Advanced fiber components for optical networks

Abstract

Due to the tremendous growth in data traffic and the rapid development in optical transmission technologies, the limits of the transmission capacity available with the conventional erbium-doped amplifiers (EDFA), optical filters and modulation techniques have nearly been reached. The objective of this thesis is to introduce new fiber-optic components to optical networks to cope with the future growth in traffic and also to bring down the size and cost of the transmission equipment. Improvements in performance and in scalability of the optical networks are studied through simulations and experimental network set-ups.

High-power single-mode laser sources operating at 980 nm are important in pumping EDFAs and Raman amplifiers. In this thesis, two new practical, fiber-coupled configurations of stable high-power cladding-pumped Yb-doped fiber sources operating at 977 nm are presented: a fiber laser and an ASE (amplified spontaneous emission) or superfluorescent source. Sources are based on high numerical aperture Yb-doped jacketed air-clad fiber and high brightness pump diodes.

L-band EDFAs are used to expand amplification bandwidth beyond the C-band wavelengths. Traditional L-band EDFAs are costly devices, which are core-pumped with expensive high-power single-mode diodes. Cladding-pumping technology brings down the cost of the pump diodes in L-band EDFAs, since high-power but low-cost multimode pump diodes can then be used. Additionally, the flexibility in designing erbium-doped fiber is improved. In this thesis, a new design for L-band EDFA based on GTWave cladding-pumping technology is introduced. Simultaneous noise reduction and transient suppression in the amplifier is achieved by using a gain-clamping seed-signal.

To increase the spectral efficiency of the optical transmission systems optical filters having square spectral response and linear phase, leading to zero dispersion both in-band and out-of-band, are required. The application of inverse scattering technique in conjunction with advanced fiber Bragg grating writing technique significantly reduces in-band dispersion and greatly improves grating characteristics. In this thesis, the in-band and out-of-band dispersion penalty of a cascade of linear-phase fiber Bragg grating (FBG) filters is experimentally measured and compared to the results with conventional apodized FBG filters.

Fiber Bragg grating based distributed feedback fiber lasers (DFB FL) are attractive alternatives to semiconductor lasers. Output power and efficiency of DFB FLs can be significantly increased by using a master-oscillator-and-power-amplifier (MOPA) configuration, consequently degrading optical signal to noise ratio (OSNR) and RIN of the master source. These trade-offs are studied in several MOPA configurations using core-pumped and cladding-pumped EDFAs as power amplifiers and compared to the results with a high-power stand-alone DFB-FLs, i.e. DFB FLs pumped with a high-power pump source.

Finally, the performance and scalability of a bidirectional and a high-density metropolitan WDM ring networks is analyzed. Results show that the scalability limitation imposed by the amplified RIN arising from the Rayleigh backscattering in bidirectional WDM ring networks can be avoided by using low gain shared-pump EDFAs and directly modulated transmitters. In high-density metropolitan WDM networks based on non-zero dispersion shifted fibers the main limiting nonlinearity is four-wave mixing. In metropolitan areas distributed Raman amplification (DRA) is the most effective means reduce the effect of four-wave mixing.