The Research On The Novel Optical Fiber Filters And Their Application In Tunable Fiber Lasers

Because the tunable lasers own many unique advantages such as wavelength adjustment, different wavelength switching and wavelength interval change, they can be widely used in the optical communications and optical sensing. Compared to the tunable semiconductor lasers, the tunable fiber lasers can obtain wide gain bandwidth, highe power, simple tuning methods, convenient connection to optical fiber system and narrow linewidth, so they are becoming one research focus in this field. In the structure of the fiber lasers, to realize the wavelength tuning, many tunable filters are introduced in the cavity, including Febry-Perot filter, dielectric film filter, acousto-optic tunable filter and fiber grating. However, in many applications, wavelength switching and wavelength interval are needed. To meet these needs, some novel tunable fiber filters is developed. In this dissertation, the filter based on different modes interference in the fiber is analyzed. Based on these interference theory, the corresponding all fiber filters are constructed. After that, these novel fiber filters are applied to the ring cavity fiber laser to realize different tuning functions. The main content of the dissertation are summarized as follows:(1) Based on the all fiber Sagnac filter which is composed of high birefringence polarization-maintaining fiber, different novel filter structures are designed and then used in the fiber ring laser to obtain different tuning function. Utilizing the Jones matrix, the reflection and transmission functions of the different novel filters is calculated. Then, the influence of the light polarization state on the reflection and transmission spectrum is simulated. According to the simulation results, two different kinds of all fiber filter based on the all fiber Sagnac filter which is composed of high birefringence polarization-maintaining fiber are built in the experiments. The experiment results verified that the spectrum of the filters can be changed by controlling the input light polarization state. When the filters are applied in the fiber ring lasers, the lasers can obtain five wavelength oscillations at most. Besides these, the lasers can not only realize wavelength switching, but also the wavelength interval can be changed. Because the effect of depth saturation in the erbium fiber is used to improve the stability of multiple wavelength oscillation at room temperature, the peak fluctuation of the laser is less than 0.5dB. These kinds of lasers own good application prospects in optical fiber communications and optical fiber sensors.(2) Based on the principle of optical fiber mode interference, an all fiber filter which is composed of two fiber tapers is researched and introduced into fiber ring laser to obtain tunable narrow linewidth laser output and wavelength switching. Using the beam propagation method(BPM), the influence of the taper parameters on the excitation modes are analyzed. The analysis results provide guidance for the taper structure optimization and tuning methods. In the experiments, the fiber filter which is composed of two fiber tapers is fabricated by the fiber splicer, and the experiment results verified the curvature tuning method can obtain wide tuning range. When the two-taper filter as a wavelength-selective component and unpumped erbium-doped fiber (EDF) as a saturable absorber(SA) are used in the fiber ring cavity, the laser obtain the tunable narrow linewidth laser output. The linewidth is less than 2 KHz, and tuning range is wider than 17 nm. If the saturable absorber is not used, the laser can also realize stable single and dual wavelength switch by loss control.(3) A compact all fiber filter which is composed of a fiber Bragg grating embedded in a fiber modal filter is designed. The fiber modal filter consists of a piece of small-core photosensitive fiber (SCPSF) which is spliced between two pieces of single mode fiber (SMF). At one splicing point in the modal filter, a core offset is introduced. When the compact filter is introduced in the fiber ring laser, stable dual-wavelength operation is achieved at room temperature and the wavelength spacing can be tuned continuously. By adjustment of the loss in the cavity, the laser can also operate in dual-wavelength or in wavelength switching modes. Using the beam propagation method(BPM), the influence of the core offset and the mismatch of the core on the excitation modes are analyzed. At the same time, the complex mode expansion method is used to optimize the extinction ratio of spectrum. The calculated results of the two methods provide guidance for the design of fiber modal filter. In the experiment, the fiber modal filter is fabricated. The responses of the filter to strain and temperature are measured. The experiment results demonstrate that the filter owns a high temperature sensitivity of 106.64 pm/ ℃ due to the use of photosensitive fiber. On the basis of this kind of fiber modal filter, a fiber Bragg grating is inscribed in the photosensitive fiber utilizing the phase mask technique, so a compact filter consists of fiber modal filter and fiber Bragg grating is formed. Because the two filters have different response to the strain, when the compact filter is used in the fiber ring laser, by the loss controlling the laser can obtain single and dual wavelength switching. In the two wavelength oscillation, when the strain applied on the compact filter changes, the interval of the two wavelengths can be tuned, and the tuning range can achieve 2nm. Therefore, in addition to the application in optical fiber communications and optical fiber sensors, this kind of fiber laser can also be used as a tunable terahertz generation source.