An IOPD Acquisition Method for Signals in an FPGA-Based FT-Raman Spectrometer

Abstract

This invention discloses an equal-OPD sampling method for interference optical signals in an FT-Raman spectrometer based on FPGA. This sampling method can perform sampling at different rates according to user needs. While accurately detecting the frequency information contained in the interferogram, it uses a low-speed A / D converter to sample the interference signal, reducing the acquisition time and spectral calculation time of the interference optical signal in the FT-Raman spectrometer. It features fast real-time operation, low cost, and diverse sampling methods.

Description

Technical Field

[0001] This invention relates to the field of spectral detection technology, and in particular to an equal OPD sampling method for interference light signals in a Fourier transform Raman spectrometer. Background Technology

[0002] Raman spectroscopy, a type of scattering spectroscopy, is based on the elastic scattering effect proposed by physicist C.V. Raman in 1928. When an excitation beam shines on an object, elastic and inelastic scattering occur. In elastic scattering, the spectral line energy remains unchanged, and the wavelength of the scattered light is equal to that of the incident light; this is called Rayleigh scattering. In inelastic scattering, the spectral line energy changes, causing the wavelength of the scattered light to be different from that of the incident light; this is collectively known as Raman scattering.

[0003] Raman spectroscopy is a spectroscopic method for studying molecular vibrations. It can provide information on the various normal vibrational frequencies and related vibrational energy levels within a molecule, which can then be used to identify functional groups present in the molecule.

[0004] Fourier transform Raman spectroscopy is a cutting-edge technology developed since the 1990s. It primarily consists of a laser, a Michelson interferometer, a signal detector, a computer, and various optical components. The Michelson interferometer is a precision measuring instrument based on the principle of light interference, capable of precisely measuring length and minute changes in length. The excitation light is split into two beams by the Michelson interferometer, creating a specific optical path difference. These beams are then recombine to form interference light. The interference signal detected by the signal detector contains all the frequency and intensity information of the excitation light. The computer then performs a Fourier transform on the interference signal to obtain the spectrum. Compared to traditional Raman spectrometers, Fourier transform Raman spectrometers offer advantages such as fast scanning speed, high wavenumber resolution, high light flux, high sensitivity, and a wide spectral range, and are widely used in materials science, chemical engineering, petroleum, polymer science, and environmental protection.

[0005] However, current signal sampling methods for FT-Raman spectrometers on the market include: using two laser beams, one as the excitation beam to excite the sample to produce Raman scattering; the other, usually a helium-neon laser, serves as the reference beam. When the intensity of the helium-neon laser interference signal is zero, the interference signal of the excitation beam is sampled at equal optical path differences. In addition, some researchers use high-speed A / D converters to sample the interference signal at equal time intervals. The above methods have the following shortcomings:

[0006] The wavelengths of the excitation and reference light in the FT-Raman spectrometer described above need to satisfy a certain relationship: the wavelength of the reference light must be less than or equal to half the wavelength of the excitation light to ensure that the sampling conforms to the Nyquist sampling theorem. However, in practical applications, when using a shorter wavelength excitation light to sample the interference signal, to satisfy the Nyquist sampling theorem, an even shorter wavelength reference light must be used to detect the optical path difference, thereby achieving distortion-free sampling of the interference signal. For example, if acquiring the Raman spectrum excited by a 532nm laser, a 266nm laser must be used as the reference light for zero-crossing sampling. This method has a fixed mode, lacks flexibility, has low resolution and accuracy, and the 266nm laser is expensive. When there is a certain deviation between the source of the reference light and the moving mirror speed of the interferometer, glitches will form in the interference pattern of the reference light, interfering with the acquisition of the Raman scattering interference signal. At the same time, this method uses two lasers, resulting in resource waste and increasing the cost of the Raman spectrometer.

[0007] The second type of FT-Raman spectrometer uses a high-speed A / D converter to sample the interference light signal at equal time intervals. The sampling rate of this high-speed A / D converter is typically on the order of tens of MHz, while the frequency of the interference light signal is typically on the order of kHz. Using this type of A / D converter to acquire the interference light signal generates a large amount of data, and the price of the A / D converter is related to the sampling rate; the higher the sampling rate, the higher the price. This approach not only generates redundant data, occupies a large amount of computer memory, hinders data processing, reduces data processing speed, and cannot obtain Raman spectra in real time, but also increases the cost of the spectrometer.

