Coherent signal combination based on a highly stable fiber transmission system

We present an ultra-stable wideband receiving system for distributed coherent signal combination. A 500 Mbps binary phase shift keying signal with a carrier frequency of 10 GHz is transmitted from two remote ends to the local end via two 20 km fiber links for combination. The transmission delay variation of the optical links has been detected and precisely compensated for by adopting a 27.4 GHz probe signal. Thanks to the highly stable transmission link with a stability better than 20 fs, the two signals have a relative delay variation of merely 1.95 ps. The signal-to-noise ratio of the combined signal is increased by 2.9 dB which is close to the theoretical value of 3 dB. The proposed wideband signal receiving and combination system can be widely used in distributed coherent radars and multi-antenna interferometric detection.