ABSTRACT
Optical fibre geometry confers many benefits but it also imposes some special constraints on the design of resonators. Whilst structures such as unstable resonators are not possible when single–mode fibres are used, an important benefit arises from the ease with which complicated designs of resonator can be realised in practice. An advantage of fibre geometry, which is of particular relevance to the topic of fibre resonators, is the availability of fibre-based components such as directional couplers. The theory of loss due to connectorisation using dissimilar single–mode fibres, which may have misaligned fibre ends, has been calculated in terms of the Gaussian mode approximation and is presented by Marcuse. Fibre lasers use the waveguiding properties of optical fibres, together with the gain produced by an inverted population of rare earth ions to produce the amplification necessary for lasing. This chapter discusses the constraints that the fibre geometry places on the design of laser resonators.
