Crds system and method with etalon effect influence elimination function

Abstract

The application provides a CRDS system and method with Etalon effect influence elimination function, which comprises a light source, a light switch, a light reflection cavity and a first detector; a beam splitting unit is used for splitting the measuring light emitted by the light source into a first light beam and a second light beam with the same light intensity; the first light beam passes through the first mirror, and then is reflected on the second mirror, the third mirror and the first mirror in turn, and passes through multiple reflections in this order, and finally is emitted from the second mirror and received by the first detector; the second light beam passes through the first mirror, and then is reflected on the third mirror, the second mirror and the first mirror in turn, and passes through multiple reflections in this order, and finally is emitted from the second mirror and received by the second detector; an analysis unit is used for summing a first ring-down signal output by the first detector and a second ring-down signal output by the second detector. The application has the advantages of high resolution, small volume and the like.

Description

Technical Field

[0001] This invention relates to cavity ringback technology, and more particularly to a CRDS system and method with Etalon effect elimination function. Background Technology

[0002] Currently, in CRDS systems for measuring trace gases, concentration drift is a crucial performance indicator, and the Etalon effect of the CRDS cavity is the primary factor influencing this drift. Due to the high reflectivity of the mirrors in the CRDS cavity, multiple reflections of the beam between these mirrors lead to the Etalon effect. This ultimately results in a sinusoidal modulation of the ring-down times of the signals generated by different longitudinal modes within the cavity. This has a significant impact on systems that calculate gas concentration by integrating the absorption peak area, manifesting as sinusoidal modulation of the concentration measurement.

[0003] To mitigate or eliminate the effects of the Etalon effect, existing technologies employ the following methods: 1. The method of changing the standing wave cavity with two high-reflectivity lenses to a traveling wave cavity with three lenses is currently widely used in many research and products. However, the traveling wave cavity cannot completely eliminate the influence of the Etalon effect.

[0004] 2. Typical CRDS systems only utilize the transmitted (forward) signal from the optical cavity because the forward signal has a high signal-to-noise ratio and strong signal intensity. In addition, there is a reflected (backward) signal, which is generated by defects in the optical elements within the cavity. Some studies have shown that simultaneously acquiring the forward signal and the backward signal can reduce the effects of the Etalon effect; however, in practical applications, the backward signal is very weak and has a low signal-to-noise ratio, making the effect negligible in the system. Summary of the Invention

[0005] To address the shortcomings of the existing technical solutions, this invention provides a CRDS system with Etalon effect elimination function.

[0006] The objective of this invention is achieved through the following technical solution: A CRDS system with Etalon effect cancellation capability includes a light source, an optical switch, an optical reflecting cavity, and a first detector; the CRDS system with Etalon effect cancellation capability further includes: The beam splitting unit is used to split the measurement light emitted by the light source into a first beam and a second beam with the same light intensity. The first beam and the second beam are linearly polarized light with the same polarization state. A first reflector, a second reflector, and a third reflector are disposed within the light reflection cavity. The first light beam passes through the first reflector and is then reflected sequentially by the second, third, and first reflectors, undergoing multiple reflections in this order before finally exiting from the second reflector and being received by the first detector. The second light beam passes through the first reflector and is then reflected sequentially by the third, second, and first reflectors, undergoing multiple reflections in this order before finally exiting from the second reflector and being received by the second detector. The analysis unit is used to sum the first oscillation signal output by the first detector and the second oscillation signal output by the second detector.

[0007] The present invention also aims to provide a method for eliminating the influence of the Etalon effect in CRDS systems, which is achieved through the following technical solution: A method for eliminating the influence of the Etalon effect in a CRDS system, the method being: The measurement light emitted by the light source is split into a first beam and a second beam with the same light intensity after passing through the beam splitting unit. Both the first beam and the second beam are S or P linearly polarized light. The first beam and the second beam are incident into the light reflection cavity at different angles. The first beam passes through the first reflector and is then reflected in sequence by the second reflector, the third reflector, and the first reflector, undergoing multiple reflections in this order before finally exiting from the second reflector and being received by the first detector. At the same time, the second beam passes through the first reflector and is then reflected in sequence by the third reflector, the second reflector, and the first reflector, undergoing multiple reflections in this order before finally exiting from the second reflector and being received by the second detector. The first and second light beams entering the optical reflection cavity are turned off, and the first and second detectors respectively output ring-down signals. The analysis unit sums the two oscillation signals to eliminate the influence of the Etalon effect.

