TECHNOLOGY AREA(S): Battlespace, Electronics, Sensors

ACQUISITION PROGRAM: PEO IWS 2.0, Above Water Sensors Program Office.

OBJECTIVE: Develop and demonstrate an array of quantum cascade lasers with integral (chip-level) wavelength beam combining.

DESCRIPTION: Many threats to surface ships employ infrared (IR) imagers and detectors. These include lethal threats such as anti-ship cruise missiles as well as aircraft and unmanned aerial systems performing routine surveillance. In all cases, shipboard countermeasures are needed and lasers are a fundamental component of any electro-optic/infrared (EO/IR) countermeasure suite. For compactness and simplified power and control circuitry, semiconductor lasers are a highly attractive solution. However, in order to achieve the output powers required, multiple individual laser diodes must be combined in a laser �module� with a single output. This represents a considerable cost in manufacturing as the exacting tolerances required result in high component costs and labor-intensive assembly processes. The assembly cost of the laser diode combiner accounts for as much as half the cost of the finished laser module alone.

The quantum cascade laser (QCL) has demonstrated attractive qualities that make it particularly well suited to wavelength beam combining (WBC). Wavelength beam combined QCL designs have been demonstrated as feasible in achieving acceptable output power [Ref 2, 3, 4], although the resulting laser modules are expensive. This cost can only be reduced through implementation of automated assembly processes and through higher levels of integration at the component level. Since the QCL is a solid-state device produced by the accustomed semiconductor fabrication processes, it seems logical that higher levels of integration can be applied to reduce cost, consistent with common experience across the electronics industry.

The Navy requires a technology that lowers the cost of laser modules by combining multiple QCLs and integrating the wavelength beam combining structure on the semiconductor chip (on-chip). The wavelengths of interest lie primarily in the mid-wave infrared (MWIR) wave band (3.7-4.8 �m specifically). However, the long-wave infrared (LWIR) wave band (7.8-11.5 �m) is also of interest and the technology may be demonstrated in whichever wave band is deemed the easiest to demonstrate the proposed technology. While on-chip coherent laser combining in the near-IR has been demonstrated, coherent combining is not acceptable for this effort. However, the quality of the combined output beam is highly important. It is desired that the beam exhibit nearly diffraction limited operation with M2 factor, as defined by ISO Standard 11146, of 2.0 being the minimum and M2 factor of 1.5 or less being the goal.

The overall loss in the combining structure is of great importance, as it is the demonstration of efficient on-chip combining that is the goal of this effort. For this purpose, a single QCL output power of 500 mW (minimum at room temperature) with device efficiency of 8% is considered achievable in the MWIR band. In the LWIR band, a single QCL power of 200 mW and efficiency of 5% is considered reasonable. Therefore, the combining structure should be realized in a semiconductor family suited to QCL fabrication (such as InGaAs/InAlAs quantum wells on an indium phosphide (InP) substrate or an InP-based epilayer integrated with silicon) and the individual QCLs in the device must ultimately achieve these power levels. The power handling capability of the combining structure must therefore anticipate the combination of power from an integrated array of such QCLs.

For this effort, demonstration of on-chip combining of a five QCL array is considered the minimum goal and the ability to combine up to 20 devices is highly desirable. The combining efficiency (combined optical power out divided by the sum of the power produced by the QCLs) [Ref 2, 3, 4] should be made as high as possible with a goal of 80%. Furthermore, even though coherent combining is not wanted, each individual QCL operating wavelength must be fixed and repeatable (from device to device) within the operating band. For wavelength beam combining, each QCL in the array must operate at a different wavelength that is determined by the optical path. Therefore, the combining structure (or some other structure integrated on the chip) must enable the wavelengths of the individual QCLs to be selected during design.

PHASE I: Propose a concept for an on-chip wavelength beam combined QCL array meeting the objectives and performance parameters detailed in the Description. Demonstrate feasibility by a combination of analysis, modelling, and simulation. Include in the feasibility analysis predictions of combining efficiency and output beam quality as a function of the number of individual QCLs in the array. Develop a Phase II plan. The Phase I Option, if exercised, will include the initial design specifications and capabilities description to build a prototype solution in Phase II.

PHASE II: Based on the results of Phase I and the Phase II Statement of Work (SOW), demonstrate the concept for an on-chip wavelength beam combined QCL array by production of prototype devices that meet the requirements defined in the description and are generic devices not intended for any specific system application. This is expected to be an iterative process, likely resulting in the fabrication and testing of multiple prototypes. At the conclusion of Phase II, deliver a minimum of three sample prototype devices to the Government for characterization and evaluation.

PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the technology for Government use. Since the design and prototypes resulting from Phase II are generic, the company will assist in applying the design for specific system applications such as countermeasures. This is expected to entail selection of device dimensions and adjustment of corresponding process parameters in order to produce on-chip combined QCL arrays at specific wavelengths (within the chosen IR band) that combine to produce output power determined by the number of individual QCLs integrated in the array. The final product will therefore be a related family of devices, each device being a highly integrated QCL array with a single high-quality wavelength combined output beam suitable for application in multiple DoD systems including airborne and shipboard IR countermeasures.

In non-military applications, QCLs are mainly used in scientific instruments, especially for laser spectroscopy. These devices typically contain a single QCL diode and yield low power as the cost of combing multiple QCLs is prohibitive. This technology would provide higher-power laser sources to the scientific community at reasonable cost.

REFERENCES:

1. Zhao, Yunsong, and Zhu, Lin. "On-chip coherent combining of angled-grating broad-area diode lasers.� 2012 Conference on Lasers and Electro-Optics (CLEO), May 2012. https://www.osapublishing.org/oe/abstract.cfm?uri=oe-20-6-6375

2. Razeghi, Manijeh, et al. "Recent progress of quantum cascade laser research from 3 to 12 �m at the Center for Quantum Devices.� Applied Optics 56, 1 November 2017: H30-H44. https://www.osapublishing.org/ao/abstract.cfm?uri=ao-56-31-H30

3. Vitiello, Miriam Serena, et al. "Quantum cascade lasers: 20 years of challenges.� Optics Express 23, 20 February 2015: 5167-5182. https://www.osapublishing.org/oe/abstract.cfm?uri=oe-23-4-5167

4. Razeghi, Manijeh, et al. "Recent advances in mid infrared (3-5�m) Quantum Cascade Lasers.� Optical Materials Express 3, 10 October 2013: 1872-1884. https://www.osapublishing.org/ome/abstract.cfm?uri=ome-3-11-1872

KEYWORDS: Quantum Cascade Laser; Shipboard Countermeasures; Mid-Wave Infrared; Wavelength Beam Combining; Laser Modules; On-Chip Combining