step uses steps S2 and S3 to measure the magnitude of the phase shift signal of the standard samples.

[0044] S2 uses pulsed laser 1 to generate a laser pulse with energy higher than the ablation threshold of the sample surface to ablate the sample, thereby generating the plasma to be tested in the sample cell.

[0045] The S3 continuously tunable laser 5 emits laser light with a wavelength range covering the isotope transition peaks of the sample to be tested. Its probe light A enters the sample cell and interacts with the plasma. The center frequency of the probe light A of the continuously tunable laser 5 is scanned, and the intensity signal of the light after interference is measured by the first photodetector 12 and the second detector 13. The intensity signal is amplified, filtered and mixed by the signal processing unit to obtain the magnitude of the phase shift signal that varies with the center frequency of the laser.

[0046] S4 obtains the isotope concentration of the sample to be tested by using the phase shift signal magnitude in S3 and the calibration curve.

[0047] The physical mechanism of this method is based on the phase shift generated by the interaction of laser light through plasma and atoms, and the prediction of lead isotope content based on the phase shift signal. Specifically, this invention utilizes zero-difference detection to extract the magnitude of the phase shift generated by the interaction between laser light and atoms. Phase modulation is performed in the probe optical path of the laser interferometer using an electro-optic modulator to generate a frequency of (ω0±ω). m ) sideband (ω m It is the modulation frequency, which is also the detuning of the sideband relative to the atomic transition resonance frequency. The positive and negative sideband light fields interact with the atoms to generate phase shift signals of equal magnitude but opposite sign. The phase shift signal, which is a dispersion-type signal, is obtained by differential detection by a detector, and the spectral lines of the phase shift signal change with the laser frequency. Finally, the isotope content is estimated by the magnitude of the phase shift signal.

[0048] The technical principle of this invention is as follows: When probe light A passes through plasma, the light interacts with atoms, changing the speed of light propagation and causing a phase shift, which affects atomic transitions. Caused laser phase shift The relationship between the number of atoms and the detuning of the probe light can be expressed as:

[0049] ……①

[0050] Where λ is the laser wavelength, l is the length of the probed plasma, q is the laser polarization state, and I is the spin quantum number of the atomic nucleus. It is in a state The number of atoms, V is the volume of the plasma being probed. It is the natural linewidth of the detected transition. Ω0 is the detuning of the laser relative to the transition, J' is the total angular momentum quantum number of the upper energy level, J is the total angular momentum quantum number of the lower energy level, F' is the total hyperfine angular momentum quantum number of the upper energy level, F is the total hyperfine angular momentum quantum number of the lower energy level, and m' is the total hyperfine angular momentum quantum number of the lower energy level. F It is the magnetic quantum number of the total hyperfine angular momentum of the transition to the next energy level, m F It is the magnetic quantum number of the total hyperfine angular momentum of the energy level under the transition.

[0051] If the laser is phase-modulated, when the laser carrier frequency resonates with the transition frequency, the phase shifts of the positive and negative sidebands are equal in magnitude and opposite in sign. Using a laser interferometer with modulated probe light, the positive and negative first-order sidebands ω0±ω in the probe light are differentially measured. m The phase-shifted signal, and the first-order radio frequency component in the resulting photocurrent signal that is related to the modulation frequency. for:

[0052] ……②

[0053] Atomic phase shift Reference phase difference ;in, o It is the phase of the local oscillator signal. g It is the global phase; It is the phase shift of the positive first-order sideband. It is the phase shift of the negative first-order sideband. It is positive first-order sideband laser noise. It is negative first-order sideband laser noise. m It is the modulation phase, J1 is the first-order Bessel function, and a is the modulation depth; after mixing and filtering, the pure atomic phase shift is extracted. As can be seen from formula ①, the atomic phase shift is directly proportional to the number of atoms, thus yielding the number of atoms.

