Optical homodyne RZ-QPSK transmission through wind tunnel at 3.8 and 1.55 μm via wavelength conversion

Atmospheric absorption, scattering, and turbulence are impairments in practical high-speed free-space laser communications. These atmospheric effects can be mitigated by choosing the proper transmission wavelength. It is well known that the MWIR (~3.8 μm) has many low-absorption spectral lines suitable for low-loss propagation. Also, MWIR can be more robust to turbulence in the weak-turbulence regime. Since high-speed laser transceivers are not available in the MWIR, a 3.8-μm signal can be generated and detected using a 1.55-μm telecom transceiver via wavelength conversion. Free-space transmission of optical homodyne RZ-QPSK through a turbulent channel at 3.8 μm has been investigated. A pair of Ti:PPLN-based nonlinear wavelength converters were used to down- and up-convert from 1.55 to 3.8 and back to 1.55 μm at the transmitter and at the homodyne receiver, respectively. The converted RZQPSK signal was transmitted through a tabletop wind tunnel that produces a weak turbulent path. Comparison of 1.55 and 3.8 μm transmission through the wind tunnel shows that under weak-turbulence 3.8 μm transmission is more robust than 1.55 μm. Under the same turbulence condition, the scintillation index measured at 3.8 μm is consistently lower than that at 1.55 μm. Extrapolated scintillation indexes for 3.8 and 1.55 μm using the Rytov variance (~ λ -7/6 ) and independent measurement at 632.8 nm are consistent with the RZ-QPSK scintillation data for 3.8 and 1.55 μm. Under the most severe turbulence condition, the average bit-error-rate of 3.8-μm transmission is better than that of 1.55-μm giving an estimated receiver sensitivity improvement of at least 6 dB.