Coarse Wavelength Division Multiplexing Research Articles

Silicon-Nanowire-Based 100-GHz-Spaced 16λ DWDM, 800-GHz-Spaced 8λ LR-8, and 20-nm-Spaced 4λ CWDM Optical Demultiplexers for High-Density Interconnects in Datacenters

Several types of silicon-nanowire-based optical demultiplexers (DeMUXs) for use in short-reach targeted datacenter applications were proposed and their spectral responses were experimentally verified. First, a novel 100-GHz-spaced 16λ polarization-diversified optical DeMUX consisting of 2λ delayed interferometer (DI) type interleaver and 8λ arrayed waveguide gratings will be discussed in the spectral regimes of C-band, together with experimental characterizations showing static and dynamic spectral properties. Second, a novel 800-GHz-spaced 8λ optical DeMUX was targeted for use in LR (long reach) 400 Gbps Ethernet applications. Based on multiple cascade-connected DIs, by integrating the extra band elimination cutting area, discontinuous filtering response was analytically identified with a flat-topped spectral window and a low spectral noise of <−20 dB within an entire LR-8 operating wavelength range. Finally, a 20-nm-spaced 4λ coarse wavelength division multiplexing (CWDM)-targeted optical DeMUX based on polarization diversity was experimentally verified. The measurement results showed a low excessive loss of 1.0 dB and a polarization-dependent loss of 1.0 dB, prominently reducing spectral noises from neighboring channels by less than −15 dB. Moreover, TM-mode elimination filters were theoretically analyzed and experimentally confirmed to minimize unwanted TM-mode-oriented polarization noises that were generated from the polarization-handling device. The TM-mode elimination filters functioned to reduce polarization noises to much lower than −20 dB across the entire CWDM operating window.

Ultra-compact and high-performance four-channel coarse wavelength-division (de)multiplexing filters based on cascaded Mach-Zehnder interferometers with Bezier-shape directional couplers

Using cascaded Mach-Zehnder interferometers (CMZIs) provides an attractive option for realizing coarse wavelength-division (de)multiplexing (CWDM) filters with low losses, low crosstalk, flat tops, and high scalability. However, they usually have large footprints and insufficient fabrication tolerances, due to the inferior performance of conventional directional couplers (DCs) used for MZIs. Here, a four-channel CMZI wavelength-division (de)multiplexer based on novel Bezier-shape DCs with compact footprints, broad bandwidths and decent fabrication tolerances. For the fabricated (de)multiplexer with 20-nm channel spacing, the excess loss is less than 0.5 dB and the crosstalk is lower than −19.5 dB in the 1-dB bandwidth of 12.8 nm. For the case with a core-width deviation of ±20 nm, the device still performs very well with low losses and low crosstalk. Compared to the state-of-the-art MZI-based CWDM filters, the present device has slightly high performances and a footprint of 0.012 mm2 shrunk greatly by ∼3-folds. This work can be extended for more channels and other material platforms.

High-precision magnetic field sensor based on phase-shift fiber loop ring-down technique and FBG-magnetostrictive structure

A high-precision fiber-optic magnetic field sensor based on the phase-shift fiber-optic loop ring-down (PS-FLRD) technique and the structure of fiber Bragg grating (FBG) bonding to a magnetostrictive rod is proposed and experimentally demonstrated. A coarse wavelength division multiplexer (CWDM) is inserted into the fiber loop and serves as an optical edge filter (OEF) to provide wavelength-dependent transmittance. Due to the linear response of the CWDM, the wavelength shift of the FBG induced by the external magnetic field is linearly converted to a transmittance change of the fiber loop, which is reflected in the phase shift. Consequently, a relationship between the phase shift and the magnetic field applied to the FBG is established. The experimental results show a good linear relationship between the phase shift and the magnetic field. For magnetic field strengths ranging from 0 to 880 Gs, the measured sensitivity is 0.0056 deg/Gs with a resolution of approximately 18 Gs. Furthermore, the proposed scheme demonstrates excellent stability and repeatability, remaining unaffected by power fluctuations, which provides a promising solution for magnetic field sensing.

Integrated Sensor-Optics Communication System Using Bidirectional Fiber and FSO Channels and Hybrid Deep Learning Techniques.

