Compact ultraviolet double-path optical system for measuring nitrate concentration of seawater

Abstract

The invention relates to seawater nitrate concentration measurement, in particular to a compact ultraviolet double-path optical system for seawater nitrate concentration measurement, and solves the problems of overlarge system size, complex structure and low energy transmission efficiency when an existing double-optical-path structure composed of optical fibers is adopted. The spectroscope is arranged on an emergent light path of the collimating mirror, the first window, the second window and the coupling mirror group are sequentially arranged on a transmission light path of the spectroscope, and the light path is a measurement light path for measurement; the first reflector is arranged on a transmission light path of the spectroscope, the second reflector is arranged on a reflection light path of the first reflector, the third reflector is arranged on a reflection light path of the second reflector, the coupling mirror group is simultaneously positioned on a reflection light path of the third reflector, and the light path is a reference light path; the spectrograph is arranged on a transmission light path of the coupling mirror group; and the measuring light path and the reference light path share the collimating mirror, the spectroscope and the coupling mirror group.

Description

Technical Field

[0001] The invention relates to seawater nitrate concentration measurement, in particular to a compact ultraviolet dual-path optical system for seawater nitrate concentration measurement. Background Art

[0002] In seawater, nitrate, as the main form of inorganic nitrogen, is the main cause of seawater eutrophication. In today's marine protection field, accurate monitoring of nitrate content in water bodies is related to the monitoring and management of the marine environment, and has made outstanding contributions to promoting the development of marine science, helping people to effectively warn of marine disasters, and achieving sustainable development of marine aquaculture. Nitrate sensors are an important technical means for monitoring nitrate concentration in seawater today. The principle is that in-situ nitrate sensors based on ultraviolet spectroscopy use water absorption spectra to detect nitrate concentration. Nitrate sensors have the advantages of long-term continuous observation, no need for chemical reagents, and a simple detection process. They have now become a hot topic in the study of seawater nitrate concentration measurement.

[0003] However, these nitrate sensors still have the following problems:

[0004] 1) Changes in the intensity of the ultraviolet light source will cause measurement errors, thereby reducing detection accuracy. However, most existing nitrate sensors use a single optical path structure system, which cannot effectively eliminate such errors. In order to solve the above problems, some nitrate sensors use a dual optical path structure system including a measurement path and a reference path. The reference spectrum and the continuous spectrum passing through seawater can eliminate the influence of light source changes to a certain extent. However, due to the differences in optical devices in the measurement path and the reference path, there will still be certain errors.

[0005] 2) When existing nitrate sensors adopt a dual optical path structure, optical fibers are often used for light beam separation, merging and transmission, which results in a system that is too large in size and has a complex structure.

[0006] 3) When a dual optical path structure composed of optical fibers is used, the separation of the light beams and the absorption of light energy by optical materials and films reduce the energy transmission efficiency of each optical path, thereby reducing the detection range and accuracy of nitrate concentration. Summary of the invention

[0007] The purpose of the present invention is to solve the technical problems of excessively large system size, complex structure and low energy transmission efficiency when the existing nitrate sensor adopts a dual-path structure composed of optical fibers, and to provide a compact ultraviolet dual-path optical system for measuring seawater nitrate concentration.

[0008] In order to achieve the above-mentioned invention object, the present invention provides the following technical solutions:

[0009] A compact ultraviolet two-way optical system for measuring seawater nitrate concentration, which is special in that it includes a collimator, a beam splitter, a first window, a second window, a coupling mirror group, a first reflector, a second reflector, a third reflector, a spectrometer and an ultraviolet light source;

[0010] The collimator is located on the outgoing light path of the ultraviolet light source, the beam splitter is arranged on the outgoing light path of the collimator, the first window, the second window and the coupling lens group are arranged in sequence on the transmission light path of the beam splitter; and the seawater to be measured is arranged between the first window and the second window;

[0011] The first reflector is arranged on the transmission light path of the beam splitter, the second reflector is arranged on the reflection light path of the first reflector, the third reflector is arranged on the reflection light path of the second reflector, and the coupling mirror group is also arranged on the reflection light path of the third reflector;

[0012] The spectrometer is arranged on the transmission light path of the coupling mirror assembly;

[0013] The focusing spot size DF of the coupling mirror assembly and the focal length f of the coupling mirror assembly satisfy:

[0014] 0.014≤DF / f≤0.031

[0015] Among them, the value range of f is 15mm-24mm.

[0016] Furthermore, the coupling lens group includes a first lens, a second lens and a third lens arranged in sequence along the optical path; the first lens is a biconvex lens with positive optical power, the second lens is a biconcave lens with negative optical power, the third lens is a biconvex lens with positive optical power, and the incident surface and the exit surface of the first lens, the second lens and the third lens are all spherical surfaces.

