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High-Power Tapered Amplifier Laser Diode Array With Active Phase Control Feedback Loop for Future High Energy Laser Weapons



OBJECTIVE: Develop a kilowatt class near infrared direct diode laser array based on tapered amplifier technology with emitter-level phase control for extracavity coherent beam combination.

DESCRIPTION: High-power, high brightness direct diode lasers exhibiting single mode performance and displaying high beam quality are needed to scale the output power of SWAP-constraint mobile high energy laser weapons by surpassing the fundamental efficiency limits of high energy fiber laser systems. Commonly the output power of diode lasers can be increased by increasing the semiconductor active area. However, a direct consequence is a degradation in the emitted beam quality, which severely limits laser beam propagation. Therefore, broad area diode lasers are not practical solutions for high energy laser weapons. Instead, solutions that can emit at a high power while maintaining excellent beam quality are needed.At the single emitter level, tapered amplifiers have achieved some of the highest output powers (Watts) in the field of diode lasers at brightness levels of 1000 MW/cm(2)/sr, while maintaining near single mode beam characteristics and diffraction limited beam quality in the fast axis [1,2]. Efforts to improve the beam quality of the slow axis have achieved near diffraction limited slow axis performance, and external micro-optics can match the divergence angles and address astigmatism between the two axes. A tapered amplifier is driven by a high quality seed laser, which may be integrated into each amplifier element, or a single seed may supply multiple tapered amplifiers in an active master oscillator power amplifier (MOPA) architecture [3].Realizing an array of tapered amplifiers to produce a high power laser (kW-class power, GW/cm(2)/sr-class brightness) poses two overarching challenges. First, the solution to construct a dense array, e.g. monolithically, needs to address thermal management, wall plug efficiency, seed geometry, and irregularities such as “smile” in the fabrication stages. Tradeoffs between array fill factor, external array symmetrisation and optics, as well as thermal management need to be considered in the context of SWAP and efficiency.Secondly, a method to actively control the phase relationship between N elements, which is necessary to achieve predictable coherent beam combining (CBC) results at high combining efficiencies, needs to be developed. Indeed, the added complexity of phase control at each element may in turn decrease the complexity of the optical path from laser to telescope: beam control methods such as the use of adaptive optics to address atmospheric effects, fast steering mirrors, and associated mechanical and electronic control loops may be addressed by the laser itself through its ability to control the phase at each element at kilohertz speed.

PHASE I: Design the architecture and fabrication technique to construct a kW-class (>=1kW) laser array based on tapered amplifiers. The design must include predictable wall-plug efficiency, beam quality characteristics, fabrication cost, tolerances and areas of risk. Furthermore, the design must be complete with a detailed description of the proposed CBC architecture including all hardware components from seed laser to phase detectors, packaging, as well as phase modulation and feedback algorithms to produce predictable and highly stable phase relationships between all elements of the diode array. The design must include a hardware solution for physically combining N elements into a single, coherent, high beam quality, high power laser beam.

PHASE II: Construct a kW-class (>=1kW) laser array based on the design delivered during Phase 1. To address uncertainty in cost for realizing such an array, at a minimum, a 500W array has to be delivered if the design clearly shows that scalability to over 1kW per array is straightforward, and does not pose additional technical challenges. The array shall be delivered with external beam combining hardware to demonstrate the ability to coherently combine all elements. For demonstrational purposes, the maximum level of constructive, and destructive interference in unites of output power and coherence length shall be reported on to provide quantitative results for combining efficiency. Furthermore, the beam quality of the combined beam shall be characterized. Quantitative efficiency values related to the individual amplifiers, diode array, and CBC method shall be clearly identified.

PHASE III: Optical metrology, which is an emerging field as a non-contact measurement method for military and industry will benefit from a high power tapered array with phase control, which can open the door for electronic beam steering of lasers. Tapered amplifiers are needed for nonlinear research through second harmonic generation as well as for pumping of Raman or other solid state amplifiers, requiring high power and high brightness. Furthermore, a CBC tapered amplifier array will allow for amplitude modulation of the high power laser beam, which is very difficult with current state of the art high energy lasers.

KEYWORDS: Direct Diode HEL, tapered amplifiers, high-brightness diode lasers, phased locked lasers


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