Polarization mode dispersion emulation and the impact of high first-order PMD segments in optical telecommunication systems

In this study, focus is centred on the measurement and emulation of first-order (FO-) and second-order (SO-) polarization mode dispersion (PMD). PMD has deleterious effects on the performance of high speed optical transmission network systems from 10 Gb/s and above. The first step was characterising deployed fibres for PMD and monitoring the state of polarization (SOP) light experiences as it propagates through the fibre. The PMD and SOP changes in deployed fibres were stochastic due to varying intrinsic and extrinsic perturbation changes. To fully understand the PMD phenomenon in terms of measurement accuracy, its complex behaviour, its implications, mitigation and compensation, PMD emulation is crucial. This thesis presents emulator designs which fall into different emulator categories. The key to these designs were the PMD equations and background on the PMD phenomenon. The cross product from the concatenation equation was applied in order to determine the coupling angle β (between 0o and 180o) that results in the SO-PMD of the emulator designs to be either adjustable or fixed. The digital delay line (DDL) or single polarization maintaining fibre (PMF) section was used to give a certain amount of FO-PMD but negligible SO-PMD. PMF sections (birefringent sections) were concatenated together to ensure FO- and SO-PMD coexist, emulating deployed fibres. FO- and SO-PMD can be controlled by altering mode coupling (coupling angles) and birefringence distribution. Emulators with PMD statistics approaching the theoretical distributions had high random coupling and several numbers of randomly distributed PMF sections. In addition, the lengths of their PMF sections lie within 20% standard deviation of the mean emulator length. Those emulators with PMD statistics that did not approach the theoretical distributions had limited numbers of randomly distributed PMF sections and mode coupling. Results also show that even when an emulator has high random mode coupling and several numbers of randomly distributed PMFs, its PMD statistics deviates away from expected theoretical distributions in the presence of polarization dependent loss (PDL). The emulators showed that the background autocorrelation function (BACF) approaches zero with increasing number of randomly mode coupled fibre sections. A zero BACF signifies that an emulator has large numbers of randomly distributed PMF sections and its presence means the opposite. The availability of SO-PMD in the emulators made the autocorrelation function (ACF) x asymmetric. In the absence of SO-PMD the ACF for a PMD emulator is symmetric. SO-PMD has no effect on the BACF. Polarization-optical time domain reflectometry (P-OTDR) measurements have shown that certain fibre sections along fibre link lengths have higher FO-PMD (HiFO-PMD) than other sections. This study investigates the impact of a HiFO-PMD section on the overall FO- and SO-PMD, the output state of polarization (SOP) and system performance on deployed fibres (through emulation). Results show that when the wavelength-independent FO-PMD vector of the HiFO-PMD section is greater than the FO-PMD contributions from the rest of the fibre link, the mean FO-PMD of the entire link is biased towards that of the HiFO-PMD section and the SO-PMD increases (β ≠ 0o or 180o) or remains fixed (β = 0o or 180o) depending on the coupling angle β between the HiFO-PMD section and the rest of the fibre link. In addition, the FO-PMD statistics deviates away from the theoretical Maxwellian distribution. However, experimental results show that the HiFO-PMD section has negligible influence on the SOPMD statistical distribution. An increase in the amount of FO-PMD on a HiFO-PMD section reduces the output SOP spread to a given minimum, in this study the minimum was reached when the HiFO-PMD ≥ 35 ps. However, the outcome of the output SOP spread depends on the location of the HiFO-PMD section along the fibre link length. It was found that when the HiFO-PMD section introduces SO-PMD, the bit error rate (BER) is much higher compared to when it does not introduce SO-PMD

Coexistence of optical systems on a physical layer

Tato diplomová práce se zabývá koexistencí optických systémů na společné fyzické vrstvě. Cílem této práce je analýza interakcí mezi různými optickými systémy na fyzické vrstvě, přičemž dílčím cílem je porovnání integrace těchto systémů za různých provozních podmínek. Data pro tuto práci byla získána pomocí simulačního prostředí Optsim. Na základě výsledků koexistence různých systémů za různých provozních podmínek lze vyvodit závěr, zda je možné systémy sloučit či se tato varianta nasazení nedoporučuje.This thesis deals with coexistence of optical systems on a physical layer. The main objective of this thesis is to analyse interactions between multiple optical systems at the physical layer, while partial goal is to compare the integration of these systems under different system conditions. Data for this study were obtained by computer simulation in Optsim environment. On the basis of the resulting models of coexistence of different transmission systems under various system conditions it can be concluded, whether it is recommended to combine certain systems or not

Nonlinear effects with a focus on cross phase modulation and its impact on wavelength division multiplexing optical fibre networks

