Wavelength Division Multiplexing Signals Research Articles

Multi-beam, WDM-based photonic beamformer with spectrum reuse.

This manuscript presents a wavelength-division multiplexing (WDM)-based photonic beamformer for an RF phased array antenna transmitter, capable of simultaneously generating multiple beams using the same optical spectrum. In the proposed architecture, for each RF beam, a WDM signal comprising the modulated RF sidebands undergoes complex-valued filtering, while another WDM signal with the same channels, but carrying only optical carriers, goes through an optical frequency-shifting stage. The proposed architecture allows the same WDM channels to be reused for multiple RF beams. The detection of the frequency-shifted optical carrier and the filtered RF sideband of each WDM channel at the photodetector produces a frequency-converted, correctly weighted signal to be fed to each antenna element. The features described herein are analytically derived, numerically simulated, and experimentally demonstrated. Results showcase two independent beams being transmitted in different directions.

Design and Characterization of Dispersion-Tailored Silicon Strip Waveguides toward Wideband Wavelength Conversion

One of cost-effective ways to increase the transmission capacity of current standard wavelength division multiplexing (WDM) transmission systems is to use a wavelength band other than the C-band to transmit in multi-band. We proposed the concept of multi-band system using wavelength conversion, which can simultaneously process signals over a wide wavelength range. All-optical wavelength conversion could be used to convert C-band WDM signals into other bands in a highly nonlinear fiber (HNLF) by four-wave mixing and allow to simultaneously transmit multiple WDM signals including other than the C-band, with only C-band transceivers. Wavelength conversion has been reported for various nonlinear waveguide materials other than HNLF. In such nonlinear materials, we noticed the possibility of wideband transmission by dispersion-tailored silicon-on-insulator (SOI) waveguides. Based on the CMOS process has high accuracy, it is expected that the chromatic dispersion fluctuation could be reduced in mass production. As a first step in the investigation of the broadness of wavelength conversion using SOI-based waveguides, we designed and fabricated dispersion-tailored 12 strip waveguides provided with an edge coupler at both ends. Each of the 12 waveguides having different widths and lengths and is connected to fibers via lensed fibers or by lenses. In order to characterize each waveguide, the pump-probe experimental setup was constructed using a tunable light source as pump and an unmodulated 96-ch C-band WDM test signal. Using this setup, we evaluate insertion loss, input power dependence, conversion bandwidth and conversion efficiency. We confirmed C-band test signal was converted to the S-band and the L-band using the same silicon waveguide with 3 dB conversion bandwidth over 100-nm. Furthermore, an increased design tolerance of at least 90 nm was confirmed for C-to-S conversion by shortening the waveguide length. It is confirmed that the wavelength converters using the nonlinear waveguide has sufficiently wide conversion bandwidth to enhance the multi-band WDM transmission system.

Unidirectional Ring-Based WDM Fiber Network for Both Downlink and Uplink Signal Access

In the paper, a dual-bidirectional ring-type wavelength-division-multiplexing (WDM) access network with downlink and uplink signal access simultaneously using a single fiber backbone in clockwise and counterclockwise directions, respectively. The proposed network architecture is simple and easy to implement via the designed remote node (RN) and optical line termination (OLT) modules, but it also can double the downlink traffic using the original WDM downlink wavelengths. The presented ring-type WDM network can also avoid the Rayleigh backscattering (RB) beat noise when the same wavelengths are applied as downlink and uplink channels concurrently. In the measurement, 50 km long-reach and 15 km short-reach fiber transmission lengths are achieved for the symmetrical 10 and 28 Gbit/s on-off keying (OOK) data access, respectively. In addition, based on the obtained power budgets of eight downlink WDM signals and network design at the forward error correction (FEC) threshold, 16 optical network units (ONUs) can be supported simultaneously.