[0017] Furthermore, the distance between the center of the exit surface of the collimator and the center of the beam splitting surface of the beam splitter on the optical axis is 24 mm-27 mm;

[0018] The distance between the center of the beam splitter surface of the beam splitter and the center of the incident surface of the first window on the optical axis is 39.4 mm-51 mm;

[0019] The distance between the center of the exit surface of the first window and the center of the incident surface of the second window on the optical axis is 8mm-10mm;

[0020] The distance between the center of the exit surface of the second window and the incident surface of the coupling lens assembly on the optical axis is 14 mm-19 mm;

[0021] The distance between the center of the beam splitter surface of the beam splitter and the center of the incident surface of the first reflector on the optical axis is 18mm-20mm;

[0022] The distance between the center of the exit surface of the first reflector and the center of the incident surface of the second reflector on the optical axis is 60.5 mm-86 mm;

[0023] The distance between the center of the exit surface of the second reflector and the center of the incident surface of the third reflector on the optical axis is 10.8 mm-13 mm;

[0024] The distance between the center of the exit surface of the third reflector and the incident surface of the coupling mirror assembly on the optical axis is 5.5 mm-7 mm.

[0025] Further, the distance between the center of the exit surface of the first lens and the center of the incident surface of the second lens on the optical axis is 0.15 mm-0.35 mm;

[0026] The distance between the center of the exit surface of the second lens and the center of the incident surface of the third lens on the optical axis is 30mm-35mm;

[0027] The distance between the center of the exit surface of the third lens and the focusing surface of the spectrometer on the optical axis is 3.7 mm-4.8 mm.

[0028] Furthermore, the first window and the second window have the same structural dimensions, and the light aperture DW and thickness TH1 of the first window satisfy:

[0029] 2≤DW / TH1≤2.6

[0030] Among them, the value range of DW is 6mm-13mm; meeting the above conditions can make the system armament have good pressure resistance characteristics, the working depth can reach 4000m underwater, and it is conducive to the processing and forming of the two windows;

[0031] The light aperture DS2 of the coupling mirror assembly and the total length TTL of the coupling mirror assembly satisfy:

[0032] 0.21≤DS2 / TTL≤0.32

[0033] Among them, the value range of DS2 is 8mm-14mm. Meeting the above conditions can improve the beam focusing ability of the coupling mirror group and control the lateral and longitudinal dimensions of the entire system within a reasonable range.

[0034] Furthermore, the effective clear aperture DE2 and the curvature radius R2 of the exit surface of the first lens satisfy:

[0035] 0.65≤DE2 / R2≤1.43

[0036] Among them, the value range of R2 is 10.5mm-11.5mm; satisfying the above relationship can reduce the difficulty of lens processing, while controlling the deflection of light by the coupling lens group and enhancing the light beam focusing ability;

[0037] The focal length f of the coupling lens group and the focal length f3 of the third lens satisfy:

[0038] 0.41≤f3 / f≤0.85

[0039] Satisfying the above relationship can make the focal length distribution of each lens in the coupling lens group more reasonable, improve the processing technology of each lens, and reduce the processing cost.

[0040] Furthermore, the interval between the optical axis of the transmitted light beam of the beam splitter and the optical axis of the light beam reflected by the first reflector is defined as SP1, and SP1 and the light aperture DS3 of the beam splitter satisfy:

[0041] 0.19≤DS3 / SP1≤0.36

[0042] Among them, the value range of SP1 is 18mm-20mm; meeting the above conditions can make the layout of various components in the system more reasonable and reduce the lateral size of the system;

[0043] The distance between the center of the collimator mirror exit surface and the incident surface of the coupling mirror group on the optical axis is defined as SW1, and the interval between the optical axis of the transmitted light beam of the beam splitter and the optical axis of the light beam reflected by the third reflector is defined as SP2. The SW1 and SP2 satisfy:

[0044] 0.03≤SP2 / SW1≤0.08

[0045] Among them, the value range of SW1 is 90mm-120mm; meeting the above conditions is conducive to controlling the system length within a reasonable range and improving the system compactness;

[0046] The interval between the center of the exit surface of the first lens and the center of the incident surface of the second lens on the optical axis is defined as SP3, and the curvature radius R3 of SP3 and the incident surface of the second lens satisfies:

[0047] 30≤R3 / SP3≤76.7

[0048] Among them, the value range of R3 is 10.5mm-11.5mm. Meeting the above relationship can enhance the ability of the coupling mirror group to deflect the light beam, while reducing the difficulty and sensitivity of system adjustment;

[0049] Define the interval between the center of the second lens exit surface and the center of the third lens incident surface on the optical axis as SP4, then SP3 and SP4 satisfy:

[0050] 0.0043≤SP3 / SP4≤0.0175;

[0051] Meeting the above relationship can prevent the entire coupling lens group from being too long, thereby improving the focusing effect of the system;

[0052] Define the distance between the center of the exit surface of the third lens and the focusing surface of the spectrometer on the optical axis as SP5. The SP5 and the radius of curvature R5 of the incident surface of the third lens satisfy:

[0053] 1.1 ≤ R5 / SP5 ≤ 1.89

[0054] Wherein, the value range of R5 is 5.3 mm - 7 mm; satisfying the above relationship can reduce the size of the focusing spot and facilitate the adjustment of the focusing position at the same time.

