N221-041 TITLE: Compact High Power Mid-Wave Infrared Laser System

OUSD (R&E) MODERNIZATION PRIORITY: Directed Energy (DE)

TECHNOLOGY AREA(S): Sensors

Updated 12/10/21 - The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

OBJECTIVE: Develop a compact high power laser system that provides broad spectral coverage across both atmospheric transmission windows in the mid-wave infrared band.

DESCRIPTION: Infrared (IR) passive sensors, essentially IR cameras, have been widely used for intelligence, surveillance, reconnaissance, and tracking (ISRT) and are increasingly being deployed for targeting. For both purposes, the mid-wave infrared (MWIR) band is particularly attractive for conditions of poor visibility and for nighttime use. These sensors vary in sophistication but use arrays of photodetectors that are generally sensitive to IR radiation across a wide wavelength band. For high resolution ISRT applications, the detectors are large two-dimensional focal plane arrays (FPAs) with pixel counts in the millions. Driven by energetic research in the area, IR FPA format, efficiency, noise performance, and spectral sensitivity are steadily improving. Targeting sensors may not require such large format detectors but nonetheless benefit from the drive to produce ever larger, more sensitive, and more efficient IR FPAs at lower costs. In either case, the result is the proliferation of increasingly sophisticated, completely passive imaging sensors that can cover the entire MWIR band with high resolution, yet require little electrical power and are, overall, increasingly compact, and affordable.

Comparable active IR systems, whether active imaging sensors, range finders, illuminators, beacons, or robust countermeasures, are not so fortunate. Active IR systems � at least those exhibiting the most basic level of sophistication, require IR lasers as sources. Lasers, whether gas lasers, chemical dye lasers, or solid state lasers, emit radiation at very specific and stable wavelengths. This is one of their chief strengths. For some applications however, this is their chief drawback. Building an active IR laser source that emits across a wide band (a set of multiple discrete wavelengths) is difficult, typically requiring a separate laser for every wavelength desired within the band. True wavelength agility � that is, selecting individual wavelengths "on the fly" or by system presets is expensive because it typically requires complex tuning mechanisms or includes multiple individual lasers that are simply turned off when not needed. For example, a laser illuminator in which individual systems are programmed to emit at certain sub-bands so as to be distinguished from (or not interfere with) other such systems is inherently complex, heavy, and costly, especially if the emitted power is significant. Semiconductor lasers offer some relief since they are inherently compact. However, to achieve appreciable power levels and broad spectral coverage, multiple semiconductor laser diodes must be combined.

Complicating matters, the MWIR spectrum is divided into two main sub-bands separated by a region of virtually complete atmospheric absorption. Even within the sub-bands, atmospheric transmission is still highly variable with changes in absorption occurring with both season and latitude. Passive imaging sensors are not greatly handicapped by this fragmented spectrum of usable wavelengths because they still "see" across all the wavelengths that are transmitted and benefit from time integration of the received IR power. However, MWIR lasers, if unfortunate enough to be chosen at the wrong wavelength, simply waste their power trying to burn through atmosphere that is largely opaque to them. Consequently, active MWIR systems must be carefully designed at properly chosen wavelengths spanning the MWIR spectrum (or at least the portions of the spectrum of interest) in order to assure availability of operation. Multiple individual lasers are therefore an unavoidable consequence of agile active MWIR systems.

Multi-laser systems suffer from one additional constraint. To be tactically useful, they should emit a high-quality beam from a single aperture. Good beam quality (near diffraction limited) also provides for optimal system optics and beam propagation and therefore makes maximum use of the available system power. A single, well-formed beam at the output aperture simplifies the beam director, both mechanically and optically, reducing overall system size, weight, power, and cost (SWaP-C). System SWaP-C is often the overriding consideration in how widely a tactical system will be deployed.

The Navy needs a compact, high quality, broad-spectral laser source in the MWIR band. In this context, "laser source" is understood to mean an integrated assembly of individual lasers and beam combining optics. The laser source should cover both MWIR atmospheric sub-bands, nominally 3.5-4.1 microns and 4.6-4.9 microns, with a combined output optical power of 100 W minimum in continuous wave (CW) operation. Alternately, quasi-CW operation is allowable where the laser source is pulsed at a minimum of 100 kHz and a sufficiently high duty factor to achieve 100 W of average power output. The output power may be weighted toward either sub-band but each sub-band must include at least 30% of the total emitted power. Coverage of the sub-band is defined as at least three (more is preferred) discrete spectral lines across each sub-band (while taking into account the objective of maximizing atmospheric propagation). More lines may be chosen so as to spread the power more evenly across the sub-bands or to decrease the power required from each individual laser.

It should be noted that a scalable solution in which spectral lines can be added to future versions of the laser source is highly desirable. Spectral lines should be chosen for maximum atmospheric transmission in a tropical maritime environment as defined by MODTRAN® or an equivalent atmospheric propagation code evaluated for 10 km range at sea-level.