This paper introduces a new bidirectional integration approach that combines fiber sensor/free space optics (FSO) communication using an intensity and wavelength division multiplexer (IWDM) techniques-based long-distance fiber Bragg grating (FBG) sensor strain-sensing system. By implementing coarse wavelength division multiplexing (CWDM), the system achieves the simultaneous transmission of optical communication and fiber optical sensor (FOS) sensing signals, resulting in a highly capable, flexible, and cost-effective solution. The proposed FSO transmission technique addresses complex fiber cable installation concerns with topographical limitations. This bidirectional structure ensures the reliability and stability of the long-distance FBG sensor system, supported by extensive research and experimentation. A hybrid stacked gated recurrent units and long short-term memory (SGRU-LSTM) model is proposed to enhance strain measurement accuracy by predicting and measuring the central wavelength of overlapped strain-sensing FBG sensor signals. The results demonstrate the superiority of the proposed model in peak wavelength detection accuracy. The primary benefit of integrating communication and sensing is the significant reduction in construction costs by eliminating the requirement for two individual fiber optic systems, as the integration allows for a single system to fulfill both functions, resulting in more efficient and cost-effective implementation. Overall, this paper contributes to advancing long-distance FBG sensor systems by integrating fiber sensor/FSO communication and deep learning techniques, improving transmission distance, multiplexing capacity, measurement accuracy, system survivability, and cost-effectiveness.

Anti-reflection grating-assisted contra-directional coupler with large corrugation width

The grating-assisted contra-directional coupler (contra-DC) is one of the commonly used add-drop filters serving for coarse wavelength-division-multiplexing (CWDM) systems due to its flat-top and wide-band response. However, the free-spectral-range (FSR) of contra-DC is limited by the back reflection to the input port caused by the intra-waveguide coupling. In this paper we propose an anti-reflection grating-assisted contra-DC by simply increasing the sidewall corrugation width. The intra-waveguide coupling coefficient can be decreased to 0 when the corrugation width is increased to a certain value. The simulation result shows that the transmission at drop port is −0.42 dB, while the back reflection to the input port is only −22.0 dB. The 4-channel cascaded contra-DCs with wavelength range of ∼60 nm are also simulated, all the channels are not affected by the intra-waveguide coupling. We believe the proposed contra-DC with large corrugation width can be applied to the CWDM system requiring a broad spectrum range.

An improved curved grating demultiplexer based on silicon-on-insulator platform

This article proposes two basic principles to address the astigmatism problem of traditional Rowland gratings. Ray tracing method is employed to analyze the correlation between the optical performance of gratings and the structural parameters, and further, optimize the grating structure. Using the proposed principles, we design a 1×4 demultiplexer with an ultracompact footprint of 97 × 73 μm2, which is suitable of the coarse wavelength division multiplexing (CWDM) system, where the wavelength spacing is 10 nm. The numerical calculation results show that the channel crosstalk between the four channels of the device is less than −40 dB, the insertion loss is between −1.35 dB and −1.80 dB, and the channel uniformity is better than 0.45 dB. The structure proposed curved grating in this article offers broad applications in high-density integrated photonic circuits.

Silicon-Nanowire-Type Polarization-Diversified CWDM Demultiplexer for Low Polarization Crosstalk.

Coarse wavelength division multiplexing (CWDM)-targeted novel silicon (Si)-nanowire-type polarization-diversified optical demultiplexers were numerically analyzed and experimentally verified. The optical demultiplexer comprised a hybrid mode conversion-type polarization splitter rotator (PSR) and a delayed Mach-Zehnder interferometric demultiplexer. Si-nanowire-based devices were fabricated using a commercially available Si photonics foundry process, exhibiting nearly identical spectral responses regardless of the polarization states of the input signals under the PSR. The experiment demonstrated a low insertion loss of 1.0 dB and a polarization-dependent loss of 1.0 dB, effectively suppressing spectral crosstalk from other channels by less than -15 dB. Furthermore, a TM-mode rejection-filter-integrated optical demultiplexer was designed and experimentally validated to mitigate unwanted TM-mode-related polarization crosstalk that arose from the PSR. It exhibited an improved polarization crosstalk rejection efficiency of -25 dB to -50 dB within the whole CWDM spectral range.

Perencanaan Sistem Optical Transport Network (OTN) Pada Sistem Transmisi DWDM

In the era of rapid development of information and communication technology, modern society requires reliable and sophisticated means of communication. The required means of communication must be aimed at meeting current service needs, but also oriented towards meeting current and future service needs. To meet this need, a reliable network is needed, with a large bandwidth capacity with easy capacity additions, better performance, high levels of availability, and good flexibility. Fiber Optic Network is a network that is trusted to be able to handle this problem. Developments in this network were preceded by two types of transmission systems used, namely Plesiochronous Digital Hierarchy (PDH) and Synchronous Digital Hierarchy (SDH), and currently developing Dense Wavelength Division Multiplexing (DWDM) and Coarse Wavelength Division Multiplexing (CWDM) technologies.

