The EML as Analogue Radio-Over-Fiber Transceiver—A Coherent Homodyne Approach

Analogue radio-over-fiber schemes avoid transmission overhead as they dismiss digitization and processing of radio signals at head- and tail-end of the mobile optical fronthaul. Typically laid out following an intensity-modulation/direct-detection methodology, such systems do not scale well in terms of optical spectral efficiency and sensitivity. Here, I propose and experimentally demonstrate an opto-electronic engine for coherent homodyne radio-over-fiber transmission. The complexity is greatly reduced by relying on a cost-effective component that is well adopted in telecommunication and datacenter networks: the externally modulated laser (EML). Taking advantage of all-optical injection locking of its monolithic integrated laser section, homodyne reception is facilitated through the electro-absorption property of the EML. The radio carrier frequency is exactly translated between the electrical and the optical domain despite the use of coherent reception without additional signal processing. Using a commercial off-the-shelf EML device, a low error vector magnitude of 4.6% is achieved for radio-over-fiber transmission of a 100-MHz 64-ary quadrature amplitude modulated orthogonal frequency division multiplexed radio signal over a 27.5 km reach, 1:128 split fronthaul network. Extension to 1-GHz wideband radio signals is investigated. On top of this, full-duplex analogue radio-over-fiber transmission is implemented with a single EML through simultaneous use of its modulation property. Signal integrity can be retained through frequency division duplex, as it is evidenced by a small error vector magnitude penalty of 0.7% for downlink reception while transmitting a symmetric uplink radio signal with the same EML. With this, a cost-effective full-duplex radio-over-fiber solution based on a coherent analogue remote radio head interface with single fiber and single radio frequency port for its opto-electronics is provided.