A dual-parameter fiber sensor achieved by cascading a fiber Bragg grating with a no-core fiber (NCF) is used for simultaneously detecting both the temperature and index physical parameters. The main index sensing mechanism of NCF is based on the wavelength shift of multimode signals’ interference (MMI), and the temperature-sensing mechanism is determined by the Bragg wavelength shift and MMI wavelength shift. As the testing index value approaches the cladding index of the optical fiber, an MMI-induced loss-dip is thus created with a sensitivity of 899 nm/RIU due to the phase-match condition of MMI being satisfied. By coating the thin films of different materials, this kind of sensor can be applied in a wide range of different sensing systems.
We propose a spectral flat-top, single resonant, and ultrabroadband-more than 180 nm in a −20-dB bandwidth-long-period fiber grating (LPG) filter. The ultrabroadband LPG is based on a thin cladding layer LPG synthesized by the Lagrange multiplier optimization (LMO) algorithm. As the bandwidth and resonant spectra cover a very wide band, both material dispersion and waveguide dispersion were included in the calculations of the LMO method. To the best of our knowledge, the bandwidth of the designed flat-top LPG filter in the −20-dB coupling is the broadest currently existing in the literature. Such designed LPG devices can be very useful for a variety of applications in broadband optical communication systems.
A novel and simple in-line fiber polarizer is presented. The proposed device is fabricated by tapering an
anisotropic flat-cladding birefringent micro-fiber surrounding with low-dispersion optical-liquid cladding. We also
presented a theoretical analysis for dispersive birefringence of flat-clad micro-fiber with liquid overlay. The proposed
device can be useful as all-fiber polarizer for optical communications. Simulation results show the birefringence of the
device can be enhanced when the aspect is larger. In the experimental measurement for polarization extinction ratio
(PER) of the proposed device, a fiber-pigtailed 1549.25 nm DFB laser light was used as light source. The PER about 30
dB was demonstrated when the liquid with refractive index nD = 1.45 was used.
This paper presents a new synthesis method for designing complex fiber Bragg gratings (FBGs). The method is based on a multiobjective Lagrange-multiplier-constrained optimization (LMCO), to which various constraints on the designed filters can be added in consideration of practical application demands and fabrication requirements. The maximum amplitude of the index modulation profiles of the designed FBGs can be substantially reduced under constrained conditions. In contrast with the layer-peeling (LP) algorithm, the LMCO method can easily incorporate different types of requirements in terms of a user-defined cost function. Compared to stochastic approaches such as genetic algorithms, the proposed method is likewise a direct optimization method, but without using random numbers, and therefore has a smoother coupling coefficient profile as well as faster convergence. A theoretical model and investigation have been made in this study. A narrowband dispersionless FBG filter for optical fiber communication was designed, and its simulation results were compared with those of the LP algorithm. The study results demonstrate that the LMCO algorithm can provide an alternative for practical and complex fiber grating filters.
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