CN208013060U - A kind of more gas detecting systems of wave-length coverage and wavelength continuously adjustable - Google Patents

Abstract

A multi-gas detection system with continuously tunable wavelength range and wavelength, including a fiber laser, an acousto-optic modulator, a collimator, a gas absorption cell, a photodetector, a spectral signal analysis module, a laser drive control module, and a radio frequency drive system; A fiber laser, an acousto-optic modulator, a collimator, a gas absorption cell and a photodetector are arranged in sequence; the fiber laser is connected to the output end of the laser drive control module, and the photodetector is connected to the spectral information analysis module; the radio frequency drive system is connected to the acousto-optic The modulator is connected with the spectral signal analysis module; the acousto-optic modulator is generated with the frequency synthesizer, and the frequency synthesizer is connected with the microprocessor in the spectral signal analysis module. The system has fast analysis speed; fast conversion speed; the detection laser wavelength can be continuously tuned; real-time online detection can also be used for remote analysis and monitoring; good detection stability and accuracy; low cost, only one tunable laser can be completed Detection of most gas components.

Description

A wavelength range and wavelength continuously tunable multi-gas detection system

technical field

The utility model relates to a system for detecting multiple gases, which adopts a combination structure of a continuously tunable fiber laser with a wavelength range and an optical fiber coupled acousto-optic modulator to realize multi-gas detection with a continuously tunable wavelength, and belongs to the technical field of laser gas detection.

Background technique

In the field of gas spectral absorption, tunable diode laser absorption spectroscopy (Tunable diode laser absorption spectroscopy, TDLAS) is a high-sensitivity, high-resolution and fast-response gas detection technology, widely used in gas monitoring in industrial processes, ambient atmosphere Detection and scientific research and other fields.

When detecting the concentration of multiple gas components, it is desirable to be able to simultaneously measure the spectral absorption lines of multiple gas components. The disadvantage of existing infrared tuned lasers is that the tuning range is limited, generally on the order of 200cm ^-1 , and the fast current tuning width is about 1-2cm ^-1 . In actual gas detection applications, the detection of gas is usually achieved by current tuning. However, the tuning range of the existing tunable lasers is small, and usually cannot cover the laser wavelengths absorbed by all gases; and because the temperature tuning cannot achieve fast tuning, the method of temperature tuning the semiconductor laser It takes a long time to realize the detection of multiple spectral absorption lines. In practical applications, it is generally necessary to work with multiplex technology for each gas that needs to be detected. At present, the multiplex technology used for multi-component detection technology can be divided into three types, namely time division multiplex technology and wavelength division multiplex technology. And frequency division modulation multiplexing technology.

Time-division multiplexing technology is the most direct method. It selects the measurement beam corresponding to the wavelength of the measured gas from a series of lasers and introduces it into the detection optical path to sequentially detect the concentration of various gases; WDM technology is to combine different wavelengths The combined laser beams are combined by a wavelength division multiplexer or a fiber optic beam combiner, and the combined multiple beams pass through the absorption cell along the same optical path. Receiving, to realize simultaneous detection of multiple groups of gases; frequency modulation technology is to use the narrow-band filtering characteristics of phase-sensitive detection to modulate multiple lasers with sinusoidal current signals of different frequencies, and then combine the laser beams and pass along the same optical path The gas absorption cell is finally focused on the same detector. Then, by demodulating the signal at the corresponding frequency, the contribution of each wavelength laser can be extracted and separated from the combined beam signal, so as to analyze the concentration of the corresponding gas component.

Wavelength division multiplexing technology solves the problem of slow detection speed of simultaneous detection of multi-component gases, but in fact this method has not improved much compared with multiple sets of instrument detection systems, but only shares a gas absorption cell and detection Just the route. Moreover, beam combiners or wavelength division multiplexers and separate optical splitting elements also reduce the compactness and stability of the system, and the use of diffractive elements may also introduce noise and deviation. The main problems of the frequency division modulation multiplexing technology are the need to eliminate the cross-interference between the detection channels and the limitation of the number of multiple lasers. Compared with the above two methods, the time-division multiplexing technology is very simple in the detection system structure. It only needs one detector, one detection device and one signal processing system, and there is no limit to the number of multi-components in the detected gas. However, most of the traditional time division multiplexing technology uses a mechanical structure to convert the beam source, which greatly reduces the measurement efficiency, and this method makes the stability of the light intensity poor. Moreover, the traditional time-division multiplexing technology cannot continuously, quickly and accurately detect the multi-component gas concentration at a certain moment due to the limitation of the laser source.

