CN212748729U - Optical fiber photo-thermal gas sensing device - Google Patents
Optical fiber photo-thermal gas sensing device Download PDFInfo
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- CN212748729U CN212748729U CN202021031546.0U CN202021031546U CN212748729U CN 212748729 U CN212748729 U CN 212748729U CN 202021031546 U CN202021031546 U CN 202021031546U CN 212748729 U CN212748729 U CN 212748729U
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Abstract
本实用新型公开并提供了一种一次谐波信号解调后没有背景噪音信号的光纤光热气体传感装置。本实用新型中的光纤光热气体传感装置包括中红外泵浦激光器（1）、近红外探测激光器（2）、耦合组件、红外宽波段空芯光纤（3）以及光热信号探测解调组件（4），所述中红外泵浦激光器（1）以及所述近红外探测激光器（2）通过所述耦合组件与所述红外宽波段空芯光纤（3）相连接，所述红外宽波段空芯光纤（3）通过所述耦合组件与所述光热信号探测解调组件（4）相连接，所述红外宽波段空芯光纤（3）内填充待测气体。本实用新型适用于光纤光热气体传感领域。
The utility model discloses and provides an optical fiber photothermal gas sensing device having no background noise signal after demodulation of the first harmonic signal. The optical fiber photothermal gas sensing device in the utility model comprises a mid-infrared pump laser (1), a near-infrared detection laser (2), a coupling component, an infrared wide-band hollow-core optical fiber (3), and a photothermal signal detection and demodulation component (4), wherein the mid-infrared pump laser (1) and the near-infrared detection laser (2) are connected to the infrared wide-band hollow-core optical fiber (3) through the coupling component, and the infrared wide-band hollow-core optical fiber (3) is connected to the photothermal signal detection and demodulation component (4) through the coupling component, and the infrared wide-band hollow-core optical fiber (3) is filled with a gas to be detected. The utility model is applicable to the field of optical fiber photothermal gas sensing.
Description
Technical Field
The utility model relates to an optic fibre light and heat gas sensing device.
Background
Absorption spectroscopy is a commonly used gas measurement method, in which a portion of light energy is absorbed by a gas to be measured when light of a specific wavelength passes through the gas to be measured, the absorption energy is positively correlated with 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 performs high-frequency modulation on the wavelength of incident light, a transmission light signal of the incident light after gas absorption contains a series of harmonic signals, the harmonic signals are demodulated to obtain gas concentration information, and meanwhile, the 1/f noise is reduced by adopting high-frequency detection. If the transmitted light signal is demodulated at the modulation frequency, the first harmonic signal 1f is obtained, and the 1f signal has a strong background signal, because the wavelength modulation usually adopts a method of modulating the driving current of a laser diode, and 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 techniques typically demodulate the 2f harmonic signal of transmitted light, but WMS-2f signal strength is typically much less than 1 f.
SUMMERY OF THE UTILITY MODEL
The utility model aims to solve the technical problem that overcome prior art not enough, provide a do not have background signal's optic fibre light and heat gas sensing device behind harmonic signal demodulation.
The utility model discloses a well infrared pump laser instrument, near-infrared detection laser instrument, coupling subassembly, infrared broadband hollow optic fibre and light and heat signal detection demodulation subassembly, well infrared pump laser instrument and near-infrared detection laser instrument passes through the coupling subassembly with infrared broadband hollow optic fibre is connected, infrared broadband hollow optic fibre passes through the coupling subassembly with light and heat signal detection demodulation subassembly is connected, the intussuseption of infrared broadband hollow optic fibre is filled with the gas that awaits measuring.
The coupling assembly comprises a near-infrared-intermediate-infrared double-path coupling assembly and a near-infrared single-path coupling assembly, the intermediate-infrared pump laser and the near-infrared detection laser are connected with the infrared broadband hollow optical fiber through the near-infrared-intermediate-infrared double-path coupling assembly, and the infrared broadband hollow optical fiber is connected with the photo-thermal signal detection demodulation assembly through the near-infrared single-path coupling assembly.