[0054] Example 3: A system for detecting isotopes using differential measurement of phase shift signals by an interferometer, with the following configuration: a continuously tunable laser, selected according to the transition frequency of the isotope to be measured; its output frequency can be continuously tuned near the transition resonance frequency of the isotope to be measured, and the laser linewidth is smaller than the displacement of the isotope sample. For example, in this example, for testing lead isotopes in soil, a semiconductor laser with a wavelength of 405 nm and a linewidth of approximately 100 kHz is selected.

[0055] A high-power pulsed laser is selected based on the type of sample to be tested. The energy of the laser must be greater than the sample ablation threshold. In this embodiment, a pulsed laser with a pulse energy of 200 mJ and a wavelength of 1064 nm is selected.

[0056] An electro-optic modulator is selected based on the laser wavelength, modulation frequency, and modulation depth. In this embodiment, an electro-optic modulator with a wavelength range of 400-600 nm, a modulation frequency of DC-100MHz, and a maximum modulation depth of 35Vpp is selected.

[0057] Two photodetectors are used. The appropriate high-speed detector is selected based on the response speed required by the signal to be measured and the shot noise limit that the system aims to achieve.

[0058] Three reflectors are included. In this embodiment, two of them are coated with a 405nm high-reflectivity film, one of which is equipped with a PZT coating, and the last one is coated with a 1064nm high-reflectivity film. Two beam splitters are included. In this embodiment, beam splitters for 405nm lasers are selected. One convex lens is included. In this embodiment, a broadband film lens with a focal length of 100mm and capable of withstanding 100mJ pulse energy is selected.

[0059] The sample cell is selected based on the laser optical path arrangement and the requirements of the detection environment, and has a suitable glass window, air inlet and air outlet. In this implementation case, a hexahedral vacuum cavity is selected as the sample cell, and air at standard atmospheric pressure is selected as the detection environment.

[0060] Two mixers are used. The appropriate mixer is selected based on parameters such as the local oscillator frequency and conversion gain. In this embodiment, an active mixer covering the local oscillator frequency is used. Two bandpass filters are also used. The appropriate bandpass filter is selected based on the frequency range of the signal to be extracted (the modulation frequency in the experiment and twice the modulation frequency).

[0061] The lead-containing sample is placed in a sample cell, with the inlet and outlet closed, maintaining a pressure of 1 atmosphere. A 1064nm high-power pulsed laser emits a laser pulse, which is then directed perpendicularly into the sample cell via a mirror and lens to generate plasma. A 405nm continuously tunable laser is used to configure a laser interferometer, with one path serving as the reference beam and the other as the probe beam. An electro-optic modulator is placed on the probe beam path for phase modulation. Finally, two photodetectors convert the interferometric optical signal into an electrical signal, which is then processed using a differential amplifier, filter, and mixer.

[0062] The high-power pulsed laser was set to operate at a voltage that generated a 40 mJ laser pulse to ablate the sample, producing plasma in the sample cell; the modulation frequency was set to a 6p transition frequency. 2 3 P0-6p7s 3 P1 0 Five times the natural linewidth, with a modulation depth of 20Vpp; 206 Using the Pb transition frequency as the center, the center frequency of the tunable laser is scanned within a range of ±3 GHz to obtain the curve of the phase shift signal changing with the laser center frequency. The peak-to-peak value of the curve is proportional to the number of atoms.

[0063] The specific estimation method is as follows: prepare a series of standard samples with known concentrations, use the above system to detect them, obtain the phase shift signal magnitude corresponding to the concentration, and establish a calibration curve; then use the above system to detect the concentration of the unknown sample and obtain the phase shift signal magnitude; finally, use the calibration curve and the phase shift magnitude measured for the unknown sample to estimate the lead concentration in the unknown sample.

