November 2022
Spotlight Summary by Francesco Morichetti
Mid-infrared polarization-insensitive grating coupler
Mid-infrared (Mid-IR) is a wavelength range of increasing interest for many applications, including free-space optical communications, advanced systems for positioning and ranging, molecular spectroscopy and sensing. To implement these functionalities in small, portable or wearable devices, chip-scale photonic circuits are required. However, efficient coupling of Mid-IR light with optical waveguides is still an open challenge, especially if signals with a broad optical spectrum are used and if the polarization of the light is not controlled.
In this work, Gao and coauthors report on a broad-band mid-IR grating coupler with an extremely low polarization dependence. To this aim, they borrowed from the more-consolidated near-IR silicon photonic platform the concept of a polarization-diversity scheme, which is based on a pair of 2D polarization-splitting grating couplers. The key to reach best-in-class performance in the Mid-IR range is the use of a suspended sub-wavelength waveguide structure, where the bottom silica oxide is removed and Mid-IR absorption is reduced. In addition, an ultra-thin grating coupler structure is adopted to reduce backreflections. Results demonstrate state-of the art performance, showing a coupling efficiency of –11.5 dB around a wavelength of ∼2200 nm, a 1-dB bandwidth of ∼148 nm and a polarization-dependent loss of only 2.1 dB. Notably, this device was fabricated in a silicon photonics multi-project wafer (MPW) foundry service, thus meaning suitability for high-volume, high-reproducibility, and low-cost production.
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In this work, Gao and coauthors report on a broad-band mid-IR grating coupler with an extremely low polarization dependence. To this aim, they borrowed from the more-consolidated near-IR silicon photonic platform the concept of a polarization-diversity scheme, which is based on a pair of 2D polarization-splitting grating couplers. The key to reach best-in-class performance in the Mid-IR range is the use of a suspended sub-wavelength waveguide structure, where the bottom silica oxide is removed and Mid-IR absorption is reduced. In addition, an ultra-thin grating coupler structure is adopted to reduce backreflections. Results demonstrate state-of the art performance, showing a coupling efficiency of –11.5 dB around a wavelength of ∼2200 nm, a 1-dB bandwidth of ∼148 nm and a polarization-dependent loss of only 2.1 dB. Notably, this device was fabricated in a silicon photonics multi-project wafer (MPW) foundry service, thus meaning suitability for high-volume, high-reproducibility, and low-cost production.
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Article Information
Mid-infrared polarization-insensitive grating coupler
Haoran Gao, Rongxiang Guo, Shujiao Zhang, Chunzhen Lin, Tiegen Liu, and Zhenzhou Cheng
Opt. Lett. 47(20) 5381-5384 (2022) View: Abstract | HTML | PDF