Multi-wavelength wavelength modulation method and system based on external chopping time division multiplexing

Abstract

The present invention discloses a multi-wavelength wavelength modulation method and system based on external chopping time division multiplexing. The method includes: S1. By injecting a current signal with low frequency superimposed on high frequency into different wavelength lasers in a laser group, wavelength modulation of the lasers is achieved, so that the low frequency in the emitted light signal of the lasers is a scanning signal and the high frequency is a modulation signal; S2. The emitted light of different wavelength lasers is combined, and external chopping processing is performed before beam combination, thereby realizing time-division emission of different wavelength lasers. S3. The combined laser passes through an etalon to generate an interference signal, which is received by a detector, thereby realizing calibration of the spectral frequency time response of multi-wavelength wavelength modulation. The present invention avoids the influence of unstable thermal load of the lasers on wavelength calibration through the method of external chopping, and thereby realizes accurate wavelength calibration under wavelength modulation time division multiplexing.

Description

Technical Field

[0001] The present invention relates to the technical field of spectral measurement, and particularly to a multi-wavelength wavelength modulation method and system based on external chopper time division multiplexing. Background Art

[0002] In the high-temperature and high-pressure environment of a combustion field, accurately measuring the gas temperature and component concentration is crucial for studying combustion characteristics. Due to the influence of pressure broadening and pressure frequency shift in a high-pressure environment, the traditional direct absorption narrow-band laser spectroscopy is prone to absorption line broadening and overlapping phenomena, making it difficult to meet the measurement requirements under high-pressure conditions.

[0003] Wavelength Modulation Spectroscopy (WMS) reduces low-frequency noise interference by modulating the absorption signal to the high-frequency part and obtains harmonic signals with high signal-to-noise ratio after demodulation. Therefore, it has good anti-interference performance. WMS has been widely used in gas concentration and temperature detection in complex environments.

[0004] In a high-pressure environment, WMS has significant advantages in solving the problems of absorption line broadening and overlapping and absorption feature extraction. For example, in the paper "Wavelength-modulation spectroscopy near 1.4 µm for measurements of H2O and temperature in high-pressure and-temperature gases" published by Goldstein et al. in the journal "Measurement Science and Technology" (2014), the frequency division multiplexing method with different modulation frequencies was used to modulate the laser absorption lines at wavelengths 7185.59 cm⁻¹ and 6806.03 cm⁻¹ respectively. After demodulating the signal received by the detector, the harmonic signal was separated by a low-pass filter, and the temperature and water vapor concentration under the conditions of 700 - 2400 K and 2 - 25 atm in the shock tube environment were measured by the two-line ratio method. However, for a wider temperature and pressure measurement environment, the two-line ratio measurement method has the problem of insufficient coverage range and still has room for improvement.

[0005] To broaden the temperature measurement range and improve the measurement accuracy in high-temperature environments, in the paper "High-pressure gas temperature sensing for exit plane of aero-engine combustor using tunable diode laser absorption spectroscopy" (2024) published by Huang et al. in the journal "Microwave and Optical Technology Letters", multi-wavelength measurement is achieved by selecting multiple absorption lines with large differences in low-state energy levels, and the temperature and water vapor concentration are successfully measured in an environment with a maximum pressure of 0.55 MPa and a maximum temperature of 1100 K at the exit of an aero-engine combustor. However, when the pressure further increases, direct absorption spectroscopy is significantly affected by pressure broadening, and the measurement accuracy cannot be guaranteed. Therefore, the multi-wavelength measurement method based on WMS has gradually become an effective solution for wide-temperature-range and high-pressure gas measurement.

[0006] Compared with direct absorption spectroscopy, the frequency-time response calibration of wavelength modulation spectroscopy is more complex. In the paper "A novel methodology to directly pre-determine the relative wavelength response of DFB laser in wavelength modulation spectroscopy" (2019) published by Liu et al. in the journal "Optics Express", a frequency-time response prediction model based on low-frequency sawtooth scanning superimposed on high-frequency sine modulation is proposed, and the relative wavelength response of a distributed feedback (DFB) laser can be determined with only a single fitting parameter. Du et al. published a paper in the journal "Optics Express" titled "A high-accurate and universal method to characterize the relative wavelength response (RWR) in wavelength modulation spectroscopy (WMS)" (2020), which proposed a model using low-frequency sine scanning superimposed on high-frequency sine modulation to achieve accurate calibration of the frequency-time response of a single laser. However, in a multi-wavelength scenario, when time-division multiplexing is achieved by controlling the laser current, this model cannot guarantee the accuracy of calibration.

