Essay: FSO communication systems

There has been an exponential rise in the demand for broadband applications and services [1, 2]. Optical carrier technologies can be a good solution since they potentially offer huge bandwidth [2-6]. Optical communication has gradually replaced the copper-based access network technologies. Optical fiber has many advantages (low cost, no electromagnetic interference problems, and less power loss) over the incumbent copper systems [7, 8].
Due to the optical nature of FSO, the performance of a system is greatly affected by the environmental conditions present between transmitter and receiver. As an FSO signal travels over a distance the signal degrades according to the amount of interference it encounters. This signal loss due to the interference experienced as the signal propagates through the atmosphere is known as atmospheric attenuation and is the result of the signal being either absorbed or scattered by several different properties of the air.
The level of atmospheric attenuation will determine the performance of the FSO system. This interference comes in the form of particulates, absorption, scattering, scintillation, and turbulence.
It is very difficult to predict the performance of FSO systems due to the relatively unpredictable nature of atmospherics. Weather reports with the level of accuracy needed to make accurate predictions on the performance of FSO systems are generally only collected in the proximity of airports. These reports are made public, but are limited in scope as far as area is concerned. In order to make accurate predictions of the performance of a system in a certain area it is necessary to take very accurate weather readings in the area in which the system is to be employed for an extended period of time.
Atmospheric particulates are most commonly experienced in the form of precipitation, but are also encountered as dust, smoke, volcanic ash and other pollutants. Severe weather of all types will have a detrimental effect on performance due to the combination of dense particulate and turbulence. As one might expect, as the density of the particulate increases, the performance of the system decreases. Fog has the biggest impact on signal performance. This is due to the fact that fog is composed of water droplets that are a size optimum to interfere, through scattering, with IR wavelengths [27].
Over the last 10 – 20 years, optical amplifiers have become essential components in optical communication systems and have successfully replaced electrical repeaters as a means of compensating for optical signal loss. An advantage offered by optical amplifiers over repeaters include the fairly large gain bandwidth offered by the optical amplifier which makes it practical for wavelength division multiplexing (WDM) whereby a single amplifier can amplify multiple signals on different wavelengths simultaneously, while without the optical amplifier separate repeaters would be needed for each wavelength. Additionally, optical amplifiers are easily adaptable for many bit rates and signal modulation formats without a need to replace the amplifier, while the repeaters are designed to work at a particular bit rate (or at around only one wavelength) and modulation format [1, 30]. The early research into optical amplifiers led to the development of the semiconductor optical amplifier (SOA), which was initially referred to as the semiconductor laser amplifier (SLA) [31-33]. SOAs were initially fabricated from a semiconductor laser by replacing the end mirrors with antireflective coatings (so reflections needed for laser operation were eliminated (in the case of travelling-wave amplifier (TWA)) or reduced (in the case of Fabry Perot amplifier (FPA))). While the SOA technology was only able to achieve a few dB improvement in power levels compared to the electrical repeaters, continuous research in the laboratory led to the development of the EDFA which is capable of providing a large amount (typically 30-40 dB [1, 8, 14, 34]) of optical gain over a wide spectral range (approximately 30-60 nm).
Similarly FSO communication systems can benefit from using an optical amplifier in various ways. The optical preamplifier configuration can be used to boost optical signal strength which has been degraded due to various atmospheric phenomena, to overcome the eye-limit restrictions on transmitted laser power, to suppress the limiting effect of the receiver thermal noise generated in the electronic amplifier, as well as to effectively improve receiver sensitivity.
Wavelength division multiplexing (WDM) systems allow more ONUs to be connected at high data rates and assign a distinct pair of dedicated wavelengths to each ONU such that a point-to-point connection is established between the ONU and the optical line terminator (OLT) [1]. Major drivers for the WDM PON are the potential increase in the bandwidth and the greater data security that can be offered to the ONUs compared to the TDM/TDMA system [1, 2, 10-12]. While the WDM system is technically interesting, the major challenge for its commercialisation is the higher cost of the equipment (for example arrayed waveguide grating (AWG)-based multiplexer (mux)/de-multiplexer (demux) and wavelength specific sources) compared to the TDM/TDMA system which uses power splitters and low-cost sources [1, 10, 11].
A WDM access network using FSO communications in the distribution link is a realistic proposition since both optical fibre and FSO systems operate using similar optical transmission wavelengths and system components [7, 10, 17, 19]. Therefore, the integration of both technologies may yield a cost effective and reliable hybrid optical access network solution.
For long propagation distances, the use of optical amplifiers becomes necessary. However, the optically amplified signal is accompanied by ASE noise which somewhat offsets the performance benefits of the amplifier and complicates performance calculations [20-22]. The use of optical amplifiers for extending the maximum reach and/or split in optical access networks (SuperPONs) was investigated in the 1990s [23] whilst long-range PONs, which incorporate WDM, are under investigation [9].
A comprehensive system design and performance evaluation for a WDM optical link has been proposed in this study. The aim of this design is to provide a high-speed and long range optical link in foggy weather. This thesis studies the analysis of FSO link over long range by introducing the technique of WDM under different weather conditions. It will cover the analysis of different atmospheric weather effects on the WDM based FSO system and the performance of the FSO link under different beam divergence and laser power. The semiconductor optical amplifier has also been used in a preamplifier configuration to increase the power efficiency of laser and to enhance the link range of the system.
1.4 Thesis Organization
The remainder of this thesis is organized as follows. Chapter II provides review of literature containing critical review which describes the previous work with references to update the reader about the current status of research finding in FSO and delivers the knowledge from basic to advance level. It begins with a brief history, initial problems, addresses the capabilities and limitations, and ends with a discussion of general FSO system construct. Chapter III is a materials and methods. This chapter will provide sufficient detail to allow the work to be reproduced. Methods already published have indicated by a reference and only relevant modifications have been described. Chapter IV describes results and discussions. Finally, Chapter V includes the conclusion and recommendations for future exploration and development of FSO communication capabilities.