Ytterbium Hybrid Fiber/Bulk 343-nm Laser System for High Altitude Wind Lidar Measurements

Gust load alleviation (GLA) techniques involve adapting wing shapes based on forward wind measurements, crucial for upcoming elongated-wing aircraft designs aimed at reducing greenhouse gas emissions [1]. High-altitude wind lidar measurements rely on UV pulses, analyzing the Doppler frequency shift of Rayleigh signals backscattered by molecules. To achieve this, our project focuses on developing a 343 nm laser (Figure 1.a) delivering 10 W average power in pulsed mode (25 ns, 40 kHz) [2]. Given airborne constraints, a fiber-based Master Oscillator Power Amplifier (MOP A) architecture is preferred in order to limit the use of free space components. However, at these pulse durations (exceeding phonon lifetimes), peak power in fibers is limited by stimulated Brillouin scattering (SBS). To overcome this SBS limitation, our approach employs several strategies [3], including the use of large mode area (LMA) fibers, limited fiber length and broadened laser spectrum. Unfortunately, efficient spectral broadening techniques like white noise or pseudo-random binary sequence (PRBS) phase modulation are incompatible with Doppler lidar measurements. As an alternative, we use sinusoidal phase modulation generating a spectral comb that matches the free spectral range (FSR) of a Quadri-Mach-Zehnder Interferometer (QMZI), the chosen lidar receiver [2]. Then, our frequency comb system allows to push forward the SBS limitation while maintaining compatibility with lidar measurement.