[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) - Speckle Noise Reduction by Fiber Scrambling for Improving the Measurement Precision of an Airborne Wind Lidar System

摘要： Optical guidance of coherent light in multimode fibers involves the generation of a speckle pattern at the fiber output due to the interference of the modes traveling in the fiber. Since the speckle pattern is sensitive to external perturbations (temperature, stress) as well as polarization and wavelength changes of the guided light, speckle noise is introduced limiting the quality and precision of fiber-coupled devices employed in various spectroscopic and imaging applications such as optical coherence tomography or differential absorption lidar. Speckle noise was also identified as one of the major contributors to the random error of an airborne direct-detection Doppler wind lidar which is a key instrument for the validation of the first spaceborne wind lidar Aeolus providing vertical wind profiles of the Earth. Significant reduction of the internal reference response fluctuations has been accomplished by integrating a fiber scrambler in front of the multimode fiber. The mode scrambler (GiGa Concept Inc.) is based on two counter-rotating step motors which squeeze a portion of a fiber at a time constant of a few milliseconds. Compressional and torsional stress of the fiber gives rise to local and stress-induced birefringence during these motions. This leads to a decorrelation of the modes supported by the fiber, thus homogenizing the intensity distribution at the fiber output and diminishing speckle noise. As a result, the response variations of the internal reference signal are lowered to 1.1 MHz (Mie) and 4.2 MHz (Rayleigh), which corresponds to a reduction in random wind error by a factor of about five (0.2 m/s) and two (0.8 m/s), respectively. In addition, the intensity fluctuations are decreased by 55% (Mie) and 22% (Rayleigh). Smoothing of the speckle pattern hence has a large impact on the measurement precision of the wind lidar and demonstrates the potential of fiber scramblers for improving the performance of fiber-coupled spectroscopic and imaging instruments suffering from speckle noise in general.