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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 11, NOVEMBER 2003
Laser “Firefly” Clustering; A New Concept in Atmospheric Probing Debbie Kedar and Shlomi Arnon
Abstract—Profiling the chemical and particulate composition of the atmosphere is of growing importance as awareness of the role of pollution monitoring increases and deliberate contamination by hostile elements becomes a sad reality. Low cost, flexible, safe and mobile solutions are sought. In this letter, we propose an innovative concept of distributed probes in which miniature, low power lasers are deployed in situ at the altitude of interest. We term these laser transmitters “fireflies.” The emitted light is at eye safety levels and is modulated by orthogonally coded signals, which make it possible to distinguish between the transmitters. The light is scattered by the aerosols and molecules in the locality of the firefly and received on the ground. Analysis of the received signals makes it possible to obtain a spatial map of the atmospheric constituents without having to perform mechanical scanning It is shown that signal-to-noise ratios (SNRs) of above 10 dB can be achieved when using more than 1700 fireflies at an altitude of 1 km for a midlattitude summer clear night scenario. At half the altitude, 350 fireflies suffice to achieve the same SNR for the same environment. Index Terms—CDMA, distributed sensing, lidar.
I. INTRODUCTION
T
HERE is a growing need for collecting data on the aerosol and molecular composition of the atmosphere for pollution monitoring, meteorological forecasting, and scientific research. With the threat of unconventional warfare, the need for easily deployable probing equipment, often in unpredictable and remote locations, has become urgent and existing methods are inadequate due to their bulk and cost. The two common techniques for obtaining data on atmospheric constituents are in situ probing by means of balloons or aircraft, and remote sensing using lidar systems. With in situ probing, the location of the atmosphere under investigation is known, but must be accessible, and the balloon or aircraft are exposed and vulnerable. Light detection and ranging (lidar) is a widely used atmospheric probing technique whereby a high energy pulse of laser light is launched into the atmosphere and backscattered from gas molecules and particulates suspended at different altitudes. The time delay of the return pulse indicates the range of the probed data, and the intensity of the detected signal attests to the amount of scattering particles. Safety considerations restrict the use of lidar and the equipment is expensive, but the results can be very accurate and spatial resolution is high. In this letter, we propose an innovative lidar-style probing method, where the backscattering effect of particulates genManuscript received March 27, 2003; revised June 18, 2003. This work was supported by the DIP Fund (Israeli-German research fund) for the BLISL project and by the Ontar Corporation for supplying the MODTRAN program. The authors are with the Satellite and Wireless Communication Laboratory, Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, IL-84105 Beer-Sheva, Israel (e-mail:
[email protected];
[email protected]). Digital Object Identifier 10.1109/LPT.2003.818682
Fig. 1. Firefly scenario.
erates return signals from emitted laser light. The laser light is produced in situ, at the altitude of interest, by clusters of miniature, low power laser transmitters. The vertically emitted laser beam is modulated by a uniquely coded pulse sequence, while a second, downward signal, at a different wavelength but with the same code, facilitates synchronization (see Fig. 1). The backscattered light is detected at four terrestrially located receivers, which may be fixed or mobile. The distinction between the received backscattered signals from each laser firefly is accomplished by virtue of the orthogonality of the code sequences, using CDMA analysis methods [1]. The position of each laser firefly is calculated by measuring the code sequence time delay at the four receivers and using triangulation methods. Thus the features of remote and in situ sensing are combined to yield precise aerosol spatial characteristics in a safe, secure and potentially inexpensive way. We call our in situ distributed transmitters “firefly” clusters.
