RU2753612C1 - Multichannel fiber-optic near-infrared heterodyne spectroradiometer - Google Patents

Abstract

FIELD: high-resolution infrared spectroradiometry.

SUBSTANCE: invention relates to the field of high-resolution infrared spectroradiometry and concerns a multichannel fiber-optic heterodyne near-infrared spectroradiometer. The spectroradiometer includes a system containing at least two optical receiving devices, a chopper for the received optical signal, a heterodyne with high-precision frequency stabilization of optical radiation and a single-mode fiber output, quartz single-mode fiber splitters, a reference channel consisting of an optical cell and an electronic unit for recording an optical signal. The radiometer also contains a system of analytical channels consisting of a photodiode and a preamplifier with a narrow bandwidth, a signal processing unit at an intermediate frequency, including a system of analog-to-digital converters and an FPGA-based computing unit, and a control electronics unit including a tunable local oscillator current source.

EFFECT: invention shortens the signal accumulation time, increases the signal-to-noise ratio, increases the sensitivity and accuracy of the measurements.

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Description

The invention relates to high-resolution infrared spectroradiometry, which allows recording isolated absorption lines of molecular gases with ultra-high spectral resolution in the near infrared range. The invention can be used for remote measurement of the concentration of greenhouse gases (GHG) and pollutants (Pollutants) contained in the Earth's atmosphere. Measurements can be carried out both from ground-based measuring stations intended for local measurements of the long-term dynamics of changes in the content of GHGs and pollutants, and from mobile platforms, for example, unmanned or guided aerial vehicles and orbital probes, to cover vast territories and build maps of the global distribution of the GHG content. and pollutants in the atmosphere. Ultra-high spectral resolution allows one to obtain the spatial distribution of GHGs and pollutants in the atmospheric column at the measurement point, as well as to determine the projection of the air flow velocity on the direction of the line of sight. Thus, the presented invention is a reliable, compact and high-precision measuring device for studying the composition and structure of the atmosphere and remote monitoring of the content of GHGs (CH ₄ , CO ₂ , H ₂ O and O ₂ ) and pollutants (CO and NO ₂ ).

The essence of the method of heterodyne registration of the signal under study is mixing the investigated radiation with the radiation of a local oscillator (heterodyne) on a quadratic receiver and analyzing the beats that occur at the intermediate frequency. Heterodyne spectroscopy enables highly accurate measurements of weak signals with high spectral resolution. Devices operating on the basis of the heterodyne signal registration method are widely used in the radio frequency range.

A number of fiber heterodyne spectroradiometers are known from the prior art, for example, US Pat. the main greenhouse gases (CH ₄ , CO ₂ and H ₂ O) in the atmosphere by the absorption spectrum in the near infrared range. The listed analogs have a number of disadvantages:

1. Significantly lower spectral resolution, which, in turn, reduces the information content of measurements.

2. The given analogs do not have an additional system for stabilizing the frequency of the local oscillator, which leads to a drift of the local oscillator frequency during the measurement and a decrease in the accuracy of the results obtained.

3. In electrical circuits for signal processing at an intermediate frequency in the described analogs, amplitude detectors based on semiconductor diodes are used, which introduce nonlinearity into the detected signal and reduce the accuracy of processing the measured data.

4. These analogs operate in a single-channel mode, therefore, the power of the detected signal is limited by the conditions of the antenna theorem (3), which leads to an increase in the signal accumulation time required to obtain a satisfactory signal-to-noise ratio.

The listed disadvantages do not allow achieving high spectral resolution, the required accuracy, speed and reliability of the algorithms for processing the measured results, which are provided by the claimed invention.

Closest to the claimed invention is a near infrared heterodyne fiber optic spectroradiometer described in patent No. RU 121927 (U1) (4), which is used to measure the concentration of methane in the atmosphere in the vicinity of a wavelength of 1.651 μm.

Single mode silica fiber is used as an optical system. The combination of wavefronts of the radiation of the recorded signal and the local oscillator is realized through the use of a fiber single-mode quartz splitter, which is used as a diplexer. Also, a fiber single-mode quartz splitter is used to separate the heterodyne radiation between the measurement channels. The registered radiation is fed into the optical system using a short-focus lens. A tunable distributed feedback diode laser operating at a wavelength of 1.651 μm is used as a local oscillator. The device has two measuring channels: analytical and reference. The reference channel is a glass single-pass cell filled with methane with an electronic unit for signal registration and amplification. The analytical channel is a semiconductor photodetector with an electronic unit for amplifying and filtering a signal in a narrow band at intermediate frequencies. According to the results given in (5), the considered utility model allows in the direct observation of the Sun to measure the methane content in the atmosphere with an accuracy of no worse than 1%. The signal accumulation time is 20 minutes, at which the signal-to-noise ratio is about 100.

