N22A-T013 TITLE: Damage-Free High Power Emission from Indium Phosphide-Based Solid State Waveguides in the Long Wave Infrared

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

TECHNOLOGY AREA(S): Sensors

OBJECTIVE: Develop a capability that enables reliable emission of high power, single lateral mode, long wave infrared laser beams from Indium Phosphide-based solid state waveguides.

DESCRIPTION: Infrared (IR) photonic integrated circuits, especially those incorporating solid state laser diodes operating in the long wave infrared (LWIR) band, often employ the Indium Phosphide (InP) III-V semiconductor system. Optical signals are transmitted in solid state waveguides fabricated directly in epilayers grown on the InP substrate, which are usually designed for light propagation in a single lateral mode. In many applications, the optical power may be emitted to free space at an edge facet or from some other surface. However, the emitted power is sometimes quite high and the maximum power density at the center of the beam can be exceedingly intense. Furthermore, the efficient extraction of optical power from the facet is typically aided by the deposition of an anti-reflection (AR) coating that minimizes the reflection of light back into the waveguide.

Current InP-based waveguides operating in the 9-11 �m spectral band are susceptible to optical damage at the AR-coated output facet, which limits the maximum continuous wave or average power that can be emitted to less than 2 W. This limitation severely constrains the usefulness of technologies that could otherwise enable higher levels of integration, such as beam combining by an arrayed waveguide grating (AWG). Therefore, the Navy needs an LWIR InP-based waveguide and output coupling technology that reliably increases the maximum power that can be emitted to at least 10 W.

The goal is to demonstrate damage-free operation in both the waveguide and at the output interface over long term operation. Propagation in the waveguide shall be in a single lateral mode and the transmission at the output surface should be at least 90%. The output should be in a nearly diffraction-limited beam with maximum M2 factor of 2.0 (M2 defined according to ISO Standard 11146). The output interface is considered to be to the atmosphere, at sea-level.

Methods for injecting optical power into the waveguide for testing are not a subject of this effort. However, accurate measurement of the output coupling efficiency is expected. In addition, the ability to vary the transmitted power, incrementally or continuously, in order to "test to failure" is highly desirable. Prototype solutions may be demonstrated at any wavelength (or combination of multiple wavelengths) between 9 and 11 �m. However, test wavelengths should be chosen for maximum atmospheric transmission in order to minimize uncertainties in testing and all prototypes should be tested at the same wavelengths. While testing at all wavelengths across the LWIR band is not required, the solution should be suitable for applications that combine multiple LWIR wavelengths spanning the entire upper LWIR band (8-14 �m) in the same beam. Solutions that are "tuned" to specific wavelengths or narrow bands are unacceptable.

Potential solutions may include improvements in ridge geometry, improved AR coatings with lower absorption in the LWIR, tapering of the waveguide along one or both axes, improved heat dissipation at the output surface, surface-emitting (versus edge-emitting) geometries, or other solutions employing innovative architectures and materials. However, acceptable solutions must be capable of fabrication through normal integrated circuit manufacturing processes and work flow. The objective is to develop a technology that can be incorporated into multiple photonic integrated circuit designs. Therefore, coatings and bonding processes are acceptable but solutions that require the addition of "off-chip" elements or require labor-intensive "touch time" assembly are unacceptable. Assembly steps that are performed solely to incorporate diagnostic elements or are performed for fixturing or calibration and do not form a part of the actual technical solution are acceptable. For example, process and assembly steps required to inject optical power into the device for demonstration and testing are not considered to be part of the solution.

As this effort is assumed to be necessarily iterative in nature, it is expected that multiple prototype devices will be produced during its course. In addition, a staged approach in which prototypes capable of 5 W output are first demonstrated and then extensively tested over long term cyclical operation (a minimum of 100 hours of operation with 50 on-off cycles) to assess cumulative damage effects is highly desirable. Testing will be performed in a laboratory environment provided by the proposer. At the end of the effort, the five best performing prototype devices (which have not been "tested to failure") shall be delivered to the Naval Research Laboratory (NRL). Any specialized equipment (e.g., power sources, test equipment and test fixtures, calibration standards, etc.) specifically built or acquired for testing of the devices, along with test data on the devices, shall also be delivered to NRL.

PHASE I: Develop a concept for a high-power LWIR InP-based waveguide technology with transmission, out-coupling, and power-handling characteristics that meet the objectives stated in the Description. Define the architecture and materials required for the concept, and demonstrate its feasibility for meeting the Navy need. Feasibility shall be demonstrated by a combination of analysis, modelling, and simulation. Identify key manufacturing steps and challenges. Define the test configuration, including the method for injecting and measuring the power introduced to the waveguide. The Phase I Option, if exercised, will include formulation of the device specification, test specifications, interface requirements, and the manufacturing requirements necessary to build and evaluate device prototypes in Phase II.

PHASE II: Develop and deliver a prototype high-power LWIR InP-based waveguide transmission and out-coupling technology based on the concept, analysis, architecture, and specifications resulting from Phase I. Demonstrate that the prototype waveguides operate without damage as detailed in the Description. Demonstrate the technology through production and testing of prototypes in a laboratory environment provided by the proposer. It is expected that multiple prototypes will be produced during execution of this Phase as the design process is assumed to be necessarily iterative in nature. At the conclusion of Phase II, five samples employing the best-performing prototype solution (or solutions) shall be delivered to the Naval Research Laboratory, along with complete test data and any specialized equipment needed to replicate testing.

PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the technology for Government use. Identify specific manufacturing steps and processes that require further development, mature those steps and processes, establish a hardware configuration baseline, create production-level documentation, and insert the technology into specific semiconductor fabrication processes. Assist the government in integrating the technology into specific photonic integrated circuit designs meeting requirements supplied by the government and transitioning those designs into production.

Commercial, and scientific applications include use in laser spectroscopy for remote detection of chemicals and explosive compounds, and free-space optical communications (backhaul networks).

REFERENCES:

1. Hitaka, M., et al. "Stacked quantum cascade laser and detector structure for a monolithic mid-infrared sensing device." Applied Physics Letters, Vol. 115, Issue 16, October 2019. https://aip.scitation.org/doi/full/10.1063/1.5123307.
2. Sin, Y., et al. "Catastrophic Degradation in Quantum Cascade Lasers Emitting at 8.4 �m." 2014 IEEE Photonics Society Summer Topical Meeting Series, Montreal, 14-16 July 2014. https://ieeexplore.ieee.org/document/6902994.
3. Phillips, Mark C., et al. "Standoff detection of chemical plumes from high explosive open detonations using a swept-wavelength external cavity quantum cascade laser." Journal of Applied Physics 128, Issue 16, 27 July 2020. https://aip.scitation.org/doi/abs/10.1063/5.0023228.
4. Johnson, Stephen, et al. "High-speed free space optical communications based on quantum cascade lasers and type-II superlattice detectors." Proceedings of the SPIE, Quantum Sensing and Nano Electronics and Photonics XVII: 11288, San Francisco, 2-6 February 2020. https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11288/1128814/High-speed-free-space-optical-communications-based-on-quantum-cascade/10.1117/12.2548348.short.

KEYWORDS: Long Wave Infrared; Anti-Reflection Coating; Beam Combining; Indium Phosphide; Solid State Waveguides; Photonic Integrated Circuits.