Polarization modulation based off switching optical cavity ring-down system and measurement method

Abstract

The present application relates to the technical field of optical cavity ring-down spectroscopy detection, and discloses a kind of optical cavity ring-down system and measurement method based on polarization modulation cut-off, the system is sequentially arranged polarization modulator and polaroid between laser source and resonant cavity along light path, and main control unit acquires the acquisition value corresponding to the output light intensity of resonant cavity in real time;When acquisition value rises to preset initial light intensity threshold, control polarization modulator to switch polarization direction to block incident light path and simultaneously start timing;When acquisition value drops to bottom noise, stop timing, and obtain the ring-down time corresponding to the environment;The concentration of the gas to be measured is calculated by the ring-down time in the vacuum and the gas environment to be measured twice.The present application can realize light path cut-off without stopping laser, and guarantee the long-term stability of light source;The cut-off speed reaches nanosecond level, and the extinction is complete, combined with threshold synchronous trigger mechanism to improve measurement accuracy;Overall optical path and control structure are simple, easy to integrate and debug.

Description

Technical Field

[0001] This invention relates to the field of optical cavity ring-down spectroscopy detection technology, and more specifically, to an optical cavity ring-down system and measurement method based on polarization modulation shutdown. Background Technology

[0002] Cavity ring-down spectroscopy (CRDS) is a highly sensitive spectroscopic detection technique primarily used to measure extremely low concentrations of substances or weak optical absorption in gases. Its core principle is to invert the absorption characteristics of a sample by measuring the ring-down time of light within an optical resonant cavity—that is, the time required for the light intensity to decay to 1 / e of its initial value. During CRDS measurements, the timing of the ring-down time requires the incident laser to be instantaneously interrupted. If the incident laser continues to be injected into the resonant cavity after the timing begins, the light intensity within the cavity will not exhibit a single exponential decay, and the ring-down waveform will be interfered with by the superimposed continuous injection components, leading to systematic deviations in the measurement results. Therefore, rapid interruption of the incident light signal is a crucial step in ensuring the accuracy of the ring-down time measurement.

[0003] Currently, methods for achieving rapid optical signal shutdown are mainly divided into two categories: electrical shutdown and optical shutdown. Electrical shutdown typically achieves laser shutdown by directly turning off the laser driver, resulting in very high shutdown speeds. However, the frequent start-up and shutdown of the laser source negatively impacts its output stability, making it difficult to guarantee the reliability of long-term continuous measurements. Therefore, there is an urgent need in this field for a cavity ringing-down scheme that achieves rapid optical shutdown without turning off the laser driver, but rather by directly shutting down the optical path. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides an optical cavity ring-down system and measurement method based on polarization modulation shutdown. It employs a polarization modulator in conjunction with a polarizer to achieve optical path shutdown, eliminating the need to shut down the laser and ensuring long-term stability of the light source. The shutdown speed reaches nanosecond levels, ensuring complete extinction. Combined with a light intensity threshold for synchronous triggering of shutdown and timing, it achieves high measurement accuracy. The overall optical path and control structure are simple and easy to integrate and debug.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A cavity ring-down system based on polarization modulation shutdown includes a laser source, a resonant cavity, a gas path unit, a photoelectric detection unit, a data acquisition unit, a main control unit, and a host computer. The gas path unit is connected to the resonant cavity. The photoelectric detection unit is correspondingly disposed on the light-emitting side of the resonant cavity. The data acquisition unit is connected between the photoelectric detection unit and the main control unit. The main control unit is communicatively connected to the host computer. The control terminal of the laser source is electrically connected to the host computer. A polarization modulator and a polarizer are sequentially disposed along the light propagation direction on the optical path between the laser source and the resonant cavity. The light-emitting side of the polarizer corresponds to the incident coupling terminal of the resonant cavity. The control terminal of the polarization modulator is electrically connected to the main control unit.

[0007] The main control unit is used to acquire the acquisition value corresponding to the output light intensity of the resonant cavity through the data acquisition unit. When the acquisition value rises to the preset initial light intensity threshold, it controls the polarization modulator to switch the polarization direction to block the incident light path. At the same time, it starts timing and stops timing when the acquisition value drops to the noise floor to obtain the decay time of the corresponding environment.