The laser source should have a single output aperture and emit a near-diffraction limited beam with a minimum M2 factor of 2.0 and a goal of 1.5 (M2 is herein defined according to ISO Standard 11146). In order to facilitate the safe testing of prototypes at low average power, the laser source shall include an interface for pulse operation at continuously variable pulse widths (starting from a 5 �s minimum pulse width) and arbitrary duty factor. Note that, in pulse operation, the entire source (even if a quasi-CW solution is pursued) is intended for pulse operation as a unit, not individual laser lines separately. The laser source may be liquid cooled but note that the volume of cooling hardware (connectors, manifolds, cold plates) present in the source is included in the size goal for the unit. Chillers, heat exchangers, pumps and other cooling hardware required to supply the liquid coolant is considered external to the laser source and does not factor into the size and power budget calculations. Likewise, power supplies and power conditioning units are considered external to the laser source. The laser source has a desired minimum wall plug efficiency (WPE) of 10% where WPE is defined as the total CW (or quasi-CW) optical output power divided by the total electrical power provided to the source from the external power supplies. The laser source should not exceed three cubic feet in volume and gimbals, beam steering, and stabilization systems are not included as part of this effort. Testing will be done by the company in a laboratory environment.

Life-cycle cost is always a fundamental concern and the laser source should therefore be designed with affordable manufacturing, long-term reliability, and ease of use (including maintenance) in mind. Acceptable solutions are therefore assumed to incorporate solid state or semiconductor lasers and multiple-laser solutions should take into account the manufacturing cost associated with the beam combiner and the cost implications associated with a scalable architecture (scalable in spectral content and by extension, total power). However, any solution that meets the requirements (including solutions that employ single lasers as sources) is acceptable and, while the goal of this effort is not to produce manufacturing technology, the production cost of the laser source should be estimated and the key manufacturing steps and processes that are identified as cost drivers should be analyzed and prioritized for remediation under a follow-on effort with a goal of $200K as the per-unit production price.

PHASE I: Propose a concept for a compact high power MWIR laser source that meets the objectives stated in the Description. Define the laser source architecture and demonstrate the feasibility of the concept in meeting the Navy need. Feasibility shall be demonstrated by a combination of analysis, modelling, and simulation. Identify key manufacturing steps and processes and estimate the cost of the laser source in low-rate production (10 units per month). The cost estimate for the concept shall be based on an analysis of key manufacturing steps and processes, their maturity and availability within the industry, the cost and availability of key components, and by comparison to the manufacture of similar items. The Phase I Option, if exercised, will include the laser source specification, test specifications, interface requirements, and capabilities description necessary to build and evaluate a prototype in Phase II.

PHASE II: Develop, demonstrate, and deliver a prototype compact high power MWIR laser source (the laser source plus chiller and the required power supply, pulse drive, and control electronics) based on the concept, analysis, architecture, and specifications resulting from Phase I. Demonstration of the laser source shall be accomplished through production and test of a prototype (or multiple prototypes) in a laboratory environment. The analysis of key manufacturing steps and processes identified in Phase I shall be refined and updated to reflect lessons learned in fabrication and test of the final prototype. The cost estimate for low rate production shall likewise be updated. Multiple prototypes (or partial prototypes) may be produced during execution of this Phase II as the design process is assumed to be necessarily iterative in nature. However, at the conclusion of Phase II, the final (best performing) prototype laser source system demonstrator shall be delivered to the Naval Research Laboratory along with complete test data, the final manufacturing analysis, and final low-rate production cost estimate.

PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the technology for Government use. Identify specific manufacturing steps and processes that require maturation, mature those steps and processes, establish a hardware configuration baseline, produce production level documentation, and transition the laser source into production. Assist the Government in integration of the laser source into next higher assemblies and deployable systems.

The technology resulting from this effort is anticipated to have broad military application. Law enforcement, commercial, and scientific applications include use as sources for laser spectroscopy for chemical detection and identification (detection of explosive compounds, for example).

REFERENCES:

1. Mecherle, G. Stephen. "Laser Diode Combining for Free Space Optical Communication." Proceedings of the SPIE 0616 15 May 1986: 281-291. http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1241781.
2. Fan, T. Y. "Laser Beam Combining for High-Power, High-Radiance Sources." IEEE Journal of Selective Topics in Quantum Electronics 11 May-June 2005: 567-577. http://ieeexplore.ieee.org/document/1516122/.
3. Pauli, M. et al, "Power Scaling and System Improvements to Increase Practicality of QCL-Based Laser Systems", Proceedings of the SPIE 10926 27 June 2019. https://doi.org/10.1117/12.2508710.
4. Sanchez-Rubio, A. et al. "Wavelength Beam Combining for Power and Brightness Scaling of Laser Systems." Lincoln Laboratory Journal 20 2 2014; 52-66. https://www.ll.mit.edu/sites/default/files/page/doc/2018-05/20_2_3_Sanchez.pdf.

KEYWORDS: mid-wave infrared; MWIR Lasers; Laser Source; Semiconductor Lasers; Beam Combining; Wavelength Agility; Atmospheric Transmission