[0008] To address the above issues, an equal OPD sampling method for interference light signals in an FT-Raman spectrometer based on FPGA is proposed. This method can effectively solve the problems of unstable laser source amplitude and glitch caused by uneven speed of the interferometer moving mirror. While ensuring accuracy, it can greatly reduce the cost of the Raman spectrometer and effectively shorten the calculation time of the spectrum, enabling rapid acquisition and real-time display of Raman spectra. Summary of the Invention

[0009] The purpose of this invention is to address the shortcomings of existing technologies, reduce the cost of spectrometers, remove interference from glitches in reference light signals, and propose an equal OPD sampling method for interference light signals in an FT-Raman spectrometer based on FPGA. This method can perform sampling at different rates according to user needs, accurately detect the frequency information contained in the interferogram, and use a low-speed A / D converter to sample the interference signal, thereby reducing the acquisition time and spectral calculation time of the interference light signal in the FT-Raman spectrometer. It also features fast real-time operation, low cost, and diverse sampling methods.

[0010] To achieve the aforementioned objectives, the present invention provides an FPGA-based OPD acquisition system for interference signals in an FT-Raman spectrometer, comprising a conditioning circuit module one, a conditioning circuit module two, an ADC module, a comparator module, a FIFO module, a control module, a glitches filter module, a MAX21 module, an FFT module, a reference clock module, and a host computer. The comparator module, FIFO module, control module, glitches filter module, MAX21 module, FFT module, and reference clock module are implemented in the FPGA.

[0011] The conditioning circuit module 1 is used to amplify and buffer the reference light interference signal acquired by the detector so that it meets the input requirements of the subsequent A / D converter.

[0012] The conditioning circuit module 2 is used to amplify and buffer the Raman scattering interference signal acquired by the detector so that it meets the input requirements of the subsequent A / D converter.

[0013] The ADC_1 module is used to acquire the input reference light interference signal. Its bit width is N. The data sequence ADC_DATA1 obtained from the acquired reference light interference signal is sent to the comparator module and the FIFO module at the same time.

[0014] The ADC_2 module is used to acquire Raman scattering interference signals. Its bit width is N. The data sequence ADC_DATA2 obtained from the acquired Raman scattering interference signals is sent to the FIFO module.

[0015] The comparator module is used to perform over-threshold comparison on the reference light interference signal to obtain a rectangular wave signal CARD_CNV that is in phase and frequency with the input signal, and then outputs the rectangular wave signal CARD_CNV to the glitches filter module. This threshold is obtained by the system's self-test upon startup. The specific process is as follows: after the instrument under test has been powered on and stabilized for a period of time, the ADC_1 module repeatedly acquires the light intensity of the reference light interference signal, records its maximum and minimum values, and takes the average of half of the difference between the two values ​​as the threshold for the reference light interference signal threshold comparison.

[0016] The FIFO module is used to buffer two signal sequences, the reference light interference signal and the Raman scattering light interference signal, acquired by the ADC module. The FIFO has a bit width of 2N, storing the reference light interference signal in the lower N bits and the Raman scattering light interference signal in the higher N bits. When the user selects 2x sampling, the FIFO module operates in a write-read-while-read state. Each time the ADC_2 module acquires a Raman scattering interference signal and stores it in the FIFO module, the control module sends RD_EN and WR_EN signals to read the value into the FFT module. At the same time, the signal acquired by the next ADC_2 module is stored in the FIFO module. When the user selects 8x sampling, the FIFO module stores the reference interference signal for one cycle in the lower N bits and the Raman scattering interference signal at the corresponding position in the higher N bits. When the FIFO module is full, it sends a WR_FULL signal to the control module. After receiving the WR_FULL signal, the control module sends an RD_EN signal to the FIFO module to read the data stored in the FIFO module into the FFT module. The reading method is as follows: when the reference interference signal in the lower N bits of the FIFO module is equal to the pre-acquired value, the Raman scattering interference signal in the higher N bits at the corresponding position is read into the FFT module.

