WDM Channels Research Articles

Enhancing transmission reach: performance analysis of WDM RoF PON OFDM system design with varied QAM schemes

Abstract Radio over fiber is upcoming and maturing technology in terms of security, reliability, and coverage for long-distance optical-wireless communication systems. The nonlinear behavior and dispersive nature of optical channels are performance restrictive factors limiting the long reach in such systems. Along with enhancing channel capacity and bandwidth for radio signals over a long distance, the use of OFDM-based such systems can overcome various performance degrading effects like phase modulation, electrical power attenuation, chromatic dispersion, and multipath fading. Here, we propose our design of an 8 channels WDM RoF PON system using the OFDM scheme. Performance analysis is carried out in terms of performance comparison for various modulation schemes like 8, 16, 128, 256, and 512 QAM levels applied on information for long transmission reach of a system up to 500Kms. 8 channels WDM system model is developed to analyze the performance at a higher data rate. System performance is evaluated with Q factor and EVM level parameters of recovered signals at various transmission distances.

Machine learning model based on the time domain regular perturbation-based theory for performance estimation in arbitrary heterogeneous optical links

To meet the required high demands for the capacity of optical networks, there are several efforts in recent years to further reduce the system margin. To achieve this goal, a fast and reliable QoT estimation tool is needed. The key module of such QoT tool is the nonlinear interference variance estimation. This paper presents a novel machine learning tool to speed up the exact model by six orders of magnitude without scarifying the accuracy and the scalability of this semi-analytical model. Moreover, to further enhance the scalability of the model, we used the cross-correlation functions to re-write the equations. The proposed ML-based framework KerrNet, based on a bank of small ANNs, can handle any arbitrary heterogeneous link up to ten thousand km composed of different fiber span. The transmitting C-band WDM channels in both fully loaded and sparsely occupied configurations are evaluated. The crucial steps for the machine learning algorithm to converge, which are the data preparation and the choice of training data, are presented in detail.

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.

Minimization of dispersion and non-linear effects in WDM based long-haul high capacity optical communication systems

Abstract Chromatic dispersion and nonlinear impairments pose challenges in high-capacity optical transmission systems, leading to signal distortions, channel interference and diminished the transmission performances. This paper explores the utilization of dispersion compensating fibers (DCF) using different schemes to mitigate the dispersion that accumulates along the length of a fiber in a 16 × 10 Gbps high-capacity long haul WDM system over a range of 500 km. When different wavelengths of light pulses are transmitted through an optical fiber, they experience varying speeds due to the refractive index’s wavelength dependency. As a result, the light pulses become temporally spread out as they travel through the fiber, and this dispersion persists throughout the fiber’s length. Considering non-linear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM), this paper analyze and compare the three compensation schemes of DCF in a 16 channel WDM system. Moreover the impact of non-linearities based on fiber length are also analyzed and compared. The results are evaluated by comparing metrics such as Q-factor, bit error rate (BER) and eye height. The analysis concludes that the symmetrical compensation scheme of DCF outperforms the pre- and post-compensation scheme. This finding suggests that the symmetrical compensation method offers a promising solution for high-capacity access networks, providing high spectral efficiency, cost-effectiveness, and improved flexibility.

Joint time and frequency transfer through one International Telecommunication Union 100 GHz wavelength division multiplexing channel with commercial devices.

We proposed a joint time and frequency transfer scheme over a single International Telecommunication Union 100 GHz wavelength division multiplexing (WDM) channel using a normal commercial WDM device and commercial offset WDM device. A standard 100 GHz WDM channel is divided into three sub-channels with a frequency interval of more than 20 GHz for a time and frequency transfer, which could help to avoid the interference among time, frequency, and data signals in other WDM channels. A joint high-precision time and frequency transfer is, therefore, able to be performed with data transmission over WDM optical communication links without extra requirements on devices. A joint time and frequency transfer in a single 100 GHz WDM channel is experimentally demonstrated over a 60 km fiber link with the communication data transmission in the adjacent channels. The stability of the time transfer can be better than 15 ps at 1 s, and the stability of the frequency transfer can be better than 2.7×10-14 at 1 s, while the bit error rates of the adjacent channels are at the same level as the separate transmission.