The near-infrared-intermediate-infrared double-path coupling component 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 arranged at the input end of the infrared broadband hollow optical fiber in an adaptive connection mode, the dichroic mirror is arranged corresponding to the first air chamber, the transmitting end of the intermediate-infrared pump laser, the first focusing lens and the dichroic mirror are sequentially and correspondingly arranged, the input end of the first optical fiber collimating mirror is connected with the output end of the near-infrared detection laser through an optical fiber, the output end of the first optical fiber collimating mirror, the second focusing lens and the dichroic mirror are sequentially and correspondingly arranged, the near-infrared single-path coupling component comprises a second air chamber, a collimating lens and a near-infrared optical fiber coupling mirror, and the second air chamber is arranged at the output end of the infrared broadband hollow optical fiber in an adaptive mode, the second air chamber, the collimating lens and the input end of the near-infrared optical fiber coupling mirror are sequentially and correspondingly arranged, and the output end of the near-infrared optical fiber coupling mirror is connected with the photo-thermal signal detection and demodulation component through an optical fiber.
The near-infrared-intermediate-infrared double-path coupling component comprises a first pair of tail gas chambers and an infrared broadband optical fiber beam combiner, the input end of the infrared broadband hollow optical fiber is inserted into the first pair of tail gas chambers in a matching mode, the input end of the infrared broadband optical fiber beam combiner is respectively connected with the output end of the intermediate-infrared pump laser and the output end of the near-infrared detection laser in an optical fiber mode, the output end of the infrared broadband optical fiber beam combiner is connected with a first output optical fiber, the output optical fiber is inserted into the first pair of tail gas chambers in a matching mode and is arranged corresponding to the input end of the infrared broadband hollow optical fiber in a matching mode, the near-infrared single-path coupling component comprises a second pair of tail gas chambers and a second output optical fiber, the output end of the infrared broadband hollow optical fiber is inserted into the second pair of tail gas chambers in a matching mode, and the input end of the second output optical fiber is inserted into the second pair of tail gas The output end adaptation corresponds the setting, the output of second output fiber with light and heat signal detection demodulation component is connected.
The photo-thermal signal detection demodulation assembly is connected with the near-infrared single-path coupling assembly.
The utility model also comprises a polarization controller, an optical fiber beam splitter, a piezoelectric ceramic ring and a second optical fiber beam combiner, the input end of the optical fiber beam splitter is connected with the output end of the near-infrared detection laser through an optical fiber, the polarization controller is arranged between the optical fiber beam splitter and the near-infrared detection laser in a matching mode, one output end of the optical fiber beam splitter is connected with the near infrared-intermediate infrared double-path coupling component through an optical fiber, 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 wound on the piezoelectric ceramic ring in a matching way, the input end of the second optical fiber beam combiner is respectively connected with the third output optical fiber and the near-infrared single-path coupling component, 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 splitter, the input end of the electric signal splitter is connected with the output end of the near-infrared photoelectric detector, one output end of the electric signal splitter is connected with a low-pass filter, a PID (proportion integration differentiation) controller and a piezoelectric ceramic driver in sequence, the piezoelectric ceramic driver is connected with a piezoelectric ceramic ring, another output end of the electric signal splitter is connected with a phase-locked amplifier, a data acquisition card and a laser controller in sequence, and the laser controller is connected with the intermediate infrared pump laser.
The beneficial effects of the utility model reside in that: the utility model discloses a pumping-survey two light source configuration, the gaseous pumping light that absorbs the periodic modulation of awaiting measuring back because the light and heat effect refracting index takes place periodic variation, when the light path of detecting light coincides with the pumping light, the change of gas refracting index can lead to the phase place of detecting light to take place periodic variation, through the phase information of demodulation detecting light, can obtain the concentration of the gaseous body that awaits measuring. Compare in wavelength modulation absorption spectrum technique, the utility model discloses avoid direct measurement through the gaseous absorptive transmission light intensity, consequently the demodulation signal of detecting the optical phase place does not receive the influence of the surplus intensity modulation of pump light.
The utility model discloses laser instrument, photoelectric detector, optic fibre subassembly etc. that well adopted have the advantage of low price, technological maturity, adopt infrared broadband hollow optic fibre simultaneously, improvethe pump light, survey light and the gas of awaiting measuring to be confined in optic fibre micron order hollow core simultaneously, improve pump light energy density, consequently greatly improved detection sensitivity.
The infrared broadband hollow fiber also has the advantages of wide transmission spectrum range, low 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 first schematic diagram of a connection structure of the mid-near infrared-mid infrared two-way coupling assembly of the present invention;
fig. 3 is a schematic diagram ii of a connection junction of the mid-near infrared-mid infrared two-way coupling assembly of the present invention;
fig. 4 is a first schematic diagram of a connection structure of the mid-near infrared single-path coupling component according to the present invention;
fig. 5 is a schematic diagram ii of a connection structure of the mid-near infrared single-path coupling module according to the present invention;
fig. 6 is a schematic view of the connection structure of the photo-thermal signal detecting and demodulating module according to the present invention;
fig. 7 is a first schematic illustration of the mechanism of the mid-infrared hollow fiber according to the present invention;
fig. 8 is a second schematic diagram of the mechanism of the mid-infrared hollow fiber according to the present invention.