This utility model has a number of disadvantages:

1. A single-pass glass optical cell is used as a reference channel. This technical solution is suitable for measuring those gases, the content of which in the atmosphere is low. For example, to measure the concentration of carbon dioxide or water vapor in the atmosphere, the use of a single-pass cell in the reference channel is unacceptable.

2. It is known from work (5) that an amplitude detector based on a semiconductor diode is used to register a signal at an intermediate frequency, which introduces significant nonlinearity into the measured signal, which in turn leads to a complication of the processing procedure and a decrease in the accuracy of the measured data.

3. The power of the recorded signal is limited by the condition of the antenna theorem (3), which leads to the need for a long (up to 20 minutes) signal accumulation to achieve an acceptable signal-to-noise ratio equal to 100.

4. When making measurements in the heterodyne mode, it is necessary to measure the noise power level of the local oscillator, ie. a dark signal corresponding to the mode when only the heterodyne radiation falls on the photodetector. In the described model, the dark signal is measured by turning on / off the recorded signal incident on the square-law detector using a chopper in a sequential mode, which led to a twofold increase in the accumulation time of the recorded signal.

The task to be solved by the claimed invention is to create a multi-channel fiber-optic laser heterodyne spectroradiometer of high resolution near infrared range for the simultaneous measurement of several atmospheric PGs and pollutants, which would significantly reduce the signal accumulation time, increase the signal-to-noise ratio and increase the sensitivity, and the accuracy of the measurements.

The problem is solved due to the fact that a multichannel fiber-optic heterodyne spectroradiometer for the near infrared range is proposed, characterized in that it includes a system containing at least two optical receiving devices, a chopper for the received optical signal, a heterodyne with high-precision frequency stabilization of optical radiation and single-mode fiber output, quartz single-mode fiber splitters of fused and planar types, used both for separation and for combining optical signals, a reference channel consisting of an optical cell and an electronic unit for recording an optical signal, a system of analytical channels consisting of a photodiode and a narrow-band preamplifier transmission, a signal processing unit at an intermediate frequency, including an analog-to-digital converter system and an FPGA-based computing unit, and a control electronics unit including a precision tunable local oscillator current source.

Additional options for the implementation of a multichannel fiber-optic heterodyne spectroradiometer are possible, in which it is advisable that:

• the cuvette of the reference channel was a multipass optical cuvette operating according to the principle of registration of integral intracavity output radiation;

• the chopper of the received optical signal was a fiber switch based on microelectromechanical systems;

• the optical receiver was a single-mode fiber collimator with a gradient lens.

• the number of optical receiving devices and analytical channels ranges from 2 to 8 units.

The technical result provided by the above set of features is a significant increase in the sensitivity of determining the concentrations of PG or pollutants in a shorter period of time, due to the use of a multichannel system for recording the signal under study and synchronous measurement of dark and mixed signals.

The claimed invention has a number of advantages over its closest analogue (4):

1) The possibility of detecting weak spectral lines, atmospheric gases, such as CO ₂ and H ₂ O, has appeared due to the use of a multi-pass optical cell in the reference channel, based on the principle of registration of integral intracavity output radiation, which makes it possible to achieve an effective optical path of 30 m. of such an optical system is described in (6).

2) As a signal analyzer at an intermediate frequency, an electronic signal analysis unit developed by the author's team is used based on a programmable logic integrated circuit (FPGA), which does not introduce additional distortions into the signal during its processing.

3) To overcome the condition of the antenna theorem limiting the field of view of the heterodyne spectroradiometer, the concept of multichannel is applied, which consists in the simultaneous use of several independent, identical channels that register the signal. This technical solution allows for a fixed accumulation time to increase the signal-to-noise ratio in proportion to the square root of the number of channels.

4) The signal chopping is implemented on the basis of a fiber switch in such a way that the dark and mixed signals are measured during one scan cycle of the local oscillator frequency. Thus, in parallel mode, two signals are continuously recorded - dark and mixed. This modification makes it possible to halve the signal accumulation time.

The essence of the invention is illustrated by drawings, which show:

FIG. 1 is a schematic block diagram of a multichannel fiber-optic heterodyne spectroradiometer for the near infrared range;

FIG. 2 - shows the dependence of the atmospheric transmission on the wavenumber for the absorption line of methane;

FIG. 3 shows the dependence of the atmospheric transmission on the wavenumber for the absorption line of carbon dioxide.