5G Mobile Wireless Access and Digital Channeling with RF Over Fiber for Long-Haul 64-QAM Communication

This paper presents the experimental results of a novel radio-over-fiber (RFoF) architecture that is capable of transmitting data at 20 Gb/s over a 600 km fiber. Such high bit rates required by 5G systems for long-haul communication. This is accomplished by overcoming the limitation of single mode fibers (SMF) of power attenuation and chromatic dispersion. This is achieved by interspersing sections of the SMF with a dispersion compensating filter (DCF) and embedding midway of the fiber length with chirped fiber Bragg grating (CFBG). Moreover, the proposed approach is shown to enhance the total bandwidth over a long-haul gigabit passive optical network (GPON) using integrated channel CWDM. In the experiment, a 64-quadrature amplitude modulation (64-QAM) signal was modulated onto coarse wavelength division multiplexing (CWDM) signal. With the proposed RFoF architecture the signal power received over a 600 km fiber is greater than a conventional optical architecture by 19 dB for identical applied input optical power of 20 dBm. This translates to significant power savings that can be achieved with the proposed architecture over existing architectures that should reduce the carbon footprint of long-haul optical communications systems. The optical receiver is shown to have a sensitivity of −28.3 dBm for a bit-error rate (BER) of 10−9. These results demonstrate the viability of the proposed RFoF architecture in providing significant improvement in the bit rate over a transmission distance.

Monolithically Integrated Tunable Three- Section All-Active DBR Laser Diodes With 30 nm Tuning Range

We present a monolithically integrated three-section all-active AlGaInAs/InP tunable distributed Bragg reflector (DBR) ridge waveguide laser diode that uses thermal tuning mechanism and mode-hopping tuning principle to achieve wide-range wavelength tuning. It is similar to the traditional three-section DBR laser structure, including a gain section, a phase section, and a DBR grating section. The difference is that the phase section and the DBR section incorporate the same active quantum well structure as the gain section. In this case, weak effective refractive index changes of the three active sections affected by the current injection will occur since the carrier density is clamped by the threshold gain condition of the DBR laser diode. Despite this limitation, a noncontinuous tuning up to ∼30 nm (1564–1594 nm) can be observed using only current injection at a constant temperature of 25 °C. Significant thermal effects occurred during the tuning process. In addition, a discontinuity is introduced in the tuning because it does not contain a passive region for phase adjustment. However, the device still provides a stable single longitudinal mode output with nine channels, a wavelength spacing of about 4 nm, and a side mode suppression ratio (SMSR) of more than 30 dB. Furthermore, the DBR laser diode can be easily fabricated utilizing the conventional distributed feedback laser diode fabrication process without secondary epitaxy and heater metal. The all-active DBR laser diode will be expected to be applied in the coarse wavelength division multiplexing (CWDM) systems with the advantage of low cost.

Achromatic Broadband Multi‐Layer Diffraction Mode Multiplexing

AbstractEfforts have been made in recent years to achieve broadband mode‐division multiplexing (MDM) for wavelength and mode hybrid multiplexing optical interconnect. However, most multiplexers are suffering from either strong chromatic aberration or material dispersion, like planar lightwave circuits, mode selective couplers, and single‐layer diffraction elements. The multi‐layer diffraction structure provides a method for achieving both mode conversion and broadband properties. However, the reported multi‐layer diffraction MDM can usually only achieve high efficiency working at a specific narrow bandwidth due to the monochromatic coherent light design. Here, a general achromatic broadband multi‐layer diffraction model is proposed to compensate the dispersion by wavefront matching of discrete multi‐wavelength optical fields. Using this model, an achromatic orbital angular momentum beam generator and an achromatic linearly polarized mode multiplexer for coarse wavelength‐division multiplexing (CWDM) optical interconnections are designed. Compared with the monochromatic designs, the generator exhibits lower wavelength dependence and the multiplexer doubles the bandwidth with mode purity exceeding 91%. Additionally, signal transmission over a 5‐km fiber is demonstrated experimentally in the CWDM‐MDM hybrid communication system. This work represents a solid step toward practical implementation of achromatic MDM and may find applications in the design of various achromatic devices.

High speed radiation tolerant optical links based on coarse wavelength division multiplexing

The upgrade of CERN’s accelerator complex requires improved beam instrumentation systems that will generate an increased volume of data to be transferred from the radiation areas to the back-end. A solution that increases the throughput of already deployed fibers, with minimal increase of the system complexity, consists in implementing the coarse wavelength division multiplexing (CWDM) technique where independent optical carriers at properly spaced wavelengths are multiplexed into a single fiber. In this paper we describe the CWDM Link project, whose scope is to design and build a high-speed radiation-tolerant optical link based on four CWDM channels. We will also provide details of commercially-available CWDM components and their radiation resistance.

