|Title:||Novel splice techniques and micro-hole collapse effect in photonic crystal fibers|
|Subject:||Hong Kong Polytechnic University -- Dissertations.|
|Department:||Department of Electrical Engineering|
|Pages:||viii, 159 p. : ill. ; 30 cm.|
|Abstract:||Photonic crystal fibers (PCFs), which are also named microstructured optical fibers or holey fibers, represent one of the most active research areas today in the field of fiber optics. Because of the freedom they offer in their design and novel wave-guiding properties, PCFs have resulted in a number of novel devices, communication and sensing applications that are difficult to achieve with conventional fibers. The micro-holes of PCFs can allow for the infusion of materials, thus the combination of PCFs with new materials provides a new platform for ultra-compact photonic devices. In practical applications, low-loss connection PCFs with conventional fibers is a key issue for integrating PCF devices into existing fiber optic systems. However, connecting PCFs to conventional fibers without incurring too much loss is a very challenging problem. The previous methods to solve this problem are time-consuming and expensive. So it is very important to find a simple and low-cost way to splice different PCFs with conventional single mode fibers (SMFs). Two novel techniques were proposed to solve this problem in the thesis. One is fusion splicing technique; the other is micro-tip technique. First, fusion splicing technique for PCFs is investigated in detail. The splice loss is generally due to two reasons: one is the mode field mismatch between PCFs and SMFs; the other is that the air holes in PCFs may completely collapse in the vicinity of the splice joint during the splicing process, which significantly increases the coupling loss by destroying the light guiding structure of PCF near the joint interface. Different kinds of PCFs have different micro-hole structures, and the properties of heat-induced collapse when splicing are quite different. One solution which is suitable for one kind of PCF will fail when applying to other kinds of PCF. So a detailed study about the effect of micro-hole collapse on the splice loss for different kinds of PCFs is very important. For fusion splicing SMFs and PCFs having similar mode field diameters, a low-loss joint with good mechanical strength can be formed by choosing a suitably weak fusion current, short fusion time, offset and overlap to minimize the collapse of air holes and well melt two fibers together. For small-core PCFs, an optimum mode field match at the interface of PCF/SMF and an adiabatic mode field variation in the longitudinal direction of the small-core PCF can be achieved by repeated arc discharges applied over the splicing joint to gradually collapse the air holes of the small-core PCF. Low-loss fusion splicing of five different PCFs with SMFs are achieved, including large mode PCF, hollow-core PCF, nonlinear PCFs with low and high air-filling fraction and polarization maintaining PCF. The other novel technique is using micro-tips. The method is based on growing photopolymer micro-tips directly on the end face of SMFs. The advantages of this micro-tip fabrication method are its simplicity, controllability, reproducibility and being inexpensive. The shape and the size of the tips can be controlled, by adjusting the laser power, the exposure time and the oxygen diffusion concentration for polymerization, to match its mode field to the small-core PCFs. A photopolymer micro-tip integrated on the end face of a SMF is used to reduce the mode field diameter and increase the numerical aperture of the light beam coming out from the SMF, so that there is a better match to the small mode field diameter and the large numerical aperture of small-core PCFs. A 5 dB improvement in coupling efficiency between a SMF and a commercial small-core, highly nonlinear PCF is demonstrated. This compact and efficient butt-coupling method is particularly suitable for PCF gas sensor applications where holes in the PCF need to be kept open at the joint for easier access to the evanescent field. Micro-hole collapse effect can be used to fabricate selective injection PCFs. The suitable arc discharge energy can cause the cladding holes to collapse while leaving the central hollow core to remain open. Thus a simple method for selective filling the central hole of PCFs based on a conventional fusion splicer is developed. The opening and closing of the central hole and the holes in the cladding can be controlled to a certain degree by controlling the fusion current, the fusion duration and the fusion offset position. Experiments show that this method can be used to make hybrid polymer/silica PCFs with the central hole filled by a polymer. The quality of the hybrid fiber is good and the fabrication process is highly reproducible. This novel structure not only introduces an effective way for micro-fluidics sensing applications, but also opens new perspectives for nonlinear applications by filling various functional materials into the central hole of a hollow-core PCF. Hybrid PCF guides light by a novel guiding mechanism, which is a combination of index-guiding and bandgap-guiding. The properties of the hybrid PCF are systematically investigated, including modal effective index, mode field area, confinement loss, group velocity dispersion and birefringence. The hybrid PCF can be fabricated by selectively collapsing micro-holes and filling a row of air holes with high-index liquid. The potential applications are discussed.|
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