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Abstract Optical layer attacks on optical fiber communication networks are one of the weakest reinforced areas of the network, allowing attackers to overcome security software or firewalls when proper safeguards are not put into place. Encrypting data using a random phase mask is a simple yet effective way to bolster the data security at the physical layer. Since the interactions of the random phases used for such encryption heavily depend on system properties like data rate, modulation format, distance, degree of phase randomness, laser properties, etc., it is important to determine the optimum operating conditions for different scenarios. In this work, assuming that the transmitter and the receiver have a secret pre-shared key, we present a theoretical study of security in such a system through mutual information analysis. Next, we determine operating conditions which ensure security for 4-PSK, 16-PSK, and 128-QAM formats through numerical simulation. Moreover, we provide an experimental demonstration of the system using 16-QAM modulation. We then use numerical simulation to verify the efficacy of the encryption and study two preventative measures for different modulation formats which will prevent an eavesdropper from obtaining any data. The results demonstrate that the system is secure against a tapping attack if an attacker has no information of the phase modulator and pre-shared key.

Visible light communication (VLC) has emerged as a viable supplement to existing radio frequency (RF) communication systems. The limited modulation bandwidth of sources is one of the significant challenges in the multiuser (MU) VLC systems, which prohibits transmission at high data rates for each user. Since an array of LEDs is used to illuminate a room, a MU VLC system utilizing massive multiple-input multiple-output (mMIMO) orthogonal frequency-division multiplexing (OFDM) is proposed for achieving high data rates in this paper. Since the distances of the multiple transmitter-receiver links are different, their temporal delays are also different, resulting in complex channel gain and phase differences when transformed to the frequency domain in the OFDM technique. Complex channel vectors associated with different users are mutually spatially orthogonal as the number of transmitters increases. Therefore, inter-user interference (IUI) can be eliminated with simple linear signal processing, and more users can simultaneously communicate in the same time-frequency resource. In this paper, three linear precoding methods, including the maximum ratio transmission (MRT), the minimum mean square error (MMSE), and the zero-forcing (ZF), are investigated for VLC systems based on proposed MU-mMIMO-OFDM in intensity-modulation and direct-detection link. We evaluate and compare the performance of the aforementioned precoding methods in terms of the achievable spectral efficiency and total downlink optical power for different scenarios. Moreover, we derive a closed-form expression for the lower bound on the average achievable sum-rate, which we confirm the accuracy of the derived expression with the numerical simulation. The results demonstrate that the performance of the proposed method in this paper for all precoder techniques, i.e., ZF, MMSE, and MRT, is better than the previous works since the complex channel in the frequency domain is used in this paper. Furthermore, the ZF and MMSE methods are better than MRT when the ratio of the number of LEDs to the number of users is large. At the same time, when this ratio is relatively small, the MRT precoding technique outperforms the ZF and MMSE techniques.

Abstract Mode‐division multiplexing (MDM) using few‐mode fibre (FMF) has received increasing attention to address the exponential growth of data traffic in long‐haul optical communication systems. Also, combining the MDM with wavelength‐division multiplexing (WDM) is a promising approach for dramatically growing the transmission capacity in such systems. However, a major barrier in this regard is the FMF nonlinear effects, which can significantly reduce the link performance. In this paper, in order to alleviate the FMF nonlinear effects, we focus on power allocation in FMF links by optimizing the input power of each optical WDM channel of each spatial mode, which leads to maximizing the total capacity transmission and also the minimum signal to noise ratio (SNR) margin. The FMF nonlinearity has been already modelled as the Gaussian noise (GN) for which no closed‐form formulation has been developed so far. Here, we derive a closed‐form GN model for this problem and verify it by comparing with the integral‐form GN model and split‐step Fourier method. In this approach, an optimal power is independently determined for each channel of each mode by optimizing a capacity maximization and a minimum SNR margin maximization problem in convex forms. The performance of different links including the single mode fibre‐WDM, MDM‐single channel, and MDM‐WDM are compared using computer simulations. These systems are comprehensively investigated in equal/non‐equal required SNR as well as flat/non‐flat amplifier gain scenarios. It is shown that optimized power allocation to each channel of each mode has a significant enhancement in the minimum SNR margin maximization scheme compared to the best equal power allocation. Furthermore, this improvement is much more in non‐equal required SNR and the non‐flat amplifier gain scenarios, showing the efficiency of the established approach in practical communication links.

