Devices for generating Mach-Zehnder interferometry, photothermal spectroscopic gas sensing devices and detection methods based on it

Abstract

This application discloses a device for generating Mach-Zehnder interferometry, a photothermal spectral gas sensing device based thereon, and a detection method thereof, belonging to the field of laser spectral gas sensing technology. The interferometer device sequentially includes an optical frequency comb and a first optical beam splitter in the optical transmission path. A movable-arm mirror group is provided in the optical path of one beam B, which is split into two beams by the first beam splitter. Beam B changes its path after passing through the movable-arm mirror group and intersects with the optical path of the other beam C. A beam combiner is provided at the intersection. The movable-arm mirror group is connected to a driving mechanism, which drives the movable-arm mirror group to perform linear motion, the trajectory of which is parallel to the line containing beam B. The interferometer device of this application does not require multi-heterodyne interference using a local oscillator optical comb and a signal optical comb. The downconversion factor can be directly calculated using the moving speed of the movable-arm mirror group in the Mach-Zehnder interferometer structure, mapping each longitudinal mode of the signal optical comb at the optical frequency to the radio frequency component.

Description

Technical Field

[0001] This application belongs to the field of laser spectral gas sensing technology, and specifically relates to a device for generating Mach-Zehnder interferometry, a photothermal spectral gas sensing device based thereon, and a detection method thereof. Background Technology

[0002] With the continuous development of gas sensing technology, photothermal spectroscopy has gradually become an important means of trace gas sensing due to its advantages such as high sensitivity, zero background, and low gas consumption. However, traditional photothermal spectroscopy usually relies on narrowband laser sources, making it difficult to achieve multi-component, wide-band gas measurement, thus limiting its application range. Since the beginning of this century, the rapid advancement of mode-locked laser technology has promoted the widespread application of optical frequency combs. Optical frequency combs have characteristics such as wide spectral coverage, high frequency accuracy, and excellent stability, providing an ideal light source for high-resolution spectral measurement of multi-component gases. Combining optical frequency combs with photothermal spectroscopy technology, while retaining the original advantages of photothermal spectroscopy, has opened up a new path for broadband, multi-component gas detection, becoming one of the emerging high-sensitivity, zero-background gas detection technologies in recent years.

[0003] However, existing optical frequency comb photothermal spectroscopy gas sensing schemes still have limitations, stemming from the relatively slow relaxation process of gas molecules. To match the modulation frequency of the photothermal signal with the molecular relaxation process, the excitation of the photothermal signal by the signal light typically requires multiheterodyne interference between the local oscillator optical comb and the signal optical comb to convert the optical frequency down-convert to a suitable radio frequency range. Such schemes often rely on high-precision dual-comb locking loops, resulting in large system size, cumbersome operation, and high cost, making it difficult to meet the requirements of field measurement and miniaturized integration. Summary of the Invention

[0004] To address the aforementioned issues, this application provides a device for generating Mach-Zehnder interferometry, a photothermal spectral gas sensing device based thereon, and a detection method thereof.

[0005] The first objective of this application is to provide a device for generating Mach-Zehnder interference, which includes an optical frequency comb and a first optical beam splitter in sequence in the optical transmission path;

[0006] In the two beams split by the first beam splitter, a movable arm reflector group is provided on the optical path of one beam B, and the path of beam B is changed by the movable arm reflector group and intersects with the optical path of the other beam C, and a beam combiner is provided at the intersection.

[0007] The boom reflector assembly is connected to a drive mechanism, which drives the boom reflector assembly to move in a straight line. The trajectory of the straight line is parallel to the line where the light beam B is located.

[0008] In a specific embodiment of this application, the boom reflector assembly includes two reflectors with their mirror surfaces facing each other, and the mirror surface of one reflector receives the illumination of the light beam B.

[0009] In a specific embodiment of this application, the straight line containing the boom reflector group is perpendicular to the optical path of the beam B.

[0010] A second objective of this application is to provide a method for generating Mach-Zehnder interferometry, implemented based on the aforementioned apparatus for generating Mach-Zehnder interferometry, comprising:

[0011] The light output from the optical frequency comb is split into two beams by the first optical beam splitter, resulting in beam B and beam C.

[0012] The beam B changes its path after passing through the boom reflector group and is combined with the beam C by the beam combiner.

[0013] The boom reflector assembly moves in a straight line, and the trajectory of the straight line is parallel to the line where the light beam B is located, thereby applying intensity modulation at different frequencies to each longitudinal mode component of the signal optical comb pulse.

[0014] After the boom reflector assembly stops moving, beam B and beam C are combined by a beam combiner, and the combined beam with intensity modulation information is output, thus completing the mapping from the optical frequency domain to the radio frequency domain.

[0015] In a specific embodiment of this application, the moving distance of the boom reflective lens group within a single pulse acquisition time is... L max The following requirements must be met:

[0016]

[0017] in, u The moving speed of the boom reflector assembly. n It is the refractive index of the environment in which the apparatus that produces the Mach-Zehnder interferometer is located. T The time interval between signal light pulses. c It is the speed of light in a vacuum.