[0064] To verify the effectiveness of the method of the present invention, a simulation experiment was conducted in this embodiment using the proposed differential phase shift measurement method with a laser interferometer to simulate the phase shift magnitude. The continuous laser power was set to 5mW, the beam waist radius to be 20mm, and the plasma electron temperature to be 10000K, the plasma radial radius to be 10mm, and the plasma containing 10E+11 lead atoms. The simulation was performed according to formula (1), and the results showed that the phase shift magnitude was 2.1 rad. Figure 2 As shown; the continuous laser power is set to 5mW, the beam waist radius is 20mm, the plasma electron temperature is assumed to be 10000K, the plasma radial radius is 10mm, and it contains 10E+5 lead atoms. Simulation is performed according to formula (1), and the results show that the phase shift is 2.1E-6 rad. Figure 3 As shown. Comparison Figure 2 and Figure 3 It can be seen that the phase shift is proportional to the number of atoms. Based on the phase shift, the lead atom concentration in the sample can be estimated, and high-precision and high-stability detection of weak atomic signals can be achieved.

[0065] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A system for detecting isotopes using differential measurement of phase-shift signals with an interferometer, characterized in that: The system includes a sample cell (8), a high-power laser ablation module, a laser interferometer module, a phase modulation module, a signal detection module, and a signal processing unit. The high-power laser ablation module includes a pulsed laser (1), whose laser pulses reach the sample cell (8) via a first optical element and ablate the sample to be tested to obtain the plasma to be tested. The laser interferometer module includes a continuously tunable laser (5), a first beam splitter (6), and a second beam splitter (11). The laser emitted by the continuously tunable laser (5) is split into a probe beam A and a reference beam B by the first beam splitter (6). The probe beam A is then split into a phase modulation module and a phase modulation module. After the bit modulation module reaches the sample cell (8), it maintains spatial overlap with the plasma to be tested; the reference light B enters the second beam splitter (11) through the third reflector (10) with PZT; the reference light B and the probe light A overlap and interfere at the second beam splitter (11); the signal detection module includes a first photodetector (12) and a second detector (13), the first photodetector (12) and the second detector (13) respectively correspond to the two output optical paths of the second beam splitter (11); the first photodetector (12) and the second detector (13) are processed by the signal processing unit.

2. The system for detecting isotopes using differential measurement of phase-shift signals with an interferometer according to claim 1, characterized in that: The phase modulation module includes an electro-optic modulator (7) and a first function generator (20). The first function generator (20) provides radio frequency signals to the electro-optic modulator (7), and the electro-optic modulator (7) is placed in the optical path of the probe light A and performs phase modulation.

3. The system for detecting isotopes using differential measurement of phase-shift signals with an interferometer according to claim 2, characterized in that: The electro-optic modulator (7) modulates the probe light A to generate a sideband with a frequency of ω0±nωm, where ω0 is the laser carrier frequency, n is an integer, and ωm is the modulation frequency; the probe light A is reflected to the sample cell (8) through the second mirror (4).

4. The system for detecting isotopes using differential measurement of phase-shift signals with an interferometer according to claim 3, characterized in that: The signal processing unit includes a differential amplifier (14), a first-order sideband processing circuit, and a second-order sideband processing circuit. The differential amplifier (14) performs differential processing on the signals from the first photodetector (12) and the second detector (13) and then transmits them to the first-order sideband processing circuit and the second-order sideband processing circuit. The second-order sideband processing circuit extracts the phase shift signal related to 2ωm. The first-order sideband processing circuit extracts the phase shift signal related to ωm.

5. The system for detecting isotopes using differential measurement of phase-shift signals with an interferometer according to claim 4, characterized in that: The first-order sideband processing circuit includes a first bandpass filter (16) and a first mixer (15). The first bandpass filter (16) receives the signal from the differential amplifier (14) and transmits the processed signal to the first mixer (15). The local oscillator signal of the first mixer (15) is provided by the first function generator (20).

6. The system for detecting isotopes using differential measurement of phase-shift signals with an interferometer according to claim 5, characterized in that: The second-order sideband processing circuit includes a second bandpass filter (19), a second mixer (18), and a PID servo module (17). The second bandpass filter (19) receives the signal from the differential amplifier (14) and transmits the processed signal to the second mixer (18). The processed signal from the second mixer (18) is processed by the PID servo circuit and fed back to the PZT for phase locking. The local oscillator signal of the second mixer (18) is provided by the second function generator (21).

