Silicon Photonic Integrated, Compressive, High Bandwidth, Low-Latency Wavefront Sensing Using Modal Decomposition

The Air Force faces a critical challenge in developing airborne high energy laser weapon systems that can operate effectively in tactical environments. The turbulent airflow around an aircraft in flight creates a complex, dynamic density field that induces distortions in the optical wavefront of a transmitted laser beam. Development of a new wavefront sensing technology that can operate at bandwidths of 250 kHz or greater would provide a breakthrough capability for airborne directed energy systems. We propose to use coherent detection to measure the wavefront by beating the return wavefront with a local oscillator. This technique has been matured significantly due to commercialization of digital coherent optical communication. Coherent detection has to potential to reach shot-noise limited sensitivity. Laser linewidth requirement relaxes as the speed of wavefront sensing using coherent detection increases. The faster the wavefront sensing speed, the broader the laser linewidth can be tolerated. This is especially important for directed-energy applications in which the laser source may have relative broad linewidth. This technique will be directly applied with spatial sampling. We further propose to use modal decomposition to achieve compressive wavefront sensing. By decomposing wavefront into its (spatial) modal contents, wavefront which typically require 1000s pixels can be represented by complex amplitudes of <100 modes. Compressive sensing further facilitates high-speed wavefront sensing. We also propose to use time-division multiplexing to simplify the compressive wavefront sensor. Wavefront sensing using coherent detection (either spatial sampling or compressive modal decomposition) can be implemented using silicon photonic integration circuits (PICs) and electronics that have been developed for the commercial optical communications industry. We already have a silicon PICs for wavefront sensing, having 16 sampling point. This PIC will be used to experimental demonstrate high-speed wavefront sensing at >250 kHz rate in Phase I The size, weight and power consumption of the proposed integrated wavefront sensor will be much lower than the requirements in the BAA.