RU2375686C2 - Multichannel spectrometre - Google Patents

Abstract

FIELD: physics; optics.

SUBSTANCE: invention relates to analytical instrument making. The spectrometer has an entrance slit lying at the focus of a concave input mirror, optically connected to a plane diffraction grating which lies at a distance of 0.85 of the focal length of the concave output mirror, at the focus of which there is a multi-element photodetector, which is connected to an electronic unit for controlling the said photodetector and processing its output signals. The novelty is that, the diffraction grating and the output mirror can rotate about axes which pass through the centres of their surfaces and perpendicular the meridian plane. Arresting devices are fitted for the said diffraction grating and output mirror, which prevents the zero order of the diffraction grating, reflected from the output mirror, from falling on the working surface of the diffraction grating.

EFFECT: reduced detection limits due to reduced level of background radiation.

9 cl, 6 dwg

Description

The invention relates to analytical instrumentation, namely to spectral devices, and can be used, for example, to create a small, highly sensitive and broadband multi-channel spectrometer.

Spectral devices decompose the electromagnetic radiation of the optical range into monochromatic components for a qualitative and quantitative study of the spectral composition of the light emitted, absorbed, reflected or scattered by the substance. Studies allow us to judge the chemical composition of matter and the nature of physical processes associated with the emission or interaction of light with matter. When conducting research in the conditions of mobile laboratories, it is necessary that the spectral device meets the following requirements:

- It was compact due to the limited volume of the mobile laboratory, which can be located on the basis of a car;

- had the ability to choose the working spectral range and resolution for solving various problems associated with the detection of a wide range of substances;

- had high speed to obtain measurement results in real time;

- It had high reliability and durability, which ensures high performance with a long service life;

- had low detection limits, allowing quantitative analysis of samples with high accuracy.

A known spectral device manufactured by Carl Zeiss, comprising a housing with an entrance window, inside which an optical system is installed, consisting of an entrance slit and a concave diffraction grating, arranged according to the Paschen-Runge scheme. The output of the optical system is connected to a recording device based on a linear multi-element photodetector (http://www.tec5hellma.com/Download/Literature/Product%20Information/Spectrometers/MCSPDA_t5U.pdf). Due to the small number of optical elements, the spectral device has a low level of background radiation.

The main disadvantage of this spectrometer is the curvature of the focusing surface, which significantly degrades the spectral resolution of the spectrometer (4-5 times) due to its deviation from the flat surface of the photodetector by 0.5 mm at a focal depth of 0.15 mm.

A known spectral device containing a housing with an entrance window, inside which an optical system is installed, consisting of an entrance slit, two concave mirrors and a flat diffraction grating, located according to the horizontal Ebert-Fasti scheme. The output of the optical system is connected to a recording device based on photographic plates (see I.V. Peysakhson Optics of Spectral Instruments. Ed. 2nd, add. And revised. - L .: Mashinostroenie, 1975, 312 p.). The spectral device has a wide operating spectral range, allows you to change the spectral range by rotating the diffraction grating and has a high resolution.

The main disadvantages of this spectrometer are: firstly, the increased level of background radiation at the angles of rotation of the diffraction grating, providing the required working spectral ranges, but at the same time allowing the reflection of the zero order on the diffraction grating.

Secondly, the use of an outdated system for recording spectra based on photographic plates requires a long and complex development process, which limits the use of the device as part of a real-time mobile laboratory.

The closest to the claimed device (prototype) is a multiband spectrometer "Hummingbird" (see Labusov VA. Thesis for the degree of candidate of technical sciences "Multichannel optical image analyzers for atomic emission spectral analysis", IAE SB RAS, 2005 , 241 p.). The spectrometer consists of a housing with an entrance window, inside of which there is an optical system consisting of an entrance slit, two concave mirrors and a flat diffraction grating, arranged according to the horizontal Ebert-Fasti scheme. The output of the optical system is connected to a recording device based on a linear multi-element photodetector. The spectrometer allows measurements to be taken simultaneously in a wide spectral range in real time, and has small dimensions.

The main disadvantage of this spectrometer is that the optical scheme, like that of the above analogue, allows a “zero order” to hit the working surface of the diffraction grating, which is expanded into a “spurious” spectrum focused by the output mirror on the photodetector surface. This leads to a significant increase in the level of scattered radiation.

The objective of the claimed invention is to eliminate this drawback, namely, reducing the detection limits of the desired substances by reducing the level of background radiation.