1041-1135/03$17.00 © 2003 IEEE
KEDAR AND ARNON: LASER “FIREFLY” CLUSTERING; A NEW CONCEPT IN ATMOSPHERIC PROBING
The firefly transmitters in our study are based on very small, lightweight, low-power, potentially inexpensive semiconductor lasers with on-board power supplies (see Fig. 1). The potential cost advantage of firefly transmitters is augmented by the safety benefit as the laser light power per firefly is within eye-safety restrictions. The light is emitted at the altitude of interest and only very lower power levels of scattered light reaches the ground after atmospheric attenuation. Hence, the fireflies might be useable in urban areas or airports, where conventional lidar is hazardous. Furthermore, atmosphere-induced signal degradation, due to the round trip of the light from the transmitter to the atmosphere and back to ground in conventional lidar systems, is halved in the firefly system, where the light travels only from the stratum of interest to the ground. An additional feature of our proposed system is that a number of clusters of fireflies can be distributed with differing densities at a wide range of atmospheric locations. In this way, spatial distribution information can be obtained without having to provide a means for the receiver telescope to mechanically scan the atmosphere, as is necessary with conventional ground-based lidar probing. Alternatively, the transmitters may fall under gravitational force so that an altitude profile of atmospheric constituents can be traced. An additional option could be to obtain a temporal profile of atmospheric composition, if the fireflies are stationary. We have outlined the potential of the firefly cluster probing method conceptually. The hardware required may be implemented by means of microoptoelectromechanical systems (MOEMS) technology and simple semiconductor laser products. Methods to maintain the upright orientation of the firefly to ensure vertical emission and backscattering and to position the fireflies are not treated herein, but we are confident that these issues can be dealt with creatively by engineers in the relevant disciplines [2]. II. MATHEMATICAL MODEL A. Signal and Noise CDMA is a multiple access communication scheme based on spread spectrum methods, wherein digital data is multiplied by a data-independent, spreading signal that increases the bandwidth of the transmitted signal. A commonly used spreading signal is ) 1, which has a time-ava pseudorandom sequence of ( and eraged value of zero due to the near-equiprobability of . If a number of signals are spread using mutually orthogonal sequences, then multiple users can share the same communication channel simultaneously using the same carrier frequency, since each signal can be extracted only by correlating with its unique code. The length of the sequence, measured in chips, determines the number of orthogonal codes that can be generated. The spreading signal is, thus, characterized by its chip rate (or the reciprocal, chip time), which is approximately the modulation bandwidth. In the case of a firefly cluster, we adopt the CDMA multiple access technique to enable exclusive extraction of a given firefly signal from the total power detected at the receiver. In a laser firefly cluster, the transmitted power from the th laser firefly is given by (1)
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where is the time-averaged power and is the pseudo random sequence for laser firefly , which is orthogonal to all . other The power received by the th receiver from the th firefly due to molecular and aerosol scattering is described by
(2) is the distance between the th firefly and the where th receiver, is the receiver aperture size, and are the backscattering cross sections for molecules and aerosols respectively per unit atmospheric length and at the , is the attenuation coefficient; position defined by is the code sequence chip duration and is the speed of light. The received signal is converted to an electronic current and by an avalanche photodiode (APD) with responsivity gain , and multiplied by the appropriate sequence. Assuming that the modulation power has a mean value of zero, the current . from the output of the multiplier is The molecular scattering cross section is further dewhere fined as is the differential Rayleigh backscattering cross section per “average” gas molecule in units of , and is the number of gas molecules per unit volume [3], [4]. For the mixture of the atmospheric gases which occurs below about 100 km this has been found empirically to be well approx, where imated by is assumed to be m , and is in microns. Aerosol particles are too large to be treated as Rayleigh scatterers and Mie scattering is operative. The aerosol scattering cross section is a complex function of the size parameter of each particle (the ratio between actual radius and radiation wavelength), the particle shape and the index of refraction of the particle. The backscattering is an oscillatory function of size parameter with damped oscillations and lower values as absorption increases. Approximate values for the backscattering cross sections have been tabulated for various atmospheric conditions, and these will act as a source for the numerical examples [5]. Assuming all noise sources are independent, the overall noise power accompanying the signal from firefly “ ” at the th reorthogceiver, including multiuser interference from the onally coded fireflies can be described by [6] (3) where
is the signal shot noise, given by (4)
is the electron charge, is the signal bandwidth and is the excess noise factor of the APD. is the thermal noise given by , where is Boltzmann’s constant, is the equivalent temperature and is the receiver reis the background noise given by sistance.
where
(5)
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 11, NOVEMBER 2003
where the expression in brackets is the background noise power , due to solar or lunar and stellar light sources, defined by the radiance at receiver in units of ( m), , the optical filter bandwidth in microns and , the telescope field , is the dark current noise given by of view in steradians., , where is the dark current in is the multiuser interference noise, given the th receiver. by (6) is given by and is the modulation rate where of the code. In summary, the signal-to-noise ratio (SNR) can be described by Fig. 2. Graph of SNR versus number of fireflies within cluster for three different probed altitudes from numerical example.