[0008] Compared with the prior art, the beneficial effects of the present invention are as follows: The effect of eliminating the Etalon effect is good; The first and second beams with the same intensity and polarization state enter the first mirror of the optical reflection cavity at different angles, and then are reflected in opposite directions on the three mirrors. After multiple reflections, they exit from the second mirror respectively. When the light is turned off, the first detector and the second detector output oscillation signals with the same intensity and a phase difference of π. The influence of the Etalon effect is removed by summation. It has a high signal-to-noise ratio that eliminates the influence of the Etalon effect and a lower detection limit for trace gases; 2. Simple structure; The beam splitter, optical switch, detector and analysis unit used in the CRDS system are all conventional components, with simple structure and good reliability. Attached Figure Description

[0009] The disclosure of this invention will become more readily understood with reference to the accompanying drawings. It will be readily understood by those skilled in the art that these drawings are merely illustrative of the technical solutions of this invention and are not intended to limit the scope of protection of this invention. In the drawings: Figure 1 This is a schematic diagram of the structure of a CRDS system with Etalon effect elimination function according to an embodiment of the present invention; Figure 2 This is a schematic diagram of summing two oscillation signals according to an embodiment of the present invention; Figure 3 This is a schematic diagram illustrating the elimination effect of the Etalon effect according to an embodiment of the present invention. Detailed Implementation

[0010] Figures 1-3 The following description illustrates optional embodiments of the invention to teach those skilled in the art how to implement and reproduce the invention. Some conventional aspects have been simplified or omitted to teach the technical solutions of the invention. Those skilled in the art should understand that variations or substitutions derived from these embodiments will be within the scope of the invention. Those skilled in the art should understand that the following features can be combined in various ways to form multiple variations of the invention. Therefore, the invention is not limited to the optional embodiments described below, but is defined only by the claims and their equivalents.

[0011] Example 1

[0012] The CRDS system with Etalon effect elimination function in the embodiments of the present invention, such as Figure 1 As shown, the CRDS system with Etalon effect elimination function includes: The light source 11, the optical switch 21, and the first detector 51 are all existing technologies in the CRDS field; The beam splitting unit 31 is used to split the measurement light emitted by the light source 11 into a first beam 39 and a second beam 38 with the same light intensity. The first beam 39 and the second beam 38 are linearly polarized light with the same polarization state. A first reflector 41, a second reflector 42, and a third reflector 43 are disposed within the light reflection cavity 40. The first light beam 39 passes through the first reflector 41, and is then reflected sequentially by the second reflector 42, the third reflector 43, and the first reflector 41, undergoing multiple reflections in this order before exiting from the second reflector 42 and being received by the first detector 51. The second light beam 38 passes through the first reflector 41, and is then reflected sequentially by the third reflector 43, the second reflector 42, and the first reflector 41, undergoing multiple reflections in this order before exiting from the second reflector 42 and being received by the second detector 52. Analysis unit 61 is used to sum the first oscillation signal output by the first detector 51 and the second oscillation signal output by the second detector 52, such as... Figure 2 As shown.

[0013] To obtain a first beam 39 and a second beam 38 with the same light intensity and polarization state, the beam splitting unit 31 further includes: The first half-wave plate 32 and the polarizing beam splitter 33, the measuring light passes through the first half-wave plate 32, and then the first beam 39 and the second beam 38 are split on the polarizing beam splitter 33. The second half-wave plate 34, through which the second beam 38 passes, is incident into the light reflection cavity.