[0008] As a further preferred embodiment of the present invention, the gas path unit includes an injection valve and a vacuum pump, wherein the outlet end of the injection valve is connected to the inlet of the resonant cavity, and the pumping end of the vacuum pump is connected to the outlet of the resonant cavity.

[0009] As a further preferred embodiment of the present invention, the photoelectric detection unit includes a detector, a transimpedance amplifier, and a conditioning circuit connected in sequence; the photosensitive surface of the detector corresponds to the output coupling terminal of the resonant cavity, and the output terminal of the conditioning circuit is connected to the input terminal of the data acquisition unit; the data acquisition unit includes an analog-to-digital converter.

[0010] As a further preferred embodiment of the present invention, the main control unit is a field-programmable gate array, and the host computer is used to send measurement parameters to the main control unit, receive ring-down time data, and control the start-up and shutdown of the laser source and the output wavelength.

[0011] The present invention also provides a method for measuring cavity ringback toll based on polarization modulation shutdown, comprising:

[0012] S1: Close the gas injection valve of the resonant cavity and turn on the vacuum pump to pump the gas pressure in the resonant cavity to the preset value;

[0013] S2: Start the laser source to output a beam of a specified wavelength, adjust the laser source frequency to match the longitudinal mode frequency of the resonant cavity; control the output polarization direction of the polarization modulator to be consistent with the transmission direction of the polarizer, so that the incident light path is open; start data acquisition and preset the initial light intensity threshold.

[0014] S3: Real-time acquisition of the output light intensity value of the resonant cavity; when the acquired value rises to the initial light intensity threshold, a turn-off control signal is sent to the polarization modulator to switch its polarization direction to block the light path incident on the resonant cavity, and timing is started simultaneously; when the acquired value drops to the noise floor, timing stops, and the ring-down time under vacuum is obtained. ;

[0015] S4: Inject the gas to be tested into the resonant cavity, and repeat steps S2 to S3 to obtain the ring-down time under the gas environment. ;

[0016] S5: Based on the two measured ring-down times, light speeds, and the absorption cross-section of the gas to be tested at the laser wavelength, the concentration of the gas to be tested is calculated.

[0017] As a further preferred embodiment of the present invention, the noise floor used in S3 to determine the stop timing is the inherent noise of the detector and its amplification circuit.

[0018] As a further preferred embodiment of the present invention, in S3, the optical signal output by the resonant cavity is first converted into a current signal by a detector, and then converted into a digital acquisition value by an analog-to-digital converter after current-to-voltage conversion, amplification and filtering by a transimpedance amplifier and conditioning circuit.

[0019] As a further preferred embodiment of the present invention, S4 specifically involves: turning off the vacuum pump, opening the gas injection valve to inject the gas to be tested into the resonant cavity, and repeating S2 to S3 after the gas state in the cavity has stabilized.

[0020] As a further preferred embodiment of the present invention, in S5, the concentration of the gas to be measured... The calculation formula is: In the formula, At the speed of light, This represents the absorption cross section of the gas to be tested at this laser wavelength.

[0021] As a further preferred embodiment of the present invention, the polarization modulator control, data acquisition and timing operations are performed by a field-programmable gate array, and the laser source control, parameter sending and data receiving processing are performed by a host computer.

[0022] The beneficial technical effects of this invention are:

[0023] By using a polarization modulator and a polarizer, the on / off control of the incident light path is achieved. The laser driver does not need to be turned off during the shutdown process, and the laser source can continue to operate stably. This fundamentally avoids the stability degradation caused by frequent start-stop of the laser source and is suitable for applications requiring high repeatability continuous measurement.

[0024] Based on the electro-optic response characteristics of polarization modulation, the optical path turn-off response speed can reach the nanosecond level, which can meet the high precision requirements of optical cavity ring-down measurement for turn-off timing. At the same time, based on the principle of polarization extinction, it can basically achieve complete optical path turn-off, effectively avoiding the interference of light leakage on the natural decay process of light intensity in the cavity, ensuring the authenticity of the ring-down curve, and meeting the measurement requirements of high-precision optical cavity ring-down system.

