CN111504945B - Optical fiber photo-thermal gas sensing device and method - Google Patents
Optical fiber photo-thermal gas sensing device and method Download PDFInfo
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- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N2021/3196—Correlating located peaks in spectrum with reference data, e.g. fingerprint data
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Abstract
The invention discloses and provides an optical fiber photo-thermal gas sensing device and method without background noise signals after demodulation of first harmonic signals. The optical fiber photo-thermal gas sensing device comprises a middle-infrared pumping laser (1), a near-infrared detection laser (2), a coupling component, an infrared broadband hollow fiber (3) and a photo-thermal signal detection demodulation component (4), wherein the middle-infrared pumping laser (1) and the near-infrared detection laser (2) are connected with the infrared broadband hollow fiber (3) through the coupling component, the infrared broadband hollow fiber (3) is connected with the photo-thermal signal detection demodulation component (4) through the coupling component, and the infrared broadband hollow fiber (3) is filled with gas to be detected. The invention is suitable for the field of optical fiber photo-thermal gas sensing.
Description
Technical Field
The invention relates to an optical fiber photo-thermal gas sensing device and an optical fiber photo-thermal gas sensing method.
Background
The absorption spectrometry is a common gas measurement method, and when light with a specific wavelength passes through a gas to be measured, a part of light energy is absorbed by the gas to be measured, the absorption energy is positively related to the concentration of the gas to be measured, and the absorbance is a function of the wavelength of incident light. The wavelength modulation spectrum technology WMS carries out high-frequency modulation on the wavelength of incident light, the transmitted light signal of the incident light contains a series of harmonic signals after gas absorption, the gas concentration information can be obtained by demodulating the harmonic signals, and meanwhile, 1/f noise is reduced by adopting high-frequency detection. If the transmitted light signal is demodulated at the modulation frequency, a first harmonic signal 1f,1f signal has a strong background signal, because wavelength modulation generally adopts a method of modulating the driving current of a laser diode, the laser intensity is modulated at the same time of modulating the wavelength, which is called residual intensity modulation, so that a strong background is brought to the 1f absorption signal. While 2f is less affected by background signals, WMS technology typically demodulates 2f harmonic signals of transmitted light, but WMS-2f signal intensities are typically much less than 1 f.
Disclosure of Invention
The invention aims to solve the technical problem of overcoming the defects of the prior art and providing an optical fiber photo-thermal gas sensing device and method without background signals after demodulation of first harmonic signals.
The optical fiber photo-thermal gas sensing device comprises a middle infrared pumping laser, a near infrared detection laser, a coupling component, an infrared broadband hollow fiber and a photo-thermal signal detection demodulation component, wherein the middle infrared pumping laser and the near infrared detection laser are connected with the infrared broadband hollow fiber through the coupling component, the infrared broadband hollow fiber is connected with the photo-thermal signal detection demodulation component through the coupling component, and the infrared broadband hollow fiber is filled with gas to be detected.
The coupling assembly comprises a near infrared-middle infrared double-way coupling assembly and a near infrared single-way coupling assembly, the middle infrared pumping laser and the near infrared detection laser are connected with the infrared broadband hollow fiber through the near infrared-middle infrared double-way coupling assembly, and the infrared broadband hollow fiber is connected with the photo-thermal signal detection demodulation assembly through the near infrared single-way coupling assembly.
The near infrared-middle infrared double-way coupling assembly comprises a first air chamber, a dichroic mirror, a first focusing lens, a second focusing lens and a first optical fiber collimating mirror, wherein the first air chamber is connected and arranged at the input end of an infrared broadband hollow fiber, the dichroic mirror is arranged corresponding to the first air chamber, the emitting end of a middle infrared pumping laser, the first focusing lens and the dichroic mirror are arranged corresponding to each other in sequence, the input end of the first optical fiber collimating mirror is connected with the output end of a near infrared detection laser through optical fibers, the output end of the first optical fiber collimating mirror, the second focusing lens and the dichroic mirror are arranged corresponding to each other in sequence, the near infrared single-way coupling assembly comprises a second air chamber, a collimating lens and a near infrared optical fiber coupling mirror, the second air chamber is connected with the output end of the infrared broadband hollow fiber through optical fibers, the second air chamber, the collimating lens and the input end of the near infrared optical fiber coupling mirror are arranged corresponding to each other in sequence, and the output end of the optical fiber coupling mirror is connected with a near infrared detection optical fiber demodulation optical fiber assembly through a near infrared demodulation optical fiber.
