Research On Fabrication And Sensing Properties Of L-type Micro-hole Structure In Fiber By Femtosecond Laser

Fiber-based refractive index sensors are attractive for environmental, medicaland chemical applications because of their small size, light weight andconvenience for real-time monitoring and remote measurements. In order toachieve high-precision, low-cost liquid refractive index measurement, a liquidrefractive index sensing structure based on an L-type micro-hole in thesingle-mode fiber was fabricated by water-assisted femtosecond laser ablationmethod.Firstly we deduced the characteristic equation of the transmission mode in thefiber core, simulated the optical field distribution in the single-mode fiber with amicro-hole sensing structure by use of beam propagation theory, and discussed theimpact of refractive index and the length of micro-hole on the optical fielddistribution.Then an L-type micro-hole was fabricated. We put forward a feasible method tofabricate a high-quality micro-hole, studied the impact of different processingparameters upon experimental results, and fabricated L-type micro-holes withdifferent lengths used for liquid refractive index sensing.In the end, the responses of transmission loss of the sensing structure to thewavelength, refractive index, and micro-hole length were investigated. By use ofray theory, the sensing mechanism was analyzed. The disturbance of temperatureand melting scrap on the sensor was also discussed.The experiment results show that micro-holes with uniform diameter andcomplicated structure can be fabricated by water-assisted femtosecond laserablation method. In addition, the L-type micro-hole structure has a good linearsensing response in the refractive index range of1.333-1.413. The sensitivity ofthe sensing structure can achieve157.48dB/RIU and is less susceptible totemperature change. Compared with other fiber refractive index sensingstructures, L-type micro-hole structure in optical fiber has the outstandingadvantages, including compact, easy to fabricate, high sensitivity, insensitive totemperature and low cost, etc. Therefore it can have great application in the fieldsof biochemical measurement.