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Architecture for Diode High Energy Laser Systems (ADHELS)
Program Manager: Dr. Joseph Mangano
Optically-pumped solid-state HEL systems use arrays of diode lasers to pump the final laser gain medium. Existing DARPA programs, like SHEDS, aim to increase the individual wall plug efficiency of each of these diodes thus reducing the waste heat that must be removed from the lasing material and leading to improved overall HEL system efficiency. Nevertheless, even a SHEDS-based HEL architecture suffers from the low optical-to-optical conversion efficiency (i.e. < 30%) inherent in optical pumping.
DARPA's Architecture for Diode High Energy Laser Systems (ADHELS) is a new program dedicated to investigating alternate, direct architectures, i.e. electric-to-optical conversion, to produce a new generation of compact high energy, high efficiency laser systems. The program aims to develop a technology to produce high power, high brightness laser beams in an architecture that is readily scalable to power levels of militarily interest by combining the beams of lower power, high efficiency laser diodes/arrays/stacks.
ADHELS addresses diode efficiency and diode beam combining efficiency with the added challenge of maintaining the high beam quality and low beam divergence expected for tactical applications. Specifically, the program seeks to develop diodes that are efficient (> 60%) and to produce high-quality beams with low divergence (~ 1.2x DL). Together with an efficient technique (> 80%) for beam combining, the end-of-program goal is a 10 kW output beam demonstration at laser wavelength(s) corresponding to efficient atmospheric transmission. Such laser system architectures may lead to 100-kilowatt-class laser beam power with near-diffraction-limited beam propagation characteristics suitable for tactical applications.
Key parameters of the program are (1) total optical output power, (2) diode efficiency, (3) beam combining efficiency and (4) beam quality. The diode efficiency, ηD, is defined as the total optical output power from all the diodes divided by the electrical power used to drive the diodes. The beam combining efficiency, ηBC,is defined as the combined beam optical power radiated from the effective near-field exit aperture of the laser system divided by the total optical output power from all the diodes. The beam quality of the beam is characterized by a Beam Propagation Factor (BPF) which is defined as the laser optical output power in a specified far-field bucket divided by the total optical output power radiating from the effective near-field exit aperture of the combined laser beam. For ADHELS, the far field bucket is defined as 1.44 times the diffraction-limited spot area, A<sub>DL</sub> = (π/4)(ƟDL f) 2, where ƟDL = 2.44 λ/D, λ is the laser center wavelength, f is the focal length of the optical system used to form the far field spot, and D is the effective exit aperture of the combined laser beam.
Since the success of the program lies in maximizing the above metrics simultaneously, an additional combined metric, the Laser Performance Metric (LPM), defined as the product of the diode efficiency, the beam combining efficiency and the BPF,LPM Ξ ηD ηBC BPF, has been developed to characterize the performance of these beam-combined laser diode systems.
ADHELS is structured as a 36-month, two phase program to develop a diode-laser-based approach for a HEL system. Phase 1 will result in the demonstration of high efficiency beam combining of 500 W of laser power. This combined beam has a BPF requirement of > 0.25 (at full power) and a combined LPM of ≥ 0.10. Physically speaking, this means that 25% of the output power must be deposited in the DARPA-defined far field bucket, Abucket = 1.44 ADL. Additionally, diode lifetimes, defined as the time for the diode power level to fall below 80% of the initial diode power level in continuous operation, of at least 200 hours must also be demonstrated.
The minimum performance objectives are summarized in Figure 1.