Feed-forward phase and frequency estimation in coherent digital and analog photonic links using digital signal processing

One of the biggest obstacles in coherent optical communication used to be the synchronization of transmit and local oscillator (LO) laser. Especially for homodyne systems, the need for optical phase-locked loops posed stringed requirements on the fine-width of transmit and LO lasers. To overcome these requirements, digital signal processing techniques have been proposed, which can be used to synchronize a free-running LO laser with the received optical signal in the digital domain. Only recently, advances in digital signal processor (DSP) technology allowed for lab demonstration of DSP-aided coherent optical receivers. In this paper, we will review some of the techniques for realizing robust coherent receivers with free-running LO lasers utilizing digital signal processing techniques. We will consider digital as well as analog links.