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Femto Second Laser Adaptive Optics


TECHNOLOGY AREAS: Air Platform, Sensors


The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), which controls the export and import of defense-related material and services. Offerors must disclose any proposed use of foreign nationals, their country of origin, and what tasks each would accomplish in the statement of work in accordance with section 3.5.b.(7) of the solicitation.

OBJECTIVE: To develop a rugged Adaptive Optics (AO) system which will enable Ultra-short Pulse Lasers (USPL), femto second class, to deliver the maximum fluence on target, without ionizing the atmosphere along the beam path, at tactically significant distances under a wide variety of atmospheric turbulence conditions.

DESCRIPTION: USPL have achieved a level of reliability, energy, size, efficiency, and ease of use which have made them attractive for a wide variety of applications critical to DoD missions. In particular, several Laser Guided Energy (LGE) applications have been proposed, some of which include using laser-produced plasma channels for guiding high voltage discharges, remote sensing of chem/bio agents using supercontinuum or terahertz generation, plasma waveguides for electromagnetic energy, and generic countermeasures. In all of these applications, a common feature is the requirement to efficiently transfer the very high (typically >10^12 Watts/cm2) peak intensity levels available with the USPL over distances ranging from several tens of meters to many kilometers under a variety of atmospheric conditions. The reason for this, of course, is that the unique features of these lasers at such high intensities is their ability to induce nonlinear responses in materials, including air, which result in ionization, ultra-wideband frequency generation, and white light generation, and to do so remotely and predictably. It is precisely this aspect which demands the use of Adaptive Optics.  For this project we require the adaptive optics be able to produce the maximum fluence and ionization at a specific point in space while minimizing the ionization trail along the beam path.

The atmosphere is not the quiescent, benign medium it appears to be on a pleasant sunny day. Temperature gradients result in index of refraction cells which cause laser beams to break apart as if they were traveling through a series of lenses, and reduces the intensity on target since the beam now spreads out. Aerosols from numerous sources can also cause scattering, reducing the energy deposited on target, and thus, the intensity. Since all the processes alluded to above are nonlinear, some of which depend on the intensity raised to the eighth or ninth power, it is obvious that one cannot tolerate these kinds of losses.

AO uses low power light sources to determine the wavefront deviations near the path to be taken by the higher power laser. Through a closed loop series of algorithms, a deformable mirror (DM) compensates for these distortions such that the beam travels through the atmosphere and arrives at the target with the theoretical minimum spot size or highest achievable spatial resolution. This technique is identical to that used in astronomy to correct for the aberrations that occur in the observation of distant stars (twinkling).

Several aspects of USPL make the choice and production of AO a challenge. First, typical USPL have relatively broad bandwidths due to their short (less than 10^-12 seconds) pulse width. The deformable mirror must not introduce uncorrectable dispersion in the beam, since that would limit the available temporal width of the USPL. Second, typical peak powers in these lasers are on the order of several Terawatts. At these power levels, beams width diameters of mm size will damage any material currently used in coatings. Therefore, the size of the DM and number of actuators must be compatible with this limitation. Third, since the requirement is to produce the minimum spot size possible at a variety of distances during an engagement, the temporal response of the DM as well as the time to process the wavefront data from the guide star must be compatible with the change in the engagement range.  Of particular importance is the ability to maintain the focusing ability on rapidly moving objects.

PHASE I: Perform a trade study of existing technology and components and compare the capabilities to the requirements stated above for minimizing plasma channel and ionization except at the target point. The result of this study is to be a series of specifications and recommendations for an adaptive optics system which can be used in existing LGE and USPL systems. Demonstrations at this phase are encouraged if practical.

PHASE II: Based on the results and findings of Phase I, demonstrate the technology by fabricating and testing a prototype in a laboratory environment. Assemble a proof-of-principle device and demonstrate the proposed technology and its ability to signal an attack warning and to identify its characteristics. Identify and address technological hurdles. The proposed development and demonstration should be limited to what can be demonstrated in a Phase II program and should identify the means necessary to transition the technology.

PHASE III: This technology could be used in a broad range of military and commercial applications such as rapid remote chemical analysis. The final embodiment of this device would be a standalone hardware package and set of specifications that could be integrated into a mobile military platform.

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