CN112345077B - Real-time calibration method for optical path difference of photoelastic modulation type Fourier transform spectrometer - Google Patents
Real-time calibration method for optical path difference of photoelastic modulation type Fourier transform spectrometer Download PDFInfo
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Abstract
The invention belongs to the field of data processing of photoelastic modulation interference signals, and particularly relates to a real-time calibration method for optical path difference of an photoelastic modulation type Fourier transform spectrometer, which comprises the following steps: simultaneously sampling a reference laser interference signal modulated by the photoelastic and a light source signal to be detected; extracting data of a periodic reference laser interference signal through a frame header and a frame tail, and transmitting a digital signal obtained by sampling to an upper computer through filtering processing; and (4) carrying out parallel processing on the calibration and the spectral signal to be measured, and finishing the calibration of the optical path difference for processing and combining the polychromatic light interference signals. The method uses the laser with short wavelength as a reference light source to generate a reference interference pattern, carries out zero-crossing counting on the acquired digital signal of the reference interference pattern in the upper computer, calculates the transient maximum optical path difference, and realizes the calibration of the recovery wavelength. The invention is used for calibrating the optical path difference.
Description
Technical Field
The invention belongs to the field of data processing of photoelastic modulation interference signals, and particularly relates to a real-time calibration method for optical path difference of an photoelastic modulation type Fourier transform spectrometer.
Background
The spectrometer is divided into two types of light splitting and modulation according to the working principle, one of the light splitting type spectrometers is a dispersion type spectrometer composed of a prism, the resolution ratio of the dispersion type spectrometer is low, the dispersion type spectrometer is sensitive to temperature and humidity, and the environment requirement is strict, and the other type of the dispersion type spectrometer composed of gratings adopts advanced grating carving and copying technology, so that the resolution ratio is improved, the measurement wave band is widened, and the environment requirement is reduced. The modulation type spectrometer is a Fourier transform spectrometer and has wide measurement range, high precision and high resolution.
At present, in infrared and visible light wave bands, a Fourier transform spectrometer with a Michelson interferometer structure is generally adopted, a movable mirror needs to be pushed to complete spectrum measurement, the measurement speed is limited, generally, single measurement time cannot break through ms magnitude, and the movable mirror is mechanically pushed and swept and is sensitive to environmental vibration. The elasto-optical modulation Fourier transform spectrometer is used as a spectral measurement technology with high speed, high sensitivity and wide spectral range, overcomes the defect, has wide application in the fields of chemical analysis, environmental remote sensing, military and the like, and has potential application value and development prospect in the fields of transient spectral detection such as transient temperature measurement, spectral measurement in an explosion process and the like. In order to realize fast Fourier transform and wavelength calibration of interference signals, the optical path difference of the photoelastic modulator needs to be calibrated in real time.
In the prior art, an ultra-high-speed comparator is used for converting an interference signal into a square wave and carrying out zero-crossing counting on the square wave. But when the resolution of the interference signal is higher than 20cm -1 In the process, a comparator with higher speed is difficult to find, and although a laser with larger wavelength can be used as a reference light source, the calculation accuracy of the maximum optical path difference is reduced at the moment. And when the FPGA is used for counting, the requirement on the frequency of the crystal oscillator is higher, the requirement on the response time of the high-speed comparator is also higher, and the transient maximum optical path difference cannot be transmitted in real time.
Disclosure of Invention
Aiming at the technical problems that the existing method for converting interference signals into square waves by using an ultra-high-speed comparator has poor calculation accuracy, has higher requirements on the frequency of a crystal oscillator and the response time of the high-speed comparator and cannot transmit transient maximum optical path difference in real time, the invention provides a method for converting the interference signals into the square waves by using the ultra-high-speed comparator
In order to solve the technical problems, the technical scheme adopted by the invention is as follows:
a photoelastic modulation type real-time calibration method for optical path difference of Fourier transform spectrometer comprises the following steps:
s1, simultaneously sampling a reference laser interference signal modulated by an elastic light and a light source signal to be detected;
s2, extracting data of a periodic reference laser interference signal through the frame head and the frame tail, and transmitting a digital signal obtained by sampling to an upper computer through filtering processing;
and S3, after filtering, performing matlab programming zero-crossing counting on the upper computer, simultaneously performing optical path difference calibration on the recovered spectrogram, multiplying the counting value obtained by the upper computer by the calibrated wavelength, calculating the optical path difference of the photoelastic modulation light source to be measured so as to complete the optical path difference calibration, and transmitting the optical path difference calibration to polychromatic light algorithm processing in real time after the optical path difference calibration is completed.
