Shipborne single-photon fluorescence oceanic lidar: instrumentation and inversion.

Laser-induced fluorescence (LIF) technology has been widely applied in remote sensing of aquatic phytoplankton. However, due to the weak fluorescence signal induced by laser excitation and the significant attenuation of laser in water, profiling detection becomes challenging. Moreover, it remains difficult to simultaneously retrieve the attenuation coefficient (K l i d a r m f) and the fluorescence volume scattering function at 180° (ßf) through a single fluorescence lidar. To address these issues, a novel all-fiber fluorescence oceanic lidar is proposed, characterized by: 1) obtaining subsurface fluorescence profiles using single-photon detection technology, and 2) introducing the Klett inversion method for fluorescence lidar to simultaneously retrieve K l i d a r m f and ßf. According to theoretical analysis, the maximum relative error of ßf for the chlorophyll concentration ranging from 0.01 mg/m3 to 10 mg/m3 within a water depth of 10 m is less than 20%, while the maximum relative error of K l i d a r m f is less than 10%. Finally, the shipborne single-photon fluorescence lidar was deployed on the experimental vessel for continuous experiments of over 9 hours at fixed stations in the offshore area, validating its profiling detection capability. These results demonstrate the potential of lidar in profiling detection of aquatic phytoplankton, providing support for studying the dynamic changes and environmental responses of subsurface phytoplankton.

Simultaneous sensing profiles of beam attenuation coefficient and volume scattering function at 180° using a single-photon underwater elastic-Raman lidar.

Lidar has emerged as a promising technique for vertically profiling optical parameters in water. The application of single-photon technology has enabled the development of compact oceanic lidar systems, facilitating their deployment underwater. This is crucial for conducting ocean observations that are free from interference at the air-sea interface. However, simultaneous inversion of the volume scattering function at 180° at 532 nm (ßm) and the lidar attenuation coefficient at 532 nm (K l i d a r m) from the elastic backscattered signals remains challenging, especially in the case of near-field signals affected by the geometric overlap factor (GOF). To address this challenge, this work proposes adding a Raman channel, obtaining Raman backscattered profiles using single-photon detection. By normalizing the elastic backscattered signals with the Raman signals, the sensitivity of the normalized signal to variations in the lidar attenuation coefficient is significantly reduced. This allows for the application of a perturbation method to invert ßm and subsequently obtain the K l i d a r m. Moreover, the influence of GOF and fluctuations in laser power on the inversion can be reduced. To further improve the accuracy of the inversion algorithm for stratified water bodies, an iterative algorithm is proposed. Additionally, since the optical telescope of the lidar adopts a small aperture and narrow field of view design, K l i d a r m tends to the beam attenuation coefficient at 532 nm (cm). Using Monte Carlo simulation, a relationship between cm and K l i d a r m is established, allowing cm derivation from K l i d a r m. Finally, the feasibility of the algorithm is verified through inversion error analysis. The robustness of the lidar system and the effectiveness of the algorithm are validated through a preliminary experiment conducted in a water tank. These results demonstrate that the lidar can accurately profile optical parameters of water, contributing to the study of particulate organic carbon (POC) in the ocean.

Full-day profiling of a beam attenuation coefficient using a single-photon underwater lidar with a large dynamic measurement range.

A compact underwater lidar system, utilizing a single-photon detection technology, is proposed to effectively eliminate interference from the sea-air interface and enhance the accuracy of water optical property measurements. However, the high sensitivity of the single-photon detector poses challenges, including daytime operation difficulties due to strong solar radiation noise and detector saturation from near-field lidar signals. To address these issues, the laser and optical receiver of the lidar are optimized to suppress solar radiation noise, and a dual-telescope structure is introduced to improve the dynamic measurement range beyond 70 dB. In addition, a Monte Carlo simulation establishes the relationship between beam attenuation coefficients (c) and lidar attenuation coefficients (Klidar), enabling the retrieval of c profiles from Klidar. A field experiment conducted in the South China Sea, spanning from inshore to offshore waters, demonstrates the effectiveness of the lidar. The results highlight its potential applications, including the assessment of subsurface particulate organic carbon (POC).

Sensing profiles of the volume scattering function at 180° using a single-photon oceanic fluorescence lidar.

