Tunable Optical Filter Based on Mechanically Induced Cascaded Long Period Optical Fiber Grating

We have proposed and demonstrated experimentally a novel and simple tunable optical filter based on mechanically induced and cascaded long period optical fiber gratings. In this filter variable FWHM and center wavelength is provided by cascading long period and ultralong period optical fiber gratings with different periods in a novel fiber structure. We report here for the first time to our knowledge the characterization of mechanically induced long period fiber gratings with periods up to several millimeters in novel multimode-single-mode-multimode fiber structure. We have obtained maximum loss peak of around 20？dB. 1. Introduction Long period fiber grating (LPFG) is the special case of fiber Bragg grating (FBG). It was first suggested by Vengsarkar and coworkers in 1996 [1]. LPFG can be formed by introducing periodic longitudinal perturbations of refractive index along the core of a single-mode fiber. It can couple light between fundamental core mode and copropagating cladding modes at specific resonance wavelength. The period of a typical LPFG ranges from 100？μm to 1000？μm. The cladding modes of LPFG are absorbed by the polymer coating of the fiber hence, the transmission spectrum consists of a number of rejection bands at the resonance wavelengths. In contrast to the Bragg grating, LPFG does not produce reflected light and can serve as spectrally selective absorber. Therefore, it is also called transmission grating. LPFGs are relatively easy to produce. Their spectral properties, that is, resonance wavelength, bandwidth, and so forth, can be varied in a wide range. All these features make LPFG very attractive for many applications in telecommunication, laser, and sensor system. LPFG can be induced optically or mechanically [2–4]. Optically induced gratings are permanent, whereas mechanically induced gratings are reversible. LPFGs formed by a mechanically induced technique have generated great interest due to their versatility in the process of fabrication. In these gratings, the fiber is subject to periodical stress, which results in alternated regions under compression and stretching that modulate the refractive index via the photoelastic effect. Mechanically induced long period fiber gratings (MLPFGs) induced by pressure need neither a special fiber nor an expensive writing device for fabrication. These gratings also offer advantages of being simple, inexpensive, erasable, and reconfigurable and also give flexible control of transmission spectrum. 2. LPFG Mathematical Model Due to their highly multimode nature, cladding modes are located closely