Industrial Applications of Laser Remote Sensing

Laser remote sensing systems have been developed as laser radar or lidar (Light Detection And Ranging). The spatial distribution of dust aerosol particles, water droplets, atomic and molecular components of low concentration could be detected efficiently. Various meteorological applications were developed, and effective sensing techniques have been accumulated in the field of atmospheric, oceanic and terrestrial studies, ranging from in situ, local, to global sensing. In this chapter, the basic principles, performance, and historical progress of laser remote sensing techniques are briefly introduced. Optical interaction processes used in lidar and other remote sensors are discussed and their detection sensitivities are compared. The basic concept can be applied to the design of laser sensing techniques and systems for various industrial applications. As examples of laser sensors for industrial applications, Mie lidars in the eye-safe near infrared and ultraviolet spectral regions for dust plume monitoring of urban and industrial areas are introduced.

Nowadays, measurement of air flow and certain gas species in near range are needed for safety and environmental monitoring. Lidar is the appropriate tool for these applications. However, conventional lidar optics has a blind area because of the distance necessary to overlap the transmitted beam and the receiver’s field of view. To detect the near range lidar echo with a narrow field of view, the optical design should be compact and simple. The near range from zero to a few hundred meters ( < a few km) is the target distance. In this chapter, the theoretical calculation of the lidar echo for the near range measurement lidar is presented. The inline optics, which has common optics for the transmitting and receiving optics, is introduced. The signal-to-noise ratio is also estimated from the viewpoint of lowering the transmitted beam power for eye-safety. The actual near range lidar setup, which is based on the analysis, is also presented. The calculated results are compared and evaluated with the optical specification of the lidar. Some studies using the near range lidar, and several types of near range lidars are introduced.

Gas detection is important for industrial safety, environmental protection, and environmental monitoring. Absorption spectroscopy is a widely used technique for gas detection. By applying laser remote sensing techniques, gas sensing with large standoff distances becomes possible. The industry standard method uses the double frequency technique, in which the laser wavelength is modulated about a center wavelength corresponding to the absorption peak of the target gas species, and the optical signal is detected at the second harmonic of the modulation frequency. Alternatively, by selecting two wavelengths which correspond to strong and weak absorption of the target gas species, two-dimensional imaging or visualization of gas leaks becomes possible. These techniques have been applied to detection and imaging of natural gas (methane gas) leaks. Laser sensing is also used for in situ gas analysis in power plants and incineration plants. Recent progress in quantum cascade lasers has enabled detection of minor species in harsh environments such as flue gas.

The Raman shift is a characteristic property of a molecule, and the intensity of Raman scattering is proportional to the density of the molecule. Therefore, gas sensing using Raman scattering can detect and identify gases. The method is especially useful for detection of hydrogen gas, because the hydrogen molecule does not have absorption bands from the near ultraviolet to near infrared that can be used for optical detection using absorption. On the other hand, the hydrogen molecule exhibits a strong Raman effect, so Raman scattering is a suitable method for hydrogen gas detection. In this chapter, the fundamentals of Raman scattering, detection of hydrogen gas by Raman scattering, development of lidar systems for detection, imaging, and concentration measurement of hydrogen gas, are presented. The concentration of hydrogen gas leaked into the open air can be remotely measured by simultaneous measurement of the Raman scattering signals from the hydrogen gas and atmospheric nitrogen. Hydrogen gas leak detection using Coherent Anti-Stokes Raman Scattering (CARS) is also presented.

Remote sensing using pulsed lasers (lidar) has mainly been applied to atmospheric observation, but it can also be applied to marine observation. In this chapter, lidar applications to marine observation, including bathymetry, detection of oil spills on sea surfaces, water quality inspection, monitoring of marine organisms, are presented. The present state and future prospects on these applications are presented.

