Fabricating Of All-optical Fiber Sensors And Its Sensing Application Research

Optical fiber sensors have attracted increasing attention from theacademia and industry for their unique advantages such as immunity toelectromagnetic interference, light weight and compact size. They are becomingthe focus in sensor field. Previous work on the progress of fiber sensors athome and abroad is summarized in the thesis, and five fiber opticinterferometric sensors are proposed, mainly employed for the refractive index,temperature, strain and liquid level sensing, respectively. Their sensitivityand other performance indexes are greatly improved for the proposed fibersensors. The major contributions in this thesis are summarized as follows.(1) The characteristics of femtosecond laser micromachining are studied,a micro hole is machined on a PCF-based Mach-Zehnder interferometer (MZI)sendor using femtosecond laser, then the fabricated sensor is used inrefractive index and strain sensing test. Then two experiments of micro-holeetching are done using hydrofluoric acid, the refractive index is tested aftereach etching and the data are recorded. The refractive index sensitivity ineach etching step is contrasted and the result is analysed. Finally, a doublemicro-hole ablation process is operated on the PCF-based MZI with the samesize, and the refractive index sensitivity is tested.(2) A fusion-splicing method is applied for the preparation of two opticfiber Michelson interferometer (MI) sensors. One of them is consisting of asingle peanut taper and a free reflective fiber end, used for temperaturesensing, besides that the effection of different combinations of weldingparameters on signal loss introduced by single tapers is analyzed. The otherone is a sandwich-type MI sensor fabricated with the multi-mode fiber, thenthe liquid depth is tested with the fabricated sensor.(3) A single-spot irradiation method using the CO2laser is applied forthe fabrication of two optic fiber interferometric sensors. The first deviceis a single taper MI, applied for refractive index sensing. The second deviceis a combined MZI, and it is used for the sensing of refractive index andtemperature. The main innovation of this paper is as follows:(1) A femtosecond laser processing methods is proposed to improve theoriginal MZI sensor performance index, and applied for refractive indexsensing. A new criterion is proposed to determine whether the micromachiningcan be ceased throuth the observation of attenuation peak in transmissionspectrum of the interferometer. It is found that micro-hole ablation on thephotonic crystal fiber can improve the refractive index sensitivity of thepervious sensor in the study, i.e. the sensitivity of the device with singlehole ablated is twice of the non-hole device, and the device with double holeablated is five times larger than the non-hole device. The number ofmicro-holes can affect the rise of refractive index sensitivity directly. Inaddition, hydrofluoric acid etching can not increase the refractive indexsensitivity of the sensor.(2) A single peanut-type taper MI sensor fabricated by fusion-splicingmethod is proposed, and the maximum attenuation of16dB for the temperaturesensing is achived. This new sensor can be used to measure temperature rangingfrom25to1000℃, especially in the range of300-1000℃, a temperaturesensitivity of78pm/℃can be achieved, higher than other types of sensors.Meanwhile it has good thermal stability. The problem why thedifferenttemperature sensitivity performed at low temperatures and hightemperatures is carefully discussed and analyzed theoretically.(3) An optic fiber MI sensor based on Single-mode-Multimode-Singlemode(SMS) fiber structure is proposed and used to detect the liquid level andrefractive index. Since the length of multimode fiber affects the attenuationpeak of the signal directly, a method of determining the length of multimodefiber is thus brought forward by comparing the loss of signal introduced bythe multimode fiber. And a sensor with the maximum attenuation peak of13dBis fabricated to test liquid level in three kind of liquid with differentrefractive index, the measured level sensitivity has a maximum of-0.368nm/mm,and the refractive index sensitivity is-41.69nm/RIU.(4) A single-taper MI sensor is fabricated by single spot irradiationmethod using CO2laser and subsequently coated with a thin layer. The refractiveindex sensitivity of the device was-197.3nm/RIU after fiber surface coating, which is much higher than that of ordinary long-period grating sensors anddouble peanut-taper type MZI sensor.(5)A processing method to fabricate double peanut-taper type MZI sensorsis improved by single spot irradiation method. The new method can increasethe refractive index sensitivity greatly. The interferometer fabricated inthe new method has a refractive index sensitivity of69.59nm/RIU, more than3times of the device made by original method. Later the sensor is appliedfor temperature sensing, its extinct wavelength shift with temperatureincreases exhibits a good linearity in the range of500-800℃with a highsensitivity of0.1299nm/℃. Thus, the proposed combined type sensor issuitable for high temperature measurement.