TDLAS gas detection system based on multiple modulation frequencies

Abstract

The invention discloses a TDLAS (Tunable Diode Laser Absorption Spectroscopy) gas detection system based on multiple modulation frequencies. The TDLAS gas detection system comprises a triangular wave generator, a multi-channel sine wave generator, an adder, a semiconductor laser, a detector, a multi-channel multiplier, a multi-channel low-pass filter, a multi-channel integrator and a processor, the triangular wave generator and the multi-channel sine wave generator are respectively connected with an adder, the adder is connected with a semiconductor laser, and after laser output by the semiconductor laser passes through gas, signal acquisition is carried out by a detector; the multi-channel sine wave generator is connected with the multi-channel multiplier through the multi-channel frequency multiplier; and the detector, the multi-channel multiplier, the multi-channel low-pass filter, the multi-channel integrator and the processor are connected in sequence. According to the invention, the detection speed of the TDLAS is improved, and the detection time is shortened.

Description

Technical Field

[0001] The invention relates to the technical field of optical communication, and in particular to a TDLAS gas detection system based on multiple modulation frequencies. Background Art

[0002] TDLAS is the abbreviation of Tunable Diode Laser Absorption Spectroscopy. This technology mainly utilizes the narrow line width and wavelength of tunable semiconductor lasers that change with the injected current. By modulating the wavelength of the laser, the wavelength of the laser is scanned across the absorption peak of the gas molecules to be measured. Based on the Beer-Lambert law, the gas molecules absorb the modulated laser, and the concentration of the gas molecules is measured according to the absorption amount.

[0003] Tuned semiconductor absorption spectroscopy (TDLAS) technology is a type of laser absorption spectroscopy (LAS) technology. According to the different driving forms of the laser, laser absorption spectroscopy (LAS) technology can be divided into: direct absorption method and modulation absorption method. The modulation absorption method has a higher detection efficiency than the direct absorption method because it uses the second harmonic 2f signal to filter the noise. The specific method is: a sawtooth wave driving current signal needs to be given to the laser, and a 2f harmonic signal needs to be loaded onto the driving current. It has high sensitivity and can avoid low-frequency interference. It is further divided into wavelength modulation and frequency modulation. The wavelength modulation type requires a larger tuning range, and the frequency modulation type requires a very high scanning frequency and modulation frequency. The technology is complex and the sensitivity is higher.

[0004] TDLAS needs to provide a slower triangle wave and a faster sine wave for superposition; since the output laser frequency of the semiconductor laser will change with the power, the ultimate purpose of the above modulation is to modulate the frequency of the laser and scan the absorption peak. In addition, in order to improve the sensitivity and stability of detection, it is necessary to accumulate and average the results of a single measurement, for example, accumulate the signals in 100 triangle waves and divide them by 100. Generally speaking, the longer the accumulation time, the higher the sensitivity of the detection. In order to obtain sufficient sensitivity, the above average is inevitable, so it will increase the detection time. Summary of the invention

[0005] In view of the above problems, the present invention provides a TDLAS gas detection system based on multiple modulation frequencies, which improves the detection speed of TDLAS and reduces the detection time.

[0006] The technical solution of the present invention is: a TDLAS gas detection system based on multi-modulation frequency, including a triangle wave generator, a multi-channel sine wave generator, an adder, a semiconductor laser, a detector, a multi-channel multiplier, a multi-channel low-pass filter, a multi-channel integrator and a processor;

[0007] The triangle wave generator and the multi-channel sine wave generator are connected to the adder respectively.

[0008] The adder is connected to a semiconductor laser,

[0009] After the laser output by the semiconductor laser passes through the gas, the detector collects the signal;

[0010] The multi-channel sine wave generator is connected to the multi-channel multiplier via a multi-channel frequency multiplier;

[0011] The detector, the multi-channel multiplier, the multi-channel low-pass filter, the multi-channel integrator and the processor are connected in sequence.

[0012] The processor is used to add the integrated multi-channel data and obtain the measurement level after averaging, thereby obtaining the corresponding gas concentration.

[0013] The multi-channel sine wave generator is synthesized from a single-channel sine wave generator.

[0014] The multi-channel multiplier adopts AD633 four-quadrant analog multiplier.

[0015] The multi-channel frequency multiplier adopts the ADL5391 chip.

