RU171813U1 - Device for detecting molecular impurities in the atmosphere by vibrational-rotational spectra - Google Patents

Device for detecting molecular impurities in the atmosphere by vibrational-rotational spectra Download PDF

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Publication number
RU171813U1
RU171813U1 RU2016114756U RU2016114756U RU171813U1 RU 171813 U1 RU171813 U1 RU 171813U1 RU 2016114756 U RU2016114756 U RU 2016114756U RU 2016114756 U RU2016114756 U RU 2016114756U RU 171813 U1 RU171813 U1 RU 171813U1
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Russia
Prior art keywords
device
interferometer
lines
vibrational
atmosphere
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RU2016114756U
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Russian (ru)
Inventor
Михаил Павлович Иванов
Юрий Александрович Толмачев
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Федеральное государственное бюджетное образовательное учреждение высшего образования "Санкт-Петербургский государственный университет" (СПбГУ)
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/01Arrangements or apparatus for facilitating the optical investigation

Abstract

The utility model relates to devices for measuring the concentration of harmful molecular impurities in the gaseous phase, mainly in the atmosphere surrounding us, by spectroscopic methods. The device includes a source of a continuous spectrum and a comb optical filter based on a Fabry-Perot interferometer, passing a complete set of lines of the vibrational-rotational spectrum of a molecule, as well as a recording system and a device for simultaneously modulating the entire set of transmission frequencies of the interferometer due to periodic changes in the optical thickness of the gap between it mirrors. An objective and an aperture are mounted at the output of the interferometer, which summarize the intensity of all the missed lines, which ensures an increase in the measured signal by at least an order of magnitude and a corresponding increase in the sensitivity of the device for detecting molecular impurities. The technical result consists in the possibility of repeatedly increasing the sensitivity of the measuring system and its mechanical stability. 2 ill.

Description

The utility model relates to devices for measuring the concentration of harmful molecular impurities in the gaseous phase, mainly in the atmosphere surrounding us, by spectroscopic methods. The high efficiency of spectroscopic methods for searching and detecting gaseous substances under various conditions (both terrestrial and extraterrestrial, including astronomical) is well known. Increasing the sensitivity of such methods has been and will remain an important scientific and technical task. The urgent need to solve the problem of determining the content of molecular impurities, for example, greenhouse gases, in the atmosphere determines the multiplicity of proposals for devices for its solution. The general principle of their work is based on measuring the fraction of radiation absorbed by the molecules coming from a certain source.

A device [1] is known in which an infrared emitting diode is used to measure absorption. The spectral emission band of LEDs is known to have a width far exceeding the width of the entire absorption band of the gas under study, and since the integral absorption in the band is determined by the ratio of the total width of the absorption lines to the width of the source radiation band, the sensitivity of such a system is very small and can only be increased by increasing the length of the absorption path, for example, by using a multi-pass cell, which sharply reduces the mechanical stability of the system.

A device [2] is also known for spectral measurements by sequentially tuning a specially designed laser between components of the vibrational-rotational spectrum of methane and other molecules. Thus, in reality, at any given moment in time, measurements are taken along only one spectral line. The need for the laser itself, which is a complex precision optical-mechanical system, the combination of additional standards and pumping the test gas through a measuring cell deprives this system of the possibility of real use in field (mine) conditions, limiting the range of laboratory applications.

A device [3] for detecting (determining the presence of) gases is also known, in which a light source containing a source with a white or broadband spectrum and a scanned comb filter create a spectrum with regularly spaced wavelengths or lines that correspond to the distance between the absorption spectrum lines or measured gas and in which a multi-line output radiation from a light source propagates in the studied gas zone. The technical result of this device is the ability to correctly take into account the structural features of molecular spectra, namely, the almost exact equidistance of absorption lines in the vibrational-rotational spectrum, which is realized by using the Fabry-Perot interferometer as a narrow-band filter. The construction principle of the device is close to optimal, therefore we have taken this device as a prototype. A common feature of the known device and the claimed utility model is that they use a Fabry-Perot interferometer as a spectral filter that transmits all or most of the emission (absorption) lines of the spectrum of the molecule.