Experimental study of four-wave mixing based on a quantum dot semiconductor optical amplifier

The article is devoted to the study of four-wave mixing (FWM) in a semiconductor optical amplifier with quantum dots (QD-SOA). Experimental measurements of FWM signals are characterized in nonlinear media of GaAs-based QD-SOA with gain in the 1,550 nm region. Theoretical part of nonlinear effect of FWM is studied and important all-optical communication applications are listed. Experimental studies of a four-wave system are described and an analysis of FWM signals is given for various input powers of pump signals and injection currents. The devices are compared in terms of such parameters as conversion efficiency and signal-to-noise ratio. The results of the study made it possible to reveal the possibility of the effect of FWM signals on useful signals in channels with spectral division multiplexing wavelength division multiplexing (WDM) in the downstream and upstream in a semiconductor optical amplifier. Based on the experimental results, it was concluded that FWM does not affect adjacent channels of WDM signals and does not generate additional optical noise when scaling the WDM gigabit passive optical network (GPON) up to 60 km using semiconductor optical amplifiers (SOAs).

Compensation of the Distorted WDM Signals by Symmetric Dispersion Map with Nonuniform Zero-Crossing Place of Accumulated Dispersion in Midway-OPC System

The nonlinear Kerr effect and chromatic dispersion are the fundamental causes of optical signal degradation in single-mode fiber (SMF) and erbium-doped fiber-amplification (EDFA)-based wavelength division multiplexing (WDM) transmission. Dispersion management combined with a midway optical phase conjugator among the technologies for compensating for such optical signal distortion is known to not be limited by the modulation format and multiplexing technology. Optimization of the dispersion map can partially alleviate the capacity and maximum transmission distance limitations of the SMF and EDFA system. In this paper, we propose various types of symmetric dispersion maps in which the position of zero-crossing place of the cumulative dispersion is not constant, and analyze the effect of each dispersion map configuration on 40 Gb/s × 24-channel WDM signal distortion compensation. When designed with the residual dispersion per span (RDPS) around 400 ps/nm, it is confirmed that most of the proposed dispersion maps are more effective in compensating the distorted WDM signal than conventional dispersion map. In particular, we confirm that, among the proposed dispersion maps, the dispersion map in which the RDPS is designed uniformly for all fiber spans can increase the power margin of WDM channel and expand the range of the total residual dispersion in the dispersion-managed link.

Effect of Gain Saturation on Wideband WDM Signal in PPLN-Based Optical Parametric Amplifier

The periodically poled LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (PPLN) based optical parametric amplifier (OPA) is a technology that contributes to the construction of multi-band wavelength-division multiplexing (WDM) optical networks thanks to its wide amplification bandwidth and wavelength conversion function. In addition, the fast response of the OPA means it is unaffected by sudden changes of wavelength channels due to dynamic channel add-drop. However, such fast response leads to nonlinear signal distortion when operating in the gain saturation region. This paper experimentally investigates the conversion of gain saturation into the nonlinear amplitude distortion in a 64-Gbaud 64QAM WDM signal. We characterized the input power tolerance of a PPLN-based OPA in wideband WDM applications and found that the nonlinear amplitude distortion can be suppressed by increasing the number of WDM channels. For a 75-GHz-spacing 64-channel WDM configuration, the total input power tolerance was improved by ∼10 dB compared to a single-channel case. We also confirmed that there is almost no wavelength dependence of the effect of gain saturation on the signal distortion. These results show that PPLN-based OPAs have a wide total input power range for amplifying wideband WDM signals in terms of the nonlinear distortion.

Multi-Wavelength Photonic Neuromorphic Computing for Intra and Inter-Channel Distortion Compensations in WDM Optical Communication Systems

