Toward a high concentration Yb-Er phosphate glass optical amplifier for eye-safe compact LIDAR

LIDAR systems offer a powerful remote sensing technique that has been successfully employed for several applications. The key component of a LIDAR system is the laser source whose main parameters contributes to overall system performance. An advantageous approach to realize a high power LIDAR source is the MOPA configuration, in which a master oscillator produces a highly coherent beam and an optical amplifier is used to boost the beam output power while preserving its main spectral properties. The NATO SPS project “CALIBER” (CompAct eye safe Lidar source for AirBorne lasER scanning) aims to develop a compact, lightweight and low cost version of a LIDAR source that can be placed on small UAVs or in specific locations of premises where a small footprint equipment is required. Following the requests of a high degree of compactness while maintaining high performance and low cost, the choice for the optical amplifier fell on an Yb/Er co doped phosphate glass based waveguide. Phosphates are recognized to be an ideal host material for engineering the amplification stage of a pulsed MOPA thanks to their ability to maximize energy extraction and minimize the nonlinearities. They enable extremely high doping levels of rare--earth ions to be incorporated in the glass matrix without clustering, thus allowing the fabrication of compact active devices with high gain per unit length (> 5 dB/cm). In this work we report on the design and fabrication of a series of Yb/Er--doped phosphate glasses to be used as active materials for the core of a fiber amplifier. The fabricated glasses were thoroughly characterized and the best composition selected for the fabrication of the first amplifier prototype. Suitable cladding compositions were explored and the final core/cladding glass pair was used to realize a multi--mode optical fiber. Preliminary results of optical amplification are presented using a CW source as seed laser.