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When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON Faculty of Physical and Applied Sciences Optoelectronics Research Centre (ORC) Pump Conditioning and Optimisation for Erbium Doped Fibre Applications by Ee Leong Lim Thesis for the degree of Doctor of Philosophy Sep 2012 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF ENGINEERING AND APPLIED SCIENCE OPTOELECTRONICS RESEARCH CENTRE Doctor of Philosophy PUMP CONDITIONING AND OPTIMISATION FOR ERBIUM DOPED FIBRE APPLICATIONS by Ee Leong Lim This thesis presents my investigation into in-band pumped erbium doped fibre amplifiers (EDFAs) and their performance under high power continuous wave (cw) operation and high energy low repetition rate pulsed operation. In addition, Q-switched erbium doped fibre lasers were investigated and used as the seed laser for a high energy low repetition rate EDFA system. Furthermore, the power scaling of all-fibre frequency doubled fibre lasers based on periodically poled silica fibre (PPSF) was also investigated. In Q-switched fibre lasers, the multiple-peak phenomenon (MPP) is an undesirable effect in which the Q-switched pulse develops sub-structure or even breaks into multiple sub pulses. I demonstrated that the MPP can be eliminated by increasing the acousto-optic modulator rise time. An experimentally validated numerical model was also used to explain the origin of MPP. Next, I showed that the interplay between MPP and modulation instability (MI) changes the detail of the spectral evolution of the Q-switched pulses. The in-band EDFAs were investigated using 1535 nm pump fibre lasers. For cw operation, a highly efficient (~ 80%), high power (18.45 W) in-band, core pumped erbium/ytterbium co-doped fibre laser was demonstrated. Using a fitted simulation model, I showed that the significantly sub-quantum limit conversion efficiency of in-band pumped EDFAs observed experimentally can be explained by concentration quenching. I then numerically studied and experimentally validated the optimum pumping configuration for power scaling of in-band, cladding pumped EDFAs. My simulation results indicate that a ~ 77% power conversion efficiency with high output power should be possible through cladding pumping of current commercially available pure erbium doped active fibres providing the loss experienced by the cladding guided 1535 nm pump due to the coating absorption can be reduced to an acceptable level by better coating material choice. The power conversion efficiency has the potential to exceed 90% if concentration quenching of erbium ions can be reduced via improvements in fibre design and fabrication. For low repetition rate pulsed operation, I demonstrated and compared high-energy, in-band pumped EDFAs operating at 1562.5 nm under both a core pumping scheme (CRS) and a cladding pumping scheme (CLS). The CRS/CLS sources generated smooth, single-peak pulses with maximum pulse energies of ~1.53/1.50 mJ, and corresponding pulse widths of ~176/182 ns respectively, with an M2 of ~1.6 in both cases. However, the conversion efficiency for the CLS was >1.5 times higher than the equivalent CRS variant operating at the same pulse energy due to the lower pump intensity in the CLS that mitigates the detrimental effects of concentration quenching. With a longer fibre length in a CLS i implementation a pulse energy of ~2.6 mJ was demonstrated with a corresponding M2 of ~4.2. Using numerical simulations I explained that the saturation of pulse energy observed in my experiments was due to saturation of the pump absorption. For the frequency doubling work, the fundamental pump source of the PPSF was a master oscillator power amplifier seeded with a tuneable external cavity laser. During the high power operation, the heat deposition along the PPSF shifted the optimal quasi-phase matched wavelength to a longer wavelength. This shift must be compensated to achieve optimal performance of the PPSF under test and was achieved in my experiment by tuning the central wavelength of the pump source. At the end of the high power experiment, the PPSF samples degraded to ~40% of their pristine PPSF normalised efficiencies. The glass property of the PPSF had also been changed by the high power exposure. A high power all-fibre frequency doubled laser was demonstrated with 1.13 W of second harmonic average power with ~27% internal conversion efficiency. ii Table of contents Chapter 1 Introduction ............................................................................................................................ 1 1.1 The motivation ............................................................................................................................ 1 1.2 Structure of the thesis ................................................................................................................. 5 Chapter 2 Background ............................................................................................................................ 7 2.1 Optical properties of optical fibres ............................................................................................. 7 2.1.1 Wave equations for optical fibres ...................................................................................... 7 2.1.2 Weakly guided step index fibres ........................................................................................ 8 2.1.3 Fibre modes in multimode fibres ..................................................................................... 10 2.1.4 Guided modes in actual fibres .......................................................................................... 12 2.2 Optical nonlinearity .................................................................................................................. 14 2.2.1 Nonlinear induced polarisation ........................................................................................ 14 2.2.2 Third order nonlinear processes ....................................................................................... 15 2.2.3 Nonlinearities in optical fibre .......................................................................................... 18 2.3 Erbium doped fibre amplifiers .................................................................................................. 26 2.3.1 Emission and absorption of light...................................................................................... 26 2.3.2 Pump wavelengths for erbium ions .................................................................................. 28 2.3.3 Pump coupling scheme for active fibres .......................................................................... 29 2.3.4 Overlap factor .................................................................................................................. 32 2.3.5 Quasi-three level fibre amplifier model ........................................................................... 33 2.3.6 Amplified spontaneous emission ..................................................................................... 33 2.3.7 Master oscillator power amplifier .................................................................................... 35 Chapter 3 Q-switched fibre lasers ......................................................................................................... 39 3.1 Point model of Q-switched laser ............................................................................................... 39 3.2 The multiple-peak phenomena in core-pumped Q-switched fibre laser ................................... 41 3.2.1 Experiment ....................................................................................................................... 41 3.2.2 Simulation ........................................................................................................................ 44 3.3 The MPP and nonlinear effects in Q-switched cladding-pumped fibre lasers .......................... 51 3.3.1 MPP in Q-switched cladding-pumped fibre lasers ........................................................... 51 iii 3.3.2 Nonlinear effects in Q-switched cladding-pumped fibre lasers ........................................ 52 3.3.3 The interplay of MPP and nonlinearity ............................................................................ 53 3.4 Conclusion ................................................................................................................................ 55 Chapter 4 Continuous wave in-band pumped fibre amplifiers .............................................................. 57 4.1 Background ............................................................................................................................... 57 4.1.1 Net cross