Digital signal processing (DSP) has been widely applied in optical communication systems to mitigate various signal distortions and has become one of the key technologies that have sustained data traffic growth over the past decade. However, the strict energy budget of application-specific integrated circuit-based DSP chips has prevented the deployment of some powerful but computationally costly DSP algorithms in real applications. As a result, fiber nonlinearity-induced signal distortions impede fiber communication systems, especially in wavelength-division multiplexed (WDM) transmission systems. To solve these challenges in DSP, there has been a surge of interest in implementing neural networks-based signal processing using photonics hardware (i.e., photonic neural networks). Photonic neural networks promise to break performance limitations in electronics and gain advantages in bandwidth, latency, and power consumption in solving intellectual tasks that are unreachable by conventional digital electronic platforms. This work proposes a photonic recurrent neural network (RNN) capable of simultaneously resolving dispersion and both intra and inter-channel fiber nonlinearities in multiple WDM channels in the photonic domain, for the first time to our best knowledge. Furthermore, our photonic RNN can directly process optical WDM signals in the photonic domain, avoiding prohibitive energy consumption and speed overhead in analog to digital converters (ADCs). Our proposed photonic RNN is fully compatible with mature silicon photonic fabrications. We demonstrate in simulation that our photonic RNN can process multiple WDM channels simultaneously and achieve a reduced bit error rate compared to typical DSP algorithms for all WDM channels in a pulse-amplitude modulation 4-level (PAM4) transmission system, thanks to its unique capability to address inter-channel fiber nonlinearities. In addition to signal quality performance, the proposed system also promises to significantly reduce the power consumption and the latency compared to the state-of-the-art DSP chips, according to our power and latency analysis.

A Silicon-Based On-Chip 64-Channel Hybrid Wavelength- and Mode-Division (de)Multiplexer

An on-chip 64-channel hybrid (de)multiplexer for wavelength-division multiplexing (WDM) and mode-division multiplexing (MDM) is designed and demonstrated on a 220 nm SOI platform for the demands of large capacity optical interconnections. The designed hybrid (de)multiplexer includes a 4-channel mode (de)multiplexer and 16-channel wavelength-division (de)multiplexers. The mode (de)multiplexer is comprised of cascaded asymmetric directional couplers supporting coupling between fundamental TE mode and higher-order modes with low crosstalks in a wide wavelength range. The wavelength-division (de)multiplexers consist of two bi-directional micro-ring resonator arrays for four 16-channel WDM signals. Micro-heaters are placed on the micro-resonators for thermal tuning. According to the experimental results, the excess loss is &lt;3.9 dB in one free spectral range from 1522 nm to 1552 nm and &lt;5.6 dB in three free spectral ranges from 1493 nm to 1583 nm. The intermode crosstalks are −23.2 dB to −33.2 dB, and the isolations between adjacent and nonadjacent wavelength channels are about −17.1 dB and −22.3 dB, respectively. The thermal tuning efficiency is ∼2.22 mW/nm over one free spectral range.

Over 100 nm Bandwidth Orbital Angular Momentum Modes Amplification For MDM and WDM Transmission With a Ring-Core Bi/Er Co-Doped Fiber

We designed and fabricated a ring-core Bi/Er co-doped fiber (RC-BEDF), and thus investigated the broad-spectrum amplification of orbital angular momentum (OAM) modes. The deviation from the average gain of the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> modes enabled gains of <1.8 dB and <1.9 dB respectively, and the differential mode gain (DMG) between the two modes was <1.0 dB over 100 nm bandwidth (1525–1626 nm). In particular, the 3 dB bandwidth of the gain spectra exhibited 74 nm (1552–1626 nm) and 69 nm (1557–1626 nm), respectively. Furthermore, four beams of different wavelengths, i.e., 1525, 1560, 1590, and 1626 nm, were separately extracted from the broad spectra to detect the amplified modes, by which the interferograms of first- and second-order OAM modes were obtained. The amplified OAM mode purities of the wavelengths were all above 95%. Moreover, dual-channel combination of the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> mode at 1550 nm and OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mode at 1600 nm was amplified, which indicated realization of different-OAM-modes amplification with wavelength-division multiplexing (WDM). Herein, for the first time, we extended the C+L band OAM modes amplification to 1626 nm and achieved OAM multiplexed amplification of WDM signals. This ultra-broad-spectrum and ultra-low-DMG OAM amplification has been widely considered as a promising technology for long-haul mode division multiplexing and WDM fiber-optics communication systems and networks.

A Bidirectional Wavelength Division Multiplexed (WDM) Free Space Optical Communication (FSO) System for Deployment in Data Center Networks (DCNs).