[0017] The control module is used to control the reading and writing of the FIFO module, receive instructions from the host computer, send control signals to control the MAX21 module to select an appropriate drive clock source for the ADC_2 module; at the same time, it outputs a control signal to the glitch filtering module based on the input from the host computer to determine the width of the glitch to be filtered.

[0018] The glitch filtering module is used to filter out glitch in the rectangular wave signal CARD_CNV output by the comparator module. The resulting rectangular wave signal CARD_FREQ after glitch filtering is sent to the MAX21 module and the control module.

[0019] The MAX21 module is used to select one of the reference clock module and the glitch filter module as the driving clock source for the ADC_2 module according to the control command output by the control module, so as to acquire the Raman scattering light interference signal.

[0020] The FFT module is used to perform Fourier transform on the signal, thereby generating a spectrum. Specifically, the received acquisition data sequence ADC_DATA2 is subjected to FFT operation, and the spectrum of each point is recorded, including the real part Re[ ] and imaginary part Im[ Two parts, =1,2,…,N, using the formula X[k]=(|Re[k]| 2 +|Im[k]| 2 ) 1 / 2 The frequency energy at point N was calculated. Then, output the spectrum sequence of N points to the host computer;

[0021] The reference clock module provides a reference clock clk_fre as the driving clock source for the ADC_2 module to acquire Raman scattered light, and also provides a reference clock clk_ref to the glitch filter module as the clock source for filtering out glitches.

[0022] After receiving the N-point spectrum sequence output from the FFT module, the host computer displays the obtained Raman spectrum results on the screen in real time.

[0023] This invention relates to an FPGA-based signal acquisition system. The input signal passes through a conditioning circuit module to meet the requirements of subsequent circuits. The ADC_1 module acquires the reference light interference signal and simultaneously inputs it into the lower N bits of the comparator module and the FIFO module. A rectangular wave signal of the same frequency is obtained from the comparator module. After filtering out glitches by the glitches filter module, it is input into the MAX21 module. The host computer inputs instructions to the control module, selecting a clock from the glitches filter module and the reference clock module as the driving clock source for the ADC_2, so that the ADC_2 acquires the Raman scattering light interference signal and stores it in the higher N bits of the FIFO. The control module controls the reading and writing of the FIFO module. When the data read into the FFT module reaches the set value, the FFT operation is performed, and the result is transmitted to the host computer through the PCI-e interface to obtain the Raman spectrum.

[0024] This invention addresses the practical problems of FT-Raman spectrometers by utilizing FPGA to acquire and process Raman interference signals. It effectively solves the problems of unstable laser source amplitude and glitch caused by uneven speed of interferometer moving mirrors, reduces the calculation time in the spectrum formation process, achieves real-time acquisition of Raman spectra, and reduces the cost of spectrometers. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the FT-Raman spectrometer of the present invention;

[0026] Figure 2 This is a structural diagram of a specific implementation of the OPD acquisition system for signals in an FPGA-based FT-Raman spectrometer according to the present invention;

[0027] Figure 3 This is a schematic diagram of the equal OPD sampling method in an embodiment of the present invention. Detailed Implementation

[0028] The specific embodiments of the present invention will now be described with reference to the accompanying drawings to enable those skilled in the art to better understand the invention. It should be particularly noted that in the following description, detailed descriptions of known functions and designs that might obscure the main content of the invention will be omitted here.

[0029] Example

[0030] Figure 1 This is a structural diagram of a novel FT-Raman spectrometer proposed in this invention. Figure 1 As shown, the FT-Raman spectrometer includes detector 1, detector 2, laser 3, laser narrowband pass filter 4, beam splitter 5, reflector 6, Michelson interferometer assembly 7, reflector 8, dichroic mirror 9, and sample cell 10. The Michelson interferometer assembly 7 includes a fixed reflector 701, a beam splitter 702, and a movable reflector 703.

