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.
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.
It has been widely recognized that the underlying principles of quantum mechanics could be used to enable secure communications, without any additional conventional cryptographic systems, while utilizing the properties of the photons. Considerable research efforts have been invested to develop an efficient quantum key distribution (QKD) scheme over either free-space optical or fiber-optics channels. Most of these research efforts have been focused on a two-dimensional QKD, commonly realized by the use of the polarization state of photons. However, the data rates for the quantum key exchange in two-dimensional QKD are still low, while transmission distance is limited. On the other hand, it is well known that photons can carry both the spin angular momentum (SAM) and the orbital angular momentum (OAM), associated with the polarization and azimuthal phase of the complex electric field, respectively. Accordingly, we can define the combined-OAM-SAM state of a photon as |l, σ〉, where l and σ correspond to OAM and SAM indexes, respectively. Since the OAM eigenstates are orthogonal, an arbitrary number of bits per single photon can be transmitted, which could considerably increase the total secure bit rate. To improve secure data rates, we propose two types of protocols, namely, nonentangled-based (such as weak coherent state) and entanglement-assisted protocols, both employing the photon combined-OAM-SAM state, which can be used for secure key distribution over free-space optical and few-mode fiber channels. Two types of entanglement-assisted protocols are described, namely, two-basis and (D + 1)-basis protocols ( D is dimensionality of corresponding Hilbert space). We further describe how to implement the qudit gates required for implementation of these protocols. Finally, we discuss the security issues of the proposed protocols and determine both infinite and finite secret key fraction rates.
We propose a chaotic spatial symbol transforming encryption method for enhancing the physical layer security in fiber-optic systems based on space-division multiplexed (SDM) transmission. The designed 3-D chaotic sequences are used to scramble the real and imaginary part of quadrature amplitude modulation (QAM) symbols, permute the symbol-to-space mapping, and control iteration times between the two processes. The encrypted transmission of 200 Gb/s per mode polarization-division-multiplexed 16QAM signals has been simulated over 1000-km two-mode fiber. The results show that the eavesdropper cannot recover the original information with a part of the security keys or a part of the transmitted signals when the iteration times are beyond 5. We found the error spreading phenomenon can be combined with the mode-dependent loss (MDL) to realize the system security enhancement. When eavesdroppers coupled signals out from SDM waveguide by bending-induced fiber tapping, they need reduce the bend radius to receive more spatial leak modes. The bending-induced fiber tapping inherently causes MDL. Although the error spreading phenomenon leads to bit-error-rate increasing by 1.3 dB than conventional 16QAM transmission, it will be a promising solution to overcome the weakness that the huge amount of data can be compromised by taping out the portion of the signal in single-mode transmission.
Society as we know it today would not have been possible without the explosive and astonishing development of telecommunications systems, and optical fibers have been one of the pillars of these technologies. Despite the enormous amount of data being transmitted over optical networks today, the trend is that the demand for higher bandwidths will also increase. Given this context, a central element in the design of telecommunications networks will be data security, since information can often be confidential or private. Quantum information emerges as a solution to encrypt data by quantum key distribution (QKD) between two users. This technique uses the properties of nature as the fundamentals of operation rather than relying on mathematical constructs to provide data protection. A popular alternative to performing QKD is to use the relative phase between two individual photon paths for information encoding. However, this method was not practical over long distances. The time-bin- based scheme was a solution to the previous problem given its practical nature, however, it introduces intrinsic losses due to its design, which increases with the dimension of the encoded quantum system. In this thesis we have designed and tested a fiber-optic platform using spatial-division- multiplexing techniques. The use of few-mode fibers and photonic lanterns are the cornerstone of our proposal, which also allow us to support orbital angular momentum (OAM) modes. The platform builds on the core ideas of the phase-coded quantum communication system and also takes advantage of the benefits proposed by the time-bin scheme. We have experimentally tested our proposal by successfully transmitting phase-coded single-photon states over 500 m few-mode fiber, demonstrating the feasibility of our scheme. We demonstrated the successful creation of OAM states, their propagation and their successful detection in an all in-fiber scheme. Our platform eliminates the post-selection losses of time-bin quantum communication systems and ensures compatibility with next-generation optical networks and opens up new possibilities for quantum communication.
