Light detection and ranging (lidar) have been valuable tools in remote sensing of aerosols near the ground. For the operation of a lidar system, the laser, as a transmitter, plays a vital role in the whole system. Laser Diodes (LD) and Diode-Pumped Solid State (DPSS) laser technology have evolved, making the lidar system more compact compared to Nd:YAG laser sources. However, the lidar system’s long time and continual operation need maintenance to keep the laser source output stable. Also, the laser source is vulnerable to static electricity and needs to stabilize electric power. In this work, a multiwavelength lidar system with a Light Emitting Diode (LED)-based light source is designed and developed to monitor aerosol distribution in the near-ground atmosphere during continuous observation. The LED light source does not require any heat dissipation system and can emit light for long periods with constant output. The LED lamp light sources with wavelengths of 365, 450, 525, and 630 nm (peak power of up to 2W) are used as lidar transmitters. This lidar system visualizes rapid activities of aerosols in the near-range measurement due to its repetition frequency of over 250 kHz. Analysis of the backscattering light intensity with four wavelengths from this LED lidar system produces real time extinction coefficient and size distribution in the near-ground atmosphere. This report discusses the design and practical test of the multi-wavelength LED Lidar.
Visualization of the wind field in the lower atmosphere and its dynamics are important for understanding the mixing and interaction between geology and atmosphere. The dispersion of that dust is a major problem not only for environmental protection, but also for human health, such as respiratory diseases, and air pollution caused by man-made dust in urban areas, so great demands for visualization and monitoring of wind fields and dust flow near the ground surface are raised. To observe atmospheric flows on small spatio-temporal scales near ground, we are developing a low coherence Doppler lidar. Low coherence Doppler lidars can capture the dynamics of the lower atmosphere because of the high spatial and temporal resolutions of 1 m and 5 ms, respectively. Dust flow measurements can be made, while the system is not sensitive enough to measure the atmospheric wind itself. That is current task. In this paper, we discuss the efficiency improvements of the lidar transmitting and receiving optical systems and the receiving system itself of fiber coupling by two orders of intensity with a fiber-type distributed feedback laser diode. The beam quality of this light source was higher than the previous bulk type one, and the interference light intensity was six times higher. The Doppler shift frequency measurement with a rotating target showed a larger signal-to-noise ratio of approximately 70 dB, 30dB higher than the previously reported system.
In this study, LED based mini-lidar to install into Mars rover has been developed. Its aim is to capture the dynamics of dust twister called “Dust Devil”. For Mars rover installation, there are large restrictions of 10cm3 optics size, 1kg weight, 1W power consumption, etc. We have developed its concept model, and conducted the demonstration experiment. The LED mini-lidar is equipped with a high speed photon counter, of which data sampling interval is 1ns, corresponding to a range bin of 0.15m, and a high repetition pulsed LED beam, of which repetition frequency is greater than 500kHz. Transmitting optical pulse power was <10nJ/10ns (= 1W). For summation time of greater than 0.2s, surface atmosphere activity in the order of seconds can be visualized with this high resolution. The dynamics measurement of an artificial twister was successfully conducted with this mini-lidar. In this report, the quantitative measurement method to distinguish dust samples in relation to Zirconia beads measurements is proposed. The relationship between the shaking off dust weight and its scattering cross section is obtained. The dust chamber experiment was conducted with several kinds of dust particles, too. These lidar echo results were compared with transmissometer, and they were well coincided due to the concentration change. The nature fog observation was conducted with this mini-lidar, too. The liquid water content was estimated under the consideration of the result of the quantitative measurement.
In this work, we propose a low-coherence Doppler lidar (LCDL) to measure particle velocity at near-surface atmosphere. To measure particle velocity information at near-surface atmosphere, it is necessary to conduct the measurement at high frequency and resolution. Low coherence light source can satisfy this high-resolution criterion. We discuss the system efficiency to detect atmospheric echoes by theoretical analysis. In the particle velocity experiment, Doppler shifted frequency is about 5 MHz and the particle velocity calculated from the peak Doppler signal is 2.7 m/s, while minimum and maximum velocity are 1.85 and 2.92 m/s, respectively. Anemometer result shows good agreement with LCDL results.
The attenuation of backscattered light through human tissue makes it difficult to measure the interferometric signal with high accuracy. We present a new method that combines the originally developed Time Domain-OCT (TD-OCT) technique with Ghost Imaging (GI) technique to detect targets at a certain depth in the scattering medium. Our method's first result shows that sample images can be reconstructed through a homogeneous mixture of 0.5% milk solution (1 cm thickness) scattering medium. Theoretical calculations show that it is possible to reconstruct images in higher milk solutions (about 20%) with optical properties close to human tissues.