[0016] The triangular wave generator generates a triangular wave with an amplitude of V1 to V2 and a frequency of F;

[0017] The multi-channel sine wave generator generates an amplitude of V3, a center frequency of ω0, and a phase difference of ω i The N frequencies are added together to form the following signal:

[0018]

[0019] Where f1(t) is the synthetic electrical signal function, ω i is the angular frequency of the ith channel, and t is the time;

[0020] The above sine wave and triangle wave are added, and the added signal is provided to the semiconductor laser as power. The laser power and frequency of the semiconductor laser are modulated, and after being absorbed by the gas, the detector collects a signal to form:

[0021]

[0022] Where f2(t) is the 2-fold frequency signal function, and A is the proportional factor;

[0023] At the same time, the signal generated by the multi-channel sine wave generator is passed through a multi-channel frequency multiplier to generate a reference signal:

[0024]

[0025] In the formula, f r (t) is the reference signal function;

[0026] The multi-channel multiplier multiplies equation (2) and equation (3), where the terms with the same frequency in the sine function generate 2N DC terms according to the product-to-sum-difference formula, while the terms with different frequencies do not generate DC terms, and the following signal is obtained:

[0027]

[0028] The signal in the above formula is filtered through a multi-channel low-pass filter to remove the high-frequency cosine term, and then A is integrated through a multi-channel integrator, and then the N frequencies are averaged.

[0029] The sine wave signals generated by the multi-channel sine wave generator are multiplied to synthesize the signal.

[0030] In the operation of the present invention, the triangular wave generator generates a triangular wave, and the multi-channel sine wave generator generates multiple sine wave signals, which are then superimposed with the triangular wave through an adder. The above signal is provided to a semiconductor laser, so that the laser output by the semiconductor laser generates corresponding power and frequency modulation. The laser is absorbed after passing through the gas, and because of the frequency modulation, the absorption peak is continuously scanned to generate a high-frequency signal with a frequency of 2 times. At the same time, the signal of the multi-channel sine wave generator is multiplied by a multi-channel frequency multiplier to generate a high-frequency signal with a frequency of 2 times. These two signals are input into a multi-channel multiplier, and the single-channel signal output by the detector is multiplied by the multi-channel signal output by the multi-channel frequency multiplier. After passing through a multi-channel low-pass filter, a stable multi-channel signal is obtained, which is then integrated, and the integrated multi-channel data is added, and the measurement level is obtained after averaging, corresponding to the gas concentration. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a principle block diagram of the present invention,

[0032] Figure 2 is an output signal diagram of the multi-channel sine wave generator in Example 1,

[0033] Figure 3 This is the circuit schematic diagram of a single-channel sine wave generator.

[0034] Figure 4 This is the schematic diagram of the multi-channel adder circuit.

[0035] Figure 5 is the output signal diagram of the adder in Example 1,

[0036] Figure 6 This is a diagram of one output signal of a multi-channel low-pass filter.

[0037] Figure 7 is an output signal diagram of the multi-channel sine wave generator in Example 2,

[0038] Figure 8 This is a diagram of the adder output signal in Example 2. DETAILED DESCRIPTION

[0039] The present invention Figure 1 As shown, a TDLAS gas detection system based on multi-modulation frequency includes a triangle wave generator, a multi-channel sine wave generator, an adder, a semiconductor laser, a detector, a multi-channel multiplier, a multi-channel low-pass filter, a multi-channel integrator and a processor;

[0040] The triangle wave generator and the multi-channel sine wave generator are connected to the adder respectively.

[0041] The adder is connected to a semiconductor laser,

[0042] After the laser output by the semiconductor laser passes through the gas, the detector collects the signal;

[0043] The multi-channel sine wave generator is connected to the multi-channel multiplier via a multi-channel frequency multiplier;

[0044] The detector, the multi-channel multiplier, the multi-channel low-pass filter, the multi-channel integrator and the processor are connected in sequence.

[0045] The processor is used to add the integrated multi-channel data and obtain the measurement level after averaging, thereby obtaining the corresponding gas concentration. Figure 1 Not shown in FIG.

[0046] Specifically, the triangle wave generator generates a triangle wave with an amplitude of V1 to V2 and a frequency of F. The multi-channel sine wave generator generates a triangle wave with an amplitude of V3, a center frequency of ω0, and a phase difference of ω. i The N frequencies are added together to form the following signal, which has 2N frequencies:

[0047]

[0048] Where f1(t) is the synthetic electrical signal function; ω i is the angular frequency of the ith channel; t is the time;

[0049] The sine wave and the triangle wave are added together. The specific analog circuit is a classic analog adder such as a Wien bridge oscillator circuit, or a digital signal adder. The added signal is provided to the semiconductor laser as a power source. The laser power and frequency of the semiconductor laser are modulated, and after being absorbed by the gas, a signal is collected by the detector to form a signal. The signal contains

[0050] The proportional factor A in this formula is proportional to the gas concentration.