Data centers are crucial to the growth of cloud computing. Next-generation data center networks (DCNs) will rely heavily on optical technology. Here, we have investigated a bidirectional wavelength-division-multiplexed (WDM) free space optical communication (FSO) system for deployment in optical wireless DCNs. The system was evaluated for symmetric 10 Gbps 16-quadrature amplitude modulation (16-QAM) intensity-modulated orthogonal frequency-division multiplexing (OFDM) downstream signals and 10 Gbps on-off keying (OOK) upstream signals, respectively. The transmission of optical signals over an FSO link is demonstrated using a gamma-gamma channel model. According to the bit error rate (BER) results obtained for each WDM signal, the bidirectional WDM-FSO transmission could achieve 320 Gbps over 1000 m free space transmission length. The results show that the proposed FSO topology offers an excellent alternative to fiber-based optical interconnects in DCNs, allowing for high data rate bidirectional transmission.

Beam Steering and Divergence Control Using Variable Focus Liquid Lenses for WDM FSO Communications

Optical beam steering and divergence control is a critical function in free-space optical (FSO) communications. Variable focus liquid lenses (VFLLs) can be used for simultaneous realization of beam steering and adaptive beam divergence control. Two off-axis VFLLs are employed to steer the optical beam in 2 dimensional directions, while one on-axis VFLL adjusts the beam divergence adaptively to channel conditions. This VFLL-based system has been studied only for single-wavelength FSO communications. In this letter, we explore the possibility of using the VFLL-based system for wavelength-division-multiplexed (WDM) FSO communications. For this purpose, we investigate the wavelength dependence of VFLL-based optical beam steering and divergence control through experiment and theory. We also demonstrate the transmission of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times 10$ </tex-math></inline-formula> -Gb/s WDM signals over a 104-m FSO link.

Wideband PPLN-Based Phase-Sensitively Amplified Transmission of 20-Channel 96-Gbaud WDM Signal

A periodically poled LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (PPLN)-based phase-sensitive amplifier (PSA) has the potential of ultra-low-noise and high-gain optical amplification. For the wideband operation of PSAs in optical fiber transmission, the chromatic dispersion (CD) that causes gain ripples is required to be compensated at the PSA input. We proposed a method to accurately estimate CD from the gain characteristic of a PSA. Estimated CD can be compensated by feed backing it to a spatial light modulator-based CD controller. This paper demonstrates a 2-THz wavelength-division multiplexing (WDM) single-span transmission using a PPLN-based PSA with CD pre-equalization based on our proposed method. The 20-channel WDM signal was modulated with single-polarized 96-Gbaud PS-64QAM, and its net data rate was 400 Gbps/channel. 2-THz signal bandwidth (4-THz bandwidth including an idler band) was the widest bandwidth in the fiber transmission using the PSA. We also discuss the system design of phase-sensitively amplified links for a high-symbol-rate and wideband WDM signal.

Optically powered 5G WDM fronthaul network with weakly-coupled multicore fiber.

The number of base stations (BSs) for the fifth generation (5G) wireless network is substantially increased, as each coverage is greatly reduced. Therefore, both the miniaturization and the densification of BSs suffer from the challenges of electrical power supply and deployment cost. Here, we present an optically powered 5G fronthaul network, in support of the co-propagation of spatial-division-multiplexing (SDM) energy light and wavelength-division-multiplexing (WDM) 5G new radio (NR) signals over the weakly-coupled multicore fiber (WC-MCF). When the 60-W energy light at 1064.8-nm is equally distributed among the outer six cores, and the 9-Gbit/s 5G NR WDM signals are transmitted over the central core of 1-km WC seven-core fiber (WC-7CF), we can collect total 11.9-W electrical power at the remote node, for the purpose of optically powered small cells. Meanwhile, the error-vector magnitude (EVM) values of 1.5-Gbit/s 5G NR 64-level quadrature amplitude modulation orthogonal frequency division multiplexing (64QAM-OFDM) signals at the central frequency of 3.5 GHz fluctuate within a range of 0.3%∼0.39%, under a received electrical power of -25 dBm, for all six-wavelength channels. Six optically powered small cells are equipped with the characteristics of centralized management and flexible access-rate.

