Fluorescence Lidar Research Articles

Remote Sensing of Plant Fluorescence Spectrum and Lifetime by Laser-induced Fluorescence Lidars

Remote Sensing of Plant Fluorescence Spectrum and Lifetime by Laser-induced Fluorescence Lidars

Metastable helium Faraday filter for helium lidar to measure the density of the thermosphere.

We demonstrate a metastable helium Faraday optical filter operating on the 23S1 - 23P1 and 23S1 - 23P2 transition at 1083 nm by using a 3 cm long helium cell. The influence of the magnetic field and gas pressure of the helium cell on the filter characteristics is experimental studied. When the magnetic field is 230 Gs and the gas pressure of helium cell is about 110 Pa, the peak transmission corresponding to the two energy level transitions is about 32% and 57%, respectively. The equivalent noise bandwidth (ENBW) under this working condition is about 1.9 GHz. The metastable helium Faraday filter can be used to improve the optical inefficiency of a helium resonance fluorescence lidar to achieve the metastable helium density detection at 200-1000 km thermosphere.

Pseudo-random modulation continuous-wave lidar for the measurements of mesopause region sodium density.

The sodium fluorescence lidar utilizes a 589 nm narrowband pulse laser system to measure mesopause region atomic sodium density, atmospheric temperature, and wind. However, this system is complicated and unstable. The continuous-wave (CW) sodium laser system can achieve ultra-narrow bandwidth, all-solid-state, and small compact size, as such it is extremely valuable for mobile, aircraft, and space-borne applications. In this study, we developed the first pseudo-random modulated CW (PMCW) sodium lidar by using an electro-optic modulated narrowband 589 nm CW laser with an output power of ∼1.2W. A pseudorandom M-sequence-code with a length of 127 is used to achieve altitude information by modulating laser and then decoding photon signals. Also, a biaxial structure with 9 m separation between the optical axes of the transmitter and receiver is designed to suppress the strong near-ground signals, which are crucial for improving the signal-to-noise ratio (SNR) of the PMCW lidar system. Nighttime measurements on December 2-4, 2019 show that the SNR at sodium layer peak is more than 10, corresponding to a statistical uncertainty of less than 10% in sodium density with temporal and spatial resolutions of 5 min and 1.05 km respectively. The comparison of vertical profiles of sodium density simultaneously observed by PMCW lidar and collocated pulse lidar shows good agreement.

Combined use of Mie–Raman and fluorescence lidar observations for improving aerosol characterization: feasibility experiment

Abstract. To study the feasibility of a fluorescence lidar for aerosol characterization, the fluorescence channel is added to the LILAS multiwavelength Mie–Raman lidar of Lille University, France. A part of the fluorescence spectrum induced by 355 nm laser radiation is selected by the interference filter of 44 nm bandwidth centered at 466 nm. Such an approach has proved to have high sensitivity, allowing fluorescence signals from weak aerosol layers to be detected and the fluorescence backscattering coefficient from the ratio of fluorescence and nitrogen Raman backscatters to be calculated. Observations were performed during the November 2019–February 2020 period. The fluorescence capacity (ratio of fluorescence to elastic backscattering coefficients), measured under conditions of low relative humidity, varied in a wide range, being the highest for the smoke and the lowest for the dust particles. The results presented also demonstrate that the fluorescence measurements can be used for monitoring the aerosol inside the cloud layers.

FULL-SCALE STUDIES OF FLUORESCENCE FEATURES OF SURFACTANT FILMS ON EUTROPHIC WATER WAVY SURFACE

The current paper presents the results of a full-scale study of the fluorescence features of pollutants ( diesel fuel, Urals oil), as well as an alcoholic solution of oleic acid, which imitates oil by visco-elastic characteristics. The experiment was carried out on the Gorky Reservoir under conditions of wind waves. Local areas of artificial slicks were created for multiple intersections at different angles to the wind and waves with continuous lidar sounding of the upper water layer. Signals were recorded by the ninth generation UVL-9 laser ultraviolet fluorescent lidar developed by the Institute of Oceanology of the Russian Academy of Sciences. At signal analysis, spectral intensities and fluorescence features of the studied substances are determined, possibilities of slick detecting as well as film thickness estimation are made, examples of surfactant film detecting behind the oleic slick where visual observation of pollutants does not give a result are showed. The obtained results are useful for the further development of fluorescent methods and devices for operational remote sensing of surface pollution on wave water surface.

