Homodyne OFDM with Optical Injection Locking for Carrier Recovery

Homodyne detection provides the simplest digital signal processing (DSP) solution to optical coherent detection and minimizes the receiver bandwidth requirements. These features make it promising for high spectrally efficient formats such as optical orthogonal frequency domain multiplexing (OFDM), which has a flat optical spectrum and which is thus inherently sensitive to high-frequency distortions, e.g., due to limited detector bandwidth. The key to homodyne detection is recovery of the carrier from the received signal all optically (as opposed to frequency offset compensation via DSP. Herein, we use optical injection locking (OIL) in conjunction with carrier tone-assisted OFDM to achieve this. In contrast to previous reports, we show that OIL carrier recovery with subsequent homodyne detection can operate without the need for any optical prefiltering. First, we evaluate the performance as a function of the carrier tone guardband bandwidth. Further, we improve the robustness of this technique using a slow-phase lock loop that compensates for drift in the laser´s temperature/current control electronics. Using this improved setup, we compare our all-optical-carrier-recovered homodyne and the “traditional” DSP-assisted intradyne detection for the case of OFDM-16QAM signals. Finally, we compare the computing complexity necessary for both approaches and estimate the intradyne performance limitations due to the carrier-local oscillator frequency offset.