Range Dependent Turbulence Characterization by Co-operating Coherent Doppler Lidar with Direct Detection Lidar

Transcription

1 Range Dependent Turbulence Characterization by Co-operating Coherent Doppler idar with Direct Detection idar Sameh Abdelazim(a), David Santoro(b), Mark Arend(b), Sam Ahmed(b), and Fred Moshary(b) (a)fairleigh Dickinson University(FDU) (b)the City College of New York (CCNY)

2 Outline Introduction System overview CD backscattered signals range dependence Analytical vs. experimental SNR range dependence Wind measurement Direct detection vs. CD Sameh Abdelazim 18th Coherent aser Radar Conference

3 INTRODUCTION A CD system for wind sensing was developed and operated at the Remote Sensing aboratory at the City College of New York (CCNY) The system is capable of measuring vertical wind velocity for up to 3km A direct detection lidar is also operated at the CCNY and is used to validate the CD measurements. Effects of range and turbulence on signal to noise ratio (SNR) of backscattered signals of the CD system are examined. Sameh Abdelazim 18th Coherent aser Radar Conference 3

4 All fiber based eye safe laser system. Mobile wind speed measuring station. A 1.5 µm polarization maintained fiber optics. Heterodyne balanced detection to suppress the RIN (Relative Intensity Noise) ADC (Analog to Digital Converter) using a data acquisition card with FPGA on board. Sameh Abdelazim 18th Coherent aser Radar Conference 4

5 Direct Detection idar System at The City College of New York For comparing vertical profile of aerosols Detection, control, data storage equipment Telescope, laser, optics Sameh Abdelazim 18th Coherent aser Radar Conference 5

6 The CD Backscattered Signals Range Dependence The concept of back projected local oscillator (BPO) is used to evaluate the SNR range dependence of a CR monostatic system. The BPO is the imaginary local oscillator field distribution projected at the target side of the receiver aperture, receiver lens, originating from the detector [4,7,8]. Analysis shows that the SNR is proportional to the product of direct detection power and heterodyne efficiency Sameh Abdelazim 18th Coherent aser Radar Conference 6

7 Range Dependence cont d The calculation of received power and SNR requires mutual coherence function of the backscattered field incident on the receiver. As for natural aerosol targets, backscattered field at each aerosol particle has a random phase, and the mutual coherence function of the total backscattered field is the integration of all mutual coherence functions from each aerosol particle. The SNR range dependence [5] is expressed as: where: / ) ( ) ( hb D K E SNR D ) ( 4 ) ( 1 1 ) ( o S D C A D C A D F total Sameh Abdelazim 18th Coherent aser Radar Conference 7

8 Range Dependence cont d The system efficiency factor describes the loss of the SNR due to a mismatch between the O and backscattered fields as a result of loss of coherence of the backscattered field. The loss of coherence is caused by: D ( ) 1 a) incoherent aerosol targets, which causes its field phase-front curvature and O to mismatch 1 F total ( A D) C 4 Sameh Abdelazim 18th Coherent aser Radar Conference 8 A D C S o ( ) b) propagation through the atmospheric refractive turbulence, which also causes an expansion of the transmitted beam resulting a larger incoherent image at the target

9 SNR (db) Analytical vs. Experimental SNR Range Dependence SNR (experimental) SNR (analytical) Distance (m) The parameters used are listed in Table 1. Sameh Abdelazim 18th Coherent aser Radar Conference 9

10 CD Backscattered Signals Range Dependence Table 1. Parameters corresponding to analytical estimation of wideband SNR range dependence Parameter Description Value Range (m) B Bandwidth 100 MHz λ Wave length μ E Pulse Energy 7 μj D Effective aperture Diameter 0.1 m τ Pulse width 00 ns β atmospheric backscatter coefficient 8.3x10-7 /m/sr K one way atmospheric transmittance 0.95 /km F Focal Range of Optical Antenna 5 km A c Correction Factor Ref. [10] 0.76 Cn Refractive Index Structure Constant x10-14 m -/3 η total Total system efficiency -6 db S o () Transverse coherent length ~ (1.1 kw Cn ) -/3 kw Wave number π/ λ Sameh Abdelazim 18th Coherent aser Radar Conference 10

11 Signal power (a) and Vertical Wind velocity (b) vs. height and time measured on 7/1/01 Sameh Abdelazim 18th Coherent aser Radar Conference 11

12 Range corrected signal power vs. height and time compared with 1 μm direct detection measured on 7/1/01 Coherent Detection Direct Detection Sameh Abdelazim 18th Coherent aser Radar Conference 1

13 Vertical Wind velocity (a) and signal power (b) vs. height and time measured on 8/17/011 a b Sameh Abdelazim 18th Coherent aser Radar Conference 13

14 Range corrected signal power vs. height and time compared with 1 μm direct detection measured on 8/17/011 Coherent Detection Direct Detection Sameh Abdelazim 18th Coherent aser Radar Conference 14

15 CONCUSION Turbulence and range effects on CD backscattered signals were corrected for to allow for comparison signals power with those of a direct detection lidar. Both lidars show a strong agreement of aerosol profiles and cloud pattern which validates measurements of the developed CD instrument Sameh Abdelazim 18th Coherent aser Radar Conference 15

16 REFERENCES [4] M. J. Kavaya and F. G. Rod, "Coherent laser radar performance for general atmospheric refractive turbulence," Appl. Opt., vol. 30, pp , [5] S. F. Clifford and S. Wandzura, "Monostatic heterodyne lidar performance: the effect of the turbulent atmosphere," Appl. Opt., vol. 0, pp , [6] S. M. Wandzura, "Meaning of quadratic structure functions," J. Opt. Soc. Am., vol. 70, pp , [7] M. J. Kavaya, R. T. Menzies, D. A. Haner, U. P. Oppenheim and P. H. Flamant, "Target reflectance measurements for calibration of lidar atmospheric backscatter," Appl. Opt., vol., p , [8] Y. Zhao, M. J. Post and R. M. Hardesty, "Receiving efficiency of monostatic pulsed coherent lidars. : Applications," Applied Optics, vol. 9, pp , [9] D.. Fried, "Optical heterodyne detection of an atmospherically distorted signal wave front," IEEE, vol. 55, pp , [10] S. Kameyama, T. Ando, K. Asaka, Y. Hirano and S. Wadaka, "Performance of discrete- Fourier-transform-based velocity estimators for a wind sensing coherent Doppler lidar system in the Kolmogorov turbulence regime," IEEE Transanction on Geoscience and Remote Sensing, vol. 47, no. 10, pp , 009. Sameh Abdelazim 18th Coherent aser Radar Conference 16

17 THANK YOU Q& A Sameh Abdelazim 18th Coherent aser Radar Conference 17