The Photonic Lantern

Photonic lanterns are made by adiabatically merging several single-mode cores into one multimode core. They provide low-loss interfaces between single-mode and multimode systems where the precise optical mapping between cores and individual modes is unimportant.Comment: 45 pages; article unchanged, accepted for publication in Advances in Optics and Photonic

Mode Evolution in Fiber Based Devices for Optical Communication Systems

Space division multiplexing (SDM) is the most promising way of increasing the capacity of a single fiber. To enable the few mode fiber (FMF) or multi-mode fiber (MMF) transmission system, several major challenges have to be overcome. One is the urgent need of ideal mode multiplexer, the second is the perfect amplification for all spatial modes, another one is the modal delay spread (MDS) due to group velocity difference of spatial modes. The main subject of this dissertation is to model, fabricate and characterize the mode multiplexer for FMF transmission. First, we designed a novel resonant mode coupler (structured directional coupler pair). After that, we studied the adiabatic mode multiplexer (photonic lantern). 6-mode photonic lantern using graded-index (GI) MMFs is proposed and demonstrated, which alleviates the adiabatic require-ment and improves mode selectivity. Then, 10-mode photonic lantern is demonstrated using novel double cladding micro-structured drilling-hole preform, which alleviates the adiabatic requirement and demonstrate a feasible way to scale up the lantern modes. Also, multi-mode photonic lantern is studied for high order input modes. In addition, for the perfect amplification of the modes, cladding pump method is demonstrated. The mode selective lantern designed and fabricated can be used for the characterization of few mode amplifier with swept wavelength interferometer (SWI). Also, we demonstrated the application of the use of the few mode amplifier for the turbulence-resisted preamplified receiver. Besides, for the reduction of MDS, the long period grating for introducing strong mode mixing is demonstrated

Development of photonic technologies for astronomical instruments using ultrafast laser inscription

Recently there has been a desire to apply photonic concepts and technologies to astronomical applications, with the aim of replacing traditional bulk optic instruments. This astrophotonic approach is envisioned to produce compact devices that have the potential to provide the unprecedented precision and stability required for current astronomical goals, such as the detection of Earth-like exoplanets capable of supporting life. The work in this thesis covers the investigation of the technique of Ultrafast Laser Inscription (ULI) to create the building blocks that may lead to a fully integrated compact spectrograph for astronomy. Unlike conventional fabrication technologies, ULI allows custom three-dimensional optical devices to be directly inscribed within a bulk substrate. Volume gratings with high first order diffraction efficiencies optimised for a variety of wavelengths are demonstrated, with a view to providing efficient gratings for the midinfrared wavelength range. Initially the mid infrared transmitting material GLS was used to create gratings with a first order efficiency of 63 % up to a wavelength of 1.35 μm. Anti-reflection coatings were applied to GLS and gratings with an efficiency of 95 % at 1.02 μm were produced. A second material, IG2 was used and diffraction gratings with a first order efficiency of 63 % were produced, which were efficient up to a wavelength of 2.5 μm, with thicker gratings produced which have yet to be characterised in a mid-infrared setup. These developments show that practical mid-infrared volume gratings can be produced by the process of ULI. Photonic reformatters have also been developed to reshape a multimode telescope point spread function into a pseudo-slit, suitable as an input for a diffraction-limited spectrograph. Two device designs were investigated. The first was a fully integrated ULI component which, tested in the laboratory reformatted a multimode input at 1550 nm into a slit, single mode in one axis and highly multimode in the orthogonal axis with an efficiency of 66 %. The device was tested on-sky at the William Herschel Telescope and performed with an efficiency of 19.5 % over the wavelength range 1450 to 1610 nm. The second, improved device combined a ULI component with a multicore fibre component, and performed with a similar performance in the laboratory demonstrating an efficiency of 69 %, but a much improved on sky efficiency of 53 % showing a potential for such devices to be used as an input for a diffraction limited spectrograph