Research On Complex Mode Theory For Tilted Fiber Gratings And Fiber Bundle Beam Coupling Technology For Laser Diode Array

Tilted fiber Bragg grating (TFBG) refractive index sensors are miniature, mini-mally invasive, can be interrogated remotely and are very promising to achieve a quasi-distributed multi-point sensing network. They are widely applicable in areas such as envi-ronmental monitoring, industrial process monitoring, chemical, biological and biomedical applications. However, a versatile theory for tilted fiber gratings is still absent, which re-stricts the cognition of the tilted fiber gratings and limits the practicality of the TFBG based devices. High power fiber lasers are widely used in areas of military, medical, in-dustrial processing etc. due to their outstanding merits, such as compactness, good ther-mal property, high beam quality etc. However, the beam emitted from the pump source i.e. laser diode array has lots of unfavorable features. Thus, collimation, shaping and focusing of the pump beam and finally coupling the light into the fiber are of vital impor-tance for the implementation of high power fiber lasers. The thesis is mainly devoted to the development of complex mode theory for tilted fiber gratings, theoretical research on the tilted fiber Bragg grating refractometers and experimental research on the fiber bundle beam coupling technology for laser diode array. The main achievements of this thesis are listed as follows.1. An improved full-vector finite difference (FD) complex mode solver for optical fiber is proposed. Rigorous interface conditions and perfectly matched layer (PML) technique are applied in this mode solver. This mode solver shows significant im-provement in terms of accuracy, rate of convergence and self-consistence, in com-parison with the conventional FD scheme. It can be used to analyze the full-vector guided and leaky modes of optical fibers. The mode solver also provides a new open waveguide model, in which the continuous radiation modes can be approximated by the discrete complex (quasi-leaky) modes; hence the analysis of radiation mode coupling can be greatly simplified. This open waveguide model is the foundation of the complex mode theory for tilted fiber gratings.2. A comprehensive full-vector complex mode theory for fiber gratings is developed. It includes complex mode matching method (CMMM) for fiber gratings and com-plex coupled mode theory for tilted fiber gratings. This theory is based on the open waveguide model offered by the complex mode solver, in which the treatments of discrete guided modes and continuous radiation modes in the mode coupling prob-lems are unified. Hence the coupling to the radiation modes is greatly simplified. The complex mode theory can deal with the complicated mode coupling (includes guided cladding and unguided radiation modes) problems for the cases when the outer-cladding index of the fiber is smaller, equal and higher than that of the inner-cladding.3. The influence of mode loss on the feasibility of grating-assisted optical fiber sur-face plasmon resonance (SPR) refractive index (RI) sensors is investigated. It is demonstrated that the grating length should be smaller than or comparable with the propagation length of surface plasmon polarition (SPP) mode in order to achieve effective coupling. More generally, in order to achieve effective mode coupling with the help of waveguide grating, the grating length is bounded by the shortest propagation length of the modes in lossy waveguides.4. The titled fiber Bragg grating (TFBG) refractometer is investigated comprehen-sively. The polarization effects in TFBGs and their influences on the performance of TFBG refractometers are investigated in detail. The possible paths to reduce the influence of polarization effects are discussed. For large surrounding refrac-tive index (SRI) changes, the cutoff resonance wavelength can be used as sensing parameter to achieve a polarization insensitive SRI measurement with linear sen-sitivity of 500nm/u.r.i and SRI uncertainty of～10-3 over SRI range of 0.1; for tiny SRI changes the measurement can be refined by monitoring proper individual resonance (close to the cutoff resonance wavelength) with accuracy approaching 2.5×10-5 over a SRI range of 0.001.5. The fiber bundle coupling technology for laser diode array is investigated experi-mentally. A silica cone array is fabricated to couple light emitted from a fast-axis collimated laser diode array. A method for the measurement of beam intensity dis-tribution at the facet of laser diode array with bare silica fiber probe is proposed. The fabrication process of silica based V groove array with optical lithography for fiber bundle coupling of laser diode array is investigated. A silica V-groove array with 200 micron spacing is fabricated with this method. This work offers a method to fabricate simple and economic fiber bundle coupling devices for laser diode ar-ray.