Uniform Long-period Fiber Grating Research Articles

Ambient refractive index sensitivity of long-period fiber grating (LPFG) with reduced cladding thickness using three-layer fiber geometry approach

Abstract This article presents the refractive index (RI) sensitivity study of cladding reduced uniform long-period fiber grating. The displacement of resonant wavelength with the change in ambient RI is observed to inspect the sensitivity. Here, the grating period is fixed at a value (550 μm) to analyze RI sensitivity of cladding modes (HE13-HE14). However, it varies in earlier mathematical studies to adjust the resonant wavelength of all modes at a specific wavelength. The three-layer fiber geometry-based mathematical approach is employed to estimate the core and cladding modes. The obtained results support decline in quantity of cladding modes sustained by a single-mode fiber. The decrease in cladding radius has shown enhancement in RI sensitivity. For a reduction of 22 μm in cladding radius, the highest ordered HE14 cladding mode has shown 5.8 times improvement in its sensitivity.

Recent progress in design and fabrication of novel long-period fiber grating

Long-period fiber grating (LPFG) is a kind of wide-range transmission passive photonic device with extensive applications in the field of fiber communication and fiber sensing. In this review, from the angle of refractive index spatial modulation, we extract three characteristic parameters of LPFG:grating period length, index modulated depth and normal orientation of grating plane, and classify LPFG as two types:uniform LPFG (none of these three parameters changes) and nonuniform LPFG (at least one of them changes), and analyze the deficiency of LPFG, including larger size than fiber Bragg grating, no reflection peak, too large bandwidth, polarization loss from single-side exposure, etc. We define the concept of novel LPFG (NLPFG) as the LPFGs based on general LPFG but having new structures and new characters by importing new factors from different aspects, like grating formed mechanism, grating structure, making material, processing technique, application performance, etc. Then we point out that the research significance of NLPFG lies in improving and exploring its real usable property, and making it practical by overcoming the defects of general LPFG in structure, property and application. We expound new techniques of LPFG fabrication, such as multi-exposure, apodized exposure, outfield action, coating and filling, fiber incised and welded, multi-dimensional modulation, and show some NLPFG examples written with these techniques. We build the spatial model of NLPFG to expand the refraction index modulation region from only fiber core to both core and cladding, and to correctly mark the direction of grating plane with tilted angle and azimuth angle. On this basis, we propose the design theory of NLPFG by adding those two angles into the coupling mode coefficient and solving the coupling mode equation. We also expound three different NLPFG design processes, as the direct design to start from given factors of grating, the reserve design to calculate the factors back from expected function or spectrum, and the direct-reserve design combined by them. Meanwhile, we introduce some typical design methods of NLPFG, like geometrical structure changed method, materials changed method, medium coated and embedded method, etc. In addition, we review the recent fabrication and typical application of NLPFG, then introduce different LPFG devices based on excentric core LPFG, multi-core LPFG, few-mode LPFG, stagger LPFG, mismatched LPFG, over-melted LPFG, phase-shift LPFG, tuning LPFG, coupled LPFG and cascaded LPFG, and show their sensing applications in strain, twisting, bending, temperature, displacement, gas concentration and biology. Finally, we provide a developing prospect of the research on NLPFG and give three possible means to improve the research, as innovating new gating structures, exploring new design methods and developing new fabrication techniques.

Broadband single-peak filtering characteristics of coated long-period fiber gratings

A coated long-period fiber grating (LPFG) operating at the phase-matching turning point couples the fundamental core mode to a higher-order cladding mode, producing a single broad-band whose 3dB-bandwidth is dependent on the difference in dispersion between the core mode and a cladding mode, grating length and central wavelength. The variations of film refractive index and thickness influence the difference in dispersion between the core mode and cladding mode and thus, the bandwidth of loss peak. The central wavelength of loss peak also varies with the changes of film parameters. When the film refractive index is 1.57 and the film thickness is 350 nm, the bandwidth of loss peak reaches 302 nm. The bandwidth can be further improved to 334 nm by reducing the grating length based on the fact that the loss at the central wavelength is guaranteed to be more than 6 dB. A further investigation shows that introducing a π phase shift into a uniform LPFG at a proper position that is away from the grating center can increase the bandwidth to 372 nm and more.