[0055] Furthermore, the beam splitting surface of the beam splitter is coated with a beam splitting film;

[0056] The surfaces of the first mirror, the second mirror, and the third mirror are all coated with an ultraviolet high-reflection emissivity metal film.

[0057] Furthermore, the collimator is an off-axis parabolic mirror, and the ultraviolet light source is located on one side of the reflective surface of the off-axis parabolic mirror.

[0058] Furthermore, the manufacturing material of the off-axis parabolic mirror is 6061-T6 aluminum. The reflection focal length fr of the off-axis parabolic mirror satisfies 5 mm < fr < 10 mm; the bottom diameter Dp and the reflection angle Rg of the off-axis parabolic mirror satisfy: 0.04 < Dp / Rg < 0.08, and the reflection angle Rg is 90°;

[0059] The beam splitter is a multi-transmission and few-reflection mirror, the ratio of its transmittance to emissivity is 7:3, and its length, width, and height are all 6 mm. The manufacturing material is fused silica;

[0060] The length, width, and height of the first mirror, the second mirror, and the third mirror are all 10 mm. The manufacturing material is H-K9L, and all three are right-angle mirrors;

[0061] The thickness of the first window and the second window is both 5.6 mm, the diameter is 13 mm, and the manufacturing material is sapphire;

[0062] The manufacturing materials of the first lens and the third lens are calcium fluoride, the manufacturing material of the second lens is fused silica, and the first lens and the second lens adopt a double-separation structure.

[0063] Compared with the prior art, the beneficial effects of the present invention are:

[0064] In the compact ultraviolet two-path optical system for measuring seawater nitrate concentration provided by the present invention, a collimator is located on the exit light path of an ultraviolet light source, a beam splitter is arranged on the exit light path of the collimator, a first window, a second window and a coupling mirror group are arranged in sequence on the transmission light path of the beam splitter, and this light path is a measurement light path for measurement, a first reflector is arranged on the transmission light path of the beam splitter, a second reflector is arranged on the reflection light path of the first reflector, a third reflector is arranged on the reflection light path of the second reflector, and the coupling mirror group is simultaneously located on the reflection light path of the third reflector, and this light path is a reference light path, and a focusing spot size DF of the coupling mirror group and a focal length f of the coupling mirror group satisfy: 0.014≤DF / f≤0.031, wherein the value range of f is 15mm-24mm. The measurement optical path and the reference optical path share a collimator, a beam splitter and a coupling mirror group, which simplifies the system composition, reduces the structural complexity, and greatly reduces the error when the reference optical path performs light intensity correction; the collimator, the beam splitter, multiple reflectors and the coupling mirror group are used to realize the collimation, separation, merging and coupling of the light beam, optimize the light beam collimation and focusing characteristics, and compared with the dual optical path structure composed of optical fibers, the light energy loss of each of the two optical paths is reduced, while ensuring the compactness of the system, the light energy transmission efficiency is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 The optical path diagram of an embodiment of a compact ultraviolet dual-path optical system for measuring seawater nitrate concentration of the present invention;

[0066] Figure 2 A spot diagram of an embodiment of a compact ultraviolet dual-path optical system for measuring nitrate concentration in seawater according to the present invention;

[0067] Figure 3 This is a coupling efficiency simulation effect diagram of an embodiment of the compact ultraviolet dual-path optical system for measuring seawater nitrate concentration of the present invention.

[0068] The following are the descriptions of the reference numerals:

[0069] 1-collimator, 2-beam splitter, 3-first window, 4-second window, 5-coupling mirror group, 51-first lens, 52-second lens, 53-third lens; 6-first reflector, 7-second reflector, 8-third reflector, 9-spectrometer, 10-ultraviolet light source. DETAILED DESCRIPTION

[0070] The present invention will be further described below in conjunction with the accompanying drawings and exemplary embodiments.

[0071] Reference Figure 1The compact ultraviolet two-path optical system for measuring the nitrate concentration in seawater of the present invention comprises a collimator 1, a beam splitter 2, a first window 3, a second window 4, a coupling mirror group 5, a first reflector 6, a second reflector 7, a third reflector 8, a spectrometer 9 and an ultraviolet light source 10, and the collimator 1, the beam splitter 2, the first window 3, the second window 4, the coupling mirror group 5, the first reflector 6, the second reflector 7, and the third reflector 8 can all effectively work in the ultraviolet band of 200-400nm.