Optimization of 16 wdm channel of 40 gbps optical link

The demand for higher capacities in fifth-generation networks and upcoming generations can be met with technology such as Wavelength Division Multiplexing (WDM). A study has been conducted to enhance the performance of a system comprising of sixteen light waves that carry high data rates of 40 Gbps for each data source associated with one of the wavelengths. The key focus of this study was to optimize the system parameters, including channel spacing, power levels, and dispersion compensation, to increase the signal-to-noise ratio and reduce signal degradation, which would be compatible with high data transfer speeds. The aim of this work is to improve the efficiency and performance of the proposed system. Using the simulation program OptiSystem, the proposed optical communications system based on 16 WDM channels for the 40 Gbps optical link was improved. Several parameters were adjusted to improve the performance of the proposed system. By carefully treating variables that can significantly affect system performance and striving to maximize the signal-to-noise ratio, minimize signal degradation, including dispersion compensation strategies, and optimize power levels. This work can determine the ideal configuration for the 16 WDM channels by evaluating and comparing the performance of different optimization strategies to obtain better results compared to previous work. The results show the extent of the impact of the optimization process on improving the system performance, which makes the proposed system suitable for communications systems in the next generations.

Enhanced Pre/Post Compensation of Fiber Link Nonlinear Distortion by a Stepwise Parameter Profiled Fiber and Phase Conjugation Module

Compensation of nonlinear distortion in long distance communication links from the power induced Kerr effect is demonstrated at the transmitter or receiver by the combination of optical phase conjugation (OPC) and propagation in a stepwise parameter profiled fiber (SPF). The SPF tailored with an increasing nonlinearity and decreasing dispersion parameter on average with distance as well as Raman gain induces a distortion more closely matching that of the link to improve propagation symmetry for enhancing overall distortion cancellation by OPC. Measurements show for a module inserted at the transmitter or receiver with a 60 km long SPF comprising of three different fibers, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q²</i> -factor is improved by 2.3 and 1.7 dB, respectively, compared to without for transmitting 5 channel WDM dual polarization 12 Gbaud 16-QAM signals in 2×160 km fiber spans that both include dispersion compensation. Similarly, recirculating loop transmission of the same signal over 18×80 km (1440 km) shows improved <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q²</i> -factors of 1.9 and 1.3 dB, respectively. This corresponds to extending the transmission reach by 125 and 80%, respectively, for matched performance at equal launch power, highlighting the greater nonlinear distortion tolerance achievable with advanced OPC designs.

Temperature-insensitive and fabrication-tolerant coarse wavelength division (de)multiplexing on a silica platform using an angled multimode interferometer.

Wavelength division (de)multiplexing (WDM) device is a crucial component for optical transmission networks. In this paper, we demonstrate a 4 channel WDM device with a 20 nm wavelength spacing on silica based planar lightwave circuits (PLC) platform. The device is designed using an angled multimode interferometer (AMMI) structure. Since there are fewer bending waveguides than other WDMs, the device footprint is smaller, at 21 mm × 0.4 mm. Owing to the low thermo-optic coefficient (TOC) of silica, a low temperature sensitivity of 10pm/°C is achieved. The fabricated device exhibits high performance of an insertion loss (IL) lower than 1.6 dB, a polarization dependent loss (PDL) lower than 0.34 dB, and the crosstalk between adjacent channels lower than -19 dB. The 3 dB bandwidth is 12.3∼13.5 nm. Moreover, the device shows a high tolerance with a sensitivity of central wavelength to the width of multimode interferometer < 43.75 pm/nm.

Demonstration of 5.12-Tbps THz-over-fiber transmission in 80 channel WDM system

Demonstration of 5.12-Tbps THz-over-fiber transmission in 80 channel WDM system

Asymmetry Optimization for 10 THz OPC Transmission over the C + L Bands Using Distributed Raman Amplification.

An optimized design for a broadband Raman optical amplifier in standard single-mode fiber covering the C and L bands is presented, to be used in combination with wideband optical phase conjugation (OPC) nonlinearity compensation. The use of two Raman pumps and fiber Bragg grating reflectors at different wavelengths for the transmitted (C band) and conjugated (L band) WDM channels is proposed to extend bandwidth beyond the limits imposed by single-wavelength pumping, for a total 10 THz. Optimization of pump and reflector wavelength, as well as pump powers, allows us to achieve low asymmetry across the whole transmission band for optimal nonlinearity compensation. System performance is simulated to estimate OSNR, gain flatness and nonlinear Kerr distortion.

