Studies and design lidar system using 40 cm telescope

Abstract. Light Detection And Ranging (LIDAR) technology has been developed in Vietnam. We plan to develop LIDAR system with a 40 cm telescope at HNUE in order to measure density at the higher atmosphere. The telescope 40 cm Meade LX200 is the biggest telescope in Vietnam right now, so we have to design optical system difference with the previous LIDAR systems in Vietnam. This article focuses on developing a compact LIDAR receiver system to be used on a ground bases-rover for the detection of aerosols and atmosphere population.

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JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2016-0034 Mathematical and Physical Sci., 2016, Vol. 61, No. 7, pp. 75-79 This paper is available online at STUDIES AND DESIGN LIDAR SYSTEM USING 40 CM TELESCOPE Nguyen Quynh Lan1, Luc Huy Hoang1, Nguyen Van Khanh1, Nguyen Van Minh1, Nguyen Anh Vinh1 and Dinh Van Trung2 1Faculty of Physics, Hanoi National University of Education 2Institute of Physics Abstract. Light Detection And Ranging (LIDAR) technology has been developed in Vietnam. We plan to develop LIDAR system with a 40 cm telescope at HNUE in order to measure density at the higher atmosphere. The telescope 40 cm Meade LX200 is the biggest telescope in Vietnam right now, so we have to design optical system difference with the previous LIDAR systems in Vietnam. This article focuses on developing a compact LIDAR receiver system to be used on a ground bases-rover for the detection of aerosols and atmosphere population. Keywords: Compact LIDAR receiver system, ground bases-rover, aerosols, atmosphere population. 1. Introduction Light Detection And Ranging (LIDAR) is a measurement technique using laser radiation in the ultraviolet wavelength region, the visible and near-infrared. The use of techniques and optical spectroscopy allows the application of LIDAR to measure the physical characteristics of the atmosphere continuously over space and time with a high resolution. Spatial resolution capabilities can reach every meter and the ability to resolve highly time may come every second. This is the basic advantages compared to tools such as atmospheric measurements balloon (balloon radiosondes) because these tools only allow measurements in a certain number of times and a height limit (30 km for the balloon). In recent times LIDAR has become an indispensable tool for studying physics and chemistry of the atmosphere (up to 100 km altitude), the atmospheric phenomena such as gravity waves (gravity waves), distribution temperature with altitude. At the same time LIDAR technique has also been widely used for environmental monitoring, such as determining the density of dust, aerosols (aerosols), density and distribution space time of ozone gas or hazardous waste pollutant gases such as SO2, NO2, benzene, toluene. Light detection and ranging is one of the most important methods for detection of the middle and upper atmosphere. This article describes principle of LIDAR. We have been calculating and simulating the model for the LIDAR system in order to have the best design, and this Lidar system allows us to measure at atmosphere up to more than 10 km. Our calculation lets us know that this LIDAR system can be measure up to 70 km of the high altitude of the atmosphere. Received October 15, 2016. Accepted October 24, 2016. Contact Nguyen Quynh Lan, e-mail address: nquynhlan@hnue.edu.vn 75 N. Q. Lan, L. H. Hoang, N. V. Khanh, N. V. Minh, N. A. Vinh and D. V. Trung 2. Content 2.1. Overall design of the light detection and ranging system Figure 1. Schematic of the LIDAR system using 40 cm telescope Figure 2. Profile of 532 nm channel in photon counting mode, 20-minute average with telescope diameter 25 cm 76 Studies and design lidar system using 40 cm telescope LIDAR used to study atmospheric in most case can be regarded to be at infinite distance from the telescope. The telescope will have no significant problems in focusing backscattered light onto a detector. This luxury is not available in near field LIDAR since objects in the near field will be focused at different distances from the focusing mirror. It is readily seen in the lens formula: 1 f = 1 z1 + 1 z2 (2.1) Where z1 is the distance from object to the focusing mirror and z2 is the distance from focusing mirror to the image plane and f is the focal length of the focusing mirror. The general form of the LIDAR equation can be shown as: P (z) = E β(z) z2 exp[ ∫ t 0 σ(z′)dz′] (2.2) where: E: Transmitted power per pulse, P(z): Returned powers relative to the outgoing laser beam, β(z): Backscattering coefficient, σ(z): Extinction coefficient, z: Distant (km). We use a Nd:YAG laser (Quantel Brilliant model) emitting 360 mJ at 1064 nm and 180 mJ at the second harmonic 532 nm as the transmitter of our LIDAR system. The laser beams at these wavelengths are directed into the atmosphere by a pair of high energy laser mirrors M1 and M2. A half wave plate allows the reorientation of the polarization direction of the laser beam at 532 nm. That allows us to measure the depolarization of the LIDAR signal at 532 nm. The backscattered light is collected by a Cassegrain telescope of diameter 40 cm and f/D=10. The field of view of the telescope is selected by using a pinhole of appropriate diameter located at the focal plane of the telescope. In our measurements we usually use a field of view of 2 mrad, which corresponds to a pinhole size of 4 mm. A dichroic mirror then separates the backscattered light into 532 nm and 1064 nm channels. We also use a polarizing beamsplitter cube to split the backscattered light into parallel and perpendicular