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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Up-Scaling the Power of Pulsed Single Frequency Fibre Amplifiers for Coherent LIDAR Applications
摘要： In optical sensing applications, the laser sources are strongly constrained by the sensing system requirements. In the case of coherent LIDARs, that measure wind speed thanks to the Doppler shift (by means of a coherent detection), highly coherent high power pulsed lasers with good output beam quality are necessary to guarantee a proper measurement over long distances. The pulsed emission ensures the spatial resolution along the line of sight whereas the high coherence accounts for the wind speed resolution (measured in the frequency domain). Thus, the challenge when designing laser sources for such instruments resides in the difficulty of improving the output power while not altering the other characteristics. Fibre lasers and amplifiers have proven their interest in high power laser systems thanks to their compactness and their stability against temperature and mechanical fluctuations. The increase of emitted optical power is currently limited by nonlinear effects due to the intrinsically high confinement of the optical field in fibres. Among all the nonlinear effects, the Stimulated Brillouin Scattering (SBS) predominates in the case of single frequency or narrow spectrum optical fields. In order to increase the power of single frequency fibre lasers, many developments have been done in the past decades to mitigate SBS. We can class the different SBS mitigation techniques in two categories depending on whether they act on the optical field properties or on the optical fibre itself. All the techniques belonging to the first category are based on an active control of the phase or the frequency of the incident optical wave, in order to spread the incident energy in the spectral domain. In the second category, we can maximize the core radius of the fibre, minimize the fibre length, apply temperature gradients, dopant concentrations gradients or strain gradients to broaden the SBS spectrum in a controlled manner and thus lower its effective gain. We review here the different techniques we use to mitigate the SBS in pulsed single frequency fibre amplifiers and discuss their conceptual and technical limitations. Among all the existing mitigation techniques, we present here different realizations of fibre amplifiers designed for coherent LIDAR systems, based on strain gradients [1] and specifically designed highly doped and large core active fibres [2]. Figure 1 presents the typical achievements obtained with our patented tensile and compressive strain gradients techniques when applied to active fibers. We are able to increase the output power by a factor 2 to 3 while keeping a high beam quality on a highly coherent laser emission. We emphasize the robustness of such developments thanks to the absence of active devices that make them suitable for embedded sensing applications.
关键词： coherent LIDAR,high power pulsed lasers,Stimulated Brillouin Scattering,SBS mitigation,single frequency fibre amplifiers
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - All-Fiber 100-ns Single-Frequency 0.5mJ Anisotropic MOPA with Double Clad Tapered Yb-Doped Gain Fiber
摘要： High peak power, single-frequency pulsed fiber MOPA sources with extremely narrow line and nanosecond pulses are in highly demand to date. Such sources are required for various applications, including coherent LIDARs systems, remote sensing, and frequency conversion [1]. Stimulated Brillouin scattering (SBS) arising is a main obstacle to further peak power and pulse energy scaling [2]. There are several strategies for SBS suppression in the fiber MOPA have been described in the literature to date. These include exploiting a longitudinal temperature or mechanical stresses gradient in a gain fiber [3] or by using a birefringent fiber with both eigenstates excited. In [4] Stolen has demonstrated doubling of SBS threshold in birefringent fiber with equally excited fast and slow waves. Tapering of a fiber is significantly increases a SBS threshold [5].Usually, authors have used only one of abovementioned strategies for SBS mitigation. Meanwhile, all these measures might be imposed simultaneously by using a short, active, tapered, end pumped (aiming strong temperature gradient) fiber with large mode field diameter fiber. The experimental set up is shown in Fig.1. In conclusion, we have presented short (1.2m), tapered, birefringent, ytterbium doped, end pumped double clad fiber MOPA system with 40μm mode field diameter for amplification of single-frequency long pulses (100ns) for coherent LIDAR application. We have reached record 0.522mJ pulse energy and demonstrated highest to date 5kW peak power by using anisotropic active tapered double clad fiber as a gain media and exciting both slow and fast polarization waves.
关键词： single-frequency,tapered fiber,SBS suppression,fiber MOPA,coherent LIDAR
更新于2025-09-11 14:15:04