Fibre Raman Amplification of Bessel and Orbital Angular Momentum Higher Order Modes in Step Index Fibres

Abstract

Large mode area (LMA) has been a cornerstone of high power fibre amplifiers and lasers thanks to the power scaling capability without exceeding the intensity threshold for nonlinear effects and optical damage. Many applications require single-mode operation, and conventionally, these fibres are designed to operate with the fundamental mode. Alternatively, a single higher order mode (HOM) with the intensity distribution of a zeroth order Bessel function LP0m (usually m ≥ 4) can propagate stably in a highly multimode step-index fibre. These HOMs are also reported to be more resistant to bending induced distortions and mode coupling than the fundamental mode. Fibre amplifiers for these modes may have higher mode area scaling capability and can then offer higher power threshold for nonlinearities. So far, rare-earth (RE) doped fibre amplifiers for these HOMs have been demonstrated, while fibre Raman amplifiers (FRAs) for such modes have not been reported prior to my thesis work. FRAs are unique in that they are wavelength agile. More importantly for my thesis work, they may preserve the target HOM better than a RE-doped fibre amplifier does thanks to not only better control of fibre perturbations, but also to the absence of local gain saturation. Additionally, FRAs for HOMs can be better than cladding pumped FRAs in that they fulfil the area ratio requirement for efficient first order Stokes conversion while allowing stable single mode operation.
This thesis is dedicated to the demonstration of the mode preservation and brightness enhancement capability of FRAs for HOMs. I start with investigations of signal mode purity degradation via numerical simulations and compare the performance of typical Yb-doped fibre amplifiers (YDFAs) and FRAs both with the same HOM. Results show that the mode purity is preserved better in the FRA than in the YDFA for incoherent signal and pump. My experimental demonstrations are restricted to FRAs and start with pulsed Raman amplification of a single HOM in large core step index fibres with lengths of ~10 m. The signal output sees 18 dB gain and is ~14 dB brighter in terms of maximum power in a single mode than the launched multimode pump, with good mode purity preservation and weighted conversion efficiency. Then a pulsed FRA in 335 m of large core step index fibre is demonstrated with over 23 dB amplification to the signal and ~16 dB brightness enhancement from the launched pump. The purity degradation is modest and is likely to be the result of the emergence of 2nd order Stokes. The low pump brightness also points towards direct diode laser pumping, which I attempted with an 877-m large core step index fibre and spectrally combined pumping from five diode lasers. A 6 dB Raman gain of the signal is achieved with quasi-CW pump with ~170 W combined peak power, which is expected to be improved with further optimizations. These results successfully demonstrate the capability and potential of power scaling and brightness enhancement of FRAs with HOMs.
The advantages of FRAs are also utilized with fibre modes carrying orbital angular momentum (OAM). These OAM modes (OAMM) can be viewed as pairs of spatially coherent degenerate modes, in which the angular momentum improves the stability to some perturbations. Although in this case, the saturation of RE-doped fibre amplifiers may aid in suppressing the growth of degenerate parasitic modes, notably those of conjugate charges, FRAs are still preserving the target OAMM well, which is evident by the experimental results presented in the thesis. I first describe a pulsed FRA for a charge-2 OAMM in a 5-m multimode step index fibre with 15 dB maximum gain with an amplified signal mode purity of 83.2% dropping from 95.8% when unamplified. The purity degradation is attributed to coupling into the conjugate charge which I expect to decrease for higher charges. The subsequent experiment of Raman amplification of a charge-15 OAMM in a 30-m large core step index fibre confirms this. The signal gain reached 9.2 dB, with only modest purity degradation from 84.3% to 78.4% and negligible coupling into the conjugate charges.
The results of both HOMs and OAMMs have proven the feasibility of FRAs for power scaling of these modes with great signal mode preservation, making FRAs an attractive alternative to RE-doped fibre amplifiers.

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Identifiers

ORCID for Sheng Zhu: orcid.org/0000-0002-4256-6307

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

ORCID for Jonathan HV Price: orcid.org/0000-0003-0256-9172

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