Use of Fiber Optic Technologies to Verify the Operational Parameters of Meteorological Lidars
Meteorological lidars are currently one of themost universal and informative devices for noncontact atmospheric research. Depending on the type, they are able to provide information on the altitude of the lower cloud cover, the number and density of cloud layers, the atmospheric dynamic parameters (wind speed and direction, wind shear, vortex traces, etc.). Despite the growing demand and a wide application range of meteorological lidars, today there is no sufficiently universal and practical method for verifying their operational parameters. The article describes a promising method for verification the operational parameters of meteorological lidars based on the use of fiber-optic technologies.
A. A. Kim1, V. S. Luginya1, M. A. Konyaev1, A. E. Orlov2, D. N. Vasiliev2
Baltic State Technical University «VOENMEH» named after D. F. Ustinova, St. Petersburg, Russia
Laser systems JSC, St. Petersburg, Russia
Meteorological lidars are currently one of the most universal and informative devices for non-contact atmospheric research. Depending on the type, they are able to provide information on the altitude of the lower cloud cover, the number and density of cloud layers, the atmospheric dynamic parameters (wind speed and direction, wind shear, vortex traces, etc.). Despite the growing demand and a wide application range of meteorological lidars, today there is no sufficiently universal and practical method for verifying their operational parameters. The article describes a promising method for verification the operational parameters of meteorological lidars based on the use of fiber-optic technologies.
Keywords: meteorological lidar, characteristics verification, verification method, fiber optic delay line, imitation line
Meteo lidar (LIDAR – LIght Detection And Ranging) is an atmospheric backscattering profiler. An optical probe pulse is emitted into the atmosphere, after which an optical signal is detected backscattered from each point of the atmospheric path at the lidar photodetector. The electrical signal from the photodetector undergoes analog-to-digital conversion with subsequent high-level processing. Coherent Doppler lidars are also capable of measuring the projection of the air mass velocity on the sounding axis by detecting the Doppler shift of the radiation frequency during heterodyne reception [1–3].
Today, lidars are widely used in meteorology, climatology, ecology, wind energy and flight safety: they are recommended by the International Civil Aviation Organization (ICAO) for inclusion in the measurement and information systems of airport equipment [4–6]. One of the important nuances of the metrological support of meteorological lidars is the difficulty of their operational characteristics verification associated with the inability to create a reference atmospheric path and the lack of comparable reference meters.
The company Laser Systems JSC (St. Petersburg, Russia), which is a leading domestic developer of meteorological lidars, is also actively working on creating modern facilities of their metrological support. Significant results in the field of metrological support were achieved thanks to the use of fiber-optic technologies to simulate some metrologically significant atmospheric parameters.
The main operational characteristics of meteo lidars, depending on their type, can be conditionally classified into primary parameters and parameters according to their intended purpose (see table).
From the classification it can be seen that for meteo lidars of all basic types, the primary parameters are exclusively spatio-temporal, while the parameters for the intended purpose are more complex.
Verification of the primary parameters of the lidar
To verify the primary parameters of the lidar, it was proposed to create an optical path that allows imitating objects remote from the lidar at known distances. The first «object» is located on the border of the blind zone of the lidar, the second is removed from it by the amount of spatial resolution, all subsequent imitated objects are removed from the lidar by known distances (Fig. 1).
It was possible to realize such an optical path using a fiber optic delay line of a ring structure containing linear elements and splitters with selected splitting ratio. An illustration of the principle of operation is shown in Fig. 2 .
The optical pulse of the probe lidar radiation is injected into the fiber optic line and supplied to one or more coils connected by a ring. In this case, part of the optical power of the pulse is diverted from the line at each passage of the optical length of the coil through the splitter. At the output, a sequence of fading pulses is formed, delayed in time relative to each other. Since all the lengths of the optical fibers are constant and known with high accuracy, it can be argued that all the time intervals between the light pulses at the exit from the line are also known and equivalent to the corresponding distances to the imitated conditional objects.
The distance to the conditional n-th mark is uniquely determined by a simple relation:
where is the total equivalent length of the optical fibers, is its effective refractive index. Strictly speaking, and forms the so-called nonlinear dispersion dependence, however, in this case, when working at the same radiation wavelength, this influence can be neglected.
The use of fiber-optic technologies makes it possible to place all optical elements in a rather compact package and to verificate the metrological characteristics of lidars in the field without removing the lidar from the operating site (Fig. 3).
A typical lidar signal when connecting such a imitation line contains a set of specific peaks equivalent to reflection from imaginary objects that are remote from each other and from the lidar at precisely known distances in the entire measurement range (Fig. 4 a) and in the near zone (Fig. 4b). The analysis of the graphs allows us to talk about the correspondence of the primary (spatio-temporal) parameters of the lidar with the declared ones in particular: range and accuracy of distance measurement, spatial resolution and the size of the blind zone.
