The Fabrication Of Micro-structured Optical Fiber Sensors Based On Femtosecond Laser Machining

Optical fiber sensor is a new photosensitive device,the light interacts with the external parameters,then it becomes the modulated light as some optical properties(intensity,phase,wavelength,etc.)changed,so the external parameters can be measured by demodulation.Optical fiber sensor has many advantages,such as small size,high sensitivity,intrinsic insulation,anti-electromagnetic interference and distributed measurement.So it has been widely used in the field of safety monitoring,fire alarming to large-scale facilities and so on.More and more attention has been paid on the fabrication of optical fiber sensing devices.Sensors are required for good structure and high sensitivity on the basis of high stability.However some new problems appear in the process of research.Firstly,due to the limitation of traditional processing technology,the performance of FabryPerot(F-P)interferometric sensor will be affected by the roughness and reflectivity of fiber surface.In addition,the coating in the fiber Bragg grating(FBG)cladding is often disturbed by the low sensitivity and the film is easy to fall off.Under this background,this paper will study two different types of optical fiber sensors based on femtosecond laser,including F-P temperature sensor and FBG hydrogen sensor.The main work is divided into three parts as follows:(1)The mechanism of femtosecond laser have been analyzed,F-P temperature sensing model and FBG strain model are established,which lays a theoretical foundation for the design of sensors.(2)The influence of parameters such as attenuation mode,aperture,laser frequency on the surface of microstructure is searched by femtosecond laser,then the processing rules have been summed up under different parameters.The method of laser processing assisted by water is proposed.,which can overcome the shortcomings such as low reflectivity and rough surface in the air.Experiments to optimize the processing parameters in water have been achieved,and kinds of temperature sensor with different cavity length are fabricated.On the other hand,composite microstructure FBG sensor based on femtosecond laser machining has been designed,it contains straight-trench structures and spiral micro-pit structures.The machining and coating process of composite structure are discussed in detail.This compositestructure not only increases the sensitivity,but also improves the bonding force between the film and the fiber.By changing the laser energy,numbers of trench and spiral pitch,three groups of sensor with different parameters have been designed.(3)The temperature experiment shows that F-P sensors have good performance with 60 mW of energy,5s of duration time and 10 of aperture.Fabricating in water has obvious advantage,sensor with 100 ?m length has achieved the sensitivity of11.4 pm/? in the range of 50?~ 500?,nearly fivefold more sensitive compared with machining in air.To FBG hydrogen sensors,experiment shows that the composite structure has better sensitivity enhancement with 2~4 folds sensitive than single structure,it comprised of straight trenches(n = 8)with 50 mW laser energy and spiral micro-pits with pitches(p = 60?m)have the strongest response to hydrogen gas.