Novel Fiber Laser And Fiber Sensor Based On Fiber Filter

With the advance of the optical fiber communication technology, fiber laser and fiber sensor with high performance have become the focus in the recent years. Fiber filters play an important role in the development of the novel fiber laser and fiber sensor with high performance. Supported by National Nature Science Fundation Program and Funamental Research Funds for the Central Universities, this dissertation mainly focus on the theoretical and experimental investigations on the fiber filters based on Rayleigh backscattering(RBS), twin core fiber(TCF), taper, core-offset splice, and tilted long period fiber grating(TLPG). Employing these novel filters, we proposed and experimentally demonstrated the tunable single longitudibal mode(SLM) fiber laser, multi-wavelength SLM fiber laser, dual-wavelength fiber laser, tunable multiwavelength fiber alser, tunabel narrow linewidth fiber laser, high sensitive refractive index sensor, and high sensitive bending sensor with wide measuring range.The main innovative achievements of the dissertation are listed below:1. A novel sinlge longitudinal mode fiber laser (SLM-FL) has been proposed and experimentally demonstrated based on RBS in single mode fiber (SMF-RBS). With the establishing of the therotical model of SLM operation based on SMF-RBS, a single laser cavity was formed by the structure of sharing light path of main cavity and the RBS feedback cavity. Utilizing the RBS in660m SMF and the tunable filter with tunable bandwidth of0.5nm, the SLM-FL can operate at7wavelengths with linewidth of2.5-3.0kHz, side mode suppression ratio (SMSR) of72dB, and threshold power of30mW. Futuremore, muli-wavelength SLM laser based on SMF-RBS have been proposed and experimentally demonstrated by using the structure with cascaded fiber Bragg gratings and semiconductor optical amplifier.2. A stable dual-wavelength Er-doped fiber laser (EDFL) with continuously tunable wavelength spacing was proposed and experimentally demonstrated by using a TCF coupler (TCF-Coupler) filter at room temperature. By using a1.1m TCF-Coupler filter, The wavelength spacing of the dual-wavelength laser was continuously tuned from0.1nm to1.2nm with a with a SMSR of40-50dB, power fluctuation of0.25dB, and wavelength drift of0.004nm. A further application of the TCF-Coupler filter in curvature sensor was investigated. By utilizing a TCF-Coupler filter with length of1.1m and free spectral range of49nm, a high sensitive fiber cuvature sensor with the wide measuring range can be achieved. Its sensitivity can reach to-14.7nm/m-1and the curvature range can be measured from0to9.30m-1.3. A multiwavelength fiber laser based on nonlinear optical loop mirror (NOLM) was proposed and experimental demonstrated by using a twin core fiber-based Mach-Zehnder interferometer (TCF-MZI).40lasing wavelengths with a signal noise ratio (SNR) of28dB can operate around the wavelength of1560nm when the length of EDF is6m. Further more,30lasing wavelengths with a SNR of20dB were achieved around the wavelength of1530nm for the first time when the length of EDF is2m. In order to exptend the range of operation wavelength, a tunable multiwavelength fiber laser based on NOLM was proposed and experimental demonstrated by using a compound filer which cascaded a standard MZI and a TCF-Coupler filer with TCF length of11.5cm,. By utilizing a standard MZI with optical path difference of4mm and a TCF-Coupler filter with length of11.5cm, the lasing waveband can been linearly tuned over a rang of-24nm form1542nm to1566nm with a channel spacing of0.4nm, a SNR of39dB, a wavelength drift of0.04nm and a power fluctuation of±0.3dB.4. A tunable fiber laser based on an two-taper Mach-Zehnder interferometer (TT-MZI) filter was realized, and the effect of beam waists of the tapers on performance of the laser was investigated including the SMSR, bandwidth, tunable range, and tunable step. Experimental results show that the tunable laser can cover the wavelength range of16.1nm with tuning steps of0.07-0.5nm, a bandwidth of10pm, and a SMSR of40-50dB by using TT-MZI with arm length of1m and taper beam waist of49nm. Compared with other tunable lasers based on taper, this tunable laser not only obtains a larger tunning range and a higher SMSR, but also decreases the tunning step to increase the wavelength number.5. A novel high sensitive refractive index (RI) sensor was proposed and experimentally demonstrated by using an asymmetrical fiber Mach-Zehnder interferometer based on taper and core-offset splice (TC-AMZI). Experimental results show that the RI sensitivity of the proposed sensor is28.2nm/RIU (refractive index unit) and59.2nm/RIU for arm length of30mm and50mm, respectively, which are2times and3times as large as those of TT-MZI with the same arm length. Meantime, the phenomenon that the attenuation peaks jumped from "blue shift" to "red shift" can be weakened in TC-AMZI. This makes TC-AMZI suitable for the large range RI measurement.6. The mode coupling in TLPG was analyzed by using the full vector complex coupled mode theory. The polarization sensitive coupling betwteen the LP01and LP1m modes was investigated in detail, including degenerate vector modes TEo0m, TM0m and HE2m.Numerical results show that the maximum coupling between LP01and LP1m modes occurs at tilt angle of～87°. Compared with the non-titled LPQ new resonance bands corresponding to the LP1m cladding modes were observed in the transmission spectrum of the TLPG. The wavelength and transmission of the new resonance bands are effected by the the state of polarization (SOP) of the input light. Also, the RI sensitivity of the new resonance wavelength is much higher than that of the normal resonance wavelength corresponding to the LPom cladding modes. Such TLPG would have a number of potential applications in chemical and biological sensing field.