Micromirrors are mirrors with micrometer scale that are widely utilized in a variety of applications, primarily optical instruments, optical scanners, and fiber-optic telecommunications. Photonic chips, which can be seen as the miniature versions of the macroscopic optical benches, can incorporate micromirrors.
Silicon micromirrors coupling efficiency measurements using single-mode optical fibers fed from a monochromatic light source, a directional coupler, and an optical detector. Image Credit: The Authors doi 10.1117/1.JOM.2.3.034001
Micromirrors are crucial components of cross couplers, variable optical attenuators, and external cavity tunable lasers in optical communication. The effectiveness of coupling light into and out of these micromirrors is a crucial performance indicator dictating the signal quality in all of those applications.
Micromirrors are crucial components of optical interferometers and optical resonators in instrumentation. The coupling efficiency in these situations is also a crucial performance indicator impacting the metrological features.
Researchers from Ain Shams University in Egypt, led by Yasser Sabry, examined how micromirror behavior varied depending on their form, height, and surface quality in a study published in the Journal of Optical Microsystems.
Furthermore, they examined the effects of incident light misalignment, considering both angular and off-axis misalignment.
Due to limitations in microfabrication, most micromirrors are flat, and the associated height is often restricted to 80 m. Beyond this point, the etched surface's verticality and roughness diminish.
The light spot size must be kept below the mirror height to obtain a satisfactory throughput. Although deeper micromirrors are in demand, making them is challenging. Curved micromirrors, although more challenging to make, are in theory more intriguing than flat mirrors.
Numerous methods that have lately been revealed showed how to manufacture such micromirrors in both 2D and 3D forms. Therefore, the researchers suggested a thorough evaluation of the potential of such curved mirrors.
They used both flat and curved micromirrors to thoroughly investigate the free-space coupling of Gaussian light beams. Analysis was done on the theoretical background and the non-ideal effects, including the restricted micromirror extent, asymmetry in the spherical micromirrors’ curvature, misaligned axis, and imperfections on the micromirror surface.
The behavior of flat (1D), cylindrical (2D), and spherical (3D) micromirrors was examined and contrasted theoretically and empirically using the developed formulas. The investigation concentrated on a range of dimensions, where the curved micromirror's radius of curvature is equivalent to the incident beam's Rayleigh range, which also corresponded to a reference spot size,
For generic micro-optical systems, the researchers developed transfer matrix-based field and power coupling coefficients that account for various matrix characteristics in the tangential and sagittal planes of the microsystem while taking into consideration potential non-idealities.
To keep the findings broad and relevant to all scenarios, they reported the results in terms of normalized quantities. The coupling efficiency in visible and near-infrared wavelengths was also experimentally validated using silicon micromirrors constructed with regulated forms.
Journal Reference:
Sabry, Y. M., et al. (2022) Critical analysis of in-plane free-space light beam coupling using photonic curved micromirrors. Journal of Optical Microsystems. doi:10.1117/1.JOM.2.3.034001.