A compact and efficient pump-probe optical system

Abstract

The present invention discloses a compact and efficient pump-probe optical system, which relates to the field of pump-probe technology. The optical path structure includes a first reflector, a second reflector, an ultraviolet reflector, and a multi-pass cell cavity. The first reflector and the second reflector are respectively provided with a first circular hole and a second circular hole at their centers. The ultraviolet reflector is mounted at the first circular hole in the center of the first reflector, and its working surface is convex and faces the second reflector. The incident beam of the photolysis laser enters through the second circular hole and is incident on the ultraviolet reflector along the optical axis of the multi-pass cell. It is then diverged and reflected by the ultraviolet reflector to the second reflector. It is reflected and collimated by the second reflector into a collimated beam, which covers the probe laser reflected back and forth in the multi-pass cell. The present invention has high mirror surface utilization, dense reflected light spots, and the photolysis beam completely covers the probe beam. It can obtain an effective overlapping optical path of up to 100 meters with a short base length, achieving a small size and high sensitivity.

Description

Technical Field

[0001] The present invention relates to the field of pump-probe technology, in particular to a compact and efficient pump-probe optical system. Background Art

[0002] Atmospheric ozone (O3) pollution is formed during the atmospheric photochemical oxidation process, and the mechanism is very complex. Hydroxyl radical (OH), as the most important oxidizing substance in the troposphere, determines the generation and removal of atmospheric pollutants and is the main driver of tropospheric atmospheric chemistry. OH ') is a comprehensive evaluation of the concentration of active substances in the atmosphere that can react with OH and participate in the degree of atmospheric photochemical oxidation. It is an extremely important photochemical parameter and plays a vital role in the study of the causes of atmospheric O3 pollution and pollution prevention and control. OH reactivity is defined as the total pseudo-first-order loss rate caused by the reaction of OH with reactive gases in ambient air and can be described by the following formula:

[0003]

[0004] Among them, X i The concentration is [X i ] reactive substances, is the chemical reaction rate constant of OH with various substances.

[0005] Laser flash photolysis coupled with laser-induced fluorescence is a commonly used technique for directly measuring OH reactivity (Stone, et al., Measurement of OH reactivity by laser flash photolysis coupled with laser-induced fluorescence spectroscopy, Atmospheric Measurement Techniques, 2016, 9, 2827-2844). This technique draws atmospheric air into a flow tube and adds a small flow of zero-air containing oxygen and water vapor to the gas path. A 266nm Nd:YAG pulsed laser is used to illuminate the sample in the flow tube along the axis of a multi-pass cell. This photolysis of the oxygen and water vapor in the sample produces a high concentration of OH. Simultaneously, a laser-induced fluorescence device is used to measure the OH concentration signal, generating a time-varying decay curve corresponding to the reaction between OH and the reactive gas. However, the laser flash photolysis-laser induced fluorescence method is extremely complex. A large pump group is required to maintain the measurement pressure of the cavity at about 5 mabr. The 308nm pump dye laser used for OH fluorescence excitation costs up to several million yuan and requires frequent maintenance and addition of dye consumables. It has poor stability. In addition, the laser induced fluorescence device and the flow tube for photolysis of atmospheric samples are relatively separated. The length and width of the instrument are 3-5 square meters, and the overall size is huge, making it difficult to transport and quickly deploy at field sites.

[0006] Laser flash photolysis-magnetic rotation spectroscopy is a new, recently proposed technique for measuring OH reaction activity. Its main structure consists of a 266nm photolysis optical path, a 2.8μm multi-pass detection optical path, and a magnet. The magnetic field generated by the magnet is used to overlap the detection optical path to enhance the detection signal. The photolysis laser beam overlaps with the infrared detection laser beam, which is reflected multiple times within the multi-pass cell. Therefore, the OH produced by the photolysis of the O3 and water vapor system by the 266nm pulsed laser in the photolysis optical path can be simultaneously detected by the overlapping infrared detection optical path, directly obtaining the OH decay curve and, in turn, the OH reaction activity. Laser flash photolysis-magnetic rotation spectroscopy uses a distributed feedback laser diode and a semiconductor-cooled photodiode to detect OH. The pressure requirement for measuring the multi-pass cell is low, resulting in a low-cost device and no complex maintenance.

