Collective synchronization and phase locking of fs fiber amplifiers: Requirements and potential solutions
Active coherent beam combination (CBC) of femtosecond pulses requires knowledge of the absolute phase between all the optical pulses: pulse synchronization and phase locking are both mandatory and must be measured and controlled. To assess controller requirements, we study fiber amplifier phase noise for various packaging and power levels. The main contributors are thermal noise (with large amplitude phase noise and frequencies below 5 Hz) and vibrations (with low amplitude phase noise and frequencies from 5 Hz to 2 kHz), leading to a woofer/tweeter pair of actuator. The amplifier packaging is of major importance: a simple PID model shows that in order to achieve a residual phase error of λ/100, the required controller bandwidth ranges from 0.5 Hz to 1 kHz, mostly depending on amplifier packaging. We also shortly review common phase locking techniques either based on pupil-plane sensor or focal-plane sensor. Common focal-plane techniques as SPGD or LOCSET are not directly compatible with pulsed femtosecond operation. Pupil-plane fringe-sensor techniques are more promising and route towards collective synchronization for femtosecond regime are proposed. Following the example of telescope interferometry, the final system might include both a pupil-plane, fringe-sensor and a focal-plane phase-diversity sensor, with pupil-plane sensor for synchronization and phase lock, and focal-plane sensor to control and maintain required focalized beam quality.
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