Stimulated Brillouin scattering (SBS) is useful, among others for generating slow light, sensing and amplification. SBS was previously viewed as a poor method due to the limitation on optical power in high-powered photonic applications. However, considering the many possible applications using SBS, it is now of interest to enhance SBS in areas of Brillouin frequency shift together with Brillouin Gain. A numerical model, using a fully vectorial approach, by employing the finite element method, was developed to investigate methods for enhancing SBS in optical fiber. This paper describes the method related to the numerical model and discusses the analysis between the interactions of longitudinal, shear and hybrid acoustic modes; and optical modes in optical fiber. Two case studies were used to demonstrate this. Based on this numerical model, we report the influence of core radius, clad radius and effective refractive index on the Brillouin frequency shift and gain. We observe the difference of Brillouin shift frequency between a normal silica optical fiber and that of a microfiber - a uniformed silica fiber of a much smaller core and cladding dimensions where nonlinearities are higher. Also observed, the different core radii used and their respective Brillouin shift. For future work, the COMSOL model can also be used for the following areas of research, including simulating "surface Brillouin shift" and also to provide in-sights to the Brillouin shift frequency vB of various structures of waveguides, e.g circular, and triangular, and also to examine specialty fibers, e.g. Thulium and Chalcogenide doped fibers, and their effects on Brillouin shift frequency.
A new approach for filtering an optical band-pass in optical amplifier is proposed using a macro bending. The proposed filter leverages the bending loss of higher order modes at shorter wavelengths. At longer wavelengths, the filter increases fiber's bending loss as the fundamental mode 'tail' is leak out from the cladding. The combination of wavelength dependent loss at longer and shorter wavelength gives rise to the optical band-pass filter characteristic inside the fiber. The simulated spectral response of the filter is found to be in good agreement with the experimental results. Subsequently, the proposed optical band-pass filter is applied in Thulium-doped fiber amplifiers (TDFA) system for gain and noise figure enhancements. The filter functions to suppress both the amplified spontaneous emission (ASE) at 800 nm and 1800 nm wavelength regions and thus improves both gain and noise figure performances in S-band region. By bending of the gain medium, gain and noise figure of the TDFA are improved by about 2 dB and 0.5 dB respectively, within a wavelength region from 1440 and 1500 nm when the 1050 nm pump power is fixed at 250 mW.
This paper presents short wavelength operation of tunable thulium-doped mode-locked lasers with sweep ranges of 1702 to 1764 nm and 1788 to 1831 nm. This operation is realized by a combination of the partial amplified spontaneous emission suppression method, the bidirectional pumping mechanism and the nonlinear polarization rotation (NPR) technique. Lasing at emission bands lower than the 1800 nm wavelength in thulium-doped fiber lasers is achieved using mode confinement loss in a specially designed photonic crystal fiber (PCF). The enlargement of the first outer ring air holes around the core region of the PCF attenuates emissions above the cut-off wavelength and dominates the active region. This amplified spontaneous emission (ASE) suppression using our presented PCF is applied to a mode-locked laser cavity and is demonstrated to be a simple and compact solution to widely tunable all-fiber lasers.