The verification of the spatio-temporal parameters of a pulsed Doppler wind lidar operating in the backscattering profiler mode goes in a similar way. The probe radiation through the breaking mirror is inserted into the receiving collimator and sent to the imitation line (Fig. 5).
In Fig. 6, the plots of the spatial profile of the backscattering coefficient β and SNR signal are presented when an imitation line is connected to the lidar. Equidistant peaks are also clearly visible on the graph. The estimated distance between them is 1 000 m and is determined by the equivalent length of the optical fiber imitation line.
The use of quartz optical fiber as a medium for the propagation of probe lidar radiation is justified in almost all cases when the radiation wavelength exceeds 850 nm, i. e. for most meteorological lidars. This is due to the prevalence of the optical fiber component base and the fact that the attenuation coefficient in this wavelength range allows you to create imitation lines of a sufficiently large total length; the use of polymer fibers can be considered impractical due to the excessively high attenuation coefficient .
The developed stand for metrological support of meteorological lidars was sertified as a measurind standard for a unit of length in the range of values from 10 to 12 000 m (the number in the Register of approved standards for units of Rosstandart 3.6.BNL.0001.2017).
Imitation line approaching the real atmosphere
The propagation of probe radiation and the signal energy at the lidar photodetector is described by the so-called lidar equation in the single scattering approximation (2) .
where P0 – is the peak power of the laser pulse, r – is the distance with which the signal is received, ηall – is the overall efficiency of the lidar system, c – is the speed of light, τ – is the duration of the laser pulse, ηg(r) – is the geometric factor (depends on geometry of the lidar optical system, the maximum value is 1), D – is the area of the receiving antenna, β(r) – is the aerosol backscattering coefficient, α(r) – is the aerosol attenuation coefficient, and Рbg – is the power of the background signal. On the other hand, an expression that generally describes the energy characteristics of signals generated at the output of an imitation fiber optic line has the following form:
where B – is the constant characterizing the radiation input-output efficiency, is the light attenuation coefficient in the fiber, – is the length of the n-th fiber section, is the division coefficient of the n-th optical splitter, is the additional attenuation coefficient of the n-th output optical momentum.
× . (4)
The left side of equation (4) is a modified lidar equation with a generalized hardware coefficient A written for discrete distances rn and an isotropic atmosphere with constant coefficients β and α; the right part describes the energy of signals generated at the output of the imitation fiber optic line.
A detailed consideration of equation (4) shows that for given and constant β and α, it can have solutions over the entire range of distances rn only if: a) the imitation line has a sequential structure (Fig. 7) with the selection of independent attenuation and division ratio of splitters in each of the output arms; b) when introducing mechanisms of active regulation of optical power in loop-type structures (see Fig. 2).
The Fig. 7 shows: a receiving collimator (1), a transceiving unit (2), a calibrated-length duplex optical cord (3), fiber optic couplers (4), a blind zone delay line (5), a spatial resolution delay line (6), high-rise delay lines samples (n pieces) (7), tunable attenuators (8), fiber optic splitter (9), transmitting collimator (10).
The sequential structure of the imitation line when implementing long optical paths with a large number of splitters is excessively cumbersome and economically impractical, and passive methods of regulating its parameters significantly limit the scope. A much more promising way is the introduction of mechanisms for active regulation of optical power and feedback in loop-type structures. In their absence, it is possible to achieve satisfactory indicators only in the near or far zone (Fig. 8).
In fig. 8, the lidar equation simulates 3 isotropic atmospheres with backscattering coefficients β = 10–3; 4 ∙ 10–3; 10–4 and attenuation coefficient α = 4 ∙ 10–5. The values of these coefficients can be considered characteristic of the real atmosphere for a radiation wavelength of 1.55 μm. It can be seen from the graphs that passive methods of regulation make it possible to achieve a satisfactory correspondence between the profiles in the near or in the far zone. With active regulation, the dynamic range of attenuation tuning in this case will be about 30 dB.
Prospects and direction of development
To approximate the imitation line to the parameters of the natural atmosphere, it is advisable to provide controlled temporal broadening and amplitude profiling of the probe pulse in the fiber optic line to simulate reflection from distributed atmospheric formations (clouds, fogs), as well as a controlled relatively small frequency shift of the probe radiation to verify the parameters of the Doppler wind lidars. Both of these aspects can be implemented using fiber optic technology.
