Wavelength Division Multiplexing Research Articles

All-optical reconfigurable optical neural network chip based on wavelength division multiplexing

Optical computing has become an important way to achieve low power consumption and high computation speed. Optical neural network (ONN) is one of the key branches of optical computing due to its wide range of applications. However, the integrated ONN schemes proposed in previous works have some disadvantages, such as fixed network structure, complex matrix-vector multiplication (MVM) unit, and few all-optical nonlinear activation function (NAF) methods. Moreover, for the most compact MVM schemes based on wavelength division multiplexing (WDM), it is infeasible to employ intrinsic nonlinear effects to implement NAF, which brings frequent O-E-O conversion in ONN chips. Besides, it is also hard to realize a reconfigurable ONN with coherent MVMs, while it is much easier to implement in WDM schemes. We propose for the first time an all-optical silicon-based ONN chip based on WDM by adopting a new adjustment mechanism: optical gradient force (OGF). The proposed scheme is reconfigurable with tunable layers, variable neurons per layer, and adjustable NAF curves. In the task of classification of the MNIST dataset, our chip can realize an accuracy of 85.13% with 4 full-connected layers and only 50 neurons in total. In addition, we analyze the influence of the OGF-based NAF under fabrication errors and propose a calibration method. Compared to the previous works, our scheme has the two-fold advantages of compactness and reconfiguration, and it paves the way for the all-optical ONN based on WDM and opens the path to unblocking the bottleneck of integrated large-dimension ONNs.

Closed-form formula for unequal channel allocation to reduce FWM crosstalk in DWDM long-haul networks

Dense wavelength division multiplexing (DWDM) is considered as a mainstream solution for 5G optical transmission networks to confront increasing bandwidth demand. In such a dense multiwavelength system, the four wave mixing (FWM) non-linear effect limits the achievable transmission distance and channel capacity. For this reason, and to the best of our knowledge, a new unequal channel allocation (UCA) has been introduced in this paper based on new closed-form equations to reduce the FWM crosstalk in the DWDM optical long-haul network. The proposed channel allocation has been investigated based on several design parameters that affect FWM nonlinear crosstalk, such as total transmission distance, channel frequency spacing, the number of WDM channels, and the input power per channel. The suggested technique has further been compared with a previous model proposed by other authors to prove the effectiveness of this work. The simulation results have been examined and validated by analysing the optical spectrum analyser and the power of the FWM spectral products. Results demonstrate the reliability and the efficiency of the developed method.

Temperature-insensitive and cost-effective distributed NP-Doped optical fiber sensors

This paper presents the development of a cost-effective distributed optical fiber sensor for temperature-insensitive assessment of mechanical disturbances along an optical fiber cable. The proposed sensor system uses a nanoparticle (NP)-doped optical fiber with enhanced Rayleigh backscattering to provide higher sensitivity and spatial resolution using the transmission and reflection analysis (TRA) approach, where the transmitted and backscattered optical powers are analyzed as a function of the mechanical disturbance. In addition, Fiber Bragg Gratings (FBGs) are used as wavelength filters to provide the wavelength division multiplexing of the proposed device, which enable the use of 3 different NP-doped optical fiber sections for simultaneous detection of multiple curvature conditions in a cost-effective distributed sensing approach. The sensor characterization tests are performed by means of applying curvature angles from 360° to 1080° at different positions along NP-doped fibers (namely 25 mm, 100 mm and 175 mm) at 4 different temperatures of 25 °C, 30 °C, 40 °C and 50 °C. The results indicate the feasibility of the proposed approach, where the temperature variations lead only to a wavelength shift of the Bragg wavelength, whereas the mechanical disturbances (the curvatures) lead only to variations in the transmitted and reflected optical powers. Thus, by analyzing the transmitted and reflected optical powers in conjunction with the Bragg wavelength shift, it is possible to estimate both the mechanical disturbance amplitude (i.e., curvature angle) and the position along each NP-doped optical fiber section. Results indicate a relative error of around 3 % and 4 % for the mechanical disturbance location and absolute value, respectively. Moreover, the temperature cross-sensitivity in this case is below 2 % considering both amplitude and location of the mechanical disturbance. The proposed approach can be applied in structural health monitoring of different types of structures by integrating the fibers in the structures themselves with the possibility of measuring the strain distribution along the fibers (instead of in different points along the fiber) using a lower cost hardware when compared with similar distributed optical fiber sensing approaches.