Detailed Description
The first embodiment is as follows:
as shown in fig. 1, the utility model provides an optic fibre light and heat gas sensing device includes well infrared pump laser 1, near-infrared detection laser 2, coupling subassembly, infrared broadband hollow optic fibre 3 and light and heat signal detection demodulation subassembly 4, well infrared pump laser 1 and near-infrared detection laser 2 passes through the coupling subassembly with infrared broadband hollow optic fibre 3 is connected, infrared broadband hollow optic fibre 3 passes through the coupling subassembly with light and heat signal detection demodulation subassembly 4 is connected, pack the gas that awaits measuring in infrared broadband hollow optic fibre 3 intussuseption.
The coupling component couples the intermediate infrared pump light and the near infrared probe light with the infrared broadband hollow fiber 3; the laser emitted by the intermediate infrared pump laser 1 is used for exciting the photothermal 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 of the gas after the photothermal effect; the fiber core of the infrared broadband hollow fiber 3 is hollow and is filled with gas to be detected; the photo-thermal signal detection and demodulation assembly 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-intermediate-infrared double-path coupling assembly 5 and a near-infrared single-path coupling assembly 6, the intermediate-infrared pump laser 1 and the near-infrared detection laser 2 are connected with the infrared broadband hollow-core optical fiber 3 through the near-infrared-intermediate-infrared double-path coupling assembly 5, and the infrared broadband hollow-core optical fiber 3 is connected with the photo-thermal signal detection demodulation assembly 4 through the near-infrared single-path coupling assembly 6.
As shown in fig. 2 and 4, the near-infrared-intermediate-infrared two-way 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 fiber collimating lens 55, where the first air chamber 51 is adapted to be connected to the input end of the infrared broadband hollow fiber 3, the dichroic mirror 52 is disposed corresponding to the first air chamber 51, the emitting end of the intermediate-infrared pump laser 1, the first focusing lens 53, and the dichroic mirror 52 are sequentially disposed corresponding to each other, the input end of the first fiber collimating lens 55 is connected to the output end of the near-infrared detection laser 2 through an optical fiber, and the output end of the first fiber collimating lens 55, the second focusing lens 54, and the dichroic mirror 52 are sequentially disposed corresponding to each other. Near-infrared single pass coupling subassembly 6 includes second air chamber 61, collimating lens 62 and near-infrared optical fiber coupling mirror 63, second air chamber 61 adaptation sets up the output of infrared broadband hollow optic fibre 3, second air chamber 61 collimating lens 62 and the input of near-infrared optical fiber coupling mirror 63 is corresponding the setting in proper order, the output of near-infrared optical fiber coupling mirror 63 pass through optic fibre with light and heat signal detection demodulation subassembly 4 is connected.
As shown in fig. 6, the photothermal signal detection demodulation element 4 includes a near-infrared photodetector 41, and the near-infrared photodetector 41 is connected to the near-infrared single-path coupling element 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 splitter 72, a piezoelectric ceramic ring 73 and a second optical fiber combiner 74, wherein the input end of the optical fiber splitter 72 is connected to the output end of the near-infrared detection laser 2 through an optical fiber, the polarization controller 71 is disposed between the optical fiber splitter 72 and the near-infrared detection laser 2 in a matching manner, one output end of the optical fiber splitter 72 is connected to the near-infrared-middle-infrared dual-path coupling module 5 through an optical fiber, the other output end of the optical fiber splitter 72 is connected to the photo-thermal signal detection demodulation module 4 through a third output optical fiber 75, one section of the third output optical fiber 75 is wound on the piezoelectric ceramic ring 73, the input end of the second optical fiber combiner 74 is connected to the third output optical fiber 75 and the near-infrared optical fiber coupling mirror 63 respectively, the output end of the second optical fiber combiner 74 is connected to the input end of the near infrared photodetector 41.
