Proceedings Volume 6457

Free-Space Laser Communication Technologies XIX and Atmospheric Propagation of Electromagnetic Waves

Steve Mecherle, Olga Korotkova
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Proceedings Volume 6457

Free-Space Laser Communication Technologies XIX and Atmospheric Propagation of Electromagnetic Waves

Steve Mecherle, Olga Korotkova
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 6 February 2007
Contents: 6 Sessions, 27 Papers, 0 Presentations
Conference: Lasers and Applications in Science and Engineering 2007
Volume Number: 6457

Table of Contents

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Table of Contents

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  • System Demonstrations
  • Acquisition, Pointing, and Tracking
  • Receiver and Modulation Technologies
  • Modeling, Simulation, and Experiments Involving Propagation of Electromagnetic Waves in Atmospheric Turbulence
  • Mitigation of Atmospheric Effects, Adaptive Optics, and Applications for Laser Radar Systems and Laser Communications through the Atmosphere
  • Wave Propagation in Non-classical (Non-Kolmogorov) Turbulence and Propagation of Pulses
System Demonstrations
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Overview of the inter-orbit and the orbit-to-ground laser communication demonstration by OICETS
Takashi Jono, Yoshihisa Takayama, Koichi Shiratama, et al.
The experiment results on the inter-orbit laser communications between OICETS and a geostationary satellite and the results of two kinds of orbit-to-ground laser communications between OICETS and ground stations are summarized. The geostationary satellite for the inter-orbit demonstrations is the European Space Agency's geostationary satellite, ARTEMIS, and the ground stations for the orbit-to-ground demonstrations are of the National Institute of Information, and Communications Technology (NICT) in Japan and the German Aerospace Center (DLR), respectively. The descriptions of those experiments contain some statistically analyzed results as well as data samples measured during the demonstrations. The authors present the overview of these demonstration progresses and discuss on the results.
Homodyne BPSK-based optical inter-satellite communication links
Robert Lange, Berry Smutny
Summer 2007, Tesat will verify laser communication terminals based on homodyne BPSK (binary phase shift keying) in-orbit. A 5.625 Gbps LEO-LEO laser communication link, established between the German satellite TerraSAR-X and the US satellite NFIRE, shall demonstrate the performance and advantages of laser communication. End of 2006, a further program has been kicked-off to demonstrate the performance of ~2 Gbps LEO-GEO laser communication links. The link is part of a data relais from the German LEO satellite TanDEM-X via a Geo satellite to ground. The LEO-to-GEO laser commmunication link can be extended to further ~2 Gpbs GEO-GEO, and GEO-to-ground links.
Results of the optical downlink experiment KIODO from OICETS satellite to optical ground station Oberpfaffenhofen (OGS-OP)
Optical LEO downlinks from the Japanese OICETS to the optical ground station built by the German Aerospace Center (DLR) near Munich have been performed. This was the first optical LEO downlink on European grounds. The ground station received a 50-Mbit/s OOK signal at 847 nm on its 40-cm Cassegrain telescope and sent two spatially displaced beacon beams towards OICETS. Five out of eight trials could be performed successfully while the other three were hindered by cloud blockage. A BER of 10-6 has been reached. The elevation angle above the horizon ranged between 2° and 45°. The Fried parameter and the scintillation were measured with instruments inside the ground station. The beacon power received by the LUCE Terminal onboard OICETS has also been recorded. This paper describes the setup of the experiment and highlights the results of the measurement trials.
