Combined effect of turbulence and aerosol on free-space optical links

[EN] Despite the benefits of free-space optical (FSO) communications, their full utilization is limited by the influence of atmospheric weather conditions, such as fog, turbulence, smoke, snow, etc. In urban environments, additional environmental factors such as smog and dust particles due to air pollution caused by industry and motor vehicles may affect FSO link performance, which has not been investigated in detail yet. Both smog and dust particles cause absorption and scattering of the propagating optical signal, thus resulting in high attenuation. This work investigates the joint impact of atmospheric turbulence and dust particle-imposed scattering on FSO link performance as part of the last-mile access network in urban areas. Propagation of an optical wave is at first analyzed based on the microphysic approach, and the extinction caused by small particles is determined. An experimental measurement campaign using a dedicated test chamber is carried out to assess FSO link performance operating wavelengths of 670 nm and 830 nm and under dust and turbulent conditions. The measured attenuation and the 𝑄Q factor in terms of the velocity of particle flow and turbulence strength are analyzed. We show that for an airflow of 2 m/s, the 𝑄Q factor is almost 3.5 higher at the wavelength of 830 nm than at 670 nm. However, for a wavelength of 670 nm, the FSO link is less affected by the increase in airflow compared to 830 nm. The 𝑄 factor reduces with turbulence. Under similar turbulence conditions, for ash particles, the 𝑄Q factor is higher than that of sand particles.European Social Fund (ESF) (CZ.1.07/2.3.00/30.0034); Ministerio de Economia y Competitividad (MINECO) (JCI-2012-14805); European Cooperation in Science and Technology (COST) (IC 1101); Ceske Vysoke Uceni Technicke v Praze (CVUT) (SGS14/190/OHK3/3T/13).Libich, J.; Perez, J.; Zvanovec, S.; Ghassemlooy, Z.; Nebuloni, R.; Capsoni, C. (2017). Combined effect of turbulence and aerosol on free-space optical links. Applied Optics. 56(2):336-341. https://doi.org/10.1364/AO.56.000336S336341562Khalighi, M. A., & Uysal, M. (2014). Survey on Free Space Optical Communication: A Communication Theory Perspective. IEEE Communications Surveys & Tutorials, 16(4), 2231-2258. doi:10.1109/comst.2014.2329501Wang, C.-X., Haider, F., Gao, X., You, X.-H., Yang, Y., Yuan, D., … Hepsaydir, E. (2014). Cellular architecture and key technologies for 5G wireless communication networks. IEEE Communications Magazine, 52(2), 122-130. doi:10.1109/mcom.2014.6736752Parca, G. (2013). Optical wireless transmission at 1.6-Tbit/s (16×100  Gbit/s) for next-generation convergent urban infrastructures. Optical Engineering, 52(11), 116102. doi:10.1117/1.oe.52.11.116102Kedar, D., & Arnon, S. (2004). Urban optical wireless communication networks: the main challenges and possible solutions. IEEE Communications Magazine, 42(5), S2-S7. doi:10.1109/mcom.2004.1299334Awan, M. S., Horwath, L. C., Muhammad, S. S., Leitgeb, E., Nadeem, F., & Khan, M. S. (2009). Characterization of Fog and Snow Attenuations for Free-Space Optical Propagation. Journal of Communications, 4(8). doi:10.4304/jcm.4.8.533-545Nauerth, S., Moll, F., Rau, M., Fuchs, C., Horwath, J., Frick, S., & Weinfurter, H. (2013). Air-to-ground quantum communication. Nature Photonics, 7(5), 382-386. doi:10.1038/nphoton.2013.46Perez, J., Zvanovec, S., Ghassemlooy, Z., & Popoola, W. O. (2014). Experimental characterization and mitigation of turbulence induced signal fades within an ad hoc FSO network. Optics Express, 22(3), 3208. doi:10.1364/oe.22.003208Kim, I. I., McArthur, B., & Korevaar, E. J. (2001). Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications. Optical Wireless Communications III. doi:10.1117/12.417512Rekab-Eslami, M., Esmaeili, M., & Aaron Gulliver, T. (2017). Generic Linear Network Code Construction Using Transversal Matroids. IEEE Communications Letters, 21(3), 448-451. doi:10.1109/lcomm.2016.2619706Corrsin, S. (1951). On the Spectrum of Isotropic Temperature Fluctuations in an Isotropic Turbulence. Journal of Applied Physics, 22(4), 469-473. doi:10.1063/1.1699986Ghassemlooy, Z., Le Minh, H., Rajbhandari, S., Perez, J., & Ijaz, M. (2012). Performance Analysis of Ethernet/Fast-Ethernet Free Space Optical Communications in a Controlled Weak Turbulence Condition. Journal of Lightwave Technology, 30(13), 2188-2194. doi:10.1109/jlt.2012.2194271Clifford, S. F., Ochs, G. R., & Lawrence, R. S. (1974). Saturation of optical scintillation by strong turbulence*. Journal of the Optical Society of America, 64(2), 148. doi:10.1364/josa.64.00014