[0007] This limitation mainly stems from the non - linear characteristics of the laser during the moment of light output, which are difficult to accurately describe through a specific model. The active region inside the laser will experience changes in thermal load with different injection currents, resulting in temperature fluctuations. Especially when the current suddenly jumps from 0 to a relatively high current, the impact of temperature fluctuations on wavelength stability is particularly significant.

[0008] The temperature response speed of the laser is slow, and temperature fluctuations will act for a long time after the current jump (usually caused by a switching operation), leading to irregular changes in the output light wavelength, posing a great challenge to the calibration of the frequency - time response. In addition, the non - linear characteristics of different lasers vary, making it difficult to describe them with a unified model, further limiting the accuracy of the frequency - time response calibration. Summary of the Invention

[0009] The purpose of the present invention is to provide a multi - wavelength wavelength modulation method and system based on external chopping time - division multiplexing to solve the problems of discontinuous laser scanning in the case of time - division multiplexing and inaccurate wavelength calibration caused by the non - linear characteristics of the laser due to the current jump in the injection laser.

[0010] To this end, on the one hand, the present invention provides a multi - wavelength wavelength modulation method based on external chopping time - division multiplexing, including: S1. By injecting a low - frequency superimposed high - frequency current signal into lasers with different wavelengths in a laser group, wavelength modulation of the lasers is achieved, so that the low - frequency component in the output light signal of the laser is a scanning signal and the high - frequency component is a modulation signal; S2. The output lights of lasers with different wavelengths are combined, and external chopping processing is performed before combination, thereby achieving time - division output of lasers with different wavelengths.

[0011] According to another aspect of the present invention, a multi - wavelength wavelength modulation system based on external chopping time - division multiplexing is provided for implementing the above - mentioned multi - wavelength wavelength modulation method based on external chopping time - division multiplexing. The system includes a laser group, a laser control circuit, a timing control circuit, an external chopping module, a chopping module controller, and a wavelength - division multiplexer. The timing control circuit provides timing to the laser control circuit to control the injection of a low - frequency superimposed high - frequency current signal into the laser group to achieve wavelength modulation of the lasers. The output lights of the laser group converge to the wavelength - division multiplexer for combination. An external chopping module is provided on the optical paths of the output lights of lasers with different wavelengths in the laser group. The timing control circuit also provides timing to the chopping module controller to control different external chopping modules to perform chopping processing to achieve time - division output.

[0012] The present invention also provides a multi-wavelength wavelength modulation system based on external light-cutting time-division multiplexing, which is used to implement the above-mentioned multi-wavelength wavelength modulation method based on external light-cutting time-division multiplexing. The system includes a laser group composed of lasers of different wavelengths, a laser control circuit, a timing control circuit, and an optical switch cluster. The timing control circuit provides timing to the laser control circuit to control the low-frequency superimposed high-frequency current signal injected into the laser group to achieve laser wavelength modulation. The light output of the laser group converges to the optical switch cluster for beam combining. The timing control circuit also provides timing to the optical switch cluster to control the optical switch cluster to perform light-cutting processing to achieve time-sharing light output.

[0013] The time-division multiplexing multi-wavelength wavelength modulation spectroscopy technology based on external light cutting according to the present invention optimizes the frequency time response calibration accuracy. The present invention realizes the time-division light output control of the laser through the external light cutting module, significantly improves the accuracy of wavelength calibration, and can be used for temperature and gas concentration measurement in high temperature and high pressure environments of combustion fields, and is suitable for high-precision multi-wavelength spectroscopy measurement scenarios such as combustion diagnosis, industrial process control and environmental monitoring.