[0014] In order to obtain a first beam 39 and a second beam 38 with the same light intensity and polarization state, the measurement light is further S or P linearly polarized light, and the beam splitting unit 31 includes a semi-transparent and semi-reflective mirror.

[0015] In order to achieve more reflections between the three mirrors, the first mirror 41 and the second mirror 42 are further arranged symmetrically with respect to the normal on the third mirror 43, which is a concave mirror.

[0016] To control whether light enters the light reflection cavity, the light switch 21 is further disposed in the optical path between the light source 11 and the beam splitting unit 31.

[0017] The method for eliminating the influence of the Etalon effect in a CRDS system according to an embodiment of the present invention is as follows: The measurement light emitted by the light source 11 is split into a first beam 39 and a second beam 38 with the same light intensity after passing through the beam splitting unit 31. Both the first beam 39 and the second beam 38 are S or P linearly polarized light. The first beam 39 and the second beam 38 are incident on the light reflection cavity 40 at different angles. The first beam 39 passes through the first reflector 41 and is then reflected in sequence on the second reflector 42, the third reflector 43 and the first reflector 41. After multiple reflections in this order, it finally exits from the second reflector 42 and is received by the first detector 51. At the same time, the second beam 38 passes through the first reflector 41 and is then reflected in sequence on the third reflector 43, the second reflector 42 and the first reflector 41. After multiple reflections in this order, it finally exits from the second reflector 42 and is received by the second detector 52. The first beam 39 and the second beam 38 entering the light reflection cavity 40 are turned off, and the first detector 51 and the second detector 52 respectively output oscillation signals. Analysis unit 61 sums the two oscillation signals, such as Figure 2 As shown, this eliminates the influence of the Etalon effect.

[0018] In order to obtain a first beam 39 and a second beam 38 with the same light intensity and polarization state, the linearly polarized light emitted by the light source 11 passes through the first half-wave plate 32 and the polarizing beam splitter 33 in sequence. The split second beam 38 passes through the second half-wave plate 34 and then enters the light reflection cavity, while the split first beam 39 enters the light reflection cavity 40. Rotate the first half-wave plate 32 so that the light intensity of the first beam 39 and the second beam 38 entering the light reflection cavity 40 is the same.

[0019] In order to obtain a first beam 39 and a second beam 38 with the same light intensity and polarization state, the linearly polarized light emitted by the light source 11 is further separated into a first beam 39 and a second beam 38 with the same intensity after passing through a semi-transparent and semi-reflective mirror.

[0020] To control whether light enters the light reflection cavity, a light switch 21 is further provided between the light source 11 and the beam splitting unit 31 to control whether light enters the light reflection cavity 40.

[0021] To achieve more reflections between the three mirrors, the first mirror 41 and the second mirror 42 are further arranged symmetrically about the normal of the third mirror 43, which is a concave mirror.

[0022] Example 2

[0023] Application examples of the CRDS system and method with Etalon effect elimination function according to embodiments of the present invention.

[0024] In this application example, such as Figure 1As shown, the light source 11, optical switch 21, and beam splitting unit 31 are arranged sequentially. The light source 11 is a semiconductor laser that outputs S-polarized light. The optical switch 21 is an acousto-optic crystal that controls whether light passes through. Within the beam splitting unit 31, a first half-wave plate 32, a polarizing beam splitter prism 33, and a second half-wave plate 34 are arranged sequentially. The polarizing beam splitter prism 33 splits the first beam 39 into S-polarized light, and the second beam 38 into P-polarized light, which is converted to S-polarized light after passing through the second half-wave plate 34. By rotating the first half-wave plate 32, the light intensities and polarization states of the first beam 39 and the second beam 38 (which has become S-polarized light) are made the same. The second beam is reflected sequentially by the fourth reflector 35 and the fifth reflector 36 before entering the optical reflection cavity 40.