So, for example, the temporal broadening of the probe pulse is easily provided by a parallel array of matched delay lines, the discreteness of which is equal to its duration. Amplitude profiling is achieved by including an external fiber optic amplitude modulator into the line. A controlled shift of the radiation frequency can be achieved by using acousto-optical modulators or fiber electro-optical phase modulators operating in the linear phase modulation mode [10, 11].
The use of fiber-optic technologies and element base today is one of the most promising directions for solving the problem of meteorological lidars parameters verification. In the absence of comparable reference meters and the impossibility of creating a reference atmospheric path, fiber-optic imitation lines are almost the only practical and universal facilities of verifying the characteristics of meteorological lidars.
The functionality of the fiber optic line is not limited to the creation of known time delays. They can also provide temporary broadening and profiling of probe pulses, frequency shift of radiation, etc. Actually, this leads to the fact that at the moment it becomes possible to create an imitation of the atmospheric path with standard properties and parameters that can be changed widely.
A verification kit developed by Laser Systems JSC for monitoring the parameters of meteorological lidars is sertified as a metrology standard and is used for initial and routine verification procedures.
Andreev M., Vasil’ev D., Penkin M., Smolencev S., Borejsho A., Klochkov D., Konyaev M., Orlov A., CHugreev A. Kogerentnye dopplerovskie lidary dlya monitoringa vetrovoj obstanovki. Photonics Russia. 2014; 6(48): 20–29.
Boreysho A. S., Kim A. A., Konyaev M. A., Luginya V. S., Morozov A. V., Orlov A. E. Modern Lidar Systems for Atmosphere Remote Sensing. Photonics Russia. 2019; 13(7): 648–657. DOI: 10.22184 / 1993–7296.FROS.2019.13.7.648.657.
Protopopov V. V., Ustinov N. D. Laser heterodyning / ed. by Ustinov N. D. – M.: Nauka, 1985.
Regional implementation of Electronic Terrain and Obstacle data (e-TOD). – International Civil Aviation Organization SAM / IG / 13; South American Regional Office: procs. of the Thirteenth Workshop / Meeting of the SAM Implementation Group (SAM / IG / 13) – Regional Project RLA / 06 / 901, Lima, Peru, April 21–25, 2014.
Air traffic planning and maintenance guidance: ICAO document: DOC9426-AN / 924. – International Civil Aviation Organization. 1984. Part 2, Ch. 3, appendix A. 636 c.
Low Altitude Wind Shear Remote Detection Aerodrome Systems: ICAO Document: Doc A39-WP / 287. – International Civil Aviation Organization. 2016, March 25.
Patent RU № 2636797. Method for monitoring and verification of Meteorological Lidar Ceilometers-Type Equipment and the Device for its Implementation. IPC G01S7 / 497, G01C25 / 00 priority of 01 / 19 / 2017 / Kim A. A., Klochkov D. V.
Boreysho A. S., Kim A. A., Strakhov S. Yu. Limitations in the application of fiber-optic technologies for remote transmission of energy. Radio industry Russian. 2017; 4: 34–41. DOI:10.21778 / 2413–9599–2017–4–34–41.
Mezheris R. Laser remote sensing / Translation from English I. G. Gorodetsky; under the editorship of A. B. Karaseva. – M.: Mir. 1987. 550 p.
De Paula R. P., Moore E. L. Review of all-fiber phase and polarization modulators. Fiber Optic and Laser Sensors II: proc. SPIE of 1984. Technical Symposium East. USA. 1984; 478: 3–11. DOI:10.1117 / 12.942649.
Jenoptik. Integrated-optical modulators. Technical information and instructions for use Updated: 12.08.2019 [Electronic resource]: Available at: https://www.jenoptik.us//media/websitedocuments/optics/modulators/modul atorfibel_en.pdf.
Kim A., Ph.D, BSTU «VOENMEH» named after D. F. Ustinov, St. Petersburg.
V. Luginya, BSTU «VOENMEH» named after D. F. Ustinov, St. Petersburg.
M. Konyaev, Ph.D, BSTU «VOENMEH» named after D. F. Ustinov, St. Petersburg.
A. Orlov, Ph.D, Laser Systems JSC, St. Petersburg.
D. Vasiliev, Ph. D. Laser Systems JSC, St. Petersburg.
Contribution by the members
of the team of authors
The article was prepared on the basis of many years of work by all members of the team of authors. Meteorological lidars – operating principles, development: Konyaev M. A., Orlov A. E.; Imitation fiber optic line – concept, development, mathematical model: Kim A. A.; Experiments: Kim A. A., Orlov A. E.; Metrological support, certification: Luginya V. S., Vasiliev D. N.
Development and research are carried out at the expense of Laser Systems JSC.
Conflict of interest
The authors claim that they have no conflict of interest.