Optical Frequency Reconstruction with Microcavity Assemblies

Precise frequency reconstruction is essential for many scientific and industrial applications with particular attention for the growing field of quantum technologies. Instruments that use computational methods for spectral information retrieval from pseudo-random patterns enable high-resolution measurements in compact devices with improved dynamic range [1]. Speckle is obtained via interference of multiple reflections/refractions by directing the analyzed light through a disordered medium. The resultant tiny spectral alterations cause random changes in speckle patterns and provide strong spectral variability. Among diffusers, multimode fibers and integrating spheres used as a speckle pattern source, the latter offers the highest intrinsic sensitivity (sub-100~fm) which is one order of magnitude better than for the second-best multimode fibers [2]. Natural variation of whispering gallery mode (WGM) microresonators converts the microscale size cavity into a scattering medium with extremely long (≳100 m) optical path and a source for unique spectral pattern. However, this pattern is confined to the vicinity of resonance peaks and suffers from spectral gaps and periodicity, limiting its spectral certainty to a narrow range, typically a few GHz. In this work, we propose a novel method for optical frequency reconstruction of high precision with a compact and affordable instrument. It is based on fabrication of dense and disordered assemblies of optical microcavities on a single chip providing unique spectral pattern for the broad wavelength range. In particular, we employ PMMA microspheres with dimensions ≤ 100 μm and loaded Q-factors of up to 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> as individual optical cavities. Total number of up to ten thousand cavities are randomly allocated on the chip area of 10×10 mm where they are immersed in the low refractive index material (Fig. 1(a)). The multiplexed microresonator imaging is then utilized to acquire unique spectral patterns by probing the whole assembly on the chip, as previously introduced for environmental sensing [3]–[5].