Novel fiber design for broadband long period gratings (original) (raw)
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Design and Fabrication of Novel Broadband Long-Period Fiber Gratings Using Synthesis Techniques
Journal of Lightwave Technology, 2011
We present results of novel broadband long-period gratings (LPGs) that were fabricated axially symmetric in singlemode fiber with a carbon-dioxide laser. The discrete layer-peeling (DLP) technique and a genetic algorithm (GA) were used to design the LPGs. Numerical simulations were used to indicate the performance and absolute error of reconstructing a complex spectral profile for a particular synthesis technique. We found that the DLP technique has the highest performance and executes in the least amount of time. However, a GA could not efficiently synthesize an LPG, but produced a refractive index change profile that can be implemented using a common LPG fabrication system. It is shown that the experimental results obtained with the GA are superior to that obtained with the DLP technique. Index Terms-Carbon-dioxide (CO 2) laser, complex spectrum, genetic algorithm (GA), layer-peeling technique, long-period gratings (LPGs). I. INTRODUCTION O VER the years, it has become important to design appropriate optical filters, for example, fiber gratings, to achieve a desired spectral response. It is important, when designing grating structures, to strictly monitor the complexity of the index modulation of the grating structure, such that it can be practically realized in the optical fiber core during the grating fabrication process. At present, long-period gratings (LPGs) contribute a great deal to the high-speed optical fiber communication industry with their guided-to-cladding mode power exchange. Due to the low insertion loss, low backreflection, and ease of fabrication of LPGs [1], these grating structures are popular in the gain flattening of erbium-doped fiber amplifiers [2] and add-drop multiplexing [3], [4]. Currently, two classes of the optical filter design process exist, namely, filter analysis and filter synthesis [5]. During the analysis of optical filters, the spectral response, dispersion, and grating strength properties are calculated, given a physical Manuscript
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]:
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.
A Review on Optical Fiber Long Period Grating, its applications in Optical Communication System
IJARCCE, 2015
Innovations in optical fiber technology are revolutionizing world communications. As we can se that optical fiber long period grating can be used in designing of devices which are used to meet the growing demands for various ranges in the field of optical communication systems. Thus, this paper deals with the descriptive study of long period fiber grating (LPG) and its applications in emerging field of optical communication systems. LPG forms an important component of optical communication. The paper covers the analysis of long fiber grating and their fabrication. This paper also deals with the cladding mode analysis of the fiber which describes the inaccuracies of two layer model of the fiber and implementation of three layer fiber geometry to calculate the effective refractive index of the fiber so that it can effectively couple the signal for its efficient transmission. In the cladding mode analysis the whole mathematical derivation along with the requisite mathematical expressions are explained to find the effective refractive indices of the various cladding modes being supported by the fiber which are used to plot the transmission spectrum of the LPG designed for a particular frequency or wavelength used for telecommunication purposes.
Journal of Lightwave Technology, 2003
A new method to control the free spectral range (FSR) of a long-period fiber grating (LPFG) is proposed and theoretically analyzed. As the refractive index decreases radially outward in the silica cladding by graded doping of fluorine, waveguide dispersion in the cladding modes was modified to result in the effective indexes change and subsequently the phase-matching conditions for coupling with the core mode in a LPFG. Enlargement of the FSR in a LPFG was theoretically confirmed.
Photonics
In this paper, we have briefly review the developing history and recent advances made with regard to helical long-period fiber gratings (HLPGs) in three aspects, i.e., the mode-coupling theories, the fabrication techniques, and the applications. It is shown that, due to the intrinsic helicity characteristics, which are especially suitable to control the loss, polarization, and orbit-angular-momentum (OAM) states of the light in optical fiber, HLPGs have recently attracted great research interest and have found various applications, such as the mode-converters, the torsion sensors, the band-rejection filters, wave plates, linear- and circular-light polarizers, and OAM mode generators, etc. It is believed that HLPGs and the HLPGs-based devices would find further applications to not only the fields of optical sensors and optical communication, but also other fields such as ultrahigh precision measurement, quantum optics, and biochemistry, etc.
Analysis of transmission characteristics of long period gratings in tapered optical fibers
Applied Optics, 2009
We report an analysis of the transmission characteristics of long period gratings written in a linearly tapered optical fiber. Such gratings are best analyzed by a coupled mode theory that includes several cladding modes simultaneously. However, the numerical solution needs to be modified to take into account the changing propagation constants along the length of the fiber. The transmission spectra show certain distinctive features that depend on the grating period and choice of the phase-matched coupled cladding modes. For low-order modes, the resonances broaden and tend to vanish as the taper of the angle increases. For higher-order modes, the resonances remain well defined but shift toward higher or lower wavelengths and broaden. The transmission spectrum is also highly sensitive to a change in the ambient atmosphere.
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 fiber gratings with overlay of variable refractive index
IEEE Photonics Technology Letters, 2000
A theoretical analysis is presented of a long-period fiber grating (LPFG) with an overlay of variable refractive index. The highest sensitivity of the resonance wavelengths to variations in the refractive index of the overlay can be optimized. There are two key points for a good design: the selection of an overlay refractive index close to that of the cladding of the LPFG and the overlay thickness. The problem is analyzed with a numerical method based on coupled-mode theory.
Light Modulation Based on Fiber Cladding Mode Coupling Between Concatenated Long-Period Gratings
IEEE Photonics Technology Letters, 2000
In-fiber efficient light modulation using two concatenated long-period gratings (TCLPG), written in single-mode fiber, is reported. The device is compressed in the middle of both gratings against a corrugated surface, with a piezoelectric, producing a transfer of energy between cladding modes which results in modulated light transmission. The TCLPG operates at 1542 nm, with a bandwidth of 29 nm and an insertion loss of 0.3 dB. The modulation depth is 20 dB and the rise time is 7.5 ms at a repetition rate of kilohertz.