This chapter describes the application of laser-induced fluorescence (LIF) spectroscopy to monitoring of living plants. Several applications in agriculture, horticulture, and forestry are described along with specific LIF monitoring systems. The following topics are covered. 1) The basics of LIF spectroscopy for plant monitoring. If an ultraviolet laser is used as an excitation source, the LIF of the plant receiving the light consists of blue-green fluorescence and red-far red fluorescence. Because LIF is a physiologically based optical phenomenon exchanging absorbed optical energy among living molecules, it can include information about living status. 2) A lettuce leaf and a sasanqua (Camellia sasanqua) leaf were monitored using long–term LIF. The growth status of the lettuce was shown by variation in the LIF intensity at 460 nm and 530 nm. Symptoms of water stress in the sasanqua leaf also appeared at the same wavelengths. Such variation can be used for quality control of their products and understanding the process of the development of stress in plants. 3) A laser-induced fluorescence spectrum (LIFS) light detection and ranging (lidar) was developed for monitoring a large tree. The entire LIF spectrum of a zelkova (Zelkova serrata Makino) tree growing outside was monitored at different growth stages. The formation of chlorophyll is discussed. 4) A LIF imaging system and LIFS imaging lidar were developed for laboratory use and for remote monitoring, respectively. LIF images of a spinach leaf indicated plant activity and productivity. The LIFS imaging lidar successfully created a chlorophyll distribution map of a whole poplar tree. The effectiveness of LIF imaging is described. 5) Based on these results, “a Green-Cross; general hospital for plants” and “Optical Farming” are proposed for future development.

Coherent Doppler Lidar (CDL) systems are suitable for wind sensing of localized regions under clear weather conditions. Especially, a 1.5 μm all-fiber wind sensing CDL system has many advantages since telecomm fibers and fiber-based components, which have reached technical maturity, can be used. We have developed all-fiber CDL systems during this decade, and in this chapter we introduce our development in addition to the basic theory for system design. After the demonstration of wind sensing performance using the prototype model, we have developed the product model by refining the prototype model. The product model consists of a compact fiber-based optical transceiver unit, an optical antenna unit that includes a wedge scanner, and a Field Programmable Gated Array (FPGA)-based real-time signal processing unit. All-fiber CDL systems can be used for various industrial applications, such as meteorological monitoring, wind survey for wind power generation, and aviation safety.

3D laser radar is a high-speed figure recognition system that measures the position and speed of objects in real time. It can function despite poor weather conditions as it works by measuring the distance to an object with a laser pulse. By applying 3D laser radar for use in road and rail traffic safety, IHI has realized a system that can measure the position and speed of vehicles and pedestrians. Currently, more than 800 of these systems are in operation in Japan and this number will highly likely increase in future, contributing to improvements in traffic safety.

A laser-based remote sensing system (LRSS) for detecting defects in concrete has been developed. The diffraction efficiency of a photorefractive crystal (PRC) in the LRSS was increased by an applied electric field. A stabilization system to stabilize the interference pattern in the PRC using the running hologram technique was constructed. Defects in concrete can be located using initiation and detection of impact echo and standing Lamb waves (or natural mode of vibration). The prototype of the LRSS was assembled and set on a small truck. Field experiments were carried out to investigate real concrete defects of a bridge in bullet-train line in Japan. The LRSS scan concrete surfaces to produce a two-dimensional map of real concrete defects. The observed predominant frequency of concrete vibration was consistent with data from impact hammering method.

Laser-induced breakdown spectroscopy (LIBS) is attractive for fast, on-site, and remote measurement of trace elements with high spatial resolution. The measurement of chlorine concentration in concrete, which can be useful for the evaluation of durability of reinforced concrete structures, was performed with a sensitivity of better than 0.18 kg/m3, which is below the threshold chlorine concentration of 0.6 kg/m3 at which the reinforcing bars in concrete structures start to corrode. In addition, ultrashort laser pulses have several advantages for application to LIBS. The propagation of an ultrashort high-intensity laser pulse in the atmosphere produces a bundle of filaments, which can be generated for a distance of more than several hundreds of meters and have sufficient intensity for producing plasmas at various targets. LIBS using filaments, called filament-induced breakdown spectroscopy (FIBS), is very useful for remote measurement of trace elements. Remote detection and identification of microparticles in air by FIBS at a distance of 16 m was demonstrated. In addition, as a new application of LIBS, remote measurement of the electric field is presented.