Fiber amplifiers, directly modulated transmitters and a ring network structure for optical communications

The three technologies that are considered the key elements in building a metropolitan area optical network are studied in this thesis. They are optical amplification, high-speed low cost transmitters and ring network structures. These studies concentrate on cost reduction of these three technologies thus enabling the use of optical networks in small customer base metropolitan areas. The research on optical amplification concentrated first on the solution doping process, at present the most used method for producing erbium doped fiber. It was found that separationing the soot growth and the sintering improved the uniformity of the porous layer. This made the homogeneity of the doping concentration in the fiber core better. The effects of index profile variations that arise from the non-ideal solution doping process were also simulated. In the search for a better doping method a new nanoparticle glass-forming process, the direct nanoparticle deposition, was developed. In this process the doping is done simultaneously with glass formation. Utilizing this new process it was possible to improve the uniformity of the doping resulting in higher usable doping levels and shorter erbium doped fiber lengths in the amplifiers. There were fewer limitations in the amplifier caused by optical non-linearities and polarization mode dispersion since shorter fiber lengths were needed. The double cladding fiber, which avoids the costly coupling of the pump laser into a single mode waveguide, was also studied. This pumping scheme was found to improve the inversion uniformity in the erbium doped fiber core thereby enhancing the power conversion efficiency for the long wavelength band amplifier. In characterizing the erbium doped fiber amplifier the gain and noise figure was measured with a temporal filter setup. It was made of simple, low cost components but yielded accurate measurements since the noise originating from the amplified spontaneous emission was measured at the signal wavelength. In the study of fiber amplifier controlling schemes the input power of the fiber amplifier was successfully used to regulate the pump laser. This feed-forward control scheme provides a simple, low cost control and managment system for the erbium doped fiber amplifier in metropolitan area network applications that require flexible adding and dropping of wavelength channels. The transmitter research focused on the DFB laser due to its simplicity and low cost structure. A solid state Fabry-Perot etalon made from double polished silicon chip was used as a frequency discriminator in the chirp analyser developed for the DFB lasers. This wavelength discriminator did not require repeated calibration or active stabilisation and was controled electrically enabling automatic measurements. The silicon Fabry-Perot etalon was also used for simultaneous spectral filtering and wavelength control of the laser. The usable dispersion limited transmission length was increased when the filter was used in conjunction with the directly modulated distributed feedback laser transmitter. The combination of spatial multiplexing and dense wavelength division multiplexing in ring topology was investigated in the course of the research on the ring network as the feeder part of the metropolitan network. A new way to organize different wavelengths and fibers was developed. This ring network structure was simulated and an experimental ring network built. The results of the studies demonstrated that the same limitations effecting uni-directional ring structures also are the main limitations on the scalability of the spatial and wavelength division multiplexed ring networks based on bi-directional transmission when the node spacing is short. The developed ring network structure demonstrated major cost reductions when compared with the heavy use of wavelength division multiplexing. The node structure was also greatly simplified resulting in less need for different wavelength transmitters in each node. Furthermore the node generated only minor losses for the passing signals thus reducing the need for optical amplification.reviewe

Complete characterization of ultrashort pulse sources at 1550 nm

This paper reviews the use of frequency-resolved optical gating (FROG) to characterize mode-locked lasers producing ultrashort pulses suitable for high-capacity optical communications systems at wavelengths around 1550 nm, Second harmonic generation (SHG) FROG is used to characterize pulses from a passively mode-locked erbium-doped fiber laser, and both single-mode and dual-mode gain-switched semiconductor lasers. The compression of gain-switched pulses in dispersion compensating fiber is also studied using SHG-FROG, allowing optimal compression conditions to be determined without a priori assumptions about pulse characteristics. We also describe a fiber-based FROG geometry exploiting cross-phase modulation and show that it is ideally suited to pulse characterization at optical communications wavelengths. This technique has been used to characterize picosecond pulses with energy as low as 24 pJ, giving results in excellent agreement with SHG-FROG characterization, and without any temporal ambiguity in the retrieved puls

All-optical logic circuits based on the polarization properties of non-degenerate four-wave mixing