[0025] Within the light-reflecting cavity 40, the first reflecting mirror 41 and the second reflecting mirror 42 are symmetrical about the third reflecting mirror 43. Both have inner surfaces coated with a film, achieving a reflectivity of 99.995% for light in the 1603nm wavelength band, and their outer surfaces are coated with a high-transmittance film. The angle between the first reflecting mirror 41 and the second reflecting mirror 42 is 44.4 degrees. The first reflecting mirror 41 and the second reflecting mirror 42 are plane mirrors, while the third reflecting mirror 43 is a concave reflecting mirror with a radius of curvature of 1m. The optical path length of the first beam 39 and the second beam 38 traveling one circle between the three reflecting mirrors is 480mm.

[0026] The first detector 51 and the second detector 52 are both PDs.

[0027] The method for eliminating the influence of the Etalon effect in a CRDS system according to this invention, that is, the working method of a CRDS system with the function of eliminating the influence of the Etalon effect in this embodiment, is as follows: The measurement light (linearly polarized light) emitted by the light source 11 enters the beam splitting unit 31 after passing through the optical switch 21; Within the beam-splitting unit 31, the measuring light passes through the first half-wave plate 32 and enters the polarizing beam splitter 33. The resulting first beam 39 is S-polarized light, and the second beam 38 is P-polarized light. However, after passing through the second half-wave plate 34, the second beam 38 is converted to S-polarized light. It is then reflected sequentially by the fourth reflecting mirror 35 and the fifth reflecting mirror 36 before entering the light reflection cavity 40. By rotating the first half-wave plate 32, the light intensities of the first beam 39 and the S-polarized second beam 38 are made the same.

[0028] The first beam 39 and the second beam 38 are incident on the light reflection cavity 40 at different angles. The first beam 39 passes through the first reflector 41, and is then reflected in sequence by the second reflector 42, the third reflector 43 and the first reflector 41, and undergoes multiple reflections in this order before finally passing through the second reflector 42 and being received by the first detector 51. At the same time, the second beam 38 passes through the first reflector 41, and is then reflected in sequence by the third reflector 43, the second reflector 42 and the first reflector 41, and undergoes multiple reflections in this order before finally passing through the second reflector 42 and being received by the second detector 52. When the measurement light is turned off by the optical switch 21, no light enters the optical reflection cavity 40. The first detector 51 receives the first oscillation signal and, by scanning the current points, obtains the oscillation duration Tau of 50 consecutive FSRs, which presents a sinusoidal distribution with phase k.

[0029] Meanwhile, the second detector 52 receives the second oscillation signal and, by scanning the current point, obtains the oscillation time Tau of 50 consecutive FSRs, which exhibits a sinusoidal distribution with a phase of (k+π).

[0030] Analysis unit 61 sums the first and second oscillation signals. The sinusoidal distributions of the oscillation times Tau of the two signals have equal signal intensities and a phase difference of π. After summing, they cancel each other out, essentially eliminating the Etalon effect. Figure 2 As shown.

[0031] exist Figure 2 In the graph, the horizontal axis represents the laser current point corresponding to every FSR (Freeze-Rate Response) ring-down signal, and the vertical axis represents the processed Loss (inversely proportional to Tau). The sinusoidal phases of the first and second ring-down signals are exactly opposite, and the light intensities of the first beam 39 and the second beam 38 are the same. After adding the signals, a relatively flat curve with eliminated Etalon can be obtained. The irregular bulges of the sinusoidal curve originate from factors such as single-point fluctuations, uneven cavity mirror coating, and unresolved absorption.

[0032] like Figure 3 As shown, the upper curve represents the Allen variance of only the data corresponding to the second oscillation signal, while the lower curve represents the Allen variance of the superposition of the two oscillation signals. The conclusion is that not only has the starting position of the single-point accuracy been optimized, but the averaging time has also been optimized.

[0033] Example 3

[0034] An application example of the CRDS system and method with Etalon effect elimination function according to an embodiment of the present invention differs from Embodiment 2 in that: A semi-transparent and semi-reflective mirror is installed in the beam splitting unit to replace the first half-wave plate 32, the polarizing beam splitter prism 33, and the second half-wave plate 34. After the linearly polarized light emitted by the light source 11 passes through the semi-transparent and semi-reflective mirror, it is split into a first reflected beam 39 and a second transmitted beam 38 with the same light intensity.