56 Gbps externally modulated widely tunable lasers with SOA boosters heterogeneously integrated on silicon

We demonstrate externally modulated widely tunable lasers co-integrated with semiconductor optical amplifiers (SOAs) heterogeneously integrated on silicon. The widely tunable laser enables continuous single-mode operation over a tuning range of approximately 40 nm, with a side-mode suppression ratio (SMSR) of at least 50 dB and an average waveguide-coupled optical power of 5 mW. The integrated electro-absorption modulator (EAM) exhibits an extinction ratio (ER) of 16 dB when reversed biased at -2 V. The bit-error-rate (BER) measurements conducted across the available optical bandwidth (15 nm) showcase error-free transmission at 32 Gbps using non-return-to-zero (NRZ) signals for the majority of wavelengths in a back-to-back (B2B) configuration. Additionally, transmission measurements over distances of up to 10 km through a standard single-mode fiber (SSMF) have been successfully demonstrated. Dynamic extinction ratio (DER) values exceeding 4.5 dB are achieved for all wavelengths. Open-eye diagrams were measured up to 56 Gbps. These results demonstrate that this compact mono-epitaxial externally modulated tunable laser with integrated optical amplification can be a cost-effective transmitter solution for dense wavelength division multiplexing (DWDM) metropolitan and access networks.

Performance analysis of a 400-Gbps DWDM-FSO system using advanced modulation formats and under adverse weather conditions

Free space optical (FSO) systems offer an attractive and cost-effective solution for providing communication services in remote regions, as they allow secure transmission without the need for licensing and with lower deployment costs. However, the performance of FSO systems can be significantly impacted by atmospheric turbulences, creating considerable challenges to their deployment. To meet the expanding bandwidth requirements in optical networks, dense wavelength division multiplexing (DWDM) has emerged as a viable option. The development of a 400-Gbps DWDM-FSO system with advanced modulation formats is the subject of this paper. To ensure efficient energy conservation in such a system, power consumption needs to be minimized while maintaining performance level; this calls for optimization of different components within the system. The system is made up of 10 channels and each channel can transmit data at 40 Gbps. Various modulation schemes like carrier-suppressed return-to-zero, modified duo binary return-to-zero, differential phase shift keying, and duo binary return-to-zero are studied for their impact on system performance parameters Q-factor and bit error rate (BER) in C-band around 1550 nm wavelengths. The assessment is also extended to the effects that changing FSO length, input power, and data rate have on these two parameters as well as an evaluation regarding how differing atmospheric conditions influence the FSO system’s effectiveness.

Integrating angled multimode interferometer with Bragg grating filters for coarse wavelength division (de)multiplexing with optimized shape factor

Abstract We propose and experimentally demonstrate an angled multimode interferometer (AMMI) integrated with cascaded phase-shifted Bragg gratings (PSBGs) for coarse wavelength division multiplexing (CWDM), based on a 400-nm-thick silicon nitride waveguide platform. Due to the design constraints, the spectral response of a standard AMMI filter suffers from the drawbacks of limited optical bandwidth, high inter-channel crosstalk, and non-ideal shape factor. Two techniques were used to improve performance. Firstly, it was found that increasing the input waveguide width of the AMMI is beneficial for the optical bandwidth while maintaining good insertion loss and crosstalk performance. Experimental results show that the 1 dB bandwidth increases from 7.6 nm to 9.7 nm. To achieve an improved shape factor and decreased crosstalk, the PSBG filters were utilized as the second-stage filters by integrating them at the output waveguides of the AMMI. Simulation results demonstrate an average crosstalk reduction of approximately 15.1 dB at the center wavelengths of the four channels and shape factor improvement by about 0.29. However, measurement results show an average crosstalk decrease of only about 5.5 dB, which is possibly due to the fabrication imperfection of the PSBGs and the unsatisfactory film quality of the SiN wafers used. Despite deviations between measured and simulated performance, the shape factor is increased from 0.26 to 0.74 using these proposed techniques, showing the feasibility of such an integration method.

