Description
Objective:The objective of the activity is to develop scintillation mitigation techniques based on femtosecond pulses for optical feeder uplinks. Their performance will be experimentally verified using the fine acquisition sensor on Alphasat's laser communication terminal.Targeted Improvements:25dB link budget improvement under strong turbulence conditions.Description:The adoption of optical feeder links will largely depend on their ability to support in practice large enough bandwidths that allow to reduce and simplify the ground segment. To achieve large bandwidths, optical beams need to overcome significant distortion due to atmospheric turbulence, the same phenomenon that makes stars twinkle at night. This is particularly critical for the uplink where the light transmitted by a ground station is first distorted by the atmosphere and then spread over a significant distance before it reaches the satellite. As a result, the received signal suffers strong intensity fluctuations (fades and surges) that seriously limit the achievable data rate. This is in contrast to the down to Earth link where the atmosphere is crossed at the end of the path leaving no significant distance to spread the beam after distortion. Atmospheric turbulence in the uplink may be mitigated using adaptive optics. Theyconsist of constantly measuring on ground the distortions that will cause intensity fluctuations and pre-compensating for them on the feeder uplink beam. Although technically possible, this technique is costly and its operational feasibility is not yet fully understood. Hence this activity will explore a different approach. Rather than relying on pre-compensation, the idea is to minimise the distortion by minimising the volume in which the signal interacts with the atmosphere. This is achieved using femtosecond pulses (100fs) whose short coherence length (30um) reduces the interaction volume to a 'thin disc' instead of the 'cylinder/thick disc' experienced by wider pulses. Published theoretical papers and experimental results indicate a strong gain in the uplink budget in the order of 25dB under strong turbulence conditions. This activity will develop scintillation mitigation techniques based on femtosecond pulses for optical feeder uplinks. To experimentally verify the techniques, a femtosecond laser will be installed in an ESA's optical ground station and a beacon will be transmitted to Alphasat's (GEO) tracking receiver. The dependency of the link budget gain withrespect to the pulse duration will be determined by varying the pulse duration from femtoseconds to nanoseconds and recording the received signal intensity on the satellite.