Extending the Reach of Short-Reach Optical Interconnects with DSP-Free Direct Detection

Coherent detection is the usual solution to achieve high bit rate transmission over long distance. However, it requires a local oscillator and energy consuming digital signal processing that make it not suitable for cost and energy sensitive interconnections and short haul links. In these applications, direct detection is the preferred solution and the main impairment is the chromatic dispersion of optical fibers. In theory, combined amplitude-phase shift (CAPS) codes are able to defeat any amount of chromatic dispersion. However, while they can be detected with a very simple direct detection receiver, their generation is not trivial. Their complexity grows with the order of the code which, in turn, increases with the chromatic dispersion. In this paper, the IQ-duobinary modulation scheme, a novel technique for approximating an order-3 CAPS code, is presented. Such a modulation scheme allows us to bridge up to 18 km at a bit-rate of 50 Gb/s over a standard single mode fiber using a simple direct-detection receiver without the need of any digital signal processing technique.