7. The system for detecting isotopes using differential measurement of phase-shift signals with an interferometer according to claim 1 or 6, characterized in that: The first optical element includes a first reflector (2) and a first lens (3). The pulsed laser (1) generates a laser pulse with energy higher than the ablation threshold of the sample surface and converges it to the sample surface through the first reflector (2) and the first lens (3).

8. The system for detecting isotopes using differential measurement of phase-shift signals with an interferometer according to claim 1, characterized in that: The sample cell (8) is set on the three-dimensional translation stage (9). The sample cell (8) is provided with an air inlet and an air outlet, and a glass window is provided on the sample cell (8).

9. A method for detecting isotopes using differential measurement of phase-shift signals with an interferometer, characterized in that: The system for detecting isotopes using differential measurement of phase-shift signals with an interferometer, as described in any one of claims 1 to 8, comprises the following steps: S1 is configured with a series of standard samples with known concentrations. The system is used to measure their phase shift signals, obtain the magnitude of the phase shift signal corresponding to the concentration, and establish a calibration curve. S2 uses a pulsed laser (1) to generate a laser pulse with energy higher than the ablation threshold of the sample surface to ablate the sample, thereby generating the plasma to be tested in the sample cell. The S3 continuously tunable laser (5) emits laser light with a wavelength range covering the isotope transition peaks of the sample to be tested. Its probe light A enters the sample cell and interacts with the plasma. The center frequency of the probe light A of the continuously tunable laser (5) is scanned, and the light intensity signal after interference is measured by the first photodetector (12) and the second detector (13). The light intensity signal is amplified, filtered and mixed by the signal processing unit to obtain the phase shift signal magnitude that varies with the laser center frequency. S4 obtains the isotope concentration of the sample to be tested by using the phase shift signal magnitude in S3 and the calibration curve in S1.

10. The method for detecting isotopes using differential measurement of phase-shift signals with an interferometer according to claim 9, characterized in that: When probe light A passes through the plasma, the interaction between the light and atoms alters the speed of light propagation, causing a phase shift that affects atomic transitions. Caused laser phase shift The relationship between the number of atoms and the detuning of the probe light can be expressed as: ……① Where λ is the laser wavelength, l is the length of the probed plasma, q is the laser polarization state, and I is the spin quantum number of the atomic nucleus. It is in a state The number of atoms, V is the volume of the plasma being probed. It is the natural linewidth of the detected transition. Ω0 is the detuning of the laser relative to the transition, J' is the total angular momentum quantum number of the upper energy level of the transition, J is the total angular momentum quantum number of the lower energy level of the transition, F' is the total hyperfine angular momentum quantum number of the upper energy level of the transition, F is the total hyperfine angular momentum quantum number of the lower energy level of the transition, m'F is the magnetic quantum number of the total hyperfine angular momentum of the upper energy level of the transition, and mF is the magnetic quantum number of the total hyperfine angular momentum of the lower energy level of the transition. If the laser is phase-modulated, when the laser carrier frequency resonates with the transition frequency, the phase shifts of the positive and negative sidebands are equal in magnitude and opposite in sign. Using a laser interferometer with modulated probe light, the phase shift signals of the positive and negative first-order sidebands ω0±ωm in the probe light are differentially measured, and the first-order radio frequency component related to the modulation frequency in the obtained photocurrent signal is obtained. for: ……② Atomic phase shift Reference phase difference ; in, o is the phase of the local oscillator signal. g is the global phase; It is the phase shift of the positive first-order sideband. It is the phase shift of the negative first-order sideband. It is positive first-order sideband laser noise. It is negative first-order sideband laser noise. m is the modulation phase, J1 is the first-order Bessel function, and a is the modulation depth; After mixing and filtering, pure atomic phase shifts are extracted. As can be seen from formula ①, the atomic phase shift is directly proportional to the number of atoms, thus yielding the number of atoms.

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