Photonic Crystal Fiber Sensor Based On Modal Interference

In the past ten years, photonic crystal fiber (PCF) has attracted more and more attention of researchers due to its unique optical properties. In sensing area, a PCF sensor based on modal interference (MI) has become a hot topic of research in recent years. In this thesis, we have designed several PCF sensors based on MI with different structures and applied them into refractive index (RI), temperature, magnetic field and hydrogen sensing.Details are as below:1. We design PCF RI sensors based on MI. We firstly splice a short piece of PCF between two single mode fibers (SMFs) with a fusion splicer to form a modal interferometer (MIR). Then we design two miniature PCF RI sensors based on MI with acid microdroplets etching and flame brushing methods separately. In our experiment, the RI sensitivity can reach～775 nm/RIU when the PCF diameter is etched to～70 μm with the acid microdroplets etching method and the RI sensitivity can reach～907 nm/RIU when the PCF diameter is tapered to～45 μm with the flame brushing method. Both methods decrease the PCF diameter, increase the PCF sensor RI sensitivity based on MI and keep its original temperature insensitive property.2. We design PCF temperature sensors based on MI. We firstly fabricate a high sensitive PCF temperature sensor based on MI by filling and sealing liquid with high thermo-optical coefficient (isopropanol) into the micro holes of the PCF. The temperature sensitivity can reach-166 pm/℃, which increases dozens of times than those of previous conventional PCF temperature sensors based on MI. But the RI sensitivity still reaches up to 275 nm/RIU, which will interfere with temperature sensing. In order to eliminate the RI influence on temperature sensing, we also utilize a soft glass PCF with a very high RI to fabricate a RI insensitive soft glass PCF temperature sensor based on MI. The temperature sensitivity is 17 pm/℃, which still increases several times than those of previous conventional PCF temperature sensors based on MI. Meanwhile, the RI sensitivity is only-1 nm/RIU, which will not interfere with temperature sensing.3. We design a PCF magnetic field sensor based on MI. We bond an aluminium (Al) wire with a PCF MIR together to fabricate an Ampere force based PCF magnetic field sensor based on MI. When an electrical current is applied in the Al wire and a magnetic field is applied perpendicular to the Al wire, the Al wire will bend under the Ampere force and induce the PCF to bend. As the PCF MIR is very sensitive to bend, it can be utilized for magnetic field sensing detection. In our experiment, when the electrical current applied in the A1 wire is 100 mA, the magnetic field sensing sensitivity can reach up to 32.4 pm/mT.4. We design a PCF hydrogen sensor based on MI. We coat a palladium (Pd) film on the end face and side face of the PCF, and then we fusion splice the PCF with a SMF to fabricate a reflective PCF hydrogen sensor based on MI. When the sensor is applied into hydrogen atmosphere, the Pd film will absorb hydrogen and change its complex RI. And therefore the reflection spectra will be changed.In our experiment, when hydrogen concentration changes 1% in the range of 0 to 5%, the resonant wavelength will change 0.25 nm, which is much higher than the sensitivities of traditional FBG based hydrogen sensors.In conclusion, we do some research in PCF sensing area. We design some PCF sensors based on MI with different structures and applications. We believe this thesis will provide some reference meanings to PCF sensing area in the future.