All-fiber miniature non-contact photoacoustic probe based on photoacoustic remote sensing microscopy for vascular imaging in vivo.

Photoacoustic (PA) remote sensing (PARS) microscopy represents a significant advancement by eliminating the need for traditional acoustic coupling media in PA microscopy (PAM), thereby broadening its potential applications. However, current PARS microscopy setups predominantly rely on free-space optical components, which can be cumbersome to implement and limit the scope of imaging applications. In this study, we develop an all-fiber miniature non-contact PA probe based on PARS microscopy, utilizing a 532-nm excitation wavelength, and showcase its effectiveness in in vivo vascular imaging. Our approach integrates various fiber-optic components, including a wavelength division multiplexer, a mode field adaptor, a fiber lens, and an optical circulator, to streamline the implementation of the PARS microscopy system. Additionally, we have successfully developed a miniature PA probe with a diameter of 4 mm. The efficacy of our imaging setup is demonstrated through in vivo imaging of mouse brain vessels. By introducing this all-fiber miniature PA probe, our work may open up new opportunities for non-contact PAM applications.

Wavelength-division multiplexing (WDM) FSO communications with soliton microcombs

This study highlights the benefits of soliton microcombs (SMCs) in FSO communication systems. By employing a microring resonator coupled with an add/drop filter, we achieved soliton microcombs with a precise Free Spectral Range (FSR) of 1870 pm and a Full Width at Half Maximum (FWHM) of 60 pm, tailored for Wavelength Division Multiplexing (WDM) channels. The system utilized 100 lines for the WDM channel with an inter-channel spacing of 0.5 nm and PolSK modulation to encode the data. This combination achieved a bit error rate (BER) of 10 − 9 , significantly lower than the 10 − 7 observed with traditional systems. For performance simulation, the channel was characterized using the gamma-gamma model, which assessed the impact of atmospheric turbulence on data transmission. The results show a high channel capacity ( 10 3 Gbit/s) and a low loss probability ( 10 − 8 ), ensuring reliable data transmission even in the presence of atmospheric disturbances. This methodology provides substantial improvements in stability and efficiency for FSO networks.

W-band photonic-aided reconfigurable multichannel vector mm-wave transmission enabled by the digital subcarrier multiplexing technique.

As multimedia service demands rise, radio-over-fiber (RoF) systems necessitate a flexible wireless resource allocation in multi-user scenarios. Optical wavelength division multiplexing (WDM) can meet multi-user application scenarios but has the problems of high hardware cost and complexity. In this Letter, we propose a photonic-aided reconfigurable multichannel vector millimeter-wave (mm-wave) transmission scheme in the RoF system enabled by the digital subcarrier multiplexing (DSCM) technique. The scheme can flexibly adjust the number and frequency spacing of subcarriers as well as the bandwidth and modulation format allocated to each subcarrier, and it can be applied to multi-user scenarios without increasing the cost and complexity of hardware implementation. The off-line experiments verify the system transmission performance with different numbers, bandwidth, and frequency spacing of subcarriers, fully verifying the feasibility of the scheme. Furthermore, the Letter delves into the effects of the subcarrier number, bandwidth, and frequency spacing on overall system performance, highlighting the potential of this scheme for flexible and cost-effective wireless resource allocation in RoF systems.