In the S1, the reference laser interference signal and the light source signal to be detected of the photoelastic modulation are sampled by adopting a high-speed acquisition card with two channels, one channel is used for sampling the reference laser interference signal, the other channel is used for sampling the light source signal to be detected, and the sampling mode of the high-speed acquisition card is differential sampling.
And the modulation period of the reference laser interference signal in the S2 is determined by the modulation frequency of the elastic optical modulator or by the sampling data quantity of an interference pattern of an incident light signal.
The filtering processing method in the S2 comprises the following steps: before counting by the upper computer, digital filtering and analog filtering are carried out on the reference laser interference signal.
And the filtering processing in the S2 adopts a preprocessing module, the preprocessing module comprises a large bandwidth amplifier and a filter, the large bandwidth amplifier is connected with the filter, the large bandwidth amplifier is connected with an infrared detector, and the filter is connected with a high-speed acquisition card.
The zero-crossing counting method in the S3 comprises the following steps: and inputting reference laser interference signal data, counting in matlab programming, judging the data one by one, if the number of the previous data and the number of the next data are different, multiplying the result to be negative, and adding one to the count until all data are judged.
The calculation formula of the optical path difference of the photoelastic modulated light source to be measured in the S3 is as follows: l =0.5 lambda x n, wherein lambda is the wavelength of the reference laser interference signal, n is the zero-crossing times of the reference laser interference signal in one period, and L is the optical path difference of the light source to be measured.
Compared with the prior art, the invention has the following beneficial effects:
1. the method uses the laser with short wavelength as a reference light source to generate a reference interference pattern, carries out zero-crossing counting on the acquired digital signal of the reference interference pattern in the upper computer, calculates the transient maximum optical path difference, and realizes the calibration of the recovery wavelength. The counting mode can overcome the defect that when the spectral resolution is high, the ultra-high speed comparator is difficult to find by zero-crossing comparison counting of laser interference signals, and compared with the traditional calibration, the counting mode has high accuracy, good real-time performance and simple counting method, and is only related to the wavelength of a reference light source;
2. the invention adopts a high-speed acquisition card with two channels, wherein one channel acquires reference laser, the other channel acquires polychromatic light, when the wavelength calibration is carried out on the recovered spectrum, the calibration and the spectral signal to be measured are processed in parallel, and the calibration of the optical path difference is completed for processing and combining the polychromatic light interference signals, thereby avoiding the problem of low transmission speed by using a serial port in the past and improving the accuracy and speed of the recovered spectrum;
3. according to the invention, the interferogram is sampled, the upper computer zero-crossing counting is carried out on the interferogram in a period, and the counting can be carried out when the frequency of an interference signal is higher, so that the problem that a high-speed comparator meeting the requirement is difficult to find is avoided, and the problem that the clock of an FPGA has a larger requirement when the counting is programmed by the FPGA is avoided;
4. the invention adopts the laser with short wavelength as the reference light source for sampling and calibration, has very good autocorrelation, and has higher spectral resolution ratio higher than 20cm -1 In time, the calculation accuracy of the optical path difference calibration is still high, and the spectrum can be restored with high precision.