[0072] The ultraviolet light source 10 adopts a xenon flash lamp, the collimator 1 is located on the exit light path of the ultraviolet light source 10, the beam splitter 2 is arranged on the exit light path of the collimator 1, the first window 3, the second window 4 and the coupling lens group 5 are arranged in sequence on the transmission light path of the beam splitter 2, and the seawater to be measured is located between the first window 3 and the second window 4. The above optical path is the measurement optical path for measurement, and the distance between the center of the exit surface of the collimator 1 and the center of the beam splitter surface of the beam splitter 2 on the optical axis is 24mm-27mm. In this embodiment, is 26.7mm; the distance between the center of the splitting surface of the beam splitter 2 and the center of the incident surface of the first window 3 on the optical axis is 39.4mm-51mm, and is 44.5mm in this embodiment; the distance between the center of the exit surface of the first window 3 and the center of the incident surface of the second window 4 on the optical axis is 8mm-10mm, and is 9.8mm in this embodiment, and the distance between the center of the exit surface of the second window 4 and the incident surface of the coupling lens group 5 on the optical axis is 14mm-19mm, and is 17.2mm in this embodiment.

[0073] The first reflector 6 is arranged on the transmission light path of the beam splitter 2, the second reflector 7 is arranged on the reflection light path of the first reflector 6, the third reflector 8 is arranged on the reflection light path of the second reflector 7, and the coupling mirror group 5 is also located on the reflection light path of the third reflector 8. This light path is the reference light path, and the distance between the center of the beam splitting surface of the beam splitter 2 and the center of the incident surface of the first reflector 6 on the optical axis is 18mm-20mm, and is 18mm in this embodiment; the distance between the center of the exit surface of the first reflector 6 and the center of the incident surface of the second reflector 7 on the optical axis is 60.5mm-86mm, and is 76.9mm in this embodiment; the distance between the center of the exit surface of the second reflector 7 and the center of the incident surface of the third reflector 8 on the optical axis is 10.8mm-13mm, and is 12mm in this embodiment; the distance between the center of the exit surface of the third reflector 8 and the incident surface of the coupling mirror group 5 on the optical axis is 5.5mm-7mm, and is 5.8mm in this embodiment.

[0074] The collimating mirror 1, the beam splitter 2 and the coupling mirror group 5 are shared by the measurement optical path and the reference optical path, which simplifies the system composition, greatly reduces the error when the reference optical path performs light intensity correction, reduces the light energy loss of each path at the same time, avoids the problems of complex system structure and too large size, and helps to improve the system compactness. The spectrometer 9 is arranged on the transmission optical path of the coupling mirror group 5, and the size DF of the focused spot of the coupling mirror group 5 and the focal length f of the coupling mirror group 5 satisfy:

[0075] 0.014 ≤ DF / f ≤ 0.031

[0076] Among them, the value range of f is 15 mm - 24 mm.

[0077] The collimating mirror 1 is an off-axis parabolic mirror. The ultraviolet light source 10 is located on the reflection surface side of the off-axis parabolic mirror. When in use, the outgoing optical path of the ultraviolet light source 10 is aligned with the focal position of the off-axis parabolic mirror, which is beneficial to generating a good collimated beam. The diameter of the spot after collimation is about 4 mm, and the divergence angle is guaranteed to be within 1°. The collimating mirror 1 is made of 6061-T6 aluminum, and a metal high-reflection film is coated on the parabolic surface, which reduces the processing difficulty and cost of the off-axis parabolic mirror, is beneficial to the assembly with other components, and improves the light energy utilization rate at the same time. The reflection focal length fr of the collimating mirror 1 satisfies 5 mm < fr < 10 mm. Satisfying the above relational expression can avoid the too large lateral size of the system and improve the system compactness. Specifically, in this embodiment, its reflection focal length fr is 6.35 mm, the parent focal length is 3.175 mm, and the bottom diameter Dp and the reflection angle Rg of the collimating mirror 1 satisfy: 0.04 < Dp / Rg < 0.08, where Rg = 90°, the bottom diameter Dp is 6 mm in this embodiment, the collimating mirror 1 adopts a reflective structure, and the reflection angle Rg is set to 90°, which optimizes the reflection focal length fr of the collimating mirror 1, and improves the light energy utilization rate of the collimating mirror 1 through the coating on its parabolic surface, realizing a compact design.

[0078] Since seawater itself has strong light absorption, the beam splitter 2 in this embodiment is a multi-transmission and less-reflection mirror, with a length, width and height of 6 mm, made of fused quartz, and the ratio of the transmittance to the emissivity is 7:3. Its shape is a right-angled prism, and a beam splitting film is coated on the beam splitting surface, while the other surfaces are not coated. After being split by the beam splitter 2, the energy ratio of the light entering the measurement optical path and the reference optical path is approximately between 7 and 9. Meeting these conditions is beneficial to improving the light energy transmission efficiency of the system and increasing the energy transmission consistency at different wavelengths. Because the measurement optical path needs to pass through the seawater to be measured, its light energy attenuation ratio is greater than that of the reference optical path that does not pass through the seawater to be measured. Meeting the above ratio can allocate more light energy to the measurement optical path, reduce the energy difference between the two paths caused by seawater absorption, and enhance the measurement accuracy.