[0007] In the laser flash photolysis-magnetic rotation spectroscopy device, the pump-probe optical path structure determines the effective absorption optical path of the overlap of the photolysis beam and the multi-pass detection optical path, which directly affects the system detection sensitivity and device size, and is the most critical component of the device. The device reported by Wei et al. in 2020 (Wei, et al., Time-resolved laser-flash photolysis Faraday rotation spectrometer: a new tool for total oh reactivity measurement and free radical kinetics research, Analytical Chemistry, 2020, 92, 4334-4339) uses a Herriot-type multi-pass cell combined with an ultraviolet beam expander to construct a pump-probe optical path. In this structure, the detection laser forms 25 reflection points distributed in a single circle on the surface of the multi-pass cell lens. The 266nm photolysis laser is expanded to 30mm by an external beam expander, passes through the multi-pass cell from the center once, and forms an effective overlapping optical path of 25m with the multi-reflected detection laser. However, the effective overlap rate between the photolysis beam and the detection beam of this pump-detection optical path mechanism is only 41%, that is, most of the detection optical path is not utilized. Therefore, in order to ensure the effective absorption optical path that meets the atmospheric measurement application, the spacing between the Herrtiott multi-pass cell reflectors reaches 1220mm, resulting in a large overall device volume. In addition, a large solenoid magnet is used, which has high Joule heat and requires water cooling, resulting in high power consumption.

[0008] Chinese invention patent publication number CN109856065A discloses a device for measuring the activity of hydroxyl radical reaction, which is similar to the device reported by Wei et al., and its pump-detection optical path structure is as follows: Figure 1 As shown, it consists of an optical multi-pass cell and an external UV beam expander. The expanded UV beam passes through the center of the optical multi-pass cell's annular reflector in a single pass. This device also suffers from low effective overlap, large size, and high power consumption. Summary of the Invention

[0009] In order to overcome the defects in the above-mentioned prior art, the present invention provides a compact and efficient pump-detection optical system with high mirror utilization, dense reflected light spots, and complete coverage of the detection beam by the photolysis beam. It can obtain an effective overlapping optical path of up to 100 meters with a short base length, achieving small size and high sensitivity.

[0010] To achieve the above object, the present invention adopts the following technical solutions, including:

[0011] A compact and efficient pump-probe optical system, comprising: an optical path structure, a probe laser and a photolysis laser;

[0012] The optical path structure includes a first reflecting mirror, a second reflecting mirror, an ultraviolet reflecting mirror and a multi-pass cell cavity;

[0013] The working surfaces of the first reflector and the second reflector are concave, and the concave surfaces are mounted facing each other at the two ends of the multi-pass pool cavity to form a multi-pass pool; the edges of the first reflector and the second reflector are respectively provided with a first conical oblique light hole and a second conical oblique light hole; the centers of the first reflector and the second reflector are respectively provided with a first circular hole and a second circular hole;

[0014] The ultraviolet reflector is installed at the first circular hole in the center of the first reflector, and the working surface of the ultraviolet reflector is convex and faces the second reflector;

[0015] The incident beam of the detection laser enters the multi-pass cell from the first tapered oblique light hole and reflects back and forth between the first reflector and the second reflector of the multi-pass cell;

[0016] The incident beam of the photolysis laser enters from the second circular hole and is incident on the ultraviolet reflector along the optical axis of the multi-pass cell. It is then diverged and reflected by the ultraviolet reflector to the second reflector, reflected and collimated by the second reflector into a collimated beam, covering the detection laser reflected back and forth in the multi-pass cell.

[0017] Preferably, the first reflector and the second reflector are both plano-concave reflectors with a diameter of 50 mm, the flat surface being the non-working surface and the concave surface being the finely polished working surface.

[0018] Preferably, the UV reflector is a plano-convex reflector with a diameter of 10 mm, the flat surface is the non-working surface, and the concave surface is the finely polished working surface.

[0019] Preferably, the working surfaces of the first reflector and the second reflector are both coated with a damage-resistant dielectric film, and the reflectivity of the damage-resistant dielectric film at 266 nm, 633 nm and 2.8 μm is greater than 0.99.

[0020] Preferably, the incident beam of the detection laser is reflected back and forth between the first reflector and the second reflector in the multi-pass cell to form a dense light spot distributed in multiple circles, and then the outgoing beam of the detection laser is emitted from the second conical oblique light hole.