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FIG. 1 is a flow chart of the present invention;
fig. 2 is a waveform diagram illustrating the counting principle of the reference laser interference signal according to the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
A method for calibrating the optical path difference of a photoelastic modulation type Fourier transform spectrometer in real time is shown in figure 1 and comprises the following steps:
sampling a reference laser interference signal modulated by an elastic light and a light source signal to be detected simultaneously;
extracting data of a periodic reference laser interference signal through the frame head and the frame tail, and transmitting a digital signal obtained by sampling to an upper computer through filtering processing;
and step three, after filtering, performing programming zero-crossing counting on the light source through matlab in the upper computer, simultaneously performing optical path difference calibration on the recovered spectrogram, multiplying the counting value obtained by the upper computer by the calibrated wavelength, calculating the optical path difference of the light source to be measured of the photoelastic modulation so as to finish the optical path difference calibration, and transmitting the optical path difference calibration to polychromatic light algorithm processing in real time after the optical path difference calibration is finished.
Further, in the first step, the reference laser interference signal and the light source signal to be detected of the photoelastic modulation are sampled by adopting a two-channel high-speed acquisition card, one channel is used for sampling the reference laser interference signal, the other channel is used for sampling the light source signal to be detected, and the sampling mode of the high-speed acquisition card is differential sampling.
Further, the modulation period of the reference laser interference signal in the second step is determined by the modulation frequency of the elastic optical modulator or by the sampling data amount of the interference pattern of the incident light signal.
Further, the filtering processing method in the second step comprises: before counting by the upper computer, digital filtering and analog filtering are carried out on the reference laser interference signal.
Furthermore, the filtering processing in the second step adopts a preprocessing module, the preprocessing module comprises a large bandwidth amplifier and a filter, the large bandwidth amplifier is connected with the filter, the large bandwidth amplifier is connected with an infrared detector, and the filter is connected with a high-speed acquisition card.
Further, the zero-crossing counting method in the third step is as follows: and inputting reference laser interference signal data, counting in matlab programming, judging the data one by one, if the number of the previous data and the number of the next data are different, multiplying the result to be negative, and adding one to the count until all data are judged.
Further, in the third step, the calculation formula of the optical path difference of the photoelastic modulated light source to be measured is as follows: l =0.5 lambda multiplied by n, lambda is the wavelength of the reference laser interference signal, n is the zero crossing frequency of the reference laser interference signal in one period, and L is the optical path difference of the light source to be measured.
The working principle of the invention is as follows:
electro-optical, magneto-optical, acousto-optical, elasto-optical modulators that tune the refractive index of a material by electrical, magnetic, ultrasonic, or stress, etc., may be used as interferometers in Fourier Transform Spectrometers (FTS). Among them, the application potential of the photoelastic modulator in FTS is great. The elasto-optical modulation is an artificial birefringence phenomenon based on the elasto-optical effect. The PEM is primarily composed of a driver to generate the driving force and an elasto-optic birefringent crystal, which is typically driven by a piezoelectric driver. The basic working principle is that the optical element and the piezoelectric driver are coupled in a certain mode, a periodic stress is applied through the piezoelectric driver, so that the optical element and the piezoelectric driver perform periodic reciprocating motion in opposite directions, the whole elastic optical modulator generally works in a fundamental frequency mode, wherein v is the sound velocity in a material, and the whole vibration process of the boundary of the optical element and the piezoelectric driver belongs to simple harmonic vibration.
In the photoelastic modulation spectrometer, fourier transform is performed on an interference signal, and the power spectrum distribution of an incident beam is restored as follows:
firstly, in the photoelastic modulation spectrometer, an interferogram detected by a detector is continuously changed, and in order to realize the fourier transform described in the above formula, the interferogram needs to be sampled. Sampling the interferogram in the process that a moving mirror of the Fourier transform interferometer moves from a negative maximum optical path difference point to a positive maximum optical path difference point, and forming a complete interferogram by the collected data. And performing fast Fourier transform on the whole interference pattern to obtain a spectrogram in a certain frequency spectrum range.
