|Title:||Optical sensors and devices based on highly birefringent micro/nanofibers|
Textile fibers, Synthetic.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Electrical Engineering|
|Pages:||160 pages : illustrations ; 30 cm|
|Abstract:||Optical micro/nanofibers (MNFs) have attracted significant attention recently for their potential applications in micro/nano-scale photonic systems. Various MNF devices have been reported, and most of them are based on MNFs with circular cross-sections. The circular MNFs have little or no capability of maintaining the state of polarization during transmission, which cause instability in phase and polarization sensitive devices and systems. We proposed a simple and effective technique for fabricating air-cladding elliptical MNFs with wavelength and sub-wavelength scale diameters. The technique involves the use of a femtosecond infrared laser to cut away parts of the fiber cladding on opposite sides of a single mode fiber (SMF), and tapering-drawn the SMF by use of a flame-brushing technique. The cross-sectional shape of the "cut" region is well preserved during initial taper-drawn process and eventually turns to an approximately elliptical shape when it is drawn down to micrometer scales. These elliptical MNFs have demonstrated very large birefringence of the order of 10⁻², much larger than the conventional highly birefringent (Hi-Bi) fibers. The elliptical MNFs have SMF pigtails from which they are made, and hence can be easily integrated into conventional optical fiber systems with low loss. We also explored the potential applications of the Hi-Bi MNFs. By splicing Hi-Bi MNFs into fiber Sagnac loops, all-fiber comb filters with flat-top passing bands are demonstrated. Two different configurations are studied, one incorporates a single piece of twisted Hi-Bi MNF in the Sagnac loop, while the other includes two pieces of Hi-Bi MNFs cascaded along a fiber with a rotation of their birefringence axes. The lengths of Hi-Bi MNFs used are on the order of centimeters, one to two orders of magnitude shorter than the conventional Hi-Bi fiber-based comb filters. The birefringence of the MNFs may be tuned by varying the dimension of MNFs and the refractive index surrounding the MNFs, which provides more flexibility in designing comb-filter with desired properties. The comb filters might be used for multi-wavelength fiber lasers, optical signal processing and switching, and management for wavelength divided multiplexing networks, especially for photonic integration, computing, and nano-scale sensing. Many device and sensor applications exploited the large external evanescent field associated with the thin MNF. However, the optical performance of MNFs quickly degrades after fabrication due to surface light scattering from dust particles and from cracks induced by water vapor, resulting in large irrecoverable increases in loss and eventual mechanical failure. We developed a technique to fabricate in-line fib er-optic photonic micro cells by encapsulating MNFs with glass tubes. The encapsulation isolates MNFs from external contamination and makes them more robust for real-world applications. By splicing the encapsulated Hi-Bi MNFs into fiber Sagnac loops, gas pressure, temperature and refractive index sensors were developed. A Sagnac loop interferometer with a Hi-Bi MNF microcell demonstrated a refractive index sensitivity of ~2024 nm per refractive index unit (RIU) in gaseous environment (refractive index ~ 1) and 21231 nm/RIU when it is surrounded by water (refractive index ~ 1.33). Such micro cells may be used as low loss evanescent-wave-coupled optical absorption and amplifying cells, as well as florescent and photo-acoustic cells.|
Long period gratings (LPGs) exploit the resonant coupling between the fiber modes and have been studied extensively for sensing and communication applications. We fabricated LPGs directly on an encapsulated Hi-Bi MNF by periodically modifying the surface along one side of the MNF with a femtosecond laser. One such made LPG exhibits resonance dips at 1532.7 and 1614.2 nm for two orthogonal principal polarization states, and the corresponding grating strengths are 19.2 and 15.2 dB respectively. Higher order LPGs based on the encapsulated Hi-Bi MNF was also realized and used as a refractive index sensor in water with the sensitivity of ~4623 nm/RIU. We also fabricated LPGs by use of focused high frequency CO₂ laser pulses to periodically modify the transverse dimension of a bare Hi-Bi MNFs, and then encapsulated them within a glass capillary afterward. These LPGs may be used as robust wavelength selective polarization filters and sensors. Polarization rocking filters (PRFs) are a special type of LPGs that couple light resonantly between two principle states of polarizations in a Hi-Bi fiber. They are essential functional components in guided wave optical systems such as polarization diversity heterodyne receivers and highly sensitive fiber sensors. We have successfully fabricated PRFs in Hi-Bi MNFs. A MNF with a slight ellipticity can have very large birefringence and hence much shorter polarization beat length than the conventional Hi-Bi fibers. This means that a smaller pitch or "rocking period" and hence shorter device length to produce reasonable polarization coupling. The PRFs were fabricated by introducing permanent twist at particular locations along the MNF by heating up the twisted fiber with a CO₂ laser. High polarization extinction of ~ 20 dB was achieved for a device length of 3.12 mm. A high order PRF was tested for refractive index sensing and demonstrated a refractive index sensitivity of 32036 nm/RIU.
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