Microfiber Devices Based On Coupling Theory

Microfiber (MF) has received a lot of attention because of its low loss, large evanescent field, strong confinement, configurability and robustness.All these advantages promise MF a broad prospect of application. Large evanescent field, the unique property, causes MF easier to interact with environment, thus enlarge the sensitivities of MF based sensors. In addition, large evanescent field makes it convenient to fabricate micro/nano structures according to coupling theory which are quite useful to miniaturize fiber devices.There are two typical MF coupling structures:coupler and resonator. Coupler is fabricated by touching two MF waveguides together while resonator is demonstrated using only one MF which is bent to form loop/coil and leading coupling between adjacent turns.In this thesis, we demonstrated some novel microfiber devices based on microfiber couplers (MFC) and coil resonators (MCR), respectively. Based on a microfiber coupler Sagnac loop, we fabricated a highly sensitive micro-force sensor and a twist sensor. We investigated the temperature characteristic of microfiber resonators with respect to the optical-thermal coefficients and optical-expansion coefficients of the rod, coating polymer, and fiber materials. And then theoretically and experimentally demonstrated a Teflon coated microfiber resonator with weak temperature dependence.1, Microforce sensors based on MFCs. We fabricated the sensor by splicing output ports together to form a Sagnac loop. Compared with traditional fiber based force sensor, microfiber has a small cross section area, for the same material, the force caused strain is inversely proportional to the force region area. Therefore, the same force applied on the microfiber coupler will cause higher strain, thus larger effective refractive index and coupling region length changes (according to the photo-elastic effect and Hooke’s law), and finally result in higher force sensitivity. The sample we fabricated showed a force sensitivity 1-2 orders of magnitude larger than traditional fiber force sensors, which is 3670 nm/N.2, Twist sensors based on MFC. We also used the same structure to measure twist making use of the birefringence of MFC. The asymmetric property of coupler waist region cross section brings different coupling coefficients for x and y polarization. Thus we can find a slow modulation of the spectra envelope in coupler transmission spectrum, which can be used to measure polarization change. When a coupler is twisted, there will be a geometry variation in the waist structure as well as a refractive index change caused by torque. We presented two methods for twist sensing:one is using the envelope change and the other is measuring the coupling peak contrast variation. Both of them showed high sensitivities,0.9 nm/° and 0.16 dB/°. To compensate the temperature influence, i.e. separate the twist and temperature effect, we suggest a difference method in data processing making use of the both methods.3, Temperature sensors based on MCR. Microfiber coil resonator (MCR) has generated tremendous research progress in telecommunication and sensing with key merits of compact size, wavelength agility and tenability. The temperature characteristic of MCR is very important for temperature sensing and many other applications (calling for low temperature sensitivity). We fabricated a two-turn coil resonator by wrapping a 4～5 μm diameter microfiber on a 2-mm diameter PMMA rob and then packaged it using low refractive index polymer Teflon. The device showed high temperature sensitivity ～80 pm/℃, which is useful for temperature sensing application but harmful for others. The main contribution to the high sensitivity comes from the large optical-expansion coefficient of PMMA as we analysis.4, Temperature insensitive MCRs. In order to decrease the temperature sensitivity of the MCR, we replace the support rod by a glass one. Then we investigate theoretically and experimentally the thermal characteristics of a MCR embedded in Teflon with negative thermo-optic coefficient. The temperature dependence severely relates to the microfiber radius and it is possible to suppress the temperature dependence at a certain diameter. Our calculation shows that the ideal microfiber radius should be around 1.45μm. We fabricated a sample by embedding a three-turn MCR with～3 um-diameter microfiber in Teflon, which showed a low temperature sensitivity of<6pm/℃ in the room temperature range, almost fifty times lower than the 1μm-diamter MCR embedded in EFIRON UVF PC-373. These results are expected to be applied in fabricating temperature-insensitive MCR based filter, refractive index sensor, etc.