The near infrared-middle infrared double-way coupling assembly comprises a first pair of tail gas chambers and an infrared broadband optical fiber combiner, wherein the input ends of the infrared broadband hollow optical fibers are adaptively inserted into the first pair of tail gas chambers, the input ends of the infrared broadband optical fiber combiner are respectively connected with the output ends of the middle infrared pumping lasers and the output ends of the near infrared detection lasers, the output ends of the infrared broadband optical fiber combiner are connected with a first output optical fiber, the output optical fiber is adaptively inserted into the first pair of tail gas chambers and is correspondingly arranged with the input ends of the infrared broadband hollow optical fibers, the near infrared single-way coupling assembly comprises a second pair of tail gas chambers and a second output optical fiber, the output ends of the infrared broadband hollow optical fibers are adaptively inserted into the second pair of tail gas chambers, the input ends of the second output optical fiber are adaptively inserted into the second pair of tail gas chambers and are correspondingly arranged with the output ends of the infrared broadband hollow optical fibers, and the second output ends of the infrared broadband hollow optical fibers are in demodulation mode, and the output ends of the second output optical fiber are correspondingly arranged with the output signals of the infrared broadband hollow optical fiber demodulation assembly.
The near-infrared photoelectric detector of the photo-thermal signal detection demodulation component is connected with the near-infrared single-path coupling component.
The invention further comprises a polarization controller, an optical fiber beam splitter, a piezoelectric ceramic ring and a second optical fiber beam combiner, wherein the input end of the optical fiber beam splitter is connected with the output end of the near infrared detection laser in an optical fiber mode, the polarization controller is adaptively arranged between the optical fiber beam splitter and the near infrared detection laser, one output end of the optical fiber beam splitter is connected with the near infrared-mid infrared double-path coupling component in an optical fiber mode, the other output end of the optical fiber beam splitter is connected with the photo-thermal signal detection demodulation component through a third output optical fiber, one section of the third output optical fiber is adaptively wound on the piezoelectric ceramic ring, the input end of the second optical fiber beam combiner is connected with the third output optical fiber and the near infrared single-path coupling component respectively, and the output end of the second optical fiber beam combiner is connected with the input end of the near infrared photoelectric detector.
The photo-thermal signal detection demodulation assembly further comprises an electric signal branching device, the input end of the electric signal branching device is connected with the output end of the near infrared photoelectric detector, one output end of the electric signal branching device is connected with a low-pass filter, a PID controller and a piezoelectric ceramic driver in sequence, the piezoelectric ceramic driver is connected with the piezoelectric ceramic ring, the other output end of the electric signal branching device is connected with a lock-in amplifier, a data acquisition card and a laser controller in sequence, and the laser controller is connected with the middle infrared pumping laser.
According to the optical fiber photo-thermal gas sensing method, pumping light and detection light are combined to pass through gas to be detected, the pumping light excites the absorption of the gas to be detected, photo-thermal effect is generated, the detection light measures refractive index change generated after the gas absorbs pumping laser, the phase of the detection light periodically changes after passing through a medium of the gas to be detected, which generates the photo-thermal effect, phase information of the detection light is collected, and demodulation is carried out, so that a photo-thermal spectrum first harmonic signal is obtained.
The wavelength or power of the pump light is periodically modulated, and the wavelength of the probe light is kept fixed and far away from the absorption line of the gas to be detected.
The peak-to-peak value of the photothermal spectrum first harmonic signal is proportional to the gas concentration within the linear range of the system.
The invention has the beneficial effects that: according to the invention, the pump-detection dual-light source configuration is adopted, the refractive index of the gas to be detected periodically changes due to the photo-thermal effect after the gas to be detected absorbs the energy of the periodically modulated pump light, when the light path of the detection light coincides with the pump light, the phase of the detection light changes periodically due to the change of the refractive index of the gas, and the concentration of the gas to be detected can be obtained by demodulating the phase information of the detection light. Compared with the wavelength modulation absorption spectrum technology, the method avoids directly measuring the transmitted light intensity after gas absorption, so that the demodulation signal of the detection light phase is not influenced by the residual intensity modulation of the pump light.
The laser, the photoelectric detector, the optical fiber component and the like adopted in the invention have the advantages of low price and mature technology, and simultaneously adopt the infrared wide-band hollow fiber to simultaneously limit the pumping light, the detection light and the gas to be detected in the micron-sized hollow fiber, thereby improving the energy density of the pumping light and greatly improving the detection sensitivity.
The infrared wide-band hollow fiber also has the advantages of wide transmission spectrum range, small transmission loss and single-mode transmission maintenance.
Drawings
FIG. 1 is a schematic view of the overall structure of the present invention;
FIG. 2 is a schematic illustration of a connection structure of a near infrared-mid infrared dual-path coupling assembly of the present invention;
FIG. 3 is a schematic illustration II of a junction of a near infrared-mid infrared two-way coupling assembly of the present invention;
FIG. 4 is a schematic illustration of a connection structure of a near infrared one-way coupling assembly of the present invention;
FIG. 5 is a schematic illustration II of the connection structure of the near infrared one-way coupling assembly of the present invention;
FIG. 6 is a schematic diagram of a connection structure of the optical-thermal signal detection demodulation component according to the present invention;
FIG. 7 is a schematic illustration of the mechanism of a mid-infrared hollow core fiber in accordance with the present invention;
FIG. 8 is a schematic illustration II of the mechanism of the mid-IR hollow fiber of the present invention;
fig. 9 is a schematic diagram of typical measurement results in the present invention.