Claims

1. A CRDS system with Etalon effect cancellation function, the CRDS system with Etalon effect cancellation function comprising a light source, an optical switch, an optical reflecting cavity, and a first detector; characterized in that, The CRDS system with Etalon effect elimination function also includes: The beam splitting unit is used to split the measurement light emitted by the light source into a first beam and a second beam with the same light intensity. The first beam and the second beam are linearly polarized light with the same polarization state. A first reflector, a second reflector, and a third reflector are disposed within the light reflection cavity. The first light beam passes through the first reflector and is then reflected sequentially by the second, third, and first reflectors, undergoing multiple reflections in this order before finally exiting from the second reflector and being received by the first detector. The second light beam passes through the first reflector and is then reflected sequentially by the third, second, and first reflectors, undergoing multiple reflections in this order before finally exiting from the second reflector and being received by the second detector. The analysis unit is used to sum the first oscillation signal output by the first detector and the second oscillation signal output by the second detector.

2. The CRDS system with Etalon effect elimination function according to claim 1, characterized in that, The beam splitter unit includes: The first half-wave plate and the polarizing beam splitter, wherein the measuring light passes through the first half-wave plate and is then split into a first beam and a second beam on the polarizing beam splitter; The second half-wave plate passes through the second half-wave plate and then enters the light reflection cavity.

3. The CRDS system with Etalon effect elimination function according to claim 1, characterized in that, The measuring light is S or P linearly polarized light, and the beam splitting unit includes a semi-transparent and semi-reflective mirror.

4. The CRDS system with Etalon effect elimination function according to claim 1, characterized in that, The first and second reflectors are symmetrically arranged about the normal of the third reflector, which is a concave reflector.

5. The CRDS system with Etalon effect elimination function according to claim 1, characterized in that, The optical switch is disposed in the optical path between the light source and the beam splitter unit.

6. A method for eliminating the influence of the Etalon effect in a CRDS system, wherein the method is as follows: The measurement light emitted by the light source is split into a first beam and a second beam with the same light intensity after passing through the beam splitting unit. Both the first beam and the second beam are S or P linearly polarized light. The first beam and the second beam are incident into the light reflection cavity at different angles. The first beam passes through the first reflector and is then reflected in sequence by the second reflector, the third reflector, and the first reflector, undergoing multiple reflections in this order before finally exiting from the second reflector and being received by the first detector. At the same time, the second beam passes through the first reflector and is then reflected in sequence by the third reflector, the second reflector, and the first reflector, undergoing multiple reflections in this order before finally exiting from the second reflector and being received by the second detector. The first and second light beams entering the optical reflection cavity are turned off, and the first and second detectors respectively output ring-down signals. The analysis unit sums the two oscillation signals to eliminate the influence of the Etalon effect.

7. The method for eliminating the influence of the Etalon effect in a CRDS system according to claim 6, characterized in that, Linearly polarized light emitted from the light source passes sequentially through the first half-wave plate and the polarizing beam splitter. The first beam split off passes through the second half-wave plate and then enters the light reflection cavity. The second beam split off also enters the light reflection cavity. Rotate the first half-wave plate so that the light intensity of the first beam and the second beam entering the light reflection cavity are the same.

8. The method for eliminating the influence of the Etalon effect in a CRDS system according to claim 6, characterized in that, When linearly polarized light emitted from a light source passes through a semi-transparent mirror, the resulting first and second beams have the same intensity.

9. The method for eliminating the influence of the Etalon effect in a CRDS system according to claim 6, characterized in that, An optical switch is provided between the light source and the beam splitting unit to control whether light enters the light reflection cavity.

10. The method for eliminating the influence of the Etalon effect in a CRDS system according to claim 6, characterized in that, The first and second reflectors are symmetrically arranged about the normal of the third reflector, which is a concave reflector.

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