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The quantum key distribution (QKD) research, which is drawing attention as the next secure communication, is actively expanding from point to point system to network architecture. In QKD network system, it is important to increase the number of users who can securely communicate. Up to date, a wavelength division multiplexing (WDM) architecture has successfully expanded the number of channels without significant system loss, but there is a limitation of increasing channels considering the range of telecommunication wavelength and crosstalk noise, etc. In this article, we propose a polarizing division method that increases user channels independently of wavelengths. The proposed architecture can increase the number of wavelength multiplexed channels by a multiple of the polarization number. We identify the issues in the QKD network system that may occur when using polarization and wavelengths simultaneously, then provide solutions and optimize the system operation accordingly. Finally, we describe a field test result of a one to many QKD network system that shows successful key exchange with 3% QBER.

This paper presents an investigation of the electromagnetic signature and the coupling mechanism of quadcopter drones with incident electromagnetic (EM) wave and radar cross section (RCS) analysis. Coupling analysis is performed based on the dominant coupling path: when an incident EM wave with a magnitude of 50 kV/m contacts a commercial quadcopter drone, its motor power wires are identified as the dominant coupling path. Higher coupling voltages are obtained for frequencies that have large impedance values at both ends of the load on the motor power wire. This induced voltage can affect the integrated circuit chip on a printed circuit board, as well as parallel plate resonances. Furthermore, the RCS of a quadcopter drone is measured in the frequency range of 0.5-3 GHz. The internal-component vulnerability characteristics of quadcopters can spike at specific frequencies with high RCS values and can be analyzed with or without motor power wires. We verified these hypotheses via 2D inverse synthetic aperture radar images, and we analyzed the results by comparing the empirical and full-wave simulation values.

We have implemented a practical plug-and-play measurement-device-independent quantum key distribution (MDI QKD) system with polarization division multiplexing (PDM). MDI QKD is known as a secure protocol that can inherently prevent detection loopholes. Significant efforts have been made toward implementing MDI QKD systems. Recently, a plug-and-play architecture has been proposed for implementing the MDI QKD system, and its feasibility has been experimentally verified in both free space and fiber channel. However, in order to apply it to the real world, it is necessary to develop a more practical architecture including multiplexing methods and self-compensation techniques. In this paper, we have proposed and implemented a practical plug-and-play MDI QKD architecture that can be implemented regardless of whether the quantum channel lengths of Alice and Bob are symmetric or asymmetric using PDM and can be operated even under ambient-temperature-changing environments through an optical path-length self-compensation technique. Experimentally, we have achieved $6.25\times 10^{-6}$ bits per pulse as the key rate and the quantum bit error rates under 3% on 25-km quantum channels.

We propose a practical perfect absorber comprising three dielectric layers for prism coupling with monolayer graphene embedded in the middle layer. Since the loss rate can be controlled by the graphene position within the middle layer, the limitation on the material index choice is greatly relieved, overcoming the problem of the previously studied similar structure. It is shown that F2 glass-silica-quartz structure enables perfect absorption if the silica layer thickness and the graphene position are properly chosen. The proposed structure can adopt a thin layer of any absorbing material for perfect absorption owing to the adaptive loss rate feature.

We numerically investigated the effect of an optically ultrathin top metal grating in an absorber structure composed of an amorphous silicon (a-Si) thin film and a metal reflector for efficient solar cells. The ultrathin metal grating-based structure shows nearly polarization-insensitive net absorption due to a great improvement for TE polarized wave, which is about 2.5 times enhanced compared with a relatively thick metal grating case. In particular, it is also shown that the absorption angle of the proposed solar cell is as wide as 80° owing to light coupling into an active layer through a leaky mode of flat dispersion characteristic for TM polarization.

A simple tool for evaluating the stability of the finite-element beam propagation method (FEBPM) for lossy waveguides is described. Current stability issues are explained by this method. The sufficient stability condition of the FEBPM is described. The stability of the analytical propagation function, the Crank-Nicolson propagation scheme, and various Padé approximants are analyzed using the tool. The dependence of the stability of the FEBPM on the propagation step size is also noted. [ABSTRACT FROM AUTHOR]

The general nonlinear behavior of multiple cascaded resonators is analyzed by using the coupled-mode theory in time and then compared to a single resonator from the viewpoint of optical bistable operation. Our analysis reveals that from the perspective of switching intensity, the multiple resonator structure is more advantageous than the single resonator with an equivalently increasing quality factor. It is also shown that a switching intensity-response time product, suggested as a performance metric for optical bistable devices, decreases linearly as the number of cascaded resonators increases. The effect of coupling n resonators is equivalent to a single resonator device with n-time increased quality factor in terms of the performance metric. The coupled-mode theory analysis is confirmed by a numerical study of the bistable operation of one-dimensional photonic crystal resonator structures using the finite-difference time-domain method. [ABSTRACT FROM PUBLISHER]

In this paper, a general methodology for the design of higher order coupled resonator filters in photonic crystals (PCs) is presented. In the proposed approach, the coupling between resonators is treated as though it occurs through a waveguide with an arbitrary phase shift. The coupling through the waveguide is analyzed theoretically, based on the coupled-mode theory in time. The derived theoretical model suggests a way to extend an equivalent circuit approach, previously demonstrated with a certain value of a phase shift, to the higher order filter design with an arbitrary phase shift. The validity of the proposed approach is confirmed by the design of a third-order Chebyshev filter having a center frequency of 193.55 THz, a flat bandwidth of 50 GHz, and ripples of 0.3 dB in the passband. The characteristics of the designed filter are suitable for wavelength-division-multiplexed (WDM) optical communication systems with a 100-GHz channel spacing. The performance of the designed filter is numerically calculated using the two-dimensional (2-D) finite-difference time-domain (FDTD) method. [ABSTRACT FROM PUBLISHER]

