We are pleased to publish a Special Issue of the IEEE Photonics Technology Letters (PTL) by selecting papers that were presented at the Asia Communications and Photonics Conference (ACP) 2021 during October 24-27, 2021. The invited papers, best student papers, best innovation papers, best poster papers, and post-deadline papers of ACP 2021 were invited to submit the extended version to this Special Issue. The articles collected in this Special Issue provide additional technical results and further discussions and insights. All manuscripts submitted to this issue went through the standard review process according to the PTL review guidelines.