Multichannel high-data-rate transmission between ground and airborne platforms
This paper describes the results of an experiment where an FSO link was established between a ground terminal and an airborne terminal on a tethered aerostat. In the first part of the experiment, four 10 Gb/s WDM channels were transmitted from the aerostat to the ground terminal, demonstrating error-free performance over many 5 minute windows. In the second part of the experiment, both one and two WDM channels, each running at a native 40 Gb/s rate, were transmitted over the FSO link, demonstrating error free performance over short (30 second) timeframes, and bit error rates in the 10-6 range over longer term operation. During all tests, a 1 Gb/s optical Ethernet channel was run on a separate WDM channel. The paper will describe the test configuration as well as the results when running both single and multiple channels. The impact of the overall system design used for this experiment on the link integrity will be analyzed and discussed. It was found that the spatial dynamics of the aerostat as a function of time of day had an impact on the quality of the link - this will also be discussed. Modeling of potential penalty sources will also be discussed.
OPTEL terminal for deep space telemetry links
Guy Baister, Thomas Dreischer, Michael Tüchler, et al.
Contraves Space AG is currently developing the OPTEL family of optical terminals for free-space optical communications. The optical terminals within the OPTEL family have been designed for future opportunities open to this technology. These opportunities range from commercial optical satellite crosslinks between geostationary (GEO) satellites, deep space optical links between planetary probes and the Earth, as well as optical links between airborne platforms (either between the airborne platforms or between a platform and satellite). The OPTEL terminal for deep space applications has been designed as an integrated RF-optical terminal for telemetry links between the science probe and Earth. The integrated architecture provides increased TM link capacities through the use of an optical link, while spacecraft navigation and telecommand are ensured by the classical RF link. The optical TM link employs pulsed laser communications to achieve robustness on the link to atmospheric degradation at the optical ground station. This paper will present an overview of the system architectures that the Contraves Space family of OPTEL terminals have been designed to support and will focus on the design of the deep space optical communications terminal.
Acquisition, Pointing, and Tracking
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Tracking and pointing characteristics of OICETS optical terminal in communication demonstrations with ground stations
The tracking and pointing performance of the Laser Utilizing Communications Equipment (LUCE) equipped on the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) is presented. The operation characteristics of LUCE observed in the ground-satellite communication demonstrations are focused on. Since the angular movement of LUCE's optical antenna required in the ground-satellite trials exceeds the specification demanded in the inter-satellite communications, the marginal performance for proper tracking and pointing can be observed. During the ground-satellite communication trials, the ground stations found periodical discontinuances in the optical link. By looking at the LUCE's telemetries, the cause of the repeated breaks is revealed.
Hybrid beam steering system for laser communication between mobile platforms
For many practical needs, laser communication systems must support operation between mobile platforms. Engineering robust links; however, will depend on several innovations. In particular, successful pointing, acquisition, and tracking (PAT) require the use of a beacon signal and the capability of accurate and agile alignment of the line-of-sight (LOS) between the communicating terminals performed over a large field of regard. While mechanical devices, such as gimbals, offer relatively slow tracking over a very wide range, they lack in pointing bandwidth necessary for rejecting high frequency vibrations and beam deflection caused by the optical turbulence. In contrast, fast steering and especially non-mechanical devices, such as Bragg cells, enjoy very high bandwidth (on the order of several kHz), but their effective range is very small. Inherent limitations of both gimbals and fast steerers result in shortcomings of the entire PAT system when either of these devices is used as a sole beam steerer. Therefore, focus needs to be shifted to hybrid architectures, exploiting the advantages of the constituting elements. This paper demonstrates a system combining a robotic manipulator with two acousto-optic cells and presents the algorithm development and the simulation results.
Infrared Earth tracking for deep-space optical communications: feasibility study based on laboratory emulator
Yijiang Chen, Jeffrey Charles, Hamid Hemmati, et al.
We report on laboratory emulator experiments performed to assess the feasibility of infrared (IR) Earth image tracking for optical communications. A quantum well infrared photodetector (QWIP) is evaluated and characterized for this purpose. For proof of concept, an emulator is designed and implemented in the laboratory to generate IR Earth images over the range of 0.1 to 40 AU at the Focal Plane Array of the QWIP camera to evaluate feasibility of using IR Earth images as opticomm pointing reference. IR Earth images measured from the testbed emulator are compared with the corresponding images derived from Earth thermal modeling and the two are found to be in good agreement. IR Earth image non-uniformity is also emulated in the laboratory testbed, demonstrating that for characteristic deep space optical apertures, the image is dominated by the Earth thermal non-uniformity at short distance (<2 AU) but is overwhelmed by diffraction blurring beyond 4 AU. Furthermore, preliminary laboratory testbed investigation on centroiding accuracy of IR Earth center detection shows improved angular centroiding accuracy with increasing image size and IR signal level.
