The feasibility of an optical phased array antenna applicable for spaceborne laser communications was experimentally demonstrated. Heterodyne optical phase-locked loops provide for a defined phase relationship between the collimated output beams of three single-mode fibers. In the far field the beams interfere with a measured efficiency of 99%. The main lobe of the interference pattern can be moved by phase shifting the subaperture output beams. The setup permitted agile beam steering within an angular range of 1 mr and a response time of 0.7 ms. We propose an operational optical phased array antenna fed by seven lasers, featuring high transmit power and redundance.

No abstract

This paper focuses on optical links from ground to a geostationary satellite, using adaptive optics to pre-compensate the wave-front of the uplink beam. We present the numerical prediction of the irradiance statistics at the satellite. Uplink beam diameters, as well as the turbulence strength in the boundary layer and in the tropopause are varied. Results show, that the choice of uplink beam diameter significantly affects the fading statistics, in particular the shape of the probability density function. In a realistic worst case (30° elevation, day-time turbulence, sea level), the optimum beam diameter, showing minimum irradiance fluctuation at the target, is around 20 cm. In that case, the probability of fades larger than 6 dB (with respect to the diffraction limit) is approx. 10 -3 .A breadboard of such a closed-loop adaptive-optics pre-compensation system has been implemented. It comprises a Shack-Hartmann wavefront sensor, two tip/tilt mirrors, a 140-element deformable mirror, a far-field propagation simulator, and two rotating turbulence phase screens (simulating tropopause and atmospheric boundary layer, respectively). The link geometry is representative of the intended application in an optical feeder link for geo-stationary satellites. Preliminary test results confirm the numerical predictions.

A novel type of optical phase locked loop (OPLL), optimized for homodyne inter-satellite communication, is presented. The loop employs a conventional 180? 3 dB optical hybrid and an AC-coupled balanced front end. No residual carrier transmission is required for phase locking. The loop accepts analog as well as digital data and various modulation formats. The only requirement to the transmitted user signal is a constant envelope. Phase error extraction occurs through applying a small sinusoidal local oscillator (LO) phase disturbance, while measuring its impact on the power of the baseband output signal. First experimental results indicate a receiver sensitivity of 36 photons/bit (-55.7 dBm) for a BER of 10 ;-9, when transmitting a PRBS-31 signal at a data rate of 400 Mbit/s. The system setup employs diode-pumped Nd:YAG lasers at a wavelength of 1.06 mum.