The indicated problem is in a multichannel spectrometer containing a housing with a horizontal optical circuit including an entrance slit located in the focus of a concave input mirror optically coupled to a plane diffraction grating located at a distance of 0.85 of the focal length of the concave output mirror, in the focus of which there is a multi-element photodetector, connected to the electronic control unit of the photodetector and processing its output signals, it is decided that the diffraction grating and the output mirror are mounted on rotating platforms with the possibility of rotation about an axis passing through the centers of their surfaces and perpendicular to the meridional plane equipped with rotation limiters, while the rotation limiters of the output mirror do not allow the normal restored to the edge of the mirror surface of the output mirror closest to the input mirror to the diffraction working surface lattice.

It is known that to compensate for the coma of decentration of the central beam of the horizontal optical Ebert-Fasti scheme, there is a strict mathematical formula:

where α ₁ and α ₂ are the angles between the main rays incident and reflected from the mirrors, φ is the angle of incidence on the diffraction grating, φ 'is the diffraction angle for the average wavelength of the working spectral range. In this case, the best resolution is achieved in the middle of the working spectral range. However, when the working spectral range is changed due to rotation of the diffraction grating, for some angles of rotation, the zeroth order, falling on the output mirror and reflected from it, returns to the diffraction grating and decomposes into a “spurious” spectrum, which increases the background level in the recorded spectrum and accordingly increases (degrading) detection limits.

The possibility of rotation of the output mirror allows you to set it in accordance with formula (1) when changing the angle of incidence on the diffraction grating.

For certain angles of incidence on the diffraction grating, the position of the output mirror, satisfying formula (1), is such that the zero order reflected from it falls onto the diffraction grating. To eliminate the entry of a zero order onto the diffraction grating, the output mirror rotation limiters are installed, which specify the maximum possible angle of rotation of the output mirror. At such an angle, the normal restored to the edge of the output mirror closest to the input mirror does not fall on the working surface of the diffraction grating.

Thus, at any angle of rotation of the diffraction grating, the reflected zero order does not fall on it, thereby significantly reducing the level of background radiation, due to which the limits of detection of sample elements are increased.

As stops of rotation, mechanical stops can be used that prevent the output mirror from turning at a forbidden angle at which the normal restored to the edge of the output mirror crosses the edge of the working surface of the diffraction grating.

Also, as a rotation limiter, a manually controlled mechanical drive or an electromechanical drive controlled by a computer program can be used and allows the most accurate positioning of the output mirror with respect to the diffraction grating and thereby obtain the best spectral resolution of the device.

To simplify the adjustment of the spectrometer, the angle of rotation of the diffraction grating is changed using a rotary platform equipped with a mechanical drive with manual control or an electromechanical drive controlled by a computer program.

To prevent the radiation reflected from the photosensitive region of the photodetector from getting into the output mirror, the photodetector is mounted for rotation around its longitudinal axis. Due to this, the radiation reflected from the photodetector is directed above or below the output mirror, where it is absorbed on the anti-reflective surface of the housing, which can be made, for example, in the form of an anodized oxide coating of black color.

For accurate installation in the spectrum focusing plane, the photodetector is located on a movable platform mounted to move perpendicularly to the longitudinal axis of the detector and rotate around the normal to the meridional plane recovered from the center of the photodetector.

Thus, the elimination of the reflection of the zero order on the diffraction grating and the use of an inclined movable photodetector can significantly reduce the level of background illumination inside the spectrometer, thereby significantly lowering the detection limits of the spectrometer, which has no analogues among the technical solutions used in modern spectrometers, which means that the claimed the solution meets the criterion of "inventive step".

Figure 1 and figure 2 presents a diagram of the inventive device in the meridional plane, explaining the principle of operation of the inventive spectrometer.

Figure 3 presents the view of the photodetector mounted on a movable platform in the sagittal plane.

Figure 4 presents examples of the recorded spectral intervals for three diffraction gratings with a different number of strokes per mm.

Figure 5 shows the spectral lines in the center of the working spectral range for the prototype and for the inventive device, obtained by simulation in the Zemax program.

Figure 6 shows the background level inside the housing of the claimed spectrometer and prototype.

The spectrometer (Fig. 1) comprises a housing 1, an entrance window 2, an entrance slit of variable width 3, an input spherical mirror 4, a flat diffraction grating 5, an output spherical mirror 6, a photodetector 7, a movable platform 8, alignment screws 9, an electronic control unit and registration of signals of a multi-element photodetector 10, computer 11, input radiation 12, rotation limiters of the diffraction grating 13, rotation limiters of the output mirror 14, zero order 15, the longitudinal axis of the photodetector 16, movable platforms 17 and 18.