Most of the multi-component gas detection technologies currently used are composed of several DFB lasers (distributed feedback lasers) with different central wavelengths to form a combined multi-wavelength light source, and the multiple detection lasers emitted by the combined multi-wavelength light source are wavelength division multiplexed. After being combined by a fiber optic combiner or an optical fiber combiner, the multi-mode optical fiber in front of the pool transmits the beam-combining detection laser from the light entrance and introduces it into the gas absorption cell containing the gas to be detected. The beam-combining detection laser passes through multiple After the second reflection, it is emitted from the light outlet, and then guided by the multimode optical fiber behind the pool to the detector. The detector converts the optical signals of various gas absorption lines into electrical signals, and obtains the gas absorption signals through the operation of the preamplifier and lock-in amplifier. The first and second harmonics, and finally use the harmonic intensity to achieve the measurement of gas concentration.

Most of the above-mentioned existing multi-gas detection technologies use the same principle, using a wavelength division multiplexer or a fiber combiner to couple multiple optical paths into a single-core multi-mode optical fiber, and the optical fiber leads to a gas absorption cell for simultaneous absorption of multi-wavelength lasers. detection.

However, the above method has the following defects when realizing multi-gas simultaneous detection:

1. After multiple detection lasers with different wavelengths pass through the wavelength division multiplexer, they will be attenuated to varying degrees, and the more laser beams with different wavelengths to be coupled, the more serious the attenuation will be;

2. When multiple laser light sources are used at the same time, they occupy a large volume and have strict requirements on the detection environment;

3. It is greatly affected by the ambient temperature, and the wavelength of the semiconductor laser output laser is also affected by the temperature, which will cause certain errors;

4. The wavelength of the detection laser is discontinuous. On the basis of not changing the components of the instrument, it can detect gas fixation, and the portability of the instrument is poor.

Utility model content

The utility model aims at the deficiencies in the existing multi-gas component detection technology, and provides a multi-gas detection system with simple structure, adaptable to complex measurement environment, high gas detection sensitivity and flexible wavelength range and continuously tunable wavelength.

The wavelength range of the utility model and the multi-gas detection system with continuously tunable wavelength adopt the following technical solutions:

The system includes fiber laser, acousto-optic modulator, collimator, gas absorption cell, photodetector, spectral signal analysis module, laser drive control module and radio frequency drive system; fiber laser, acousto-optic modulator, collimator, The gas absorption cell and photodetector are arranged in sequence; the fiber laser is connected to the output end of the laser drive control module, and the photodetector is connected to the spectral information analysis module; the radio frequency drive system is connected to the acousto-optic modulator and the spectral signal analysis module; the acousto-optic modulation Generated by a frequency synthesizer and a frequency synthesizer, and the frequency synthesizer is connected with the microprocessor in the spectral signal analysis module.

The laser drive control module controls the wavelength range of the laser emitted by the fiber laser. The laser emitted by the fiber laser is modulated into the wavelength corresponding to the gas composition through the acousto-optic modulator, and then introduced into the gas absorption pool through the collimator, and then received by the laser detector. The detector converts the received optical signal into an electrical signal and transmits the electrical signal to the spectral information analysis module. The spectral analysis module analyzes and processes the received electrical signal to obtain an electrical signal related to the gas concentration, which is used to characterize the gas in the absorption cell. Concentration information for various component gases.

The fiber laser is a wavelength range tunable laser.

The laser drive control module includes a control scanning circuit, a constant temperature circuit and a protection circuit, the control scanning circuit makes the wavelength of the output laser scan within the wavelength range, the temperature control circuit is used to stabilize the temperature of the laser, and eliminate the influence of temperature on the wavelength of the output laser .

The gas absorption cell can adjust the optical path, and is provided with an air inlet and an air outlet.