[0025] The main control unit collects the output light intensity of the resonant cavity in real time. When the light intensity rises to the preset initial light intensity threshold, the optical path is turned off and the ring-down timing is triggered synchronously. When the light intensity decays to the noise floor, the timing stops. This achieves precise synchronization between the turn-off action and the timing process, effectively reducing the measurement error caused by timing deviation and improving the measurement accuracy of the ring-down time.

[0026] The shutdown module consists of only a polarization modulator and a polarizer connected in series along the optical path, resulting in a simple optical path structure and low insertion loss. The control link only requires the output signal of the main control unit to drive the polarization modulator, without the need to modulate and control the laser source itself. The system architecture is clear and easy to integrate and debug. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the system structure of the present invention. Detailed Implementation

[0028] In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0029] Combination Figure 1 The present invention provides the following embodiments:

[0030] Example 1:

[0031] A cavity ring-down system based on polarization modulation shutdown is used for high-sensitivity concentration detection of trace gases. The system consists of four parts: an optical path unit, a gas path unit, a signal acquisition unit, and a control and interaction unit.

[0032] The optical path unit consists of a laser source, a polarization modulator, a polarizer, and a resonant cavity, arranged in series along the beam propagation direction. The output end of the laser source faces the incident end face of the polarization modulator, and is used to output a continuous detection beam of a specified wavelength. Its output frequency can be adjusted by a host computer to match the longitudinal mode spacing of the resonant cavity. The polarization modulator is an electrically controlled polarization adjustment device. Its optical input end receives the beam output from the laser source, and its optical output end faces the incident end face of the polarizer. The electrical control end is connected to a control interaction unit, which can quickly change the polarization direction of the output beam through electrical signals. The polarizer is a polarization analyzer with a fixed polarization direction. Its output end face is aligned with the incident coupling mirror of the resonant cavity, allowing only polarized light with the same transmission direction as itself to pass through. It works with the polarization modulator to switch the incident optical path on and off.

[0033] Based on the principle of polarization extinction, the output polarization state of the polarization modulator is switched electronically, enabling the optical path to be open when the output light is parallel to the transmission direction of the polarizer and blocked when it is orthogonal to it. This allows for high-speed on / off control of the incident light path without shutting down the laser source. The resonant cavity is composed of high-reflectivity cavity mirrors, with the output side of its output coupling mirror aligned with the detection end face of the signal acquisition unit, providing a cavity environment for beam resonance enhancement and ring-down detection.

[0034] The gas path unit includes an injection valve and a vacuum pump. The outlet of the injection valve is connected to the inlet of the resonant cavity through a gas path pipe, which is used to control the injection of the gas to be measured into the resonant cavity. The pumping end of the vacuum pump is connected to the pumping port of the resonant cavity through a gas path pipe, which is used to evacuate the inside of the resonant cavity and create a reference measurement environment without absorption of the gas to be measured.

[0035] The gas injection valve and vacuum pump independently control the gas intake and extraction processes, respectively, which can construct a vacuum reference environment without gas absorption and a measurement environment containing the gas to be measured. By performing differential calculations on the ring-down times under the two environments, the influence of the inherent reflection loss and diffraction loss of the resonant cavity can be eliminated, which is the basis of the gas path structure for quantitative measurement of optical cavity ring-down.

[0036] The signal acquisition unit, along the direction of electrical signal transmission, sequentially includes a detector, a transimpedance amplifier, a conditioning circuit, and an analog-to-digital converter (ADC). The photosensitive surface of the detector faces the output coupling mirror of the resonant cavity, used to convert the optical signal output from the resonant cavity into a current signal of corresponding intensity. The input terminal of the transimpedance amplifier is electrically connected to the output terminal of the detector, used to convert the weak current signal into a voltage signal. The input terminal of the conditioning circuit is electrically connected to the output terminal of the transimpedance amplifier, used to amplify and filter the voltage signal to adapt to the range requirements of the back-end acquisition. The analog input terminal of the ADC is electrically connected to the output terminal of the conditioning circuit, and the digital output terminal is connected to the control interaction unit, used to convert the analog voltage signal into a digital acquisition value and output it.

