Effective performance of laser systems intended for power delivery on a distant object requires an adaptive optics system to correct the laser beam distortions caused by atmospheric perturbations along the propagation path. The turbulence-induced effects are responsible for beam wobbling, wandering, and intensity scintillation, resulting in degradation of the beam quality and decline of the power density on the target. Adaptive optics methods are used to compensate these negative effects. In its turn, operation of the Adaptive Optics System (AOS) requires a reference wave that can be generated by the beacon on the target. This report discusses the requirements to the beacon that can support optimal correction of the wavefront. Post-processing of the beacon-generated light field enables retrieval and detailed characterization of the turbulence-perturbed wavefront — data essential to control the adaptive optics module of a high-power laser system.
High-average power, ultra-broadband, mid-IR radiation can be generated in a nonlinear medium by illuminating it with a multi-line laser radiation. Propagation of a multi-line CO2 laser beam in a nonlinear medium, e.g. gallium arsenide or chalcogenide, will generate directed, broadband, IR radiation in the atmospheric window (2-13 μm). A 3-D laser code for propagation in a nonlinear medium has been developed to incorporate extreme spectral broadening resulting from the beating of several wavelengths. The code has the capability to treat coupled forward and backward propagating waves. In addition, we include transverse and full linear dispersion effects. Methods for enhancing the spectral broadening are proposed and analyzed; in particular, grading the refractive index radially will tend to guide the CO2 radiation and extend the interaction distance, allowing for enhanced spectral broadening. Finally, we show that the laser phase noise associated with the finite CO2 linewidths can significantly enhance the spectral broadening. In a dispersive medium laser phase noise results in laser intensity fluctuations. These intensity fluctuations result in spectral broadening due to the self-phase modulation mechanism.
Ultra-short pulse lasers are dominated by solid-state technology, which typically operates in the near-infrared. Efforts to extend this technology to longer wavelengths are meeting with some success, but the trend remains that longer wavelengths correlate with greatly reduced power. The carbon dioxide (CO2) laser is capable of delivering high energy, 10 micron wavelength pulses, but the gain structure makes operating in the ultra-short pulse regime difficult. The Naval Research Laboratory and Air Force Research Laboratory are developing a novel CO2 laser designed to deliver ~1 Joule, ~1 picosecond pulses, from a compact gain volume (~2x2x80 cm). The design is based on injection seeding an unstable resonator, in order to achieve high energy extraction efficiency, and to take advantage of power broadening. The unstable resonator is seeded by a solid state front end, pumped by a custom built titanium sapphire laser matched to the CO2 laser bandwidth. In order to access a broader range of mid infrared wavelengths using CO2 lasers, one must consider nonlinear frequency multiplication, which is non-trivial due to the bandwidth of the 10 micron radiation.
This paper discusses a novel type of beam director for effective laser beacon formation in deep turbulence conditions. The concept of the proposed beam director is based on an innovative approach employing a Brillouin enhanced four-wave mixing (BEFWM) mechanism for generating a tight (small spot size) laser beacon on a remote image-resolved target. The BEFWM technique enables both amplification and total (phase and amplitude) conjugation of the beacon-forming beam without the need for wavefront sensors, deformable mirrors or predictive feedback algorithms. Total conjugation is critical for beam control in the presence of strong turbulence, whereas conventional adaptive optics methods do not have this capability. The phase information retrieved from the beacon beam can be used in conjunction with an AO system to propagate laser beams in deep turbulence.
A remote atmospheric breakdown (RAB) is a very rich source of ultraviolet (UV) and broadband visible light that could provide the early warning to the presence of CW/BW agents through spectroscopic detection, identification and quantification at extended standoff distances. A low-intensity negatively chirped laser pulse propagating in air compresses in time due to linear group velocity dispersion and focuses transversely due to non-linear effects resulting in rapid laser intensity increase and ionization near the focal region that can be located kilometers away from the laser system. Proof of principle laboratory experiments are being performed at the Naval Research Laboratory on the generation of RAB and the spectroscopic detection of mock BW agents. We have demonstrated pulse compression and focusing up to 105 meters in the laboratory using femtosecond pulses generated by a high power Ti:Sapphire laser. We observed nonlinear modifications to the temporal frequency chirp of the laser pulse and their effects on the laser compression and the positions of the final focus. We have generated third harmonics at 267 nm and white light in air from the compressed pulse. We have observed fluorescence emission from albumin aerosols as they were illuminated by the compressed femtosecond laser pulse.
Conference Committee Involvement (3)
Ultrafast Bandgap Photonics III
16 April 2018 | Orlando, Florida, United States
Ultrafast Bandgap Photonics II
10 April 2017 | Anaheim, California, United States
Ultrafast Bandgap Photonics
18 April 2016 | Baltimore, Maryland, United States