蛍光ライダーの原理・トレンド・展開

蛍光ライダーの原理・トレンド・展開

Continuous Detection of Diurnal Sodium Fluorescent Lidar over Beijing in China

Based on application of the atomic filter technology in a signal detection system of lidar, the diurnal observation of sodium lidar were obtained using the system at the National Space Science Center of the Chinese Academy of Sciences at Beijing Yanqing station (40.5° N, 116° E) in April 2014. During the lidar observation period, among the 103 cases of continuous daytime observations, the longest time was 181 h. In the case of a continuous observation period of 5 days (13–18 October 2014), the signal-to-noise ratio reached to 19:1 at 12:00–13:00 Local Time of the daytime, when the spatial and time resolutions were respectively set to 96 m of 167 s. The improvements resulted in the highest detection level of any existing diurnal lidars in China. Some interesting phenomena such as the sporadic sodium layer have also been observed during the daytime. The daytime capability extended the observing time range of the earlier systems that were limited to only nighttime observations. This innovation provides a useful method for the studies of diurnal tides, photochemistry, gravity waves, and correlative modeling studies.

Dual-channel mobile fluorescence lidar system for detection of tryptophan.

We present a dual-channel mobile lidar system based on laser-induced fluorescence (LIF) for real-time standoff detection and concentration distribution analysis of tryptophan. The system employs an ultraviolet laser excitation source and signal detectors for receiving fluorescence signals within two different wavelength bands. The performed experiments measured tryptophan aerosols at two different standoff distances. Moreover, distilled water and ethanol solutions were also detected for comparison. The results show that the system can detect LIF signals of tryptophan, give early warnings, locate the diffusion sources, and monitor the variation of the aerosol concentration distribution in real time.

Novel Combined Spectral Indices Derived from Hyperspectral and Laser-Induced Fluorescence LiDAR Spectra for Leaf Nitrogen Contents Estimation of Rice

Spectra of reflectance (Sr) and fluorescence (Sf) are significant for crop monitoring and ecological environment research, and can be used to indicate the leaf nitrogen content (LNC) of crops indirectly. The aim of this work is to use the Sr-Sf features obtained with hyperspectral and laser-induced fluorescence LiDAR (HSL, LIFL) systems to construct novel combined spectral indices (NCIH-F) for multi-year rice LNC estimation. The NCIH-F is in a form of FWs* Φ + GSIs* Φ , where Φ is the Sr-Sf features, and FWs and GSIs are the feature weights and global sensitive indices for each characteristic band. In this study, the characteristic bands were chosen in different ways. Firstly, the Sr-Sf characteristics which can be the intensity or derivative variables of spectra in 685 and 740 nm, have been assigned as the Φ value in NCIH-F formula. Simultaneously, the photochemical reflectance index (PRI) formed with 531 and 570 nm was modified based on a variant spectral index, called PRIfraction, with the Sf intensity in 740 nm, and then compared its potential with NCIH-F on LNC estimation. During the above analysis, both NCIH-F and PRIfraction values were utilized to model rice LNC based on the artificial neural networks (ANNs) method. Subsequently, four prior bands were selected, respectively, with high FW and GSI values as the ANNs inputs for rice LNC estimation. Results show that FW- and GSI-based NCIH-F are closely related to rice LNC, and the performance of previous spectral indices used for LNC estimation can be greatly improved by multiplying their FWs and GSIs. Thus, it can be included that the FW- and GSI-based NCIH-F constitutes an efficient and reliable constructed form combining HSL (Sr) and LIFL (Sf) data together for rice LNC estimation.

Drone-Based Fluorescence Lidar Systems for Vegetation and Marine Environment Monitoring

We have developed two different types of drone-based fluorescence lidar systems for vegetation and marine environment monitoring, both based on violet CW diode lasers. An inelastic hyperspectral Scheimpflug lidar system was used for vegetation profiling combined with fluorescence spectral recordings. A light-weight fluorosensor set for fixed-height recordings was employed for monitoring of marine environments, featuring water Raman signals, algal chlorophyll and strong oil spill fluorescence.

Lidar Technology Based on Fiber System and its Application

The sodium atom existed in the metal layer of the earth’s atmosphere has a high atomic number density and a large scattering cross section. Sodium layer can act as a good tracer for atmospheric detection in the middle and lower-thermosphere (MLT) region. The sodium fluorescence lidar uses ultrashort pulsed laser to excite sodium atoms, which enabling simultaneous detection of wind and temperature in the middle and upper atmosphere. This paper reports on the development of sodium fluorescence laser radar in recent years, especially the integration of fiber-coupled optical switches and fiber-coupled acousto-optic frequency modulation technologies, which greatly improved the stability and reliability of lidar system and reduced the maintenance of lidar operation, laying a good foundation for the application of lidar observations under harsh environments. This technology has been applied to the sodium wind/temperature lidar in Yangbajing, Tibet and has been running stably for a long time.