Detailed Description
First embodiment:
as shown in fig. 1, the optical fiber photo-thermal gas sensing device in the invention comprises a mid-infrared pumping laser 1, a near-infrared detection laser 2, a coupling component, an infrared broadband hollow fiber 3 and a photo-thermal signal detection demodulation component 4, wherein the mid-infrared pumping laser 1 and the near-infrared detection laser 2 are connected with the infrared broadband hollow fiber 3 through the coupling component, the infrared broadband hollow fiber 3 is connected with the photo-thermal signal detection demodulation component 4 through the coupling component, and the infrared broadband hollow fiber 3 is filled with gas to be detected.
The coupling component couples the middle infrared pumping light and the near infrared detection light with the infrared broadband hollow fiber 3; the laser emitted by the mid-infrared pumping laser 1 is used for exciting the photo-thermal effect of the gas to be detected, and the laser emitted by the near-infrared detection laser 2 is used for sensing the refractive index change after the photo-thermal effect of the gas; the fiber core of the infrared broadband hollow fiber 3 is hollow and filled with gas to be measured; the photo-thermal signal detection demodulation component 4 is used for measuring the phase change of the near infrared detection laser and analyzing the concentration of the gas to be detected.
The coupling assembly comprises a near infrared-middle infrared double-way coupling assembly 5 and a near infrared single-way coupling assembly 6, the middle infrared pump laser 1 and the near infrared detection laser 2 are connected with the infrared broadband hollow fiber 3 through the near infrared-middle infrared double-way coupling assembly 5, and the infrared broadband hollow fiber 3 is connected with the photo-thermal signal detection demodulation assembly 4 through the near infrared single-way coupling assembly 6.
As shown in fig. 2 and fig. 4, the near infrared-mid infrared dual-path coupling assembly 5 includes a first air chamber 51, a dichroic mirror 52, a first focusing lens 53, a second focusing lens 54, and a first optical fiber collimator 55, where the first air chamber 51 is adaptively connected to the input end of the infrared broadband hollow fiber 3, the dichroic mirror 52 is correspondingly disposed with the first air chamber 51, the transmitting end of the mid infrared pump laser 1, the first focusing lens 53, and the dichroic mirror 52 are sequentially and correspondingly disposed, the input end of the first optical fiber collimator 55 is connected with the output end of the near infrared detection laser 2, and the output end of the first optical fiber collimator 55, the second focusing lens 54, and the dichroic mirror 52 are sequentially and correspondingly disposed. The near-infrared single-path coupling assembly 6 comprises a second air chamber 61, a collimating lens 62 and a near-infrared optical fiber coupling mirror 63, the second air chamber 61 is adaptively arranged at the output end of the infrared broadband hollow fiber 3, the input ends of the second air chamber 61, the collimating lens 62 and the near-infrared optical fiber coupling mirror 63 are correspondingly arranged in sequence, and the output end of the near-infrared optical fiber coupling mirror 63 is connected with the photo-thermal signal detection demodulation assembly 4 through optical fibers.
As shown in fig. 6, the photo-thermal signal detection demodulation component 4 includes a near-infrared photodetector 41, and the near-infrared photodetector 41 is connected to the near-infrared single-path coupling component 6 through a second optical fiber combiner 74.
As shown in fig. 1, the present invention further includes a polarization controller 71, an optical fiber beam splitter 72, a piezoelectric ceramic ring 73, and a second optical fiber combiner 74, where an input end of the optical fiber beam splitter 72 is connected to an output end of the near infrared detection laser 2, the polarization controller 71 is adaptively disposed between the optical fiber beam splitter 72 and the near infrared detection laser 2, one output end of the optical fiber beam splitter 72 is connected to the near infrared-mid infrared dual-path coupling assembly 5 through an optical fiber, another output end of the optical fiber beam splitter 72 is connected to the photo-thermal signal detection demodulation assembly 4 through a third output optical fiber 75, a section of the third output optical fiber 75 is adaptively wound on the piezoelectric ceramic ring 73, and an input end of the second optical fiber beam combiner 74 is connected to the third output optical fiber 75 and the near infrared optical fiber coupling mirror 63, and an output end of the second optical fiber beam combiner 74 is connected to an input end of the near infrared photoelectric detector 41.