A novel oceanic fluorescence lidar technique has been proposed and demonstrated for remotely sensing the volume scattering function at 180° (ßf), which can be used to further retrieve the profiles of the absorption coefficient of phytoplankton (aph) at 532 nm and chlorophyll concentration (Chl). This scheme has these features. 1) The single-photon detection technology is employed to enhance the detection sensitivity to the single-photon level, enabling the oceanic lidar to obtain fluorescence backscatter profiles. 2) In terms of algorithms, the Raman backscattered signals of the water are utilized to normalize the backscattered signals of chlorophyll fluorescence, effectively minimizing the depth-dependent variation of the differential lidar attenuation coefficient (Δ K l i d a r f r). To reduce the contamination of fluorescence signals in the Raman backscatter signals, a Raman filter with a bandwidth of 6 nm was chosen. Subsequently, a perturbation method is utilized to invert the ßf of the fluorescence lidar. Finally, aph and Chl profiles can be inverted based on empirical models. 3) The value of Δ K l i d a r f r used in inversion is obtained through a semi-analytic Monte Carlo simulation. According to theoretical analysis, the maximum relative error of ßf for Chl ranging from 0.01 mg/m3 to 10 mg/m3 is less than 13 %. To validate this approach, a field experiment was conducted aboard the R/V Tan Kah Kee in the South China Sea from September 4th to September 5th, 2022, resulting in continuous subsurface profiles of ßf, aph, and Chl. These measurements confirm the robustness and reliability of the oceanic single-photon fluorescence lidar system and the inversion algorithm.

Sensing the profile of particulate beam attenuation coefficient through a single-photon oceanic Raman lidar.

A lidar technique has been proposed and demonstrated for remotely sensing particulate beam attenuation coefficient (cp) profiles using the Raman backscattered signal from water. In Raman lidar, the backscatter coefficient at 180° can be considered constant, allowing for the determination of the lidar attenuation coefficient (Klidar) from the Raman backscattered signal. This scheme has these features. 1) The bandwidth of the filter that used to extract the Raman component from the backscattered signal of the lidar was optimized to ensure sufficient lidar signal strength while minimizing the influence of chlorophyll fluorescence on inversion. 2) A receiving telescope with narrow field of view (FOV) and small aperture was utilized to suppress multi-scattering components in the backscattered signal. 3) A relationship between the beam attenuation coefficient (c) and Klidar was established after simulations via a semi-analytic Monto Carlo. 4) The value of cp was obtained by subtracting the attenuation coefficient of pure seawater (cw) from c. According to the theoretical analysis, the maximum relative error of cp is less than 15% for chlorophyll concentrations up to 10 mg/m3. Due to the water Raman backscattered signal being several orders of magnitude lower than the elastic backscattered signal, a single-photon detector is required to significantly improve the detection sensitivity to the single-photon level. To validate this approach, a field experiment was conducted aboard the R/V Tan Kah Kee in the South China Sea from September 4th to September 5th, 2022, and continuous subsurface profiles of cp were obtained. These measurements confirm the robustness and reliability of the oceanic single-photon Raman lidar system and the inversion method.

Remote sensing oil in water with an all-fiber underwater single-photon Raman lidar.

The detection of oil in water is of great importance for maintaining subsurface infrastructures such as oil pipelines. As a potential technology for oceanic application, an oceanic lidar has proved its advantages for remote sensing of optical properties and subsea materials. However, current oceanic lidar systems are highly power-consuming and bulky, making them difficult to deploy underwater to monitor oil in water. To address this issue, we have developed a compact single-photon Raman lidar by using a single-photon detector with high quantum efficiency and low dark noise. Due to the single-photon sensitivity, the detection of the relatively weak Raman backscattered signal from underwater oil was realized with a laser with a pulse energy of 1 µJ and a telescope with a diameter of 22.4 mm. An experimental demonstration was conducted to obtain the distance-resolved Raman backscatter of underwater oil of different thicknesses up to a distance of 12 m. The results indicate the single-photon Raman lidar's potential for inspecting underwater oil pipelines.

Day and night continuous high-resolution shallow-water depth detection with single-photon underwater lidar.

