Coherent beam combine based on a single photodetector without local beam optics

In free-space optical (FSO) communications, increasing the peak power of a transmission beam is important to extend the communication range. In fiber-optic communications, transmission losses are compensated by an optical fiber amplifier in the transmission path. On the other hand, in space optical communications, fiber amplifiers cannot be used on the transmission path, so it is important to achieve high transmission power output. However, high power output from fiber amplifiers is limited by stimulated Brillouin scattering (SBS) due to fiber nonlinearity. One solution to this limitation is coherent beam combining (CBC), which spatially combines multiple laser beams in the far field. In general, to configure the CBC, the phase of each beam is detected by photodetectors and feedbacked to the transmission phase of each beam, resulting in a complex and large optical system. Therefore, a frequency dither signal is applied to each beam, and the phases of multiple beams are detected simultaneously by a single photodetector. Each beam is separated by its dither frequency. Typically, a plane-wave local beam is combined on all transmitted beams to obtain heterodyne beat signals. Therefore, to achieve higher peak power, the number of beams is increased, so the size of the optical system for emitting and combining the local beam is larger. We propose a configuration in which the local beam is combined into one of the transmitted beams and photoelectrically converted by a single photodetector with the other transmitted beams simultaneously. This can simplify the optical system by removing the optical components for emitting and combining the local beams. In this paper, we explain the proposed configuration and measurement results in detail and show their effectiveness.