[0021] Preferably, the incident beam of the photolysis laser is collimated by the second reflector in the multi-pass cell to be a collimated beam with a diameter equal to that of the second reflector, then incident on the first reflector and converged by the first reflector, and then the outgoing beam of the photolysis laser is emitted from the second circular hole.

[0022] Preferably, the concave surfaces of the first reflector and the second reflector are spherical surfaces with the same curvature, and the distance and curvature between the two reflectors are selected so that the incident beam of the detection laser breaks the paraxial approximation condition, forming a dense light spot distributed in multiple circles between the first reflector and the second reflector.

[0023] Preferably, the working wavelength is 3568.52cm -1 A mid-infrared distributed feedback diode laser is used to emit the detection laser, and a 266nm pulse laser with a pulse frequency of 1 to 10 Hz and a pulse energy of more than 10 mJ emitted by a frequency quadrupled Nd:YAG laser is used as the photolysis laser.

[0024] A laser flash photolysis-magnetic rotation spectroscopy device for detecting OH reaction activity comprises a compact and efficient pump-probe optical system as described above.

[0025] Preferably, the optical path structure is installed in a fixed frame; the multi-pass cell is sealed with a calcium fluoride window, which is used to transmit the photolysis laser and the detection laser; a magnet is coaxially installed outside the multi-pass cell to provide a magnetic field for the detection laser.

[0026] The advantages of the present invention are:

[0027] (1) Compared with the existing optical path structure based on the Herriot-type multi-pass cell, the optical path structure of the compact and efficient pump-probe optical system of the present invention has a high mirror utilization rate, dense reflected light spots, and the photolysis beam completely covers the detection beam. It can obtain an effective overlapping optical path of up to 100 meters under a short base length (the base length refers to the spacing between the multi-pass cell mirrors), achieving a small size and high sensitivity.

[0028] (2) The optical path structure of the compact and efficient pump-probe optical system of the present invention has a cavity volume of only 14% of the existing structure, requires less atmospheric samples, has a fast replacement speed, and can operate at a high flash frequency of 10 Hz, which is beneficial to improving the time resolution during atmospheric measurement.

[0029] (3) The present invention also provides a laser flash photolysis-magnetic rotation spectroscopy device for detecting OH reaction activity. The smaller optical flash-detection structure facilitates reducing the size of the electromagnet in the laser flash photolysis-magnetic rotation spectroscopy device, thereby reducing heat and overall device power consumption. Furthermore, it facilitates replacing the small electromagnet with a permanent magnet, thereby resolving power consumption and Joule heating issues.

[0030] (4) Based on the compact and efficient pump-detection optical system structure of the present invention, the miniaturization of the atmospheric OH reaction activity detection device can be achieved, which is conducive to large-scale networking applications and vehicle-mounted cruise applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the pump-probe optical path structure of Chinese patent publication number CN109856065A.

[0032] Figure 2It is a multiple reflection light spot of the detection laser in the pump-detection optical path structure with Chinese patent publication number CN109856065A.

[0033] Figure 3 Schematic diagram of a compact and efficient pump-probe optical system of the present invention.

[0034] Figure 4 It is a multiple reflection light spot of the detection laser in the optical system of the present invention.

[0035] Figure 5 Schematic diagram of a laser flash photolysis-magnetic rotation spectroscopy device for detecting OH reaction activity.

[0036] Description of reference numerals:

[0037] 1-first reflector, 2-second reflector, 3-ultraviolet reflector, 4-incident beam of detection laser, 5-photolysis laser beam, 6-outgoing beam of detection laser, 7-multi-pass cell cavity, 101-first conical oblique light hole, 102-first circular hole, 201-second conical oblique light hole, 102-second circular hole, 10-fixed frame, 11-calcium fluoride window, 12-optical path structure, 13-magnet. DETAILED DESCRIPTION

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0039] Example 1

[0040] Depend on Figure 3 As shown, this embodiment 1 provides a compact and efficient pump-probe optical system, including an optical path structure 12, a probe laser, and a photolysis laser. The optical path structure 12 includes: a first reflector 1, a second reflector 2, an ultraviolet reflector 3, and a multi-pass cell cavity 7.

