A cavity ring-down system and gas measurement method based on dual optical path correction

Abstract

The present invention provides a cavity ring-down system and a gas measurement method based on dual optical path correction, the system comprising: a first measurement optical path system, a second measurement optical path system and a dual optical path correction system; the first measurement optical path system and the second measurement optical path system share a resonant cavity, and the dual optical path correction system is used to maintain the consistency of the ring-down signal. The present invention solves the technical problem of poor stability and reliability of the cavity ring-down optical path system in the prior art.

Description

Technical Field

[0001] The present invention relates to the technical field of gas detection, and in particular to a cavity ring-down system based on dual-optical path correction and a gas measurement method. Background Art

[0002] Cavity ring-down spectroscopy detection technology is one of the most accurate and lowest detection limit methods for trace greenhouse gas spectroscopy today. Compared with common mass spectrometry-chromatography, tunable semiconductor laser absorption spectroscopy and off-axis integral cavity spectroscopy, it has the advantages of immunity to laser intensity fluctuations, long absorption path and high sensitivity.

[0003] Cavity ring-down spectroscopy detection technology usually consists of two or more lenses with high reflectivity to form a resonant cavity. By tuning the wavelength of the laser, the laser beam enters the resonant cavity through a series of external collimation and polarization light paths and is absorbed by the gas to be measured. Finally, the detector at the output end of the resonant cavity captures the absorbed light signal for concentration measurement.

[0004] From the principle of cavity ring-down spectroscopy detection technology, we can know that the cavity ring-down system achieves an ultra-long absorption optical path (ranging from 10 to 100 km) through multiple reflections of the incident laser in the ring-down cavity, thereby having a lower detection limit and detection accuracy, which requires the stability and high reliability of the entire optical path system. Summary of the invention

[0005] The purpose of the present invention is to provide a cavity ring-down system and a gas measurement method based on dual optical path correction, so as to solve the technical problems of poor stability and reliability of the cavity ring-down optical path system in the prior art.

[0006] The present invention provides a cavity ring-down system based on dual-optical path correction, which specifically includes:

[0007] A first measuring optical path system, a second measuring optical path system and a dual optical path correction system;

[0008] The first measuring optical path system and the second measuring optical path system share a resonant cavity, and the consistency of the ring-down signal is maintained through the dual optical path correction system.

[0009] Furthermore, the first measuring optical path system includes: a first near-infrared DFB laser, a first feedback rate control module, a first mode matching lens, a first dichroic mirror with an electric adjustment structure, a resonant cavity, a first focusing lens and a first photodetector.

[0010] Furthermore, the first feedback rate control module includes: a first 1 / 2 wave plate, a polarization beam splitter and a first 1 / 4 wave plate.

[0011] Furthermore, the output light emitted by the first near-infrared DFB laser passes through the first feedback rate control module, enters the resonant cavity through the first mode matching lens and the first dichroic mirror with an electric adjustment structure, and then exits along the output end of the resonant cavity and enters the first photodetector after passing through the first focusing lens.

[0012] Furthermore, the second measuring optical path system includes: a second near-infrared DFB laser, a second feedback rate control module, a second mode matching lens, a second dichroic mirror with an electric adjustment structure, a resonant cavity, a second focusing lens, a second photodetector and a beam deflecting mirror.

[0013] Furthermore, the second feedback rate control module includes: a second 1 / 2 wave plate, a polarization beam splitter and a second 1 / 4 wave plate.

[0014] Furthermore, the output light emitted by the second near-infrared DFB laser passes through the second feedback rate control module, enters the second mode matching lens through the beam deflection mirror, enters the resonant cavity through the second dichroic mirror with an electric adjustment structure, and then exits along the output end of the resonant cavity and enters the second photodetector after passing through the second focusing lens.