The spectral signal analysis module includes a preamplifier, a lock-in amplifier, an analog-to-digital converter and a microprocessor connected in sequence.

The multi-gas detection system of the present invention also includes terminal equipment, and the spectral signal analysis module transmits the gas concentration data to the terminal equipment remotely through wired or wireless means.

The utility model adopts a fast wavelength range continuously adjustable laser light source combined with an acousto-optic modulator to adjust the wavelength of the detection laser, and can realize a wide range of wavelength continuously adjustable, thereby realizing the purpose of detecting most gas components. The continuity and ultra-wide range of wavelength adjustment endow this detection system with great flexibility and portability, which can be adapted to the detection of almost all component gases.

Compared with other light-splitting techniques, the light-splitting principle of the acousto-optic modulation technology proposed by the utility model has the advantages of high resolution, firm all-cured device, rapid light-splitting and arbitrary wavelength switching. The advantages of the utility model are mainly reflected in:

1. The detection laser wavelength range can be continuously tuned, covering the corresponding wavelengths of most gases;

2. High resolution, strong all-cured device, rapid light splitting and arbitrary wavelength switching

3. Using AOTF technology for wavelength selection, the wavelength can be selected arbitrarily, and the spectral scanning speed is fast;

4. Time-sharing reuse of one optical path of the same light source and the same gas chamber can not only reduce the volume of the instrument, reduce moving parts, reduce the complexity of the system, and achieve better automatic control;

5. Real-time online detection, and remote analysis and monitoring;

6. Good detection stability and accuracy, avoiding the influence of temperature;

7. The cost is low, the number of light sources is reduced, the design of the gas chamber and the optical path are simplified, the volume of the analyzer is reduced, and the consumption of sample gas and the energy consumption of the temperature control system are reduced.

Description of drawings

Fig. 1 is a schematic diagram of the wavelength range and the multi-gas detection system with continuously tunable wavelength of the present invention.

In the figure: 1. Spectral signal analysis module, 2. RF drive system, 3. Laser drive control module, 4. Fiber laser, 5. Acousto-optic modulator (AOTF), 6. Collimator (collimating lens), 7 .Gas absorption cell, 8. Photodetector, 9. Preamplifier, 10. Lock-in amplifier, 11. Digital-to-analog converter, 12. Control scanning circuit, 13. Constant temperature circuit, 14. Protection circuit, 15. Microprocessing 16. Frequency synthesizer.

Detailed ways

The wavelength range and wavelength continuously tunable multi-gas detection system of the present invention, as shown in Fig. A detector 8, a spectral signal analysis module 1, a laser drive control module 3 and a radio frequency drive system 2.

The gas absorption pool 7 is provided with an air inlet and an air outlet. A collimator 6 is arranged in front of the gas absorption cell 7, an acousto-optic modulator 5 is arranged in front of the collimator 6, a fiber laser 4 is arranged in front of the acousto-optic modulator 5, and the fiber laser 4 is connected to the laser drive control module 3 , the acousto-optic modulator 5 is connected to the radio frequency drive system 2, a photodetector 8 is arranged behind the gas absorption cell 7, and the photodetector 8 is connected to the spectral signal analysis module 1.

The spectral signal analysis module 1 includes a preamplifier 10 , a lock-in amplifier 11 , an analog-to-digital converter 12 and a microprocessor 15 connected in sequence.

Laser drive control module 3 is used to control the laser wavelength range that fiber laser 4 sends, and laser drive control module 3 includes control scan circuit (current scan circuit) 12, constant temperature circuit 13 and protection circuit 14, and wherein control scan circuit 12 makes output laser The wavelength of the laser is scanned within the emitted laser wavelength range. The constant temperature circuit 13 is used to stabilize the temperature of the laser 4 and avoid the influence of the temperature on the wavelength of the output laser.

