Reconstruction of long-period fiber gratings from their core-to-core transmission function (original) (raw)
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Experimental reconstruction of a long-period grating from its core-to-core transmission spectrum
Optics Letters, 2005
We demonstrate, for the first time to our knowledge, reconstruction of the structure of a long-period grating from its measured core-to-core transmission spectrum intensity. The reconstruction is obtained by writing an auxiliary grating in cascade to the interrogated grating. Our reconstruction technique is based on using the Hilbert transform and a phase-retrieval algorithm. Using our method, we have reconstructed the structure of a uniform long-period grating with a 47% coupling efficiency.
Long-Period Fiber Gratings in Active Fibers
Current Trends in Short- and Long-period Fiber Gratings, 2013
2.1. Coupled mode equations considering gain/loss Similarly to an LPG in a gain/loss-less fiber, LPG assists coupling between the core and a cladding mode at the wavelength for which the phase matching condition is satisfied for real part of the mode propagation constants: 2 Re Re() , cl co (1) where β cl = (2π/λ)n cl eff+jα cl and β co = (2π/λ)n co eff +jα co are the propagation constants of the interacting cladding and core modes, respectively, λ is the wavelength, Λ is the grating period, n cl eff and n co eff are the effective refractive indices of the interacting cladding mode and the core mode, α cl and α co are the absorption (when positive) or amplification (when negative) factors of the cladding and the core modes, respectively. Propagation constants for the core and cladding modes can be straightforwardly calculated, especially when considering a step-index fiber refractive index profile [4-6]. The strength of the coupling between the two modes depends on the coupling coefficient calculated as a confinement factor between the field of the two interacting modes [7]:
Analysis of Concatenated Long Period Fiber Gratings Having Phase-Shifted and Cascaded Effects
Japanese Journal of Applied Physics, 2003
We propose an accurate model for analyzing phase-shifted and cascaded long period fiber gratings (LPFG) as well as concatenated LPFGs having the characteristics of phase-shifted and cascaded LPFGs. To analyze the concatenated LPFGs with arbitrary wavelength spacing and line width, the coupling between the core mode and multiple cladding modes was considered. In order to obtain an analytical matrix model for the concatenated LPFGs, a multiport lattice filter structure was used. To verify the validity of the proposed model experimentally, we have fabricated two LPFG structures: cascaded LPFGs and concatenated LPFGs with three uniform LPFGs having the characteristics of cascaded LPFGs. We have observed that the transmission spectra calculated using our multiport lattice filter model are close to the corresponding measured spectra in the wavelength band of interest.
Long-period gratings in special geometry fibers for high-resolution and selective sensors
Optical Engineering, 2014
Advantages for sensor applications of long-period gratings (LPGs) in special optical fibers are reported. Two consecutive LPGs separated by 60 to 100 mm interfere to improve the resolution and reduce noise in a highly doped fiber with inner cladding and in a D-shaped fiber. These gratings provide good contrast to increase the resolution for sensing applications, with or without access to the surroundings along the fiber. The mode profiles of the devices were characterized experimentally to gain deeper insight into the improved functionality.
Sensing Characteristics of a Novel Two-Section Long-Period Grating
Applied Optics, 2003
The behavior of a temperature self-compensating, fiber, long-period grating ͑LPG͒ device is studied. This device consists of a single 325-m-period LPG recorded across two sections of a single-mode B-Ge-codoped fiber-one section bare and the other coated with a 1-m thickness of Ag. This structure generates two attenuation bands associated with the eighth and ninth cladding modes, which are spectrally close together ͑ϳ60 nm͒. The attenuation band associated with the Ag-coated section is unaffected by changes in the refractive index of the surrounding medium and can be used to compensate for the temperature of the bare-fiber section. The sensor has a resolution of Ϯ1.0 ϫ 10 Ϫ3 for the refractive index and Ϯ0.3°C for the temperature. The effect of bending on the spectral characteristics of the two attenuation bands was found to be nonlinear, with the Ag-coated LPG having the greater sensitivity.
Microwave and Optical Technology Letters, 2003
In this paper, we show that a complete three-layer analysis is necessary to characterise the cladding modes in order to obtain grating period, resonances, and coupling length in the design of longperiod gratings (LPGs), and that the simplified two-layer fiber geometry used by many authors leads to incorrect designs. This is illustrated by design calculations corresponding to actual applications of long-period gratings as sensors and gain equalisation filters for EDFA.
In this paper we show the experimental characterization of optical fiber long period grating for the reduced temperature sensitivity by optimizing the refractiveindex profile of the host fiber. In Long Period Fiber grating the fundamental guided mode light couple with the into copropagating cladding modes at various wavelengths. The two layer Geometry later on the three layer geometry which is useful for refractive index sensing sensing application. The sensitivity of LPFGs to environmental parameters is influenced by the period of the LPFG by the order of the cladding mode to which coupling takes place and by the composition of the optical fiber. A novel method to compensate the temperature sensitivity using a boron co-doped germanosilicate core fiber is used as a host fiber. The Theoretical Characteristic Results matches with the experimental results which demonstrate that such gratings with reduced temperature coefficient retain their capability to detect strain and ambient index variations.
Journal of Lightwave Technology, 2003
A numerical method is presented for determining the transmittance of long-period (LP) fiber-gratings having arbitrary azimuthal/radial refractive index variations. The method uses coupled-mode theory and includes both the sine and cosine character of the LP modes. The model treats interactions between the fundamental 01 mode and high-azimuthal-order cladding modes. The method utilizes the transfer matrix method to model cylindrical layers both in the core and the cladding regions.
Analysis of the response of long period fiber gratings to external index of refraction
Journal of Lightwave Technology, 1998
This paper demonstrates that the change in wavelength of a long period fiber grating attenuation band with changes in external index of refraction can be enhanced by proper selection of the grating period. We calculate and experimentally verify that the wavelength shift caused by changing the external index from n = 1 n = 1 n = 1 to n = 1:44 n = 1:44 n = 1:44 of the attenuation band which appears in the 1400-1600 nm region in a 200-m period grating is four times that in a 350-m period grating. Changes in the spectrum over a wavelength range from 1100 to 1600 nm and 1 < n < 1:72 1 < n < 1:72 1 < n < 1:72 index range are also presented, and implications for grating design when enhanced or reduced index sensitivity is desired are discussed. Finally, we demonstrate the use of a highly index-sensitive long period fiber grating as a chemical concentration sensor.