As shown in fig. 6, the photo-thermal signal detecting and demodulating assembly 4 further includes an electrical signal splitter 42, an input end of the electrical signal splitter 42 is connected with an output end of the near infrared photodetector 41, one output end of the electrical signal splitter 42 is sequentially connected with a low-pass filter 43, a PID controller 44 and a piezoceramic driver 45, the piezoceramic driver 45 is connected with the piezoceramic ring 73, and another output end of the electrical signal splitter 42 is sequentially connected with a lock-in amplifier 46, a data acquisition card 47 and a laser controller 48, and the laser controller 48 is connected with the mid-infrared pump laser 1.
In this embodiment, the connection of the other optical fibers in the present invention is completed by the normal single mode optical fiber 8 in the near infrared band except the hollow core optical fiber.
Specific embodiment II:
the main difference between this embodiment and the first embodiment is that: as shown in fig. 3 and 5, the near infrared-middle infrared dual-path coupling assembly 5 includes a first pair of tail gas chambers 56 and an infrared broadband optical fiber combiner 57, the input ends of the infrared broadband optical fiber 3 are adaptively inserted into the first pair of tail gas chambers 56, the input ends of the infrared broadband optical fiber combiner 57 are respectively connected with the output ends of the middle infrared pump laser 1 and the near infrared detection laser 2, the output ends of the infrared broadband optical fiber combiner 57 are connected with a first output optical fiber 58, the input ends of the infrared broadband optical fiber 3 are adaptively inserted into the first pair of tail gas chambers 56 and are butt-coupled with the output ends of the first output optical fiber 58, the near infrared single-path coupling assembly 6 includes a second pair of tail gas chambers 64 and a second output optical fiber 65, the output ends of the infrared broadband optical fiber 3 are adaptively inserted into the second pair of tail gas chambers 64, the output ends of the second output optical fiber 65 are adaptively inserted into the second pair of tail gas chambers 64, and are butt-coupled with the second output optical fiber 3, and the output ends of the infrared broadband optical fiber 3 are butt-coupled with the second output optical fiber 3.
In the present invention, the infrared broadband hollow core optical fiber 3 may be a hollow core antiresonant optical fiber. As shown in fig. 7, the hollow-core antiresonant fiber includes a core region 91 and a cladding region, the cladding region being composed of an inner cladding and an outer cladding 93; the inner cladding region consists of capillaries 92 arranged in a single layer, and adjacent capillaries 92 are not contacted and have no nodes; the fiber core region 91 is filled with gas to be measured and is surrounded by the inner cladding; all materials are silica. The hollow anti-resonance fiber has a number of cladding capillaries 92 of generally 6-8, surrounding a core region 91 of 50-100 microns in diameter, and the capillaries 92 have a wall thickness of generally about 1 micron. The first antiresonant passband of the hollow antiresonant fiber extends from 1.6 microns to the mid-infrared band, which can achieve near single mode transmission. The total length of the optical fiber is 120 cm, and in order to reduce the space of the sensor, the optical fiber is coiled into a bending radius of 15 cm, and the bending loss is small.
In the invention, the mid-infrared pump laser 1 selects a mid-infrared inter-band cascade laser with the wavelength of 3.6 microns, the output wavelength of the laser can be modulated by changing the driving current, and a tunable wave band covers an absorption baseband of formaldehyde; the near infrared detection laser 2 selects a near infrared laser with the wavelength of 1.56 microns, has fixed wavelength and is far away from the absorption wavelength of formaldehyde to be detected and other interference gases (such as water). In photo-thermal sensing, a high-frequency sinusoidal modulation is superimposed on the driving current of the mid-infrared pump laser 1, the highest detection signal-to-noise ratio is obtained through testing for optimal selection of the modulation frequency and the modulation amplitude, and meanwhile, the center wavelength is swept through the characteristic absorption peak of the gas to be detected to obtain a complete spectrum signal. The laser is changed into other wavelengths, so that detection of other gases of different types can be realized.
In a first embodiment, the mid-infrared pump laser and the near-infrared detection laser are respectively coupled into the infrared broadband hollow fiber 3 from two ends of the optical fiber, the near-infrared detection laser 2 adopts an optical fiber tail fiber for outputting, the output of the mid-infrared pump laser 1 is space light, and the near-infrared-mid-infrared dual-path coupling component 5 simultaneously couples the mid-infrared pump light and the near-infrared detection light into the infrared broadband hollow fiber 3. As shown in fig. 2, the collimated light beam 10 output by the mid-infrared pump laser 1 sequentially passes through the first focusing lens 53, the dichroic mirror 52 and the first air chamber 51, and is converged at the end face of the infrared broadband hollow core optical fiber 3, and enters the fiber core; near infrared detection light is transmitted through the common single-mode communication optical fiber 8, the near infrared detection light is converted into a space beam by the optical fiber collimator 55, and sequentially passes through the second focusing lens 54, the dichroic mirror 52 and the first air chamber 51, and the final focus is located at the end face of the infrared broadband hollow core optical fiber 3 and enters the fiber core. Wherein the dichroic mirror 52 has a high transmittance for mid-infrared light and a high reflectance for near-infrared light.