As shown in fig. 6, the photo-thermal signal detection and demodulation assembly 4 further includes an electrical signal splitter 42, an input end of the electrical signal splitter 42 is connected to an output end of the near-infrared photodetector 41, an output end of the electrical signal splitter 42 is connected to a low-pass filter 43, a PID controller 44 and a piezoelectric ceramic driver 45 in sequence, the piezoelectric ceramic driver 45 is connected to the piezoelectric ceramic ring 73, another output end of the electrical signal splitter 42 is connected to a phase-locked amplifier 46, a data acquisition card 47 and a laser controller 48 in sequence, and the laser controller 48 is connected to the mid-infrared pump laser 1.
In this embodiment, the other fiber connections in the present invention are made by the common single mode fiber 8 in the near infrared band, except for the hollow core fiber.
The second embodiment is as follows:
the main differences between the first embodiment and the second embodiment are as follows: as shown in fig. 3 and 5, the near-infrared-intermediate-infrared dual-path coupling assembly 5 includes a first pair of tail gas chambers 56 and an infrared broadband optical fiber combiner 57, an input end of the infrared broadband hollow-core optical fiber 3 is inserted into the first pair of tail gas chambers 56, an input end of the infrared broadband optical fiber combiner 57 is respectively in optical fiber connection with an output end of the intermediate-infrared pump laser 1 and an output end of the near-infrared detection laser 2, an output end of the infrared broadband optical fiber combiner 57 is connected with a first output optical fiber 58, an input end of the infrared broadband hollow-core optical fiber 3 is inserted into the first pair of tail gas chambers 56 and is in butt-joint coupling with an output end 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, an output end of the infrared broadband hollow-core optical fiber 3 is inserted into the second pair of tail gas chambers 64, the input end of the second output optical fiber 65 is inserted into the second pair of tail gas chambers 64 and coupled with the output end of the infrared broadband hollow optical fiber 3 in a butt joint manner, and the output end of the second output optical fiber 65 is connected with the photo-thermal signal detection and demodulation component 4.
In the present invention, the infrared broadband hollow fiber 3 may be a hollow antiresonant fiber. As shown in fig. 7, the hollow-core antiresonant optical fiber comprises a core region 91 and a cladding region, the cladding region is 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 in contact and have no node; the core region 91 is filled with gas to be detected and surrounded by the inner cladding; all materials are silica. In the hollow-core anti-resonant fiber, the number of the cladding capillaries 92 is generally 6-8, the core region 91 with the diameter of 50-100 micrometers is surrounded, and the wall thickness of the capillaries 92 is generally about 1 micrometer. The first antiresonant pass band of the hollow-core antiresonant optical fiber extends from 1.6 microns to the mid-infrared band, and approximate single-mode transmission can be achieved. 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 utility model, the mid-infrared pump laser 1 selects the mid-infrared interband cascade laser with the wavelength of 3.6 microns, the output wavelength of the laser can be modulated by changing the driving current, and the tunable waveband covers the absorption baseband of formaldehyde; the near-infrared detection laser 2 selects a near-infrared laser with the wavelength of 1.56 microns, the wavelength is fixed, and the near-infrared detection laser is far away from the absorption wavelength of formaldehyde and other interference gases (such as water) to be detected. In photo-thermal sensing, a high-frequency sinusoidal modulation is superposed on the driving current of the mid-infrared pump laser 1, the modulation frequency and the modulation amplitude are optimized and selected by obtaining the highest detection signal-to-noise ratio through testing, and meanwhile, the central wavelength sweeps 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 different gases can be realized.
In the first specific embodiment, the mid-infrared pump laser and the near-infrared detection laser are respectively coupled into the infrared broadband hollow-core fiber 3 from two ends of the fiber, the near-infrared detection laser 2 is output by using a fiber pigtail, the output of the mid-infrared pump laser 1 is space light, and the near-infrared-mid-infrared double-path coupling component 5 simultaneously couples the mid-infrared pump light and the near-infrared detection light into the infrared broadband hollow-core fiber 3. As shown in fig. 2, the collimated light beam 10 output by the intermediate 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 surface of the infrared broadband hollow-core fiber 3, and enters the core; the near-infrared detection light is transmitted through a common single-mode communication optical fiber 8, the near-infrared detection light is converted into a spatial light beam by the optical fiber collimating lens 55, passes through the second focusing lens 54, the dichroic mirror 52 and the first air chamber 51 in sequence, and finally falls on the end face of the infrared broadband hollow optical fiber 3 to enter 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-core optical fiber 3, is emitted from the other end, and is collected to a common single-mode communication optical fiber 8 by a near-infrared single-path coupling component 6. As shown in fig. 4, the near-infrared detection laser passes through the second air chamber 61 after being output by the infrared broadband hollow fiber 3, 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 optical fiber 3 through flanges, the internal gas is communicated with the hollow optical fiber, the flange connection between the optical fiber and the air chambers is high in air tightness, the hollow optical fiber is not directly contacted with the external environment, and an optical window sheet is arranged at one side of the air chambers, close to the input/output space light beams.
