LIDAR (Light Detection and Ranging) is a remote sensing method that uses a laser not only to determine the distance between the target and the system, but also to deduce the physical properties of the target based on the light-matter interaction (scattering, absorption, and fluorescence) and optical properties (Doppler and polarization). In this special issue, we introduce the latest lidar technologies that can be applied to a broad range of areas from the atmospheric to oceanic monitoring.

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Forest canopy height is a key parameter in above ground biomass estimation, which is crucial for improving knowledge of the global carbon cycle. The lidar mission named “MOLI: Multi-footprint Observation Lidar and Imager” aimed to measure the canopy height in the dense forest. The space qualified laser is a key component for achieving the MOLI mission. The laser transmitter employed master oscillator and power amplifier design to manage thermal distortion, optical alignment stability, and laser power scaling. The lifetime test and environmental test are conducted to improve feasibility of a space qualified laser transmitter for earth observation missions. These results and status are described in this article.

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Metallic layers, containing such elements as Na, K, Li, Ca, and Fe, originating from meteors are valuable tracers for performing observations of the Earth’s upper atmosphere. The present paper provides a brief review of resonance-scattering lidars, which use laser remote-sensing technology to detect the metals occurring in these layers. In particular, two resonance-scattering lidars are introduced. The first is a highly-stable Na resonance-scattering lidar, which is easily handled even by novices. The other is a frequency- tunable resonance-scattering lidar, that enables observations of three different metallic species: K, Fe, and Ca+. The recent studies using these lidars are described.

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A high-spectral-resolution lidar (HSRL) method independently retrieves of aerosol extinction profile from aerosol backscatter profile by separately measuring the atmospheric molecular (Rayleigh) scattering from aerosol (Mie) scattering. We used spectrally narrow band lasers as a light source and such high-spectral-resolution filters such as absorption vapor filters and interferometric filters to separate Rayleigh and Mie scattering. A wide variety of combinations of light sources and spectral filters has been studied for almost 40 years. This paper outlines the history of HSRL development as well as the concept of HSRL methods for the retrieval of aerosol extinction and backscatter. We also introduce a simple HSRL method using a scanning interferometer recently developed at the National Institute for Environmental Studies.

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Coherent Doppler LIDAR, which is a laser remote sensor, measure the atmospheric wind velocity and the wind direction using the phase shift of the aerosols backscattered light from the transmitted light. It can also measure the wind under the clear air conditions. Coherent Doppler LIDAR has mainly been utilized for wind power assessment and aeronautical meteorology. Our Doppler LIDARs consist following key-technologies: a transmitter-and-receiver integrated circuit, a multi-direction TRX telescope, adaptive parameter control processing, and a waveguide laser amplifier. In this paper, we introduce the latest LIDAR technology and these applications.

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The prediction of heavy rain is critical to reduce damage and increase public safety. To improve prediction accuracy, we developed water vapor lidars that measure the vertical distribution of water vapor in the lower atmosphere. We conducted long-term observations of vertical water vapor distributions on the windward side of heavy rain areas to study the influence of moisture on convection initiation and cumulus development that cause heavy rain. With a data assimilation technique, we also evaluated the impacts of lidar-observed data on the objective analysis of water vapor field and on the prediction of the precipitation of a numerical weather prediction model.

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Raman Lidar is a powerful and practical tool for the remote probing of the atmosphere. Its capability has been extended by remote sensing in water. With underwater Raman Lidar, we can remotely identify and determine the concentration of such a particular molecular species as a gas in water. We describe our recent experimental results and the development of a marine Raman Lidar system to demonstrate the feasibility of Lidar mapping in water.

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LED mini lidar was developed for surface atmosphere observation. The lidar is equipped with a high speed photon counter of which data sampling interval is 1 ns, corresponding to a range bin of 0.15 m, and a high repetition pulsed LED beam of which repetition frequency is greater than 500 kHz. Transmitting pulse power was <10 nJ/10 ns (= 1 W). For summation time greater than 0.2 s, surface atmosphere activity could be visualized. In this study, concentration change in the order of seconds was visualized with this high resolution. Furthermore, the quantitative measurement method to distinguish dust samples in relation to Zirconia beads measurements is proposed. The ratio between weight and scattering cross section of suspended dusts is obtained and the suspension characteristics of such dusts are discussed with the results.

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Nitrogen dioxide (NO2) gas generated by indoor fires significantly impacts human health. Since smokeless fires might not be detected by fire alarms, we propose an LED sensor that measures the spatial mean concentration of NO2 by a differential optical absorption spectroscopy (DOAS) technique and developed a prototype LED sensor using LEDs whose center wavelengths are 392.4 and 624.0 nm. A linear regression between LED and commercial NO2 sensors at concentrations between 0 and 100 ppm with a 21-cm distance showed an R 2 value of 0.93 and statistical error of 8.7%, indicating reasonable agreement.

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We present a stable polarization-maintained supercontinuum that is generated via pulses from a master oscillator fiber amplifier (MOFA) setup solely based on ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fibers. The pumping source provides sub-300 fs pulses with multi-watts output power after two stages of amplification and 1 MHz repetition rate. The obtained supercontinuum spans from 350 nm to 4.5 μm and exhibits high stability.

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