[0079] In this embodiment, the first window 3 and the second window 4 have the same structural dimensions and are both made of sapphire, with a thickness of 5.6 mm and a diameter of 13 mm. Using sapphire as the manufacturing material can reduce the absorption of ultraviolet light energy by the first window 3 and the second window 4, reduce the light energy attenuation of the measuring optical path, and increase the design margin of other components.

[0080] The coupling lens group 5 includes a first lens 51, a second lens 52 and a third lens 53 arranged in sequence along the optical path. The first lens 51 is a biconvex lens with positive focal length, the second lens 52 is a biconcave lens with negative focal length, and the third lens 53 is a biconvex lens with positive focal length. The incident surfaces and the exit surfaces of the first lens 51, the second lens 52 and the third lens 53 are all spherical surfaces. With this structure, the manufacturing difficulty and cost of the coupling lens group 5 can be reduced, so that it has good focusing characteristics and improves the light beam coupling efficiency. In addition, the maximum apertures of the three lenses in the coupling lens group 5 in this embodiment are designed. In the coupling lens group 5, the incident surface of the first lens 51 has the largest aperture, which is 12 mm, which improves the light energy utilization rate of the measurement optical path and the reference optical path, so that the system has a reasonable lateral size and is easy to assemble.

[0081] The distance between the center of the exit surface of the first lens 51 and the center of the incident surface of the second lens 52 on the optical axis is 0.15mm-0.35mm; the distance between the center of the exit surface of the second lens 52 and the center of the incident surface of the third lens 53 on the optical axis is 30mm-35mm; the distance between the center of the exit surface of the third lens 53 and the focusing surface of the spectrometer 9 on the optical axis is 3.7mm-4.8mm. The first lens 51 and the second lens 52 adopt a close double separation structure, wherein the first lens 51 and the third lens 53 are made of calcium fluoride material, and the second lens 52 is made of fused quartz material, and the first lens 51 and the second lens 52 adopt a close double separation structure. Because the main absorption band of nitrate is 210nm-240nm, there are few optical materials with high transmittance in this band. Currently known high transmittance materials include fused silica and calcium fluoride, and the transmittance in this band is greater than 90%. Therefore, the coupling mirror group 5 is designed with these two materials, and no anti-reflection film is coated on the three lens surfaces. The reason is that the common coating process does not significantly improve the transmittance of light energy in the ultraviolet band of 210-240nm, but will also increase the inconsistency of energy in different bands, which is mainly reflected in the strong absorption of the film layer near 210nm, which makes its transmittance lower than that of other longer bands. The achromatic aberration ability of a single material is weaker than that of a combination of different materials. Therefore, the coupling mirror group 5 of this system adds a group of calcium fluoride-fused silica-calcium fluoride lens group to achieve system chromatic aberration control, and the entire coupling mirror group 5 only uses three lenses, which reduces the length of the system while improving the beam focusing effect.

[0082] The specific parameters of coupling lens group 5 are shown in the following table:

[0083]

[0084]

[0085] In the above table, surface type refers to the type of each optical surface, curvature radius refers to the curvature radius of each optical surface, thickness refers to the center thickness of the optical lens or the thickness of the medium before and after the lens, material refers to the material used for the optical lens, and STO stands for aperture.

[0086] The spectrometer 9 is arranged on the transmission light path of the coupling mirror group 5. The spectrometer 9 receives the focused light beam transmitted from the coupling mirror group 5 through the optical fiber. In this embodiment, a multimode optical fiber with a core diameter of 400um and a numerical aperture of 0.22 is used to receive the focused light beam. The optical fiber has an SMA905 interface and the back intercept is set to 3.998mm, which is conducive to fine-tuning the focus plane position of the spectrometer 9 after the optical fiber is inserted to ensure the focusing effect.

[0087] The first reflector 6, the second reflector 7 and the third reflector 8 are all right-angle reflectors, the length, width and height of the three are all 10 mm, and the manufacturing material is H-K9L. The surfaces of the first reflector 6, the second reflector 7 and the third reflector 8 are coated with ultraviolet high reflective reflectivity metal film, which can improve the light energy transmission efficiency of the reference light path, reduce light energy loss and improve system stability.