[0015] Further, the dual-light path correction system includes: a third focusing lens, a second four-quadrant detector, a first four-quadrant detector, and a fourth focusing lens;

[0016] The third focusing lens receives the light beam from the first dichroic mirror with an electric adjustment structure and focuses the light beam onto the second four-quadrant detector;

[0017] The fourth focusing lens receives the light beam from the second dichroic mirror with an electric adjustment structure and focuses the light beam onto the first four-quadrant detector.

[0018] A gas measurement method, applied to a cavity ring-down system based on dual optical path correction, comprises the following steps;

[0019] S11, when the output light signal of the resonant cavity meets the preset threshold condition, the power supply of the first near-infrared DFB laser and the second near-infrared DFB laser is cut off through the laser control system;

[0020] S21, collecting a ring-down signal by a first photodetector and a second photodetector;

[0021] S31, obtaining a ring-down time according to the collected ring-down signal;

[0022] S41. Obtain the absorption coefficient of the gas to be measured according to the ring-down time.

[0023] A dual-optical path correction method is applied to a cavity ring-down system based on dual-optical path correction. The correction steps are as follows:

[0024] S111, the first near-infrared DFB laser is used as the first light source of the ring-down cavity, and its output light is divided into a P1 beam and an S1 beam after passing through a first feedback rate control module and a polarization beam splitter;

[0025] The S121 and S1 beams are collected by the first photodetector; the P1 beam is collected by the first four-quadrant detector;

[0026] S131, a second near-infrared DFB laser is used as the second light source of the ring-down cavity, and its output light is divided into a P2 beam and an S2 beam after passing through a second feedback rate control module and a polarization beam splitter;

[0027] The S141 and S2 beams are collected by the second photoelectric detector; the P2 beam is collected by the second four-quadrant detector;

[0028] S151. When the external environment changes or the optical path shifts, the quadrant positions on the first photodetector and the first four-quadrant detector will be deflected, the ring-down signal will change significantly, and the ring-down signal will also be affected. At this time, the angles of the first dichroic mirror with an electric adjustment structure and the second dichroic mirror with an electric adjustment structure are adjusted according to the position deviation of the detection signal of the four-quadrant detector to control the parallelism of the light beam entering the resonant cavity and ensure the consistency of the ring-down signal.

[0029] The beneficial effects of the present invention are:

[0030] 1. Use optical feedback to narrow the line width of the laser and achieve frequency locking control of the laser.

[0031] 2. The adaptive correction of the dual-optical path system is more reliable and stable than the current single-path optical cavity ring-down system.

[0032] 3. The present invention overcomes the influence of the overlapping absorption peaks of different gases on the measurement accuracy when measuring mixed gases, and can achieve measurement with higher accuracy and lower system detection limit. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings, which constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

[0034] Figure 1 It is a schematic diagram of the system structure of the present invention;

[0035] Figure 1 middle:

[0036] 1. A first near-infrared DFB laser; 2. A first 1 / 2 wave plate; 3. A polarization beam splitter; 4. A second 1 / 2 wave plate; 5. A second near-infrared DFB laser; 6. A first 1 / 4 wave plate; 7. A second 1 / 4 wave plate; 8. A first mode-matching lens; 9. A first dichroic mirror with an electric adjustment structure; 10. A second focusing lens; 11. A second photodetector; 12. A third focusing lens; 13. A second four-quadrant detector; 14. A resonant cavity; 15. A first four-quadrant detector; 16. A fourth focusing lens; 17. A second dichroic mirror with an electric adjustment structure; 18. A second mode-matching lens; 19. A beam deflecting mirror; 20. A first focusing lens; 21. A first photodetector. DETAILED DESCRIPTION

[0037] Various exemplary embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings. It should be noted that unless otherwise specifically stated, the relative arrangement of components and steps, numerical expressions and numerical values ​​set forth in these embodiments do not limit the scope of the present invention.

[0038] At the same time, it should be understood that for the convenience of description, the sizes of the various parts shown in the drawings are not drawn according to the actual proportional relationship.

