We numerically present a method to generate a high-energy square pulse in dual-amplifier figure-eight microstructured optical fiber laser (DAF8MOFL) using a genetic algorithm (GA). We explore a large region of the parameter space to find adequate parameters that promote the generation of high-performance square pulse. We demonstrate that, by utilizing the GA-based optimization technique, the parameters of the DAF8MOFL can be conveniently optimized to achieve targeted output pulse shape according to real-parameter constraints. We show that by adequately adjusting the cavity parameters, high-energy pulse with a square profile and picosecond pulse width can be obtained without the pulse breaking.
The investigated Q-switching technique is based on the spectral overlap between a Fabry–Perot (F–P) filter and a fiber Bragg grating. This Q-switching concept is implemented in a ring erbium (Er)-doped fiber laser configuration. We use a traveling-wave analytical model to study the peak power and the time duration of the output pulses with respect to the cavity parameters, such as the Er-doped fiber length, the pumping power, and the scanning frequency of the fiber F–P tunable filter. Experimental results are used to validate the proposed Q-switched fiber laser model.
We present a widely adjustable high energy square pulse laser operating in DSR in a passively mode-locked F8L using dual Er:Yb co-doped double clad amplifiers. By manually controlling the power of each amplifier, the pulse width can be varied in a range of 360 ns without generating multi-pulsing instabilities. To ensure that DSR would dominate the modelocking mechanism, we use a 1.5 km standard single-mode fiber in the cavity. At a maximum pumping power, the laser generated square pulses with 416 ns duration and an average output power of about 1.33 W with a repetition frequency of 133 KHz corresponding to a record pulse energy of 10 μJ.
We propose to connect a single-mode fiber (SMF) to a dual-concentric cores fiber (DCCF) using an adiabatically tapered microstructured mode converter, and to evaluate the SMF LP01 mode and the DCCF LP01 and LP02 modes selective excitations performances. We theoretically and numerically study this selective excitation method by calculating the effective indices of the propagated modes, the adiabaticity criteria, the coupling loss, and the modes amplitudes along the tapered structure. This study shows that this method is able to achieve excellent selective excitations of the first two linearly polarized modes (LP01 and LP02) among the five guided modes in the DCCF with a negligible loss. The part of the LP01 and LP02 modes from the total power are 99% and 84% corresponding to 0.1 and 0.8 dB losses, respectively.
We numerically analyze the square pulse emission from a passively mode-locked figure-of-eight microstructured optical fiber laser. Numerical simulations demonstrate that the high nonlinearity of the microstructured fiber plays a key role in the output pulse duration. A dual-stage erbium-doped fiber amplifier has been used in the cavity. The first amplifier, localized in the nonlinear amplifying loop mirror, allows control of the pulse width, while the second amplifier in the unidirectional ring allows variation of the amplitude without affecting the pulse width. Our results give some physical insight to the square pulse formation and the generation of high-energy pulses. Our numerical model provides a general approach to control the properties of a square pulse, and hence could be of great importance for the design of practical high-energy fiber laser systems.
The coupling characteristics of newly proposed photonic-crystal-fiber couplers are rigorously analyzed using a 3-D finite-difference vector beam propagation method. The coupling length and efficiency of the couplers are investigated as a function of their geometrical parameters at different wavelengths. Size variability of the central hole is considered as a means of improving the coupling between the two cores. A determination of the guiding properties of the couplers, such as the propagation constants and the effective indices, is performed using a mode solver based on the plane-wave method. We show that it is possible to design very short photonic-crystal-fiber couplers with submillimeter coupling lengths. Nonuniformities are deliberately introduced in the coupler pattern to decrease the coupling length in comparison with the ideal structures. This study confirms that the designed coupler can act as an efficient ultrasmall-wavelength splitter or polarization-preserving directional coupler for wavelength division multiplexing systems.
We present experimental results on Microstructured Optical Fibre (MOF) coupling with standard Single Mode Fibre (SMF) using a simple and reliable set-up. An electric-arc splicing system is used to demonstrate its effectiveness in splicing SMF with MOFs. The measurements of the additional loss induced by transverse and longitudinal offsets at connections between SMF and MOF are reported and discussed. The influence of mode size mismatch is emphasized.
Microstructured optical fibers (MOFs) are optical fibers having a periodic air-silica cross-section. The air holes extend along the axis of the fiber for its entire length. The core of the fiber is formed by a missing hole in the periodic structure. Remarkable properties of MOFs have recently been reported. This paper presents new work in the modeling of the propagation characteristics of MOFs using the Finite Element Method (FEM) and the Galerkin Method (GM). This efficient electromagnetic simulation package provides a vectorial description of the electromagnetic fields and of the associated effective index. This information includes accurate determination of the spectral extent of the modes, cutoff properties and mode-field distributions. We show that FEM is well adapted for describing the fields at abrupt transitions of the refractive index while GM has the advantage to accurately analyze MOFs of significant complexity using only modest computational resources. This presentation will focus on the specific techniques required to determine single mode operation, dispersion properties and effective area through careful choice of the geometrical parameters of the fibers. We demonstrate that with suitable geometrical parameters, the zero dispersion wavelength can be shifted. This tool can also provide design criteria for fabricating MOFs and a corresponding map of effective area. This approach is validated by comparison with experimental results and measurements on actual MOFs fabricated at IRCOM and at Alcatel Research and Innovation Center.
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