(7)
where the nominator is the total received signal power from the cluster, summed for the four receivers, and the denominator is the total noise power for the signal. If more than one cluster is used (7) should be expanded accordingly. B. Location Measurement The unique coding of each firefly enables distinction between the backscattered signals, which enables precise local profiling of the atmosphere. In order to determine the location of a ), we measure the transit single firefly positioned at ( time delays of a signal emitted from the firefly to each of the ), ( ), four receivers which are positioned at ( ), ( ) in the same co ordinate system. The ( distances from a firefly to each receiver are not the same so that different transit times are incurred from the firefly to each receiver. The delay between receiver and receiver 1 is . Thus ) by simultaneously solving the three we can find ( , and . difference equations for III. NUMERICAL CALCULATION AND RESULTS A Midlatitude Summer, clear night model was selected for the numerical example. A densely packed cluster of fireflies (within the order of one m ) was positioned at different altitudes. The background radiation noise was calculated using the MODTRAN atmospheric transmissivity program. The backscatter coefficients and optical densities were derived by interpolating the atmospheric data from [5]. The precise molecular atmospheric composition for different altitudes is listed elsewhere in [5]. A wavelength of 400 nm was chosen to enhance the Rayleigh scattering effect. The power output of 5 mW from each laser ensures eye-safety. Assuming s, Hz, and m, we simulated the performance of the scenario shown in Fig. 1. The SNR was calculated as a function of the number of fireflies in the
cluster for three altitudes (see Fig. 2). We assume insignificant mutual scattering within the dense cluster. We observed that for a small number of fireflies in a cluster, the SNR is low due to thermal noise. As more fireflies are added to the cluster, the signal and signal-related noise (shot noise and multiuser noise) predominate and the noise-repressing characteristic of spread spectrum communications becomes evident. Beyond a “knee” point, which is altitude sensitive, the SNR levels off and is neither improved nor degraded as more fireflies are added to the cluster. For example, an SNR of 10 dB is achieved with 1700 fireflies located at an altitude of 1 km, and for 350 fireflies at an altitude of 500 m. IV. CONCLUSION A novel system for atmospheric probing has been presented using concepts from CDMA communications and classic lidar theory. It was shown that acceptable levels of SNR can be achieved for clusters of several hundred fireflies with very low power, eye-safe lasers at altitudes of several hundred meters. Future work using different environments will be performed. Another notion to be investigated is the deployment of several clusters of fireflies at different locations, which will also be the subject of future research. REFERENCES [1] N. Takeuchi, N. Sugimoto, H. Baba, and K. Sakurai, “Random modulation CW lidar,” Appl. Opt., vol. 22, no. 9, pp. 1382–1386, 1983. [2] J. Yan, S. A. Avadhanula, J. Birch, M. H. Dickinson, M. Sitti, T. Su, and R. S. Fearing, “Wing transmission for a micromechanical flying insect,” J. Micromechatron., vol. 1, no. 3, pp. 221–237, 2002. [3] E. J. McCartney, Optics of the Atmosphere. New York: Wiley, 1976. [4] R. T. H. Collis and P. B. Russell, “Lidar measurement of particles and gases by elastic backscattering and differential absorption,” in Topics in Applied Physics, E. D. Hinkley, Ed. Berlin, Germany: SpringerVerlag, 1976, vol. 14, Laser monitoring of the atmosphere, ch. 4. [5] R. A. McClatchey, R. W. Fenn, J. E. A. Selby, F. E. Volz, and J. S. Garing, “Optical Properties of the Atmosphere (Revised),” Air Force Cambridge Research, Environmental research papers, no. 354, 1971. [6] G. P. Agrawal, Fiber-Optic Communication Systems. New York: Wiley, 1997.