Performance analysis of an SDM-WDM multicore optical fiber link

Analytical evaluation of inter-channel and inter-core coupling induced crosstalk including Amplified Spontaneous Emission (ASE) noise in a single mode 7-core homogeneous multi-core optical fiber (MCF) based Space Division Multiplexed Wavelength Division Multiplexing (SDM-WDM) communication system with distributed Raman amplifier is presented. The Stimulated Raman Scattering (SRS) induced crosstalk impairment including ASE noise and inter-core and inter-channel coupling induced crosstalk at the output of any core with excitation into other core of the MCF is investigated. Signal to Crosstalk plus Noise Ratio (SCNR) is determined at the output of photo detector of the excited core. Finally, Bit Error Rate (BER) performance results are evaluated for different system parameters, viz. WDM channel number, channel spacing, propagation distance, pump power etc. It is noticed that BER deteriorates with SRS induced crosstalk and ASE noise, which is worsened further by the effect of inter-core crosstalk (IC-XT). The maximum acceptable propagation distance for a desired value of BER and other system parameters considering inter-channel crosstalk (Ich-XT), IC-XT including SRS induced crosstalk and ASE noise is also evaluated analytically. For example, to maintain a particular BER (10E-9, say) we need to increase the input signal power from 2.5dBm to 5.5dBm while increasing the WDM channels from 12 to 16 considering only the SRS and ASE noise. On the other hand, we need to increase the input signal power from 5.5dBm to 9.0dBm to increase the WDM channels from 12 to 16 when the IC-XT is added up with the SRS and ASE noise. The maximum allowable pump power considering the collective impact of Ich-XT and IC-XT including SRS and ASE noise is determined for a given value of BER (10E-9). Power penalty versus WDM channel spacing at a given BER value (10E-10), link distance and pump power at different WDM channels is also evaluated and presented.

Enhanced Compensation of Nonlinear Signal Distortion by Optical Phase Conjugation in Step-Parameter Profiled Fiber Links

Improving the efficacy of optical phase conjugation (OPC) for compensating nonlinear signal distortion in long distance optical fiber links from the Kerr effect is demonstrated. This is based on tailoring the link spans as step-parameter profiled fiber (SPF) for progressively increasing nonlinearity and decreasing dispersion with distance to mitigate the impact of propagation loss on unbalancing the phase distortion induced before and after OPC. With addition of a dispersion element after OPC, the SPF design also targets compensation in a shorter subsection of spans where induced nonlinear distortion is strongest. This enables performance benefit with less required parameter change in spans comprising just two or three segments of different regular fibers as 2-SPF and 3-SPF, respectively. Numerical simulations of 5 channel WDM dual polarization (DP) 32 Gbaud 64-QAM signals in a 12 × 130 km fiber link shows use of OPC and a 58 km 3-SPF in spans achieves 2.1 dB higher <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q²</i> -factor at 5 dB higher launch power than without. Similarly, measurements of 5 channel WDM DP 12 Gbaud 16-QAM signals in a 2 × 155 km link shows use of OPC and a 41 km 2-SPF in spans achieves 1.2 dB higher <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q²</i> -factor at 6 dB higher launch power and with low bit error rate compared to without. The improvements highlight the OPC advancements achievable by link design with regular fibers.

Investigating system performance in 16 channel WDM network at minimum attenuation window for pre-defined input noise level

Abstract Input and output optical signal-to-noise ratio in 16-channel WDM network are investigated for predefined input noise level when design is set at minimum attenuation window. By keeping the input OSNR constant, gain and noise figure are computed along with SNR at both transmitting and receiving terminals for two different data rates and also with short-to-long range of trans-receiver distance. Corresponding variations of all the network properties, i.e., maximum Q-factor, eye height and noise figure are measured for estimation of performance degradation. Optimization in terms of both bit rate and channel distance is required, as evident from simulation result, for best possible system performance. Setting up of input noise level at 20 dBm indicates the fact of real-time compatibility with optimum value of output OSNR is revealed at a particular distance, which suggests the best result for communication system design.

Nanophotonic Cavity Based Synapse for Scalable Photonic Neural Networks

The bandwidth and energy demands of neural networks has spurred tremendous interest in developing novel neuromorphic hardware, including photonic integrated circuits. Although an optical waveguide can accommodate hundreds of channels with THz bandwidth, the channel count of photonic systems is always bottlenecked by the devices within. In WDM-based photonic neural networks, the synapses, i.e. network interconnections, are typically realized by microring resonators (MRRs), where the WDM channel count ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$N$</tex-math></inline-formula> ) is bounded by the free-spectral range of the MRRs. For typical Si MRRs, we estimate <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$N \leq 30$</tex-math></inline-formula> within the C-band. This not only restrains the aggregate throughput of the neural network but also makes applications with high input dimensions unfeasible. We experimentally demonstrate that photonic crystal nanobeam based synapses can be FSR-free within C-band, eliminating the bound on channel count. This increases data throughput as well as enables applications with high-dimensional inputs like natural language processing and high resolution image processing. In addition, the smaller physical footprint of photonic crystal nanobeam cavities offers higher tuning energy efficiency and a higher compute density than MRRs. Nanophotonic cavity based synapse thus offers a path towards realizing highly scalable photonic neural networks.