Stable all-fiber photonic temporal differentiator using a long-period fiber grating interferometer

A novel configuration of all-fiber photonic temporal differentiator optimized for tens-of-gigahertz processing bandwidth is suggested and experimentally demonstrated. This device covers the gap in terms of processing bandwidth between the two previously-demonstrated all-fiber designs, namely the fiber Bragg grating-based differentiator (<20 GHz) and that based on a uniform long-period fiber grating (>100–200 GHz).

Analysis of Six-Port Optical Fiber Couplers Based on Three Parallel Long-Period Fiber Gratings

We present a coupled-mode analysis of a six-port coupler consisting of three parallel uniform long-period fiber gratings, where a longitudinal offset distance may be introduced between the gratings. In particular, we discuss the effects of the grating coupling coefficient and the evanescent-field coupling between the parallel fibers on the performance of the coupler. Our theoretical results compare well with the experimental data for several typical couplers. The method of analysis together with the numerical illustrations presented in the paper can facilitate the design of this new class of six-port all-fiber wavelength-selective couplers with applications in add/drop multiplexing, signal processing, and broadband filtering.

Picosecond and sub-picosecond flat-top pulse generation using uniform long-period fiber gratings

We propose a novel linear filtering scheme based on ultrafast all-optical differentiation for re-shaping of ultrashort pulses generated from a mode-locked laser into flat-top pulses. The technique is demonstrated using simple all-fiber optical filters, more specifically uniform long period fiber gratings (LPGs) operated in transmission. The large bandwidth typical for these fiber filters allows scaling the technique to the sub-picosecond regime. In the experiments reported here, 600-fs and 1.8-ps Gaussian-like optical pulses (@ 1535 nm) have been re-shaped into 1-ps and 3.2-ps flat-top pulses, respectively, using a single 9-cm long uniform LPG.

Ultrafast all-optical differentiators

We report the experimental realization of an ultrafast all-optical temporal differentiator. Differentiation is obtained via all-fiber filtering based on a simple diffraction grating-assisted mode coupler (uniform long-period fiber grating) that performs full energy conversion at the optical carrier frequency. Due to its high bandwidth, this device allows processing of arbitrary optical signals with sub-picosecond temporal features (down to 180-fs with the specific realizations reported here). Functionality of this device was tested by differentiating a 700-fs Gaussian optical pulse generated from a fiber laser (@ 1535nm). The derivative of this pulse is an odd-symmetry Hermite-Gaussian waveform, consisting of two linked 500-fs-long, pi-phase-shifted temporal lobes. This waveform is noteworthy for its application in advanced ultrahigh-speed optical communication systems.

Long-period fiber gratings as ultrafast optical differentiators

It is demonstrated that a single, uniform long-period fiber grating (LPFG) working in the linear regime inherently behaves as an ultrafast optical temporal differentiator. Specifically, we show that the output temporal waveform in the core mode of a LPFG providing full energy coupling into the cladding mode is proportional to the first derivative of the optical temporal signal (e.g., optical pulse) launched at the input of the LPFG. Moreover, a LPFG providing full energy recoupling back from the cladding mode into the core mode inherently implements second-order temporal differentiation. Our numerical results have confirmed the feasibility of this simple, all-fiber approach to processing optical signals with temporal features in the picosecond and subpicosecond ranges.

Analysis of Two Parallel Long-Period Fiber Gratings

A wavelength-selective coupler constructed with two parallel identical uniform long-period fiber gratings (LPFGs) is analyzed by the coupled-mode theory. The effects of introducing a longitudinal offset distance between the two gratings on the transmission characteristics of the coupler are discussed and the conditions for achieving a 100% coupling efficiency are highlighted. It is found that 100% coupling can be achieved with a practical grating length by using a proper grating design and a suitable index-matching surrounding medium. The theoretical results agree well with the experimental data.

Analysis of Concatenated Long Period Fiber Gratings Having Phase-Shifted and Cascaded Effects

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.