On the study of the FSO link performance under controlled turbulence and fog atmospheric conditions

In this paper, the effect of turbulence and fog on the free space optical (FSO) communication systems for on off keying (OOK), pulse amplitude modulation (PAM) and subcarrier intensity modulation (SIM) based on binary phase shift keying (BPSK) is experimentally investigated. The experiment is carried out in a controlled laboratory environment where turbulence and fog could be generated in a dedicated FSO chamber. In comparison to 4 PAM signal, the BPSK and OOK NRZ modulation signalling format are more robust against the fog and turbulence effects. In addition BPSK system is much less susceptible to the signal amplitude fluctuation due to turbulence compared to the other two modulation formats

Bit error rate measurement of free space optical communication links under laboratory controlled conditions

This paper outlines the experimental investigation of the fog effect on the bit error rate (BER) performance of the free space optical (FSO) communication link under a controlled laboratory environment. The link transmittance and the received signal Q-factor are measured against different levels of fog densities. The link visibility derived from fog attenuation measurement is used to characterize the fog within the chamber. Moreover, the effect of using different average transmitted optical communication power (Popt) on the transmittance and received Q-factor of the link is also studied for light and dense fog densities

Experimental Characterisation and Modelling of Atmospheric Fog and Turbulence in FSO

Free space optical (FSO) communication uses visible or infrared (IR) wavelengths to broadcast high-speed data wirelessly through the atmospheric channel. The performance of FSO communications is mainly dependent on the unpredictable atmospheric channel such as fog, smoke and temperature dependent turbulence. However, as the real outdoor atmosphere (ROA) is time varying and heterogeneous in nature as well as depending on the magnitude and intensity of different weather conditions, carrying out a proper link assessment under specific weather conditions becomes a challenging task. Investigation and modelling the ROA under diverse atmospheric conditions is still a great challenge in FSO communications. Hence a dedicated indoor atmospheric chamber is designed and built to produce controlled atmosphere as necessary to mimic the ROA as closely as possible. The experimental results indicate that the fog attenuation is wavelength dependent for all visibility V ranges, which contradicts the Kim model for V < 0.5 km. The obtained result validates that Kim model needs to be revised for V < 0.5 km in order to correctly predict the wavelength dependent fog attenuation. Also, there are no experimental data and empirical model available for FSO links in diverse smoke conditions, which are common in urban areas. Therefore, a new empirical model is proposed to evaluate the wavelength dependent fog and smoke attenuation by reconsidering the q value as a function of wavelength rather than visibility. The BER performance of an FSO system is theoretically and experimentally evaluated for OOK- NRZ, OOK-RZ and 4-PPM formats for Ethernet line data-rates from light to dense fog conditions. A BER of 10-6 (Q-factor ≈ 4.7) is achieved at dense fog (transmittance, T = 0.33) condition using 4-PPM than OOK-NRZ and OOK-RZ modulation schemes due to its high peak-to-average power ratio albeit at the expense of doubling the bandwidth. The effects of fog on OOK-NRZ, 4-PAM and BPSK are also experimentally investigated. In comparison to 4-PAM and OOK-NRZ signals, the BPSK modulation signalling format is more robust against the effects of fog. Moreover, the effects of using different average transmitted optical communication powers Popton the T and the received Q-factor using the OOK-NRZ modulation scheme are also investigated for light and dense fog conditions. The results show that for an FSO system operating at a Q-factor of 4.7 (for BER = 10-6), the required Q-factor is achieved at T of 48% under the thick fog condition by increasing Popt to 1.07 dBm, whereas the values of T are 55% and ~70% for the transmit power of 0.56 dBm and -0.7 dBm, respectively. The experimental characterisation and investigation of the atmospheric turbulence effect on the Ethernet and Fast-Ethernet FSO link is reported using different modulation schemes. The experiment is carried out in a controlled laboratory environment where turbulence is generated in a dedicated indoor atmospheric chamber. The atmospheric chamber is calibrated to mimic an outdoor turbulence conditions and the measured data are verified against the theoretical predictions. The experiment also demonstrates methods to control the turbulence levels and determine the equivalence between the indoor and outdoor FSO links. The results show that the connectivity of Ethernet and Fast-Ethernet links are highly sensitive to atmospheric turbulence. The results also show that the BPSK and OOK-NRZ modulation signalling formats are more robust against the weak atmospheric turbulence conditions than PAM signal