[0051] Where, f2(t) is the 2-fold frequency signal function;

[0052] At the same time, the signal generated by the multi-channel sine wave generator passes through the multi-channel frequency multiplier to generate a reference signal:

[0053]

[0054] In the formula, f r (t) is the reference signal function;

[0055] The multiplier multiplies equation (2) and equation (3), where the terms with the same frequency in the sine function generate 2N DC terms according to the product-to-sum-difference formula, while the terms with different frequencies do not generate DC terms, and the following signal is obtained:

[0056]

[0057] The signal in the above formula is low-pass filtered to remove the high-frequency cosine term, and then A is integrated through a multi-channel integrator, and then the N frequencies are averaged.

[0058] Formula (4) multiplies the multi-path measurement function by the reference function.

[0059] The conventional TDLAS technology only has two scanning signals for the gas spectrum for the signal integration within a triangle wave time; however, after adopting the above method, the integrated signal within a triangle wave time has 2×2N scanning signals for the gas spectrum. Therefore, the number of scanning signals increases, and the sensitivity is improved; or the detection time can be shortened to the original time while keeping the sensitivity unchanged.

[0060] The present invention can also multiply the multiple channels of the above-mentioned multi-channel sine wave generator (i.e., multiply the sine wave signals generated by the multiple channels) to synthesize the following signal:

[0061]

[0062] Where f(t) is the synthetic electrical signal function;

[0063] ω1 is the first channel angular frequency, ω2 is the second channel angular frequency, ω3 is the third channel angular frequency, and ω4 is the fourth channel angular frequency. The resulting synthetic signal is then added to the triangular wave.

[0064] From the above recursive formula, it can be seen that the synthetic frequency has Multiple frequencies, the number of frequencies is N ω =2 N In this way, the number of channels can be greatly reduced, and more synthetic frequencies can be obtained. For example, N = 10 channels can only generate N by adding. ω = 10 frequencies, and by multiplication N ω =1024 frequencies.

[0065] The present invention will be further described below by way of examples.

[0066] Example 1: Figure 2 As shown, the triangular wave generator of the present invention generates a triangular wave with V1=60mV, V2=100mV, and a repetition frequency of F=100Hz. The multi-channel sine wave generator generates an amplitude of V3=10mV, a center frequency of ω0=10kHz, and a phase difference of ω1=1kHz, ω2=2kHz, ω3=3kHz…ω 10 = N = 10 frequencies of 10kHz. The above 10 frequencies are output after addition.

[0067] Specifically, the multi-channel sine wave generator is composed of Figure 3 The single-channel sine wave generator is synthesized by a plurality of TLV9064 operational amplifiers, which form an oscillator with a resistor and a feedback loop, and the frequency is controlled by the size of the capacitor. Figure 3 The output frequency is 10KHz, and different frequencies can be obtained by changing the resistance values ​​of R3, R4, and R5. Figure 4 The following figure shows a 3-channel adder. When the configuration resistor value is: R f =R1=R2=R3, the addition operation is realized:

[0068] V o =V i1 +V i2 +V i3

[0069] In the formula, R f is the feedback resistor, V o is the output voltage signal, V i1 is the first voltage signal, V i2 is the second voltage signal, V i3 It is the third voltage signal.

[0070] Similarly, this embodiment expands the above 3-channel adder to a 10-channel adder.

[0071] Similarly, a multi-channel multiplier can be obtained by sequentially multiplying single-channel integrated circuit multipliers. In this embodiment, an AD633 four-quadrant analog multiplier is preferably used for implementation.

[0072] The signal of the above triangle wave generator and the signal of the multi-channel sine wave generator are added in the adder to obtain the corresponding signal, such as Figure 5 shown.

[0073] The above signal is input into the semiconductor laser as power supply. The emission wavelength of the semiconductor laser is 1650nm, which corresponds to the absorption peak of methane gas. After triangle wave modulation, the scanning wavelength range of the laser peak is about 10GHz, covering the absorption peak of methane gas. After the above 10 wavelengths of sinusoidally modulated laser frequencies are absorbed by the gas, they are detected by the detector and output 10 modulation frequencies of 22K, 24K, 26K, ... 40K double frequency signals, the intensity of which is proportional to the gas concentration.

[0074] The signal passing through the multi-channel frequency multiplier also has 10 frequency multiplied signals with modulation frequencies of 22K, 24K, 26K, ... 40K respectively, and the amplitude of the signal is constant.

[0075] The frequency multiplier is implemented by multiple single-channel frequency multiplication integrated circuits, and is implemented by using the ADL5391 chip. The specific method is: the input signal is divided into two paths and input into the ADL5391 chip, and multiplication is performed to obtain a high-frequency signal, and then the high-frequency signal is filtered out through a filter.

