Numerical-Aperture Optimized Microlens Array (NOMA)

The U.S. Navy seeks to reduce the size, weight, power, and cost of imaging systems. This can be accomplished through photonic imaging, which extracts additional information from the imaged scene by using the phase, as well as the amplitude, of the light. The smallest version of such a device would use a photonic integrated circuit (PIC) containing everything except the initial light collection and distribution stage. As the PICs are being improved, a new obstacle has emerged: the shape of the photonic coupler inputs, used to direct the light to the PICÆs detector array, produce astigmatism, significantly reducing the light-collection efficiency. The numerical aperture (NA) of the couplers is different along two orthogonal axes. A simple microlens array (MLA) could be used to concentrate and image the light from each coupler to the detector array if the divergence were equal; unfortunately, most commercially-available MLAÆs have the same NA along both axes. Experience with commercial MLAÆs has shown that they rarely provide reliable alignment, adding to the difficulty of using existing arrays to correct the waveguide output. To meet this need, Holoptic, LLC, is developing a new NA-Optimized Microlens Array (NOMA), based on injection-molded or compression-molded optics. The NOMA is an MLA with center-to-center spacing matching the output of the waveguide array. The NA of each lens matches the requirements of the photonic couplers, ensuring optimal coupling to the detector array. The final NOMA will exactly meet the Navy need for a high-quality, low-cost lens array for PIC. During Phase I, Holoptic designed a full MLA and fabricated a 250ū250-element prototype. This proof-of-concept NOMA prototype met the optical requirements of the final MLA and reached technology readiness level (TRL) 4. We demonstrated alignment from point to point (lens pitch accurate to ~10 nm over 10 mm, which included 100 elements); excellent surface roughness, indicating a scattering loss of <0.21%; and lens shape consistency resulting in <1% RMS variation in NA among 10,000 elements. Goniometer measurements demonstrated NA values within 6% of the goal. The two initial candidate materials, polycarbonate and borosilicate glass, should meet all the material and environmental requirements for the final NOMA. Since both materials are used in space, it is likely that they will survive the potential radiation environment as well. During Phase II, we will develop this prototype into an MLA that not only meets the specific Navy requirements but also can be manufactured reliably. We will fabricate several prototypes using a variety of materials to ensure that the NOMA will meet all the Navy needsŚoptical, mechanical, and environmentalŚwhile reaching TRL 6. Our final version will be a pre-production prototype that can be integrated into the Navy photonic imaging system, thereby achieving TRL 7; at that point the NOMA will be ready for initial production.