The growth of the Internet traffic does not appear to be levelling off any time soon and it is projected to continue to grow exponentially in the years to come. Although there are many proposals on how to deal with the incoming bandwidth capacity crunch, the security of optical networks seems to be almost completely neglected. By taping out the portion of DWDM signal, the huge amount of data can be compromised. Therefore, the security of future optical networks is becoming one of the major issues to be addressed sooner rather than later. To address the security issues of future optical networks the quantum key distribution (QKD) and chaotic cryptography have been proposed. To avoid the high cost of QKD, the properly designed fiber Bragg gratings (FBGs) as optical encryption devices have been advocated recently. In this invited paper, we follow a different strategy. It is well known that we can associate with a photon both spin angular momentum (SAM), related to polarization; and orbital angular momentum (OAM), related to azimuthal dependence of the complex electric field. Because the OAM eigenstates are orthogonal, this additional degree of freedom can be utilized for the physical-layer security in optical networks. Given that the spatial modes in spatial domain multiplexing (SDM) fibers such as few-mode fibers (FMFs), few-core fibers (FCFs), and few-mode-few-core fibers (FMFCFs) can be decomposed in terms of OAM eigenkets, the OAM can be used to enable the physical-layer security in both fiber-optics- and free-space optics-based optical networks.