The demand for faster data transmission is ever increasing. Wavelength division multiplexing (WDM) presents as a viable solution to increase the data transmission rate significantly. WDM systems are based on the ability to transmit multiple wavelengths simultaneously down the fibre. Unlike time division multiplexing (TDM) systems, WDM systems do not increase the data transfer by increasing the transmission rate of a single channel. In WDM systems the data rate per channel remains the same, only multiple channels carry data across the link. Dense wavelength division multiplexing (DWDM) promises even more wavelengths packed together in the same fibre. This multiplication of channels increases the bandwidth capacity rapidly. Networks are looking into making use of technology that will ensure no electronic signal regeneration at any point within the DWDM network. Examples are; reconfigurable optical add/drop multiplexers (ROADM) and optical cross connect (OXC) units. These components essentially enable network operators to split, combine and multiplex optical signals carried by optical fibre. WDM allows network operators to increase the capacity of existing networks without expensive re-cabling. This provides networks with the flexibility to be upgraded to larger bandwidths and for reconfiguration of network services. Further, WDM technology opens up an opportunity of marketing flexibility to network operators, where operators not only have the option to rent out cables and fibres but wavelengths as well. Cross phase modulation (XPM) poses a problem to WDM networks. The refractive index experienced by a neighbouring optical signal, not only depends on the signal’s intensity but on the intensity of the co-propagating signal as well. This effect leads to a phase change and is known as XPM. This work investigates the characteristics of XPM. It is shown that, in a two channel WDM network, a probe signal’s SOP can be steered by controlling a high intensity pump signal’s SOP. This effect could be applied to make a wavelength converter. Experimental results show that the degree of polarization (DOP) of a probe signal degrades according to a mathematical model found in literature. The pump and probe signals are shown to experience maximum interaction, for orthogonal probe-pump SOP vector orientations. This may be problematic to polarization mode dispersion compensators. Additionally, experimental results point out that the SOP of a probe signal is much more active in the presence of a high intensity pump, as compared to the single signal transmission scenario

PMD impairments in optical fiber transmission at 10 Gbps and 40 Gbps

The continuous need for greater bandwidth and capacity to support existing and emerging technologies, such as fiber-to-the-home (FTTH) and Internet Protocol Television(IPTV), drive optical-communication systems to higher and higher data rates per wavelength channel, from 10 to 40 Gbps and above. Degrading effects that tended to cause non catastrophic events at lower bit rates have become critical concerns for high-performance networks. Among them, polarization-mode dispersion (PMD) is perhaps the largest concern and, therefore, has garnered a great amount of attention. The PMD arises in an optical fiber from asymmetries in the fiber core that induce a small amount of birefringence that randomly varies along the length of the fiber. This birefringence causes the power in each optical pulse to split between the two polarization modes of the fiber and travel at different speeds, creating a differential group delay (DGD) between the two modes that can result in pulse spreading and intersymbol interference. PMD becomes a unique and challenging hurdle for high-performance systems mainly due to its dynamic and random nature. The polarization state is generally unknown and wanders with time. In general, PMD effects are wavelength (channel) dependent and can vary over a time scale of milliseconds. As a random variable, the DGD follows a Maxwellian distribution for which high-DGD points in the tail of the distribution can lead to network outages. Typically, system designers require the outage probability for high-performance networks to be 10−5 or less (penalty > 1dB for <30 min/yr). Clearly, the most straightforward approach to overcoming the effects of PMD is to employ newly manufactured low-PMD optical fibers, which have PMD values < 0.1 . However, much of the previously embedded fiber has high PMD values between 0.5 and 1 or even higher. The reality of deploying new systems over the embedded fiber means that the PMD monitoring and compensation are important for PMD mitigation. Unlike other degrading effects such as chromatic dispersion, the PMD is a time-varying random process making compensation difficult.The aim of this work is to study the trend of PMD effects over two different system, at 10 and 40 Gbps with two kind of fiber with high (0.5 ) and low (0.1 ) PMD coefficient. The first one corresponds to an old fiber’s type, that is used in the majority in the current transmission system; while the second one corresponds to a new fiber’s type, designed to have a lower response to the PMD phenomenon, making possible the transmission over long distances at high bit-rates. This work is structured as follows: After a short introduction, the second chapter is a review of PMD theory; where the PMD is faced from a theoretical point of view. It’s reported how the PMD arises in a fiber, how the DGD has a Maxwellian probability distribution, and the outage limits to design a system under the influences of PMD. In the third chapter there is a literary review over the PMD mitigation. Over the years, research groups from around the globe have proposed and/or demonstrated different strategies for PMD compensation. In this chapter an overview of these strategies shall be given, mentioning their relative merits and demerits. Following that, methods to increase the tolerance of a fiber-optic communication system to PMD, will also be discussed. After this theoretical introduction the central part’s of this study starts. In the fourth chapter the limitations imposed by the PMD are investigated. We starts probing the theoretical distance limits imposed by the only PMD, setting all other fiber’s impairments and attenuation to be negligible. Sequentially two single span optical transmission systems are compared on the basis of fiber PMD coefficient and bit-rates, to find the maximum distance that can be reached with a bit error rate of 10-10, taking in account or not the PMD and setting only the attenuation of the fiber. After this first investigation, the real impact of PMD was reported, performing a simulation of a multi span system, where the fiber’s attenuation of each section is compensated by an amplifier, so to find the maximum reachable distances over long-haul transmission and clearly see how is the PMD impact.In the last chapter a first-order polarization compensator is tested. Firstly in order to show how the compensator could works, the monitor signal’s simulation (based on the analysis of the Power Spectral Densities at selected frequency) is made, to show how the PMD level is related to the PSD. After that, the compensator is tested, performing two simulations at 10 and 40 Gbps with different value of DGD reached at the end of the fiber, to demonstrate the real capability of the compensator. The last study done is over the compensation applied to the previous multi-span system, to study how the performance of a system get increasing with a PMD compensation, and what is the system tolerance to PMD with or without compensation. All the simulation of this work are made with the use of a software package (1) used in the optical laboratory of Universitat Politècnica de Catalunya

Quantum Sensors: Improved Optical Measurement via Specialized Quantum States

Classical measurement strategies in many areas are approaching their maximum resolution and sensitivity levels, but these levels often still fall far short of the ultimate limits allowed by the laws of physics. To go further, strategies must be adopted that take into account the quantum nature of the probe particles and that optimize their quantum states for the desired application. Here, we review some of these approaches, in which quantum entanglement, the orbital angular momentum of single photons, and quantum interferometry are used to produce optical measurements beyond the classical limit