Volume holographic infra-red filters in iron doped lithium niobate
Two collimated laser beams, wavelength O-514 μm are overlapped within a crystal of heavily iron doped lithium niobate to form a one-dimensional volume holographic grating, a few mm long, designed to behave as a highly selective filter at near infra-red wavelengths. A novel recording geometry is described and a variety of diagnostic experiments are undertaken to determine the main characteristics of the grating. A filter fabricated using this method is used to obtain single mode operation from a 1-55 μm semiconductor laser. The historical survey of holography contained in chapter one emphasizes the dynamic properties and applications of photorefractive recording materials. Standard results of two mathematical theories, kinematic and coupled-wave, are used in chapter two to predict the properties of a Bragg grating when replayed with infra-red light. Chapter three details the recording materials suitable for an infra-red filter, followed by a discussion on the concept of 'scaling'. The recording mechanism of iron doped lithium niobate, including its fixing and erasure process, is presented in chapter four as a basis for selecting the most advantageous crystal characteristics. Mathematical models describing the complex dynamic recording process in iron doped lithium niobate are introduced in chapter five. A novel recording geometry involving the use of two prisms is critically described in chapter six, from initial conception to final implementation. The performance of the grating as an infra-red filter is assessed in chapter seven by measuring the angular response of the transmitted and diffracted beams at a wavelength of 1-152 microns. Chapter eight discusses the applications for volume holographic filters, with particular reference to one example. In chapter nine general conclusions are drawn and future directions for research are suggested. A number of appendices are also included.