[0018] In a specific embodiment of this application, the expression for the intensity of the interference signal resulting from the superposition of beams B and C is given by the following formula:

[0019]

[0020] in, f rep It is the repetition frequency of the optical frequency comb. v c It is the longitudinal mode frequency closest to the center frequency of the spectrum. p It is the longitudinal module number.u The moving speed of the boom reflector assembly. t For the time of travel, n It is the refractive index of the environment in which the apparatus that produces the Mach-Zehnder interferometer is located. c It is the speed of light in a vacuum. v p The absolute frequency of the optical frequency comb. It is determined by the initial optical path difference between beam B and beam C. L 0 represents the different initial phase differences caused by different longitudinal modes with index p.

[0021] In a specific embodiment of this application, the mapping relationship in completing the mapping from the optical frequency domain to the radio frequency domain is determined by a down-conversion factor of 2. nu / c Decide.

[0022] A third objective of this application is to provide a photothermal spectral gas sensing device, including the aforementioned device for generating Mach-Zehnder interferometry, and further comprising:

[0023] A collimation processing unit and a second optical beam splitter are sequentially provided in the optical path of the beam combined by the beam combiner. A first photodetector is provided in the optical path of one beam D, which is split into two beams by the second optical beam splitter. A wavelength division multiplexer and a hollow fiber air cell are sequentially provided in the optical path of the other beam E, which is split into two beams by the second optical beam splitter.

[0024] The wavelength division multiplexer also receives probe light, and a polarization controller and a three-terminal circulator are provided on the incident light path of the probe light.

[0025] One port B of the three-terminal circulator is connected to the polarization controller, one port C of the remaining two ports is connected to the wavelength division multiplexer, and the other port D of the remaining two ports is connected to the second photodetector.

[0026] In a specific embodiment of this application, the output signal of the second photodetector is divided into two signals, signal B and signal C, and signal B is connected to the laser of the detection light;

[0027] A first filter and a laser servo control system are sequentially provided in the transmission path between signal B and the laser.

[0028] In a specific embodiment of this application, the first photodetector is connected to a data acquisition card;

[0029] The signal C is sent to the data acquisition card;

[0030] A second filter is also provided on the transmission path between the signal C and the data acquisition card.

[0031] In a specific embodiment of this application, the effective bandwidth of the first photodetector and the second photodetector... All satisfy the following expression:

[0032]

[0033] in, This represents the maximum frequency of the radio frequency component of the interference light produced by the device that generates the Mach-Zehnder interference. v c The longitudinal mode frequency closest to the center of the spectrum. This indicates the maximum frequency of the longitudinal mode of the optical frequency comb. u The moving speed of the boom reflector assembly. t For the time of travel, n It is the refractive index of the medium surrounding the device that produces the Mach-Zehnder interferometer. c It is the speed of light in a vacuum. This represents the bandwidth of the contour function.

[0034] In a specific embodiment of this application, the collimation processing device includes a collimating lens group and an optical fiber collimator, which are arranged sequentially in the optical transmission direction;

[0035] And / or, the straight lens group includes a first collimating lens and a second collimating lens;

[0036] And / or, the hollow fiber optic chamber contains a Fabry-Perot interference structure.

[0037] The fourth objective of this application is to provide a detection method for photothermal spectral gas sensing, implemented based on the aforementioned photothermal spectral gas sensing device, comprising:

[0038] After being combined by the beam combiner, the beam containing intensity modulation information is processed by the collimation unit and then split into two beams by the second beam splitter, resulting in beam D and beam E.

[0039] The beam D is received by the first photodetector, and an electrical signal with intensity modulation information is obtained.

[0040] The probe light passes through a polarization controller and is transmitted from port B of the three-terminal circulator to port C of the three-terminal circulator. It is then transmitted to a wavelength division multiplexer and coupled together with the beam E into the wavelength division multiplexer. The probe light is then transmitted to the hollow fiber gas chamber and absorbed by the sample gas inside the hollow fiber gas chamber. The hollow fiber gas chamber then emits a primary return light with phase modulation information, which includes gas absorption characteristics and amplitude spectrum characteristics of beam E and the probe light.

[0041] The probe light in the primary retroreflection light interferes with the incident probe light to obtain secondary retroreflection light with periodic intensity modulation information.

[0042] The secondary reflected light is transmitted from port C of the three-terminal circulator to port D of the three-terminal circulator through the wavelength division multiplexer, and then from port D to the second photodetector to obtain an electrical signal with periodic intensity modulation information.

[0043] In a specific embodiment of this application, the electrical signal with periodic intensity modulation information is processed by a second filter and then received by a data acquisition card;

[0044] The electrical signal with intensity modulation information is received by the data acquisition card.

[0045] In a specific embodiment of this application, the electrical signal with periodic intensity modulation information is processed by a first filter and then input to the laser servo control system;

[0046] The laser servo control system receives the filtered electrical signal and locks the laser wavelength at the orthogonal working point of the interference fringes based on the received filtered electrical signal.