[0039] The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses.

[0040] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0041] Technologies, methods, and apparatus known to ordinary technicians in the relevant field may not be discussed in detail, but where appropriate, such technologies, methods, and apparatus should be considered part of the authorization specification.

[0042] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limiting. Therefore, other examples of the exemplary embodiments may have different values.

[0043] It should be noted that like reference numerals and letters refer to similar items in the following figures, and therefore, once an item is defined in one figure, it need not be further discussed in subsequent figures.

[0044] The present invention provides a cavity ring-down system based on dual-optical path correction, comprising:

[0045] A first measuring optical path system, a second measuring optical path system and a dual optical path correction system;

[0046] The first measuring optical path system and the second measuring optical path system share a resonant cavity, and the consistency of the ring-down signal is maintained through the dual optical path correction system.

[0047] It should be noted that the first measuring optical path system includes: a first near-infrared DFB laser 1, a first feedback rate control module, a first mode matching lens 8, a first dichroic mirror with an electric adjustment structure 9, a resonant cavity 14, a fourth focusing lens 20 and a second photodetector 21.

[0048] The first feedback rate control module includes: a first 1 / 2 wave plate 2 , a polarization beam splitter 3 and a first 1 / 4 wave plate 6 .

[0049] The outgoing light emitted by the first near-infrared DFB laser 1 passes through the first feedback rate control module, enters the resonant cavity 14 through the first mode matching lens 8 and the first dichroic mirror 9 with an electric adjustment structure, and then exits along the output end of the resonant cavity 14, and enters the first photodetector 21 after passing through the first focusing lens 20.

[0050] Specifically, the first near-infrared DFB laser 1 serves as the first measurement light source of the ring-down cavity. Its output light passes through the feedback rate control module composed of the first 1 / 2 wave plate 2, the polarization beam splitter 3 and the first 1 / 4 wave plate 6, and then passes through the first mode matching lens 8 and the first dichroic mirror 9 with an electric adjustment structure and enters the resonant cavity 14. After multiple reflections in the resonant cavity 14, it is emitted along the cavity output end, and after being focused by the first focusing lens 20, it enters the first photodetector 21.

[0051] It should be noted that the second measuring optical path system includes: a second near-infrared DFB laser 5, a second feedback rate control module, a second mode matching lens 18, a second dichroic mirror 17 with an electric adjustment structure, a resonant cavity 14, a second focusing lens 10, a second photodetector 11 and a beam deflecting mirror 19.

[0052] The second feedback rate control module includes: a second 1 / 2 wave plate 4 , a polarization beam splitter 3 and a second 1 / 4 wave plate 7 .

[0053] The outgoing light emitted by the second near-infrared DFB laser 5 passes through the second feedback rate control module, enters the resonant cavity 14 through the second mode matching lens 18 and the second dichroic mirror 17 with an electric adjustment structure, and then exits along the output end of the resonant cavity 14, and enters the second photodetector 11 after passing through the second focusing lens 10.

[0054] Specifically, the second near-infrared DFB laser 5 serves as the second measurement light source of the ring-down cavity. Its output light passes through the feedback rate control module composed of the second 1 / 2 wave plate 4, the polarization beam splitter 3 and the second 1 / 4 wave plate 7, and then enters the second mode matching lens 18 and the second dichroic mirror 17 with an electric adjustment structure through the beam deflection mirror 19, and enters the resonant cavity 14. After multiple reflections in the resonant cavity 14, it is emitted along the cavity output end, and after being focused by the second focusing lens 10, it enters the second photodetector 11.

