N221-079 TITLE: Low-Loss, Low-Aberration, Numerical Aperture-Matched Microlens Arrays to Improve Coupling Efficiency onto Photonic Imaging Devices.

OUSD (R&E) MODERNIZATION PRIORITY: Quantum Science

TECHNOLOGY AREA(S): Sensors

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 novel microlens arrays to improve coupling of light onto photonic imaging devices by reducing aberrations and improving mode matching to the collection aperture.

DESCRIPTION: This SBIR topic seeks to develop and fabricate microlens arrays optimized for coupling power onto a photonic imaging device to enable enhanced performance of a variety of integrated photonics systems. Most photonic imaging devices collect light and couple it into photonic integrated circuits (PIC) using grating couplers [Refs 1-2], but photonic chips can suffer significant loss from absorption and scattering within PICs. It is critical that enough light reaches the detectors for the scene to be imaged, so it is necessary to maximize the amount of usable light that couples onto the photonic chip.

Because photonic grating couplers are generally rectangular, they have axis-dependent numerical aperture (NA). The NA is greater along the shorter rectangular axis and smaller along the longer rectangular axis. Furthermore, the grating efficiency and angle of collection are both wavelength dependent. Current Commercial Off the Shelf (COTS) microlens arrays have axis-independent numerical apertures, chromatic aberrations, and also high surface roughness; making them sub-optimal for coupling light onto the chip. Integrating a low-aberration, achromatic, axis-dependent NA and pitch-matched microlens array with photonic imagers would increase the optical throughput by approximately 10 dB, increasing the detected signal.

PHASE I: Perform a design and fabrication analysis to assess the feasibility of producing low-aberration, achromatic in the near-infrared (across 700 � 900 nm) microlens arrays for integration with photonic imaging systems. The design must be able to accommodate NA values that differ significantly along two axes (for example: an NA of 0.045 along one grating axis, 0.15 in the other), while focusing light into a spot on a single plane. The thickness of the microlens array, including any substrate, must be less than 1.5mm. Array pitch should be in the 60-100 um range. Include the expected NA tolerances along both axes (no greater than 2%), pitch tolerance (within 1 micron of expected location, across 200 lenslets), and achromaticity over the near-infrared wavelength range (no more than 15 nm short of bounds). Develop a detailed plan of fabrication that identifies risks and risk mitigation strategies. The Phase I Option, if exercised, will include the initial design specifications and capabilities description to build prototype solutions in Phase II.

PHASE II: Fabricate and characterize twenty (20) prototype microlens arrays, measuring at least 1 cm by 1 cm square, with a thickness of less than 1.5 mm, to be integrated with a photonic imaging system. Surface roughness-induced loss, achromaticity, and NA should be characterized and should fall within 1% of the values determined from the work in Phase I. Evaluate the device�s thermal, vibration, and radiation sensitivities by performing tests in accordance with MIL-STD-883L [Ref 3].

Produce a final report that includes a discussion of potential near-term and long-term development efforts that would improve the technology�s performance and/or ease of fabrication. It will also include an evaluation of the cost of fabrication and how that might be reduced in the future. The prototypes should be delivered by the end of Phase II.

PHASE III DUAL USE APPLICATIONS: Based on the prototypes and continual advancement of photonics, grating-matched microlens arrays should lead to the production of a design suitable for use in integrated photonic imaging and photonic sensing applications. Support the Navy in transitioning the technology to Navy use. The lenslet arrays will be evaluated through optical characterization and testing with prototype devices. The end product technology could be leveraged to bring photonic imaging and sensing towards a more mature state with a lower Size, Weight, and Power (SWaP) profile that could make it more attractive to biomedical, navigation, and vehicle autonomy commercial markets.

REFERENCES:

1. Clevenson, Hannah A. et al. "Incoherent Light Imaging Using an Optical Phased Array", Applied Physics Letters, Vol. 116, Issue 3, 031105 (2020). https://doi.org/10.1063/1.5130697.
2. E. H. Cook et al., "Polysilicon Grating Switches for LiDAR," in Journal of Microelectromechanical Systems, vol. 29, no. 5, pp. 1008-1013, Oct. 2020, doi: 10.1109/JMEMS.2020.3004069.
3. MIL-STD-883L, Department of Defense Test Method Standard: Microcircuits (16-SEP-2019). http://everyspec.com/MIL-STD/MIL-STD-0800-0899/MIL-STD-883L_56323/.

KEYWORDS: photonic imaging; microlens array; photonic integrated circuits; numerical aperture; low-aberration, achromatic