[0047] In a specific embodiment of this application, the phase modulation of light in the periodic intensity modulation information As shown below:

[0048]

[0049] in, It is a coefficient inversely proportional to the cross-sectional area of ​​the mode field of the hollow fiber, where A is the peak absorption coefficient and L is the absorption length of the hollow fiber. It is a normalized linear function of the absorption characteristics; Let be the optical power of the p-th radio frequency comb tooth through which the signal light propagates through the hollow fiber. The expression for its magnitude is as follows:

[0050]

[0051] in, For in the radio frequency domain f p Below is the spectrum of interference light produced by the device that generates Mach-Zehnder interference.

[0052] In a specific embodiment of this application, the detection method for photothermal spectral gas sensing further includes:

[0053] Fourier transform is performed on the electrical signals with periodic intensity modulation information and the electrical signals with intensity modulation information acquired by the data acquisition card to obtain phase modulation information.

[0054] Compared with the prior art, this application has the following advantages:

[0055] The device for generating Mach-Zehnder interference in this application does not require multi-heterodyne interference between the local oscillator optical comb and the signal optical comb. Instead, it can directly use the moving speed of the movable arm mirror group in the Mach-Zehnder interference structure to calculate the downconversion factor, and map each longitudinal mode of the signal optical comb at the optical frequency to the radio frequency component.

[0056] The method for generating Mach-Zehnder interference in this application effectively avoids many problems such as the large size, cumbersome operation, and high cost of dual optical comb locking systems.

[0057] This application discloses a photothermal spectroscopy gas sensing device that combines the device for generating Mach-Zehnder interferometry with photothermal spectroscopy technology, enabling rapid inversion of gas spectral parameters. It can also be applied to the detection of mixed multi-component gases with short absorption distances, providing a new method for optical frequency comb photothermal spectroscopy gas sensing.

[0058] Other features and advantages of this application will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the application. The objectives and other advantages of this application may be realized and obtained by means of the structures pointed out in the description, claims and drawings. Attached Figure Description

[0059] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0060] Figure 1 A schematic diagram of an apparatus for generating Mach-Zehnder interference according to an embodiment of this application is shown;

[0061] Figure 2 The following diagram shows the spectral downconversion result of the signal optical comb after it has been moved through the movable arm mirror group in the Mach-Zehnder interferometer structure according to an embodiment of this application: Figure 2 (a) Simulation diagram of the original longitudinal mode power spectrum of the signal optical comb; Figure 2 (b) shows the displacement of the boom reflector assembly as a function of time. Figure 2 Image (c) shows a simulation of the time-domain interferometry result obtained by intensity modulation of the signal optical comb. Figure 2 (d) is the spectrum of the time-domain interference signal.

[0062] Figure 3The simulation results of the original amplitude spectrum and the normalized amplitude spectrum of the photothermal signal excited by the signal optical comb according to the embodiments of this application are as follows: Figure 3 (a) shows the simulation results of the original amplitude spectrum. Figure 3 Figure (b) shows the simulation results of the normalized amplitude spectrum.

[0063] In the diagram: 1. Optical frequency comb; 2. First optical beam splitter; 3. Movable arm reflector group; 4. Beam combiner; 5. Collimating lens group; 51. First collimating lens; 52. Second collimating lens; 6. Fiber collimator; 7. Second optical beam splitter; 8. First photodetector; 9. Wavelength division multiplexer; 10. Hollow-core fiber air cell; 11. Three-terminal circulator; 111. Port B; 112. Port C; 113. Port D; 12. Second photodetector; 13. First filter; 14. Laser servo control system; 15. Laser; 16. Polarization controller; 17. Second filter; 18. Data acquisition card; 19. Computer. Detailed Implementation

[0064] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0065] like Figure 1 As shown, an apparatus for generating Mach-Zehnder interference according to certain embodiments of this application includes, in sequence, an optical frequency comb 1 and a first optical beam splitter 2 in the optical transmission path;

[0066] A movable arm reflector group 3 is provided on the optical path of one of the two beams of light split by the first optical beam splitter 2, and the path of beam B is changed by the movable arm reflector group and intersects with the optical path of the other beam C, and a beam combiner 4 is provided at the intersection.

[0067] The boom reflector group 3 is connected to a drive mechanism, which drives the boom reflector group to perform linear motion. The trajectory of the linear motion is parallel to the line where the light beam B is located.

[0068] In this embodiment, the light output from the optical frequency comb 1 is split into two paths by the first optical beam splitter 2. One path passes through the motor-driven boom reflector group 3, and the other path propagates through a fixed optical path and is then re-bundled with the output light reflected by the boom reflector group 3 at the beam combiner 4 to form a Mach-Zehnder interference, thus constituting a device for generating Mach-Zehnder interference.

[0069] The movable arm reflector group 3 moves in a straight line along a direction parallel to the incident beam B. That is, the conversion factor is directly calculated by the moving speed of the movable arm reflector group 3 in the Mach-Zehnder interference structure. Each longitudinal mode of the signal optical comb at the optical frequency is mapped to the radio frequency component. There is no need to use the local oscillator optical comb and the signal optical comb for multi-heterodyne interference. This effectively avoids many problems such as the large size, cumbersome operation and high cost of the dual optical comb locking system.

