High-capacity free-space optical communications using wavelength- and mode-division-multiplexing in the mid-infrared region.

Due to its absorption properties in atmosphere, the mid-infrared (mid-IR) region has gained interest for its potential to provide high data capacity in free-space optical (FSO) communications. Here, we experimentally demonstrate wavelength-division-multiplexing (WDM) and mode-division-multiplexing (MDM) in a ~0.5 m mid-IR FSO link. We multiplex three ~3.4 µm wavelengths (3.396 µm, 3.397 µm, and 3.398 µm) on a single polarization, with each wavelength carrying two orbital-angular-momentum (OAM) beams. As each beam carries 50-Gbit/s quadrature-phase-shift-keying data, a total capacity of 300 Gbit/s is achieved. The WDM channels are generated and detected in the near-IR (C-band). They are converted to mid-IR and converted back to C-band through the difference frequency generation nonlinear processes. We estimate that the system penalties at a bit error rate near the forward error correction threshold include the following: (i) the wavelength conversions induce ~2 dB optical signal-to-noise ratio (OSNR) penalty, (ii) WDM induces ~1 dB OSNR penalty, and (iii) MDM induces ~0.5 dB OSNR penalty. These results show the potential of using multiplexing to achieve a ~30X increase in data capacity for a mid-IR FSO link.

Snapshot spectral imaging with parallel metasystems.

Spectral imagers divide scenes into quantitative and narrowband spectral channels. They have become important metrological tools in many areas of science, especially remote sensing. Here, we propose and experimentally demonstrate a snapshot spectral imager using a parallel optical processing paradigm based on arrays of metasystems. Our multi-aperture spectral imager weighs less than 20 mg and simultaneously acquires 20 image channels across the 795- to 980-nm spectral region. Each channel is formed by a metasurface-tuned filter and a metalens doublet. The doublets incorporate absorptive field stops, reducing cross-talk between image channels. We demonstrate our instrument's capabilities with both still images and video. Narrowband filtering, necessary for the device's operation, also mitigates chromatic aberration, a common problem in metasurface imagers. Similar instruments operating at visible wavelengths hold promise as compact, aberration-free color cameras. Parallel optical processing using metasystem arrays enables novel, compact instruments for scientific studies and consumer electronics.