N142-093 TITLE: High-Power Mid-Infrared Quantum Cascade Laser Array with Continuous-Wave Output Power Exceeding 100W

TECHNOLOGY AREAS: Air Platform, Chemical/Bio Defense, Electronics

ACQUISITION PROGRAM: PMA 272

OBJECTIVE: Develop a quantum cascade laser (QCL) platform with emission at MidWave IR Wavelengths based on a stack of actively�cooled QCL bars. The output power of the architecture of the laser platform should be scalable to hundreds of Watts in continuous-wave (CW) mode while maintaining excellent beam quality for directional infrared countermeasure (DIRCM) applications.

DESCRIPTION: Compact, high-power, reliable mid-wave infrared (MWIR) scalable laser platform operating in CW regime are highly desirable and critical for current and future Navy applications. Individual QCLs with several Watts CW output power have been demonstrated throughout most of the MWIR wavelength range [1, 2]. To increase the aggregate output power level of the laser sources, one can combine the multiple laser beams using either coherent beam or spectral beam combining technique. However, the beam-combined output power scales with the output power of each of the individual QCLs or QCL array within a beam combing scheme.

Therefore, to scale the aggregate beam-combined output power level of the laser sources up to the range of a few hundred of Watts to meet the future needs of DIRCM, an innovative QCL array source at MWIR Wavelength with power exceeding 100 Watts at room temperature (RT) before beam combining is desired. One potential approach to achieve the goal is to fabricate and assemble multiple QCL laser bars in a stack. A similar approach has been successfully implemented with near-infrared (NIR) diode bars and 2-dimensional stacks with raw CW output reaching 1 kiloWatt has been demonstrated [3]. However, adapting this technology from NIR diode laser array for QCL array poses many difficult challenges. For example, QCLs have higher thermal impedance, require significantly higher electrical drive voltage than NIR diode lasers, and the wall-plug efficiency of QCLs is at most 20% compared to ~60% for NIR diode lasers at RT operating at CW mode. The efficient removal of heat for QCL bar is almost six times the amount of typical NIR laser bar stack. Therefore this project is seeking the improvement of the thermal impedance of the QCLs for much better thermal conduction, and also novel actively cooling techniques that are even better than the state-of-the-art techniques such as micro-channel or micro-impingement cooling.

The need for heat removal of such a large amount exacerbates mechanical stress of the QCL stack assembly. This could lead to catastrophic failure of the QCL bar if not carefully managed and requires, for example, the selection of coefficient of thermal expansion-matched materials for the heatsinks, submounts and soldering materials. These problems are further multiplied in the case of actively cooled two-dimensional QCL stack assembly.

The goal for this program is to develop an innovative two-dimensional QCL bar stack assembly of which the RT output power can be readily scaled up to hundreds of Watts operating at CW mode. The individual emitters must have a stable output beam profile with M2< 1.5 in both fast- and slow-axis directions. The proposed approach needs to address issues such as heat extraction and mechanical stress at the chip as well as at the subsystem level. The overall reliability of the laser source needs also to be studied. Approaches that include, for example, ways to address electrically each individual emitter and/or ways to beam-combine the output of the array elements with superb combined beam quality are highly desirable and should be discussed in the proposal.

PHASE I: Design a CW QCL platform based on two-dimensional bar stack assembly capable of exceeding 100 Watts while maintaining excellent beam profile with M2< 1.5 in both directions and stable for each individual array element. The output power and wall-plug efficiency must be modeled as function of laser elements. The approach proposed must be compatible with scaling the power up to 1000 Watts. The heat extraction, mechanical stress and the reliability at level of individual emitters, bars and the entire stack as a whole must be evaluated and mitigation solutions should be presented.

PHASE II: Fabricate, demonstrate and deliver a prototype based on the design of Phase I that produces CW output power more than 100 Watts while maintaining excellent beam profile (M2< 1.5 in both directions) and stable for each individual array element.

PHASE III: Perform extensive laser module reliability and durability testing. Develop a cost-effective manufacturing process for technology transition to system integration for field deployment in a Navy platform.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The DoD undoubtedly has a need for advanced, compact, robust and low-cost MWIR lasers for applications such as current- and future-generation DIRCM, LIDAR and chemicals and explosives sensing. The commercial sector also can significantly benefit from this crucial, game-changing technology development in the areas of detection of toxic industrial gases, environmental monitoring, and non-invasive medical health monitoring and sensing.

REFERENCES:
1. Bandyopadhyay, N. et al., 2010, Applied Physics Letters, Volume 97, 131117.

2. Lyakh, A. et al., 2012, Optics Express, Volume 20, 24272.

3. See for example: http://www.nlight.net/products/diode-laser-stacks/.

4. Bai, Y. et al., 2011, Applied Physics Letters, Volume 98, 181102.

KEYWORDS: Laser Array; Qcl; micro-channel cooling; mid-infrared; lasers; aircraft protection

** TOPIC AUTHOR (TPOC) **

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