In the multi-gas component analysis of the utility model, the main component affecting the frequency of the detected light is the radio frequency drive system 2, which mainly includes a radio frequency generation channel, a power amplifier and a radio frequency switch. The microprocessor 15 accepts control and interprets various commands issued by the host computer, and controls the working state of the entire radio frequency drive system 2 by executing the commands. The radio frequency generating channel includes a signal generator, a radio frequency transformer, an LC low-pass filter and a low noise amplifier, and the switching, frequency and amplitude of the output signal of the signal generator are all controlled by the microprocessor 15 . The radio frequency signal is filtered and amplified and output, and then amplified by the power amplifier to the power range required to drive the acousto-optic modulator 5. By switching the radio frequency switch to select a suitable piezoelectric transducer, the radio frequency signal is loaded into the AOTF crystal, and the pairing is completed. Controlled by the acousto-optic modulator 5, laser light with a required wavelength is modulated for detection and analysis of the concentration of gas components.

The main component of the utility model is an acousto-optic modulator (acousto-optic tunable filter, AOTF) 5 . Acousto-optic modulator 5 is based on the principle of the acousto-optic effect. When a beam of wide spectrum passes through a crystal with optical elastic elements vibrating at high frequency, monochromatic light of a certain wavelength will be diffracted inside the crystal, and will be emitted from the crystal at a certain angle transmitted through. When the vibration frequency of the crystal changes, the wavelength of the diffracted light also changes. In practical applications, the radio frequency signal is converted into ultrasonic waves inside the crystal through the piezoelectric transducer on the surface of the crystal.

The signal used for modulation by the acousto-optic modulator 5 is generated by a frequency synthesizer 16 , and the frequency synthesizer 16 is connected with a microprocessor 15 . The modulation frequency is that the frequency (f1, f2, ... fk) at which the gas absorbs the infrared light most strongly is 5KHz-40KHz.

The demodulation frequencies are nf1, nf2, nf3...nfk, preferably n=2, that is, the second harmonic demodulation technology.

According to the vector composite relationship, the following relationship can be obtained:

In the formula:

f _e→o — RF driving frequency of incident e light diffracted into outgoing o light

f _o→e — RF driving frequency of incident o light diffracted into outgoing e light

λ—diffraction wavelength

θ _i —Incident light angle

θ _a —Ultrasonic incident angle

n _o (λ)—the refraction angle of o light

n _e (λ, θ)—refractive index of e light

v _a — ultrasonic velocity, determined by AOTF material

The tuning relationship between AOTF RF driving frequency and diffraction wavelength is derived:

θ _i —incident angle

θ _d —Diffraction angle

V—speed of sound

n _i — function about θ _i

n _d — a function of θ _d

f—sound frequency

λ—vacuum wavelength

It can be seen from the above formula that after the AOTF processing is completed, the diffraction wavelength is only controlled by the RF driving frequency. Changing the RF drive frequency can change the response diffraction wavelength.

The concrete process that the system of the present invention carries out multi-gas detection is as follows:

The utility model uses different modulation frequencies (f1, f2, ... fk) to modulate the driving signal of the laser, and uses different demodulation frequencies (nf1, nf2, ... nfk) as reference signals to demodulate in the lock-in amplifier . k is k gas components. Different modulation frequencies are used to detect different gases, the light of a specific wavelength is screened out by the AOTF, and then collimated by the collimator 6 to match the optical mode of the gas absorption cell 7 .

Use the wavelength scanning control circuit 12 to enable the fiber laser 4 to perform wavelength scanning within the tunable range, and then use the electrical tuning of the acousto-optic modulator 5 to filter out the required gas from the wide spectrum of the light source of the complex fiber laser 4. spectrum of wavelengths. The light of the specific absorption wavelength of the filtered gas interacts with the measured gas, the light intensity attenuates, and then irradiates the photodetector 8, and the photoelectric signal received by the photodetector 8 passes through the preamplifier 10, lock-in amplifier 11 and The analog-to-digital converter 12 performs pre-amplification filtering and denoising and weak signal processing (signal amplification and analog-to-digital conversion), and then obtains electrical signals related to gas concentration, so as to characterize the gas concentration of various components in the gas absorption cell concentration information.

The microprocessor 15 in the spectral signal analysis module 1 transmits the gas concentration data to the terminal device remotely through wired or wireless means, and performs frequency modulation of the next gas component.

The utility model adopts a fast wavelength range continuously adjustable laser light source combined with an acousto-optic modulator to adjust the wavelength of the detection laser, and can realize a wide range of wavelength continuously adjustable, thereby realizing the purpose of detecting most gas components. The continuity and ultra-wide range of wavelength adjustment endow this detection system with great flexibility and portability, which can be adapted to the detection of almost all component gases.