In the second embodiment, both the mid-infrared pump laser 1 and the near-infrared detection laser 2 can select fiber pigtail outputs. As shown in fig. 3, the intermediate infrared-near infrared two-way coupling component 5 uses an optical fiber element, the intermediate infrared optical path is sequentially provided with an intermediate infrared solid core optical fiber 20, the infrared broadband optical fiber combiner 57, and the first pair of tail gas chambers 56, the near infrared optical path is sequentially provided with 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, and the infrared broadband optical fiber combiner 57 combines and outputs the lasers of two wave bands into the solid core optical fiber 58. Near-infrared detection light is conducted in the infrared broadband hollow-core optical fiber 3 and emitted from the other end, as shown in fig. 5, the near-infrared detection laser single-optical-path coupling component 6 is connected to the other end of the infrared broadband hollow-core optical fiber 3, and a second pair of tail gas chambers 64 and a near-infrared common single-mode optical fiber 8 are sequentially arranged. The infrared broadband hollow optical fiber 3 and the solid optical fiber are in butt coupling, and no space optical element is used in the middle, so that 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-core optical fiber and the solid-core optical fiber, and a gap is reserved between the two optical fibers, so that the infrared broadband hollow-core optical fiber 3 can be conveniently filled with/pumped out gas to be detected.
The utility model discloses in, infrared broadband hollow optic fibre 3 also can be interior surface coating hollow optic fibre, as shown in fig. 8, interior surface coating hollow optic fibre's outmost 94 is glass or plastic protection layer, and the centre is reflector layer 95, and its material is metallic silver, is dielectric coating 96 at the most, and the material is silver iodide, and central zone 97 is to fill the gas that awaits measuring. The diameter of the central area 97 of the hollow core of the hollow-core optical fiber with the inner surface coating is generally 200-500 microns, the wavelength of the transmitted light reaches 2-16 microns, and the optical fiber has good single-mode transmission for light beams with the wavelength of more than 5 microns, but the bending loss of the hollow-core optical fiber is large, and bending is avoided.
The utility model discloses in, the gaseous fibre core of filling infrared broadband hollow optic fibre 3 through the pressure differential drive that awaits measuring, the partial energy of well infrared pumping laser is absorbed by the gas molecule, and the molecule is aroused the high energy attitude, returns the ground state through non-radiative process release energy such as molecule collision afterwards, and some energy conversion becomes molecular kinetic energy, causes local temperature to rise, and the gas refractive index changes. When the near infrared detection laser passes through the region, the phase of the near infrared detection laser changes under the influence of the changed refractive index. Finally, when the near-infrared detection light leaves the optical fiber, the phase change of the near-infrared detection light is the accumulated result of photo-thermal action on the whole length of the optical fiber.
In the first embodiment and the second embodiment, a mach-zehnder interferometer is used to detect a phase change caused by a photothermal effect. As shown in fig. 1, the output end of the near-infrared detection laser 2 is a common single-mode communication fiber 8, and is divided into two paths after passing through the fiber beam splitter 72, one path enters the infrared broadband hollow-core fiber 3 through the mid-infrared-near-infrared two-path coupling component 5, and is then coupled back to the common single-mode communication 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 piezoceramic ring 73 through a 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 on the ring is precisely controlled, and the path is called a reference arm. The two paths are integrated by the optical fiber combiner 74, interference occurs, and the phase change is converted into light intensity change by the interferometer. The polarization controller 71 is disposed between the near-infrared detection laser 2 and the fiber beam splitter 72 to ensure maximum interference contrast.
The near infrared photoelectric detector 41 is connected with the output optical fiber of the optical fiber beam combiner 74, the signal is divided into two paths by the electronic signal splitter 42, one path is input into the lock-in amplifier 46, the first harmonic signal (1 f) is demodulated, and the first harmonic signal is collected, processed and stored by the data acquisition card 47; and the other path outputs a voltage signal to a 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 an orthogonal point, and the phase detection sensitivity of the interferometer is improved. 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 pump laser 1 and controls the temperature thereof within a normal working range.