[0031] The laser emitted by laser 3 is first split into two beams by the beam splitter 5. One beam passes through the reflector 6 and reaches the dichroic mirror 9. After being reflected by the dichroic mirror 9, it reaches the object being measured. Raman scattered light emitted by the object under test reaches the Michelson interferometer assembly 7 via the dichroic mirror. The Raman scattered light is split into two parallel beams that are perpendicular to each other by the beam splitter 702. After passing through the fixed mirror 701 and the movable mirror 703, the beams return. The two parallel beams are then combined by the beam splitter 702 to form spatial heterodyne interference light, which is detected by the detector 2 to obtain the Raman scattered light interference signal. Another beam formed by the beam splitter 5 reaches the Michelson interferometer assembly 7 via the mirror 8. It is split into two parallel beams that are perpendicular to each other by the beam splitter 702 inside the Michelson interferometer assembly 7. After passing through the fixed mirror 701 and the movable mirror 703, the beams return. The two parallel beams are then combined by the beam splitter to form spatial heterodyne interference light, which is detected by the detector 2 to obtain the reference light interference signal.

[0032] Figure 2 This is a structural diagram illustrating a specific implementation of the FPGA-based FT-Raman spectrometer interferometric signal acquisition method of the present invention. Figure 2 As shown, the FPGA-based FT-Raman spectrometer interferometric signal acquisition system of this invention includes a conditioning circuit module 1 201, a conditioning circuit module 202, an ADC_1 module 203, an ADC_2 module 204, a comparator module 205, a FIFO module 206, a control module 207, a glitches filtering module 208, a MAX21 module 209, an FFT module 210, a reference clock module 211, and a host computer 212. The comparator module 205, FIFO module 206, control module 207, glitches filtering module 208, MAX21 module 209, FFT module 210, and reference clock module 211 are implemented in an FPGA. Each module will be described in detail below.

[0033] Conditioning circuit module 1 201 is used to amplify and buffer the reference light interference signal acquired by the detector so that it meets the input requirements of the subsequent ADC_1 module 203.

[0034] Conditioning circuit module 202 is used to amplify and buffer the Raman scattering light interference signal acquired by the detector so that it meets the input requirements of the subsequent ADC_2 module 204.

[0035] The ADC_1 module 203 is used to acquire the input reference light interference signal and send the data sequence ADC_DATA1 obtained from the acquired reference light interference signal to the comparator module 205 and the FIFO module 206 at the same time.

[0036] The ADC_2 module 204 is used to acquire Raman scattering light interference signals, and the data sequence ADC_DATA2 obtained from the acquired Raman scattering light interference signals is sent to the FIFO module 206;

[0037] The comparator module 205 performs an over-threshold comparison on the reference light interference signal to obtain a rectangular wave signal CARD_CNV that is in phase and frequency with the input signal. The rectangular wave signal CARD_CNV is then output to the glitch filtering module 208. If the input signal is free of interference and is a regular cosine signal, the output of the comparator module 205 is a square wave that is in phase and frequency with the input sine wave, meaning that the high and low levels each account for 50% of a cycle. This threshold is obtained through system startup self-testing. The specific process is as follows: after the instrument under test has been powered on and stabilized for a period of time, the ADC_1 module 203 repeatedly acquires the light intensity of the reference light interference signal, records its maximum and minimum values, and takes the average of half of the difference between the two values ​​as the threshold for the reference light interference signal threshold comparison.

[0038] FIFO module 206 is used to buffer two signal sequences: the reference light interference signal and the Raman scattering light interference signal acquired by ADC_1 module 203 and ADC_2 module 204. FIFO module 206 has a bit width of 2N, storing the reference light interference signal in the lower N bits and the Raman scattering light interference signal in the higher N bits. When the user selects 2x sampling, FIFO module 206 operates in a write-while-read state. Each time ADC_2 module 204 acquires a Raman scattering light interference signal and stores it in FIFO module 206, control module 207 sends RD_EN and WR_EN signals to read this value into FFT module 210. Simultaneously, the next signal acquired by ADC_2 module 204 is stored in FIFO module 206. When the user selects 8x sampling, FIFO module 206 stores one cycle of the reference light interference signal in the lower N bits and the higher N bits... The Raman scattering interference signal at the corresponding position is stored. When the FIFO module 206 is full, a WR_FULL signal is sent to the control module 207. After receiving the WR_FULL signal, the control module 207 sends an RD_EN signal to the FIFO module 206 and reads the data stored in the FIFO module 206 into the FFT module 210. The reading method is as follows: when the low N bits of the reference light interference signal of the FIFO module 206 are equal to the pre-acquired value, the high N bits of the Raman scattering interference signal at the corresponding position are read into the FFT module 210.