[0070] In this embodiment, the principle of calculating the conversion factor based on the moving speed of the boom reflector group 3 includes:

[0071] The boom reflector assembly 3 moves at a speed u Constant speed movement creates an optical path difference between beams B and C, thereby applying intensity modulation at different frequencies to the longitudinal mode components of the optical frequency comb pulse, causing the longitudinal mode components at the optical frequency to move at a proportionality coefficient of 2. nu / c Down-convert to the radio frequency domain;

[0072] The boom reflector assembly 3 is driven at a constant speed. u Moving in a straight line, the maximum distance traveled is L max The change in the optical path length of beam B causes different frequency phase modulations in each longitudinal mode of the optical frequency comb passing through the movable arm reflector group, while the optical path length of beam C remains unchanged.

[0073] In the boom reflector group 3, the light from the two reflectors coherently superimposes after beam combining to form an intensity-modulated signal light, realizing the mapping from the optical frequency domain to the radio frequency domain. Specifically, the longitudinal modes of the optical frequency comb... v p The calculation is as shown in equation (1):

[0074] (1)

[0075] In equation (1), f rep It is the repetition frequency of the optical frequency comb. v c It is the longitudinal mode frequency closest to the center of the spectrum. p It is the longitudinal module number.

[0076] Since deviations smaller than the repetition rate are negligible in the optical frequency domain, the spectral center frequency can be expressed as v. c Direct replacement. The oscillation amplitude of each longitudinal mode is determined by the spectral profile function of the optical frequency comb. G ( v p - v c The bandwidth of the contour function is determined by Δ. vB At that time, the number of longitudinal moduli Δ v B / f rep , p The range of values ​​is limited to - Δ v B / 2f rep With Δ v B / 2 f rep Therefore, the electric field of the optical comb can be expressed as a series summation, as shown in equation (2):

[0077] (2)

[0078] In equation (2), j The symbol represents the imaginary number, and exp represents taking the natural logarithm. t It refers to the experimental time and phase term. The expression is shown in equation (3):

[0079] (3)

[0080] In equation (3), The initial pulse envelope center position, The initial phase of the longitudinal mode of the optical comb, i.e. The carrier envelope phase at the location. When the movable arm mirror group 3 in the interference structure is... u When moving at a constant speed in a straight line, the change in optical path length with time ΔL (t) The expression is shown in equation (4):

[0081] (4)

[0082] The change in optical path difference causes the various longitudinal modes in the optical frequency comb to be modulated by different frequencies of phase. The calculation is shown in equation (5):

[0083] (5)

[0084] In equations (3)-(5), u The moving speed of the boom reflector assembly 3. t For the time of travel, n It is the refractive index of the medium surrounding the device that produces the Mach-Zehnder interferometer. c It is the speed of light in a vacuum. Therefore, the expression for the electric field of the optical comb after phase modulation by the movable arm reflector group 3 is shown in equation (6):

[0085] (6)

[0086] Considering the initial optical path difference between the fixed arm and the movable arm, the expression for the electric field of the optical comb pulse passing through beam C is shown in equation (7):

[0087] (7)

[0088] In equations (6)-(7), It is determined by the initial optical path difference between the two interference arms (beam B and beam C). L 0 pairs of serial numbers p The different initial phase differences caused by different longitudinal modes. This value and longitudinal mode number p This is related to the overall initial time delay of the pulsed light, as reflected in the experiment. t 0= nL 0 / c And it does not change with the movement time of the moving mirror group, which can be expressed by the following formula (see Equation 8):

[0089] (8)

[0090] The interference signal generated by the combination of two beams (beam B and beam C) can be represented by the superposition of their electric fields, as shown in equation (9):

[0091] (9)

[0092] Combining equations (6)-(9), the superimposed electric field can be expressed as a series, as shown in equation (10):

[0093] (10)

[0094] After the electric fields are superimposed, the intensity of the light emitted from the Mach-Zehnder interference structure is... I 0( t It can be expressed as the square of the electric field modulus, that is, it is equivalent to the electric field. E 0( t ) and its conjugate term Multiplication, the expression is shown in equation (11):

[0095] (11)

[0096] Where the conjugate electric field The expression is:

[0097] (12)

[0098] q For conjugate electric field E 0( tThe serial numbers corresponding to each longitudinal module. Combining equations (10), (11), and (12), we can briefly summarize them as follows: , , , The expression for light intensity is expanded as shown in equation (13):

[0099] (13)

[0100] Expanding the last two terms, the light intensity can be expressed as shown in equation (14):

[0101] (14)

[0102] In the device of this application, the effective bandwidth of the photodetector is set lower than the repetition frequency of the optical frequency comb. f rep Therefore, only when p = q The self-terminal components at time can be effectively detected. For p ≠ q The difference frequency distribution of the cross-term phase components is outside the system bandwidth and cannot be responded to, so it will not be considered in the subsequent analysis. Therefore, equation (14) can be simplified to equation (15):

[0103] (15)

[0104] Substituting equations (4) and (5) into (15), the final expression for the light intensity is shown in equation (16) (i.e., the interference signal intensity of the superposition of beams B and C):

[0105] (16)

[0106] definition Expression (16) can be written as equation (17):

[0107] (17)

[0108] The spectrum of the signal light (i.e., the interference light produced by the device generating Mach-Zehnder interference, denoted as beam D and beam E). can be The expression obtained by performing a Fourier transform is shown in equation (18):