Near infrared detection light is conducted in the infrared broadband hollow optical fiber 3, exits from the other end, and is collected to the common single-mode communication optical fiber 8 by the near infrared single-path coupling component 6. As shown in fig. 4, after being output by the infrared broadband hollow fiber 3, the near infrared detection laser passes through the second air chamber 61, is collimated by the collimating lens 62, and is finally collected by the near infrared fiber coupling mirror 63 into the common single-mode communication fiber 8. The first air chamber 51 and the second air chamber 61 are respectively fixed at two ends of the infrared broadband hollow fiber 3 through flanges, the internal air is communicated with the hollow fiber, the air tightness of the flange connection between the optical fiber and the air chamber is high, the hollow fiber is not in direct contact with the external environment, and the air chamber is provided with an optical window sheet at one side close to the input/output space light beam.
In the second embodiment, the mid-infrared pump laser 1 and the near-infrared detection laser 2 may both select fiber pigtail output. As shown in fig. 3, the mid-infrared-near-infrared dual-path coupling assembly 5 uses an optical fiber element, a mid-infrared solid optical fiber 20, the infrared broadband optical fiber combiner 57, the first pair of tail gas chambers 56 are sequentially disposed on the mid-infrared optical path, a near-infrared common single-mode optical fiber 8, the infrared broadband optical fiber combiner 57, and the first pair of tail gas chambers 56 are sequentially disposed on the near-infrared optical path, and the infrared broadband optical fiber combiner 57 combines laser beams of two wavebands into the solid optical fiber 58 and outputs the laser beams. Near infrared detection light is conducted in the infrared broadband hollow fiber 3 and exits from the other end, as shown in fig. 5, the near infrared detection laser single-light path coupling component 6 is connected with the other side of the infrared broadband hollow fiber 3, and a second pair of tail gas chambers 64 and a near infrared common single-mode fiber 8 are sequentially arranged. The infrared broadband hollow optical fiber 3 and the solid optical fiber are in butt coupling, a space optical element is not used in the middle, and the size of the sensor is greatly reduced. The first pair of tail gas chambers 56 and the second pair of tail gas chambers 64 directly butt-joint and fix the hollow optical fibers and the solid optical fibers, and a gap is reserved between the two optical fibers, so that the infrared broadband hollow optical fibers 3 can be conveniently filled with/extracted with the gas to be measured.
In the present invention, the infrared broadband hollow fiber 3 may be an inner surface coated hollow fiber, as shown in fig. 8, wherein the outermost layer 94 of the inner surface coated hollow fiber is a glass or plastic protective layer, the middle is a reflective layer 95, the material is metallic silver, the innermost is a dielectric coating 96, the material is silver iodide, and the central area 97 is filled with the gas to be measured. The hollow center area 97 of the inner surface coating hollow fiber has a diameter of 200-500 microns, the wavelength of the conducted light reaches 2-16 microns, and the hollow fiber has better single-mode transmission for light beams with a wavelength above 5 microns, but the hollow fiber has larger bending loss and is prevented from bending.
In the invention, the gas to be measured is filled into the fiber core of the infrared broadband hollow fiber 3 through differential pressure driving, part of energy of the mid-infrared pumping laser is absorbed by gas molecules, the molecules are excited to a high energy state, then the energy is released back to a ground state through non-radiation processes such as molecular collision, and part of energy is converted into molecular kinetic energy, so that local temperature rise and gas refractive index change are caused. Near infrared detection laser light passes through the region, and is affected by the changed refractive index, and the phase of the near infrared detection laser light changes. The amount of change in phase of the final near infrared probe light as it exits the fiber is the cumulative result of the photothermal effect on the entire fiber length.
In the first and second embodiments, a mach-zehnder interferometer is used to detect a phase change due to a photo-thermal effect. As shown in fig. 1, the output end of the near infrared detection laser 2 is a common single-mode communication optical fiber 8, and the output end is divided into two paths after passing through the optical fiber beam splitter 72, wherein one path of the optical fiber enters the infrared broadband hollow optical fiber 3 through the mid-infrared-near infrared two-path coupling assembly 5, and then is coupled back into the common single-mode communication optical fiber 8 through the near infrared detection laser single-path coupling assembly 6, and the path is called a sensing arm; the other path is wound around the piezoceramic ring 73 via the third output optical fiber 75, and the diameter of the piezoceramic ring 73 is changed by controlling the voltage of the piezoceramic ring 73, so that the length of the third output optical fiber 75 wound thereon is precisely controlled, and the path is called a reference arm. The two paths are integrated by the fiber combiner 74, interference occurs, and the phase change is converted into a light intensity change by the interferometer. The polarization controller 71 is provided between the near infrared detection laser 2 and the optical fiber beam splitter 72 to ensure maximum interference contrast.
