Fiber Optics

Optical fibers in metrology, telecommunications, sensors, manufacturing, and health science have gained massive research interest. The number of applications is increasing at a fast pace. This book aims to present a collection of recent advances in fiber optics, addressing both fundamental and industrial applications. It covers the current progress and latest breakthroughs in emergent applications of fiber optics. The book includes five chapters on recent developments in optical fiber communications and fiber sensors, as well as the design, simulation, and fabrication of novel fiber concepts

Data transmissions through HFC return channels

Master'sMASTER OF ENGINEERIN

Characterization of wavelength tunable lasers for use in wavelength packet switched networks

The telecom industry's greatest challenge, and the optical systems and components vendors' biggest opportunity is enabling providers to expand their data services. The solution lies in making optical networks more responsive to customer needs, i.e., making them more rapidly adaptable. One possible technique to achieve this is to employ wavelength tunable optical transmitters. The importance of tunability grows greater every year, as the average number of channels deployed on DWDM platforms increases. By deploying tunable lasers it is much easier to facilitate forecasting, planning and last minute changes in the network. This technology provides with solution for inventory reduction. It also offers solution for fast switching at packet level. The conducted research activities of the project was divided in two work packages: 1. Full static characterization-the laser used in the experiment was a butterfly-packaged Sampled Grating DBR laser with four electrically tunable sections. LabView programme was developed for distant control of the equipment and the laser itself. The parameters required for creating a look-up table with the exact currents for the four sections of the laser, namely wavelength, side mode suppression ratio and output power, were transferred to tables. Based on those tables the currents were defined for each of the 96 different accessible channels. The channel allocation is based on the 50 GHz spacing grid. A detailed analysis of the tuning mechanisms is provided. 2. Dynamic characterization and BER performance in wavelength packet switched WDM systems-a commercially available module was used supplied with the software package for controlling the wavelength channels and setting the laser to switch between any accessible channel. The laser is DBR laser without SOA integration so the dynamic tunability can be investigated. As the switching in the nanosecond regime is executed in the electrical domain, analysis of the switching parameters concerning the electrical circuit as well as laser structure is provided. The actual switching time was defined. The degradation in system performance due to spurious wavelength signals emitted from the tunable module during the switching event and their interference with other active channels was demonstrated by examining the presence of an error floor in the BER rate against received power measurements

ULTRA-WIDEBAND NONLINEAR ECHO-CANCELLATION

Hybrid fiber coaxial (HFC) networks are used around the world to distribute cable television and broadband internet services to customers. These networks are governed by the Data-Over-Cable Service Interface Specification (DOCSIS) family of standards, with the most recent version at the time of this writing being DOCSIS 3.1. A frequency division duplex (FDD) spectrum is used in DOCSIS 3.1, where the upstream and downstream signals are separated in frequency to eliminate interference. A possible method to increase signal bandwidths is to use a full-duplex (FDX) spectrum, in which the US and DS signals use the same frequencies at the same time. A main challenge faced when implementing FDX in a DOCSIS node is eliminating the interference in the received US signal caused by the transmitted DS signal. One possible method for eliminating the interference is utilizing an echo-canceling algorithm, which predicts the self-interference (SI) based on the known DS signal and cancels it from the received US signal. Although echo-cancellation algorithms exist for fundamentally similar applications, the DOCSIS FDX case is more complicated for two main reasons. First, the DOCSIS node uses a nonlinear power amplifier to amplify the DS signal. Second, the DS signal is an ultra-wideband signal spanning a frequency range of up to 1.2 GHz. Most of the amplifier modeling techniques discussed in the literature were designed for narrowband wireless signals and will have limited performance when used with ultra-wideband signals. This thesis develops an algorithm to characterize the power amplifier and to predict the harmonics it generates for a given DS signal. These predicted harmonics can be used to cancel the SI signal in a full duplex DOCSIS system. The algorithm, which is referred to as the ultra-wideband memory polynomial (UWB-MP) model, is based on the well-known memory polynomial model with adaptations which allow the model to predict harmonics for ultra-wideband signals. Since a direct implementation of the UWB-MP model in an FPGA would result in very high resource usage, system architecture recommendations are provided. Our proposed implementation of the model compensates for harmonics up to and including the 3rd order, which has a power spectrum extending above 3600 MHz. Using the techniques discussed in this thesis, it is shown that a sampling rate of 4 GHz allows for cancellation of the SI signal while providing a reasonable balance between performance and resource usage. Matlab simulations of a DOCSIS node with various parameters and PA simulation models were conducted. The simulations showed that over 75 dB of cancellation of the SI signal is possible in an idealized hardware setup. It is also demonstrated that AWGN injected into the received signal does not reduce the ability of the model to estimate the PA harmonics, although the noise itself cannot be canceled. Further simulations showed that the UWB-MP model could cancel harmonics whose power is much higher than that specified in DOCSIS. Although the UWB-MP model was designed with memory polynomial type PAs in mind, simulation results show that significant cancellation is possible with PAs that are represented by Wiener models as well. Based on the simulation results, we recommend using a filter of length 20 coefficients for each harmonic in the UWB-MP model, and 60 iterations with 500 samples for estimating the coefficients with the least squares method