[0014] The method of the present invention can simultaneously insert a short light-free period at the beginning of the wavelength modulation period by cutting off the light to monitor and eliminate background noise fluctuations and electronic DC bias, and restore the unbiased DC signal, which is helpful for accurately establishing the forward model of the absorption spectrum under wavelength modulation, thereby improving the accuracy of spectral parameters and other temperature and concentration inversion results.

[0015] The time division multiplexing method of the present invention is compatible with frequency division multiplexing, and is easy to implement the hybrid strategy of external light-cutting time division multiplexing and frequency division multiplexing, so as to meet the requirements of frequency response under multi-wavelength conditions. The present invention is suitable for relatively stable environmental conditions in the environment and industrial fields, and also has significant advantages in temperature, concentration, and pressure change environments such as combustion flow fields.

[0016] In addition to the above-described purposes, features and advantages, the present invention has other purposes, features and advantages. The present invention will be further described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0018] Figure 1 It is a flow chart of the multi-wavelength wavelength modulation method based on external light cutting time division multiplexing of the present invention;

[0019] Figure 2 It is a structural block diagram of a multi-wavelength wavelength modulation system based on external light-cutting time-division multiplexing of the present invention;

[0020] Figure 3 It is a schematic structural diagram of the multi-wavelength wavelength modulation system according to the first embodiment of the present invention;

[0021] Figure 4 It is a schematic structural diagram of the multi-wavelength wavelength modulation system according to the second embodiment of the present invention;

[0022] Figure 5 It is a laser waveform diagram of the multi-wavelength wavelength modulation method based on external chopping time-division multiplexing of the present invention;

[0023] Figure 6 is the Figure 5 amplified waveform diagram of the laser in the present invention. Detailed implementation manners

[0024] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0025] The present invention realizes time-division light output by controlling a continuous scanning laser group through external chopping, thereby avoiding the problems of difficult accurate calibration of the light output frequency time response and the stability of the continuous working heat load in the multi-wavelength wavelength modulation strategy caused by the time-division drive current of the laser group.

[0026] Referring to Figure 1 , the multi-wavelength wavelength modulation method of the present invention includes the following steps S1 - S3:

[0027] S1. By injecting a low-frequency superimposed high-frequency current signal into lasers with different wavelengths in the laser group, wavelength modulation of the lasers is realized, so that the low frequency in the laser light output signal (such as the light intensity signal) is the scanning signal and the high frequency is the modulation signal, where the frequency value of the low-frequency scanning is at least 100 times different from the frequency value of the high-frequency modulation;

[0028] S2. Combine the light output of lasers with different wavelengths, and realize time-division light output of lasers with different wavelengths through external chopping operation before beam combination;

[0029] S3. Make the combined laser pass through the etalon to generate an interference signal, which is received by the detector, and finally complete the calibration of the multi-wavelength wavelength modulation spectral frequency time response.

[0030] The multi-wavelength wavelength modulation system for realizing this method will be introduced below.

[0031] As Figure 2 shown, the multi-wavelength modulation system of the present invention is composed of a multi-wavelength laser group, a laser control circuit, a timing control circuit, an external chopping module for external chopping operation of lasers with different wavelengths, a chopping module controller, a wavelength division multiplexing module for combining the light output of lasers with different wavelengths, an etalon, and a detector.

[0032] The laser control circuit generates a current signal with low-frequency superimposed high-frequency modulation and injects it into the laser group to meet the target absorption lines, modulation depths, and scanning ranges of each laser. The external chopper module is connected to the chopper module controller through a radio frequency connection cable. The timing signal of the chopper module controller is synchronized with the frequency of the laser scanning signal and is input by the timing control circuit.

[0033] The laser optical path connects the laser pigtail to the input end of the external chopper module through an optical fiber flange, and then the output end of the external chopper module is connected to the input ends of each wavelength band of the wavelength division multiplexer, finally forming a time-division multiplexed multi-wavelength system based on external chopper control.

[0034] Lasers with different wavelength bands are correspondingly matched with external chopper modules with different applicable ranges. The external chopper module itself is controlled by the chopper module controller. The chopper module controller receives the external timing signal and modulates different chopper modules, enabling the corresponding external chopper modules to perform switching operations in sequence to achieve time sharing.