The near infrared photoelectric detector 41 is connected with an output optical fiber of the optical fiber combiner 74, signals of the optical fiber combiner are divided into two paths by the electronic signal splitter 42, one path of signals is input into the lock-in amplifier 46, a first harmonic signal (1 f) is demodulated, and the first harmonic signal is collected, processed and stored by the data collecting card 47; the other path outputs a voltage signal to the piezoelectric ceramic ring 73 of the reference arm of the interferometer through a feedback-control loop, and the optical path of the reference arm is adjusted, so that the static working point of the interferometer is kept at the right intersection point, the phase detection sensitivity of the interferometer is improved, and the feedback-control loop of the embodiment mainly comprises a low-pass filter 43, a PID controller 42 and a piezoelectric ceramic driver 45. The data acquisition card 47 outputs a modulation signal and a scanning signal to the laser controller 48, and the laser controller 48 drives the driving current of the mid-infrared pumping laser 1 and simultaneously controls the temperature of the mid-infrared pumping laser to be in a normal working range.
The optical fiber photo-thermal gas sensing method comprises the steps that pumping light and detection light are combined to pass through gas to be detected, the pumping light excites the absorption of the gas to be detected, a photo-thermal effect is generated, and the detection light measures refractive index change generated after the gas to be detected absorbs pumping laser.
In the method, the wavelength or power of the pump light is periodically modulated, the modulation frequency is f, the wavelength of the detection light is kept fixed and is far away from the absorption line of the gas to be detected, and the phase of the detection light is periodically changed after passing through the gas medium generating the photo-thermal effect.
In the method, the phase information of the detection light is collected and demodulated to obtain the first harmonic 1f signal of the photothermal spectrum.
In the method, the photothermal spectrum 1f signal is background-free, and the peak-peak value of the signal is in direct proportion to the gas concentration in the linear range of the system.
In the method, the spectrum 1f signal has no background, namely, no bias signal exists in the gas absorbed by the pumping-free laser, and the noise is near zero random noise.
The invention is further exemplified herein by the detection of formaldehyde gasDescription: firstly, filling 30ppm volume concentration standard formaldehyde mixed gas into an infrared broadband hollow fiber 3, controlling the air pressure to be 0.55 atmosphere, and selecting formaldehyde to be located at 2778.48cm -1 By adjusting the driving current of the mid-infrared pump laser 1, the wavelength of the mid-infrared pump laser is scanned from 2777.8cm -1 To 2779.2cm -1 At the same time, the driving current is modulated sinusoidally at a frequency of 8kHz, and the 1f signal is demodulated by the lock-in amplifier 46, as shown in fig. 9. The center wavelength of the mid-infrared pump laser 1 is shifted to a position far from the gas absorption peak, and the 1f signal tends to zero because the 1f signal of the photo-thermal spectrum has no background. The peak-to-peak value of the 1f signal is affected by a sinusoidal modulation index, which is defined as the ratio of the modulation depth to the half-width of the spectral line, and is adjusted by varying the sinusoidal amplitude applied to the drive current, with the peak-to-peak value of the 1f signal being highest at a modulation index of 1.8. The peak-peak value of the 1f signal is also affected by the modulation frequency, and in the kHz frequency band, the higher the modulation frequency is, the lower the peak-peak value is, in this embodiment, the signal-to-noise ratio of the 1f signal at the modulation frequency of 8kHz is the highest 163, the minimum detection concentration limit of formaldehyde is 0.18ppm, and the calculated normalized noise equivalent absorption coefficient is 4×10 according to the pumping light power of 1.6mW and the phase-locking bandwidth of 1.375Hz -9 cm -1 WHz -1/2 . The intensity of the photothermal spectrum measurement signal is in direct proportion to the pumping light power, and the mid-infrared absorption section is strong, the infrared wide-band hollow fiber 3 further improves the pumping light energy density, and when the pumping power is only 60 microwatts, the measurement of 30ppm formaldehyde still obtains the photothermal signal with the signal to noise ratio of more than 9.
The experiment successfully realizes the background-free measurement of 1f, compared with a 2f signal, the signal to noise ratio of a measurement result is improved by 2.4 times, and meanwhile, the formaldehyde gas measurement with high sensitivity is realized by the medium infrared pumping light.
The invention is suitable for the field of optical fiber photo-thermal gas sensing.