[0088] The technical specifications of the compact ultraviolet dual-path optical system for measuring seawater nitrate concentration in this embodiment are shown in the following table:

[0089]

[0090]

[0091] The basic theorem relied on by the system of the present invention when detecting the concentration of nitric acid in seawater is the Lambert-Beer theorem. According to the theorem, the wavelength of light, the outgoing light intensity and the incident light intensity satisfy the following relationship:

[0092] I out (λ)=I in ·e ∑-σ(λ)wp

[0093] Where: I in is the input light intensity, I out is the output light intensity, λ is the wavelength, σ is the molar absorption coefficient, w is the concentration, p is the optical path, and e is a natural constant.

[0094] According to the above formula, based on the light energy before and after the light intensity passes through the seawater sample, the optical path of the light beam through the sample, and the molar absorption coefficient of the absorbing substance, the concentration of nitrate in the sample can be obtained by extracting and analyzing the nitrate spectrum. However, if the nitrate concentration is calculated according to this method, the light beam incident on the sample should only be incident vertically as much as possible, so that the accuracy of the result obtained will be higher. Through the optimized design of the collimator 1, the divergence angle of the output light from the collimator 1, that is, the collimated light, is controlled at 1°, which effectively ensures the accuracy of the measurement result. The entire system works at a wavelength of ultraviolet 200nm-400nm band, which can enable the system to cover the main absorption band of nitrate, which is conducive to improving the reliability of the detection results. The distance from the center of the output surface of the collimator 1 to the focusing surface of the spectrometer 9 is 161.3mm, which effectively improves the compactness of the system and leaves enough space for adjustment of each component of the system.

[0095] At the same time, the present invention also optimizes the design of each component to improve the compactness and detection effect of the entire system. The optimized design is as follows:

[0096] 1) The light aperture DW and thickness TH1 of the first window 3 satisfy:

[0097] 2≤DW / TH1≤2.6

[0098] Among them, the value range of DW is 6mm-13mm; it can make the system have good pressure resistance characteristics, the working depth can reach 4000m underwater, and it is conducive to the processing and forming of the two windows.

[0099] 2) The clear aperture DS2 of the coupling lens group 5 and the total length TTL of the coupling lens group 5 satisfy:

[0100] 0.21≤DS2 / TTL≤0.32

[0101] Among them, the value range of DS2 is 8mm-14mm, which can improve the light beam focusing ability of the coupling lens group 5, while controlling the lateral and longitudinal dimensions of the entire system within a reasonable range.

[0102] 3) The effective aperture DE2 and the curvature radius R2 of the exit surface of the first lens 51 satisfy:

[0103] 0.65≤DE2 / R2≤1.43

[0104] Among them, the value range of R2 is 10.5mm-11.5mm; it can reduce the difficulty of lens processing, while controlling the deflection of light by the coupling lens group and enhancing the light beam focusing ability.

[0105] 4) The focal length f of the coupling lens group 5 and the focal length f3 of the third lens 53 satisfy:

[0106] 0.41≤f3 / f≤0.85

[0107] The optical power distribution of each lens in the coupling lens group 5 can be made more reasonable, the processing technology of each lens can be improved, and the processing cost can be reduced.

[0108] 5) The interval between the optical axis of the transmitted light beam of the beam splitter 2 and the optical axis of the light beam reflected by the first reflector 6 is defined as SP1, and the light aperture DS3 of SP1 and the beam splitter 2 satisfies:

[0109] 0.19≤DS3 / SP1≤0.36

[0110] Among them, the value range of SP1 is 18mm-20mm;

[0111] The distance between the center of the exit surface of the collimator 1 and the incident surface of the coupling lens group 5 on the optical axis is defined as SW1, and the interval between the optical axis of the transmitted light beam of the beam splitter 2 and the optical axis of the light beam reflected by the third reflector 8 is defined as SP2. SW1 and SP2 satisfy:

[0112] 0.03≤SP2 / SW1≤0.08

[0113] Among them, the value range of SW1 is 90mm-120mm; it can make the layout of various components in the system more reasonable and reduce the lateral size of the system.

[0114] 6) The interval between the center of the exit surface of the first lens 51 and the center of the incident surface of the second lens 52 on the optical axis is defined as SP3, and the curvature radius R3 of SP3 and the incident surface of the second lens 52 satisfies:

[0115] 30≤R3 / SP3≤76.7

[0116] Among them, the value range of R3 is 10.5mm-11.5mm; it can enhance the deflection ability of the coupling mirror group 5 to the light beam, while reducing the difficulty and sensitivity of system installation.

[0117] 7) The interval between the center of the exit surface of the second lens 52 and the center of the incident surface of the third lens 53 on the optical axis is defined as SP4, then SP3 and SP4 satisfy:

[0118] 0.0043≤SP3 / SP4≤0.0175

[0119] This can prevent the entire coupling lens group from being too long, thereby improving the focusing effect of the system.

