A tunable microstructure optical fiber for different orbital angular momentum states generation is proposed and investigated by simulation. The microstructure optical fiber is composed of a high refractive index ring and a hollow core surrounded by four small air holes. The background material of the microstructure fiber is pure silica. The hollow core and the surrounded four small air holes are infiltrated by optical functional material whose refractive index can be modulated via physical parameters, leading to the conversion between circular polarized fundamental mode and different orbital angular momentum states at tunable operating wavelengths. A theoretical model is established and the coupling mechanism is systematically analyzed and investigated based on coupled mode theory. The fiber length can be designed specifically to reach the maximum coupling efficiency for every OAM mode respectively, and can also be fixed at a certain value for several OAM modes generation under tunable refractive index conditions. The proposed fiber coupler is flexible and compact, making it a good candidate for tunable OAM generation and sensing systems.

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We propose a temperature insensitive optical fiber Fabry-Perot (FP) sensor for liquid level measurement. The sensor is based on a silicon diaphragm and borosilicate glass ferrules. Through the combination of different ferrules, the FP cavity is fabricated without a seal to eliminate the influence of the residual air in the FP cavity, thereby the temperature cross-sensitivity is significantly reduced. The opened FP cavity constitutes a differential pressure sensing structure at the same time. To verify the performance of this sensor, an experiment with an optical fiber pressure and temperature experiment system is carried out. The measuring range is 0.5-1.3 MPa. The sensor has a good monotonic linear response with an accuracy less than 0.4% F.S. (full scale). The measuring error caused by the temperature drift is lower than 0.0007 MPa/K, i.e., 0.09% F.S./K.