The spectrometer (figure 2) further comprises electric drives 19 and 20.

View A (FIG. 3) further comprises a perpendicular to the meridional plane 21, drawn through the center of the photodetector, the photodetector radiator 22.

Consider the operation of the spectrometer shown in figure 1. Radiation 12 directed to the spectrometer passes through the input window 2 and the input slit 3. From the input slit, the radiation is incident on a spherical collimating mirror 4, which converts the diverging beam into a parallel one. Next, the light enters the diffraction grating 5, which has rotation limiters 13. The grating spreads the radiation into a spectrum of different orders. In this case, the zeroth order can fall on the output mirror 6 and, reflected from it, return to the diffraction grating 5 and then decompose into a spurious spectrum, which will significantly increase the background level inside the spectrometer case (see graph 25, Fig. 6). Due to the limitation of the angle of rotation of the output mirror 6, it is impossible to get a zero order on the diffraction grating. The spectrum of the working diffraction order is directed to the output mirror 6, which converts the parallel monochromatic beams to converging, focusing on the photodetector 7. For accurate installation in the plane of focusing of the spectrum, the photodetector is located on platform 8, rotated around perpendicular 21 using adjustment screws 9. Part of the incident on the radiation photodetector is reflected, but due to the tilt of the photodetector around its longitudinal axis 16, the reflected radiation is prevented from reaching the output mirror 6. Signal from the photodetector enters into the electronic control unit 10 and signal detection multielement photodetector 7. The data from the electronic control unit 10 are transmitted to the computer 11 by Ethernet cable. Mathematical processing of the output signal, its visualization, etc. conducted using the Atom program. The possibility of rotation and change of the diffraction grating allows you to change the operating spectral range and spectral resolution of the device. This, in turn, allows you to record (see figure 4) on the inventive spectrometer as an overview spectrum with low dispersion (K ₁₅ ) for assessing the composition of the sample, and spectral ranges (M ₁₃ , M ₃₅ ) and (N ₁₂ , N ₂₄ and N ₄₅ ) with medium and high dispersion necessary for the quantitative analysis of the composition of the sample.

When rotating the diffraction grating, the output mirror must be rotated by an angle determined from formula (1), this allows one to obtain the best spectral resolution of the device at the center of the working spectral range and minimal deterioration at its edges. If the angle of incidence on the diffraction grating and the angle of rotation of the output mirror, determined from formula (1), are such that the zeroth order reflected from the output mirror hits the diffraction grating, then the rotation of the output mirror is set so that the normal restored to the edge of the output mirror closest to input mirror, did not fall on the working area of the diffraction grating. To do this, limit the rotation of the output mirror.

The spectrometer shown in FIG. 2 operates in a similar manner. In this case, the synchronous rotation of the output mirror 6 and the diffraction grating 5 is carried out using a computer 11 programmatically by changing the angles of the position of the rotary platforms 17 and 18. For example, for a detailed consideration of the spectral range with boundaries λ ₁ ... λ ₅ (see figure 4 ) obtained using a diffraction grating with a number of lines / mm - K and an incidence angle of K ₁₅ , the operator, depending on the dispersion he needs, installs a diffraction grating with a number of lines / mm - M or N in the device and then selects the analyzed spectral range by rotating the diffraction grating at an appropriate angle. Examples of used gratings, angles of incidence on them and the corresponding working spectral ranges and dispersions of the device are given in the table.

Shtr / mm Angle of incidence Spectral range Dispersion K = 320 K ₁₅ = 11.1 ° λ ₁ ... λ ₅ = 190 ... 1100 nm 28 nm / mm M = 660 M ₁₃ = 9.62 ° λ ₁ ... λ ₃ = 190 ... 645 nm 14 nm / mm M ₃₅ = 2.2 ° λ ₃ ... λ ₅ = 645 ... 1100 nm 14 nm / mm N = 1000 N ₁₂ = 7.97 ° λ ₁ ... λ ₂ = 190 ... 493 nm 9.3 nm / mm N ₂₄ = 0.77 ° λ ₂ ... λ ₄ = 493 ... 797 nm 9.3 nm / mm N ₄₅ = -5.39 ° λ ₄ ... λ ₅ = 797 ... 1100 nm 9.3 nm / mm

Computer simulation in the Zemax program (www.zemax.com) allows you to get the contour of the spectral line (see figure 5) without taking into account the resolution of the photodetector. When taking into account the discreteness, the obtained broadening by 7% of the spectral line 24 compared to line 23 practically does not degrade the spectral resolution of the device, but due to the exclusion of the reflected zero order on the diffraction grating, the background level inside the inventive spectrometer (25) decreased compared to the background level of the prototype ( 26) by 500%, which allows us to talk about a sharp decrease in the inventive spectrometer of the detection limits of elements in the sample.

