Mid-infrared data encryption and transmission beyond detector bandwidth based on frequency down-conversion temporal ghost imaging

Computational temporal ghost imaging (CTGI) allows for retrieving the fast temporal object using a slow photodetector and can naturally encrypt the transmission data to enhance communication security. Here, we first demonstrate free-space data encryption based on frequency down-conversion CTGI in the mid-infrared (MIR) atmospheric window, where commercial fast photodetectors are lacking. The target temporal signal is digitally encrypted by a secret key and modulated at 1.5 μm light by an acousto-optic intensity modulator (AOM). The encrypted signal is then optically transferred to MIR light through nonlinear frequency down-conversion. The data decryption can be realized from the correlation operation between the intensity recorded by a 1 MHz bandwidth MIR detector and the secret key. Moreover, when the data rate is significantly higher than the bandwidth of the AOM, the modulated optical signal will be severely distorted, which adds extra physical encryption. As a result, we experimentally demonstrate MIR data encryption and transmission over a 3 m free-space link at a speed more than 60 times higher than the MIR detector bandwidth in a 3.3–3.5 μm spectral region. We anticipate the frequency down-conversion CTGI could provide a new platform for secure free-space communication in the MIR region.