Real-time demonstration of 64 × 200 Gbps UDWDM-PON downstream transmission based on silicon photonic integrated transceiver

Coherent detection, high-order modulation, and dense wavelength division multiplexing (DWDM) technologies provide solutions for increasing bandwidth demand of future access networks. We experimentally demonstrate a real-time 64 × 200-Gb/s coherent ultra-dense wavelength-division (UDWDM) coherent passive optical networks (PONs) at 75-GHz channel spacing. The demonstrated downlink transmission is realized with the dual-polarization QPSK (DP-QPSK) modulation scheme. The cost is reduced by using silicon photonic integrated coherent transceiver modules and wider grid AWGs. The power budget is evaluated when all 200 channels are launched at C band. The power budget is 26 dB after 20-km G.652D standard single mode fiber (SSMF) transmission without amplifier at the receiver side under soft-decision FEC (SD-FEC) threshold of 1 × 10−2, and can achieve 32.5 dB with pre-amplified semiconductor optical amplifier (SOA) used at the receiver side.

Second-Order Microring Filter with Large Free-Spectral-Range and Wavelength-Tunable-Range over 50 nm

The high-order microring filter has been proposed for a larger free spectral range (FSR) compared with the single microring filter; therefore, it has great potential to be used in wavelength division multiplexing (WDM) systems. In this article, we have designed and fabricated a second-order microring filter made up of two rings connected in series with two Ti thermal heaters deposited above them. The large FSR of 56.8 nm is obtained by decreasing the difference of the radii between the two series rings, achieving similar FSRs to that of higher-order filters but with a simpler and more compact design. The average electrical tuning efficiencies of the two heaters are 0.186 nm/mW and 0.207 nm/mW, and the center wavelength of the filter can be tuned over the entire FSR with an applied electrical power of less than 40 mW.

All-fiber spatial and wavelength gain-flattening of few-mode EDFA via mode selective coupler

The mode division multiplexing (MDM) technique together with wavelength division multiplexing (WDM) shows the great prospect to solve the capacity crunch of current standard single mode fiber (SSMF) transmission system. However, the modal and wavelength gain fluctuation fundamentally limit both the capacity and the reach of hybrid MDM-WDM transmission system. Here, we propose an all-fiber few-mode gain-flattening filter (FM-GFF) based on mode selective coupler (MSC), to effectively mitigate the gain fluctuation of few-mode erbium doped fiber amplifier (FM-EDFA) in both spatial and wavelength dimensions. With the help of genetic algorithm (GA), variable mode dependent loss spectra of cascaded MSCs can be efficiently obtained, according to the gain profile of the used FM-EDFA. Consequently, gain-flattening among all guided linearly polarized (LP) mode groups over the C-band can be realized. Simulation results reveal that, as for the spatial and wavelength gain flattening among 4/6-LP mode groups, only 5/7 cascaded MSCs are good enough to mitigate the modal and wavelength gain fluctuation from 9.89 dB and 11.79-dB to less than 0.38 dB and 0.46 dB over the C-band. Finally, we carry out experimental verification based on 2-LP mode groups gain flattening over C-band. When 3 cascaded MSCs are involved, the gain fluctuation of FM-EDFA over the C-band can be flattened from 2.46 dB to less than 0.96 dB.

MODELLING OF A TBPS SYSTEM FOR 5G WIRELESS COMMUNICATION UTILIZING DWDM RFoF

In recent times, several sectors and businesses have been doing extensive research on the usage of Dense Wavelength Division Multiplexing (DWDM) and Radio Frequency Over Fiber (RFOF). These two technologies are considered to be the most significant features. Increasing the data rate was a significant challenge that needed to be addressed, and the goal was to successfully implement a fiber optic system that was dependable and had a high number of associated channels. As a consequence of this, a 64-channel DWDM RFOF system that is capable of supporting a larger number of data rates of 2.56 Tbps has been designed and implemented in this study. A significant number of channels that have been sampled will be chosen for inquiry based on the characteristics of Quality Factor (QF) and Bit Error Rate (BER) that have been researched. This study will be carried out with the assistance of Optisystem software. These findings would be investigated at distances ranging from sixty to one hundred eighty kilometers, with the NRZ modulation format being used and a lunched power of zero decibels per meter. Additionally, the purpose of this study would be to explore the three distinct techniques of compensation, namely pre, post, and symmetrical, to quantify the individual performance of each approach on the suggested system. According to the findings, the use of symmetrical-based compensation yielded the most favorable outcomes, with the average QF produced falling within the range of (20.33-14.09) dBm over distances ranging from (60-180) kilometers. This demonstrates the dependability of the proposed system.