In fig. 1, a reference laser interference signal from a reference laser needs to pass through a signal conditioning circuit and a silicon photodetector, and then the signal is amplified and filtered to obtain a useful signal, at this time, the useful signal can enter a high-speed acquisition card for sampling, the acquired data is cached, an upper computer performs zero-crossing counting, as long as the data before and after one number is different, the interference signal crosses zero, and the counting value is increased by one, so that the wavelength calibration of an incident signal can be directly realized by using the counting value. As shown in fig. 2, the reference laser interference signal is amplified and filtered, and then sent to the input of the spectrum recovery algorithm to perform wavelength calibration on the polychromatic light signal.
In the time modulation type Fourier transform spectrometer, when the maximum moving distance of the movable mirror is determined, the maximum optical path difference is a constant value. The dynamic model of the photoelastic modulation interferometer and the temperature drift characteristic of the resonant frequency show that the maximum optical path difference of the photoelastic modulation interferometer has larger change along with the change of the driving voltage and the temperature. Therefore, the transient maximum optical path difference is a variable quantity. The triggering frame head can obtain a whole period interference data, and the Fourier transform algorithm can be used for realizing spectrum restoration. However, to complete the analysis of the recovered spectrum, the recovered spectrum needs to be wavelength-calibrated. The parameter transient maximum optical path difference is needed when the wavelength is calibrated. Therefore, the transient maximum optical path difference of the interferogram needs to be calculated from the acquired interference data.
The high-speed acquisition card is used for sampling in a differential mode, and acquired signals are vertically symmetrical about a 0 reference position, so that zero-crossing counting can be performed by programming of an upper computer. After one period of data is cached, the data is sequentially judged, if the data before and after one number is respectively greater than 0 and less than 0, the middle part of the data has zero crossing points, and therefore the zero crossing of the interference signal can be counted.
Although only the preferred embodiments of the present invention have been described in detail, the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention within the knowledge of those skilled in the art, and all changes are included in the scope of the present invention.
Claims (1)
1. A photoelastic modulation type real-time calibration method for optical path difference of Fourier transform spectrometer is characterized by comprising the following steps: comprises the following steps:
s1, simultaneously sampling a reference laser interference signal modulated by an elastic light and a light source signal to be detected; in the S1, a reference laser interference signal and a light source signal to be detected which are modulated by the elastic light are sampled by adopting a high-speed acquisition card with two channels, wherein one channel is used for sampling the reference laser interference signal, the other channel is used for sampling the light source signal to be detected, and the sampling mode of the high-speed acquisition card is differential sampling;
s2, extracting data of a periodic reference laser interference signal through the frame head and the frame tail, and transmitting a digital signal obtained by sampling to an upper computer through filtering; the modulation period of the reference laser interference signal in the S2 is determined by the modulation frequency of the elastic optical modulator or the sampling data quantity of the interference pattern of the light source signal to be detected; the filtering processing method in the S2 comprises the following steps: before counting by the upper computer, carrying out digital filtering and analog filtering on the reference laser interference signal; the filtering processing in the S2 adopts a preprocessing module, the preprocessing module comprises a large bandwidth amplifier and a filter, the large bandwidth amplifier is connected with the filter, the large bandwidth amplifier is connected with an infrared detector, and the filter is connected with a high-speed acquisition card;
s3, after filtering, performing programming zero-crossing counting on the upper computer through matlab, simultaneously performing optical path difference calibration on the recovered spectrogram, multiplying a counting value obtained by the upper computer by a calibration wavelength, calculating the optical path difference of the photoelastic-modulated light source to be detected so as to finish the optical path difference calibration, and transmitting the optical path difference calibration to polychromatic light algorithm processing in real time after finishing the optical path difference calibration; the zero-crossing counting method in the S3 comprises the following steps: inputting reference laser interference signal data, counting in matlab programming, judging the data one by one, if the number of the previous data and the number of the next data are different, multiplying the result to be negative, and adding one to the count until all data are judged; the calculation formula of the optical path difference of the photoelastic modulated light source to be measured in the S3 is as follows: l =0.5 lambda x n, where lambda is the wavelength of the reference laser interference signal, n is the zero-crossing times of the reference laser interference signal in one period, and L is the optical path difference of the light source to be measured.
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