Claims (10)
1. An optical fiber photo-thermal gas sensing device is characterized in that: the optical fiber coupling device comprises a mid-infrared pumping laser (1), a near-infrared detection laser (2), a coupling component, an infrared broadband hollow fiber (3) and a photo-thermal signal detection demodulation component (4), wherein the coupling component comprises a near-infrared-mid-infrared two-way coupling component (5) and a near-infrared one-way coupling component (6), and the infrared broadband hollow fiber (3) is connected with the photo-thermal signal detection demodulation component (4) through the near-infrared one-way coupling component (6); the medium-infrared pump laser (1) and the near-infrared detection laser (2) are connected with the infrared wide-band hollow fiber (3) through the coupling component, the infrared wide-band hollow fiber (3) is connected with the photo-thermal signal detection demodulation component (4) through the coupling component, and the infrared wide-band hollow fiber (3) is filled with gas to be detected;
the middle infrared light path is sequentially provided with a middle infrared solid fiber (20), an infrared broadband fiber combiner (57) and a first pair of tail gas chambers (56);
the near infrared detection laser single-path coupling component (6) is connected with the other side of the infrared wide-band hollow fiber (3), a second pair of tail gas chambers (64) and a near infrared common single-mode fiber (8) are sequentially arranged, the infrared wide-band hollow fiber (3) and the solid fiber (20) are in butt coupling, and a space optical element is not used in the middle;
the optical fiber photo-thermal gas sensing device further comprises a polarization controller (71), an optical fiber beam splitter (72), a piezoelectric ceramic ring (73) and a second optical fiber beam combiner (74); the output end of the near infrared detection laser 2 is a common single-mode communication optical fiber (8), the output end is divided into two paths after passing through the optical fiber beam splitter (72), one path enters the infrared wide-band hollow optical fiber (3) through the middle infrared-near infrared two-path coupling component (5), and then is coupled back into the common single-mode communication optical fiber (8) through the near infrared detection laser single-path coupling component (6), and the path is called a sensing arm; the other path is wound on the piezoelectric ceramic circular ring (73) through a third output optical fiber (75), and the diameter of the piezoelectric ceramic circular ring (73) is changed by controlling the voltage of the piezoelectric ceramic circular ring, so that the length of the third output optical fiber (75) wound on the piezoelectric ceramic circular ring is precisely controlled, and the path is called a reference arm; the two paths are integrated by the optical fiber combiner (74) to generate interference, and the polarization controller (71) is arranged between the near infrared detection laser (2) and the optical fiber beam splitter (72) to ensure the maximum interference contrast.
2. A fiber optic photothermal gas sensing apparatus as defined in claim 1, wherein: the mid-infrared pumping laser (1) and the near-infrared detection laser (2) are connected with the infrared wide-band hollow fiber (3) through the near-infrared-mid-infrared double-path coupling assembly (5).
3. A fiber optic photothermal gas sensing apparatus as defined in claim 2, wherein: the near infrared-mid infrared double-way coupling component (5) comprises a first air chamber (51), a dichroic mirror (52), a first focusing lens (53), a second focusing lens (54) and a first optical fiber collimating lens (55), wherein the first air chamber (51) is connected with the input end of the infrared wide-band hollow fiber (3) in an adapting way, the dichroic mirror (52) is correspondingly arranged with the first air chamber (51), the emitting end of the mid infrared pumping laser (1), the first focusing lens (53) and the dichroic mirror (52) are correspondingly arranged in sequence, the input end of the first optical fiber collimating lens (55) is connected with the output end of the near infrared detection laser (2) in an optical fiber way, the output end of the first optical fiber collimating lens (55), the second focusing lens (54) and the dichroic mirror (52) are correspondingly arranged in sequence, the near infrared single-way coupling component (6) comprises a second air chamber (61), a collimating lens (62) and a near infrared coupling lens (63) which are correspondingly arranged in sequence, the second air chamber (61) and the second air chamber (61) are correspondingly arranged at the output end of the near infrared collimating lens (62) in sequence, the output end of the near infrared optical fiber coupling mirror (63) is connected with the photo-thermal signal detection demodulation component (4) through an optical fiber.
4. A fiber optic photothermal gas sensing apparatus as defined in claim 2, wherein: the near infrared-middle infrared double-path coupling assembly (5) comprises a first pair of tail gas chambers (56) and an infrared broadband optical fiber combiner (57), wherein the input ends of the infrared broadband optical fiber (3) are adaptively inserted into the first pair of tail gas chambers (56), the input ends of the infrared broadband optical fiber combiner (57) are respectively connected with the output ends of the middle infrared pump laser (1) and the output ends of the near infrared detection laser (2), the output ends of the infrared broadband optical fiber combiner (57) are connected with a first output optical fiber (58), the output optical fiber (58) is adaptively inserted into the first pair of tail gas chambers (56) and is in butt coupling with the input ends of the infrared broadband hollow optical fiber (3), the input ends of the infrared broadband optical fiber combiner (57) are respectively connected with the output ends of the middle infrared pump laser (1) and the output ends of the near infrared detection laser (2), the output ends of the infrared broadband optical fiber (3) are adaptively inserted into the first pair of tail gas chambers (56) and are in butt coupling with the input ends of the infrared broadband hollow optical fiber (3), and the output ends of the infrared broadband optical fiber (65) are in butt coupling with the second pair of the output optical fiber (64).