Achieving reliable communication over different wavelength channels and modes is one of the main goals of Mode Division Multiplexing-Wavelength Division Multiplexing (MDM-WDM) transmission. The reliability can be described by the minimum Signal to Noise Ratio (SNR) margin which depends on launch power, the gain of Few-Mode Erbium-Doped Fiber Amplifiers (FM-EDFA), and the nonlinear impairments of Few-Mode Fiber (FMF). In this paper, we develop the Enhanced Gaussian Noise (EGN) nonlinear model for FMF, which can be used in both weak and strong coupling regimes. We validate the model by comparing simulation results with those obtained through the Split-Step Fourier Method. Based on our proposed EGN model, we address the problem of joint optimized power and gain allocation based on minimum SNR margin maximization when accounting for practical FM-EDFA constraints such as saturation power and maximum gain. The problem is solved using a convex optimization approach and considering different scenarios such as the best equal power, optimized power, and joint optimized power and gain. Results demonstrate that the minimum SNR margin improvement for the joint optimized power and gain allocation compared to the best equal power allocation is $1.4~dB$ and $1.7~dB$ for MDM-single channel and single-mode fiber-WDM systems, respectively.

Grating couplers (GCs) are important for light to couple between optical fibers and photonic integrated circuits (PICs). The Bound State in Continuum (BIC) waveguides offer new functionalities for PIC. High-Q resonators, electro-optical modulator, and photodetectors can be built on it without etching steps. Due to the special waveguide geometry of BIC waveguides, a high efficiency GC is still missing. In this work, we demonstrated a subwavelength grating (SWG) GC for BIC waveguide with a coupling efficiency of 8 dB and a 1 dB bandwidth >70 nm. The device shows ultra-broadband behavior which is extremely useful for BIC-based PICs.

Wavelength-division multiplexing over optical links provides an effective solution for the bandwidth challenge of off-chip and on-chip communications. We demonstrate a compact silicon photonic four-channel optical add-drop multiplexer enabled by subwavelength-grating-based contradirectional couplers. Passbands of the device show on-chip insertion losses below 1.8 dB with wide 3 dB bandwidth of ~6.7 nm suitable for coarse wavelength-division Multiplexing in short-reach optical interconnect applications. Transmission of 10 Gb/s data stream through different channels of the multiplexer results in negligible power penalties, whereas interferometric crosstalk-induced power penalties are below 2.8 dB.

The nonlinear optical loop mirror (NOLM) has been studied for several decades and has attracted considerable attention for applications in high data rate optical communications and all-optical signal processing. The majority of NOLM research has focused on silica fiber-based implementations. While various fiber designs have been considered to increase the nonlinearity and manage dispersion, several meters to hundreds of meters of fiber are still required. On the other hand, there is increasing interest in developing photonic integrated circuits for realizing signal processing functions. In this paper, we realize the first-ever passive integrated NOLM in silicon photonics and demonstrate its application for all-optical signal processing. In particular, we show wavelength conversion of 10 Gb/s return-to-zero on-off keying (RZ-OOK) signals over a wavelength range of 30 nm with error-free operation and a power penalty of less than 2.5 dB, we achieve error-free nonreturn to zero (NRZ)-to-RZ modulation format conversion at 10 Gb/s also with a power penalty of less than 2.8 dB, and we obtain error-free all-optical time-division demultiplexing of a 40 Gb/s RZ-OOK data signal into its 10 Gb/s tributary channels with a maximum power penalty of 3.5 dB.

We demonstrate photonic generation of chirped microwave pulses (CMPs) using an integrated distributed Fabry-Pérot cavity (DFPC) in silicon-on-insulator as a spectral shaper. The DPFC is based on a pair of spatially offset chirped Bragg gratings and has a response with varying free spectral range. The spectral shaper is used to filter broadband pulses from a mode-locked fiber laser, and CMPs are obtained at the output of a photodetector following a wavelength-to-time mapping process in a dispersive medium. We generate CMPs with a central frequency of up to 20 GHz, a radio-frequency (RF) chirp of 0.012 GHz/ps, and a time-bandwidth product of 14.5.

We demonstrate alternate wavelength switching in a widely tunable, dual-wavelength Tm3+-doped fiber laser operating at 1900 nm. A comb filter based on a Sagnac loop incorporating a length of high-birefringence fiber is used to define the lasing wavelengths. By simple variation of a polarization controller within the laser cavity, alternate and sequential switching between single- and dual-wavelength output, as well as tunable operation over a range of 70 nm, is obtained.