Laser beam propagation in highly scattering media has attracted much attention for optical sensing fields because the propagation of the light is often limited to the near-surface regions of the media. To increase the sensing capability in such scattering conditions, we focus on the propagation property of an annular beam in a highly scattering media. In our previous work, the non-diffractive beam was generated by propagating an annular beam in a scattering medium of colloidal suspension (diluted milk) up to the concentration of 22%, which is near the human tissue. In this study, the propagation property of a partially blocked annular beam using a fan-like obstacle with different apex angles is investigated to realize object detection in highly scattering media such as human tissue, fog, or cloud. Simulation results show that non-diffractive beams can be generated when partially blocked annular beam with blocking angle from 0° to 30o is propagated in free space. The propagation experiment of partially blocked annular beam in scattering media shows that the center peak intensity in scattered light is detected when the apex angle is set up to 30° . By measuring scattered light at different receiving distances, the experiment results show that the center peak is kept at a certain distance away from the optical cell. This result verifies that the central peak intensity of this scattered light is a non-diffractive beam.
Nighttime clouds are detected using a camera without a NIR-cut filter by exploring the pixel value distribution in each red, green, and blue (RGB) color space from clear and 100% cloudy sky images. The removal of the NIR-cut filter enhances the pixel values of each color space due to the additional NIR signals. Cloud pixels can be separated from clear sky pixels by only using each color space and applying the appropriate threshold. Exploring different cloud detection methods is vital in our ongoing research activity collecting clear and cloudy sky images in other countries using this camera.
The influence of aerosols to the atmosphere has been discussed in the context of the Earth radiation budget and global climate change. Therefore, precise monitoring of aerosol parameters is important for better understanding of their real characteristics and impacts on the environment. In this study, we report on a novel method of concurrent measurements of aerosol near the surface level by means of slant-path (SP) and plan position indicator (PPI) lidars. The SP lidar utilizes a diode-laser-pumped Nd:YAG laser operating at 532 nm, while the PPI is based on a Nd:YLF laser at 349 nm. The PPI system including the laser transmitter and telescope section is rotated over 360° for covering all the horizontal directions with the maximum observation range up to around 3 km. At the same time, the SP lidar is employed for monitoring the near surface region that cannot be covered by vertical observation lidars. Furthermore, the backscattered signals recorded by both PPI and SP lidars are analyzed using the Fernald method to retrieve aerosol extinction coefficient by employing lidar ratios for 349 and 532 nm. These values of lidar ratio are estimated by adjusting and fitting parameters in the Mie scattering calculation (mode radius, variance, and both real and imaginary parts of refractive index) to real data from ground-based sampling instruments, namely, the scattering coefficient, absorption coefficient, and size distribution observed with an integrating nephelometer, an aethalometer, and an optical particle counter, respectively. Real-time values of the extinction coefficient inside the atmospheric boundary-layer are derived as the summation of scattering and absorption coefficients. The results are then compared with those from a vertical lidar, operated by the National Institute of Environmental Studies (NIES) on the campus of Chiba University. We discuss the observed features of aerosol characteristics that vary both temporally and spatially.
The first polarization lidar was developed in Manila in early 1997 and it has been operating regularly to the present time. The regular observations aim to monitor and characterize the urban air ofManila and to study tropical clouds as well. The fixed, vertically pointing lidar transmits l5OmJ, 532 nm polarized pulses at 20Hz. The backscatter that is collected by the 27.9 cm, 0.358 nirad telescope is transmitted through a narrow bandpass filter and polarizing beamsplitter, to a two-channel receiver with PMTs operated in analog mode. Lidar signals are averaged for 2 minutes by an 8-bit digitizing storage oscilloscope and transferred to a PC. In this paper, combined measurements on suspended aerosols and boundary layer with lidar and filter-sampling of particles will be presented. These measurements, together with the meteorological data from radiosonde soundings of the weather bureau, on particular cold and hot months will be analyzed and compared. Measurements on an episodic event such as New Year's celebrations will also be discussed. The discussions in this paper will focus on the following: a) characterization of suspended aerosols and boundary layer by lidar depolarization ratio and extinction coefficient measurements and by filter-sampling method, and b) seasonal and diurnal changes ofthe above items.
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