This thesis investigates a new class of all-optical logic circuits that are based on the polarization properties of non-degenerate Four-Wave Mixing. Such circuits would be used in conjunction with a data modulation format where the information is coded on the states of polarization of the electric field. Schemes to perform multiple triple-product logic functions are discussed and it is shown that higher-level Boolean operations involving several bits can be implemented without resorting to the standard 2-input gates that are based on some form of switching. Instead, an entire hierarchy of more complex Boolean functions can be derived based on the selection rules of multi-photon scattering processes that can form a new class of primitive building blocks for digital circuits. Possible applications of these circuits could involve some front-end signal processing to be performed all-optically in shared computer back-planes. As a simple illustration of this idea, a circuit performing error correction on a (3,1) Hamming Code is demonstrated. Error-free performance (Bit Error Rate of < 10^-9) at 2.5 Gbit/s is achieved after single-error correction on the Hamming word with 50 percent errors. The bit-rate is only limited by the bandwidth of available resources. Since Four-Wave Mixing is an ultrafast nonlinearity, these circuits offer the potential of computing at several terabits per second. Furthermore, it is shown that several Boolean functions can be performed in parallel in the same set of devices using different multi-photon scattering processes. The main objective of this thesis is to motivate a new paradigm of thought in digital circuit design. Challenges pertaining to the feasibility of these ideas are discusse

Measurement of the nonlinear refractive index (n2) and stimulated Raman scattering in optical fibers as a function of germania content, using the photorefractive beam coupling technique

One of the greatest challenges in optical communication is the understanding and control of optical fiber nonlinearities. While these nonlinearites limit the power handling capacity of optical fibers and can cause noise, signal distortion and cross talk in optically amplified transmission systems, they have been equally harnessed for the development of new generations of optical amplifiers and tunable laser sources. The two prominent parameters that characterize the nonlinear properties of an optical fiber are the nonlinear refractive index n2 and the Raman gain coefficient gR. These parameters are related to the third order nonlinear susceptibility [x(3)]. In this work, the photorefractive beam coupling technique, also called induced grating autocorrelation (IGA), has been used to measure the nonlinear refractive index (n2) and the Raman gain coefficient (gR) of short lengths (z ~ 20 m) of optical fibers. In the IGA experiment, a transform limited Gaussian pulse is propagated through a short length of an optical fiber, where it undergoes self-phase modulation (SPM) and other nonlinear distortions, and the output pulse is split into two. The two-excitation pulses are then coupled into a photorefractive crystal, where they interfere and form a photorefractive phase grating. The IGA response is determined by delaying one beam (the probe) and plotting the diffracted intensity of the probe versus the relative delay (τ).Analysis of the IGA response yields information about the nonlinear phase distortions and other calibration parameters of the fiber. Using the IGA technique the author has measured the nonlinear refractive index in several types of fibers, including pure silica, Er-Al-Ge doped fibers, DCF (dispersion compensating fiber) and the recently developed TrueWave Rs fiber, and investigated the dependence of n2 on the doping profiles of Er, Al, and Ge in optical fibers. The standard IGA model for n2 measurements was derived from the solution of the nonlinear wave equation for pulse propagation in the limit of pure self-phase modulation. This model assumed that GVD (group velocity dispersion) and other nonlinear processes such as SRS (stimulated Raman scattering) are negligible. This model has been successfully used to fit the experimental data and determine the n2 of the fiber from the time dependent phase shift. However, SRS has been observed to distort the IGA trace, thus leading to a breakdown of the standard IGA model. A new IGA model has been developed in this study from the solution of the coupled-amplitude nonlinear Schrodinger equation, using both analytical and numerical approaches. This new model successfully accounts for the SRS effects on the IGA trace, in the limit of zero GVD, and allows the direct determination of the Raman gain coefficient from the fit of the SRSdistorted IGA trace. The measured nonlinear refractive index and Raman gain coefficients are in good agreement with published results. It was also shown that in the limit of zero GVD and no Raman, the IGA technique reduces to the widely accepted spectral domain SPM technique pioneered by Stolen and Lin, but is readily applicable to shorter lengths of fiber and is sensitive to smaller phase shifts

Phase sensitive amplifiers for regeneration of phase encoded optical signal formats

We discuss the application of phase sensitive fiber optical parametric devices for the regenerative processing of high baud rate optical signals. We present recent advances in phase-sensitive amplification technology and its application to the regeneration of phase-encoded signals. By combining four wave mixing based parametric effects in highly nonlinear optical fibers and injection locking assisted synchronisation of multiple coherent lasers, we demonstrate how it possible to derive phase regeneration in signals with more than two levels of phase encoding