[0035] The external chopper module is used to meet the requirements of multi-wavelength measurement through wavelength division multiplexing and is used to avoid the non-linear characteristics of the light output moment of the current-controlled laser during time-division multiplexing, so as to improve the accuracy of frequency-time response calibration.

[0036] In the case where only time-division multiplexing is used in the present invention, the scanning frequencies and modulation frequencies of lasers with different wavelengths are guaranteed to be exactly the same under the action of the timing control circuit. When the number of applied wavelength bands is large, to ensure the overall response frequency of the system, the lasers are grouped, and different modulation frequencies are injected into the two groups of lasers respectively under the action of the timing control circuit to achieve frequency-division multiplexing. By introducing frequency-division multiplexing under the hardware limitation conditions, the number of lasers can be doubled under the same control conditions to meet higher multi-wavelength measurement requirements.

[0037] To realize the short non-light-emitting section between the time-division light emissions of each wavelength of the wavelength modulation spectroscopy, the laser control circuit delays the trigger signal of the chopper module controller, so that the external chopper module is delayed to turn on on the premise of the same timing as the corresponding laser, generating a moment when no laser passes through, thereby realizing the generation of the non-light-emitting section.

[0038] The addition of the non-light-emitting section realizes the monitoring of background radiation in practical applications. Through this design, the influence of background radiation in practical applications on the overall elevation of the signal can be solved. At the same time, the recovery of the DC component is realized under the modulation condition, further improving the inversion accuracy of the modulation spectroscopy in practical applications.

[0039] This system realizes time-sharing light emission control of the laser through an external light-cutting module, significantly improving the accuracy of wavelength calibration. It can be used to measure temperature and gas concentration in high-temperature and high-pressure environments in combustion fields, and is suitable for high-precision multi-wavelength spectral measurement scenarios such as combustion diagnosis, industrial process control and environmental monitoring.

[0040] Embodiment 1

[0041] The external light-cutting module is an AOM unit (acoustic-optic modulator), and the light-cutting module controller is an AOM controller. The selection requirements of the AOM unit are to customize the corresponding AOM unit according to the selected laser wavelength, and to meet the extinction ratio ≥ 50dB; the rise time ≤ 45ns; and the insertion loss ≤ 2dB.

[0042] This embodiment is an application example of multi-wavelength wavelength modulation spectrum time division multiplexing using an AOM unit for external light cutting.

[0043] The laser control circuit 101 can generate standard low-frequency sinusoidal scanning and high-frequency sinusoidal modulated current signals. Depending on the application scenario, the adder and multiplier in the laser control circuit can generate voltage signals with different scanning ranges and modulation depths and inject them into the laser group.

[0044] The timing control circuit 102 provides a timing signal for the system, so that the laser group and the AOM controller (acousto-optic modulator) maintain the same timing. The timing control circuit combines with the laser control circuit to provide scanning and modulation frequencies for the laser group. Depending on the usage scenario, the laser group is provided with the same modulation frequency or different modulation frequencies for frequency division multiplexing.

[0045] The AOM controller 103 performs external modulation on the AOM unit 105 by receiving the timing and high and low level signals of the timing control circuit, and controls the corresponding wavelength AOM to perform switching operations.

[0046] The laser group 104 is composed of a plurality of DFB lasers of different wavelength bands. According to the actual application scenario, that is, the temperature, concentration range and other parameter requirements of the environment to be measured, the DFB lasers with suitable wavelength bands and numbers are selected.

[0047] The DFB lasers of each wavelength band are connected to the input end of the AOM unit of the corresponding wavelength through the optical fiber flange.

[0048] When the number of selected lasers is small, the laser control circuit injects the same scan fscan and modulation frequency fmod into the DFB laser group through the timing control circuit. At this time, the corresponding band AOM units are switched on and off in turn under the external modulation of the AOM controller. This mode only uses time division multiplexing.

[0049] When a relatively large number of lasers are selected and there are high requirements for the overall response frequency of the system, the timing control circuit generates two different modulation frequencies fmod 1 and fmod 2 simultaneously on the premise of generating the same scanning frequency fscan.

