Experimental investigation of VCSEL-based optical heterodyning with PAM 4 and envelop detection for 5G fronthaul systems

Abstract

Vertical cavity surface-emitting lasers (VCSELs) are power-efficient optical sources for application in passive optical networks. Despite these sources having the potential of generating coherent high-frequency radio frequency signal, their application for high-order amplitude modulation formats is yet to be demonstrated experimentally. In this work, for the first time, we experimentally demonstrate a four-level pulse amplitude modulation (4-PAM) format on a photonically generated RF carrier signal using optical heterodyning of two independent VCSELs. Two low-power VCSELs were combined at an optical coupler and allowed to beat a PIN photodiode to generate a 10-GHz RF carrier. In the first demonstration, an 8.5 Gbps data signal was modulated on the photonically generated 10 GHz RF carrier. In the second demonstration, a 17-Gbps PAM-4 data signal was externally modulated onto the same VCSEL carrier, ultimately doubling the network bit rate. A simple digital signal processing (DSP) receiver was used as an alternative to costly receiver hardware to decode the transmitted PAM-4 data signal. A 24-km fiber transmission penalty of 7-dB was recorded when PAM-4 signal was photonically generated and transmitted. The penalty introduced by moving from binary transmission to PM-4 transmission was recorded to be 3-dB.