Design of long-period gratings: Necessity of a three-layer fiber geometry for cladding mode characteristics (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
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.
Optics Express, 2010
Two optical fiber devices have been coated in parallel: a long period fiber grating (LPFG) and a cladding-removed multimode optical fiber (CRMOF). The progressive coating of the LPFG by means of the layer-by-layer electrostatic-self-assembly, permits to observe a resonance wavelength shift of the attenuation bands in the transmission spectrum. The cause of this wavelength shift is the reorganization of the cladding mode effective indices. The cause of this modal reorganization can be understood with the results observed in the CRMOF coated in parallel. A lossy-moderesonance (LMR) is generated in the same wavelength range of the LPFG attenuation bands analyzed. Moreover, the thickness range where the wavelength shift of the LPFG attenuation bands occurs coincides exactly with the thickness range where the LMR can be visualized in the transmission spectrum. These phenomena are analyzed theoretically and corroborated experimentally. The advantages and disadvantages of both optical fiber devices are explained.
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.
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.
Optimization of sensitivity in Long Period Fiber Gratings with overlay deposition
Optics Express, 2005
The deposition of an overlay of higher refractive index than the cladding in a Long Period Fiber Grating (LPFG) permits to improve the sensitivity to ambient refractive index changes in a great manner. When the overlay is thick enough, one of the cladding modes is guided by the overlay. This causes important shifts in the effective index values of the cladding modes, and henceforward fast shifts of the resonance wavelength of the attenuations bands in the transmission spectrum. This could be applied for improving the sensitivity of LPFG sensors. The problem is analysed with a numerical method based on LP mode approximation and coupled mode theory, which agrees with so far published experimental results.
Optics Express, 2016
This work presents a refractive index sensor based on a long period fiber grating (LPFG) made in a reduced cladding fiber whose low order cladding modes have the turning point at large wavelengths. The combination of these parameters results in an improved sensitivity of 8734 nm/refractive index unit (RIU) for the LP 0,3 mode in the 1400-1650 wavelength range. This value is similar to that obtained with thin-film coated LPFGs, which permits to avoid the coating deposition step. The numerical simulations are in agreement with the experimental results.
Control of the characteristics of a long-period grating by cladding etching
Applied Optics, 2000
We have analyzed the effects of the fiber cladding radius on the characteristics of long-period fiber gratings. By etching the cladding of a common single-mode fiber, we verified the characteristics experimentally. When by etching we reduce the cladding radius of a common single-mode fiber on which a long-period refractive-index modulation has been imposed, the coupling strength of the core and the cladding modes increases. In addition, the difference in the propagation constants ͑for a fixed wave-length͒ between the core mode and the cladding modes increases; hence the resonant transmission dip wavelengths shift to longer wavelengths. The proposed method can be useful in making and detuning long-period fiber grating filters.
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.