The multichannel fiber-optic heterodyne spectroradiometer of the near infrared range consists of the following components: heterodyne 1, the role of which is played by a tunable semiconductor laser with distributed feedback and fiber output of radiation; single-mode silica fiber 2; fiber splitter welded type 3; fiber splitter planar type 4; diplexers 5, which are welded-type fiber splitters; a system of optical receiving devices 6, the role of which is played by short-focus aspherical lenses; breaker front of the received radiation 7; a system of analytical channels 8, which include a photodetector 9, a transimpedance preamplifier and a cascade of bandpass filters 10 with a narrow passband; an analyzer of a signal at an intermediate frequency 11, which includes a signal digitizing system 12 and a computing unit 13 based on an FPGA; reference channel 14, consisting of an optical cell 15 filled with a test gas, and a signal registration unit 16; electronic control unit 17.

The described device operates as follows: the local oscillator 1 is modulated by sawtooth pulses of the pump current using the control electronics 17, which provides the radiation frequency tuning to cover the spectral region near the measured absorption line. The radiation of the local oscillator 1 propagates along a single-mode quartz fiber 2 and is divided by a welded type fiber splitter 3 into two parts, one of which goes to the reference channel 14, and the other part falls on a planar type 4 fiber splitter. The planar type 4 fiber splitter equally divides the incoming radiation into two parts and sends it to the inputs of the diplexers 5. The radiation from the system of optical receiving devices 6 is mixed with the radiation of the heterodyne 1 in the system of diplexers 5 and sent to the system of analytical measuring channels 8, where one of the channels receives the mixed radiation, pre-modulated by the chopper 7. In each analytical channel, the mixed radiation is recorded by a photodetector 9, the signal from which is amplified and filtered by a transimpedance amplifier and a cascade of bandpass filters 10. The signals of the analytical channels are fed to the inputs of the signal analyzer at an intermediate frequency 11, where they are converted by analog-digital converter 12, and then fed to the inputs of the electronic computing unit based on the FPGA 13, where the variance of the noise component of the recorded signal is calculated, which is proportional to the spectral density of the analyzed radiation in the scanned frequency range of the local oscillator 1. The results of calculations of the electronic computing unit based on the FPGA 13 are transmitted to personal computer for averaging, saving and further processing. The frequency stabilization of the local oscillator 1 is provided by the reference channel 14 with high accuracy, where the radiation of the local oscillator 1 through the fiber splitter 3 enters the optical cell 15. Using the signal registration unit 16, an absorption line is observed in the optical cell 15. Based on the analysis of the line position, the frequency of the local oscillator is stabilized with accuracy in the range of 2-5 MHz, depending on the type of optical cuvette 15 used in the device. The registration of the intermediate frequency signal is carried out in a narrow preamplifier band (in a fixed range from 1 to 25 MHz) when scanning the local oscillator frequency 1 in a fixed range of 10-60 GHz.

A fiber optical switch of any type can be used as the breaker of the front of the detected radiation 7, which provides the required speed and the degree of suppression of the recorded signal into the fiber optical system in the overlapped state. As a photodetector 9 can be used any device operating in a given range of the spectrum, having a sensitivity sufficient for recording shot noise of the photocurrent in the intermediate frequency band, and an internal capacitance that provides a given band. Any single-mode tunable laser with a fiber output can be used as a local oscillator, the tuning range and accuracy of which are sufficient to provide a sweep of the spectrum of the analyzed radiation. As the cuvette 15 of the reference channel 14, an optical cuvette of any type can be used, which provides the necessary optical path for observing the spectral line of interest. The shot noise of the photocurrent should be the dominant source of system noise, i.e. should prevail over the intrinsic noise of the analytical channels 8 and over the amplitude noise of the local oscillator 1 in order to ensure the maximum sensitivity of the measuring equipment.

