Relative Humidity Sensor Based On Polarization Maintaining Loop Mirror

The measurement of humidity based on all-fiber technology is attractiveand promising for their unique advantages, such as immunity to electromagneticinterferences, compact size, and fast response. Therefore, several types of humiditysensors based on fiber are widely provided. This thesis proposes a relative humiditysensor based on polarization maintaining fiber （PMF） loop mirror of combining withhumidity sensitive material and long period fiber grating （LPG）, which can makemore accurately measuring humidity by using optical detection methods. Theproposed sensors are created through single PMF, two PMFs and polarizationmaintaining long period fiber grating （PM-LPG） as sensor unit for the humiditysensing measurement, respectively.The main work of this thesis is as follows:1. The new humidity sensor based on a high birefringent （PMF-FLM） structurefor sensing application is first demonstrated. The proposed sensor is created throughcoating a thin layer of polyvinyl alcohol （PVA）, whose refractive index varies as afunction of humidity level, onto a PMF from which a portion of the cladding isremoved. And the special sensing unit structure will improve the optical fiber sensorto humidity response sensitivity.2. As the previous humidity sensor setup with single PMF couldn’t remove theinfluence of the temperature, a novel temperature-independent humidity sensor basedon a fiber loop mirror with two high birefringent fibers structure for sensingapplication is demonstrated in this thesis. The proposed sensor is created throughcoating a thin layer of PVA onto PMF1from which a portion of the cladding is etched,and another normal PMF2is used as a reference fiber to eliminate temperature on theinfluence.3. It is necessary to measure two parameters simultaneously in the practicalapplication of sensor to obtain more information. Thus a humidity sensor based on long period grating in polarization maintaining fiber （PM-LPG） loop mirror whichable to measure relative humidity and temperature simultaneously is put forward inthis thesis. The LPG written in PMF by a CO₂laser is connected to form a fiber opticmirror with a3-dB coupler. The sensor based on PM-LPG combines the advantagesof both PMF with LPG, so it can measure two parameters simultaneously.