Uniform Fiber Gratings Research Articles

Tunable Random Fiber Laser Based on Dual-Grating Structure

In order to reduce the pumping threshold and achieve a short-cavity single-mode transmission with a narrow-linewidth random fiber laser, we propose a tunable random fiber laser based on the combination of random grating and highly reflective fiber Bragg grating (FBG). Theoretical modeling of a random refractive index-modulated fiber grating was carried out. Random grating is regarded as a linear combination of uniform fiber gratings with different periods. Simulation calculations were performed using the transfer matrix method to determine the preparation parameters. Under the premise of satisfying light localization, a point-by-point method was used to write a random grating in a single-mode fiber using a femtosecond laser according to the simulated parameters. We constructed a random fiber laser with a linewidth of 1.68 kHz and a threshold of 29.2 mW using a random grating and a highly reflective FBG combined with an erbium-doped fiber. Due to the broad scattered wavelength range of the random grating, by changing the central wavelength of the high-reflection FBG, the tunable wavelength of the output laser was realized, and the tunable range was 0.847 nm (1549.110–1549.957 nm). Moreover, the laser’s central wavelength and output power are stable for a long time. Compared with other lasers, the proposed laser has the advantages of a lower threshold, shorter cavity length, narrower linewidth, and a relatively simple structure.

Generation of orbital angular momentum light based on mechanically induced long-period fiber gratings using 3D printing technique

• An OAM beam generator was proposed with broadband and tunable operation wavelength. • OAM generation was achieved by employing 3D-printed mechanically induced few-mode fiber gratings. • By adjusting applied pressures and inclination angle of few-mode fibers, the operation wavelength can be tuned. • The operational bandwidth can be broadened by employing cascaded few-mode fiber gratings. We propose and experimentally demonstrate an orbital angular momentum (OAM) beam generation method based on mechanically induced few-mode fiber gratings fabricated using 3D printing technique. Experimental results indicate that the transmission depth at resonance wavelength of few-mode fiber gratings with axially uniform refractive index could be adjusted when different lateral pressures are applied onto the few-mode fibers, and hence mode coupling efficiency between the fundamental core mode and LP 11 mode could be controlled in this way. In addition, by changing the inclination angle of the few-mode fibers with respect to direction perpendicular to the mask grooves, the resonance wavelength of the few-mode grating could be changed as well. Besides uniform fiber gratings, we have also investigated the spectral broadening property of the few-mode cascaded fiber grating, and a 10-dB spectral bandwidth up to 114 nm is obtained experimentally. Our proposed OAM light generation method based on mechanically induced fiber gratings using 3D printing technique possesses several desirable merits such as low cost, ease of operation, flexibility in grating structure and no damage to fiber medium, which is anticipated to provide low lost and flexible solution to generation of broadband OAM light for future OAM-based communication systems.

Investigations on highly sensitive fiber Bragg gratings with different grating shapes for far field applications

The Fiber Brag Grating (FBG) optical sensor is investigated with uniform fiber grating (UFBG), superstructure fiber Bragg grating (SFBG) and tilted fiber Bragg grating (TFBG) using Finite Difference Time Domain (FDTD). The effect of variation in Refractive Index (RI) on the far field pattern of the guided light, reflection and the transmission power spectra of the gratings have been observed. TFBG reported the best performance exhibiting maximum peak amplitude of far field is 0.218 a. u., followed by UFBG is 0.0467 a. u. and SFBG is 0.0383 a. u. Further, the coupling efficiency of the gratings has been calculated from the mode overlap integral. The TFBG reported the highest value of 5.622%. SFBG followed it with 0.466% and UFBG with 0.012%. The SFBG reported the maximum value of reflected power i.e. 1.9456dB which is 3 times to that of UFBG (0.6317dB) and 5.67 times to that of TFBG (0.3433dB).

Stability and collisions of traveling solitons in Bragg-grating superstructures

We investigate stability of moving solitons in a nonlinear optical fiber with short segments of a relatively strong Bragg grating (BG) periodically inserted into it. The model is related to a class of composite and artificial optical media that have recently attracted much interest. The analysis is focused on moving solitons, as this is relevant to the experiment. By means of systematic simulations, we find that, in accordance with a qualitative consideration, the stability region for moving solitons first increases with velocity c but then decreases (in contrast to the uniform fiber BG, where the stability of the solitons depends very weakly on c). As well as in the uniform system, solitons do not exist for c exceeding the group velocity of light in the fiber, nor may they be stable with negative intrinsic frequencies. Collisions between solitons moving in opposite directions are also studied in a systematic way. A difference from the situation in the uniform fiber BG is strong shrinkage and eventual disappearance of a region where the collision results in a merger of the two solitons into a single one, which is explained too. Fiber waveguides with BG superstructures being currently available, moving solitons can be created in them by means of experimental techniques similar to those that have helped to generate solitons in uniform fiber gratings.

Growth Modeling of Fiber Gratings: A Numerical Investigation

We have developed a simple method to analyze the growth process of fiber gratings (FGs) in the core of photorefractive optical fibers. It is based on the exponential saturation model and it takes into account the type of photosensitive fiber used, the exposure times, and the incident optical power. The photorefractive fiber has been characterized by two parameters: the saturation photoinduced refractive index change and the saturation energy, which can be obtained experimentally. We carry out a numerical investigation for the growth process of a uniform FG. The experimentally observed phenomena of shifting the Bragg optical frequency with exposure times and the appearance of higher-order Bragg resonances are predicted and discussed.

Modeling and Testing of Uniform Fiber Bragg Gratings Using 1-D Photonic Bandgap Structures in Microstrip Technology

A way to model and test fiber gratings and fiber grating-based systems in the microwave range is proposed. The method rests on the equivalence relationship found between a fiber grating and a one-dimensional microstrip photonic bandgap structure that consists of a periodic pattern of circles etched in its ground plane. The equivalence assures that the frequency-response of the fiber grating is an upshifted replica of the corresponding frequency to its equivalent microstrip device. Both apodized and nonapodized uniform fiber gratings have been considered and the feasibility of the proposed method has been shown in simulation and measurement.