Advanced waveguides for high power optical fibre sources
This thesis reports on theoretical and experimental studies of wavelength-selective waveguide structures for high-power Nd3+- and Yb3+-doped fibre lasers. Cladding-pumped high-power fibre lasers based on these novel waveguide designs and operating at desired unconventional wavelengths were investigated through numerical simulations and fibre laser experiments. Rare earth doped fibres have typically multiple emission bands of different effective strengths. Stimulate emission from strong bands dominates over, and via a reduced population inversion normally even suppresses, emission from weaker bands in conventional step-index waveguides. For efficient emission and laser operation on the weaker emission bands, it is necessary to suppress unwanted stimulated emission on the strong transitions by preventing power from building up at the unwanted wavelengths. Discrete "bulk" (non-waveguide) devices at a single or a few points are ineffective, if the gain at unwanted wavelength is sufficiently high to generate high-power amplified spontaneous emission even between filters. In such cases, waveguide structures which reduce the gain at unwanted wavelengths and prevent build-up of unwanted emission can be considered. The fibre itself acts as a distributed wavelength-selective filter, and a compact all-fibre laser can be made. For short-wavelength operation when the gain at longer wavelengths needs to be suppressed, a helical core fibre is proposed. This induces a large bending loss at unwanted longer wavelengths while the bending loss at desired shorter wavelength remains relatively low. The required bending loss properties, for efficient operation at the desired shorter wavelength, can be achieved by designing the helix pitch and offset along with fibre core diameter and NA (numerical aperture). A Nd3+-doped helical fibre laser operating at 0.92 μm was investigated through computer simulations. Alternatively, there are fibres in which the fundamental mode can be cut off at a certain wavelength. I have studied fibres with a W-type refractive index profile and fibres with a hollow (air-filled) central region surrounded by a core and then a region with depressed refractive index, known as depressed-clad hollow fibre. With these fibre designs, the doped core guides the desired shorter wavelength but not the unwanted longer wavelengths. Nd3+-doped W-type fibre lasers operating at 0.92 μm were simulated and experimentally demonstrated. Also Yb3+-doped depressed-clad hollow fibre lasers operating at 0.98 μm were simulated and experimentally demonstrated. For long wavelength operation, with a suppressed gain at shorter wavelengths, modified W-type designs are proposed. By designing the refractive index profile and using ring-shaped gain regions, the net gain on an intrinsically weak long-wavelength transition may become larger than that on an intrinsically stronger short-wavelength transition. Adopting this technique, Nd3+-doped fibre amplifiers and lasers operating at 1.38 μm were simulated. While fibre lasers that generate a nearly diffraction-limited single-mode beam are normally targeted, a multimode output is often obtained, e.g., in development stages with nonideal fibres. Then it is important to characterise the modal properties of the beam. For this, two different modal power decomposition methods based on intensity measurements are proposed. The first method is based on a tomography technique that uses a Wigner function followed by an inverse Radon transform. The second method adopts a wavelength-sweeping optical source which induces beat patterns after propagation through a certain length of fibre. The feasibilities of the two proposed ideas were verified through numerical simulations.