Implementation examples

Embodiment 1. A small-sized spectrometer was manufactured according to the Ebert-Fasti optical scheme. The optical scheme of the spectrometer included an entrance slit 15 μm wide. Two mirrors with the same radius of curvature of 200 mm were used as input and output mirrors. A lens was mounted in front of the spectrometer's input window to focus the analyzed radiation on the input slit. The change in the working spectral range was carried out by turning the platforms on which the diffraction grating and the exit mirror were installed. The BLPP-369 line of photodiodes was used as a multi-element photodetector (the number of photodiodes is 2580, the placement step is 12.5 μm and the height is 1 mm, the dynamic range is 10 ⁴ ).

The photosensitive surface of the ruler is installed in the region of the best focusing of the spectrum with the help of screws with micrometric thread mounted on the edges of the platform and allowing you to align the photo detector in the registration plane. As an electronic unit, we used the electronic registration unit of a multi-channel atomic emission spectrum analyzer (MAES), which is a means of measuring the intensity of spectral lines (registered in the State Register of Measuring Instruments of the Russian Federation under No. 21013-01) and manufactured by LLC VMK-Optoelectronics, g. Novosibirsk Data from the electronic unit was transferred to the computer via an Ethernet cable. Mathematical processing of the output signal, its visualization, etc. were carried out using the Atom program (certificate of registration No. 2004611127). The spectrometer had the following technical characteristics: operating spectral range - 390-860 nm; inverse linear dispersion - 0.19 nm / diode; focal length - 100 mm; relative aperture - 1: 6; minimum exposure time - 10 ms; maximum exposure time - not limited; dimensions - 150 × 200 × 80 mm ³ ; weight - 3 kg.

Embodiment 2. The design of the spectrometer, the optical scheme and its main elements were performed similarly to Embodiment 1, but the rotary platforms of the diffraction grating and the output mirror were equipped with programmable electromechanical drives based on LIN ENGINEERING 4118M-10P stepper motors, due to by which the operator sets the required operating spectral range with greater accuracy than in embodiment 1. Other technical characteristics of the spectrometer are similar to the variant complements 1.

Thus, the claimed device allows to reduce the detection limit of radiation by reducing the level of background radiation.

Claims (9)

1. A multi-channel spectrometer comprising a housing with a horizontal optical circuit including an entrance slit located in the focus of a concave input mirror optically coupled to a flat diffraction grating located at a distance of 0.85 of the focal length of the concave output mirror, in the focus of which is a multi-element photodetector connected with an electronic control unit and registration of output signals, characterized in that the diffraction grating and the output mirror are mounted on rotary platforms with the possibility of rotation relative to the axes passing through the centers of their surfaces and perpendicular to the meridional plane equipped with rotation limiters, while the rotation limiters of the output mirror do not allow the normal, restored to the edge of the mirror surface of the output mirror closest to the input mirror, to the working surface of the diffraction grating. 2. The device according to claim 1, characterized in that mechanical stops are used as limiters for turning the rotary platform of the output mirror. 3. The device according to claim 1, characterized in that the rotary platform of the output mirror is equipped with a mechanical drive. 4. The device according to claim 1, characterized in that the rotary platform of the output mirror is equipped with a program-controlled electromechanical drive. 5. The device according to claim 1, characterized in that the rotary platform of the diffraction grating is equipped with a mechanical or electromechanical program-controlled drive. 6. The device according to claim 1, characterized in that the photodetector is mounted to rotate around its longitudinal axis. 7. The device according to claim 1, characterized in that the photodetector is located on a platform mounted to move perpendicular to the longitudinal axis of the detector and rotate around the normal to the meridional plane, restored from the center of the photodetector. 8. The device according to claim 1, characterized in that the inner surface of the housing is made anti-reflective, for example in the form of an anodized oxidized coating of black color. 9. The device according to claim 8, characterized in that the photodetector is mounted to rotate around its longitudinal axis.

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