[0050] The laser group is divided into two parts. One part is injected with the signal of fscan superimposed with fmod 1 generated by the timing control circuit, and the other part is injected with the signal of fscan superimposed with fmod 2.

[0051] The AOM controller controls the AOM unit through external modulation to turn on a pair of AOM units connected to the modulation frequencies of fmod 1 and fmod 2 respectively. And so on, switch the AOM units in pairs in sequence, so that a pair of lasers with different modulation frequencies emit light simultaneously to achieve the function of time-division multiplexing.

[0052] After the detector receives the emitted light signal, it demodulates the detection signal using different modulation frequencies to achieve frequency-division multiplexing. This mode is a combination of time-division multiplexing and frequency-division multiplexing.

[0053] The timing control circuit delays the trigger signal of the AOM control module, so that the AOM control module is delayed to turn on under the condition of the same timing as the corresponding AOM unit. Make the lasers in different wavelength bands not emit light within a short time, thereby realizing the generation of non-emitting segments.

[0054] The output end of each AOM unit is connected to the input end of the corresponding wavelength of the wavelength division multiplexing module 106, and the wavelength division multiplexing module is used to realize multi-wavelength beam combination. The combined emitted light signal can be split and used accordingly according to requirements.

[0055] The combined laser generates interference fringes through the etalon 107 and is received by the detector 108. The collected detection signal is processed to accurately calibrate the time response relationship of each wavelength frequency.

[0056] Embodiment 2

[0057] In this embodiment, an optical switch cluster is used as the external light cutting and beam combination module. The selection requirements of the optical switch cluster are to customize an optical switch cluster that meets the wavelength conditions according to the selected laser wavelength; and the insertion loss ≤ 1.5 dB; the switching time ≤ 10 ms; the extinction ratio ≥ 19 dB; the crosstalk ≤ -70 dB.

[0058] The time-division multiplexing under the condition of continuous scanning of the laser is realized by sequentially performing switch operations on each channel in the optical switch cluster. It is mainly applied to application scenarios with low requirements for the scanning frequency.

[0059] The laser control circuit 201 generates a standard low-frequency sine sweep signal and a high-frequency sine modulation signal, and modulates the signals using an adder and a multiplier in the circuit to meet the scan and modulation gain and bias of different lasers under specific absorption wavelength conditions.

[0060] The DFB laser array 203 emits light continuously and stably under the action of the laser control circuit.

[0061] The timing control circuit 202 acts on the laser control circuit and the optical switch cluster 204 simultaneously. On the premise of ensuring that the laser trigger signal is consistent with the optical switch cluster trigger signal, the opening time of different channels of the optical switch cluster is delayed to achieve no laser passing through in a short time. The generation of the non-light-emitting section can reduce the background radiation in the application and restore the DC component in the modulation, improving the measurement accuracy of wavelength modulation spectroscopy in the application.

[0062] In an application environment with a small number of selected wavelengths, the output ends of the lasers of each wavelength are connected to different input channels of the optical switch cluster through fiber optic flanges. The timing control circuit controls the sequential switching of each channel of the optical switch cluster to achieve time-division multiplexing.

[0063] In an application environment with a larger number of selected wavelengths, the timing control circuit generates two groups of signals, namely fscan superimposed with fmod 1 and fscan superimposed with fmod 2, and injects them into the grouped lasers. The output ends of the lasers are connected to each input channel of the optical switch cluster 204 through fiber optic flanges.

[0064] The timing control circuit sequentially switches two channels with different modulation frequencies in the optical switch cluster to achieve time-division multiplexing. The detected signals are demodulated using different modulation frequencies to achieve frequency-division multiplexing. This mode is a combination of time-division multiplexing and frequency-division multiplexing.

[0065] The output end of the optical switch cluster passes through a collimator, and the laser passes through the etalon 205 to generate interference peaks. Finally, it is received by the photodetector 206, and the optical signal is converted into an electrical signal for acquisition. The wavelength modulation spectroscopy multi-wavelength time-division multiplexing frequency-time response calibration is completed.