[0055] A gas measurement method, applied to a cavity ring-down system based on dual optical path correction, comprises the following steps;

[0056] S11, when the output light signal of the resonant cavity 14 meets the preset threshold condition, the power supply of the first near-infrared DFB laser 1 and the second near-infrared DFB laser 2 is cut off through the laser control system;

[0057] S21, collecting the ring-down signal by the first photodetector 21 and the second photodetector 11;

[0058] S31, obtaining a ring-down time according to the collected ring-down signal;

[0059] S41. Obtain the absorption coefficient of the gas to be measured according to the ring-down time.

[0060] As an embodiment, after the emitted optical signal meets the threshold condition, the power supply of the DFB laser is cut off by the laser control system, and after the shut-off action is completed, the ring-down signal is collected by the photodiode detector (11, 21).

[0061] The collected ring-down signal uses the formula: Fitting is performed and the ring-down time τ is obtained, where It is the light intensity that changes with time, I0 is the light intensity at the beginning of cavity ring-down, and τ is the ring-down time constant obtained by fitting. The ring-down time constant τ at a certain frequency υ can be written as Where L is the cavity length, c is the speed of light, C is the concentration of the substance to be measured, and α is the absorption coefficient of the substance to be measured. When the concentration of the substance to be measured is known, the absorption coefficient α of the medium can be calculated. The frequency of the DFB laser is tuned by the laser control system, so that the frequency scanning range of the DFB laser covers the entire absorption peak of the gas to be measured, and the absorption coefficient α at several different frequency points in the absorption peak is obtained.

[0062] In the optical feedback theory, the total length of the external optical path from the laser output end to the resonant cavity input mirror must satisfy the integral multiple of the resonant cavity length, that is, the phase matching condition. Therefore, the feedback optical path of the first near-infrared DFB laser 1 is set to 3 times the resonant cavity length, and the feedback optical path of the second near-infrared DFB laser 5 is set to 2 times the resonant cavity length. The control of the feedback rate is achieved by utilizing the birefringence effect of light in anisotropic crystals, that is, by controlling the rotation angles of the two wave plates through a combination of 1 / 2 wave plates and 1 / 4 wave plates, respectively, to achieve control of the feedback rate.

[0063] It should be noted that the dual-light path correction system includes: a third focusing lens 12, a second four-quadrant detector 13, a first four-quadrant detector 15, and a fourth focusing lens 16;

[0064] The third focusing lens 12 receives the light beam from the first dichroic mirror 9 with an electric adjustment structure, and focuses the light beam onto the second four-quadrant detector 13;

[0065] The fourth focusing lens 16 receives the light beam from the second dichroic mirror 17 with an electric adjustment structure, and focuses the light beam onto the first four-quadrant detector 15 .

[0066] A dual-optical path correction method is applied to a cavity ring-down system based on dual-optical path correction. The correction steps are as follows:

[0067] S111, the first near-infrared DFB laser 1 is used as the first light source of the ring-down cavity, and its output light is divided into a P1 beam and an S1 beam after passing through a first feedback rate control module and a polarization beam splitter 3;

[0068] The S121 and S1 beams are collected by the first photodetector 21; the P1 beam is collected by the first four-quadrant detector 15;

[0069] S131, the second near-infrared DFB laser 5 is used as the second light source of the ring-down cavity, and its output light is divided into a P2 beam and an S2 beam after passing through a second feedback rate control module and a polarization beam splitter 3;

[0070] The S141 and S2 beams are collected by the second photodetector 11; the P2 beam is collected by the second four-quadrant detector 13;

[0071] S151. When the external environment changes or the optical path shifts, the quadrant positions on the first photodetector 21 and the first four-quadrant detector 15 will be deflected, the ring-down signal will change significantly, and the ring-down signal will also be affected. At this time, the angles of the first dichroic mirror 9 with an electric adjustment structure and the second dichroic mirror 17 with an electric adjustment structure are adjusted according to the position deviation of the detection signal of the four-quadrant detector to control the parallelism of the light beam entering the resonant cavity 14 and ensure the consistency of the ring-down signal.