The Relevance of Hanle Effect on Na and Fe Lidars

A laser resonant scattering process involves two steps, excitation and emission. That emission occurs spontaneously is well accepted. That the atoms involved in the emission are excited coherently by a laser beam leading to a non-isotropic angular distribution of emission (an antenna pattern) is not well known. The difference between coherent and incoherent excitation leads to the Hanle effect. In this paper, I discuss the physics of Hanle effect, and its influences on the backward scattering intensity of Na, K, and Fe atomic transitions and the associated Na and Fe resonant fluorescence lidar systems.

Laser Systems for the Pollutants Control in the Oil and Gas Industry

The questions of the laser technologies application in oil and gas industry and the scope of laser systems for the atmosphere, sea and underground waters control are considered in this article. Options of creation and using of the lidar for environmental monitoring, technological control and search of the hydrocarbons fields in various natural and technogenic conditions are offered. The main attention is paid to the Raman lidar for the sensing of the oil hydrocarbon molecules in the atmosphere. It is shown that such a lidar measurement time for the 1015 cm−3 concentration of the studied molecules at the ranging distance up to 200 m doesn’t exceed 265 microseconds. Creation of the fluorescent lidar for the sea water and the underground waters of the coastal area monitoring will allow to solve a problem of their pollution. Recording of the pollutants concentration spatial distribution can serve as the basic data for the solution of the inverse problem of the identification of pollution sources. The suggested methods have simple hardware realization and allow to diagnose the high-concentrated flows of the weighed particles and also hydrocarbons in the sea and underground waters.

Regional Models for High-Resolution Retrieval of Chlorophyll a and TSM Concentrations in the Gorky Reservoir by Sentinel-2 Imagery

The possibilities of chlorophyll a (Chl a) and total suspended matter (TSM) retrieval using Sentinel-2/MSI imagery and in situ measurements in the Gorky Reservoir are investigated. This water body is an inland freshwater ecosystem within the territory of the Russian Federation. During the algal bloom period, the optical properties of water are extremely heterogeneous and vary on scales of tens of meters. Additionally, they vary in time under the influence of currents and wind forcing. In this case, the usage of the traditional station-based sampling to describe the state of the reservoir may be uninformative and not rational. Therefore, we proposed an original approach based on simultaneous in situ measurements of the remote sensing reflectance by a single radiometer and the concentration of water constituents by an ultraviolet fluorescence LiDAR from a high-speed gliding motorboat. This approach provided fast data collection including 4087 synchronized LiDAR and radiometric measurements with high spatial resolutions of 8 m for two hours. A part of the dataset was coincided with Sentinel-2 overpass and used for the development of regional algorithms for the retrieval of Chl a and TSM concentrations. For inland waters of the Russian Federation, such research was performed for the first time. The proposed algorithms can be used for regular environmental monitoring of the Gorky Reservoir using ship measurements or Sentinel-2 images. Additionally, they can be adapted for neighboring reservoirs, for example, for other seven reservoirs on the Volga River. Moreover, the proposed ship measurement approach can be useful in the practice of limnological monitoring of inland freshwater ecosystems with high spatiotemporal variability of the optical properties.

Design of near-Infrared reconfigurable metalens on Silicon-On-Insulator (SOI) platform with Fabry–Perrot phase shifter

A metalens is the artificial surface made of billions of sub-wavelength elements and can bend light like traditional optical lens with significant efficiency and already attracted enormous research interests. However, the focus steering mechanism as an important part of optical system, is still under investigation for metalens. In this work, we describe an optical phased array model with the focalization and beam-steering features with specific phase relationship. To realize the focus-steering function on silicon metalens, the cascaded PIN junction enhanced by Fabry–Perrot cavity as the phase shifter is designed with metalens to provide up to 2 π phase variation. Silicon metalens with a diameter of 43μm and a focus of 40μm, at 1.55 um wavelength is verified in FDTD simulation. The results show that 10μm axial displacement is achieved with a carrier injection of 1019 cm−3 with the efficiency of 44%. At the end, the beam focalization and beam steering mechanism are investigated and analyzed. This focus steering mechanism could be deployed to many attractive near-infrared applications, such as fluorescence microscopy and LIDAR.

Analyzing the effect of the incidence angle on chlorophyll fluorescence intensity based on laser-induced fluorescence lidar.

Laser-induced fluorescence (LIF) technology has been widely applied to monitor vegetation growth status and biochemical concentrations. Thus, it is important to accurately acquire the fluorescence information for the quantitative monitoring of vegetation growth status. In this study, firstly, the incidence angle's effect on chlorophyll fluorescence intensity was analyzed by using the FluorMODleaf model. Then, comprehensive experimental data on the angle dependence of the fluorescence intensity to vegetation leaf surface were collected. Numerical and experimental results showed that proposed corrected cosine expression could be used to describe the relationship between the incidence angle and the fluorescence intensity in the LIF-Lidar. Lastly, fluorescence signals at 685 and 740 nm extracted at different incident angles of excitation lights were fitted with the corrected cosine expression. The coefficient of determination (R2) of the fitting results reached a maximum value of 0.93 for Salix babylonica.