[0120] 8) The distance between the center of the exit surface of the third lens 53 and the focusing surface of the spectrometer 9 on the optical axis is defined as SP5, and the curvature radius R5 of the incident surface of the third lens 53 satisfies:

[0121] 1.1≤R5 / SP5≤1.89

[0122] Among them, the value range of R5 is 5.3mm-7mm, which can reduce the size of the focused spot and facilitate the adjustment of the focusing position.

[0123] During operation, the ultraviolet light source 10 emits ultraviolet light, which enters the beam splitter 2 after being reflected by the collimator 1. The ultraviolet light is split into a transmitted light beam and a reflected light beam by the beam splitting surface of the beam splitter 2. The transmitted light beam passes through the first window 3 and the second window 4 in sequence and enters the coupling mirror group 5, while the reflected light beam passes through the first reflector 6, the second reflector 7 and the third reflector 8 in sequence and enters the coupling mirror group 5. The two light beams are coupled in the coupling mirror group 5 and finally focused on the focusing plane of the spectrometer 9.

[0124] like Figure 2 As shown in the figure, it can be seen that the system focus spot is less than 500um, the system beam coupling efficiency is improved, and the measurement lower limit of nitrate concentration is increased, which is beneficial to increase the system signal-to-noise ratio and improve the measurement accuracy. Figure 3 As shown, it is the simulation result of the coupling efficiency of the focused light beam of this system received through the optical fiber under non-sequential ZEMAX optical software. The input optical power is 1W. It can be seen that the coupling efficiency of this system is better than 80%, and the energy transmission performance is good, which is suitable for accurate measurement of seawater nitrate concentration.

[0125] The embodiments described above are merely descriptions of specific implementation methods of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A compact ultraviolet dual-path optical system for measuring seawater nitrate concentration, characterized by: It comprises a collimator (1), a beam splitter (2), a first window (3), a second window (4), a coupling mirror group (5), a first reflector (6), a second reflector (7), a third reflector (8), a spectrometer (9) and an ultraviolet light source (10); The collimator (1) is located on the outgoing light path of the ultraviolet light source (10), the beam splitter (2) is arranged on the outgoing light path of the collimator (1), the first window (3), the second window (4) and the coupling lens group (5) are arranged in sequence on the transmission light path of the beam splitter (2), and the seawater to be measured is arranged between the first window (3) and the second window (4); The first reflector (6) is arranged on the transmission light path of the beam splitter (2), the second reflector (7) is arranged on the reflection light path of the first reflector (6), the third reflector (8) is arranged on the reflection light path of the second reflector (7), and the coupling mirror group (5) is also located on the reflection light path of the third reflector (8); The spectrometer (9) is arranged on the transmission light path of the coupling mirror group (5); The focusing spot size DF of the coupling lens group (5) and the focal length f of the coupling lens group (5) satisfy: 0.014≤DF / f≤0.031 Among them, the value range of f is 15mm-24mm.

2. The compact ultraviolet dual-path optical system for measuring seawater nitrate concentration according to claim 1, characterized in that: The coupling lens group (5) comprises a first lens (51), a second lens (52) and a third lens (53) which are arranged in sequence along the optical path; the first lens (51) is a biconvex lens with positive optical power, the second lens (52) is a biconcave lens with negative optical power, the third lens (53) is a biconvex lens with positive optical power, and the incident surface and the exit surface of the first lens (51), the second lens (52) and the third lens (53) are all spherical surfaces.

3. The compact ultraviolet dual-path optical system for measuring seawater nitrate concentration according to claim 2, characterized in that: The distance between the center of the exit surface of the collimator (1) and the center of the beam splitting surface of the beam splitter (2) on the optical axis is 24 mm to 27 mm; The distance between the center of the beam splitting surface of the beam splitter (2) and the center of the incident surface of the first window (3) on the optical axis is 39.4 mm-51 mm; The distance between the center of the exit surface of the first window (3) and the center of the incident surface of the second window (4) on the optical axis is 8 mm-10 mm; The distance between the center of the exit surface of the second window (4) and the incident surface of the coupling lens group (5) on the optical axis is 14 mm-19 mm; The distance between the center of the beam splitting surface of the beam splitter (2) and the center of the incident surface of the first reflector (6) on the optical axis is 18 mm to 20 mm; The distance between the center of the exit surface of the first reflector (6) and the center of the incident surface of the second reflector (7) on the optical axis is 60.5 mm-86 mm; The distance between the center of the exit surface of the second reflector (7) and the center of the incident surface of the third reflector (8) on the optical axis is 10.8 mm to 13 mm; The distance between the center of the exit surface of the third reflector (8) and the incident surface of the coupling mirror group (5) on the optical axis is 5.5 mm-7 mm.

