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

Abstract

The invention discloses a multi-wavelength wavelength modulation method and system based on external cut light time division multiplexing, and the method comprises the steps: S1, injecting low-frequency superposed high-frequency current signals into lasers with different wavelengths in a laser group, achieving the wavelength modulation of the lasers, enabling the low frequencies in the light output signals of the lasers to be scanning signals, and enabling the high frequencies to be modulation signals; and S2, carrying out beam combination on the emergent light of the lasers with different wavelengths, and carrying out external light cutting treatment before beam combination so as to realize time-sharing emergent light of the lasers with different wavelengths. And S3, enabling the combined laser to pass through an etalon to generate an interference signal, and receiving the interference signal by a detector, thereby realizing multi-wavelength wavelength modulation spectrum frequency time response calibration. According to the method, the influence of unstable thermal load of the laser on wavelength calibration is avoided through an external light cutting mode, and then accurate wavelength calibration under wavelength modulation time division multiplexing is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of spectrum measurement, and in particular to a multi-wavelength modulation method and system based on external light-cutting time-division multiplexing. Background Art

[0002] In the high temperature and high pressure environment of the combustion field, accurate measurement of gas temperature and component concentration is crucial to studying combustion characteristics. Traditional direct absorption narrowband laser spectroscopy technology is easily affected by pressure broadening and pressure frequency shift under high pressure environment, which easily produces absorption line broadening and overlapping phenomenon, and it is 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, so 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 overlap and absorption feature extraction. For example, in the paper "Wavelength-modulation spectroscopy near 1.4 µm for measurements of H2O andtemperature in high-pressure and-temperature gases" (2014) published in the journal "Measurement Science and Technology", Goldenstein et al. used frequency division multiplexing with different modulation frequencies to modulate the laser absorption line at wavelengths of 7185.59 cm⁻¹ and 6806.03 cm⁻¹, respectively, demodulated the signal received by the detector, separated the harmonic signal through a low-pass filter, and used the dual-line ratio method to measure the temperature and water vapor concentration under the conditions of 700-2400 K and 2-25 atm in the shock tube environment. However, for a wider temperature and pressure measurement environment, the dual-line ratio measurement method has the problem of insufficient coverage, and there is still room for improvement.

[0005] In order to broaden the temperature measurement range and improve the measurement accuracy in high-temperature environments, Huang et al. published a paper titled "High-pressure gas temperaturesensing for exit plane of aero-engine combustor using tunable diode laser absorption spectroscopy" (2024) in the journal Microwave and Optical Technology Letters. They selected multiple absorption lines with large differences in low-state energy levels to achieve multi-wavelength measurement, and successfully measured the temperature and water vapor concentration in an environment with a maximum pressure of 0.55 MPa and a maximum temperature of 1100 K at the outlet of the aero-engine combustion chamber. However, when the pressure increases further, the direct absorption spectrum is significantly affected by the 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 complicated. In the paper "A novel methodology to directly pre-determine the relative wavelength response of DFB laser in wavelength modulation spectroscopy" (2019) published in the journal Optics Express, Liu et al. proposed a frequency-time response prediction model based on low-frequency sawtooth scanning superimposed on high-frequency sinusoidal modulation, which only requires a single fitting parameter to determine the relative wavelength response of the distributed feedback (DFB) laser. In the paper "A high-accurate and universal method to characterize the relative wavelength response (RWR) in wavelength modulation spectroscopy (WMS)" (2020) published in the journal Optics Express, Du et al. proposed a model that uses low-frequency sinusoidal scanning superimposed on high-frequency sinusoidal modulation to achieve accurate calibration of the frequency-time response of a single laser. However, in multi-wavelength scenarios, when laser current control is used to achieve time division multiplexing, the model cannot guarantee the accuracy of calibration.

[0007] This limitation mainly comes from the nonlinear characteristics of the laser that are difficult to accurately describe through a specific model at the moment of light emission. The active area inside the laser will produce thermal load changes with different injected currents, resulting in temperature fluctuations. Especially when the current suddenly jumps from 0 to a higher current, the temperature fluctuation has a particularly significant impact on wavelength stability.

[0008] The temperature response speed of the laser is slow, and the temperature fluctuation will last for a long time after the current jump (generally caused by the switch operation), resulting in irregular changes in the wavelength of the light output, which brings great challenges to the frequency-time response calibration. In addition, the nonlinear characteristics of different lasers are different and difficult to describe with a unified model, which further limits 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 light-cutting time-division multiplexing to solve the problem of inaccurate wavelength calibration caused by discontinuous laser scanning and nonlinear characteristics of the laser caused by the jump of injected laser current under time-division multiplexing.

