N22A-T014 TITLE: Visible to Near Infrared Laser Array with Integral Wavelength Beam Combining

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

TECHNOLOGY AREA(S): Sensors

OBJECTIVE: Develop an array of visible to near-infrared (VNIR) lasers with integral (on-chip) wavelength beam combining for a single, high quality output beam.

DESCRIPTION: Many threats to surface ships employ imagers and detectors operating in the visible to near-infrared (VNIR) band. These include lethal threats as well as aircraft and unmanned aerial systems performing routine surveillance. To combat these threats, shipboard countermeasures are needed and, for the most sophisticated threats, lasers are the fundamental component of the electro-optic (EO) 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 solution also provides spectral coverage across the wavelength band (or a specified portion of the band) as laser diodes of different wavelengths are combined ? a highly desirable feature for countermeasure applications. However, the architecture presents a considerable cost in manufacturing as the exacting tolerances required result in high component costs and the assembly process is highly labor-intensive. The assembly cost of the laser diode combiner typically accounts for as much as half the cost of the finished laser module.

Other possible laser sources are either bulky, even more expensive, or have other undesirable characteristics such as multi-mode operation. For example, some high brightness semiconductor lasers require an additional pump source or other free-space optics which increases size and cost. Other solutions involve frequency doubling to produce single wavelength output that would still have to be combined with the output from other lasers to achieve spectral coverage. Currently, there is no off-the-shelf laser source that can produce any significant power (> 1.5 W) across the VNIR waveband at an affordable price and in a sufficiently compact form factor.

The Navy needs compact and affordable laser sources in the VNIR band, specifically the wavelengths covering 0.5 through 0.85 microns. In this context, a "laser source" is understood as being distinct from a simple laser, in that the laser source combines the output of multiple individual lasers into a single output beam. In the case of the laser module described above, this is done through the assembly, integration, and alignment of multiple individual laser diodes with external optical components that perform the beam combining. However, it may also be done by integration of the combining optics directly on the same semiconductor substrate that contains the laser diodes, creating a photonic integrated circuit that is effectively a miniature laser "module" on a chip. With the exception of packaging and alignment of the output optics, this "on-chip" combining eliminates almost all of the assembly steps required for the discrete-component laser module. And while the cost of semiconductor fabrication increases, the overall cost of the resulting laser source can be significantly reduced, provided the technical challenges of on-chip combining in the VNIR can be overcome.

The goal of this topic is to demonstrate a laser source operating in the VNIR and designed for optimum size, weight, and power (SWaP), while also reducing the cost (SWaP-C). The source should be a laser array integrated on the same chip and combined into a single output, which is considered to be the key technical achievement of the effort. The minimum required continuous wave (CW) output power is 1.5 W, and the power should be distributed in at least six spectral lines. More lines are desirable, and increasing the number of integrated lasers represents an acceptable way of scaling to the required power output. The source should cover the entire VNIR band, with at least 20% of the total output power appearing in each of the sub-bands: 0.5-0.6 microns, 0.6-0.7 microns, and 0.7-0.85 microns. The output should also be placed at spectral lines corresponding to wavelengths of maximum atmospheric transmission. While the maximum number of discrete laser diodes that can be integrated on a single chip is fundamentally limited by die size and beam-combining losses, nothing about the chosen architecture should preclude further power scaling by external (off-chip) combining of multiple integrated laser arrays. In particular, the combined beam output from the chip should be of high quality, with M2 less than 2.0 and with 1.5 as a goal (note that M2 is defined by ISO Standard 11146 for this effort).

The solution must demonstrate the laser source as a packaged prototype laser module. Of fundamental importance is low SWaP, with a size goal of less than 20 cubic inches for the entire laser module and a weight goal of less than one pound. In this context, the "laser module" comprises the integrated on-chip combined laser array (which is the laser source), the mount (including thermal stack-up), the optics required for transmitting the output beam, and the packaging (including electrical and coolant connectors), but does not include the mounting hardware or power supplies. External optics for shaping the beam are acceptable, so long as they fit within the specified total module volume. Although the prototype module produced during Phase II need not be environmentally hardened, it must be contained within a closed package rather than an open breadboard.

The laser module prototype is intended for laboratory demonstration and limited outdoor range testing. However, for ease of use and in order to inform future system concepts, the laser module will be integrated with a closed-loop cooler, power supplies, and control circuitry to form a system demonstrator prototype. The system demonstrator will accept normal 60 Hz 120 V prime power and employ air cooling (convective or forced). The system demonstrator also need not be environmentally hardened, but should be capable of operation in ambient temperatures ranging from 40 to 90�F. Other than electrical prime power, the demonstrator should be self-contained and no larger than 300 cubic inches, including the laser module. The total weight of the demonstrator is not restricted. While the laser module is an integral part of the demonstrator, it should be removable to accommodate the possibility of substituting different laser modules in the future (for example, modules emitting with different spectral line placement). As a benchmark, the demonstrator prototype should be designed to meet a cost goal of $10,000 per unit when manufactured in a volume of 1,000. At the conclusion of the effort, the demonstrator unit will be delivered to the Naval Research Laboratory.

PHASE I: Develop a concept for a compact high-power integrated VNIR 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 parameters of the Description. Feasibility shall be demonstrated by a combination of analysis, modelling, and simulation. The cost estimate for the concept shall be obtained by analyzing the 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, the laser demonstrator system specification, test specifications, interface requirements, and capabilities description necessary to build and evaluate the full system demonstrator prototype in Phase II.

PHASE II: Develop and deliver a prototype compact high-power integrated VNIR laser source based on the results in Phase I. The integrated laser source (within the laser system demonstrator) shall be demonstrated by producing and testing a prototype (or multiple prototypes) in a laboratory environment. Multiple prototypes (or partial prototypes) may be produced 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, integrated with the system demonstrator, shall be delivered to the Naval Research Laboratory along with complete test data, a final manufacturing analysis, and final production cost estimate.

PHASE III DUAL USE APPLICATIONS: Assist the Navy in transitioning the technology for Government use. Specific manufacturing steps and processes that require development will be identified. Iterative testing will establish a hardware configuration baseline, produce production level documentation, and transition the laser source into production. Assist the Government in incorporating the integrated laser source into next higher assemblies and deployable systems.

Law enforcement, commercial, and scientific applications include use of VNIR lasers as sources for laser spectroscopy in detection of hazardous materials and chemical substances. The technology should also find application in the telecommunications sector as sources for wavelength multiplexed communications.

REFERENCES:

1. Zhao, Yunsong and Zhu, Lin. "On-Chip Coherent Combining of Angled-Grating Broad-Area Diode Lasers." Optics Express, Vol. 20, Issue 6, 2012, pp. 6375-6384. https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-20-6-6375&id=229737.
2. 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.
3. Chang, Hsu-Hao, et al. "Integrated Hybrid Silicon Triplexer." Optics Express, Vol. 18, Issue 23, 2010, pp. 23891-23899. https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-18-23-23891&id=206982.
4. Latkowski, S., et al. "Monolithically Integrated Laser Sources for Applications Beyond Telecommunications." Proceedings of the SPIE, Physics and Simulation of Optoelectronic Devices XXVIII, 11274N, 2 March 2020. https://spie.org/Publications/Proceedings/Paper/10.1117/12.2552784?origin_id=x4325&start_volume_number=11200.

KEYWORDS: VNIR Lasers; Near-Infrared; Laser Source; Semiconductor Lasers; Beam Combining; Laser Diodes.