[0109] (18)

[0110] Considering the principle of Fourier transform and the actual physical meaning of the experimental results, we take the positive frequency AC term for analysis, and the expression is shown in equation (19):

[0111] (19)

[0112] In equation (19), G ( v p - v c ) is related to time t The irrelevant quantity is only related to the shape of the original spectral envelope of the optical frequency comb. It is related to the longitudinal mold number of the comb teeth. p The relevant variables are related to the movement time of the boom reflector assembly 3. t It is irrelevant; it is determined by the initial optical path difference between the two beams (beam B and beam C) in the interference structure. The signal light spectrum is composed of a series of discrete Dirac functions. Composition, namely discrete radio frequency "combs", in which each spectral line frequency is located at At this point, the frequency spacing Δ f p The expression for is shown in equation (20):

[0113] (20)

[0114] Originally, the longitudinal mode components in the optical frequency v p Downmapped to the radio frequency domain f p repetition frequency f rep Downmapped to Δ f p The mapping relationship is determined by the down-conversion factor 2. nu / c The determination is made when the refractive index of the medium is... n When it is constant, the lower conversion factor is determined only by the moving speed of the boom mirror assembly 3. u with the speed of light c The frequency components of the interference light after intensity modulation conversion can be directly detected using a radio frequency detector. Compared to optical comb pulses whose longitudinal modes are in the optical frequency domain, they are easier to detect. Therefore, they are suitable as signal light for excitation photothermal modulation. The expression for the time interval between signal light pulses is shown in equation (21):

[0115] (twenty one)

[0116] To avoid spectral aliasing, the moving distance of the boom reflective lens group 3 within a single pulse acquisition time is limited. L max Satisfying equation (22):

[0117] (twenty two)

[0118] In some embodiments of this application, the driving mechanism is a drive motor commonly used in this technical field, and this application does not specifically limit it.

[0119] In some embodiments of this application, the boom reflector group 3 is driven to move simultaneously by mounting two reflectors in the boom reflector group 3 on the same moving platform, which is connected to the output end of the drive motor.

[0120] In some embodiments of this application, the boom reflector group 3 includes reflectors with mirror surfaces facing each other. The mirror surface of one reflector receives the illumination of the light beam B, ensuring that after the light beam B passes through the reflector in the boom reflector group 3 and changes its optical path, it is reflected by the other reflector in the boom reflector group 3, so that the optical path of the reflected light is parallel to the light beam B.

[0121] In some embodiments of this application, the straight line of the boom reflector group 3 is perpendicular to the optical path of the beam B, which facilitates the calculation of the optical path of the beam B.

[0122] In some embodiments of this application, for example, the optical frequency comb 1 is an electro-optic modulation frequency comb that can freely control the repetition frequency parameter.

[0123] In some embodiments of this application, for example, the first optical beam splitter 2 is a near-infrared commercial space optical beam splitter.

[0124] In some embodiments of this application, for example, the beam combiner 4 is a near-infrared commercial beam combiner.

[0125] like Figure 1 As shown, a photothermal spectral gas sensing device according to certain embodiments of the present invention includes a device for generating Mach-Zehnder interferometry as described in the above embodiments, and further includes:

[0126] A collimation processing unit and a second optical beam splitter 7 are sequentially provided in the optical path of the beam combined by the beam combiner 4. A first photodetector 8 is provided in the optical path of one beam D of the two beams split by the second optical beam splitter 7. A wavelength division multiplexer 9 and a hollow fiber air cell 10 are sequentially provided in the optical path of the other beam E of the two beams split by the second optical beam splitter 7.

[0127] The wavelength division multiplexer 9 also receives probe light, and a polarization controller 16 and a three-terminal circulator 11 are provided on the incident optical path of the probe light.

[0128] One port B111 of the three-terminal circulator 11 is connected to the polarization controller 16, one port C112 of the remaining two ports is connected to the wavelength division multiplexer 9, and the other port D113 of the remaining two ports is connected to the second photodetector 12.

[0129] The polarization controller 16 is used to adjust the contrast of the interference fringes;

[0130] The wavelength division multiplexer 9 is used to couple the probe light and the beam E into the hollow fiber air cell 10, and to pick up the secondary backlight carrying phase modulation information that returns through the hollow fiber air cell 10.

[0131] In this embodiment, by combining the hollow fiber optic gas chamber 10, photothermal spectral gas sensing is achieved. The specific process is as follows:

[0132] The combined beam output from the device that generates the Mach-Zehnder interferometer (with phase modulation information of different frequencies obtained by each longitudinal mode of the optical frequency comb) is split again into beam D and beam E. Beam D is used for detection, and beam E is used to couple with the probe light and be absorbed by the target gas in the hollow fiber gas chamber 10. The target gas absorbs the signal light of different frequencies in the coupled light, generates nonradiative transitions and forms a photothermal effect, resulting in a change in local refractive index, thereby modulating the phase of the probe light, so that the photothermal modulation information of different frequencies on the probe light band is realized, that is, photothermal spectral gas sensing is realized.