polarization with respect to the polarization direction of the laser beam. The main component of the LIDAR transmitter system is discussed in this section. The LIDAR transmitter consists Laser Nd:YAG with output beam of wavelength at 532 nm, Energy per pulse (max):180 mJ, Pulse duration: 5 ns, Repetition rate: 10 Hz and Beam divergence: 0.5 mrad. The LIDAR receiver system consists of Telescope Meade LX200 along with various receiving optics. The telescope has a 4064 mm focal length.The telescope is connected to the receiving optics assembly by a 1-mm Polymicro Technologies fiber optic cable with a numerical aperture of 0.22 that transmits the received signal from the telescope to the receiver assembly, which houses the PMT and beam dumps, and Photon Couter Module 8 bit. The space qualifiable laser sends 20 Hz laser pulses into the atmosphere during a LIDAR experiment. The 532 nm, and 1064 nm laser outputs are always transmitted and give returns from the atmosphere, the example of the measurement is only interested in the atmospheric aerosol and cloud returns from the 532 nm laser output. The telescope collects the return pulses from the atmosphere and focuses them into a 1 mm fiber optic cable, which is connected to the receiving optics. 77 N. Q. Lan, L. H. Hoang, N. V. Khanh, N. V. Minh, N. A. Vinh and D. V. Trung Figure 3. Signal to noise ratio depends on the distant, reached 1 at 63 km with 20 minutes average at 70 km in 60 minutes Figure 4. Profile of atmospheric molecules and distance The LIDAR signal is then detected by an avalanche photodiode (Hamamatsu S9251) in the 1064 nm channel and by a photomultiplier tube (Hamamatsu R7400U) in the 532 nm channel. A block diagram showing the details of our LIDAR system is presented in Figure 1. The signals coming from the detectors are then processed in either analog or photon counting mode. In analog mode, the signals are directly digitized by a compact Pico scope 200 MHz oscilloscope and transferred to computer through the USB port. To detect the faint LIDAR signal from large distances the photon counting technique is commonly used. However, this technique requires fast electronics to process and count the short electric pulses coming out of the photomultiplier tubes. Commercially available photon counting modules are still expensive, costing upward of ten thousand USD. Therefore we choose to use a pulse stretcher with the widely available and quite inexpensive high speed USB oscilloscope for photon counting. The short single photon pulses (FWHM ≈ 1.5 ns) coming out of the photomultiplier tube R7400U (from Hamamatsu) are amplified by the high speed amplifier configured to obtain a very high gain of 90. As the result, the amplified photon pulses are stretched to a FWHM of ≈ 10 ns. These broader pulses are then sampled by the Pico scope oscilloscope at a sampling rate of 100 Ms/s. True photon pulses are then discriminated by setting a pre-selected voltage level. The counting of photons and forming histography are done either in real time by a program written in Lab view or offline by another routine written in Fortran. 78 Studies and design lidar system using 40 cm telescope 2.2. Initial measurements with the light detection and ranging system Our LIDAR system has been used to make simultaneous measurements at two wavelengths 532 nm and 1064 nm. An example of the measured LIDAR signal is shown in Figure 2. The data clearly shows the complex structure of aerosol layers, which are present up to 3000 m. In addition, the sensitivity of our system allows us to easily detect the presence of cirrus clouds located even at height of 10 -15 km. We use the inversion method of Fernald [1] and Klett [2] to derive the backscattering coefficients at 532 nm and 1064 nm. The LIDAR ratio S is the ratio of extinction to backscattering, of aerosols are assumed to be 70 at 532 nm and 50 at 1064 nm, which are typical for continental type aerosols (Ackermann [3]). LIDAR ratios dependent on properties of aerosols like size, matter of the aerosols (Kovalev and Eichinger [4]). LIDAR ratio have valid in the period 20 to 35 sr for aerosol on the sea, 35 to 70 sr for aerosol from industry or pollution and 70 to 100 sr for aerosol from agriculture (Ackerman [3]). When physical properties of aerosol like size does not change to altitude, the distribution of aerosol density with altitude z ratio the backscattering coefficients(z) will determine by LIDAR method. In particular, to reduce ratio error on calculation LIDAR equation dependent on LIDAR ratio S, we need to determine integrated atmospheric optical depth. This measurement can use sun photometer on daytime and star photometry at night. Lancioco and Fiocco (2007) have been using relatively method very effectively and LIDAR system at the 532 nm to determine accuracy ratio the backscattering coefficients and LIDAR ratio S of the aerosol. 3. Conclusion This work we have calculated and simulated the model for the LIDAR system in order to have the best design, and this LIDAR system allow us to measure at atmosphere up to more than 10 km. Our calculation lets us know that this LIDAR system can be measured up to 70 km of the High Altitude of the atmosphere. Acknowledgments. This research was support in part by the Ministry of Education Grant No. B2014-17-45. REFERENCES [1] Frederick G. Fernald, 1984. Applied Optics. Vol. 23, Issue 5, pp. 652-653. [2] James D. Klett, 1981. Applied Optics. Vol. 20, Issue 2, pp. 211-220. [3] J. Ackermann, 1998. J. Atmos. Ocean. Techn., 15, 1043 1050. [4] Vladimir A. Kovalev, William E. Eichinger, 2004. Elastic LIDAR: Theory, Practice, and Analysis Methods. 79
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