Single-photon lidar has emerged as a strong technology for bathymetric measurements. However, its heightened sensitivity additionally makes it susceptible to solar radiation noise, particularly in the green light wavelength where solar radiation is strong, posing challenges for its daytime operation. To address this issue, a single-photon underwater lidar system is proposed and demonstrated. This scheme has these features. 1) Underwater applications not only mitigate the impact of the air-water interface on laser transmission but also significantly attenuate solar radiation reaching the lidar due to the absorption and scattering properties of water. 2) The telescope is designed with a small aperture and narrow field of view to significantly suppress solar radiation. 3) A combination of a narrowband laser and narrowband filter technique is effectively employed to minimize residual solar radiation, thus enabling continuous bathymetric observation capabilities during both day and night. 4) After acquiring the backscattered signal from the bottom, a water depth extraction algorithm utilizing bi-Gaussian fitting is proposed. To demonstrate the robustness of the lidar and the effectiveness of the algorithm, the underwater single-photon lidar system is deployed on a ship to conduct cruise surveys of two bays in the nearshore area, as well as a full-day stationary observation experiment. The lidar measurements are highly consistent with the synchronized sonar observations. The full-day stationary observation experiment showcased its capability to deliver continuous measurements throughout the day and night. These results demonstrate the potential of the system in various applications, including high-precision underwater terrain mapping, obstacle avoidance for underwater platforms, and underwater target imaging.

Photon-counting distributed free-space spectroscopy.

Spectroscopy is a well-established nonintrusive tool that has played an important role in identifying and quantifying substances, from quantum descriptions to chemical and biomedical diagnostics. Challenges exist in accurate spectrum analysis in free space, which hinders us from understanding the composition of multiple gases and the chemical processes in the atmosphere. A photon-counting distributed free-space spectroscopy is proposed and demonstrated using lidar technique, incorporating a comb-referenced frequency-scanning laser and a superconducting nanowire single-photon detector. It is suitable for remote spectrum analysis with a range resolution over a wide band. As an example, a continuous field experiment is carried out over 72 h to obtain the spectra of carbon dioxide (CO2) and semi-heavy water (HDO, isotopic water vapor) in 6 km, with a range resolution of 60 m and a time resolution of 10 min. Compared to the methods that obtain only column-integrated spectra over kilometer-scale, the range resolution is improved by 2-3 orders of magnitude in this work. The CO2 and HDO concentrations are retrieved from the spectra acquired with uncertainties as low as ±1.2% and ±14.3%, respectively. This method holds much promise for increasing knowledge of atmospheric environment and chemistry researches, especially in terms of the evolution of complex molecular spectra in open areas.

Simultaneous wind and rainfall detection by power spectrum analysis using a VAD scanning coherent Doppler lidar.

Doppler wind lidar is an effective tool for wind detection with high temporal and spatial resolution. However, precise wind profile measurement under rainy conditions is a challenge, due to the interfering signals from raindrop reflections. In this work, a compact all-fiber coherent Doppler lidar (CDL) at working wavelength of 1.5 µm is applied for simultaneous wind and precipitation detection. The performance of the lidar is validated by comparison with the weather balloons. Thanks to the ability of precise spectrum measurement, both aerosol and rainfall signals can be detected by the CDL under rainy conditions. The spectrum width is used to identify the precipitation events, during which the two-peak Doppler spectrum is observed. The spectrum is fitted by a two-component Gaussian model and two velocities are obtained. By using the velocity-azimuth display (VAD) scanning technique, wind speed and rainfall speed are simultaneously retrieved. The false detection probability of wind speed in the rainy conditions is thus reduced.

Micro-pulse polarization lidar at 1.5 µm using a single superconducting nanowire single-photon detector.

An all-fiber, eye-safe and micro-pulse polarization lidar is demonstrated with a polarization-maintaining structure, incorporating a single superconducting nanowire single-photon detector (SNSPD) at 1.5 µm. The time-division multiplexing technique is used to achieve a calibration-free optical layout. A single piece of detector is used to detect the backscatter signals at two orthogonal states in an alternative sequence. Thus, regular calibration of the two detectors in traditional polarization lidars is avoided. The signal-to-noise ratio of the lidar is guaranteed by using an SNSPD, providing high detection efficiency and low dark count noise. The linear depolarization ratio (LDR) of the urban aerosol is observed horizontally over 48 h in Hefei [N31°50'37'', E117°15'54''], when a heavy air pollution is spreading from the north to the central east of China. Phenomena of LDR bursts are detected at a location where a building is under construction. The lidar results show good agreement with the data detected from a sun photometer, a 532 nm visibility lidar, and the weather forecast information.

1.5 µm polarization coherent lidar incorporating time-division multiplexing.