Load the laser drive control module 3 with a certain modulation frequency (f1, f2, ... fk), let the center of the optical path pass through the acousto-optic modulator 5, apply an electrical signal to the radio frequency drive system 2, and control the acousto-optic modulator 5 to work, That is to screen the required working infrared fundamental frequency, the outgoing beam is coupled to the gas absorption cell 7 by the collimator 6, and after reflection, it is incident on the photodetector 8, and the photoelectric signal received by the photodetector 8 is pre-amplified and filtered. Noise, the resulting transmission signal is sent to the lock-in amplifier 11. According to the harmonic order to be detected, the lock-in amplifier 11 selects different demodulation frequencies nfk for demodulation according to the modulation frequency. The demodulated signal is sent to the microprocessor 15 through the three-channel ADC converter 12, and the embedded algorithm program completes the concentration inversion and completes the deduction of the gas concentration. When other gas components are to be detected, the same detection channel is time-divisionally multiplexed by modulating different frequencies and screening infrared light of different wavelengths to detect another gas component.

The utility model has the following characteristics:

1. Using a fiber laser with continuously tunable wavelength range and acousto-optic laser to provide detection laser, the wavelength coverage is wide, and there are many types of gases that can be detected;

2. The laser light source has a temperature control system, which keeps the temperature of the laser light source at a certain suitable temperature, avoids the influence of temperature on the spectrum wavelength, and the utility model reduces the volume of the analyzer, reduces the energy consumption of the temperature control system, and the temperature Control is also more even and stable.

3. The utility model greatly simplifies the design of the optical system. Traditional TDLAS analyzers use one gas cell or one optical path for one analyte. The utility model uses one light path, does not need multiple light sources, and does not need rotating parts, which reduces the mechanical complexity of the system, saves the number of air chambers, and simplifies the design of the air chamber.

4. The gas chamber adopts the multi-reflection principle to increase the optical path, thereby improving the detection sensitivity of the system. Replacing the gas chamber or changing the length of the gas chamber can achieve different range and sensitivity requirements;

5. The system uses AOTF technology for wavelength selection, as long as its gas characteristic absorption line can be detected within the wavelength range selectable by AOTF, it does not target a certain fixed gas component and has strong portability.

6. Speed up the detection;

7. The weak signal processing technology of modulation and demodulation and phase-locked amplification is used to achieve the effect of reducing flicker noise (1/f noise).

Claims (5)

1. A multi-gas detection system with continuous tunable wavelength range and wavelength, characterized in that it includes a fiber laser, an acousto-optic modulator, a collimator, a gas absorption cell, a photodetector, a spectral signal analysis module, and a laser drive control Module and radio frequency drive system; fiber laser, acousto-optic modulator, collimator, gas absorption cell and photodetector are arranged in sequence; the fiber laser is connected to the output end of the laser drive control module, and the photodetector is connected to the spectral information analysis module; The radio frequency driving system is connected with the acousto-optic modulator and the spectral signal analysis module; the acousto-optic modulator is generated with a frequency synthesizer, and the frequency synthesizer is connected with the microprocessor in the spectral signal analysis module. 2. The wavelength range and wavelength continuously tunable multi-gas detection system according to claim 1, characterized in that: the fiber laser is a wavelength range tunable laser. 3. The wavelength range and wavelength continuously tunable multi-gas detection system according to claim 1, characterized in that: the laser drive control module includes a control scanning circuit, a constant temperature circuit and a protection circuit, and the control scanning circuit makes the output laser The wavelength is scanned in the wavelength range, and the temperature control circuit is used to stabilize the temperature of the laser and eliminate the influence of temperature on the wavelength of the output laser. 4. The wavelength range and wavelength continuously tunable multi-gas detection system according to claim 1 is characterized in that: the spectral signal analysis module includes a preamplifier, a lock-in amplifier, an analog-to-digital converter and a micro processor. 5. The wavelength range and wavelength continuously tunable multi-gas detection system according to any one of claims 1 to 4, characterized in that it also includes terminal equipment.