Parallel Long-period Fiber Gratings Research Articles

Femtosecond laser inscribed parallel long-period fiber gratings for multi-channel core mode conversion.

We propose and demonstrate the inscription of parallel long-period gratings (LPGs) in a few-mode fiber (FMF) using femtosecond lasers. Mode conversion from the fundamental (LP01) mode to high-order core modes, including LP11, LP21, LP31, LP02, and LP12, is achieved by controlling the inscription period of the gratings. Taking advantage of the highly focused femtosecond laser, LPGs with different off-axis offsets were fabricated, and the resonance wavelength and the inscription efficiency of the gratings versus the offset were investigated. Based on the off-axis writing technique and using the femtosecond laser source, we wrote parallel LPGs that contain multi-gratings in a single FMF and achieved a multi-channel core mode converter in a single FMF with flexibility in terms of the resonant wavelength and mode conversion among different modes. This approach offers a new, to the best of our knowledge, option for implementation with high integration, and a multi-channel mode converter, which could find potential applications in FMF multi-wavelength laser systems, and wavelength/mode division multiplex communication systems. Furthermore, these microstructured LPGs integrated into an optical fiber can be used as a multifunctional sensor.

Optical sensing based on light coupling between two parallel long-period fiber gratings

A new optical intensity-based sensing mechanism for the measurement of refractive index and minute displacement is proposed in this paper, which is based on modifying the amount of light coupled between two parallel long-period fiber gratings. The characteristics of this sensing mechanism with experiments and simulation results are demonstrated.

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.

Add/drop channel filter based on two parallel long-period fiber gratings coupler

A wavelength-selective coupler constructed with two parallel long-period fiber gratings (LPFGs) was investigated thoroughly. Both the numerical and experimental results verified that the filter response could be improved by adjusting the UV refractive modulation of the gratings and by controlling the external surrounding. And an add/drop channel filter with high efficiency and good spectral characteristic was achieved even though the claddings of the two gratings were not processed. This feature is very favorable to simplify the fabrication process.

Symmetric 3 x 3 optical coupler using three parallel long-period fiber gratings

We propose a new technique of realizing a symmetric 3 x 3 optical coupler, which is based on evanescent-field coupling among three identical parallel long-period fiber gratings placed side-by-side. Using three 32-mm-long over-coupled long-period fiber gratings embedded in a medium with a proper refractive index, we achieved equal power outputs from the three fibers at the resonance wavelength of the gratings with a total loss of about 1.4 dB when light was launched into any one of the fibers.

Tunable optical add-drop multiplexer based on long-period fiber gratings for coarse wavelength division multiplexing systems

We propose and experimentally demonstrate a voltage-controllable add-drop multiplexer with a tunable coupler based on long-period fiber gratings (LPFGs). The tunable coupler is based on cladding mode coupling between two parallel LPFGs with divided coil heaters, which controls the resonant wavelength positions of the two LPFGs. The wavelength tunability of the tunable coupler is measured to be -0.45 nm/degrees C in the temperature range from 20 to 120 degrees C. The large tuning range of optical signals (approximately 50.54 nm) from 1502.32 to 1552.86 nm, which covers both the short (S) and the conventional (C) bands, is achieved. The channel isolation is as high as approximately 40 dB. It has several advantages, including a broad tuning range of wavelength in both the S- and C-bands, high channel isolation, no backreflection, and so on.

Transfer-matrix method for the analysis of two parallel dissimilar nonuniform long-period fiber gratings

A transfer-matrix method based on the coupled-mode theory is developed for the analysis of an all-fiber coupler consisting of two parallel long-period fiber gratings (LPFGs), which can be dissimilar and nonuniform. Using this method, the effects of various forms of nonuniformity introduced along the gratings on the transmission characteristics of the coupler are studied. The nonuniformities considered include pitch detuning, chirping, phase shifts, and index apodization. Numerical examples are given to highlight the conditions of achieving high coupling efficiency and the possibility of suppressing ripples and side lobes in the transmission spectrum. The obtained results are useful for the understanding and the design of various kinds of long-period grating-based devices, such as filters, broadband couplers, signal taps, and add/drop multiplexers.

Broad-band optical coupler based on evanescent-field coupling between three parallel long-period fiber gratings

We propose a broad-band 3/spl times/3 optical coupler based on the principle of evanescent-field coupling between three parallel identical long-period fiber gratings. In particular, we demonstrate experimentally the situation where light is launched into one fiber and coupled equally to the other two fibers. The effects of changing the surrounding refractive index and introducing an offset distance between the gratings on the transmission characteristics of the coupler are investigated. Using three identical 35-mm-long gratings, we find that the peak coupling efficiency increases by 9 and 14 dB, as the surrounding refractive index and the offset distance increase from 1.0 to 1.440 and from 0 to 50 mm, respectively. The side-lobe suppression ratio varies between 16.0 and 23.0 dB. We achieve a total power conversion efficiency of 85% at the resonance wavelength with a surrounding refractive index of 1.420 and an offset distance of 50 mm.

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.