APPLICATION SPECIFIC SPECIALTY OPTICAL FIBERS AND WAVEGUIDES

Abstract

In this thesis some novel designs of photonic crystal fibers and rectangular waveguides have been reported for the applications: i) in which the nonlinearity need to be eliminated (such as high power fiber lasers and amplifiers); ii) in which the nonlinearity need to be enhanced (such as supercontinuum and slow light generation). Large mode area photonic crystal fiber designs for high power fiber lasers and amplifiers have been achieved by tailoring the size of the air holes and introducing down doped fused silica rods in the selective air holes in the cladding region of the structures. The proposed designs offer effective single-mode operation even with large core size. Effective single-mode operation provides good beam quality at the output of fiber lasers. In the case of LMA waveguide design the trenches of lower refractive index in cladding region have been introduced in such a way that all the propagating modes become leaky. The basic principle behind the cladding profiles is to introduce the high leakage loss to the higher order modes while nominal leakage loss to the fundamental mode, which makes the design effectively single-moded. Supercontiuum generation (the creation of broadband spectral components when an intense laser pulse passes through a highly nonlinear medium) is an area of exciting research that has been attracting scientific interest over last several decades. The midinfrared spectral domain ranging from 2 – 15 μm is mainly important because of not only it contains two important windows (3 – 5 μm and 8 – 13 μm) in which the earth’s atmosphere is relatively transparent but also the strong characteristic vibration transitions of most of the molecules in this domain. Mid-infrared molecular ‘fingerprint region’ is applicable in various important applications in different diverse fields such as medical, industry, security and astronomy. In this thesis a sincere attempt has been done to design and analyze the dispersion engineered photonic crystal fibers and rib waveguide geometries for ultra broadband mid-infrared supercontinuum sources. Ever increased supercontinuum spectrum spanning 2 – 15 μm in As2Se3 based chalcogenide photonic crystal fiber and rib waveguide has been achieved using femtosecond laser Abstract viii source of relatively low peak power. Such ultra broadband supercontinuum spectrum has also been achieved using equiangular spiral photonic crystal fiber geometry. Slow light with tunable features is investigated in doped and undoped tellurite fibers and As2Se3 based photonic crystal fiber geometries for telecommunication and computing applications. The maximum time delay up to 137 ns can be obtained using 1 meter long photonic crystal fiber pump with 100 mW. All the PCF and waveguide designs which are applicable for supercontinuum and slow light generation have been designed such that the propagating mode is strongly confined in small core of the structure, which makes the designs highly nonlinear.