We demonstrate the all-optical reconfigurable time slot interchange (TSI) of individual bits at 40 Gb/s based on four-wave mixing (FWM) and a programmable planar lightwave circuit (PLC). The PLC is used to generate different control signals (masks) that determine which bits undergo TSI. By programming the PLC to generate two different masks, two different TSI patterns are obtained. TSI is achieved using FWM between the data signal and the desired mask with bidirectional propagation in a length of highly nonlinear fiber. Error-free operation is obtained for both of the TSI patterns, with a power penalty -9.

We demonstrate RF arbitrary waveform generation based on microwave photonic filtering. We use four-wave mixing in a silicon nanowire to increase the number of taps in an N-tap microwave photonic filter (MPF). Using a programmable optical filter, we can control the tap weights and, hence, the MPF response and corresponding generated waveform. We show uniform and apodized waveforms with four taps and seven taps with tunable central frequencies.

We review recent results on exploiting microwave photonics to enable simultaneous interrogation of multiple fiber Bragg grating (FBG)-based sensors. In particular, we describe the use of (1) microwave photonic filtering and (2) chirped microwave pulse generation and compression as a means to map the wavelength (spectral) changes in the response of FBG-based sensors (specifically, an in-fiber Fabry-Pérot cavity sensor based on FBGs, FBG sensors directly, and a linearly chirped FBG sensor) to applied temperature (or strain) to the power of a radio-frequency signal (i.e., a wavelength-to-power mapping) or to the correlation peak of the compressed microwave signal. The approaches support high-resolution and high-speed interrogation and can be suitable for large scale sensing networks.

All-optical signal processing based on nonlinear optical effects allows for the realization of important functions in telecommunications including wavelength conversion, optical multiplexing/demultiplexing, Fourier transformation, and regeneration, amongst others, on ultrafast time scales to support high data rate transmission. In integrated photonic subsystems, the majority of all-optical signal processing systems demonstrated to date typically process only a single channel at a time or perform a single processing function, which imposes a serious limitation on the functionality of integrated solutions. Here, we demonstrate how nonlinear optical effects can be harnessed in a mode-selective manner to perform simultaneous multi-channel (two) and multi-functional optical signal processing (i.e., regenerative wavelength conversion) in an integrated silicon photonic device. This approach, which can be scaled to a higher number of channels, opens up a new degree of freedom for performing a broad range of multi-channel nonlinear optical signal processing functions using a single integrated photonic device.

We demonstrate rational harmonic mode-locking of a fiber optical parametric oscillator (FOPO) in which the parametric gain serves as an all-optical mode-locker. A pulse train with a repetition rate from 10 to 30 GHz is generated using a 10-GHz optical clock signal. The generated pulses have a pulsewidth of ~ 9 ps, a root-mean-square (RMS) timing jitter of ~ 300 fs, and a wavelength tunable to more than 20 nm.

Microwave photonic (MWP) systems exploit the advantages of photonics, especially with regards to ultrabroad bandwidth and adaptability, features that are significantly more challenging to obtain in the electronic domain. Thus, MWP systems can be used to realize a number of microwave signal processing functions including, amongst others, waveform generation and radio-frequency spectrum analysis (RFSA). In this paper, we review recent results on fiber and integrated approaches for simultaneous generation of multiple chirped microwave waveforms as well as multi-channel RFSA of ultrahigh repetition optical rate pulse trains.

We propose and demonstrate a novel architecture for four-wave mixing (FWM)-based recognition of spectral amplitude code (SAC) labels in optical packet-switched networks. With a proper code design, a unique FWM idler for each SAC label, referred to as a label identifier (LI), is generated in a nonlinear medium. A serial array of fiber Bragg gratings is then used to reflect the LI wavelengths. Each LI is associated with a unique amount of delay between two optical signals received at two photodiodes (PDs). Label recognition is then achieved by measuring this unique time delay (referred to as the characteristic delay). The main advantages of the proposed method include the following: no serial-to-parallel conversion is required, simple label extraction is achieved, variable-length packets are supported, and the number of PDs used in the label recognition module is reduced. Moreover, the LI wavelengths do not need to exhibit any periodicity or match a particular wavelength grid; this results in a less challenging code design with smaller spectral occupancy for label generation. An experiment is conducted, where two variable-length data packets are transmitted over a 50-km dispersion-compensated span and switched at a forwarding node. The SAC labels are successfully recognized, and we obtain error-free transmission for the switched packets with less than 0.3-dB penalty.