Adaptive optical feedforward linearization of optical transceiver for radio over fiber communication link

With the tremendous growth in numbers of mobile data subscribers and explosive demand for mobile data, the current wireless access network need to be augmented in order to keep up with the data speed promised by the future generation mobile network standards. Radio over fiber technology (RoF) is a cost effective solution because of its ability to support numerous numbers of simple structured base stations by consolidating the signal processing functions at the central station. RoF systems are analog systems where noise figure and spurious free dynamic range (SFDR) are important parameters in an RoF link. The nonlinearity of a laser transmitter is a major limiting factor to the performance of an RoF link, as it generates spurious spectral components, leading to intermodulation distortions (IMD), which limit the achievable SFDR of the analog RF wave transmissions. The device nonlinearity can be mitigated through various linearization schemes. The feedforward linearization technique offers a number of advantages compared to other techniques, as it offers good suppression of distortion products over a large bandwidth and supports high operating frequencies. On the other hand, feedforward linearization is a relatively sensitive scheme, where its performance is highly influenced by changing operating conditions such as laser aging, temperature effect, and input signal variations. Therefore, for practical implementations the feedforward system has to be real-time adaptive. This thesis aims to develop an adaptive optical feedforward linearization system for radio over fiber links. Mathematical analyses and computer simulations are performed to determine the most efficient algorithm for the adaptive controller for laser transmitter feedforward linearization system. Experimental setup and practical measurement are performed for an adaptive feedforward linearized laser transmitter and its performance is optimized. The adaptive optical feedforward linearization system has been modeled and simulated in MATLAB Simulink. The performances of two adaptive algorithms, which are related to the gradient signal method, such as least mean square (LMS) and recursive least square (RLS) have been compared. The LMS algorithm has been selected because of its robustness and simplicity. Finally, the adaptive optical feedforward linearization system has been set up with digital signal processor (DSP) as the control device, and practical measurement has been performed. The system has achieved a suppression of 14 dB in the third order IMD products over a bandwidth of 30 MHz, in a two-tone measurement at 1.7 GHz

Noise Suppression in OCDMA Networks using Nonlinear Optical Devices

Optical code division multiple access (OCDMA) is a multiplexing technique that has a number of inherent advantages that make it suitable for use in passive optical networks, such as allowing subscribers to transmit information in an asynchronous fashion over a single optical fibre. This form of multiplexing can provide a higher degree of flexibility and simplicity in comparison to other techniques. However, due to the asynchronous nature of transmission, OCDMA networks suffer from multiple access interference (MAI) and optical beat noise which severely impairs system performance. A number of solutions have been proposed to mitigate these noise sources. Increasing the optical code lengths used can reduce the level of optical beat noise, however this is generally at the expense of transmission speed and increased transmitter complexity. MAI suppression can be achieved through the use fibre-based nonlinear thresholders or optical time-gating. One problem with these solutions is the requirement of long lengths of nonlinear fibre that are susceptible to changes in environmental conditions. Therefore, this thesis focuses on the development and testing of a nonlinear optical receiver based on semiconductor devices for the suppression of noise in OCDMA systems. The nonlinear optical process of two-photon absorption (TPA) in a commercially available 1.3 micron Fabry-P´erot laser is investigated as a method for optical thresholding in an OCDMA system. It is shown that the use of a saturable absorber (SA) directly before the TPA-based detector can provide additional suppression of MAI noise. However, the level of beat noise that is present on the optical signal can be increased due to the nonlinear responses of both devices. As a result, a gain-saturated semiconductor optical amplifier (SOA) is demonstrated as a method for the reduction of optical beat noise. It is shown that error-free performance can be achieved in an optical testbed designed to simulate an OCDMA system using an SA-SOA-TPA-based receiver. The performance improvement due to the suppression of MAI and beat noise using an SA-SOA receiver is examined in relation to a current fibre-based thresholding technique; a Mamyshev filter. It is shown that the SA-SOA receiver can offer a similar level of improvement when compared to the performance of a Mamyshev filter