Hitless two-dimensional focal plane switch array based on Mach-Zehnder interferometer-embedded micro-ring resonators

Empowered by compact micro-ring (MRR) arrays, wavelength-division-multiplexing (WDM)-based focal plane switch array (FPSA) chip is a promising solution for light detection and ranging (LiDAR) due to its parallelism and high-level integration. However, an MRR channel with a shifting central wavelength may interrupt another channel by dropping an optical signal with an unexpected wavelength, which causes discontinuous beam scanning. To overcome this bottleneck, we propose and fabricate a hitless two-dimensional (2-D) FPSA chip based on a 2 × 8 Mach-Zehnder interferometer-embedded micro-ring resonator (MZER) array. The FPSA chip realizes a beam divergence of 0.18° × 0.05°, a field of view (FoV) of 9.07° × 6.42° with single-ended emission, and a FoV of 10.75° × 6.42° with dual-ended field-of-view splicing technology. Besides, we demonstrate the hitless function of our FPSA chip by performing continuous wavelength sweeping and further applying it to a free-space optical communication link. The experimental results validate the feasibility of our proposed hitless FPSA chip, which efficiently prevents signal interruptions during beam steering.

Quantum dot fourth-harmonic colliding pulse mode-locked laser for high-power optical comb generation

Quantum-dot mode-locked lasers have advantages such as high temperature stability, large optical bandwidth, and low power consumption, which make them ideal optical comb sources, especially for wavelength-division multiplexing (WDM) telecommunications, and optical I/O applications. In this work, we demonstrate an O-band quantum dot colliding pulse mode-locked laser (QD-CPML) to generate optical frequency combs with 200 GHz spacing with maximum channels of 12 within 3 dB optical bandwidth. To achieve the high output power of individual comb lines, four channel conditions are implemented at central wavelength of 1310 nm for WDM transmission experiments. Each channel exhibits more than 10 dBm output power with 200 Gb/s PAM-4 and 270 Gb/s PAM-8 modulation capability via thin-film LiNbO3 Mach–Zehnder interferometer modulator without the requirement of any optical amplifications. This high-order QD-CPML is an ideal comb source for power-efficient optical interconnects and large bandwidth optical data transmission.

Design and simulation of an enhanced optical demultiplexer using photonic crystal resonators

The purpose of this paper is to examine the design, simulation, and optimization of a demultiplexer optical system using photonic crystal resonators. Utilizing the distinct properties of photonic crystals, the proposed device achieves high-quality resonances, efficient wavelength separation, and minimal crosstalk. These characteristics are crucial for enhancing the performance of dense wavelength division multiplexing (DWDM) systems, which require precise and efficient channel separation to manage increasing data traffic. Extensive simulations conducted with the RSoft CAD tool explore the impact of varying resonator parameters such as refractive index, geometric dimensions, and material composition on the device's performance. The results indicate substantial improvements in transmission efficiency and quality factor, with average values of 98.675% and 3131.125, respectively. Furthermore, the ability to fine-tune these parameters allows for the customization of the demultiplexer to specific wavelength ranges, making it highly adaptable for various telecommunications applications. This research aims to propel the development of compact, high-performance photonic devices that meet the growing demands of modern communication networks, ultimately contributing to more efficient and reliable optical communication systems.

Development of a Technique for Mitigating Four-Wave Mixing Effect in Dense Wavelength Division Multiplexing System Using DP-QPSK with Channel Spacing