Free Space Optics: Current Applications and Future Challenges

FSO is a communication system where free space acts as medium between transceivers and they should be in LOS for successful transmission of optical signal. Medium can be air, outer space, or vacuum. This system can be used for communication purpose in hours and in lesser economy. There are many advantages of FSO like high bandwidth and no spectrum license. The transmission in FSO is dependent on the medium because the presence of foreign elements like rain, fog, and haze, physical obstruction, scattering, and atmospheric turbulence are some of these factors. Different studies on weather conditions and techniques employed to mitigate their effect are discussed in this paper

SCATTERING EFFECTS BY SiO2 NANO-MICROPARTICLE AND FREE SPACE ATTENUATION MODELLING FOR DIVERSE WEATHER CONDITIONS

This article presents few empirical models to be used prediction of atmospheric attenuation due to airborne suspended particles such as sand, dust, fog and mist. The attenuation can be determined using the proposed models as standard methods considering various types of weather conditions including silicadominant sandstorm, dust storm, fog or mist. As atmospheric attenuation heavily affects the transmission of data using visible light communication, we study the effect of air suspended particles to scattering amplitude and turbulence phase of the light beam. The proposed models are compared with existing attenuation models in the case of attenuation under foggy weather condition. The proposed models especially proposed Model 2 is seen to be best fit for prediction of atmospheric attenuation under dense to very light fog weather

Imaging through obscurants using time-correlated single-photon counting in the short-wave infrared

Single-photon time-of-flight (ToF) light detection and ranging (LiDAR) systems have emerged in recent years as a candidate technology for high-resolution depth imaging in challenging environments, such as long-range imaging and imaging in scattering media. This Thesis investigates the potential of two ToF single-photon depth imaging systems based on the time-correlated single-photon (TCSPC) technique for imaging targets in highly scattering environments. The high sensitivity and picosecond timing resolution afforded by the TCSPC technique offers high-resolution depth profiling of remote targets while maintaining low optical power levels. Both systems comprised a pulsed picosecond laser source with an operating wavelength of 1550 nm, and employed InGaAs/InP SPAD detectors. The main benefits of operating in the shortwave infrared (SWIR) band include improved atmospheric transmission, reduced solar background, as well as increased laser eye-safety thresholds over visible band sensors. Firstly, a monostatic scanning transceiver unit was used in conjunction with a single-element Peltier-cooled InGaAs/InP SPAD detector to attain sub-centimetre resolution three-dimensional images of long-range targets obscured by camouflage netting or in high levels of scattering media. Secondly, a bistatic system, which employed a 32 × 32 pixel format InGaAs/InP SPAD array was used to obtain rapid depth profiles of targets which were flood-illuminated by a higher power pulsed laser source. The performance of this system was assessed in indoor and outdoor scenarios in the presence of obscurants and high ambient background levels. Bespoke image processing algorithms were developed to reconstruct both the depth and intensity images for data with very low signal returns and short data acquisition times, illustrating the practicality of TCSPC-based LiDAR systems for real-time image acquisition in the SWIR wavelength region - even in the photon-starved regime.The Defence Science and Technology Laboratory ( Dstl) National PhD Schem