[0039] The control module 207 is used to control the reading and writing of the FIFO module 206, receive instructions from the host computer 212, and send control signals to control the MAX21 module 209 to select an appropriate driving clock source for the ADC_2 module 204; at the same time, it outputs a control signal to the glitch filtering module 208 based on the input of the host computer 212 to determine the width of the glitch filtering.

[0040] The glitch filtering module 208 is used to filter out glitch in the rectangular wave signal CARD_CNV output by the comparator module 205, and the resulting glitch-filtered rectangular wave signal CARD_FREQ is sent to the MAX21 module 209.

[0041] Due to the instability of the laser source amplitude and the uneven running speed of the moving mirror, the output signal of ADC_1 module 203 may exceed the threshold multiple times within one cycle. The comparator output signal CARD_CNV will contain a narrow pulse width. If it is used as the clock source of ADC_2 module 204 to acquire Raman scattered light signals, it will lead to non-uniform OPD sampling, which will interfere with the acquisition of Raman scattered light interference signals. Therefore, it is necessary to filter out the narrow bandwidth.

[0042] When the laser source is unstable or the moving mirror moves at an uneven speed, the rectangular wave signal CARD_CNV output by comparator module 205 contains a narrow pulse width component, which can be considered as a glitch with a width of [missing information]. According to Fourier transform optics, if the light source is absolutely stable and the moving mirror's speed is absolutely uniform, then the frequency of the reference light intensity signal detected by the detector will be... ,in This represents the wavenumber of the reference light, which is the reciprocal of its wavelength. Indicates the speed at which the moving mirror operates. Period Considering the operating speed of the moving mirror The deviation, i.e., the running speed is ( If the frequency of the reference light intensity signal detected by the detector is... ( ),cycle Assuming m is the number of reference clocks required to filter out glitch components, called the mask value, then only... As long as the time length of the reference clock is greater than the glitch length, then the mask value... and the frequency of the reference clock Need to meet ,at this time ( However, due to the mask value If the value is too large, it may filter out the original signal in the rectangular wave signal, therefore it is necessary to make... ,at this time ( Considering the normal input signal, it is also necessary to make... Therefore, in summary, the mask value and frequency The conditions that need to be met are: ,at this time ( ).

[0043] The MAX21 module 209 is used to select one of the reference clock module 211 and the glitch filter module 208 as the driving clock source for the ADC_2 module 204 according to the control command output by the control module 207, so as to acquire the Raman scattering interference signal.

[0044] The FFT module 210 is used to perform Fourier transform on the signal to form a spectrum. Specifically, the received acquisition data sequence ADC_DATA2 is subjected to an FFT operation, and the spectrum of each point is recorded, including the real part Re[ ] and imaginary part Im[ Two parts, =1,2,…,N, using the formula X[k]=(|Re[k]|2 +|Im[k]| 2 ) 1 / 2 The frequency energy at point N was calculated. Then, the spectrum sequence of N points is output to the host computer.

[0045] The reference clock module 211 provides a reference clock clk_fre as the driving clock source for the ADC_2 module 204 to acquire the Raman scattering light interference signal, and also provides a reference clock clk_ref to the glitch filtering module 208 as the clock source for filtering out glitch components.

[0046] After receiving the N-point spectrum sequence output from the FFT module 210, the host computer 212 displays the obtained Raman spectrum results on the computer screen.

[0047] As described above, this invention combines a novel FT-Raman spectrometer with an FPGA-based signal acquisition method. First, the reference light interference signal is sampled and de-glitched to obtain a rectangular wave signal with the same frequency and phase as the original signal. The user selects the driving clock source of the ADC_2 module 204 for acquiring the Raman scattering light interference signal on the host computer. The selected signal is used as the clock source to acquire the Raman scattering light interference signal. Finally, FFT operation is performed, and the signal is transmitted to the host computer for display.