[0133] To avoid signal distortion, the effective bandwidth of the first photodetector 8 and the second photodetector 12 is... All must meet the minimum requirements, the expression for which is shown in (23):

[0134] (twenty three)

[0135] In equation (23), This represents the maximum frequency of the radio frequency component D of the light beam. This indicates the maximum frequency of the longitudinal mode of the optical frequency comb.

[0136] For the p Gas absorption caused by the root radio frequency comb, and phase modulation of the probe light along the incident direction of beam E. The phase modulation expression in the detection signal is shown in equation (24), which is detected by the second photodetector 12:

[0137] (twenty four)

[0138] In equation (24), It is a coefficient inversely proportional to the cross-sectional area of ​​the mode field of the hollow fiber, where A is the peak absorption coefficient. L The absorption length of the hollow fiber. It is a normalized linear function of the absorption characteristics. It is the first time that the signal light propagates through the hollow optical fiber. p The optical power of the root radio frequency comb teeth, and the expression for their magnitude relationship, is shown in equation (25):

[0139] (25)

[0140] Fixed in the hollow fiber optic cell structure ( and L Assuming the signal light is constant, the phase modulation characteristics of the probe light depend on the signal light (beam E): its modulation frequency inherits the radio frequency characteristics of the signal light, while the modulation amplitude is simultaneously affected by the amplitude-frequency distribution of beam E. Gas absorption line The influence of the peak absorption coefficient A.

[0141] In a specific embodiment of this application, the output signal a of the second photodetector 12 is divided into two signals. One signal B is connected to the laser 15 of the detection light. In the transmission path between signal B and the laser 15, a first filter 13 and a laser servo control system 14 are sequentially provided. The first filter 13 is used to extract the low-frequency (<100Hz) component in the signal B. The laser servo control system 14 is used to lock the wavelength of the laser 15 at the orthogonal working point of the interference fringes according to the extracted low-frequency component signal.

[0142] The laser servo control system 14 is a laser servo control system well known in the art, and its specific structure is not specifically limited in this application.

[0143] In some embodiments of this application, the first photodetector 8 is connected to a data acquisition card 18; signal C of the two signals is sent to the data acquisition card 18 to collect the signal detected by the first photodetector 8 and the signal detected by the second photodetector 12.

[0144] In some embodiments of this application, a second filter 17 is provided on the transmission path between the signal C and the data acquisition card 18. The second filter 17 is used to extract the high-frequency (>100Hz) component in the signal A.

[0145] In some embodiments of this application, the collimation processing device includes a collimating lens group 5 and an optical fiber collimator 6, which are arranged sequentially in the optical transmission direction;

[0146] The collimation processing device ensures that the combined light output from the device that generates Mach-Zehnder interference is shaped so that the beam can be completely incident on the fiber collimator 6 and efficiently coupled into the fiber.

[0147] In some embodiments of this application, the collimating lens group 5 includes a first collimating lens 51 and a second collimating lens 52 for guiding light into the fiber optic collimator.

[0148] In some embodiments of this application, the hollow fiber gas cell 10 contains a Fabry-Perot interference structure, which can form a stable multi-beam interference field on a small scale, serving as a micro gas sample cell and a photothermal effect excitation and amplifier.

[0149] In some embodiments of this application, the fiber collimator 6 is a commercially available fiber collimator commonly used in this technical field, used to receive the light incident from the collimating lens group 5 and couple it into the second optical beam splitter 7.

[0150] In some embodiments of this application, for example, the second optical beam splitter 7 is a near-infrared commercial space optical beam splitter.

[0151] In some embodiments of this application, the laser 15 is, for example, a commercially available narrow-linewidth continuous laser.

[0152] In some embodiments of this application, for example, the data acquisition card 18 is a commercial multi-channel data acquisition card, wherein two channels are used to acquire the output signals of the first photodetector 8 and the second photodetector 12, respectively.

[0153] In some embodiments of this application, the photothermal spectral gas sensing device further includes a computer 19;

[0154] The computer 19 is used to: analyze the phase modulation information of different frequencies carried by Fourier transform, invert the amplitude spectrum of the beam E, and complete the upconversion and spectral normalization process.

[0155] In some embodiments of this application, for example, computer 19 is a general commercial computer.

[0156] A method for generating Mach-Zehnder interferometry according to certain embodiments of this application includes:

[0157] The light output from the optical frequency comb 1 is split into two beams by the first optical beam splitter 2, resulting in beam B and beam C.

[0158] The beam B changes its path after passing through the movable arm reflector group 3, and is combined with the beam C by the beam combiner 4 to form a Mach-Zehnder interference structure.

[0159] In some embodiments of this application, a method for generating Mach-Zehnder interferometry further includes:

[0160] The boom reflector group 4 moves in a straight line along a direction parallel to the incident beam B, thereby applying intensity modulation at different frequencies to each longitudinal mode component of the signal optical comb pulse.

[0161] After the boom reflector group 3 stops moving, the beam B and the beam C are combined by the beam combiner 4, and the combined beam with intensity modulation information is output, thus completing the mapping from the optical frequency domain to the radio frequency domain.

