|Title:||Suspended-core photonic microcells made by post-processing micro-structured optical fibers|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Electrical Engineering|
|Pages:||xxi, 154 p. : ill. ; 30 cm.|
|Abstract:||Micro-structured Optical Fibers (MOFs) have attracted plenty of research interests for their unusual properties such as endlessly single mode operation, high birefringence, high non-linearity, and novel dispersion. The MOFs with holey structure also provides degrees of freedom to modify their properties by post-processing techniques. In this dissertation, a novel technique has been demonstrated for fabricating in-line suspended-core (SC) photonic microcells by post-processing holey MOFs. The technique is highly flexible in making SC microcells with various cross-sectional shapes. These microcells may find versatile applications in photonic sensors and devices. The technique involves opening of selected air-hole at the end of a MOF, pressurization of the air-holes, and tapering the MOF at selected locations. The outcome is a photonic microcell with a SC section with reduced core dimension and significantly inflated air-holes. The microcells possess the unique properties of the SC fibers (SCFs), but avoid the problem of large connection loss with standard single mode fibers. The insertion loss resulted from the adiabatic transitions between the SC region and the MOF pigtails are typically less than ~0.2 dB at 1550 nm. This makes the microcells easier to be integrated into standard fiber optic systems. The air-holes surrounding the SC of a microcell can be made significantly larger and hence the outer cladding walls thinner than those of a typical SCF, making it easier to fill fluidic materials into the holes transversely through side holes, or fabricate micro/nano structures on the SC by, for example, using a focused femtosecond (fs) laser to scan across the fiber from side. Six types of SC microcells with different core-shape and core-number have been made by the novel post-processing technique. Numerical models based on the finite element method (FEM) have been developed to study the mode properties of three types of the microcells, namely the 6-, 4-, and 3-hole microcells made by inflating the innermost ring of holes of the PCF. As examples of applications, in-fiber accelerometers, refractive index (RI) sensors, optical gain cells, and long period gratings (LPGs) are made based on these cells.|
Based on a photonic microcell with a hexagon-like SC surrounded by six air-holes, a robust micro-cantilever accelerometer within the microcell was made by use of an fs laser micromachining system. The device demonstrated a linear response to acceleration with sensitivity ~2.5 mV/g and a flat frequency response up to 2.5 kHz. A photonic microcell with a rhombus-like SC and four surrounding holes exhibits a very high group birefringence of about 5 x10³. By filling RI oil into the holes and testing the microcell within a Sagnac loop, it is shown that the birefringence of the SC is highly sensitive to RI. With an oil-filled microcell, a temperature sensor with sensitivity ~3.04 nm/°C (corresponding to RI sensitivity ~9.1 x10³ nm/RIU) is demonstrated. Based on the RI change of gas under pressure, a pressure sensor with a sensitivity of ~0.31 nm/Bar (corresponding to RI sensitivity ~1.5 x10³ nm/RIU) is implemented. By use of a photonic microcell with a triangular-like SC and three surrounding holes, an optical gain cell was made by drilling holes from the side-walls of microcell and circulating the ethylene glycol solution of rhodamine 6G in the air-columns. A net optical gain of ~ 2.04 dB was experimentally demonstrated at 633 nm with a 1 cm-long microcell side-pumped by a continuous 100mW 532 nm green laser. LPGs in the SCs of 3-hole microcells were also made by use of an fs laser and a point-by-point inscription technique.
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