A Field Trial of Multi-Homodyne Coherent Detection Over Multi-Core Fiber for Encryption and Steganography

The concept of multi-homodyne coherent detection is used for stealth and secured optical communications, demonstrating a transmission rate of 20 Gbps under negative (in dB) optical signal-to-noise ratio (OSNR), in a deployed multi-core fiber (MCF) network. A 10 GBd QPSK signal is assigned with a bandwidth of multi-hundreds GHz, optically encoded with spectral phase mask, and transmitted below ASE noise in the MCF channel. To enable such transmission scheme, a semiconductor based gain-switched laser (GSL) frequency comb, used as both the signal carrier and local oscillator (LO), are then mixed in intradyne coherent receiver. The ultra-short time-domain overlapping between the pulsed signal and the pulsed LO allows the authorized user to collect the information, while discriminating most of the noise. Consequently, the electrical SNR is dramatically improved compared to the optical SNR, resulting in optical processing gain. Nevertheless, ultra-short pulses mixing is rather susceptible to a chromatic dispersion (CD), which imposes a major challenge in this spread-spectrum transmission technique. In this contribution, the effect of CD in multi-homodyne coherent detection is analyzed in an analytic model and in experiments. The model suggests a modified CD transfer function, comprised of the optical sampling of a conventional CD transfer function. In addition, the SNR penalty associated with the CD effect is quantified, as well as its dependency in propagation length and overall GSL's bandwidth. To circumvent the devastating effect of the CD, multi-core fibers are proposed. Two different weakly-coupled cores of a homogeneous MCF allow the signal and the LO to co-propagate through a very similar dispersive channel. A field trial in a deployed 6.29 km MCF network has validated an encrypted and stealthy 20 Gbps coherent transmission, at −2 dB OSNR.