We demonstrate coherent beam combining of two femtosecond fiber chirped-pulse amplifiers seeded by a common oscillator. Using a feedback loop based on an electro-optic phase modulator, an average power of 7.2 W before compression is obtained with a combining efficiency of 90%. The spatial and temporal qualities of the oscillator are retained, with a recombined pulse width of 325 fs. This experiment opens up a way to scale the peak/average power of ultrafast fiber sources.

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Coherent Lidars are able to finely measure the vibration velocity of remote targets. This allows Operative Modal Analysis (OMA) of potentially damaged buildings, for their diagnosis at a safe distance after a seismic event. As a next step from our previous work validating this method for modal frequency determination, we have assessed its capability to extract the full modal parameters of RC buildings, including mode shapes, using multiple ambient vibrations measurements by Lidar on the entire structure. We report on the development and field trial of a 3-path Lidar vibrometer for this purpose. After a description of the system, we show that application-related constraints are fulfilled: low velocity noise, real-time signal processing, compact and laser safety. Then, we present the results of a real-scale trial on 3 buildings in Grenoble, France. We discuss the reliability of this technique for remote structural diagnosis with a comparison of modal parameters, as measured by Lidar at 200m range and in situ velocimeters.

We report what we believe to be the first experimental demonstration of coherent beam combining of two fiber amplifiers in a 100 ns pulse regime using a signal leak between the pulses. Pulses of ∼100 W stimulated-Brillouin-scattering limited peak power are combined with 95% efficiency, a residual phase error of λ/27, and no significant beam quality degradation.