Receiver and Modulation Technologies
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Signal acquisition and timing for a free space laser communications receiver
George Zogbi, Lawrence M. Candell
NASA anticipates a significant demand for long-haul communications service from deep-space to Earth in the near future. To address this need, a substantial effort has been invested in developing a novel free-space laser communications system that can be operated at data rates that are 10-1000 times higher than current RF systems. We will focus here on the receiver design which consists of a distributed array of telescopes, each with a Geiger-mode Avalanche Photo Diode (APD) array capable of detecting and timing individual photon arrivals to within a fraction of a nanosecond. Using an array of telescopes has the advantage of providing a large collection area without the cost of constructing a very large monolithic aperture. A key challenge of using a distributed array receiver is combining the detected photons from each of the telescopes so that the combined system appears as a single large collector. This paper will focus on the techniques employed by the receiver to spatially acquire a deep-space downlink laser signal, synchronize the timing of all the photon arrivals at each telescope, and combine the photon detections from each telescope into a single data stream. Results from a hardware testbed utilizing this receiver concept will be shown that demonstrate an efficiency of less than one incident photon per bit at data rates up to 14 Mbps, while operating within 1 dB of the channel capacity.
Mitigation of optical turbulence effects using a modified simplex optimization approach: experimental study
Dynamically changing turbulence in the atmosphere distorts the wavefront of the laser beam propagating through it. The resulting spatial and temporal fields of the refractive index lead to performance degradation in the form of reduced signal power and increased BER, even for short link ranges. An electrically addressed liquid crystal spatial light modulator (SLM) can be used to correct the optical path difference (OPD) pattern resulting from the atmospheric distortions. Approximating the phase profile of the distorted beam using well-known Zernike formalism reduces the complexity of controlling each pixel of the SLM. Real time correction of the wavefront can be achieved using the Simplex optimization procedure by Nelder and Mead. Previously, some modifications have been proposed to overcome the local minima problems as well as the faster convergence. Yet the better and faster performance could be achieved by more accurate prediction of the simplex initialization along with the modifications in the simplex procedure. This paper presents the experimental results of such modifications to the earlier proposed system.
Improving bit error rate through multipath differential demodulation
Yannick Keith Lize, Louis Christen, Scott Nuccio, et al.
Differential phase shift keyed transmission (DPSK) is currently under serious consideration as a deployable datamodulation format for high-capacity optical communication systems due mainly to its 3 dB OSNR advantage over intensity modulation. However DPSK OSNR requirements are still 3 dB higher than its coherent counter part, PSK. Some strategies have been proposed to reduce this penalty through multichip soft detection but the improvement is limited to 0.3dB at BER 10-3. Better performance is expected from other soft-detection schemes using feedback control but the implementation is not straight forward. We present here an optical multipath error correction technique for differentially encoded modulation formats such as differential-phase-shift-keying (DPSK) and differential polarization shift keying (DPolSK) for fiber-based and free-space communication. This multipath error correction method combines optical and electronic logic gates. The scheme can easily be implemented using commercially available interferometers and high speed logic gates and does not require any data overhead therefore does not affect the effective bandwidth of the transmitted data. It is not merely compatible but also complementary to error correction codes commonly used in optical transmission systems such as forward-error-correction (FEC). The technique consists of separating the demodulation at the receiver in multiple paths. Each path consists of a Mach-Zehnder interferometer with an integer bit delay and a different delay is used in each path. Some basic logical operations follow and the three paths are compared using a simple majority vote algorithm. Receiver sensitivity is improved by 0.35 dB in simulations and 1.5 dB experimentally at BER of 10-3.