[0010] To this end, on one hand, the present invention provides a multi-wavelength wavelength modulation method based on external light cutting time division multiplexing, including: S1, by injecting low-frequency superimposed high-frequency current signals into different wavelength lasers in the laser group, laser wavelength modulation is realized, so that the low frequency in the laser light output signal is a scanning signal and the high frequency is a modulation signal; S2, the light output of lasers of different wavelengths is combined, and external light cutting processing is performed before the combination, so as to realize the time-sharing light output of lasers of different wavelengths.

[0011] According to another aspect of the present invention, a multi-wavelength wavelength modulation system based on external light-cutting time-division multiplexing is provided, which is used to implement the above-mentioned multi-wavelength wavelength modulation method based on external light-cutting time-division multiplexing, and the system includes a laser group, a laser control circuit, a timing control circuit, an external light-cutting module, a light-cutting module controller, and a wavelength division multiplexer, wherein the timing control circuit provides timing to the laser control circuit to control the current signal of the injected low frequency superimposed high frequency injected into the laser group to achieve laser wavelength modulation, and the output light of the laser group is converged to the wavelength division multiplexer for beam combining, and an external light-cutting module is provided on the output optical path of the lasers of different wavelengths in the laser group, and the timing control circuit also provides timing to the light-cutting module controller to control different external light-cutting modules to perform light-cutting processing to achieve time-division light 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: 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; 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; Figure 3is a schematic structural diagram of a multi-wavelength wavelength modulation system according to a first embodiment of the present invention; Figure 4 is a schematic structural diagram of a multi-wavelength wavelength modulation system according to a second embodiment of the present invention; Figure 5 It is a laser waveform diagram of the multi-wavelength wavelength modulation method based on external light cutting time division multiplexing of the present invention; Figure 6 The present invention Figure 5 A magnified view of the laser waveform. DETAILED DESCRIPTION

[0018] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0019] The present invention realizes time-sharing light emission by continuously scanning the laser group through external light cutting control, thereby avoiding the problem of difficulty in accurately calibrating the light emission frequency time response and continuous working thermal load stability in the multi-wavelength wavelength modulation strategy caused by the time-sharing of the laser group driving current.

[0020] Reference Figure 1 The multi-wavelength modulation method of the present invention comprises the following steps S1-S3: S1. By injecting low-frequency superimposed high-frequency current signals into lasers of different wavelengths in the laser group, the laser wavelength modulation is realized, so that the low frequency in the laser output light signal (such as light intensity signal) is a scanning signal and the high frequency is a modulation signal, wherein the frequency value of the low-frequency scanning differs from the frequency value of the high-frequency modulation by at least 100 times; S2, combining the light from lasers of different wavelengths, and realizing time-sharing light output of lasers of different wavelengths by external light cutting operation before combining the light; S3, the combined laser beam passes through the standard tool to generate an interference signal, which is received by the detector, and finally the multi-wavelength wavelength modulation spectrum frequency time response calibration is completed.

[0021] The multi-wavelength modulation system for implementing this method is introduced below.

[0022] like Figure 2 As 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 light cutting module for external light cutting operations of lasers of different wavelengths, a light cutting module controller, a wavelength division multiplexing module for combining the output light of lasers of different wavelengths, an etalon and a detector.

[0023] The laser control circuit generates a current signal with low frequency superimposed high frequency modulation, which is injected into the laser group to meet the target absorption line, modulation depth and scanning range of each laser. The external light cutting module is connected to the light cutting module controller through an RF connection line. The timing signal of the light cutting module controller is synchronized with the laser scanning signal frequency and input by the timing control circuit.

[0024] The laser optical path connects the laser pigtail to the input end of the external light-cutting module through the optical fiber flange, and then the output end of the external light-cutting module is connected to the input ends of each band of the wavelength division multiplexer, finally forming a time-division multiplexing multi-wavelength system based on external light-cutting control.

[0025] Lasers of different wavelengths are matched with external light-cutting modules of different applicable ranges. The external light-cutting modules themselves are controlled by the light-cutting module controller. The light-cutting module controller receives external timing signals and modulates different light-cutting modules, so that the corresponding external light-cutting modules are switched on and off in turn to realize time-sharing.

[0026] The external light-cutting module is used to meet the needs of multi-wavelength measurement through wavelength division multiplexing, and to avoid the nonlinear characteristics of the light-emitting instant of the current-controlled laser during time division multiplexing, so as to improve the accuracy of frequency-time response calibration.