[0162] A method for photothermal spectral gas sensing according to certain embodiments of this application, implemented based on the aforementioned photothermal spectral gas sensing device, includes:

[0163] After being combined by beam combiner 4, the beam with intensity modulation information is processed by collimation processing device and then split into two beams by second beam splitter 7, resulting in beam D and beam E.

[0164] The beam D is received by the first photodetector 8, and an electrical signal with intensity modulation information is obtained.

[0165] The probe light passes through the polarization controller 16 and is transmitted from port B 111 of the three-terminal circulator 11 to port C 112 of the three-terminal circulator. It is then transmitted to the wavelength division multiplexer 9 and coupled together with the beam E into the wavelength division multiplexer 9. The probe light is then transmitted into the hollow fiber gas cell 10 and absorbed by the sample gas inside the hollow fiber gas cell 10. The hollow fiber gas cell 10 then emits a primary return light with phase modulation information, which includes gas absorption characteristics and amplitude spectrum characteristics of the beam E and the probe light.

[0166] The probe light in the primary retroreflection light interferes with the incident probe light to obtain secondary retroreflection light with periodic intensity modulation information.

[0167] The secondary reflected light is transmitted from port C 112 of the three-terminal circulator 11 to port D 113 of the three-terminal circulator through the wavelength division multiplexer 9, and then from port D 113 to the second photodetector 12 to obtain an electrical signal with periodic intensity modulation information.

[0168] In some embodiments of this application, the electrical signal with periodic intensity modulation information is processed by the second filter 17 and then received by the data acquisition card;

[0169] The electrical signal with intensity modulation information is received by the data acquisition card 18.

[0170] In some embodiments of this application, the electrical signal with periodic intensity modulation information is processed by the first filter 13 and then input to the laser servo control system 14;

[0171] The laser servo control system 14 receives the filtered electrical signal and locks the wavelength of the laser 15 at the orthogonal working point of the interference fringes according to the received filtered electrical signal.

[0172] In some embodiments of this application, the method for photothermal spectral gas sensing further includes:

[0173] Fourier transform is performed on the electrical signal with periodic intensity modulation information and the electrical signal with intensity modulation information acquired by the data acquisition card 18 to obtain phase modulation information.

[0174] The above-mentioned photothermal spectral gas sensing device and detection method are applied to practical applications as follows:

[0175] In the actual embodiment, the signal optical comb has a center wavelength of 1535.4 nm, a repetition frequency of 1200 MHz, and contains 400 longitudinal mode components with amplitudes corresponding to Gaussian envelopes. The spectrum conversion result of the acquired beam is shown in the figure below. Figure 2 As shown in (a). The movable lens group in the Mach-Zehnder interferometer structure... u Moving at a speed of 0.1 m / s, the maximum distance moved is 1.5 m, as shown in the curve. Figure 2 As shown in (b). The initial optical path difference between the single interference arm (i.e., beam B) and the fixed arm (i.e., beam C) is assumed to be... c / 4 f rep n The time-domain interference signal obtained after intensity modulation of the obtained signal optical comb pulse is as follows: Figure 2 As shown in (c), periodic radio frequency pulses appear. A Fourier transform of the time-domain interferogram yields the converted radio frequency spectrum under the signal comb spectrum, as shown below. Figure 2 As shown in (d), this process establishes a one-to-one mapping model of the signal optical comb from the optical frequency longitudinal mode component to the radio frequency longitudinal mode component, with a downconversion factor of 2. nu / c , here n = 1, c It is the speed of light in a vacuum.

[0176] The intensity-modulated time-domain interferometric signal was used as the signal light, incident on a 7cm long hollow fiber gas cell containing 2000ppm acetylene gas (nitrogen as the balancing gas) with a Fabry-Perot interferometer structure. The phase modulation information carried by the probe light was analyzed. Based on the acetylene gas absorption characteristic simulation results from the HITRAN database, the absorption characteristics in the spectral range of 1534.5 nm to 1536.3 nm were analyzed (intensity only was considered here). The original simulated spectrum of the photothermal signal is shown below. Figure 3 As shown in (a), the absorption region exhibits a radio frequency component distribution influenced by both the signal comb power spectrum envelope and the absorption line shape. After normalizing the baseline shift caused by the signal comb power spectrum envelope characteristics, the original photothermal signal image is obtained, as detailed in [reference needed]. Figure 3As shown in (b), the distribution of the radio frequency components closely matches the shape of the linear envelope of the acetylene absorption. The above simulation examples demonstrate the feasibility of single-optical frequency comb photothermal spectroscopy gas sensing based on Mach-Zehnder interferometry.

