Co-phasing a large array of segmented lasers via interferometry and coherent beam combination methods

The Breakthrough Starshot Initiatives aims to launch a nanocraft at 20 percent the speed of light to Proxima Centauri b. The spacecraft will be accelerated with a 1064 nm laser beam of 100 GW propagated from a launch projector of several kilometer diameter. We consider a projector architecture constructed as a dense co-phased array of 2 m telescopes transmitting the light from many thousands of laser power amplifiers slaved to a single maser oscillator (MO). The resulting system is intended to produce a diffraction-limited beam at a distance of 0.3 AU. Implementation of adaptive optics in the segmented beam propagation systems is essential to overcome atmospheric aberration and phase jitter between the lasers. We investigate the use of coherent beam combination and pairwise interferometry methods to measure and correct phase errors between the drive lasers at the projection apertures. Path length differences must be measured and corrected to below λ/20 to properly phase the sub-apertures. A small portion of light will be sampled from the output of each amplifier and coherently mixed with frequency modulated reference light from the MO, then demodulated to measure the phase error. We then feed the error back to an EOM to correct the phase jitter in closed loop. This method can detect small phase excursions below λ/30 with relatively low bandwidth requirements (∼ 10⁶ Hz). Proceeding from the beam transport optics, the light must be expanded through the launch projection apertures where additional co-phasing errors will inevitably be introduced. Piston sensing between segments will be carried out by a separate system looking at light from Proxima Centauri itself seen through coherently combined pairs of sub-apertures spanning the segment gaps. The phase of the interference fringes will be the piston error from the lower lying atmosphere and slowly varying instrumental components.