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.

垂直腔面发射激光器 (VCSEL) 是用于无源光网络的高能效光源。尽管这些源具有产生相干高频射频信号的潜力，但它们在高阶幅度调制格式中的应用尚待实验证明。在这项工作中，我们首次使用两个独立 VCSEL 的光外差在光子生成的射频载波信号上通过实验证明了四级脉冲幅度调制 (4-PAM) 格式。两个低功率 VCSEL 在光耦合器上组合在一起，并允许击败 PIN 光电二极管以生成 10-GHz 射频载波。在第一个演示中，8.5 Gbps 数据信号在光子生成的 10 GHz 射频载波上进行调制。在第二次演示中，一个 17 Gbps PAM-4 数据信号被外部调制到相同的 VCSEL 载波上，最终使网络比特率加倍。使用简单的数字信号处理 (DSP) 接收器替代昂贵的接收器硬件来解码传输的 PAM-4 数据信号。当 PAM-4 信号以光子方式生成和传输时，记录到 24 公里的光纤传输损失为 7-dB。从二进制传输到 PM-4 传输引入的损失记录为 3-dB。