Thomas Plank

Analysis and evaluation of optimum wavelengths for free-space optical transceivers

The objective of this paper is to analyse and present the latest results obtained for free-space optics (FSO) within the EU COST Action IC-0802 and within the European Space Agency (ESA) contract. First, the FSO technology is briefly discussed and some performance evaluation criteria for FSO are provided. Some optical signal propagation experiments through the atmosphere (including the recent investigations in airborne and satellite application for FSO) are also shown. In the main part, considerations on suitability of different optical wavelengths are brought into question. The wavelength selection is dependent on the atmospheric effects and on the availability of receiver and transmitter components. Discussion on the available receiver(s) and transmitter(s) includes the focus on advantages and mainly the costs of the different systems. In the final part, we examine the latest practical results (carried out within the COST Action IC-0802) on modelling of the FSO channel under fog conditions and other atmospheric effects. Additionally, recent results, showing major performance improvement, based on the hybrid system and specific modulation and coding schemes are presented.

Challenges in establishing free space optical communications between flying vehicles

Establishing free space optical links between flying vehicles is a technically tough challenge and needs enhancements both in the technology and its features. For unmanned aerial vehicle (UAV) swarms to evolve to effective usage in civil and military applications, a means of high data rate communications need be established between them. The free space optical links due to their huge bandwidth and license free spectrum provide a viable communications solution between the UAV swarm. The swarm UAV scenario due to the continuous motion and changing relative speeds of all its members provide extremely challenging conditions to maintain a line-of-sight free space optical link and this paper reviews the existing related scientific developments and talks about the challenges of utilizing FSO in the swarm UAV environment. Further, this survey paper discusses the utility and challenges of developing free space optical links between HAPs.

Recent developments on free space optical links and wavelength analysis

New results derived from the research in the field of Free Space Optics (FSO) for the qualification of different wavelengths with reference to space applications are presented. At the beginning, a brief introduction to the FSO technology and the explanation of some performance evaluation criteria is presented. The main part deals with a thorough discussion of the different optical wavelengths used either for terrestrial as well as for near-Earth and deep space FSO links. Some optical signal propagation experiments through the atmosphere (including the recent investigations in airborne and satellite application for FSO) are also shown. Practical results of COST Action IC-0802 are implemented in the modelling of the FSO channel under deteriorating conditions like rain, snow, dust, fog, clouds and other atmospheric effects. It is intended to interconnect well-proved technologies like 850 nm and 1,064 nm wavelength with new technologies under development like 10 µm wavelength. Quantum Cascade Lasers (QCLs) are currently experiencing a strong progression. Semiconductor quantum cascade lasers can operate at various wavelengths between 3 µm up to 17 µm what makes them attractive in view of the 10 µm technology. Finally, some recent major performance improvement results obtained by employing some specific modulation and coding schemes are presented.

Free Space Optics in different (civil and military) application scenarios in combination with other wireless technologies

In this paper Free Space Optics (FSO) in combination with other wireless technologies (including WLAN and satellite communications in civil-military-cooperation (CIMIC)) is presented. At first modular communication systems are shown, which allows worldwide access to the Internet or other networks by combining satellite communications, FSO, Wireless LAN, Local Multipoint Distribution System (LMDS) and DVB-T (terrestrial digital video broadcast). Wireless LAN offers connectivity to mobile users in a network cell, Free Space Optics allows quick installation of broadband fixed wireless links instead of cables and satellite communications provides a backbone between distant locations in the world. DVB-T is the current video broadcast standard (instead of former analogue TV) and will also be used for Internet-access (see project SEE TV-WEB). Different scenarios (and results) using modular wireless technologies are shown. Free Space Optics and wireless applications (including satellite) installed at a civil-military exercise in Styria (southern part of Austria) and other application scenarios are presented. In this civil-military cooperation a mobile satellite earth station (equipped with FSO and Wireless LAN) was used for videoconferencing between military and civil organizations. Applications with DVB-T will supplement a few scenarios in combining wireless technologies with FSO.