Chaotic communication in radio-over-fiber system based on optoelectronic feedback semiconductor laser
Meng-Chiao Tsai, Fan-Yi Lin
Chaotic communication in radio-over-fiber (ROF) system based on optoelectronic feedback (OEF) semiconductor laser is studied numerically. Performance of different message encoding schemes, including additive chaotic modulation (ACM) and on-off shift keying (OOSK), are compared. By taking into account the attenuation, nonlinearity, dispersion, and amplifier spontaneous emission noise in the fiber module and the path loss, additive white Gaussian noise, and multipath effect in the wireless channel, system performance including quality factor Q and bit-error-rate for different transmission lengths and message bit-rates are investigated. While both schemes are possible to achieve a BER < 10-9 of the benchmark set by the conventional communication, we have demonstrated that the OOSK scheme is better suitable for long distance RoF transmission. On the contrary, while the ACM scheme is sensitive to the perturbation in the wireless channel, it shows moderate decoding results in the fiber-only scenario. In this paper, effects of parameters mismatch between the synchronized transmitter and receiver lasers and the security of this communication system are also addressed.
Design and evaluation of a virtual quadrant receiver for 4-ary pulse position modulation/optical code division multiple access (4-ary PPM/O-CDMA)
A. J. Mendez, V. J. Hernandez, R. M. Gagliardi, et al.
M-ary pulse position modulation (M-ary PPM) is an alternative to on-off-keying (OOK) that transmits multiple bits as a single symbol occupying a frame of M slots. PPM does not require thresholding as in OOK signaling, instead performing a comparison test among all slots in a frame to make the slot decision. Combining PPM with optical code division multiple access (PPM/O-CDMA) adds the benefit of supporting multiple concurrent, asynchronous bursty PPM users. While the advantages of PPM/O-CDMA are well known, implementing a receiver that performs the comparison test can be difficult. This paper describes the design of a novel array receiver for M-ary PPM/O-CDMA (M = 4) where the received signal is mapped onto an xy-plane whose quadrants define the PPM slot decision by means of an associated control law. The receiver does not require buffering or nonlinear operations. In this paper we describe a planar lightwave circuit (PLCs) implementation of the receiver. We give detailed numerical simulations that test the concept and investigate the effects of multi-access interference (MAI) and optical beat interference (OBI) on the slot decisions. These simulations provide guidelines for subsequent experimental measurements that will be described.
Modeling, Simulation, and Experiments Involving Propagation of Electromagnetic Waves in Atmospheric Turbulence
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Simulating partially coherent fields and other special beam classes in turbulence
Optical coherence theory traditionally deals with the properties of randomly fluctuating fields over long time averages. For certain applications (such as optical communications) and special beam classes (such as vortex beams), however, the averaging process can obscure important physical aspects of the field behavior. We demonstrate a new method of simulating partially coherent fields of nearly arbitrary spatial and temporal coherence. Over sufficiently long time intervals, these simulations produce the expected average properties. The results are used to gain insight into the propagation properties of partially coherent fields through atmospheric turbulence.
Propagation of beams with any spectral, coherence, and polarization properties in turbulent atmosphere
The combination of the angular spectrum representation (in space-frequency domain) and of the Rytov perturbation theory is applied for description of the second-order statistical properties of arbitrary (coherent and partially coherent) stochastic fields (whether scalar or electromagnetic) which propagate in turbulent atmosphere. The analysis is restricted to weak regime of atmospheric fluctuations. We first introduce the new method for scalar fields and derive expressions for the cross-spectral density function, from which the spectral and the coherence properties of the propagating fields can be determined. Next we extend the new technique to electromagnetic domain, i.e. we derive expressions for the elements of the 2x2 cross-spectral density matrix of the electric field from which its spectral, coherence and polarization properties can then be found. We illustrate the new method by applying it to propagation of several model beams through the atmosphere. In particular, we consider Gaussian beam, Bessel beam, Gaussian Schell-model beam in their scalar or electromagnetic versions. We find that the results obtained on the basis of the new theory are in good agreement with those obtained earlier by standard techniques.