[0037] The light signal output by the resonant cavity is weak and is an analog light signal, which cannot be directly recognized by the digital main control unit. The detector first completes the photoelectric conversion to generate a current signal, which is then converted to a voltage by a transimpedance amplifier and initially amplified. After filtering and noise reduction by the conditioning circuit and matching the acquisition range, it is finally converted into a digital signal by the analog-to-digital converter. This can ensure the signal-to-noise ratio and accuracy of the weak light signal acquisition, and provide reliable data for subsequent threshold judgment and timing.

[0038] The control and interaction unit includes a field-programmable gate array (FPGA) and a host computer. The FPGA serves as the system's main control unit. Its data input is electrically connected to the digital output of the analog-to-digital converter (ADC), its control output is electrically connected to the electrical control of the polarization modulator, and its communication terminal is bidirectionally connected to the host computer. The FPGA receives the light intensity acquisition values ​​transmitted by the ADC in real time, presets and compares the initial light intensity threshold with the final light intensity threshold, and outputs a control signal to switch the polarization state of the polarization modulator to block the incident light path when the acquired value reaches the initial threshold. Simultaneously, it starts an internal high-precision timer, stopping the timer when the acquired value drops to the noise floor, thus obtaining the ring-down time under the corresponding environment. The host computer is electrically connected to both the laser source and the FPGA, controlling the start / stop and output wavelength of the laser source, sending measurement configuration parameters to the FPGA, and receiving the ring-down time data uploaded by the FPGA for subsequent processing and storage.

[0039] Field-Programmable Gate Arrays (FPGAs) feature parallel computing and hardware-level response, low input / output latency, and high timing control precision. They can simultaneously complete data acquisition, threshold comparison, polarization control output, and high-precision timing, ensuring nanosecond-level synchronization between shutdown and timing start. The host computer handles parameter configuration, data post-processing, and human-machine interaction, achieving layered adaptation of high-speed real-time control and system management functions.

[0040] Example 2:

[0041] Example 2 is based on the optical cavity ring-down system described in Example 1 and is used for quantitative detection of the concentration of the gas to be measured. The specific measurement steps are as follows:

[0042] Step 1: Construct a vacuum reference environment

[0043] Close the gas injection valve of the resonant cavity and turn on the vacuum pump to evacuate the inside of the resonant cavity until the gas pressure inside the resonant cavity drops to the preset vacuum reference value, forming a measurement reference environment without absorption of the gas to be measured.

[0044] Step 2: System Initialization and Parameter Configuration

[0045] The host computer sends a start command to activate the laser source, enabling it to output a detection beam of the specified wavelength. Simultaneously, the host computer sends measurement configuration parameters to the FPGA to complete system initialization. Upon receiving the parameters, the FPGA outputs a conduction control signal to the polarization modulator, adjusting the laser source frequency to match the longitudinal mode frequency of the resonant cavity. This ensures that the outgoing polarization direction of the polarization modulator aligns with the transmission direction of the polarizer, ensuring the incident light path is fully conductive and allowing the laser to normally enter the resonant cavity. Simultaneously, the FPGA activates the analog-to-digital converter (ADC), enabling it to continuously acquire the voltage signal corresponding to the light intensity.

[0046] The ring-down process of an optical cavity follows the exponential decay law of light intensity. The time it takes for the light intensity to decay from its initial value to the noise floor is the standard physical definition of the ring-down time of an optical cavity. By setting the dual thresholds in this proportion, the intrinsic ring-down time can be directly measured, which is in line with the basic principle of optical cavity ring-down spectroscopy and ensures that the physical meaning of the measurement results is consistent with industry standards.

[0047] Step 3: Vacuum environment ring-down time measurement

[0048] In the conducting state, the laser continuously enters the resonant cavity and is reflected and resonated multiple times within the cavity. The light intensity at the output end of the resonant cavity gradually increases as the beam accumulates. The detector receives the output light from the cavity in real time and converts it into a current signal. After current-to-voltage conversion, amplification, and filtering by the transimpedance amplifier and conditioning circuit, it is converted into a digital acquisition value by the analog-to-digital converter and transmitted to the FPGA in real time.

