TECHNOLOGY AREA(S): Air Platform

ACQUISITION PROGRAM: PMA272 Tactical Aircraft Protection Systems

OBJECTIVE: Develop an optimized fabrication process for quantum cascade lasers (QCLs), such as facet passivation and high thermal conductivity coatings, in order to mitigate the impact of the QCL�s operating lifetime due to catastrophic optical damage (COD).

DESCRIPTION: Employment of high-power QCLs for aircraft protection against shoulder-fired heat-seeking missiles is among the most critical applications for these devices. The ongoing Common Infrared Countermeasures (CIRCM) program represents the first program of record for the QCL technology. The program puts QCLs on the path toward wide acceptance in DoD applications.

CIRCM is focused on the development of, and transition to, a large throughput production of a compact and lightweight QCL-based Directed Infrared Counter Measure (DIRCM) system, capable of addressing the threats posed to rotary wing aircraft by the proliferation of the Man-portable air defense system (MANPADs). A relatively low (conservative) continuous wave optical power on the order of 1W is targeted in this program. However, a higher optical power, exceeding 5W for a single emitter, would significantly improve system characteristics.

The maximum optical power level for state-of-the-art QCLs is primarily limited by COD of the output laser facet: QCLs tend to fail at optical power densities on the order of 10MW/cm2, which roughly corresponds to total power level of 3W for narrow-ridge (10micron-wide) devices. Despite the fact that QCLs are projected to be the cornerstone of a number of next generation infrared systems for various DoD applications, COD failure mechanisms have not been studied for these devices. The lack of reliable experimental data on laser failure and the absence of a practical COD model make it impossible to properly evaluate mean time between failure (MTBF) for future infrared products comprising QCLs.

Preliminary QCL COD elevations [Ref 1] show that the two most typical failure scenarios for high-power buried-heterostructure QCLs mounted epi-down on submounts with a high thermal conductivity are: (1) Rapid degradation (on a scale of microseconds) in laser performance occurs when optical power density at the output facet significantly exceeds 10MW/cm2. Output facet inspection in this case shows a significant damage with a drop of melted material often observed near the active region area. The inspection results suggest that, similar to short-wave infrared diode lasers, the QCL damage occurs due to a thermal runaway process that results in the active region material melting, an irreversible damage to the laser. The positive feedback loop responsible for the QCL rapid degradation has never been clarified for QCLs and there is no active research being carried out to increase the COD threshold and, therefore, increase optical power level for traditional Fabry-Perot emitters. (2) For a lower power level, in the range from 5 to 10MW/cm2, hermetically packaged QCLs can reliably operate for >1,000h [Ref 2]. However, eventually they still catastrophically fail. The most likely explanation for the failure is device aging accompanied by defect diffusion to the output facet at elevated temperatures, which in turn lowers COD threshold. Again, the natures of the defects, time-scale for their development, and conditions that influence their formation have never been studied for QCLs.

Therefore, it is the goal of this topic to develop optimum fabrication processes, such as facet passivation and high thermal conductivity coatings that will mitigate the aforementioned reliability issues.

PHASE I: Design and demonstrate feasibility of a model capable of identifying the positive feedback loop responsible for the thermal runaway in QCLs, describing the rapid output facet degradation, and determining the COD threshold for typical edge emitting QCLs. The model development will require fitting to thermal and time-resolved optical experimental data. The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Develop a prototype fabrication process employing the model designed in Phase I. Perform experimental data collection to refine the model via study of immediate (microsecond scale) damage at high power level, long term degradation, and defects formation analysis. Based upon the improved device model, develop QCLs with increased COD threshold (higher power single emitters) and estimate MTBF for various operational conditions.

PHASE III DUAL USE APPLICATIONS: Develop a cost-effective process for manufacturing high-reliability QCLs that are to be transitioned and integrated into DIRCM systems for field deployment in a Navy platform.

Commercialize the technology based on the reliability evaluations from this program for law enforcement, marine navigation, commercial aviation enhanced vision, medical applications, and industrial manufacturing processing.

REFERENCES:

1. Lyakh, A., Maulini, R., Tsekoun, A., Go, R., and Patel, C.K.N. �Tapered 4.7�m quantum cascade lasers with highly strained active region composition delivering over 4.5 watts of continuous wave optical power.� Optics Express, 2012, Vol. 20, Issue 4, pp. 4382-4388. https://doi.org/10.1364/OE.20.004382

2. Miftakhutdinov, D., Bogatov, A., and Drakin, A. �Catastrophic optical degradation of the output facet of high-power single-transverse-mode diode lasers.� Quantum Electronics, Vol 40, No 7, 2010. http://iopscience.iop.org/article/10.1070/QE2010v040n07ABEH014340

3. Hu, Y., Wang, L., Zhang, J., Li, L., Liu, J., Liu, F., and Wang, Z. �Facet temperature distribution of a room temperature continuous-wave operating quantum cascade laser.� Journal of Physics D: Applied Physics, Vol 45, No 32, 15 August 2012. http://iopscience.iop.org/article/10.1088/0022-3727/45/32/325103

4. MIL-STD-810G, Department of Defense Test Method Standard for Environmental Engineering Considerations and Laboratory Tests. United States Department of Defense. 31 October 2008. http://everyspec.com/MIL-STD/MIL-STD-0800-0899/MIL-STD-810G_12306/

KEYWORDS: QCL; Wall-Plug Efficiency; Thermal Load; Scaling; MWIR; Brightness