[0048] To better illustrate the present invention, two specific embodiments are used to explain the process of the present invention.

[0049] Example 1

[0050] In this example, it is assumed that the ADC_1 module has a sampling rate of 203. The resolution is 16-bit, the FIFO module 206 uses 32-bit, and the laser wavelength used is 532 nm. The motion mirror speed is 1 The deviation is 2%, and the reference clock module 211 provides the reference clock frequency to the glitch filter module 208. The value is 1M. The specific workflow of the interferometric signal acquisition system in the FPGA-based FT-Raman spectrometer in this embodiment is as follows:

[0051] Step 1: When the user selects 2× sampling in the host computer, i.e., the reference light interference signal is the threshold D, the Raman scattering light interference signal is sampled. The host computer 12 sends a low reset valid signal RST_N=0 to reset the comparator module 205, FIFO module 206, control module 207, glitch filter module 208, MAX21 module 209, FFT module 210, and reference clock module 211 inside the FPGA. At the same time, the system starts self-test. After the test instrument has been powered on and stabilized for a period of time, the ADC_1 module 203 repeatedly collects the light intensity of the reference light interference signal, records its maximum light intensity MAX_D and minimum light intensity MIN_D, and takes the average of half of the difference multiple times as the threshold D for the reference light interference signal threshold comparison.

[0052] Step 2: After the reset is complete, the host computer 212 sends a reset end signal RST_N=1. The user inputs the deviation of the moving mirror's running speed on the host computer interface, and the host computer calculates the mask value m based on the user's input. The calculation method is as follows: If the moving mirror's running speed is uniform, When the moving mirror's speed is uneven, ( =1917.192, based on the range of values ​​for m It can be calculated The value range is [260.7, 266.01], so m is set to 261. The host computer sends the mask value m for filtering out glitches to the control module 207. At the same time, the control module 207 sends a signal valid_m=1 and the mask value m to the glitch filtering module 208, so that the glitch filtering module 208 is in an effective working state.

[0053] Step 3: The reference optical interference signal is amplified and buffered by the conditioning circuit module 201 to meet the input requirements of the subsequent ADC_1 module 203. Then, it is output as a rectangular wave signal CARD_CNV with glitches by the comparator module 205. After the glitches are detected by the rising edge of the mask value valid signal valid_m, the glitches filtering module 208 constructs a one-dimensional array of length 261 with all values ​​of 1. This array is then subjected to a sliding bitwise AND with the rectangular wave signal CARD_CNV obtained by comparison with the threshold D. After filtering out the glitches, the rectangular wave signal CARD_FREQ is obtained. The rectangular wave CARD_FREQ is then output to the MAX21 module 209 and the control module 207.

[0054] Step 4: When the control module 207 receives the rectangular wave signal CARD_FREQ from the glitch filtering module 208, it outputs a MAX_VALID=0 signal to the MAX21 module 209, and outputs a WR_EN signal and an RD_EN signal to the FIFO module 206. When the MAX21 module 209 receives the MAX_VALID=0 signal, it uses the rectangular wave signal CARD_FREQ as the driving clock source for the ADC_2 module 204 to start acquiring the Raman scattering interference signal. At the same time, after receiving the WR_EN signal and the RD_EN signal from the control module 207, the FIFO module 206 writes the acquired Raman scattering interference signal into the FIFO module 206 and reads it into the FFT module 210. At this time, the counter N in the FFT module 210 increments by 1.

[0055] Step 5: When the number of points N=18796 is read from the FFT module 210, stop reading and writing, perform FFT operation on the acquired Raman scattering interference signal, and record the spectrum of each point, including the real part Re[ ] and imaginary part Im[ Two parts, =1,2,…,N, using the formula X[k]=(|Re[k]| 2 +|Im[k]| 2 ) 1 / 2 The frequency energy X[k] of point N is calculated, and then the spectrum sequence of point N is output to the host computer 12. The host computer 12 displays the final result on the screen.