Propagation of partially coherent beams in a convective medium
Atmospheric conditions change throughout the day, from turbulence to convective conditions depending on solar irradiation and soil temperature. It is a necessity for optical telecommunications technologies to study propagation of partially coherent beams under convective atmospheric conditions because these are predominant during daytime in tropical regions. In this work, a theoretical and experimental study of the propagation of partially coherent J0 Bessel beams trough a convective medium is presented. Experimental results show that phase singularities are preserved.
Atmospheric turbulence profiling by detecting the wave function of a test beam
We demonstrate for the first time that the wave-function of a laser beam that has been distorted by a turbulent atmosphere can be measured instantaneously with a phase-shifting interferometer. Direct detection of the wave-function enables deriving the correlation function of various orders, including the mutual coherence function and other parameters that cannot be measured directly by existing methods. Experimental results are presented employing the proposed method to characterize a beam that was transmitted through a random phase-screen that imposed a Kolmogorov phase perturbation spectrum onto the beam.
Scintillation index for N Gaussian laser beams with different wavelengths in weak atmospheric turbulence
We study the scintillation index of N partially overlapping lowest order Gaussian laser beams with different wavelengths in weak atmospheric turbulence. Assuming a Von Karman turbulence spectrum and slow detector response and using the Rytov approximation we calculate the longitudinal and radial components of the scintillation index for typical free-space laser communication setups. We find the initial beam separation that minimizes the longitudinal scintillation and corresponds to the optimal beam configuration. Further reduction of the longitudinal scintillation is obtained by optimizing with respect to both initial beam separation and initial spot size. The longitudinal scintillation of the optimal N-beam configurations is inversely proportional to N, resulting in a 92% reduction for a 9-beam system compared with the single beam value. The existence of the minimum of longitudinal scintillation is not very sensitive to the form of the turbulence spectrum. Moreover, the radial scintillation values for the optimal N-beam configurations are found to be significantly smaller than the corresponding single beam values, and this reduction effect also grows with increasing N.
Mitigation of Atmospheric Effects, Adaptive Optics, and Applications for Laser Radar Systems and Laser Communications through the Atmosphere
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Tunable high-power high-brightness VECSELs as partially coherent sources for lasercom
J. V. Moloney, A. Peleg, P. Polynkin, et al.
We propose using high power, high brightness optically pumped vertical-external-cavity semiconductor lasers (VECSELs) as sources to build a partially coherent beam for laser communications applications. VECSELs are compact wavelength tunable, multi-Watt sources emitting light in a near TEM00 mode. Our theory suggests that the scintillation index at a remote receiver can be significantly reduced by filling the transmitter aperture with an array of beams. An experiment will be reported on that confirms the theory predictions and demonstrates further that the reduction in scintillation index carried through to case of strong turbulence where our perturbation theory fails.
Dual-frequency multifunction lidar
Rosemary Diaz, Sze-Chun Chan, Jia-Ming Liu
The design and performance of a multifunction continuous wave dual-frequency lidar system is presented. The system is based on the use of the nonlinear dynamics of an optically injected semiconductor laser. Under proper operating conditions, the laser emits a dual-frequency beam with a broadly tunable microwave separation. The two optical lines are coherently locked to each other using an external microwave synthesizer, resulting in a stable microwave beat frequency. The lidar system is capable of simultaneous velocity and range measurement of remote targets. The velocity is measured from the Doppler shift of the microwave beat frequency. The stability of the microwave beat frequency enables accurate measurement of low velocities. In addition, the stable locking enables long-range measurements because of the long microwave coherence length. Ranging is accomplished by extracting the time-of-flight information carried on the residual microwave phase noise. We demonstrate preliminary measurements of velocities as low as 26 &mgr;m/s and range measurements of 7.95 km with 2 % accuracy.
