We research the refractive index (RI) sensing characteristic based on the bandpass spectrum caused by the self-imaging effect in the single-mode-multimode-single-mode (SMS) fiber structure theoretically and experimentally. A new selectable parameter, i.e., no-core fiber (NCF) length, is investigated for improving the sensitivity of the sensor. The results show that the sensor’s sensitivity will be enhanced by shortening the NCF length when the self-imaging number remains constant. Experimentally, a maximum sensitivity of 1923 nm/RIU (RI unit) has been achieved when the RI ranges from 1.334 to 1.434. This work demonstrates a method to improve the sensitivity of SMS-fiber-structure-based RI sensors featuring a low cost, compact size, low insert loss, and high sensitivity optical fiber RI sensor.
Magneto-optical fiber plays an important role in magneto-optical devices. The fiber has larger Verdet constant will lead to a larger Faraday rotation per unit length fiber and applied field. In order to increase the magneto-optical characteristic, especially the Verdet constant of photonic crystal fiber, a magneto-optical fiber device based on combination of the magnetic fluid and the tunable photonic bandgap effect of photonic crystal fiber is proposed. The magnetic fluid is filled into the air holes of the cladding layer in the photonic crystal fiber using a new air pressure-filled method. Because the magnetic fluid prepared in this experiment has higher refractive index (>1.45), and is filled into the air-holes of photonic crystal fiber, as a result, the index guiding fiber is converted into photonic bandgap fiber. A magneto-optic system based on the Stokes polarization parameters method is designed which could analyze the Faraday effect. The corresponding Faraday rotation could be measured in the external magnetic field with different magnetic intensity by this magneto-optic system. The Faraday rotation of the photonic crystal fiber filled with magnetic fluid is approximately 5 times than that of the single mode optical fiber. The proposed magneto-optical fiber device takes full advantage of the ultrahigh sensitivity characteristic of photonic bandgap fiber and the large Verdet constant of magneto-optical fiber, can be used for high sensitive magnetic field sensor, magneto-optical switch, and magneto-optical modulator, etc.