[0066] The above are only embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A multi-wavelength wavelength modulation method based on external chopping time division multiplexing, characterized in that, Including: S1. By injecting a low-frequency superimposed high-frequency current signal into lasers with different wavelengths in a laser group, wavelength modulation of the lasers is achieved, such that the low frequency in the light output signal of the lasers is a scanning signal and the high frequency is a modulation signal; S2. The light outputs of lasers with different wavelengths are combined, and external chopping processing is performed before beam combination to achieve time-division light output of lasers with different wavelengths, Step S1 includes: S11. The low-frequency frequencies of the injected currents of lasers with different wavelengths are kept consistent, and the high-frequency frequencies are kept as the same kind; S12. The lasers in the laser group are divided into pairs in the odd-even arrangement order. The low-frequency frequencies of the injected currents of all lasers with different wavelengths are kept consistent, and the high-frequency frequencies are divided into two kinds. The high-frequency frequencies of the injected currents of the odd-numbered lasers in each pair of lasers are kept as the same kind, and the high-frequency frequencies of the injected currents of the even-numbered lasers in each pair of lasers are kept as another kind, Step S2 includes: When the high-frequency frequencies of the injected currents of all lasers with different wavelengths are kept as one kind, one-by-one time-division light output of lasers with different wavelengths is achieved through external chopping operation before beam combination; or When the high-frequency frequencies of the injected currents of all lasers with different wavelengths are kept as two kinds, pair-by-pair time-division light output of lasers with different wavelengths is achieved through external chopping operation before beam combination.

2. The multi-wavelength wavelength modulation method based on external chopping time division multiplexing according to claim 1, wherein It further includes: S3. Making the combined laser pass through an etalon to generate an interference signal, which is received by a detector, thereby achieving multi-wavelength wavelength modulation spectral frequency time response calibration.

3. The multi-wavelength wavelength modulation method based on external chopping time division multiplexing according to claim 1, wherein The injected current is composed of the superposition of a low-frequency sine / sawtooth waveform and a high-frequency sine waveform.

4. The multi-wavelength wavelength modulation method based on external chopping time division multiplexing according to claim 1, wherein By cutting off the light outputs of all lasers to generate a light-off time slot, an off-light segment is inserted between the light outputs of lasers in adjacent two bands to monitor the radiation background and restore the DC component during wavelength modulation.

5. A multi-wavelength wavelength modulation system based on external chopping time division multiplexing, which is used to implement the multi-wavelength wavelength modulation method based on external chopping time division multiplexing according to any one of claims 1 to 4, characterized in that, This system includes a laser group, a laser control circuit, a timing control circuit, an external chopping module, a chopping module controller, and a wavelength division multiplexer, The timing control circuit provides timing to the laser control circuit to control the low-frequency superimposed high-frequency current signal injected into the laser group to achieve wavelength modulation of the lasers. The light outputs of the laser group converge to the wavelength division multiplexer for beam combination, An external chopping module is provided on the light paths of the lasers with different wavelengths in the laser group. The timing control circuit also provides timing to the chopping module controller to control different external chopping modules to perform chopping processing to achieve time-division light output.

6. The multi-wavelength wavelength modulation system based on external chopping time division multiplexing according to claim 5, wherein The external chopping module is an AOM unit, and the chopping module controller is an AOM controller.

7. The multi-wavelength wavelength modulation system based on external chopping time division multiplexing according to claim 5, wherein It further includes an etalon connected to the wavelength division multiplexer and a detector connected to the etalon for multi-wavelength wavelength modulation spectral frequency time response calibration.

8. A multi-wavelength wavelength modulation system based on external chopping time division multiplexing, for implementing the multi-wavelength wavelength modulation method based on external chopping time division multiplexing according to any one of claims 1 to 4, characterized in that, This system includes a laser group composed of lasers with different wavelengths, a laser control circuit, a timing control circuit, and an optical switch cluster, The timing control circuit provides timing to the laser control circuit to control the current signal of low frequency superimposed on high frequency in the injection laser array, so as to achieve laser wavelength modulation. The light output of the laser array converges to the optical switch cluster for beam combination. The timing control circuit also provides timing to the optical switch cluster to control the optical switch cluster to perform optical switching processing, so as to achieve time-sharing light output.

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