Integration of FSO in local area networks - Combination of Optical Wireless with WLAN and DVB-T for last mile internet connections

Nowadays FSO (Free Space Optics) also called Optical Wireless, is a well-established technology for closing the last mile gap. Optical Wireless is best suited to be directly connected to fibre optics, like the extension of a fibre cable for access networks. But also combinations with RF- and wireless networks are of main interest in current applications. Various technologies have different influences on weather conditions, because of the used wavelengths (frequencies) and the differences in the propagation behaviour and the transmission channel. This contribution will show at first the selection of best suited wavelengths for optical wireless communications (OWC) for the last mile access in relevance to the weather effects and the influences of beam divergence and beam shaping. The second part is showing the usage of FSO in combination with other wireless technologies (as a special scenario with WLAN and DVB-T), available for regions of low Internet coverage. FSO allows a broadband access with high data rate for users not connected so far by fibre cables. It is a well-known fact that peripheral regions lack of reasonable broadband Internet services. WLAN and FSO are license-free transmission technologies and DVB-T has become a geographically widespread technology. This paper will show an alternative Internet access method using WLAN and FSO systems as a data uplink and DVB-T as a downlink channel for broadband data access. The work was carried out within international co-operations in the frame of a special programme of European Union called SEE (South-East-Europe).

Wavelength-selection for high data rate Free Space Optics (FSO) in next generation wireless communications

New results derived from the research in the field or Free Space Optics (FSO) for the qualification of different wavelengths with reference to future communication (including space) are presented. The contribution deals with a thorough discussion of the different optical wavelengths used either for terrestrial as well as for near-Earth and deep space FSO links. Practical results of COST Action IC-0802 are implemented in the modelling of the FSO channel under deteriorating conditions like rain, snow, dust, fog, clouds and other atmospheric effects. It is intended to interconnect well-proved technologies like 850 nm and 1064 nm as well as 1550 nm wavelength with new technologies under development like 10 μm wavelength. Quantum Cascade Lasers (QCLs) are currently experiencing a strong progression. Finally, some recent major performance improvement results obtained by employing some specific modulation and coding schemes are presented.

Reliable Optical Wireless Links used as feeder links between earth and satellite

The use of optical communication links is a growing and promising technology, with its applications spreading over many areas of telecommunication. ESAs involvement in the development of optical communication terminals for high data rate links between satellites already resulted in the successful demonstration of the SILEX programme using optical inter-satellite links, using the ARTEMIS satellite in the GEO orbit and experimental programmes like the OPTEL terminal. The Optical Ground Station (OGS) facility on the Canary Islands and communication links through the Earths atmosphere have been demonstrated successfully. In this paper we show improvements on Optical Wireless Links for future Broadband networks. Most of the results have been carried out within international co-operations like COST and EU-projects and ESA-contracts.

Design of a hardware channel emulator as lab demonstrator for detailed verification of long-distance FSO systems

The field of Free Space Optical (FSO) communications has to deal with several challenges, which have to be considered. One of the most serious amongst them is fog attenuation. In this paper, an effective method for investigation of a long-distant FSO systems performance in the presence of fog is described. The proposed hardware channel emulator for laboratory measurements is based on a controllable optical attenuator, providing uncomplicated experimental environment in terms of fog conditions. An overview of already accomplished fog measurement campaigns is given as well as a rough summary of the self-developed MATLAB channel models. The findings were incorporated into the custom-made voltage driver (attenuator control unit), which controls the optical attenuator and represent sufficient basis for further laboratory long-distance FSO link verifications.

Link budget optimization of free space optical systems in relation to the beam diverging angle

The performance of a Free Space Optical (FSO) communication system is highly dependent on the precise selection and adjustment of its operation parameters. The appropriate determination of the angle of divergence can noticeably compensate the negative impact of deviations of the optical beam from its main direction caused by a variety of deteriorative influences. Mathematical equations for calculating the optimum diverging angle are derived within this work. Regarding the obtained results, the link budget and the link margin of FSO systems are calculated. In addition, the dependence of the optimum divergence on the power and the wavelength of the optical source is investigated. In relation to this, enough figures showing evidently the better reliability of a FSO system in condition of beam wandering as well as other degradative impacts are attached.

FSO system performance evaluation based on calibrated atmospheric channel emulation

A novel approach for testing Free Space Optical (FSO) systems in a controlled laboratory condition is proposed. Based on fibre optics technology, our testbed could effectively emulate the operation of real wireless optical communication systems combined with various atmospheric perturbation effects such as fog and clouds. The suggested architecture applies an optical variable attenuator as a main device representing the tropospheric influences over the launched Gaussian beam in the free space channel. In addition, the current scheme involves an attenuator control unit with an external Digital Analog Converter (DAC) controlled by self-developed software. To obtain optimal results in terms of the presented setup, a calibration process including linearization of the non-linear attenuation versus voltage figure is performed. Finally, analytical results of the attenuation based on real and simulated measurements with the hardware channel emulator (= testbed) in a laboratory controlled environment are shown.