[0027] In the case where only time division multiplexing is used in the present invention, the scanning frequency and modulation frequency of lasers of different wavelengths are guaranteed to be completely consistent under the action of the timing control circuit. When the number of applied wavelengths is large, in order 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 realize frequency division multiplexing. By introducing frequency division multiplexing under hardware constraints, the number of lasers can be doubled under the same control conditions to meet higher multi-wavelength measurement requirements.

[0028] In order to realize the brief period of no light between the time-sharing light emission of each wavelength in the wavelength modulation spectrum, the laser control circuit delays the trigger signal of the light-cutting module controller, so that the external light-cutting module is delayed to start at the same timing as the corresponding laser, resulting in a brief moment without laser passing, thereby realizing the generation of the no light segment.

[0029] The addition of the non-emitting light segment enables the monitoring of background radiation in practical applications. This design can solve the impact of background radiation on the overall signal elevation in practical applications. At the same time, it can realize the recovery of the DC component under modulation, further improving the inversion accuracy of the modulation spectrum in practical applications.

[0030] 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.

[0031] Embodiment 1 The external light-cutting module is an AOM unit (acousto-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 meet the extinction ratio ≥ 50dB; rise time ≤ 45ns; insertion loss ≤ 2dB.

[0032] This embodiment is an application example of multi-wavelength wavelength modulation spectrum time division multiplexing using an AOM unit for external light cutting.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] When a large number of lasers are selected and a high requirement is placed on the overall response frequency of the system, the timing control circuit generates two different modulation frequencies fmod 1 and fmod 2 simultaneously under the premise of generating the same scanning frequency fscan.

[0040] The laser group is divided into two parts, one of which is injected with the signal of fscan superimposed on fmod 1 generated by the timing control circuit, and the other is injected with the signal of fscan superimposed on fmod 2.

[0041] The AOM controller controls the AOM units through external modulation, turning on a pair of AOM units connected to modulation frequencies fmod 1 and fmod 2. Similarly, the AOM units are switched on and off in pairs, so that a pair of lasers with different modulation frequencies emit light at the same time, thus realizing the function of time division multiplexing.

[0042] After the detector receives the outgoing 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.

[0043] The timing control circuit delays the trigger signal of the AOM control module, so that the AOM control module is turned on with a delay under the same timing as the corresponding AOM unit, so that lasers of different wavelength bands do not emit light for a short period of time, thereby achieving the generation of a light-free section.

[0044] The output end of each AOM unit is connected to the input end of the wavelength corresponding to the wavelength of the wavelength division multiplexing module 106, and the wavelength division multiplexing module is used to realize multi-wavelength beam combining. The combined light signal can be correspondingly split and used according to the needs.

[0045] The combined laser beams generate interference fringes through the etalon 107 and are received by the detector 108. The collected detection signals are processed to achieve accurate calibration of the frequency-time response relationship of each wavelength.

[0046] Embodiment 2 This embodiment uses an optical switch cluster as an external light cutting and beam combining 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 is ≤1.5dB; the switching time is ≤10ms; the extinction ratio is ≥19dB; and the crosstalk is ≤-70dB.

[0047] The time division multiplexing under the condition of continuous scanning of the laser is realized by switching each channel in the optical switch cluster in sequence. It is mainly used in application scenarios where the scanning frequency requirement is not high.

[0048] The laser control circuit 201 generates a standard low-frequency sinusoidal scanning signal and a high-frequency sinusoidal modulation signal, and modulates the signal using the adder and multiplier in the circuit to meet the scanning and modulation gain and bias of different lasers under specific absorption wavelength conditions.

[0049] The DFB laser group 203 emits light continuously and stably under the action of the laser control circuit.

[0050] The timing control circuit 202 acts on the laser control circuit and the optical switch cluster 204 at the same time. Under 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 for a short period of time. The generation of the non-light-emitting section can reduce background radiation in the application and realize the recovery of the DC component in the modulation, thereby improving the measurement accuracy of the wavelength modulation spectrum in the application.

[0051] In the application environment where a small number of wavelengths are selected, the output end of each wavelength laser is connected to different input channels of the optical switch cluster through an optical fiber flange. The timing control circuit controls the switching of each channel of the optical switch cluster in sequence to realize time division multiplexing.

[0052] In the application environment where more wavelengths are selected, the timing control circuit is used to generate two groups of signals, namely fscan superimposed fmod 1 and fscan superimposed fmod 2, which are injected into the grouped lasers. The laser output end is connected to each input channel of the optical switch cluster 204 through an optical fiber flange.