The utility model discloses a theory of operation:
pumping light and detection light are combined to pass through the gas to be detected, the pumping light excites the gas to be detected to absorb, a photothermal effect is generated, and the detection light measures the refractive index change generated after the gas absorbs the pumping laser.
The wavelength or power of the pumping light is periodically modulated, the modulation frequency is f, the wavelength of the probe light is kept fixed and is far away from an absorption line of the gas to be detected, and the phase of the probe light periodically changes after passing through a gas medium generating a photothermal effect.
And collecting the phase information of the detection light, and demodulating to obtain a photo-thermal spectrum first harmonic 1f signal.
The photothermal spectroscopy 1f signal is background-free, and the peak-to-peak value of the signal is proportional to the gas concentration in the linear range of the system.
The spectrum 1f signal is background-free, that is, there is no bias signal in the gas without the absorption of the pump laser, and the noise is random noise close to zero.
The utility model discloses use formaldehyde gas's detection as the example here and further explain: firstly, filling a standard formaldehyde mixed gas with the volume concentration of 30ppm into the infrared broadband hollow optical fiber 3, controlling the air pressure to be 0.55 atmospheric pressure, and selecting the formaldehyde to be positioned at 2778.48cm-1By adjusting the drive current of the mid-infrared pump laser 1, scanning its wavelength from 2777.8cm-1To 2779.2cm-1And simultaneously, the drive current is subjected to sinusoidal modulation with the frequency of 8kHz, and the phase-locked amplifier 46 demodulates to obtain a 1f signal. When the central wavelength of the mid-infrared pump laser 1 is shifted to a position away from the gas absorption peak, the 1f signal tends to zero because the 1f signal of the photothermal spectrum has no background. The peak-to-peak value of the 1f signal is influenced by a sinusoidal modulation index, the modulation index is defined as the ratio of the modulation depth to the half width at half maximum of the spectral line, the modulation is carried out by changing the amplitude of the sine loaded on the driving current, and when the modulation index is 1.8, the peak-to-peak value of the 1f signal is the highest. The peak-to-peak value of the 1f signal is also affected by the modulation frequency, the higher the modulation frequency is in the kHz band, the lower the peak-to-peak value of the signal is, in this embodiment, the highest signal-to-noise ratio of the 1f signal is 163 at the modulation frequency of 8kHz, the minimum detection concentration limit of formaldehyde is 0.18ppm, and according to the pump light power of 1.6mW and the phase-locked bandwidth of 1.375Hz, the calculated normalized noise equivalent absorption coefficient is 4 × 10-9cm-1WHz-1/2. The photo-thermal spectrum measurement signal intensity is in direct proportion to the pump light power, and the mid-infrared absorption cross section is strong, the infrared broadband hollow optical fiber 3 further improves the pump light energy density, and when the pump power is only 60 microwatts, 30ppm formaldehyde is measured, and photo-thermal signals with the signal-to-noise ratio of more than 9 are still obtained.
The experiment successfully realizes the 1f background-free measurement, compared with a 2f signal, the signal-to-noise ratio of the measurement result is improved by 2.4 times, and meanwhile, the mid-infrared pump light realizes the high-sensitivity formaldehyde gas measurement.
The utility model is suitable for an optic fibre light and heat gas sensing field.
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN111504945A (en) * | 2020-06-08 | 2020-08-07 | 朗思科技有限公司 | Optical fiber photo-thermal gas sensing device and method |
| CN113588624A (en) * | 2021-08-03 | 2021-11-02 | 武汉理工大学 | Hollow optical fiber-based vacuum air cavity and gas detection system |
| CN115112603A (en) * | 2022-06-24 | 2022-09-27 | 浙江师范大学 | Novel quasi-distributed optical fiber gas detection method |
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN111504945A (en) * | 2020-06-08 | 2020-08-07 | 朗思科技有限公司 | Optical fiber photo-thermal gas sensing device and method |
| CN113588624A (en) * | 2021-08-03 | 2021-11-02 | 武汉理工大学 | Hollow optical fiber-based vacuum air cavity and gas detection system |
| CN115112603A (en) * | 2022-06-24 | 2022-09-27 | 浙江师范大学 | Novel quasi-distributed optical fiber gas detection method |
| CN115112603B (en) * | 2022-06-24 | 2024-06-11 | 浙江师范大学 | Novel quasi-distributed optical fiber gas detection method |
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