4. The compact ultraviolet dual-path optical system for measuring seawater nitrate concentration according to claim 3, characterized in that: The distance between the center of the exit surface of the first lens (51) and the center of the incident surface of the second lens (52) on the optical axis is 0.15 mm-0.35 mm; The distance between the center of the exit surface of the second lens (52) and the center of the incident surface of the third lens (53) on the optical axis is 30 mm-35 mm; The distance between the center of the exit surface of the third lens (53) and the focusing surface of the spectrometer (9) on the optical axis is 3.7 mm-4.8 mm.

5. The compact ultraviolet dual-path optical system for measuring seawater nitrate concentration according to claim 4, characterized in that: The first window (3) and the second window (4) have the same structural dimensions, and the light aperture DW and thickness TH1 of the first window (3) satisfy: 2≤DW / TH1≤2.6 Among them, the value range of DW is 6mm-13mm; The light aperture DS2 of the coupling lens assembly (5) and the total length TTL of the coupling lens assembly (5) satisfy: 0.21≤DS2 / TTL≤0.32 Among them, the value range of DS2 is 8mm-14mm.

6. The compact ultraviolet dual-path optical system for measuring seawater nitrate concentration according to claim 5, characterized in that: The effective aperture DE2 and the curvature radius R2 of the exit surface of the first lens (51) satisfy: 0.65≤DE2 / R2≤1.43 Among them, the value range of R2 is 10.5mm-11.5mm; The focal length f of the coupling lens group (5) and the focal length f3 of the third lens (53) satisfy: 0.41≤f3 / f≤0.

85.

7. The compact ultraviolet dual-path optical system for measuring seawater nitrate concentration according to claim 6, characterized in that: The interval between the optical axis of the transmitted light beam of the beam splitter (2) and the optical axis of the light beam reflected by the first reflector (6) is defined as SP1, and SP1 and the light aperture DS3 of the beam splitter (2) satisfy: 0.19≤DS3 / SP1≤0.36 Among them, the value range of SP1 is 18mm-20mm; The distance between the center of the exit surface of the collimator (1) and the incident surface of the coupling lens group (5) on the optical axis is defined as SW1, and the interval between the optical axis of the transmitted light beam of the beam splitter (2) and the optical axis of the light beam reflected by the third reflector (8) is defined as SP2. SW1 and SP2 satisfy the following conditions: 0.03≤SP2 / SW1≤0.08 Among them, the value range of SW1 is 90mm-120mm; The interval between the center of the exit surface of the first lens (51) and the center of the incident surface of the second lens (52) on the optical axis is defined as SP3, and the curvature radius R3 of SP3 and the incident surface of the second lens (52) satisfies: 30≤R3 / SP3≤76.7 Among them, the value range of R3 is 10.5mm-11.5mm; The interval between the center of the exit surface of the second lens (52) and the center of the incident surface of the third lens (53) on the optical axis is defined as SP4, and SP3 and SP4 satisfy: 0.0043 ≤ SP3 / SP4 ≤ 0.0175; Define the distance between the center of the exit surface of the third lens (53) and the focal plane of the spectrometer (9) on the optical axis as SP5. The SP5 and the radius of curvature R5 of the incident surface of the third lens (53) satisfy: 1.1 ≤ R5 / SP5 ≤ 1.89 Among them, the value range of R5 is 5.3 mm - 7 mm.

8. The compact ultraviolet dual-path optical system for measuring seawater nitrate concentration according to claim 1, characterized in that: The beam-splitting surface of the beam splitter (2) is coated with a beam-splitting film; The surfaces of the first mirror (6), the second mirror (7), and the third mirror (8) are all coated with an ultraviolet highly reflective emissive metal film.

9. The compact ultraviolet dual-path optical system for measuring seawater nitrate concentration according to claim 2, characterized in that: The collimating mirror (1) is an off-axis paraboloidal mirror, and the ultraviolet light source (10) is located on the reflection surface side of the off-axis paraboloidal mirror.

10. The compact ultraviolet dual-path optical system for measuring seawater nitrate concentration according to claim 9, characterized in that: The production material of the off-axis paraboloidal mirror is 6061-T6 aluminum; the reflection focal length fr of the off-axis paraboloidal mirror satisfies 5 mm < fr < 10 mm; the bottom diameter Dp and the reflection angle Rg of the off-axis paraboloidal mirror satisfy: 0.04 < Dp / Rg < 0.08, and the reflection angle Rg is 90°; The beam splitter (2) is a multi-transmission and less-reflection mirror, the ratio of its transmittance to emissivity is 7:3, and its length, width, and height are all 6 mm. The production material is fused quartz; The length, width, and height of the first mirror (6), the second mirror (7), and the third mirror (8) are all 10 mm. The production material is H-K9L, and all three are right-angle mirrors; The thicknesses of the first window (3) and the second window (4) are both 5.6 mm, the diameter is 13 mm, and the production material is sapphire; The production materials of the first lens (51) and the third lens (53) are calcium fluoride, the production material of the second lens (52) is fused quartz, and the first lens (51) and the second lens (52) adopt a double-separation structure.

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