[0072] As an embodiment, the adaptive correction of the dual-light path system is specifically implemented as follows:

[0073] The first near-infrared DFB laser 1 is used as the first light source of the ring-down cavity. After its output light passes through the feedback rate control module composed of the first 1 / 2 wave plate 2, the polarization beam splitter 3 and the first 1 / 4 wave plate 6, the polarization beam splitter 3 will split the light beam into two paths, P1 light and S1 light.

[0074] The S1 light passes through the first mode matching lens 8 and the first dichroic mirror 9 with an electric adjustment structure and enters the resonant cavity 14 . After multiple reflections in the resonant cavity 14 , it is emitted along the cavity output end and is focused by the first focusing lens 20 and enters the first photodetector 21 .

[0075] The P1 light enters the first four-quadrant detector 15 through the beam deflecting mirror 19 , the second mode matching lens 18 , the second dichroic mirror 17 with an electric adjustment structure, and the fourth focusing lens 16 .

[0076] The second near-infrared DFB laser 5 serves as the second light source of the ring-down cavity. After its output light passes through the feedback rate control module composed of the second 1 / 2 wave plate 4, the polarization beam splitter 3 and the second 1 / 4 wave plate 7, the polarization beam splitter 3 will split the light beam into two paths, P2 light and S2 light.

[0077] The P2 light passes through the beam deflecting mirror 19, the second mode matching lens 18, the second dichroic mirror 17 with an electric adjustment structure, and enters the resonant cavity 14. After multiple reflections in the resonant cavity 14, it is emitted along the cavity output end, and after being focused by the second focusing lens 10, it enters the second photodetector 11.

[0078] The S2 light passes through the first mode matching lens 8 , the first dichroic mirror 9 with an electric adjustment structure, and the third focusing lens 12 and then enters the second four-quadrant detector 13 .

[0079] After the emitted optical signal meets the threshold condition, the laser control system cuts off the power supply of the DFB laser, and after the shutdown action is completed, the photodiode detector, the second photodetector 11, and the second four-quadrant detector 13 collect the ring-down signal.

[0080] Because the reflectivity of the cavity mirror is consistent with the reflectivity of the two lasers, the decay time of the two optical systems should be consistent. When the external environment changes or the optical path of the instrument shifts, the quadrant position of the first photodetector 21 and the first four-quadrant detector 15 of the photodetector will be deflected, the decay signal will change significantly, and the decay signal will also be affected. At this time, the angles of the first dichroic mirror 9 with an electric adjustment structure and the second dichroic mirror 17 with an electric adjustment structure are adjusted according to the position deviation of the detection signal of the four-quadrant detector to control the parallelism of the light beam entering the resonant cavity 14 and ensure the consistency of the decay signal.

[0081] The two ring-down optical paths of the present invention do not interfere with each other, so two different types of gases can be measured simultaneously, and the influence of the overlapping absorption peaks of different gases on the measurement accuracy when the two gases are mixed can be overcome in combination with an algorithm, so that measurements with higher accuracy and lower system detection limit can be achieved.

[0082] The beneficial effects of the present invention are:

[0083] 1. Use optical feedback to narrow the line width of the laser and achieve frequency locking control of the laser.

[0084] 2. The adaptive correction of the dual-optical path system is more reliable and stable than the current single-path optical cavity ring-down system.

[0085] 3. The present invention overcomes the influence of the overlapping absorption peaks of different gases on the measurement accuracy when measuring mixed gases, and can achieve measurement with higher accuracy and lower system detection limit.

[0086] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

[0087] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the process, method, commodity or device including the elements.