Atmospheric depolarization ratio and wind velocity are measured simultaneously by a single versatile coherent Doppler lidar (CDL). Backscattering components at parallel and perpendicular polarization states are obtained by using a single balanced detector, adopting time-division multiplexing technique. Thus systematic error induced by the non-uniform response of different detectors in traditional lidars is avoided. The operation mode of the instrument can be switched from polarization CDL to traditional CDL by the user depending on atmospheric conditions and desired performance. As demonstrated, the perpendicular component of the backscattering, usually wasted, not only can be used to retrieve the ADR, but also can be used to improve the carrier to noise ratio in wind detection. In the traditional mode, given a tolerance of 0.5 m/s precision, a detection range of 6 km is achieved by using a 300 ns laser pulse with energy of 100 µJ, where the temporal and spatial resolution of 2 s and 60 m, respectively. Continuous wind detection of the atmospheric boundary layer over 26 hours is presented to demonstrate the robustness and stability of the system. Dynamic evolution and wind structure are recorded.

Dual-frequency Doppler lidar for wind detection with a superconducting nanowire single-photon detector.

A dual-frequency direct detection Doppler lidar is demonstrated using a superconducting nanowire single-photon detector (SNSPD) at 1.5 µm. The so-called double-edge technique is implemented by using a dual-frequency laser pulse, rather than using a double-channel Fabry-Perot interferometer. Such a modification to the reported lidars enhances the frequency stability in the system level. Using the time-division multiplexing method, only one piece of SNSPD is used in the optical receiver, making the system simplified and robust. The SNSPD is adopted to enhance the temporal resolution since it offers merits of high quantum efficiency, low dark count noise, no after-pulsing probability, and a high maximum count rate. Two telescopes that point westward and northward at a zenith angle of 30° are used to detect the line-of-sight wind components, which are used to synthesize the horizontal wind profile. Horizontal wind profiles up to an altitude of about 2.7 km are calculated with vertical spatial/temporal resolution of 10 m/10 s. Wind dynamic evolution and vertical wind shears are observed clearly.

Fully integrated free-running InGaAs/InP single-photon detector for accurate lidar applications.

We present a fully integrated InGaAs/InP negative feedback avalanche diode (NFAD) based free-running single-photon detector (SPD) designed for accurate lidar applications. A free-piston Stirling cooler is used to cool down the NFAD with a large temperature range, and an active hold-off circuit implemented in a field programmable gate array is applied to further suppress the afterpulsing contribution. The key parameters of the free-running SPD including photon detection efficiency (PDE), dark count rate (DCR), afterpulse probability, and maximum count rate (MCR) are dedicatedly optimized for lidar application in practice. We then perform a field experiment using a Mie lidar system with 20 kHz pulse repetition frequency to compare the performance between the free-running InGaAs/InP SPD and a commercial superconducting nanowire single-photon detector (SNSPD). Our detector exhibits good performance with 1.6 Mcps MCR (0.6 µs hold-off time), 10% PDE, 950 cps DCR, and 18% afterpulse probability over 50 µs period. Such performance is worse than the SNSPD with 60% PDE and 300 cps DCR. However, after performing a specific algorithm that we have developed for afterpulse and count rate corrections, the lidar system performance in terms of range-corrected signal (Pr2) distribution using our SPD agrees very well with the result using the SNSPD, with only a relative error of ∼2%. Due to the advantages of low-cost and small size of InGaAs/InP NFADs, such detector provides a practical solution for accurate lidar applications.

Micro-pulse upconversion Doppler lidar for wind and visibility detection in the atmospheric boundary layer.

For the first time, to the best of our knowledge, a compact, eye-safe, and versatile direct detection Doppler lidar is developed using an upconversion single-photon detection method at 1.5 µm. An all-fiber and polarization maintaining architecture is realized to guarantee the high optical coupling efficiency and the robust stability. Using integrated-optic components, the conservation of etendue of the optical receiver is achieved by manufacturing a fiber-coupled periodically poled lithium niobate waveguide and an all-fiber Fabry-Perot interferometer (FPI). The double-edge technique is implemented by using a convert single-channel FPI and a single upconversion detector, incorporating a time-division multiplexing method. The backscatter photons at 1548.1 nm are converted into 863 nm via mixing with a pump laser at 1950 nm. The relative error of the system is less than 0.1% over nine weeks. In experiments, atmospheric wind and visibility over 48 h are detected in the boundary layer. The lidar shows good agreement with the ultrasonic wind sensor, with a standard deviation of 1.04 m/s in speed and 12.3° in direction.

All-fiber upconversion high spectral resolution wind lidar using a Fabry-Perot interferometer.