[0049] Optical signals are analog optical quantities and cannot be directly used for digital logic judgment and timing triggering. Through photoelectric conversion, transimpedance transformation, filtering and amplification, and analog-to-digital conversion, weak light intensity signals can be converted into stable digital quantities, while suppressing circuit noise and environmental interference, and improving the accuracy of threshold triggering and timing precision.

[0050] When the FPGA detects that the acquired value has risen to the preset initial light intensity threshold, it immediately outputs a shutdown control signal to the polarization modulator, causing the polarization modulator to quickly switch the outgoing polarization direction. At this time, the outgoing light polarization direction is orthogonal to the transmission direction of the polarizer, and the beam is completely blocked by the polarizer, and the light path incident on the resonant cavity is quickly shut off. At the same time, the FPGA starts its internal high-precision timer to begin timing.

[0051] After the optical path is turned off, no new light beam is injected into the resonant cavity. The light intensity inside the cavity continues to decay naturally due to reflection loss and medium absorption, and the corresponding acquired value gradually decreases synchronously. When the FPGA detects that the acquired value has dropped to the noise floor, it immediately controls the timer to stop timing. The time interval recorded by the timer is the ring-down time in the vacuum environment. The background noise is used to determine when to stop timing, and its value is the inherent noise of the detector and its amplification circuit.

[0052] Step 4: Measurement of the decay time of the gas under test

[0053] Turn off the vacuum pump, open the gas injection valve, and inject the gas to be tested into the resonant cavity. After the gas state in the cavity stabilizes, repeat steps 2 to 3 above to measure the ring-down time in the environment filled with the gas to be tested. .

[0054] Stopping the gas extraction before injecting gas avoids airflow disturbances and uneven concentration distribution within the cavity caused by simultaneous extraction and injection. Starting the measurement after the gas state has stabilized eliminates interference from airflow fluctuations and concentration gradients on the light intensity signal, ensuring the accuracy and repeatability of the gas decay time measurement results.

[0055] Step 5: Calculation of the concentration of the gas to be measured

[0056] Based on the vacuum decay time obtained from the two measurements The decay time of the gas to be tested Combined with the speed of light and the absorption cross section of the gas to be tested at the current laser wavelength. The molecular number concentration of the gas to be tested can be calculated using the following formula. : .

[0057] The formula is derived from the basic principle of cavity ring-down spectroscopy: the reciprocal of the ring-down time is proportional to the total loss within the cavity. In a vacuum environment, only the inherent loss of the cavity is included; after filling with gas, additional gas molecule absorption loss is added. Subtracting the two equations eliminates the inherent loss term, and the molecular concentration of the analyte gas can be directly inferred from the difference in ring-down rates. This is a classic physical model for quantitative detection of ring-down gases in a cavity. The calculated concentration data is uploaded from the FPGA to the host computer for display and storage.

[0058] This embodiment adopts a hierarchical control architecture, in which the field-programmable gate array performs polarization modulator control, data acquisition and timing operations, and the host computer performs laser source control, parameter sending and data receiving and processing.

[0059] The real-time control, acquisition, and timing functions that require high timing accuracy are handled by the field-programmable gate array (FPGA), ensuring nanosecond-level timing synchronization accuracy. The system management, parameter configuration, and data processing functions that require less real-time performance are handled by the host computer, balancing measurement accuracy and system usability.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A cavity ring-down system based on polarization modulation shutdown, comprising a laser source, a resonant cavity, a gas path unit, a photoelectric detection unit, a data acquisition unit, a main control unit, and a host computer, wherein the gas path unit is connected to the resonant cavity, the photoelectric detection unit is correspondingly disposed on the light-emitting side of the resonant cavity, the data acquisition unit is connected between the photoelectric detection unit and the main control unit, the main control unit is communicatively connected to the host computer, and the control terminal of the laser source is electrically connected to the host computer; characterized in that, A polarization modulator and a polarizer are sequentially arranged along the light propagation direction on the optical path between the laser source and the resonant cavity, with the light-emitting side of the polarizer corresponding to the incident coupling end of the resonant cavity; the control end of the polarization modulator is electrically connected to the main control unit. The main control unit is used to acquire the acquisition value corresponding to the output light intensity of the resonant cavity through the data acquisition unit. When the acquisition value rises to the preset initial light intensity threshold, it controls the polarization modulator to switch the polarization direction to block the incident light path. At the same time, it starts timing and stops timing when the acquisition value drops to the noise floor to obtain the decay time of the corresponding environment.