Claims

1. A cavity ring-down system based on dual optical path correction, characterized in that: include: A first measuring optical path system, a second measuring optical path system and a dual optical path correction system; The first measuring optical path system and the second measuring optical path system share a resonant cavity, and the consistency of the ring-down signal is maintained through the dual optical path correction system; The first measuring optical path system comprises: a first near-infrared DFB laser (1), a first feedback rate control module, a first mode matching lens (8), a first dichroic mirror with an electric adjustment structure (9), a resonant cavity (14), a first focusing lens (20) and a first photodetector (21); The first feedback rate control module comprises: a first 1 / 2 wave plate (2), a polarization beam splitter (3) and a first 1 / 4 wave plate (6); The output light emitted by the first near-infrared DFB laser (1) passes through a first feedback rate control module, enters a resonant cavity (14) through a first mode matching lens (8) and a first dichroic mirror (9) with an electric adjustment structure, then exits along an output end of the resonant cavity (14), passes through a first focusing lens (20), and enters a first photodetector (21); The second measuring optical path system comprises: a second near-infrared DFB laser (5), a second feedback rate control module, a second mode matching lens (18), a second dichroic mirror with an electric adjustment structure (17), a resonant cavity (14), a second focusing lens (10), a second photodetector (11) and a beam deflecting mirror (19); The second feedback rate control module comprises: a second 1 / 2 wave plate (4), a polarization beam splitter (3) and a second 1 / 4 wave plate (7); The output light emitted by the second near-infrared DFB laser (5) passes through the second feedback rate control module, enters the second mode matching lens (18) through the beam deflection mirror (19), enters the second dichroic mirror (17) with an electric adjustment structure, enters the resonant cavity (14), and then exits along the output end of the resonant cavity (14), passes through the second focusing lens (10), and enters the second photodetector (11); The dual-light path correction system comprises: a third focusing lens (12), a second four-quadrant detector (13), a first four-quadrant detector (15), and a fourth focusing lens (16); The third focusing lens (12) receives the light beam from the first dichroic mirror (9) with an electric adjustment structure, and focuses the light beam onto the second four-quadrant detector (13); The fourth focusing lens (16) receives the light beam from the second dichroic mirror (17) with an electric adjustment structure, and focuses the light beam onto the first four-quadrant detector (15).

2. A gas measurement method, applied to a cavity ring-down system based on dual optical path correction as claimed in claim 1, characterized in that: The method comprises the following steps: S11, when the output light signal of the resonant cavity (14) meets a preset threshold condition, cutting off the power supply of the first near-infrared DFB laser (1) and the second near-infrared DFB laser (5) through the laser control system; S21, collecting a ring-down signal by a first photodetector (21) and a second photodetector (11); S31, obtaining a ring-down time according to the collected ring-down signal; S41. Obtain the absorption coefficient of the gas to be measured according to the ring-down time.

3. A dual-optical path correction method, applied to a cavity ring-down system based on dual-optical path correction as claimed in claim 1, characterized in that: The calibration steps are as follows: S111, a first near-infrared DFB laser (1) is used as a first light source of the ring-down cavity, and its output light is divided into a P1 light beam and an S1 light beam after passing through a first feedback rate control module and a polarization beam splitter (3); The S121 and S1 light beams are collected by the first photoelectric detector (21); the P1 light beam is collected by the first four-quadrant detector (15); S131, a second near-infrared DFB laser (5) is used as the second light source of the ring-down cavity, and its output light is divided into a P2 light beam and an S2 light beam after passing through a second feedback rate control module and a polarization beam splitter (3); The S141 and S2 light beams are collected by a second photoelectric detector (11); the P2 light beam is collected by a second four-quadrant detector (13); S151. When the external environment changes or the optical path shifts, the quadrant positions on the first photodetector (21) and the first four-quadrant detector (15) will be deflected, the ring-down signal will change significantly, and the ring-down signal will also be affected. At this time, the angles of the first dichroic mirror (9) with an electric adjustment structure and the second dichroic mirror (17) with an electric adjustment structure are adjusted according to the position deviation of the detection signal of the four-quadrant detector to control the parallelism of the light beam entering the resonant cavity (14) and ensure the consistency of the ring-down signal.

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