[0076] The signal output by the detector is multiplied by one of the signals in the multi-channel frequency multiplier mentioned above, and then the signal is obtained after low-pass filtering. Figure 6 The signal shown;

[0077] Repeat the above multiplication process, switch the signal frequency in the multi-channel multiplier, and obtain 10 groups of signals in total. Due to the presence of noise, the above 10 groups of signals are slightly different. After passing through the multi-channel integrator, V1, V2, V3…, V 10 There are 10 voltage values ​​in total. Add the above voltage values ​​and divide by 10 to get the average value. This average value is proportional to the gas concentration.

[0078] Embodiment 2: The triangular wave generator of the present invention generates a triangular wave with V1=60mV, V2=100mV, and a repetition frequency of F=100Hz. The multi-channel sine wave generator generates N=4 frequencies with an amplitude of V3=10mV, a center frequency of ω0=10KHz, and a phase difference of ω1=1KHz, ω2=2KHz, ω3=3KHz, and ω4=4KHz. The above 4 frequencies are output after multiplication.

[0079] The above 4 frequencies are converted into products and differences according to formula (5) to become ω=ω0±ω1±ω2±ω3±ω4=10KHz±1KHz±2KHz±3KHz±4KHz, and there are 16 frequencies in total: 0KHz, 8KHz, 6KHz, 14KHz, 4KHz, 12KHz, 10KHz, 18KHz, 2KHz, 10KHz, 8KHz, 16KHz, 6KHz, 14KHz, 12KHz, 20KHz. After removing repeated frequencies and frequencies of zero, there are 10 frequencies left, such as Figure 7 shown.

[0080] The signal of the triangle wave generator and the signal of the multi-channel sine wave generator are added in the adder to obtain the following Figure 8 signal shown in.

[0081] The remaining steps are the same as the above example. This embodiment uses 4 channels to generate 10 frequencies, reducing the number of channels of the multi-channel sine wave generator and obtaining more synthetic frequencies.

[0082] The present invention reduces the average time of TDLAS and improves the detection speed.

[0083] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be covered by the scope of the claims of the present invention.

Claims

1. A TDLAS gas detection system based on multi-modulation frequency, characterized in that: Including triangle wave generator, multi-channel sine wave generator, adder, semiconductor laser, detector, multi-channel multiplier, multi-channel low-pass filter, multi-channel integrator and processor; The triangle wave generator and the multi-channel sine wave generator are connected to the adder respectively. The adder is connected to a semiconductor laser, After the laser output by the semiconductor laser passes through the gas, the detector collects the signal; The multi-channel sine wave generator is connected to the multi-channel multiplier via a multi-channel frequency multiplier; The detector, the multi-channel multiplier, the multi-channel low-pass filter, the multi-channel integrator and the processor are connected in sequence. The processor is used to add the integrated multi-channel data and obtain the measurement level after averaging, thereby obtaining the corresponding gas concentration.

2. A TDLAS gas detection system based on multi-modulation frequency according to claim 1, characterized in that: The multi-channel sine wave generator is synthesized from a single-channel sine wave generator.

3. A TDLAS gas detection system based on multi-modulation frequency according to claim 1, characterized in that: The multi-channel multiplier adopts AD633 four-quadrant analog multiplier.

4. A TDLAS gas detection system based on multiple modulation frequencies according to claim 1, characterized in that: The multi-channel frequency multiplier adopts the ADL5391 chip.

5. A TDLAS gas detection system based on multi-modulation frequency according to claim 1, characterized in that: The triangular wave generator generates a triangular wave with an amplitude of V1 to V2 and a frequency of F; The multi-channel sine wave generator generates an amplitude of V3, a center frequency of ω0, and a phase difference of ω i The N frequencies are added together to form the following signal: Where f1(t) is the synthetic electrical signal function, ω i is the angular frequency of the ith channel, and t is the time; Adding the sine wave and the triangle wave, and providing the added signal as a power source to the semiconductor laser; The laser power and frequency of the semiconductor laser are modulated, absorbed by the gas, and collected by the detector to form a signal: Where f2(t) is the 2-fold frequency signal function, and A is the proportional factor; At the same time, the signal generated by the multi-channel sine wave generator is passed through a multi-channel frequency multiplier to generate a reference signal: In the formula, f r (t) is the reference signal function; The multi-channel multiplier multiplies equation (2) and equation (3), where the terms with the same frequency in the sine function generate 2N DC terms according to the product-to-sum-difference formula, while the terms with different frequencies do not generate DC terms, and the following signal is obtained: The signal in the above formula is filtered through a multi-channel low-pass filter to remove the high-frequency cosine term, and then A is integrated through a multi-channel integrator, and then the N frequencies are averaged.

6. A TDLAS gas detection system based on multiple modulation frequencies according to claim 1, characterized in that: The sine wave signals generated by the multi-channel sine wave generator are multiplied to synthesize the signal.

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