[0056] Example 2

[0057] In this example, it is assumed that the ADC_1 module has a sampling rate of 203. The resolution is 16-bit, the ADC_2 module 204 has a resolution of 16-bit, the FIFO module 206 uses 32-bit resolution, and the laser wavelength used is 532 nm. The motion mirror speed is 1 With a deviation of 2%, the reference clock module 211 provides the reference clock frequency to the glitch filter module 208 and the MAX21 module 209. The value is 1M. The specific workflow of the interferometric signal acquisition system in the FPGA-based FT-Raman spectrometer in this embodiment is as follows:

[0058] Step 1: When the user selects 8× sampling in the host computer, the host computer module 212 sends a low-reset active signal RST_N=0 to reset the comparator module 205, FIFO module 206, control module 207, glitches filter module 208, MAX21 module 209, FFT module 210, and reference clock module 211 inside the FPGA. At the same time, the system starts a self-test. After the test instrument has been powered on and stabilized for a period of time, the ADC_1 module 203 repeatedly samples the light intensity of the reference light interference signal and records its maximum light intensity MAX_D and the optical path difference X at this time. max The minimum light intensity MIN_D and the optical path difference X at this time min And take half of the difference D=(MAX_D-MIN_D) / 2, X D =(X max- X min ) / 2, and take the average after multiple records as the threshold for subsequent sampling points.

[0059] Step 2: After the reset is complete, the host computer 212 sends a reset end signal RST_N=1. Since the control module 207 does not receive the mask value m from the host computer 212, it sends a signal valid_m=0 to the glitching filter module 208, causing the glitching filter module 208 to disable. Since the control module 207 does not receive the rectangular wave signal from the glitching filter module 208, it sends a signal MAX_VALID=1 to the MAX21 module 209, causing the MAX21 module 209 to select a 1MHz clock provided by the reference clock module 211 as the driving clock source for the Raman scattering interference signal acquired by the ADC_2 module 204.

[0060] Step 3: The reference light interference signal is amplified and buffered by conditioning circuit module 201 to meet the input requirements of subsequent ADC_1 module 203. The ADC_DATA1 signal generated by ADC_1 module 203 is stored in the lower 16 bits of FIFO module 206, and the Raman scattering light interference signal ADC_DATA2 acquired by ADC_2 module 204 is stored in the higher 16 bits of FIFO module 206. FIFO module 206 selects the Raman scattering light interference signal ADC_DATA2 corresponding to the optical path difference based on the lower 16 bits of the reference light interference signal ADC_DATA1. The specific process is as follows:

[0061] Once the FIFO module 206 has stored one cycle of the reference optical interference signal ADC_DATA1, when the stored signal is the positive half-cycle of a sinusoidal signal, such as... Figure 3 As shown, the optical path difference X obtained in step 1 is the maximum light intensity MAX_D. max and X D Amplitude D) / 2) Select D. MAX_D When the lower 16 bits of data ADC_DATA1 in FIFO module 206 are exactly equal to the four values ​​mentioned above, the higher 16 bits of Raman scattering interference signal at the corresponding position in FIFO module 206 are sent to FFT module 210. At this time, the counter N in FFT module 210 increments by 1. If the stored signal is the negative half-cycle of a sinusoidal signal, the optical path difference X of the minimum light intensity MIN_D obtained in step 1 is used. min and X D Amplitude D ) / 2) Select D. MIN_D When the lower 16-bit data ADC_DATA1 in the FIFO module 206 is exactly equal to the above four values, the higher 16-bit Raman scattering interference signal at the corresponding position in the FIFO module 206 is sent to the FFT module 210 for FFT calculation.

[0062] Step 4: When the number of points N=75184 is read from the FFT module 210, stop reading and writing, perform FFT operation on the acquired Raman interference optical signal, and record the spectrum of each point, including the real part Re[ ] and imaginary part Im[ Two parts, =1,2,…,N, using the formula X[k]=(|Re[k]| 2 +|Im[k]| 2 ) 1 / 2 The frequency energy X[k] of point N is calculated, and then the spectrum sequence of point N is output to the host computer 212. The host computer 212 displays the final result on the screen.