5. A fiber optic hot gas sensing device according to claim 3 or 4, wherein: the photo-thermal signal detection demodulation assembly (4) comprises a near infrared photoelectric detector (41), and the near infrared photoelectric detector (41) is connected with the near infrared single-path coupling assembly (6).
6. The optical fiber photo-thermal gas sensing device according to claim 5, wherein: the input end of the optical fiber beam splitter (72) is in optical fiber connection with the output end of the near infrared detection laser (2), the polarization controller (71) is adaptively arranged between the optical fiber beam splitter (72) and the near infrared detection laser (2), one output end of the optical fiber beam splitter (72) is in optical fiber connection with the near infrared-mid infrared double-path coupling component (5), the other output end of the optical fiber beam splitter (72) is connected with the photo-thermal signal detection and adjustment component (4) through a third output optical fiber (75), one section of the third output optical fiber (75) is adaptively wound on the piezoelectric ceramic ring (73), the input end of the second optical fiber beam combiner (74) is respectively connected with the third output optical fiber (75) and the near infrared single-path coupling component (6), and the output end of the second optical fiber beam combiner (74) is connected with the input end of the near infrared photoelectric detector (41).
7. The fiber optic photo-thermal gas sensing device of claim 6, wherein: the photo-thermal signal detection demodulation assembly (4) further comprises an electric signal branching device (42), the input end of the electric signal branching device (42) is connected with the output end of the near infrared photoelectric detector (41), a low-pass filter (43), a PID controller (44) and a piezoelectric ceramic driver (45) are sequentially connected to one output end of the electric signal branching device (42), the piezoelectric ceramic driver (45) is connected with the piezoelectric ceramic ring (73), and a lock-in amplifier (46), a data acquisition card (47) and a laser controller (48) are sequentially connected to the other output end of the electric signal branching device (42), and the laser controller (48) is connected with the middle infrared pumping laser (1).
8. An optical fiber photo-thermal gas sensing method capable of realizing the functions of the optical fiber photo-thermal gas sensing device according to any one of claims 1 to 7, characterized in that: the method comprises the steps that a pumping light and a detection light are combined to pass through a gas to be detected, the pumping light excites the absorption of the gas to be detected to generate a photo-thermal effect, the detection light measures refractive index change generated after the gas absorbs pumping laser, the phase of the detection light periodically changes after passing through a medium of the gas to be detected which generates the photo-thermal effect, phase information of the detection light is collected, and demodulation is carried out to obtain a photo-thermal spectrum first harmonic signal; the middle infrared light path is sequentially provided with a middle infrared solid fiber (20), an infrared broadband fiber combiner (57) and a first pair of tail gas chambers (56);
the near infrared detection laser single-path coupling component (6) is connected with the other side of the infrared wide-band hollow fiber (3), a second pair of tail gas chambers (64) and a near infrared common single-mode fiber (8) are sequentially arranged, the infrared wide-band hollow fiber (3) and the solid fiber (20) are in butt coupling, and a space optical element is not used in the middle;
the optical fiber photo-thermal gas sensing device further comprises a polarization controller (71), an optical fiber beam splitter (72), a piezoelectric ceramic ring (73) and a second optical fiber beam combiner (74); the output end of the near infrared detection laser 2 is a common single-mode communication optical fiber (8), the output end is divided into two paths after passing through the optical fiber beam splitter (72), one path enters the infrared wide-band hollow optical fiber (3) through the middle infrared-near infrared two-path coupling component (5), and then is coupled back into the common single-mode communication optical fiber (8) through the near infrared detection laser single-path coupling component (6), and the path is called a sensing arm; the other path is wound on the piezoelectric ceramic circular ring (73) through a third output optical fiber (75), and the diameter of the piezoelectric ceramic circular ring (73) is changed by controlling the voltage of the piezoelectric ceramic circular ring, so that the length of the third output optical fiber (75) wound on the piezoelectric ceramic circular ring is precisely controlled, and the path is called a reference arm; the two paths are integrated by the optical fiber combiner (74) to generate interference, and the polarization controller (71) is arranged between the near infrared detection laser (2) and the optical fiber beam splitter (72) to ensure the maximum interference contrast.
9. The optical fiber photo-thermal gas sensing method according to claim 8, wherein: the wavelength or power of the pump light is periodically modulated, and the wavelength of the probe light is kept fixed and far away from the absorption line of the gas to be detected.
10. The optical fiber photo-thermal gas sensing method according to claim 9, wherein: the peak-to-peak value of the photothermal spectrum first harmonic signal is proportional to the gas concentration within the linear range of the system.
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