As an example of a specific implementation of the described invention, a multichannel fiber-optic heterodyne spectroradiometer of the near infrared range can be presented, including a double-duplicated heterodyne spectroradiometer circuit described above, for simultaneous measurement of the concentration of two greenhouse gases: methane and carbon dioxide. Each part of the spectroradiometer, which is responsible for measuring a separate gas, contains: 1 - diode laser with distributed feedback and a wavelength of 1.605 µm for measuring carbon dioxide and 1.655 µm for measuring methane; 2 - single-mode silica fiber 10/125 microns; 3 - fiber splitter of welded type based on single-mode quartz fiber 10/125 µm with division ratio 10/90; 4 - planar type fiber splitter based on 10/125 µm single-mode quartz fiber; 5 - a pair of diplexers, the function of which is performed by fiber couplers of a welded type based on a single-mode quartz fiber 10/125 µm with a division ratio of 10/90; 6 - a pair of aspherical lenses with a diameter of 25 mm and a focal length of 50 mm; 7 - interrupter of the front of the received radiation based on a microelectromechanical system, integrated into a compact case with dimensions of 20 × 50 mm, with fiber leads based on a single-mode quartz fiber 10/125 µm; 8 - a pair of analytical channels, including 9 - InGaAs photodiode with a sensitive area of 4 mm ² and a housing for mounting fiber tips of the FC / PC type, and 10 - a transimpedance preamplifier with a 4 kOhm feedback resistance and a cascade of bandpass filters with a passband of 200 kHz - 3 MHz; 11 - signal analyzer at an intermediate frequency, including 12 - a signal digitizing system consisting of 4 analog-to-digital converters (ADC) with a width of 8 bits (two ADCs for measuring each gas) and 13 - an electronic computing unit based on FPGAs of the Cyclone III series that performs signal processing simultaneously from two analytical channels 8, measuring carbon dioxide, and two analytical channels 8, measuring methane; 14 - reference channel, including 15 - single-pass optical glass cuvette 40 mm long, filled with methane at a pressure of 1000 Pa in the methane measurement channel, and 15 - multi-pass optical cell operating on the principle of recording integral intracavity output radiation, 0.3 m long and effective optical path 30 m, filled with carbon dioxide at a pressure of 1000 Pa, for measuring carbon dioxide, as well as 16 - a unit for recording a signal passed through an optical cell, consisting of an InGaAs photodiode with a sensitive area of 4 mm ² and a transimpedance amplifier with a feedback resistance of 1 kΩ ; 17 - control electronics unit based on NI-USB-6215.

With the help of the developed device, the transmittance spectra of the atmosphere in the near infrared range with the absorption line of methane (Fig. 2) and carbon dioxide (Fig. 3) were obtained.

The proposed spectroradiometer can be used as part of various field, airborne and industrial systems designed for monitoring the state of the environment and studying the climate; The invention provides information on the content of the gas under study in the atmosphere with high accuracy and high spectral resolution. The measured data are highly informative, since high spectral resolution makes it possible to additionally extract information on the structure and dynamics of the atmosphere, namely, on the vertical distribution of the concentration of the gas under study and on the vertical projection profile of the air flow velocity in the troposphere and lower stratosphere.

The concept of multichannel is most important when using the heterodyne technique for sounding the atmosphere from orbiters in the solar eclipse mode, where one observation session can last no more than 60 seconds and the time for signal accumulation is limited.

Sources of

1. US patent No. US 8699029 (B2).

2. China Patent No. CN 106382987 (A).

3. A.E. Siegman, "The antenna properties of optical heterodyne receivers," Appl. Opt. 54 (10), 1588-1594 (1966).

4. RF patent No. RU 121927 (U1).

5. A. Rodin, A. Klimchuk, A. Nadezhdinskiy, D. Churbanov, and M. Spiridonov, "High resolution heterodyne spectroscopy of the atmospheric methane NIR absorption," Opt. Express 22 (11), 13825-13834 (2014).

6. E.J. Moyer, D.S. Sayres, G.S. Engel, J.M.St. Clair, F.N. Keutsch, N.T. Allen, J.H. Kroll, and J.G. Anderson, "Design considerations in high-sensitivity off-axis integrated cavity output spectroscopy," Appl. Phys. B 92, 467-474 (2008).

Claims (5)

1. Multichannel fiber-optic heterodyne spectroradiometer of the near infrared range, characterized in that it includes a system containing at least two optical receiving devices, a chopper for the received optical signal, a heterodyne with high-precision frequency stabilization of optical radiation and a single-mode fiber output, quartz single-mode fiber splitters of fused and planar types, used both for separating and for combining optical signals, a reference channel consisting of an optical cell and an electronic unit for recording an optical signal, a system of analytical channels consisting of a photodiode and a preamplifier with a narrow bandwidth, a signal processing unit for intermediate frequency, including a system of analog-to-digital converters and a computing unit based on FPGA, and a control electronics unit, including a precision tunable local oscillator current source. 2. The spectroradiometer according to claim 1, characterized in that the cuvette of the reference channel is a multi-pass optical cuvette operating on the principle of registration of integral intracavity output radiation. 3. The spectroradiometer according to claim 1, characterized in that the chopper of the received optical signal is a fiber switch based on microelectromechanical systems. 4. The spectroradiometer according to claim 1, characterized in that the optical receiving device is a single-mode fiber collimator with a gradient lens. 5. The spectroradiometer according to claim 1, characterized in that the number of optical receiving devices and analytical channels is from 2 to 8 units.

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