[0063] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for acquiring equal OPD of interference signals in an FPGA-based FT-Raman spectrometer, characterized in that... The system includes a conditioning circuit module 1, a conditioning circuit module 2, an ADC module, a comparator module, a FIFO module, a control module, a glitches filter module, a MAX21 module, an FFT module, a reference clock module, and a host computer. The comparator module, FIFO module, control module, glitches filter module, MAX21 module, FFT module, and reference clock module are implemented in an FPGA. The conditioning circuit module 1 is used to amplify and buffer the reference light interference signal acquired by the detector so that it meets the input requirements of the subsequent ADC_1 module. Conditioning circuit module 2 is used to amplify and buffer the Raman scattering interference signal acquired by the detector so that it meets the input requirements of the subsequent ADC_2 module; The ADC_1 module is used to acquire the input reference light interference signal. Its bit width is N. The data sequence ADC_DATA1 obtained from the acquired reference light interference signal is sent to the comparator module and the FIFO module at the same time. The ADC_2 module is used to acquire Raman scattering interference signals. Its bit width is N. The data sequence ADC_DATA2 obtained from the acquired Raman scattering interference signals is sent to the FIFO module. The comparator module is used to perform over-threshold comparison on the reference light interference signal to obtain a rectangular wave signal CARD_CNV with the same frequency and phase as the input signal. The rectangular wave signal CARD_CNV is then output to the glitch filter module. The threshold is obtained by the system's self-test upon startup. The specific process is as follows: after the instrument under test has been powered on and stabilized for a period of time, the ADC_1 module repeatedly collects the light intensity of the reference light interference signal, records its maximum and minimum light intensity values, and takes the average of half of the difference between the two values ​​as the threshold for the reference light interference signal threshold comparison. The FIFO module buffers two signal sequences: the reference light interference signal and the Raman scattering light interference signal acquired by the ADC module. The FIFO module has a bit width of 2N, storing the reference light interference signal in the lower N bits and the Raman scattering light interference signal in the higher N bits. When the user selects 2x sampling, the FIFO module operates in a write-while-read state. Each time the ADC_2 module acquires a Raman scattering light interference signal and stores it in the FIFO module, the control module issues RD_EN and WR_EN signals to read the Raman scattering light interference signal into the FFT module. Simultaneously, it reads the signal from the next acquisition by the ADC_2 module. The data is stored in the FIFO module. When the user selects 8x sampling, the FIFO module stores the reference light interference signal for one cycle in the lower N bits and the Raman scattering light interference signal at the corresponding position in the higher N bits. When the FIFO module is full, it sends a WR_FULL signal to the control module. After receiving the WR_FULL signal, the control module sends an RD_EN signal to the FIFO module to read the data stored in the FIFO module into the FFT module. The reading method is as follows: when the reference light interference signal in the lower N bits of the FIFO module is equal to the pre-acquired value, the Raman scattering light interference signal in the higher N bits at the corresponding position is read into the FFT module. The control module is used to control the reading and writing of the FIFO module, receive instructions from the host computer, send control signals to control the MAX21 module to select an appropriate drive clock source for the ADC_2 module, and output a control signal to the glitch filtering module according to the input of the host computer to determine the width of the glitch filtering. The glitch filtering module is used to filter out glitch in the rectangular wave signal CARD_CNV output by the comparator module. The resulting rectangular wave signal CARD_FREQ after glitch filtering is sent to the MAX21 module and the control module. The MAX21 module is used to select one of the reference clock module and the glitch filter module as the driving clock source for the ADC_2 module according to the control command output by the control module, so as to acquire the Raman scattering light interference signal. The FFT module is used to perform Fourier transform on the signal to form a spectrum. Specifically, the received data sequence ADC_DATA2 is subjected to FFT operation, and the spectrum of each point is recorded, consisting of the real part Re[k] and the imaginary part Im[k], k=1,2,…,N. The formula X[k]=(|Re[k]| 2 +|Im[k]| 2 ) 1 / 2 The frequency energy X[k] of N points is calculated, and then the spectrum sequence of N points is output to the host computer. The reference clock module provides a reference clock clk_fre as the driving clock source for the ADC_2 module to acquire the Raman scattering light interference signal, and also provides a reference clock clk_ref to the glitch filtering module as the clock source for filtering out glitch components. After receiving the N-point spectrum sequence output from the FFT module, the host computer displays the obtained Raman spectrum results on the screen in real time.

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