Description:
TECHNOLOGY AREA(S): Sensors, Electronics, Weapons
OBJECTIVE: To develop novel adaptive optics concepts to control and extend the atmospheric propagation of laser pulses with peak powers sufficient to access nonlinear self-focusing in air and laser bandwidths sufficient to enable significant temporal compression over kilometer-scale propagation distances.
DESCRIPTION: The overall goal of this program is to produce adaptive optics for an ultrashort pulse laser system that will enable laser filamentation at controllable ranges up to multiple kilometers in the atmosphere. Emerging laser systems will produce short pulses (~psec or less) with peak powers >GWs and average powers in the hundreds of watts. At these powers, nonlinear self-focusing in the air and within the optical train of the laser compromises the effectiveness of conventional adaptive optics (AO) for correcting phase distortions due to atmospheric turbulence. For example, conventional adaptive optics are founded on the concept of reciprocity, i.e., a remote beacon on the target can be reproduced by transmitting the conjugate field of the beacon at the receiver [1]. However, there is a breakdown in reciprocity between a low-power AO beacon on the target and the transmitted ultra-short pulse laser (USPL) that is propagating near the critical power for self-focusing in air. The critical power of air, from the near-IR to the mid-IR spectrum, scales approximately as the laser wavelength squared. It is several GW in the near-IR (0.8 to 1 microns) and up to 100 GW in the mid-IR (3-5 microns) [2]. The Navy is interested in transmitting ultra-short laser pulses long distances through the atmosphere. Similar to conventional high-power Continuous-wave (CW) laser systems, this will require adaptive optics for turbulent environments. However, adaptive optics for conventional CW laser systems will need to be modified to function with ultra-short, high-peak intensity laser pulses and related non-linear phenomena and AO update rates. Present-day USPL systems can access these peak powers at wavelengths within the near-IR and mid-IR atmospheric transmission bands. In addition, ultrashort laser pulses have large bandwidths (~ tens of THz) that can be used to control the peak power of the pulse as it propagates through the air, e.g., a chirped pulse can be made to temporally compress in air due to group velocity dispersion [3]. We seek AO concepts and algorithms that are effective for high-peak power USPL systems. In particular, we are looking for ways to control the nonlinear focal range and beam quality over long-range propagation in turbulent atmospheres with Rytov variances approaching unity, and with significant aerosol extinction. The AO concept should be able to focus the laser to attain fluences of a few Joules per cm2. Concepts should address laser systems (powers, bandwidths, and wavelengths) that are projected to be achievable within a 10-year development window.
PHASE I: The Phase I effort will define and develop the Adaptive Optics for Nonlinear Atmospheric Propagation of Laser Pulses concept and identify the required technology to implement it. Approaches should address the advantages and disadvantages of operating in various atmospheric transmission windows from the near-IR (~1-2 micron) to mid-wave “IR (~3-5 micron) to long-wave-IR (~8-12 microns), however a down-select for the wavelength will occur before prototype development based on the merits of the concept and source availability. The Phase I concept should demonstrate, through analysis and simulations, the feasibility of producing the required fluence on target through turbulent, dispersive, aerosol environments as discussed above. Cooperative targets with a pre-existing beacon may be considered, but AO concepts for non-cooperative targets are preferred. Required Phase I deliverables will include a detailed report showing how the proposed concept can meet the requirements. If possible, the report should discuss how the concept could be validated, either through field testing or scaled laboratory experiments that will be carried out in Phase II or III.
PHASE II: The Phase II effort will develop and implement the best combined hardware and software approach from the Phase I effort at the selected laser wavelength based on the merits of the concept and source availability for demonstration. Once the Adaptive Optics for Nonlinear Atmospheric Propagation of Laser Pulses prototype is developed, demonstrations to validate the proposed concept are to be performed using the available USPL source in a suitable laboratory-scale experiment or in a controlled field test. The tests should demonstrate the controlled creation of sub-diffraction-limited focal spots on a target after propagating through strong turbulence. The operation and limitations of the system will be characterized for a variety of atmospheric and turbulence conditions and the statistical properties of the laser pulse on the target will be determined.
PHASE III: Phase III will ruggedize and reduce the SWaP requirement of the prototype fabricated in Phase II for operation in a shipboard environment and for operational demonstration in a maritime environment. AO systems have presently not been deployed on Navy platforms. Astronomical systems typically consist of a deformable mirror and racks of CPUs for processing. A Naval system would require more processing power but also need to be more compact and significantly more ruggedized. GPU-based systems could provide this requirement. The Phase III deformable mirror hardware product will be tested for operability and survivability against shipboard vibrations and jitter, and maritime environmental degradation, in conjunction with a future potential USPL POR source yet to be determined. Preliminary testing of the device can be done in the laboratory, but the ultimate goal is to demonstrate operability of the AO system with a USPL, on a maritime platform in sea state, that can deliver sub-diffraction-limited laser pulses through a turbulent maritime environment characterized by a scintillation index approaching unity. Private Sector Commercial Potential: A commercialized AO system based on this prototype could facilitate development of new applications and basic research including remote sensing using laser-induced breakdown and high-intensity laser-matter interactions where beam quality can be a limiting factor.
REFERENCES:
1. Ultrashort Laser Pulse Phenomena, J-C. Diels and W. Rudolph, Academic Press, 1996.
2. Adaptive Optics for Astronomical Telescopes, John W Hardy, Oxford University Press, 1998.
3. Nonlinear Optical Model of Air Medium in the Problem of Filamentation of Femtosecond Laser Pulses of Different Wavelengths, V. Yu. Fedorov and V. P. Kandidov, Optics and Spectroscopy 105, 280 (2008).
4. Propagation of intense short laser pulses in the atmosphere, P. Sprangle, J.R. Peñano, B. Hafizi, Phys. Rev. E, 66 046418 (2002).-
KEYWORDS: Laser; Lasers; Ultra-short Pulsed Lasers; Adaptive Optics; AO; USPL; Nonlinear Propagation; Filament; Filamentation; Nonlinear Self-focusing; NLSF; Turbulence; Reciprocity