In this paper, the formula of an optimal compensation vector, which is defined as an input polarization-mode dispersion (PMD) vector of the compensator, for second-order PMD compensation has been derived from the minimization of the root-mean-square (rms) pulse-broadening factor. The derived optimal compensation vector is a linear combination of frequency-averaged PMD vectors and output states of polarization, which shows a similar trend to the previously reported optimal first-order PMD compensation. The rms pulse-broadening factor after optimal second-order PMD compensation has been semianalytically calculated and compared with the previously reported simulation result. They are in good agreement. The formula of the optimal second-order compensation vector derived in this paper provides indispensable information for feedforward second-order PMD compensation. [ABSTRACT FROM PUBLISHER]

Discusses the theoretical modeling and fabrication of polarization-insensitive semiconductor optical amplifiers that use a multi-quantum-well structure as the gain media. Effects of strain on the band structure of quantum wells; Saturation characteristics of semiconductor optical amplifiers.

Bidirectional optical cross connects (BOXCs) using a single arrayed waveguide grating router and tunable fiber Bragg gratings (FBGs) have been proposed for multiwavelength bidirectional WDM ring networks. Two types of structures in fold-back and loop-back configurations have been considered and their feasibility has been demonstrated experimentally. The performances of two proposed structures have been investigated and compared in terms of crosstalk characteristics and backscattering suppression capability. The performance of the fold-back configuration is less vulnerable to the crosstalk of the arrayed waveguide grating (AWG) router. On the contrary, the BOXCs in loop-back configuration suppress backscattered signals effectively without using optical bandpass filters and, thus, have good scalability and are more cost effective. Therefore, the choice between two structures will depend on the crosstalk requirement in the BOXCs and the characteristics of the AWG router. In transmission of 2.5-Gb/s signals, the power penalty of 0.5 dB has been observed at BER of 10-9 in both structures. Expansion of the proposed 2×2 structure to N × N adopting multistage structure has been discussed. The limitation on the maximum scale due to spectral bandwidth narrowing has been considered [ABSTRACT FROM PUBLISHER]

This paper proposes a dense wavelength division multiplexing (DWDM)-based fiber-to-the-curb (FTTC) access network architecture for broad-band services. The reaches of four carrierless amplitude phase (4-CAP)-modulated very high-speed digital subscriber line (VDSL) signals over the hybrid-fiber twisted pair (HFTP) were simulated. The simulation results show that the effect of noise from the optical link is negligible as long as the signal-to-noise ratio (SNR) of the optical link output signal is 9-10 dB greater than the required SNR of a VDSL modem. The bit error rate (BER) of the VDSL transmission over HFTP was measured. Measurement results show that insertion of the optical link between the modem and twisted pair has no adverse effect on the transmission [ABSTRACT FROM PUBLISHER]

A new family of space/wavelength/time spread three-dimensional (3-D) optical codes for optical code-division multiple-access (OCDMA) networks has been proposed. Two types of 3-D codes have been constructed: 3-D codes with single pulse per plane and 3-D codes with multiple pulses per plane. Both codes are based on the prime sequence algorithm and have shown improved performance compared to the previously proposed two-dimensional (2-D) prime code. Effective implementation of the 3-D code has also been proposed. In order to eliminate the requirement of fiber ribbons and multiple star couplers in space/wavelength/time spread 3-D code based optical networks, a wavelength2/time scheme has been suggested, in which the periodic property of an arrayed waveguide grating (AWG) is used. It has been shown that the system performance can be maximized for given resources with a proper choice of the wavelength2/time scheme. Due to the improved performance of the 3-D code and the effective architecture of the wavelength2/time scheme, the feasibility of the OCDMA network is much enhanced [ABSTRACT FROM PUBLISHER]

We report the theoretical modeling and the fabrication of polarization-insensitive optical amplifiers at 1300 nm in AlInGaAs-InP material system. Gain calculations, using the k.p method, show that the introduction of 0.33% tensile strain into a three-quantum-well structure can achieve gain-matching over a wide energy spectrum. The amplifiers, fabricated and tested, show excellent polarization insensitivity (less than 0.3 dB) at 1280 nm with a gain of 11 dB at 150 mA. Gain-bandwidth needs to be improved by employing antireflection coatings to suppress the facet reflectivity [ABSTRACT FROM PUBLISHER]

We propose bidirectional fiber-optic code division multiple access networks based on the multiwavelength optical code, and the feasibility of the network is demonstrated experimentally. Encoders/decoders are implemented with AWG routers and fiber delay lines, and incoherent light sources are used. Due to the loop-back configuration of the AWG router-based encoder/decoder, bidirectional transmission is possible in the proposed access network. Decoding of broad-spectrum encoded signals (Δf=1.6 THz, Δλ=12.8 nm at 1550 nm) after transmission over a 40-km fiber link was performed with dispersion compensation [ABSTRACT FROM PUBLISHER]

For compact optical code-division multiple access (CDMA) networks, a novel cyclic optical encoder/decoder based on an arrayed waveguide grating (AWG) router is proposed. The proposed encoder/decoder treats multiple codewords of the two-dimensional Reed-Solomon code simultaneously using the cyclic property of the AWG router and the code, and multiple subscribers can share the encoder/decoder in networks. With the proposed encoder/decoder, optical CDMA networks become compact and cheap, since the number of encoders/decoders required in the network is reduced. The feasibility of the proposed encoder/decoder is demonstrated experimentally [ABSTRACT FROM PUBLISHER]