Long Period Fiber Grating Based Band-Pass Filters And Its Application In Fiber Lasers

With the development of optical fiber communication，sensing technology and opticalinformation processing, all-fiber passive and active devices are attracting more and moreattentions. With the natural compatibility of the fiber grating with fiber systems, the fibergrating based optical filter and fiber laser are of great interests. Developing highperformance, tunable and low-cost optical fiber filter and fiber lasers have been a hotresearch topic in fiber optics field.Long period fiber grating (LPFG) is normally a transmissive band-reject filter.However, with special structure designs, the LPFG based band-pass filter (BPF) can beachieved. In this paper, two types of all-fiber tunable band-pass filters (i.e., phase shiftedand multimode interference (MMI)) based on LPFG are investigated. Furthermore, basedon the phase-shift LPFG (PS-LPFG), an arbitrarily switchable multi-wavelengthcontinuous-wave (CW) Yb-doped fiber laser (YDFL) has been achieved. The main worksand originality are as follows:1. Based on the theory of LPFG, the band-pass filter is achieved with theintroduction of a phase shift in the grating. The effect of the exposure method of thisPS-LPFG on polarization dependence loss (PDL) has been analyzed. Based on asymmetricexposure method, a1060nm-band multi-wavelength band-pass filter has been achieved.The PDL and tuning characteristics of this BPF have been investigated.2. An arbitrarily switchable multi-wavelength YDFL based on the1060nm-bandmulti-wavelength BPF is proposed and demonstrated. Each or any combination of themulti-wavelength can be switched independently by controlling the waveplates in thesystem. In addition, the wavelength tuning of the multi-wavelength YDFL output is also demonstrated by changing the curvature of the PS-LPFG without adjusting any of thepolarization controlling components in the laser cavity. The contribution of the PS-LPFG,especially, the physical mechanisms of the capability of the stable operation andwavelength switching are discussed.3. A novel all-fiber band-pass filter (BPF) based on a fiber multimode interferencestructure sandwiched with two long period fiber gratings (LPFG) is proposed anddemonstrated. For optimizing the working length of the multimode fiber (MMF), asimulation based on the beam propagation method is performed and key parameters of thesystem are achieved. It is further show that both the wavelength and the full width at halfmagnitude (FWHM) of the passband can be tuned by the temperature and ambientrefractive index, respectively. Such device with low cost and simple structure is useful fortunable fiber lasers and spectral control in mode-locked fiber lasers.