Pulse-pumping of cascaded Raman fibre amplifiers

Abstract

In this thesis, I investigate cascaded Raman fibre amplifiers (RFAs) pumped with shaped optical pulses delivered from a Yb doped fibre MOPA source. RFAs offer the potential to generate gain at any arbitrary wavelength with an appropriate pump source, limited only by the fibre’s transparency range. The use of a counter-propagating signal and pump creates a continuous gain, despite the instantaneous nature of stimulated Raman scattering (SRS). A high power Yb doped fibre source emitting around the 1050 to 1100 nm region offers a flexible pump source that can in principle be used to generate gain for any signal from ~1100 to 2000 nm in a silica-based fibre via cascaded SRS. This opens up opportunities for an ultrabroadband amplifier with unmatched spectral width. Furthermore, by using a pump source that is in a MOPA configuration there is a high degree of control over the output characteristics which offers the potential of near-instantaneous electronic control of ultra-broadband Raman gain spectra. In the simplest configuration, cascaded Raman wavelength shifting across a wide range of wavelengths using single-level pump pulses (i.e., approximate super-Gaussian pulses) is investigated. Using a silica-based highly nonlinear fibre (HNLF), cascaded Raman wavelength shifting up to seven Stokes orders is demonstrated and counter-propagating gain measurements are made across all seven Stokes orders. From a pump wavelength of 1064 nm, the peak gain of the 7th Stokes order was ~1575 nm which demonstrated the potential for gain covering more than 500 nm. I believe this is the first time such a measurement has been undertaken. Other fibre types were also studied for comparison. Furthermore, the noise performance and gain saturation properties of cascaded RFAs were investigated, as well. In a more advanced configuration, the Raman gain spectra produced from pumping the HNLF with step-shaped pump pulses are investigated. Such pulses consist of multiple levels with different, controllable, instantaneous powers. By adjusting the power of each step appropriately I show that different parts of the pulse transfer their energy to different Stokes orders, leading to a controllable gain spectrum covering multiple Stokes orders at the same time. I further study how the gain spectrum can be controlled by manipulating the individual duty cycle of each section of the step-shaped pump pulses as well as using multiple pump wavelengths in a time-division multiplexed pumping scheme. Single and dual wavelength pumping of various fibres with step-shaped pulses was experimentally demonstrated. Raman gain spectra spanning two and three Stokes orders and covering over 100 nm were realised. Computer simulations are also carried out for pumping with more than two pump wavelengths and for gain spectra targeting gain simultaneously up to seven Stokes orders and covering up to ~500 nm. This shows that the use of step-shaped pulses and multiple pump wavelengths allow for further increase and control of the useable bandwidth.

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ORCID for Johan Nilsson: orcid.org/0000-0003-1691-7959

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