[0177] Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A photothermal spectral gas sensing device, characterized in that, The device includes a means for generating Mach-Zehnder interference, wherein the means for generating Mach-Zehnder interference includes, in sequence, an optical frequency comb and a first optical beam splitter in the optical transmission path; In the two beams split by the first beam splitter, a movable arm reflector group is provided on the optical path of one beam B, and the path of beam B is changed by the movable arm reflector group and intersects with the optical path of the other beam C, and a beam combiner is provided at the intersection. The boom reflector assembly is connected to a drive mechanism, which drives the boom reflector assembly to perform linear motion. The trajectory of the linear motion is parallel to the line where the light beam B is located. Also includes: A collimation processing unit and a second optical beam splitter are sequentially provided in the optical path of the beam combined by the beam combiner. A first photodetector is provided in the optical path of one beam D, which is split into two beams by the second optical beam splitter. A wavelength division multiplexer and a hollow fiber air cell are sequentially provided in the optical path of the other beam E, which is split into two beams by the second optical beam splitter. The wavelength division multiplexer receives the probe light, and a polarization controller and a three-terminal circulator are also provided on the incident light path of the probe light. One port B of the three-terminal circulator is connected to the polarization controller, one port C of the remaining two ports is connected to the wavelength division multiplexer, and the other port D of the remaining two ports is connected to the second photodetector.

2. The photothermal spectral gas sensing device according to claim 1, characterized in that, The boom reflector assembly includes two reflectors with their mirror surfaces facing each other, and one of the reflectors receives the illumination of the light beam B.

3. The photothermal spectral gas sensing device according to claim 1, characterized in that, The straight line containing the boom reflector group is perpendicular to the optical path of the beam B.

4. The photothermal spectral gas sensing device according to claim 1, characterized in that, The output signal of the second photodetector is divided into two signals, signal B and signal C, and signal B is connected to the laser of the detection light; A first filter and a laser servo control system are sequentially provided in the transmission path between signal B and the laser.

5. The photothermal spectral gas sensing device according to claim 4, characterized in that, The first photodetector is connected to a data acquisition card; The signal C is sent to the data acquisition card; A second filter is also provided in the transmission path between the signal C and the data acquisition card.

6. The photothermal spectral gas sensing device according to claim 1, characterized in that... Effective bandwidth of the first photodetector and the second photodetector All satisfy the following expression: in, This represents the maximum frequency of the radio frequency component of the interference light produced by the device that generates the Mach-Zehnder interference. v c The longitudinal mode frequency closest to the center of the spectrum. This indicates the maximum frequency of the longitudinal mode of the optical frequency comb. u The moving speed of the boom reflector assembly. t For the time of travel, n It is the refractive index of the medium surrounding the device that produces the Mach-Zehnder interferometer. c It is the speed of light in a vacuum. This represents the bandwidth of the contour function.

7. The photothermal spectral gas sensing device according to claim 1, characterized in that, The collimation processing device includes a collimating lens group and an optical fiber collimator, which are arranged sequentially in the optical transmission direction; And / or, the straight lens group includes a first collimating lens and a second collimating lens; And / or, the hollow fiber optic chamber contains a Fabry-Perot interference structure.

8. A detection method for photothermal spectroscopy gas sensing, characterized in that, Based on the photothermal spectral gas sensing device according to claim 5, it includes: After being combined by the beam combiner, the beam containing intensity modulation information is processed by the collimation unit and then split into two beams by the second beam splitter, resulting in beam D and beam E. The beam D is received by the first photodetector, and an electrical signal with intensity modulation information is obtained. The probe light passes through a polarization controller and is transmitted from port B of the three-terminal circulator to port C of the three-terminal circulator. It is then transmitted to a wavelength division multiplexer and coupled together with the beam E into the wavelength division multiplexer. The probe light is then transmitted to the hollow fiber gas chamber and absorbed by the sample gas inside the hollow fiber gas chamber. The hollow fiber gas chamber then emits a primary return light with phase modulation information, which includes gas absorption characteristics and amplitude spectrum characteristics of beam E and the probe light. The probe light in the primary retroreflection light interferes with the incident probe light to obtain secondary retroreflection light with periodic intensity modulation information. The secondary reflected light is transmitted from port C of the three-terminal circulator to port D of the three-terminal circulator through the wavelength division multiplexer, and then from port D to the second photodetector to obtain an electrical signal with periodic intensity modulation information.

9. The detection method for photothermal spectral gas sensing according to claim 8, characterized in that, The electrical signal with periodic intensity modulation information is processed by the second filter and then received by the data acquisition card. The electrical signal with intensity modulation information is received by the data acquisition card.

10. The detection method for photothermal spectral gas sensing according to claim 8, characterized in that, The electrical signal with periodic intensity modulation information is processed by the first filter and then input to the laser servo control system. The laser servo control system receives the filtered electrical signal and locks the laser wavelength at the orthogonal working point of the interference fringes based on the received filtered electrical signal.

11. The detection method for photothermal spectral gas sensing according to claim 8, characterized in that, Phase modulation of light in the periodic intensity modulation information As shown below: in, It is a coefficient inversely proportional to the cross-sectional area of ​​the mode field of the hollow fiber, where A is the peak absorption coefficient. L The absorption length of the hollow fiber. It is a normalized linear function of the absorption characteristics; It is the first time that the signal light propagates through the hollow optical fiber. p The optical power of the root radio frequency comb teeth is expressed as follows: in, For in the radio frequency domain f p Below is the spectrum of interference light produced by the device that generates Mach-Zehnder interference.

12. The detection method for photothermal spectral gas sensing according to any one of claims 8-11, characterized in that, Also includes: Fourier transform is performed on the electrical signals with periodic intensity modulation information and the electrical signals with intensity modulation information acquired by the data acquisition card to obtain phase modulation information.

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