For microscopy, high numerical apertures are necessary to resolve key features of a sample. For polarization microscopy, a high NA can result in significant polarization variation over the pupil. These systems are often challenging to calibrate for quarter-wave plates that have a strong angular dependence. This talk will describe a full system calibration method for a short-wave infrared microscope utilizing a rotating quarter-wave plate for polarization characterization of scattered light. We draw a particular distinction between calibration for direct imaging when compared to pupil imaging of dipole like objects.
Light from a dipole source (e.g., a single molecule) has long been known to possess a spatially varying polarization state, but no current method exists to emulate this response. Simulations of an engineered scattering element in a photonic integrated circuit have demonstrated the concept of a synthetic dipole. Through the AIM Photonics Foundry, photonic integrated circuits with synthetic dipoles were fabricated and characterized under a custom SWIR microscope setup. The polarization response of designed transverse dipoles was characterized using quantitative imaging and rotating quarter-wave plate polarimetry.
Efficient packaging of fabricated photonic integrated circuits (PICs) has been a daunting task given the breadth of applications and skill required for scalable manufacturing. One particular challenge has been accurately assessing the polarization state at various points in a PIC during the test, assembly, and packaging process. Polarimetric monitoring is necessary for optimizing fiber alignment, for verifying the quality of PIC components and for polarization-related functional testing. We analyze and demonstrate small-footprint engineered scattering elements for polarization monitoring. We find that small scatterers placed above or below a Si or SiN waveguide provide the best polarization integrity in a way that preserves foundry compatibility. The polarization response of these elements along with proper placement provides an optical test point that can be utilized for optimized fiber coupling into waveguides.
Polarimetric microscopy has become a useful tool in multidimensional microscopy, especially for single molecule imaging where the orientation of a molecule is related to its polarization. Using an engineered scattering element in a photonic integrated circuit, we have produced a synthetic dipole source whose orientation is controllable through amplitude and phase of each waveguide.
Accurate monitoring of the polarization within a waveguide is key to verifying the polarization fidelity in photonic integrated circuits and in verifying polarization during fiber alignment. Current testing methods require either a combined input/output fiber attachment or a tap/detector combination. These measure the coupled power but cannot monitor the polarization state near individual components. We have designed and tested foundry-compatible optical test-points for polarization monitoring. We study scatterers both for SiN and Si waveguides fabricated in the AIM Photonics foundry. We observe strong polarization effects in the light scattered from these elements when designed in certain geometries. When viewed with a short-wave infrared microscope, captured images displayed extinction values up to 30x between orthogonal polarizations for the engineered scattering elements. Finite-difference time-domain simulations were performed for each scattering element, corroborating experimental measurements but showing that even higher extinctions may be possible with further refinement.
Optical interconnects using a silicon-on-insulator integrated circuit platform have become the basis for many modern communications platforms. One limiting factor in interconnect technology is creating a consistent, reliable method for measuring the amount of coupled light from optical fibers into waveguides in a photonic integrated circuit. Monitoring the coupling efficiency before, during, and after would be the ideal scenario for fiber bonding. Using a foundry compatible engineered scattering element developed by our lab, we have been able to monitor the degree of fiber alignment by recording the relative power scattered by the engineered element. Recorded powers are then compiled to generate a heat map of the optimal fiber position for coupling. These scattering elements are also polarization sensitive, thus allowing for the fast axis of polarization maintaining fibers to be monitored and optimized for coupling.
Quantitative measurements of the polarization state of light at various points within a photonic integrated circuit is a challenging but important problem in the packaging and testing of photonic systems. We analyze and experimentally test polarimetric microscopy of several types of engineered subwavelength scatterers designed for use in a silicon photonics foundry process.
Photonic integrated circuits (PICs) are important to reduce the size and increase the capacity of optical systems. Testing of coupling loss, waveguide and bend loss, coupler splitting ratio, and polarization state are all needed for maintaining the quality of foundry-produced PICs. The use of photodetectors, loopbacks and grating couplers accomplishes some of these functions, but at the cost of chip real estate. Image capture from scattered light via a microscope set is possible but the light being emitted is random in nature thus not viable to accurately monitor light within the circuit. To solve this problem, we introduce deterministic, subwavelength scattering elements into the circuits for SWIR camera-based testing of PICs. These elements are designed for negligible footprint, foundry compatibility, and to produce little loss in the circuit while carrying polarization information in the scattered light. Finite-difference time-domain simulations were performed analyzing the use of these scattering element within the system, with subsequent numerical propagation to extend the fields into the microscope observation plane. Using PICs fabricated in the AIM Photonics foundry, we observe light from engineered scatterers that is greater than 20x brighter than the background scatter while also providing polarization sensitivity. Integration of these components with methods for circuit metrology will allow for faster processing of circuit layouts when packaging and distributing PICs.
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