An all-fiber, micro-pulse and eye-safe high spectral resolution wind lidar (HSRWL) at 1.5 µm is proposed and demonstrated by using a pair of upconversion single-photon detectors and a fiber Fabry-Perot scanning interferometer (FFP-SI). In order to improve the optical detection efficiency, both the transmission spectrum and the reflection spectrum of the FFP-SI are used for spectral analyses of the aerosol backscatter and the reference laser pulse. Taking advantages of high signal-to-noise ratio of the detectors and high spectral resolution of the FFP-SI, the center frequencies and the bandwidths of spectra of the aerosol backscatter are obtained simultaneously. Continuous LOS wind observations are carried out on two days at Hefei (31.843 °N, 117.265 °E), China. The horizontal detection range of 4 km is realized with temporal resolution of 1 minute. The spatial resolution is switched from 30 m to 60 m at distance of 1.8 km. In a comparison experiment, LOS wind measurements from the HSRWL show good agreement with the results from an ultrasonic wind sensor (Vaisala windcap WMT52). An empirical method is adopted to evaluate the precision of the measurements. The standard deviation of the wind speed is 0.76 m/s at 1.8 km. The standard deviation of bandwidth variation is 2.07 MHz at 1.8 km.

Long-range micro-pulse aerosol lidar at 1.5 µm with an upconversion single-photon detector.

A micro-pulse lidar at eye-safe wavelength is constructed based on an upconversion single-photon detector. The ultralow-noise detector enables using integration technique to improve the signal-to-noise ratio of the atmospheric backscattering even at daytime. With pulse energy of 110 µJ, pulse repetition rate of 15 kHz, optical antenna diameter of 100 mm and integration time of 5 min, a horizontal detection range of 7 km is realized. In the demonstration experiment, atmospheric visibility over 24 h is monitored continuously, with results in accordance with the weather forecasts.

Stratospheric temperature measurement with scanning Fabry-Perot interferometer for wind retrieval from mobile Rayleigh Doppler lidar.

Temperature detection remains challenging in the low stratosphere, where the Rayleigh integration lidar is perturbed by aerosol contamination and ozone absorption while the rotational Raman lidar is suffered from its low scattering cross section. To correct the impacts of temperature on the Rayleigh Doppler lidar, a high spectral resolution lidar (HSRL) based on cavity scanning Fabry-Perot Interferometer (FPI) is developed. By considering the effect of the laser spectral width, Doppler broadening of the molecular backscatter, divergence of the light beam and mirror defects of the FPI, a well-behaved transmission function is proved to show the principle of HSRL in detail. Analysis of the statistical error of the HSRL is carried out in the data processing. A temperature lidar using both HSRL and Rayleigh integration techniques is incorporated into the Rayleigh Doppler wind lidar. Simultaneous wind and temperature detection is carried out based on the combined system at Delhi (37.371°N, 97.374°E; 2850 m above the sea level) in Qinghai province, China. Lower Stratosphere temperature has been measured using HSRL between 18 and 50 km with temporal resolution of 2000 seconds. The statistical error of the derived temperatures is between 0.2 and 9.2 K. The temperature profile retrieved from the HSRL and wind profile from the Rayleigh Doppler lidar show good agreement with the radiosonde data. Specifically, the max temperature deviation between the HSRL and radiosonde is 4.7 K from 18 km to 36 km, and it is 2.7 K between the HSRL and Rayleigh integration lidar from 27 km to 34 km.

Mobile Rayleigh Doppler lidar for wind and temperature measurements in the stratosphere and lower mesosphere.

A mobile Rayleigh Doppler lidar based on the molecular double-edge technique is developed for measuring wind velocity in the middle atmosphere up to 60 km. The lidar uses three lasers with a mean power of 17.5 W at 355 nm each and three 1 m diameter telescopes to receive the backscattered echo: one points to zenith for vertical wind component and temperature measurement; the two others pointing toward east and north are titled at 30° from the zenith for zonal and meridional wind component, respectively. The Doppler shift of the backscattered echo is measured by inter-comparing the signal detected through each of the double-edge channels of a triple Fabry-Perot interferometer (FPI) tuned to either side of the emitted laser line. The third channel of FPI is used for frequency locking and a locking accuracy of 1.8 MHz RMS (root-mean-square) at 355 nm over 2 hours is realized, corresponding to a systematic error of 0.32 m/s. In this paper, we present detailed technical evolutions on system calibration. To validate the performance of the lidar, comparison experiments was carried out in December 2013, which showed good agreement with radiosondes but notable biases with ECMWF (European Centre for Medium range Weather Forecasts) in the height range of overlapping data. Wind observation over one month performed in Delhi (37.371° N, 97.374° E), northwest of China, demonstrated the stability and robustness of the system.