Free-space optical (FSO) communication provides a promising alternative solution to the traditional radio frequency (RF) communications. Compared to the RF technology, there are many advantages of FSO communications, such as ultra-high bandwidth, license-free spectrum, low inter-channel interference, and energy-efficient transmission. The potential applications for FSO communication include the last-one-mile access link from the fiber-based backbone network, low earth orbit (LEO) satellite communications, high-altitude base station platforms or vehicles, deep-space communications, and in-building automation connections. As the FSO technology becomes mature and increasingly involved, the security and reliability requirements of such applications also become crucial. Regarding the security issues, although the FSO communication is inherently more secure than the RF counterpart due to high directivity of its line-of-sight (LOS) link, it can still suffer from optical wire-tapping attack. There are many different application schemes that an eavesdropper can still probe the FSO link. Traditionally, to protect the communication, many works have been done on encryption layer and upper network layer, such as computationally intensive cryptographic algorithms and message exchanging protocols. In recent years, physical-layer security (PLS) has been getting more attentions as it provides an extra layer of security against eavesdropping. The PLS schemes, in general can be categorized into two main categories: classical PLS and quantum key distribution (QKD) protocols. The classical PLS is based on the information theoretic security analysis, while the QKD security is guaranteed by the quantum mechanics laws. Both techniques have been studied for FSO channels. In the first half of this dissertation, we study the PLS in a LOS FSO channel using orbital angular momentum (OAM) multiplexing and show that higher secrecy capacity can be achieved using OAM multiplexing technology in the presence of atmospheric turbulence effects. Then, we proposed an adaptation scheme for QKD system over FSO channel with different QKD protocols, i.e. BB84 and decoy state protocols. By adapting the source brightness based on the channel condition, we optimized the secret key rate (SKR) of the QKD system. Furthermore, we proposed a multiple spatial modes-based QKD system with backpropagation method to increase the total SKR through parallel channels. Regarding the reliability of an optical communication system as well as information reconciliation for both PLS and QKD schemes, the forward error correction (FEC) becomes an essential technique to enable high-speed transmission. The FEC with low-density parity-check (LDPC) codes has been studied for decades from LDPC block codes to the most recent spatially-coupled (SC) LDPC codes. For FSO communications, due to the time-varying nature of FSO channels, the capability of rate adaptation is important to achieve a consistent reliable transmission. In the second half of this dissertation, we proposed unified FPGA-based architecture to implement both quasi-cyclic (QC) LDPC codes and SC-LDPC codes with rate adaptation capability. The performance is verified through FPGA emulation to show good error floor properties. To further validate the rate adaptation capability, we have built a spatial light modulator (SLM)-based FSO channel emulator to test the adaptive coding scheme. Furthermore, we have conducted a fiber-based experiment with our rate adaptative coding scheme to show that the proposed adaptive coding scheme can be also used in dynamic fiber-based networks.
Optical multicore and multimode fibers (MCF/MMF) are promised to pace with the capacity crunch of back-bone networks by their spatial domain using space division multiplexing (SDM) [1]. Although multiple channels propagate within the same media in parallel, material inhomogenities and bendings induce inter- and intra-core crosstalk (XT), the compensation of which is crucial for long-haul SDM-fiber deployment [2]. In addition, the inherent XT provides random scattering to be exploited for physical layer security (PLS), either as physical un-clonable function (PUF) [3] or secret key generation (SKG) for end-to-end security [4], [5]. For both, SDM and PLS, the main challenge is to recover the spatial amplitude and phase contributions of each channel at the receiver-side in real-time, e.g. by digital signal processing (DSP). With increased number of SDM channels the computational effort in DSP-based channel diagonalization arises.
The increased demand of data services within the last few years has prompted a commensurate explosion in transportable bandwidth, most of which contain sensitive information, personal data, bank accounts, credit card numbers, proprietary documents, and more. As a result, there are two types of security in communications, securing user data and securing the physical network. The first is addressed with encryption algorithms, the unbreakability of which does not depends so much on the sophistication of the algorithm but on the difficulty to break it using a sequential computer; that is, it capitalizes on the inability of the attacker to use a fast supercomputer. The second is not a well addressed topic, particularly in modern fiber-optic networks. One attempt to create an unbreakable cipher-text is quantum cryptography that emanated from quantum mechanics; this method capitalizes on the no-cloning principle of particular characteristics of photons, such as polarization. In this paper, we examine the security of advanced optical networks, fiber-optic and free space optical (FSO). We describe quantum cryptography and the quantum key distribution, including the principle of teleportation, we identify vulnerabilities and we discuss detection mechanisms and countermeasure strategies against attacks on the physical network.