[0053] The timing control circuit switches the two channels with different modulation frequencies in the optical switch cluster in turn to realize time division multiplexing. The detected signal is demodulated using different modulation frequencies to realize frequency division multiplexing. This mode is a combination of time division multiplexing and frequency division multiplexing.

[0054] The output end of the optical switch cluster passes through the collimator, and the laser passes through the etalon 205 to generate an interference peak. Finally, the photodetector 206 receives the light and converts the light signal into an electrical signal for collection. The wavelength modulation spectrum multi-wavelength time division multiplexing frequency time response calibration is completed.

[0055] The above description is only an embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A multi-wavelength wavelength modulation method based on external light-cutting time division multiplexing, characterized in that: include: S1. By injecting a low-frequency superimposed high-frequency current signal into lasers of different wavelengths in the laser group, the laser wavelength modulation is realized, so that the low frequency in the laser light signal is the scanning signal and the high frequency is the modulation signal; S2. The light outputs of lasers with different wavelengths are combined, and external light cutting processing is performed before the combination to achieve time-sharing light output of lasers with different wavelengths.

2. The multi-wavelength modulation method based on external light cutting time division multiplexing according to claim 1 is characterized in that: Also includes: S3, the combined laser beam is passed through an etalon to generate an interference signal, which is received by a detector, thereby realizing the frequency-time response calibration of a multi-wavelength wavelength modulation spectrum.

3. The multi-wavelength modulation method based on external light cutting time division multiplexing according to claim 1 is characterized in that: The injected current signal is composed of a low-frequency sine / sawtooth waveform superimposed with a high-frequency sine waveform.

4. The multi-wavelength modulation method based on external light cutting time division multiplexing according to claim 1, characterized in that: By cutting off the light output of all lasers to generate a time slot without light, a light-free period is inserted between the light outputs of the lasers of two adjacent bands to monitor the radiation background and restore the DC component during the wavelength modulation process.

5. The multi-wavelength modulation method based on external light cutting time division multiplexing according to claim 1, characterized in that: Step S1 includes: S11. The low frequency of the injection current of lasers with different wavelengths is kept consistent, and the high frequency is kept the same; S12. Divide the laser groups into pairs according to the odd-even arrangement order, the low-frequency frequencies of the injection currents of all lasers with different wavelengths remain consistent, and the high-frequency frequencies are divided into two types. The high-frequency frequencies of the injection currents of the odd-numbered lasers in each pair of lasers remain the same, and the high-frequency frequencies of the injection currents of the even-numbered lasers in each pair of lasers remain another type.

6. The multi-wavelength modulation method based on external light cutting time division multiplexing according to claim 5, characterized in that: Step S2 includes: When the high frequency of the injection current of all lasers with different wavelengths is kept the same, the time-sharing light emission of lasers with different wavelengths is realized one by one by external light cutting operation before beam combining; or When the high-frequency frequencies of the injection currents of all lasers with different wavelengths are kept at two, the lasers with different wavelengths can emit light one by one in time division by external light cutting operation before beam combining.

7. A multi-wavelength wavelength modulation system based on external light cutting time division multiplexing, used to implement the multi-wavelength wavelength modulation method based on external light cutting time division multiplexing according to any one of claims 1 to 6, characterized in that: The system includes a laser group, a laser control circuit, a timing control circuit, an external light-cutting module, a light-cutting 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 laser wavelength modulation. The light output of the laser group is converged to the wavelength division multiplexer for beam combining. An external light-cutting module is provided on the light-emitting optical paths of lasers of different wavelengths in the laser group, and the timing control circuit also provides timing to the light-cutting module controller for controlling different external light-cutting modules to perform light-cutting processing to achieve time-sharing light emission.

8. The multi-wavelength modulation system based on external light-cutting time division multiplexing according to claim 7, characterized in that: The external light cutting module is an AOM unit, and the light cutting module controller is an AOM controller.

9. The multi-wavelength modulation system based on external light-cutting time division multiplexing according to claim 7, characterized in that: It also includes an etalon connected to the wavelength division multiplexer and a detector connected to the etalon, which are used for multi-wavelength wavelength modulation spectrum frequency time response calibration.

10. A multi-wavelength wavelength modulation system based on external light cutting time division multiplexing, used to implement the multi-wavelength wavelength modulation method based on external light cutting time division multiplexing according to any one of claims 1 to 6, characterized in that: The system includes a laser group consisting 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.

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