[0041] The first reflector 1 and the second reflector 2 are plano-concave reflectors with a diameter of 50 mm. The flat surface is a non-working surface and does not need to be polished, while the concave surface is a finely polished working surface.

[0042] The concave surfaces of the first reflector 1 and the second reflector 2 are mounted at two ends of the multi-pass cell cavity 7 facing each other to form a multi-pass cell.

[0043] A first tapered oblique light-through hole 101 and a second tapered oblique light-through hole 201 are respectively formed at the edges of the first reflector 1 and the second reflector 2 , and a first circular hole 102 and a second circular hole 202 are respectively formed at the centers of the first reflector 1 and the second reflector 2 .

[0044] The UV reflector 3 is a plano-convex reflector with a diameter of 10 mm. It is installed at the first circular hole 102 in the center of the first reflector 1 and forms a Cassegrain reflective beam expansion structure with the second reflector 2. The convex surface of the UV reflector 3 faces the second reflector 2. The convex surface is a finely polished working surface, and the flat surface is a non-working surface and does not need to be polished.

[0045] The concave surfaces of the first reflector 1 and the second reflector 2 are spherical surfaces with the same curvature, and the distance and curvature between the two reflectors are selected so that when the incident light beam 4 of the detection laser enters the multi-pass cell off-axis, the concave curved surface caused by the curvature of the reflectors cannot be ignored, breaking the paraxial approximation condition and forming a dense multi-circle light spot arrangement with high mirror utilization on the reflectors.

[0046] The working surfaces (concave surfaces) of the first reflector 1 and the second reflector 2 are both coated with multi-wavelength high-reflectivity damage-resistant dielectric films, which can simultaneously meet the reflectivity greater than 0.99 at 266nm, 633nm and 2.8μm.

[0047] In this embodiment 1, the working wavelength is stabilized at 3568.52 cm -1 The mid-infrared distributed feedback diode laser at the position emits a detection laser, and the incident beam 4 of the detection laser enters the multi-pass cell from the first tapered oblique light hole 101 at the edge of the first reflector 1, and is reflected back and forth between the first reflector 1 and the second reflector 2 of the multi-pass cell for multiple times (hundreds of times), forming the following Figure 4 After the dense light spots are distributed in multiple circles, the outgoing beam 6 of the detection laser is emitted from the second tapered oblique light hole 201 at the edge of the second reflector 2. The multi-pass pool with dense light spots can enable the detection laser to achieve a long optical path in a very small volume, improving the compactness of the optical path structure. Figure 2 and Figure 5 It can be seen that the number of light spots of the present invention is significantly greater than that of the existing structure.

[0048] In this embodiment 1, a 266nm pulsed laser with a pulse frequency of 10Hz and a pulse energy of 20mJ emitted by a quadrupled frequency Nd:YAG laser is used as a photolysis laser. The incident light beam 5 of the photolysis laser enters from the second circular hole 201 at the center of the second reflector 2, and is incident on the working surface (convex surface) of the ultraviolet reflector 3 along the optical axis of the multi-pass cell. It is then diverged and reflected by the ultraviolet reflector 3 to the concave surface of the second reflector 2, and is reflected by the second reflector 2 and re-collimated into a collimated light beam with a diameter of about 50mm, covering the detection laser reflected multiple times in the multi-pass cell, thereby realizing efficient utilization of the detection laser. The collimated light beam is incident on the first reflector 1 and converged by the first reflector 1, and then the outgoing light beam of the photolysis laser is emitted from the second circular hole 202.

[0049] Example 2

[0050] The compact and efficient pump-probe optical system of the present invention can be used in a laser flash photolysis-magnetic rotation spectroscopy device for detecting OH reaction activity.

[0051] Depend on Figure 5 As shown, this embodiment 2 provides a laser flash photolysis-magnetic rotation spectroscopy device for detecting OH reaction activity, including a compact and efficient pump-probe optical system provided in the above-mentioned embodiment 1. The optical path structure 12 is installed in a fixed frame 10. Large-sized calcium fluoride windows 11 are set at both ends of the optical path structure 12 to seal the cavity, ensuring that 266nm, 633nm, and 2.8μm pulsed lasers (photolysis lasers) can pass through simultaneously. An annular rare earth permanent magnet 13 is coaxially installed on the outside of the multi-pass cell to provide a magnetic field for the detection optical path.