The disadvantage of the prototype device is the need for sequential scanning of radiation at the frequencies of the absorption lines, i.e. sequential enumeration of several tens of lines, which under real measurement conditions, with a limited time, sharply reduces the sensitivity of the entire detection and measurement system of the molecules due to the fact that most of the scanning time is spent on switching from one frequency to another, which is a consequence of the shape of the spectral transmission circuit interferometer. In addition, the use of a scanning system highly sensitive to vibration of the interferometer inevitably leads to additional mechanical instability of the system.

The technical result of the claimed utility model is the ability to repeatedly (at least 10-20 times) increase the sensitivity of the measuring system and its mechanical stability. The implementation of the technical result is achieved by the rational use of the light flux of the source by optical summation of the radiation intensity of all or most spectral lines and the rejection of sequential tuning from one absorption line to another — the measurement technology justified in [4], as well as the introduction of a system for the simultaneous modulation of all transmission lines over a frequency that optimizes the noise characteristics of a receiving and recording system.

The essence of the claimed utility model is illustrated in FIG. 1 and FIG. 2.

In FIG. 1 shows a general diagram of a device for detecting molecular impurities in the atmosphere from vibrational-rotational spectra.

In FIG. Figure 2 shows the spectrum of radiation transmitted through a tested Fabry-Perot interferometer and detected by a photodetector.

The claimed model consists of two main blocks A and B: a radiation source block A with a multi-line spectrum and a registration block B, combined with a control system, which are placed in a common housing, but can be separated in space. Block A contains an emitter 1 with a continuous spectrum, a collimator 2 (a mirror collimator was actually used), a Fabry-Perot interferometer consisting of two mirrors 3 separated by a gap 4 filled with a transparent substance, the optical thickness of which is chosen from the condition that the interval between the transmission frequencies of the interferometer is equal the distance between the absorption lines of the vibrational-rotational spectrum of the studied molecule, and the optical thickness can be changed using the control system 11, the propagation medium of the radiation 5 and the registration unit B, consisting of a collector lens 6 (in real experience - a mirror), aperture 7, a photodetector 8, a signal processing system 9, and a signal indicating (recording) device 10.

The operation of the claimed utility model is as follows: the radiation of a source 1 with a continuous spectrum by a collimator lens 2 turns into a parallel beam of light that illuminates the Fabry-Perot interferometer, consisting of two mirrors 3 parallel to each other and the gap between them 4. The optical thickness of this gap nb, where n is the refractive index of the substance filling the gap, and b is its geometric thickness, is chosen equal to half the inverse distance between the lines of the spectrum of the molecule, expressed in inverse imetrah, released from the interferometer light beam thus consists of a set of narrow lines that coincide with the absorption lines of the molecule, for which measurements are taken. After passing through the atmosphere 5 containing the molecules under study, the radiation enters the receiving device, where it is focused by the lens 6, passes through a small circular aperture 7, which is installed in the focal plane of the lens 6 and which emits a central interference maximum, and then is detected using the photodetector 8, its signal is amplified and processed in block 9 and sent to the indicator 10 including - to print the results. The operation of the entire device is controlled by a control device 11, the signal of which can be used to control a Fabry-Perot interferometer in order to modulate it in frequency by changing the optical thickness of the gap by methods known in spectroscopy, for example, by periodically changing the refractive index n of the gap material. As a result, the interferometer tuning frequency is periodically changed and thereby the intensity of radiation transmitted through the atmosphere is modulated, the spectrum of which exactly coincides with the absorption spectrum of the molecule, making it possible to use the well-known radio engineering methods for isolating an alternating signal with a known frequency. In the inventive device for detecting molecular impurities in the atmosphere by vibrational-rotational spectra, the radiation of all lines of the molecular band simultaneously forms on the axis superimposed radiation maxima due to the above choice of the optical thickness of the gap between the mirrors. There is a summation of the intensity of all useful lines and a multiple increase in the detection signal.

The claimed utility model was tested in laboratory conditions of St. Petersburg State University in real time. A germanium plate with a high refractive index in the infrared region of the spectrum was used as an interferometer, which made it possible to use Fresnel reflection of radiation from surfaces as two mirrors, and as a radiation source with a continuous spectrum, a globar, the dimensions of the emitting surface of which were limited by a small aperture.