2. The optical cavity ring-down system based on polarization modulation shutdown according to claim 1, characterized in that, The gas path unit includes an injection valve and a vacuum pump. The outlet of the injection valve is connected to the inlet of the resonant cavity, and the pumping end of the vacuum pump is connected to the outlet of the resonant cavity.

3. The optical cavity ring-down system based on polarization modulation shutdown according to claim 1, characterized in that, The photoelectric detection unit includes a detector, a transimpedance amplifier, and a conditioning circuit connected in sequence; the photosensitive surface of the detector corresponds to the output coupling terminal of the resonant cavity, and the output terminal of the conditioning circuit is connected to the input terminal of the data acquisition unit; the data acquisition unit includes an analog-to-digital converter.

4. The optical cavity ring-down system based on polarization modulation shutdown according to claim 1, characterized in that, The main control unit is a field-programmable gate array (FPGA), and the host computer is used to send measurement parameters to the main control unit, receive ring-down time data, and control the start / stop and output wavelength of the laser source.

5. A method for measuring cavity ringback toll based on polarization modulation shutdown, characterized in that, include: S1: Close the gas injection valve of the resonant cavity and turn on the vacuum pump to pump the gas pressure in the resonant cavity to the preset value; S2: Start the laser source to output a beam of a specified wavelength, adjust the laser source frequency to match the longitudinal mode frequency of the resonant cavity; control the output polarization direction of the polarization modulator to be consistent with the transmission direction of the polarizer, so that the incident light path is open; start data acquisition and preset the initial light intensity threshold. S3: Real-time acquisition of the output light intensity value of the resonant cavity; when the acquired value rises to the initial light intensity threshold, a shutdown control signal is sent to the polarization modulator to switch its polarization direction to block the light path incident on the resonant cavity, and timing is started simultaneously; when the acquired value drops to the noise floor, timing stops, and the ring-down time under vacuum is obtained. ; S4: Inject the gas to be tested into the resonant cavity, and repeat steps S2 to S3 to obtain the ring-down time under the gas environment. ; S5: Based on the two measured ring-down times, light speed, and absorption cross-section of the gas to be tested at the laser wavelength, the concentration of the gas to be tested is calculated.

6. The cavity ringback measurement method based on polarization modulation shutdown according to claim 5, characterized in that, The noise floor used in S3 to determine when to stop timing is the inherent noise of the detector and its amplifier circuit.

7. The cavity ringback measurement method based on polarization modulation shutdown according to claim 5, characterized in that, In S3, the optical signal output from the resonant cavity is first converted into a current signal by the detector, and then converted into a digital acquisition value by the analog-to-digital converter after current-to-voltage conversion, amplification and filtering by the transimpedance amplifier and conditioning circuit.

8. The cavity ringback measurement method based on polarization modulation shutdown according to claim 5, characterized in that, Specifically, S4 involves: turning off the vacuum pump, opening the gas injection valve to inject the gas to be tested into the resonant cavity, and repeating S2 to S3 after the gas state in the cavity has stabilized.

9. The cavity ringback measurement method based on polarization modulation shutdown according to claim 5, characterized in that, In S5, the concentration of the gas to be measured The calculation formula is: In the formula, At the speed of light, This represents the absorption cross section of the gas to be tested at this laser wavelength.

10. The cavity ringback measurement method based on polarization modulation shutdown according to claim 5, characterized in that, The polarization modulator control, data acquisition, and timing operations are performed by the field-programmable gate array, while the laser source control, parameter sending, and data reception and processing are performed by the host computer.

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