Coherent Beam Combining Using an Internally Sensed Optical Phased Array of Frequency-Offset Phase Locked Lasers

Lyle Roberts,Robert Ward,Craig Smith,Daniel Shaddock

doi:10.3390/photonics7040118

Abstract

Coherent beam combining can be used to scale optical power and enable mechanism-free beam steering using an optical phased array. Coherently combining multiple free-running lasers in a leader-follower laser configuration is challenging due to the need to measure and stabilize large and highly dynamic phase differences between them. We present a scalable technique based on frequency-offset phase locking and digitally enhanced interferometry to clone the coherence of multiple lasers without the use of external sampling optics, which has the potential to support both coherent and spectral beam combining, and alleviates issues of voltage wrapping associated with actuating feedback control using electro-optic modulators. This technique was demonstrated experimentally using a tiled-aperture optical phased array in which the relative output phase of three free-running lasers was stabilized with an RMS output phase stability of λ/104.

Highlights

Optical phased arrays (OPAs) manipulate the distribution of optical power in the far field by controlling the phase of light emitted by multiple spatially separate transmit apertures
The first is split into multiple channel, each connected to a waveguide electro-optic modulator used to encode the phase of the light with a pseudo-random bit sequence (PRBS) for digitally enhanced heterodyne interferometry [22]
We have presented a technique for coherently combining multiple free-running follower lasers based on frequency-offset phase locking

Summary

Introduction

Optical phased arrays (OPAs) manipulate the distribution of optical power in the far field by controlling the phase of light emitted by multiple spatially separate transmit apertures. By precisely controlling the relative optical phase at the output of the array, OPAs can be used to deliver a large amount of optical power to a specific location in the far-field (e.g., for directed energy applications [1]); and rapidly steer the interfered beam within its field-of-regard without the use of moving parts (e.g., for solid-state light detection and ranging (LiDAR) [2], adaptive optics [3], free-space laser communications [4], satellite interferometry [5], and materials processing [6]). [14]) to actively stabilize the relative phase of multiple independent follower lasers to a single free-running leader laser at a specific offset frequency. We refer to this as a leader-follower laser configuration. One advantage of this architecture is the ability to arbitrarily define the offset frequencies between follower lasers, enabling it to support both coherent and spectral beam-combining

Methods

Results

Conclusion