Learning-based digital back propagation to compensate for fiber nonlinearity considering self-phase and cross-phase modulation for wavelength-division multiplexed systems

We propose a learning-based digital back propagation (LDBP) technique that addresses self-phase modulation (SPM) and cross-phase modulation (XPM) of a wavelength-division multiplexed (WDM) optical signal. The LDBP has a structure defined for each of the individual channels of a WDM signal, and connections between the channels address the XPM to effectively compensate for nonlinear waveform distortion of the signal in long-distance optical transmission systems. We attempt to optimize the limited number of parameters used in the structure such as the dispersion, nonlinear coefficient, and walkoff parameter to compensate for the nonlinear phase shift induced by the XPM. We derive equations to update the parameters through an iterative process based on the stochastic gradient descent algorithm. We verify the effectiveness of the proposed LDBP technique through a transmission experiment that uses an 11-channel WDM, 32-Gbaud, dual-polarization 16 quadrature amplitude modulation (QAM), and probabilistically shaped (PS) 64QAM signals. With the focus on the LDBP with a 1-step/span configuration, we operate the learning process using the received 16QAM signal in the experiment. We confirmed the successful convergence of particular parameters of the model of the transmission line after the developed learning procedure. We apply the LDBP with fixed optimized parameters to the received waveforms of the PS-64QAM signals and compare the performance with some DBPs. We observed that the proposed LDBP technique that considers XPM with 1-step/span configuration exhibits the best performance in compensating for nonlinear waveform distortion. Additionally, the learning process is effective for the case considering both SPM and XPM compared with the case of SPM only. Finally, we investigate the computational complexity of the LDBP and reveal that the total calculation cost is of the same order as that of a conventional DBP considering only SPM with a 2-step/span configuration.

Frequency-Packed Multiband-Coherent Transceiver With Symbol Rate-Adaptive Nyquist WDM Signals

In the next-generation flexible point-to-multipoint optical networks such as optical access systems, it is desirable for one transceiver to handle various Internet-of-Things traffic streams. A symbol rate-adaptive multiband-coherent-transceiver, that can transmit and receive wavelength division multiplexing (WDM) signals arranged at the Nyquist spacing after Nyquist spectrum shaping, achieves flexibility by changing the number of subbands (SB) and the symbol rate per SB. Band-asymmetric coherent access systems with wide bandwidth optical transceivers on the optical-line-terminal side and narrowband optical transceivers on the optical-networking-unit side have been reported for uplink and downlink transmissions. We also examined the multiband-coherent-transceiver for ultra-dense WDM signals with a narrow analog-device bandwidth and narrow SB spacing to improve cost and spectral efficiency. To the best knowledge of the authors, there is no other research on a Nyquist spectrally shaped WDM-passive optical network system that applies these two functions and flexibly changes the symbol rate per SB with a SB spacing less than the Nyquist spacing.

Pre-Filtering Techniques for Spectrum Narrowing Caused by Optical Node Traversal in Ultra-Dense WDM Networks

To improve the spectral efficiency and increase network capacity, wavelength signals must be multiplexed densely, i.e., ultra-dense wavelength-division-multiplexing (WDM) networks. However, the spectrum of each WDM signal is narrowed by the wavelength-selective switches placed in optical nodes, so the transmissible distance and node hop count of the signal are strictly limited. To counter this spectrum narrowing problem caused by optical node traversal in ultra-dense WDM networks, pre-filtering techniques have been proposed. This paper comprehensively investigates the performance of four pre-filtering techniques: conventional Nyquist filtering, pre-emphasis filtering, partial-response filtering, and the combination of pre-emphasis and partial-response filtering. We conduct extensive computer simulations and discuss the optimality of pre-filtering techniques. The simulation results show that the adaptive use of pre-filtering can substantially extend the maximum attainable transmission distance and hop counts of 4/16/64-QAM signals.