As a result of the experiments, the achievement of the indicated technical result was confirmed, namely, providing in the infrared region of the spectrum a simultaneous selection of a set of lines located at equal frequency distances in the frequency region 1000 cm -1 . As the results of testing showed, the spectra were reproduced with high accuracy for half a day with significant changes in room temperature and without the use of any special means of reducing vibration interference.

An example showing the selection of a system of spectral lines is shown in FIG. 2. As can be seen from FIG. 2, the lines of transmitted radiation are equidistant. The contrast of the observed interferogram is less than that required in the final real device, but this is due to both the absence of mirror coatings, for example, from gold, and the non-parallelism of the rays illuminating germanium. Nevertheless, experience has shown the simultaneous presence on the axis of a large number of frequency-equidistant emission lines in the output beam, which will increase the sensitivity of the detection system of harmful and dangerous molecular additives in the atmosphere in proportion to the number of lines used. Similar devices can be used in any spectral region from microwave to ultraviolet with an appropriate choice of characteristics of optical elements

Technical and economic efficiency of the invention consists in increasing the selectivity and sensitivity of the device, which allows to establish the presence of only a given type of molecules in a complex mixture of molecules in the atmosphere. The increase in sensitivity occurs due to the summation of the intensities, in proportion to the number of simultaneously used lines, which provides an increase in the detection rate of harmful and hazardous impurities of molecular gases in the atmosphere. The increase in mechanical stability is determined by the fact that the device for detecting molecular impurities in the atmosphere contains a system of rigid optical components that require only a single adjustment. A device for detecting molecular impurities in the atmosphere does not have components requiring high supply voltages and, with appropriate structural refinement, can be used in conditions of increased danger, for example, in coal mines.

List of references

1. Patent RU 2451285 "Gas analyzer and the optical unit used in it."

2. Patent RU 2010141803 "Method and device for gas analysis using an interference laser."

3. Patent US 4941747 A "Optical sensing arrangements".

4. Permyakova E.S., Tolmachev Yu.A. The use of optimal signal reception methods and the pulse method of analyzing the operation of optical systems for the development of a new method of spectral analysis // Chemical Physics, 2015, v. 34, No. 8, p. 78-82.

Claims (1)

  1. A device for detecting molecular impurities in the atmosphere by vibrational-rotational spectra of gases, comprising a housing, a power supply unit, a light-emitting element with devices providing the ability to control the current flowing through it, and optical elements forming a parallel source radiation beam, a Fabry-Perot interferometer, optical thickness the gap between the mirrors of which is equal to half the reciprocal distance between the frequencies of the lines of the vibrational-rotational spectrum of the molecules and can be modulated during of time according to a periodic law with a known frequency, a condenser lens and means for measuring the intensity of transmitted radiation, including a photodetector, and a control device, the light-emitting element, interferometer, optical elements and a photodetector installed on one optical axis, characterized in that the focal plane of the condenser lens is installed aperture to highlight the central maximum of the interference pattern.
RU2016114756U 2016-04-15 2016-04-15 Device for detecting molecular impurities in the atmosphere by vibrational-rotational spectra RU171813U1 (en)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4941747A (en) * 1988-02-10 1990-07-17 Plessey Overseas Limited Optical sensing arrangements
US6064488A (en) * 1997-06-06 2000-05-16 Monitor Labs, Inc. Method and apparatus for in situ gas concentration measurement
RU2010141803A (en) * 2008-04-03 2012-05-10 Интопто АС (NO) Method and device for gas analysis using an interference laser
RU2451285C1 (en) * 2010-11-12 2012-05-20 Общество С Ограниченной Ответственностью "Оптосенс" Gas analyser and optical unit used therein

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4941747A (en) * 1988-02-10 1990-07-17 Plessey Overseas Limited Optical sensing arrangements
US6064488A (en) * 1997-06-06 2000-05-16 Monitor Labs, Inc. Method and apparatus for in situ gas concentration measurement
RU2010141803A (en) * 2008-04-03 2012-05-10 Интопто АС (NO) Method and device for gas analysis using an interference laser
RU2451285C1 (en) * 2010-11-12 2012-05-20 Общество С Ограниченной Ответственностью "Оптосенс" Gas analyser and optical unit used therein

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