Investigation of Inter-Satellite OWC Incorporating Dense WDM and Polarization Interleaving (PI)

Inter-satellite communication based on lasers (LIC) is a possible choice for future satellite networks because to ultra-high capacity and speed systems that may be achieved using compact, light-weight, and low-power optical equipment. The performance of a 64 × 40 GB/s optical wireless system with polarisation diversity for dense WDM is investigated in this research paper. Using a DWDM system with a channel spacing of 50 GHz, a high-speed system is achieved to meet the broad bandwidth needs of data services in satellites. A polarisation diversity approach is implemented in the system to decrease polarisation crosstalk and nonlinearities among nearby WDM channels. Furthermore, a comparison of the polarisation interleaved and traditional DWDM-OWC systems is performed. In addition, it was discovered that a system with varying states of polarisation across neighbouring channels performs better in terms of Q-factor and BER than a traditional system.

Investigation of FSO Using Polarization Diversity Under Clear Weather, Haze and Rain

Free space optical communication (FSO) is cost effective and potential technology for realization of transmission in hilly areas and for roof top communication in congested areas. In this work, a 32 channel WDM system employing polarization diversity technique is proposed in free space optical communication. Atmospheric turbulences plays vital role in system performance such as clear weather, haze and rain. Investigation has been done for different turbulences and effects are analysed. It is observed that system performs worst and better in case of rain and clear weather respectively.

New Lower Bounds on the Capacity of Optical Fiber Channels via Optimized Shaping and Detection

Constellation shaping is a practical and effective technique to improve the performance and the rate adaptivity of optical communication systems. In principle, it could also be used to mitigate the impact of nonlinear effects, possibly increasing the information rate beyond the current limit dictated by fiber nonlinearity. However, this appealing idea is frustrated by the difficulty of designing an effective shaping strategy that takes into account the nonlinearity and long memory of the fiber channel, as well as the possible interplay with other nonlinearity mitigation strategies. As a result, only little progress has been made so far, while the optimal shaping distribution and the ultimate channel capacity remain unknown. In this work, we describe a novel technique to optimize the shaping distribution in a very general setting and high-dimensional space. For a simplified block-memoryless nonlinear optical channel, the capacity lower bound obtained by the proposed technique can be expressed analytically, establishing the conditions for an unbounded growth of capacity with power. In a more realistic scenario, the technique can be implemented by a rejection sampling algorithm driven by a suitable cost function, and the corresponding achievable information rate estimated numerically. The combination of the proposed technique with an improved (non-Gaussian) decoding metric yields a new capacity lower bound for the dual-polarization WDM channel.

Wavelength dimension in waveguide-based photonic reservoir computing.

Existing work on coherent photonic reservoir computing (PRC) mostly concentrates on single-wavelength solutions. In this paper, we discuss the opportunities and challenges related to exploiting the wavelength dimension in integrated photonic reservoir computing systems. Different strategies are presented to be able to process several wavelengths in parallel using the same readout. Additionally, we present multiwavelength training techniques that allow to increase the stable operating wavelength range by at least a factor of two. It is shown that a single-readout photonic reservoir system can perform with ≈0% BER on several WDM channels in parallel for bit-level tasks and nonlinear signal equalization. This even when taking manufacturing deviations and laser wavelength drift into account.

On‐Chip Metamaterial Enabled Wavelength (De)Multiplexer

AbstractWavelength‐division multiplexing (WDM) technology can offer considerable parallelism for large‐capacity data communications. While several configurations have been demonstrated to realize on‐chip WDM systems, their practical applications might be hindered by large footprints or compromised performances. Recently, metamaterial‐assisted silicon photonics is emerging for on‐chip light manipulation by subwavelength‐scale control of optical wavefronts. They can reach more compact footprints and broadband functionalities beyond the classical waveguide‐based architectures. Herein, wavelength (de)multiplexers are experimentally demonstrated in the subwavelength‐structured metamaterials regime with highly compact footprints. Two‐dimensional metamaterials composed of quasi‐periodic dielectric perturbation arrays patterned on a multimode waveguide are proposed to stimulate multiple high‐order waveguide modes at different wavelengths, which are then coupled to different WDM channels by cascading mode (de)multiplexers. The four‐channel wavelength (de)multiplexer is demonstrated in a box‐like spectrum with a compact footprint of 2.5 × 250 µm2, with the measured losses less than 2 dB and channel crosstalk less than –14.3 dB. By varying the patterned metamaterial structures, the proposed devices also have the merits of flexible operating wavelengths and bandwidths. The concept features large scalability, compactness, and competitive performance, which can offer versatile on‐chip light manipulation and significantly improve the integration density for various on‐chip WDM optical systems.

Gas-pressure tuning of wavelength of photon pair emitted by Four-Wave-Mixing in Nanofibers

We present experimental results demonstrating the possibility to tune the wavelength of the photon pair emitted through four wave mixing in a nanofiber, using the pressure of a gas surrounding the nanofiber. Using Argon, a shift of idler wavelength of -1.1nm/bar is measured demonstrating fine adjustment possibility of emission wavelength, allowing to choose between different WDM channels.