Performances of liquid crystal spatial light modulator (LC-SLM) as a wave-front corrector for atmospheric turbulence compensation
Liquid crystal spatial light modulator (LC-SLM) has a series of attractive characteristics as a wave-front corrector of adaptive optical system such as compactness, high density integration, low cost and possibility of batch production. However, it also has some limitations such as effective only on polarized light, chromatic aberration and low response speed and limited phase stroke. In order to investigate these pros and cons in detail, a series of experiments and analyses have been conducted using a LC-SLM Model P256 of Boulder Nonlinear System. The nonlinear phase response to applied voltage was measured for 1different wavelengths. The coupling between neighboring elements and response time were also measured. The error of phase wrapping for multi-wavelength wave-front of broad spectral band light was studied. The fitting capability to Zernike polynomials was demonstrated. An adaptive optical system with this LC-SLM and Shack-Hartmann sensor was close-looped. As a result, the applicability of LC-SLM for different applications will be evaluated and discussed.
On the use of Gaussian filter functions for adaptive optics
For adaptive optic systems, the use of aperture filter functions calculated using various Zernike modes can be useful in describing lower-order aberrations caused by atmospheric turbulence. Traditionally, these filter functions are calculated using the step function depicting a hard aperture that introduces integrals that are sometimes difficult to integrate and must be done numerically. The Gaussian method, equivalent to the ABCD matrix method, can be used in place of the conventional method for calculating the aperture filter functions. Evaluation of the Gaussian approximation for modeling a finite receiver aperture can be made by comparison of reduction in phase variance with results achieved using the conventional method. The validity of Gaussian approximation in this application is demonstrated by the consistency of results between the two methodologies. Comparison of reduction in scintillation by the two methodologies reveals several benefits derived from utilization of Gaussian approximation. The Gaussian approximation produces data that can be interpreted analytically. This paper will first examine the use of statistical models for predicting atmospheric turbulence and then the use of Zernike polynomials in adaptive optics. Next, this paper compares the reduction of phase variance and scintillation using the conventional method with the Gaussian approximation to evaluate the effectiveness of the new filter functions. The results of these comparisons are presented both as mathematical expressions and graphically.
The black fringe wavefront sensor: white light real time analog phase control
Richard J. Tansey, Avinash A. Honkan, Henry M. Chan
White light interferometry techniques to obtain 3D surface profiles or rms surface roughness measurements for microscopic instruments are used to develop an adaptive optics wavefront sensor for long range correction of lasers and images. Combining these techniques with a high power incoherent or multiline laser and a radial shear interferometer, a black fringe wavefront sensor (bfwfs) has been developed over the last two years at Lockheed Martin's Advanced technology Center. The bfwfs will be described, and results of recent tests shown using a 16 channel device. The 16 ch system is used to obtain measurements of open loop influence functions, and closed loop Bode plots using a Mems mirror. The bfwfs device can be used for adaptive optics at long ranges on weight or volume limited platforms because it allows high power incoherent lasers or other broadband sources to be combined with a parallel architecture and inherently analog servo system. Results are reported in which a superluminescent laser diode (SLD) and a multiline cw Argon laser are combined with a radial shear interferometer to measure phase at 200 Hz with 1/20 pv accuracy.