[0052] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A compact and efficient pump-probe optical system, characterized in that: include: Optical path structure (12), detection laser and photolysis laser; The optical path structure (12) comprises a first reflecting mirror (1), a second reflecting mirror (2), an ultraviolet reflecting mirror (3) and a multi-pass cell cavity (7); The working surfaces of the first reflector (1) and the second reflector (2) are concave, and the concave surfaces are mounted facing each other at the two ends of the multi-pass pool cavity (7) to form a multi-pass pool; the edges of the first reflector (1) and the second reflector (2) are respectively provided with a first conical oblique light-through hole (101) and a second conical oblique light-through hole (201); the centers of the first reflector (1) and the second reflector (2) are respectively provided with a first circular hole (102) and a second circular hole (202); The ultraviolet reflector (3) is installed at the first circular hole (102) in the center of the first reflector (1), and the working surface of the ultraviolet reflector (3) is convex and faces the second reflector (2); An incident light beam (4) of the detection laser enters the multi-pass cell from the first tapered oblique light hole (101), and is reflected back and forth between the first reflector (1) and the second reflector (2) of the multi-pass cell; The incident light beam (5) of the photolysis laser enters from the second circular hole (202) and is incident on the ultraviolet reflector (3) along the optical axis of the multi-pass cell, and is then diverged and reflected by the ultraviolet reflector (3) to the second reflector (2), and is reflected and collimated by the second reflector (2) to form a collimated light beam, covering the detection laser reflected back and forth in the multi-pass cell; An incident light beam (4) of the detection laser is reflected back and forth between a first reflector (1) and a second reflector (2) in the multi-pass cell to form a dense light spot distributed in multiple circles, and then an outgoing light beam (6) of the detection laser is emitted from the second tapered oblique light hole (201); The incident light beam (5) of the photolysis laser is collimated by the second reflector (2) in the multi-pass cell to become a collimated light beam having a diameter equal to that of the second reflector (2), then incident on the first reflector (1) and converged by the first reflector (1), and then the outgoing light beam of the photolysis laser is emitted from the second circular hole (202).

2. A compact and efficient pump-probe optical system according to claim 1, characterized in that: The first reflector (1) and the second reflector (2) are both plano-concave reflectors with a diameter of 50 mm, the planar surface being a non-working surface and the concave surface being a finely polished working surface.

3. A compact and efficient pump-probe optical system according to claim 1, characterized in that: The ultraviolet reflector (3) is a plano-convex reflector with a diameter of 10 mm, the flat surface is a non-working surface, and the concave surface is a finely polished working surface.

4. A compact and efficient pump-probe optical system according to claim 1, characterized in that: The working surfaces of the first reflector (1) and the second reflector (2) are both coated with an anti-damage dielectric film, and the reflectivity of the anti-damage dielectric film at 266 nm, 633 nm and 2.8 μm is greater than 0.

99.

5. A compact and efficient pump-probe optical system according to claim 1, characterized in that: The concave surfaces of the first reflector (1) and the second reflector (2) are spherical surfaces with the same curvature, and the distance and curvature between the two reflectors are selected so that the incident light beam (4) of the detection laser breaks the paraxial approximation condition, forming a dense light spot with multiple circles of distribution between the first reflector (1) and the second reflector (2).

6. A compact and efficient pump-probe optical system according to claim 1, characterized in that: The working wavelength is 3568.52cm -1 A mid-infrared distributed feedback diode laser is used to emit the detection laser, and a 266nm pulse laser with a pulse frequency of 1 to 10 Hz and a pulse energy of more than 10 mJ emitted by a frequency quadrupled Nd:YAG laser is used as the photolysis laser.

7. A laser flash photolysis-magnetic rotation spectroscopy device for detecting OH reaction activity, characterized in that: The invention comprises a compact and efficient pump-probe optical system as described in any one of claims 1 to 6.

8. The laser flash photolysis-magnetic rotation spectroscopy device for detecting OH reaction activity according to claim 7, characterized in that: An optical path structure (12) is installed in a fixed frame (10); a calcium fluoride window (11) is used to seal the multi-pass cell, and the calcium fluoride window (11) is used to transmit the photolysis laser and the detection laser; a magnet (13) is coaxially installed outside the multi-pass cell to provide a magnetic field for the detection laser.

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