![]() In these systems without wavelength conversion, the transmitter modulates data directly onto the mid-IR wavelengths and the receiver directly detects the data-carrying mid-IR wavelengths to recover the data 7, 8, 9, 10. There have been reports of mid-IR communication systems that have used native mid-IR transmitters/receivers (e.g., quantum cascade lasers) to achieve up to 11-Gbit/s data rate using direct detection 7, 8, 9, 10. For example, it was reported that 3–5 μm and C-band wavelengths could have ~24% and ~40% atmospheric attenuation after 2 km sea-level horizontal free-space propagation with hazy weather conditions, respectively 5. Moreover, mid-IR wavelengths tend to have better penetration through inclement weather conditions 6. For example, it was reported that 3–5 μm and C-band wavelengths could have ~7% and ~11% atmospheric attenuation after 2 km sea-level horizontal free-space propagation with clear weather, respectively 5. For free-space optical communication links, the mid-IR has several transmission windows that provide a relatively low atmospheric absorption in comparison to the C-band (1530–1565 nm) 4. There is growing interest in the mid-IR region for potential applications in communications, sensing, and imaging in both free space and fiber 1, 2, 3. These results show the potential of using multiplexing to achieve a ~30X increase in data capacity for a mid-IR FSO link. 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. They are converted to mid-IR and converted back to C-band through the difference frequency generation nonlinear processes. The WDM channels are generated and detected in the near-IR (C-band). As each beam carries 50-Gbit/s quadrature-phase-shift-keying data, a total capacity of 300 Gbit/s is achieved. 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. Here, we experimentally demonstrate wavelength-division-multiplexing (WDM) and mode-division-multiplexing (MDM) in a ~0.5 m mid-IR FSO link. 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. ![]()
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