A Dense Wavelength Division Multiplexing (DWDM) system is a vital component in the optical network for the transmission of high data over a long distance. However, the non-linear effect of Four-Wave Mixing (FWM), one of the critical non-linear effects, limits the channel capacity and the amount of data that the DWDM system transmits. Intensity Modulation formats and Channel Shuffling algorithms being used to overcome the impact of FWM in DWDM are unable to sufficiently suppress the FWM effect due to limited capacity. This research, therefore, examined the mitigation of the FWM effect in the DWDM system using Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) and channel arrangement. The DP-QPSK was designed and implemented using 100 GHz for both Equal Space Channel Allocation (ESCA) and Unequal Space Channel Allocation (USCA). It was analysed on a 64-channel DWDM system operating at 100 Gbps capacity over a transmission distance of 1000 km. The DP-QPSK-based ESCA and USCA techniques were simulated using OptiSystem 17 simulation software and Python for data analysis. The performance of the system was evaluated using bit error rate (BER), optical signal-to-noise ratio (OSNR) and Electrical Constellation Diagram. Validation was done with other advanced modulation formats: Quadrature Phase Shift Keying (QPSK) and Differential-Quadrature Phase Shift Keying (D-QPSK). The DWDM system using DP-QPSK gave better performance due to the highest OSNR and BER values obtained when compared with ESCA-based QPSK and D-QPSK techniques. The technique can be used to suppress the effect of FWM in optical communication systems.

Performance analysis of nonlinear crosstalk of WDM systems using modulation schemes criteria

Abstract Nonlinearities in optical fibers are regarded as the most significant barriers that endanger the effectiveness of the optical transmission system and pose a threat to communication quality. Four-wave mixing (FWM) is one of the most important nonlinear effects that greatly reduces the wavelength-division multiplexing (WDM) system performance at high data rates over extended transmission distances. This research examines, and assesses, numerically, the behavior of a 4-channel, 40 Gbps WDM system under the effect of the FWM under various tuning parameters, including dispersion, input power, and wavelength spacing. The system model was built using OptiSystem software, and then three different modulation formats, namely, Non-return-to-zero-frequency shift keying, Return-to-zero frequency shift keying, and differential phase shift keying (DPSK) are used to assess the FWM power penalty. The results demonstrate that the FWM power penalty obtained with 1 nm wavelength separation in the DPSK method is dramatically reduced to −35 dBm. This study also demonstrates that when power variation is taken into consideration, the DPSK modulation scheme delivers a lower bit error rate in comparison to other modulation schemes.

Tunable narrow band-rejection and band-pass filter based on long-period waveguide grating on a thin-film lithium niobate rib waveguide

We propose and demonstrate experimentally an electro-optic (EO) and thermo-optic (TO) tunable wavelength filter with band-rejection and band-pass dual-function. Our proposed filter is based on a long-period waveguide grating (LPWG) formed on a lithium niobate on insulator (LNOI) rib waveguide with a channel-shaped polymer cladding waveguide. The LPWG formed on the surface of the LNOI core enables efficient mode coupling between the two fundamental modes of the LNOI waveguide and the polymer cladding waveguide and hence dual-function filtering. For the z-/x-polarized input light, our fabricated best filter shows an 18.1-dB/8.2-dB band-rejection contrast and a 15.7-dB/17.4-dB band-pass contrast at 1565.26/1594.24 nm wavelength, with a relatively narrow 3-dB bandwidth of 2.5/2.3 nm, respectively. In addition, the fabricated filter also shows an EO tuning efficiency of ∼32 pm/V and a TO tuning efficiency of 0.465 nm/°C, respectively. Our proposed filter could find applications in wavelength division multiplexing, temperature sensing, and other fields of optical communication and optical information processing.

Experimental investigation of high-speed WDM-visible light communication using blue, green, and red InGaN µLEDs.

Micro-light-emitting diodes (µLEDs) hold significant promise for applications in displays and visible light communication (VLC). This study substantiates the viability of a wavelength division multiplexing (WDM)-VLC system using InGaN blue, green, and red µLED devices. The devices exhibited notable color stability and high modulation bandwidth due to the weakly polarized electric field in the blue and green semipolar devices and the stress-optimized structure in the red device. The aggregated data rate reached 11.14 Gbps. Moreover, the blue, green, and red InGaN µLEDs exhibited a wide color gamut, encompassing 119.4% of the NTSC and 89.2% of the Rec. 2020 standards, affirming the potential of blue, green, and red InGaN µLEDs for applications in full-color display and WDM-VLC systems.