Multiwavelength laser induced fluorescence (LIF) LIDAR system for remote detection and identification of oil spills

A multiwavelength laser induced fluorescence (LIF) LIDAR system, which can be employed around the clock for detecting oil spills in a marine environment, has been developed. The system incorporates a pulsed Nd: YAG laser, Nd: YAG laser amplifier and two KD*P crystals for generating 266 nm, 355 nm and 532 nm. Additionally, the LIF LIDAR system incorporates a nitrogen laser pumped dye laser system operating at 428 nm (Stilbene 420 dye), a Newtonian telescope, motorized monochromator, a photomultiplier detection system and a data acquisition system. Fluorescence intensity measurements collected from fresh and weathered oil samples using the LIF LIDAR system are presented. Strong fluorescence spectra were detected from refined and crude oils when excited with 266 nm. Finally, a brief description for the principle components algorithm used for identifying and classifying unknown oil samples is provided.

Preliminary measurements of fluorescent aerosol number concentrations using a laser-induced fluorescence lidar.

A laser-induced fluorescence lidar has been developed for detecting the concentration of fluorescent aerosols in the air. The fluorescence lidar was constructed with a pulsed fourth-harmonic Nd:YAG laser at the ultraviolet wavelength of 266nm with a repetition rate of 10Hz. A 250mm diameter custom telescope was used to collect optical spectra ranging from 260 to 560nm. Fluorescence signals with wavelengths ranging from 310-440nm were extracted, exploring a filter with a bandwidth of 130nm. The preliminary experiments were conducted at the campus of Xi'an University of Technology, in which the fluorescence signals of atmospheric fluorescent aerosols were continuously collected from 20:00 to 23:00 CST on 13 December 2017. Based on the fluorescence lidar equation, the density of fluorescence signals was calibrated using Rayleigh-Mie scattering signals and ozone (O3) concentration data at the ground level. Measured ranges show a strong dependence with the O3 concentrations due to its absorption characteristics at ultraviolet 266nm. Moreover, the concentration of the biogenic particles was also calculated based on the raw data of the fluorescence channel. Obtained results show that the concentration of biogenic particles in the Xi'an area varied greatly, ranging from 3456 particles · m-3 to 8835 particles · m-3 during winter.

Sudden Sodium Layers: Their Appearance and Disappearance

AbstractTemperature variation has been proposed to play an important role in the formation of the sporadic sodium layers (SSLs or NaS) in subtropic area, based on the observed significant correlation between SSLs and high temperatures. The icy‐dust particle, which could form in the extremely cold conditions and act as absorbers of sodium species, was proposed to be a possible candidate for the sodium reservoir of the SSLs. In this study, the University of Science and Technology of China temperature/wind lidar and the sodium fluorescence lidar at a subtropic station Hefei (31°N, 117°E), China, were used to observe sodium density, temperature, and wind profiles simultaneously throughout the SSL events. Based on the observations of two SSLs occurring on 12 and 13 May 2013, the possibility of an icy‐dust layer existing and acting as the sodium reservoir is tested for the first time in details. Both events experienced an extremely cold temperature (<150 K) several hours before the onset of SSLs, followed by a subsequently fast production of sodium atoms during a large temperature enhancement (>40 K). An empirical model including two main steps is then proposed: first, sodium species are collected by an icy‐dust reservoir and stored during the extremely cold phase; second, free sodium atoms could be released from the reservoir by a possible trigger. As a result, this kind of SSLs could possibly be regarded as a quasi‐continuous phenomenon caused and modulated by temperature variations with an icy‐dust model that can exhibit intermittent time variations related to the water vapor concentration.

High resolution LiDAR measurements reveal fine internal structure and variability of sediment-carrying coastal plume

We report results of a field survey conducted in the buoyant, sediment-carrying coastal plume generated by the discharge from the Patos Lagoon, the World's largest choked lagoon. The concentration of total suspended matter (TSM) and organic matter (as represented by total organic carbon, TOC) were mapped using an ultraviolet fluorescent LiDAR, which allowed for extensive data coverage (total of 79,387 simultaneous determinations of TSM and TOC) during 3 consecutive days. These observations were accompanied by hydrographic measurements from the ship and at a mooring station. We first describe synoptic variability of the plume, which responded energetically to wind forcing. We then analyze the TSM, TOC and hydrographic data jointly and develop a simple approach to estimate the rates of suspended matter removal from the upper layer due to gravitational settling and turbulent mixing based on relative changes in TSM and TOC concentrations. Four distinct regions within the plume exhibiting different dynamics of suspended and dissolved constituents were identified on this basis.