Optically amplified free-space optical communication systems

This thesis investigates terrestrial atmospheric FSO communication systems operating under the influence of turbulence-induced scintillation, beam spreading, optical interchannel crosstalk, amplified spontaneous emission noise and pointing errors. On-off keying-non–return-to-zero (OOK-NRZ) and digital pulse position (DPPM) are the modulation schemes used for the calculations. The possibility of using sophisticated performance evaluation techniques such as moment generating function (MGF)-based Chernoff bound (CB), modified Chernoff bound (MCB) and saddlepoint approximation (SPA) for terrestrial DPPM and OOK-NRZ–based FSO communication systems employing optical amplification are investigated and compared with the conventional Gaussian approximation (GA) method. Relative to the other techniques, the MCB can be considered a safe estimation method for practical systems since it provides an upper bound upon the BER. The turbulent optically preamplified DPPM FSO receiver employing integration over a time slot and comparing the results to choose the largest slot, is seen to give better advantage (about 7 - 9 dB) compared to an equivalent employing OOK-NRZ signalling. The atmospheric turbulence-induced spreading of the beam, ASE noise, and pointing error are seen to combine in a problematic way resulting in high BERs, depending on the size of the receiver and the beam’s jitter standard deviation. Using FSO communication for the distribution links of a passive optical network-like wavelength division multiplexing access network is investigated in the presence of atmospheric turbulence, ASE noise and interchannel crosstalk. The results show that, for clear atmosphere, FSO distribution link length up to 2000 m can be reliably used (depending on turbulence strength) to achieve human eye safety and high capacity access networks. Also, error floors occur due to turbulence accentuated crosstalk effect for the cases of (i) signal turbulent, but crosstalk not and (ii) crosstalk turbulent, but signal not

Dynamically reconfigurable optical access network

This dissertation presents the research results on a fiber-optic high-bitrate access network which enables dynamic bandwidth allocation as a response to varying subscribers' demands and bandwidth needs of emerging services. The motivation of the research is given in Chapter 1 "Introduction" together with a brief comparative discussion on currently available and future access networks. The idea of wavelength reconfigurability in the last-mile networks is described as a solution for more efficient bandwidth utilization and a subject of the Broadband Photonics project. Chapter 2 "Wavelength-flexible WDM/TDM access network - architecture" provides a comprehensive description of the proposed solution with each network element being analyzed in terms of its functionalities. This includes a colorless optical network unit and a reconfigurable optical add/drop multiplexer. An estimation of power budget is followed by the choice of wavelength set and network control and management layer overview. In Chapter 3 "Reflective transceiver module for ONU" after discussing different communication schemes and modulation formats three approaches to a colorless high-bitrate transmitter are analyzed in detail. This includes experiment and simulation results on a reflective semiconductor optical amplifier, reflective electro-absorption modulator and a Michelson-interferometer modulator. The Chapter is concluded with a comparative discussion. Chapter 4 "Reconfigurable optical add/drop multiplexer" discusses another key element in the proposed network architecture which is an integrated structure of micro-ring resonators providing wavelength reconfigurability. The measured characteristics assess the applicability of the device able to support unicast and multicast transmission. A range of possible sources of signal degradation in the access links are analyzed in Chapter 5 "Transmission and network impairments in the access network". An estimation of potential power penalties resulting from such impairments in the proposed system follow afterwards. Special attention is paid to optical in-band crosstalk penalties and improvement methods in Chapter 6 "Interferometric crosstalk in the access network with an RSOA". This subject is treated extensively with the support of mathematical considerations and experimental results. Proof-of-concept experiments of the proposed network architecture are presented in Chapter 7 "Reconfigurable WDM/TDM access network - experiments". The results of bidirectional transmission of high-bitrate WDM signals in different wavelength allocation schemes are discussed in detail. From there, by means of simulations the behavior of a full-scale network is assessed. In Chapter 8 "Migration towards WDM/TDM access network" the migration scenario from currently deployed fiber-optic access networks towards the novel solution is proposed. Afterwards, a short dispute on the economics of last-mile fiber technologies is included. Finally, the work is concluded and potential future research ideas based on this thesis are given in Chapter 9 "Conclusions and further work"