Pump conditioning and optimisation for erbium doped fibre applications

Abstract

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 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.

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Identifiers

ORCID for David J. Richardson: orcid.org/0000-0002-7751-1058

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