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In order to obtain analytical gain expression in Brillouin optical Fiber Amplifiers (BFAs), coupled intensity equations describing the interaction of pump and stokes waves must be solved simultaneously. For long optical fibers, although fiber loss is responsible for the pump depletion and nonnegligible effect, for short optical fibers less than 2 km, its effect can be discarded. In this paper, we provide an accurate analytic expression for the BFA gain for fiber lengths less than 2 km by discarding the optical fiber loss and show results of experimental validation.

Goktas, Hasan/0000-0002-2195-9531 WOS: 000424678900006 PubMed: 29400726 We report on an accurate closed-form analytical model for the gain of a Brillouin fiber amplifier that accounts for material loss in the depleted pump regime. We determined the operational model limits with respect to its relevant parameters and pump regimes through both numerical and experimental validation. As such, our results enable accurate performance prediction of Brillouin fiber amplifiers operating in the weak-pump, high-gain, and saturation regimes alike. (C) 2018 Optical Society of America

International audience; We demonstrate the effectiveness of silicon phase masks to implement spatially resolved, multispectral imaging capabilities in the range of terahertz frequencies, using a standard setup of basic interest for time-domain spectrometry with a single-cell source and a single-cell detector. Our principle primarily aims at the development of robust and inexpensive systems. It consists of appropriate space-to-time encoding, in order to ensure single-scan triggering and then take advantage of rapid and self-consistent measurements in the two-dimensional space. The process enables very efficient discrimination giving access to a relevant spatial resolution in the analysis of small size, planar assemblies made of inhomogeneous materials. Benchmark results are provided to validate the concept, thanks to prototyping phase masks with 2×2 pixels, prior evidencing actual performance limitations in the case of 3×3 pixels. Due to the frequency bandwidth of 0.1–1.5THz in our setup and to the available operating conditions, currently acceptable pixel resolutions lie in the range of 3–4mm. Numerical modeling by means of finite elements helps to discuss these numbers and to investigate the relevant theoretical issues, figuring the main propagation issues in connection with a sub-picosecond seed pulse throughout various masks. This involves diffraction and trailing edge effects when crossing the mask together with residual, parasitic reflections. Finally, we give a consistent prospective for improved performance, via realistic updates regarding the architecture of the setup and complementary post-processing. Further values for the attainable spatial resolution then range from 5×5 to 6×6 pixels.

We present a comprehensive analysis of the technique of Longitudinal-Mode-Filling (LMF) to reduce Stimulated Brillouin Scattering (SBS) limitations in Ytterbium Doped Fibre Amplifiers (YDFA), for the generation of nanosecond, temporally shaped pulses. A basic Master-Oscillator-Power-Amplifier (MOPA) system, comprising an output YDFA with 10 mu m-core active fibre, is experienced for benchmarking purposes. Input pulse-shaping is operated thanks to direct current modulation in highly multimode laser-diode seeds, either based on the use of Distributed Feed-Back (DFB) or of a Fibre Bragg Grating (FBG). These seeds enable wavelength control. We verify the effectiveness of the combination of LMF, with appropriate mode spacing, in combination with natural chirp effects from the seed to control the SBS threshold in a broad range of output energies, from a few to some tens of mu J. These variations are discussed versus all the parameters of the laser system. In accordance with the proposal of a couple of basic principles and with the addition of gain saturation effects along the active fibre, we develop a full-vectorial numerical model. Fine fits between experimental results and theoretical expectations are demonstrated. The only limitation of the technique arises from broadband beating noise, which is analysed thanks to a simplified, but fully representative description to discuss the signal-to-noise ratio of the amplified pulses. This provides efficient tools for application to the design of robust and cost-effective MOPAs, aiming to the generation of finely shaped and energetic nanosecond pulses without the need for any additional electro-optics. (C) 2014 Optical Society of America

Brussels Photonics Team (B-PHOT);Brussels-Capital Region;Fonds Wetenschappelijk Onderzoek (FWO);The Society of Photo-Optical Instrumentation Engineers (SPIE);Ville de Bruxelles Nonlinear Optics and Its Applications VIII; and Quantum Optics III -- 14 April 2014 through 16 April 2014 -- -- 110770 We present a comprehensive experimental study of the technique of Longitudinal Mode Filling (LMF) applied to the reduction of Stimulated Brillouin Scattering (SBS), in Ytterbium Doped Fibre Amplifiers (YDFA) at the wavelength of 1064 nm. Pulse durations and Mode Field Diameters (MFD) lie in the ranges of 10 - 100 ns and 10 - 35 ?m, respectively. Input pulse-shaping is implemented by means of direct current modulation in multimode Laser-Diode seeds. This evidences a number of interests in the development of robust and low cost Master Oscillator Power Amplifiers (MOPA). Highly energetic, but properly shaped, nanosecond pulses may be produced this way without any need of additional electro-optical means for in-line phase and amplitude modulation. Seeds consist of Distributed Feed- Back (DFB) and Fibre Bragg Gratings (FBG) with different fibre lengths. We demonstrate the benefit of LMF with properly controlled mode spacing, in combination with chirp effects due to fast current transients in the semiconductors, in order to deal with SBS thresholds in the range of a few to some hundred ?J. The variations of the SBS threshold are discussed versus the number of longitudinal modes, the operating conditions of the selected seed and pulse-shaping conditions. © 2015 SPIE.

The spectral and temporal evolution of distributed sensing based on stimulated Brillouin scattering (SBS) in optical fibers for several-nanosecond Stokes pulses is demonstrated by using the method of lines (MOL) solution of the transient SBS equations. A superbee flux limiter is utilized to avoid numerical damping and dispersion that would otherwise be brought on by the approximation of spatial derivatives associated with the partial differential equations (PDEs). In order to increase computational efficiency, an approach is adopted where by the sparse PDE jacobian matrix integrator option of the ODE solver(s) is employed. Simulation examples of SBS-based sensing for fibers containing sections with different Brillouin frequencies are presented. To the best of our knowledge, this MOL solution is proposed for the first time for modeling of the transient SBS equations for nanosecond Stokes pulses with different waveforms in a SBS based fiber optic sensor.

Highly accurate closed-form analytical formula, based on the undepleted pump approximation (UPA), is derived for pump, Stokes and the gain of a Brillouin fiber amplifier (BFA) operating in the moderately depleted pump regime. The material loss of the medium, which is a strong effect in optical fibers, is taken into account. To define validity ranges of the solution, the threshold power of BFA is accurately determined. The obtained results are numerically and experimentally validated. The presented analysis can be used to accurately predict the performance of moderate-gain BFAs.