Wave Propagation in Non-classical (Non-Kolmogorov) Turbulence and Propagation of Pulses
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Critical issues encountered in experiments and measurements involving optical turbulence
The successful design and operation of high energy laser (HEL) and laser communication systems require a comprehensive and thorough knowledge of the real turbulent atmosphere coupled with high-fidelity realistic laser beam propagation models. To date, modeling and simulation of laser beam propagation through atmospheric turbulence have relied upon a traditional theoretical basis that assumes the existence of homogeneous, isotropic, stationary, and Kolmogorov turbulence. The real impact of the refractive index structure parameter ( C2n ) on laser beam propagation including effects of non-classical turbulence as well as inner (lo) and outer scale (Lo) effects will be examined. Observations clearly show turbulence is often layered and is produced by wave activity and episodic events such as Kelvin-Helmholtz instabilities. Other critical turbulence issues involve the relationship between mechanical and optical turbulence and the effect of path variability of turbulence and inner scale on optical turbulence parameters over long paths. These issues will be examined from data obtained from five systems: a) a new measurement platform using a free-flying balloon that lifts a ring with a boom upon which are mounted several fine wire (1-&mgr;m diameter) sensors to measure high-speed temperature and velocity fluctuations, b) a new system using a kite/tethered blimp platform that obtains both profile and measurements at a fixed altitude over time, c) a 50 MHz radar at Vandenberg Air Force Base that senses at high temporal and spatial resolution to 20 km ASL, d) an instrumented aircraft system, and e) a suite of optical systems. The first four systems all provide estimates of C2n , the eddy dissipation rate (&Vegr;), lo and Lo. Methods of calibration and problems of interpreting results from the measurement systems are discussed.
Free space optical system performance for laser beam propagation through non-Kolmogorov turbulence
Free space laser system performance is limited by atmospheric turbulence that has been described for many years by Kolmogorov's power spectral density model because of its simplicity. Unfortunately several experiments have been reported recently that show Kolmogorov theory is sometimes incomplete to describe atmospheric statistics properly, in particular in portions of the troposphere and stratosphere. In this paper we present a Non-Kolmogorov power spectrum which uses a generalized exponent instead of constant standard exponent value 11/3 and a generalized amplitude factor instead of constant value 0.033. Using this new spectrum in weak turbulence, we carry out, for horizontal path, analysis of Long Term Beam Spread, Scintillation index, Probability of fade, mean SNR and mean BER as variation of the spectrum exponent.
Humidity contribution to Cn2 over a 600m pathlength in a tropical marine environment
We present new optical turbulence structure parameter measurements, C2n, over sea water between La Parguera and Magueyes Island (17.6N 67W) on the southwest coast of Puerto Rico. The 600 meter horizontal paths were located approximately 1.5 m and 10 m above sea level. No data of this type has ever been made available in the literature. Based on the data, we show that the C2n measurements are about 7 times less compared to equivalent land data. This strong evidence reinforces our previous argument1-4 that humidity must be accounted for to better ascertain the near surface atmospheric turbulence effects, which current visible / near infrared C2n bulk models fail to do. We also explore the generalised fractal dimension of this littoral data and compare it to our reference land data. We find cases that exhibit monofractal characteristics, that is to say, the effect of rising temperatures during the daylight hours upon turbulence are counterbalanced by humidity, leading to a single characteristic scale for the measurements. In other words, significant moisture changes in the measurement volume cancels optical turbulence increases due to temperature rises.
Enhanced performance of low-power (<60mW) femtosecond free space optical communication system over conventional CW operation
Paul Corrigan, Rainer Martini, John Cabaniss, et al.
These experiments report the first qualitative observation of dependency of typical fog transmission on pulse length and the first systematic measurement of scattering amplitude and pattern for fs-pulse propagation through strongly scattering (fog) medium. Also, we include the first reported measurement of enhancement of fs-pulse propagation in real FSO environment. We also provide a classical analysis of the Mie-Scattering predictions, and experiments which demonstrate a different transmission behavior than is predicted. This proves only in part that the enhanced fs performance is due to a different scattering formula than is expected; absorption is normally a value which is measured by default. Once the transmission and scattering properties of a stable substance are known (usually determined in two different measurements), the difference is the absorption. This is particularly hard to do in simulated fog, as the simulation itself is unstable, as is real fog. However, there is no current measurement or claim which indicates this enhanced propagation is related to different physics of absorption of Ultrafast Pulses by the atmosphere.