Physical Layer Encryption for WDM Optical Communication Systems Using Private Chaotic Phase Scrambling

Protecting the security of optical network is a major worldwide challenge. The conventional schemes for securing communications are based on cryptography at the MAC layer and its higher layers, which are facing a great threat with the development of quantum computers. Optical encryption is a promising way for building security on the physical layer of optical communications. In this work, we propose a novel optical encryption scheme for wavelength division multiplexing (WDM) fiber-optic communication systems, by utilizing the private chaotic phase scrambling. Secure transmission of 50 Gbps signal over 50-km standard single-mode fiber is experimentally and numerically demonstrated. The results show that the WDM signal is efficiently encrypted into a noise-like signal, due to the spectral broadening effect of chaotic phase scrambling. Since the processes of encryption and decryption are totally implemented in the optical domain, the proposed scheme can encrypt the entire network traffic with low latency and high speed. Moreover, the proposed encryption scheme is compatible with the existing WDM optical networks, and only one pair of encryption and decryption devices is sufficient to encrypt all WDM channels, thus the scheme can be easily implemented at low hardware cost.

Performance analysis of WDM-SDM system with employing Phase-Conjugated twin waves technique

In this paper, the phase conjugated twin waves (PCTWs) technique is proposed to reduce the nonlinear phase noise in wavelength division multiplexing (WDM) - spatially division multiplexing (SDM) system. In the proposed system, the m-ary quadrature amplitude modulation (mQAM) signals are generated and multiplexed to produce mQAM WDM signal. Similarly, the second WDM signal is created by multiplexing the phase conjugated copies of the mQAM signals and then they are spatially multiplexed by two fiber links. At receiver, they are firstly demultiplexed and then superimposed coherently to revoke the nonlinear phase noise and improve the signal to noise ratio (SNR). Moreover, the analytical model that best describes the phase noise alleviation is developed. Analytical results show that the proposed scheme significantly reduces the variance of the phase noise. Furthermore, to show the effectiveness of proposed scheme, 16-channel 4QAM WDM-SDM system with and without PCTWs method is numerically investigated. Numerical results reveal that the PCTWs scheme enhances the SNR of the received signal by more than 2.7 dB and extends the achievable transmission distance by 39.4% at 10 −5 bit error rate (BER).

40 Gsymbol/s channel-based Nyquist wavelength division multiplexing communication in a terahertz-band using optical-domain reception signal processing

Nyquist wavelength division multiplexing (WDM) using a 40 Gbit/s channel signal is applied to terahertz (THz)-wave communication with a view to increasing its spectral efficiency and capacity. A 2 × 40 Gbit/s Nyquist WDM signal in the THz-band is both generated and demultiplexed by the assistance of optical technology. The optical-domain demultiplexing is adopted mainly due to the difficulty in both the direct THz-wave-domain and the subsequent radio-frequency (RF)-domain demultiplexing. The received THz-wave Nyquist WDM signal at an antenna is divided into two signals, which are down-converted into RF signals with heterodyne detection utilizing different frequency local sinusoidal waves. Each signal in the RF-domain is again transformed into an optical signal through an optical modulator, and each 40 Gbit/s channel is extracted from one of the generated sidebands at each optical modulator output with an optical Nyquist filter. Bit error rates (BERs) on the order of 10−5 and 10−7 (below the first KP4 forward error correction threshold) were obtained for the demultiplexed two 40 Gbit/s channels. In addition to the explanation of the experimental setup and operating principle of the optical-domain demultiplexing method for the THz-wave Nyquist WDM signal, some experimental results are reported to show the validity of our proposed demultiplexing method and factors to limit the BERs.

Detecting WDM visible light signals by a single multi-color photodiode with MIMO processing

For the first time, to the best of our knowledge, we experimentally demonstrate that multiple-input-multiple-output (MIMO) processing allows using a single photodiode to detect simultaneously a wavelength-division multiplexing (WDM) visible light communications (VLC) signal. The photodiode has a triple junction, and when it is illuminated by a WDM signal, the junctions produce inherently three photocurrents that are unusable for detecting any of the WDM signals. However, by means of linear MIMO processing, we are able to recover the transmitted signals exactly. Bit error rate measurements confirm the effectiveness of the proposed solution. This opens a new scenario for practical WDM-VLC systems.