Momentum-Assisted Adjoint Method for Highly Efficient Inverse Design of Large-Scale Digital Nanophotonic Devices

Gradient-descent-based digitized adjoint method offers a way to realize the high-efficiency inverse design of digital nanophotonic devices with diverse functions. However, the vanishing gradient problem encountered in the design of high-dimension devices may lead to significant inefficiencies, making it difficult to integrate novel functions on a single chip. Here, we propose a highly efficient digitized adjoint method for large-scale inverse design, called adaptive gradient-descent with momentum. It uses the first- and second-order momentum, instead of the gradient, to update the device pattern during adjoint optimization. To demonstrate the efficiency of the proposed method, we design a coarse wavelength division multiplexer and a three-mode power divider with design dimensions of 800 and 1360, respectively, which are approximately 2-4 times that of conventional digital nanophotonic devices. The simulation results show that, compared with the conventional gradient descent method, the momentum-assisted adjoint method has about 4-6 times higher efficiency and obtains better optimization performance, which provides a powerful tool for the inverse design of novel digital nanophotonic devices.

Design and Analysis of an O+E-Band Hybrid Optical Amplifier for CWDM Systems.

Broadband amplification in the O+E-band is very desirable nowadays as a way of coping with increasing bandwidth demands. The main issue with doped fiber amplifiers working in this band such as the bismuth-doped fiber amplifier is that they are costly and not widely available. Therefore, a wideband and flat-gain hybrid optical amplifier (HOA) covering the O+E-band based on a parallel combination of a praseodymium-doped fiber amplifier (PDFA) and a semiconductor optical amplifier (SOA) is proposed and demonstrated through numerical simulations. The praseodymium-doped fiber (PDF) core is pumped using a laser diode with a power of 500 mW that is centered at a wavelength of 1030 nm. The SOA is driven by an injection current of 60 mA. The performance of the HOA is analyzed by the optimization of various parameters such as the PDF length, Pr3+ concentration, pump wavelength, and injection current. A flat average gain of 24 dB with a flatness of 1 dB and an output power of 9.6 dBm is observed over a wavelength range of 1270-1450 nm. The noise figure (NF) varies from a minimum of 4 dB to a maximum of 5.9 dB for a signal power of 0 dBm. A gain reduction of around 4 dB is observed for an O-band signal at a wavelength of 1290 nm by considering the up-conversion effect. The transmission performance of the designed HOA as a pre-amplifier is evaluated based on the bit-error rate (BER) analysis for a coarse wavelength-division multiplexing (CWDM) system of eight on-off keying (OOK)-modulated channels, each having a data rate of 10 Gbps. An error-free transmission over 60 km of standard single-mode fiber (SMF) is achieved for different data rates of 5 Gbps, 7.5 Gbps, and 10 Gbps.

Four-channel CWDM transmitter chip based on thin-film lithium niobate platform

Multi-lane integrated transmitter chips are key components in future compact optical modules to realize high-speed optical interconnects. Thin-film lithium niobate (TFLN) photonics have emerged as a promising platform for achieving high-performance chip-scale optical systems. Combining a coarse wavelength-division multiplexing (CWDM) devices using fabrication-tolerant angled multimode interferometer structure and high-performance electro-optical modulators, we demonstrate monolithic on-chip four-channel CWDM transmitter on the TFLN platform for the first time. The four-channel CWDM transmitter enables high-speed transmissions of 100 Gb/s data rate per wavelength channel (i.e., an aggregated date rate of 400 Gb/s).

Self-healing integration of fiber/FSO communication and sensor network for improving survivability

In this paper, we proposed a novel self-healing fiber/free-space optics (FSO) communication integrated with a sensor network for simultaneous transmission of optical communication signal and sensing signal as well as a function of fault protection by using a coarse wavelength-division multiplexer (CWDM). The combination of CWDM and fiber integrated with FSO transmission channel is incorporated to design a cost-effective, flexible network and increase the coverage of transmission distance. The experiment result proves that the fiber integrated with an FSO transmission channel achieves a clear constellation diagram and best error vector magnitude (EVM) value. On the other hand, the scanning-laser interrogation system is used for data acquisition in the strain-based fiber Bragg grating (FBG) sensor system. Moreover, the radial basis function neural network (RBFNN) algorithm is used to estimate the central wavelength of FBGs.

Experimental demonstration of single-span 100-km O-band 4×50-Gb/s CWDM direct-detection transmission.

We report on what is to the best of our knowledge the longest 50-Gb/s/λ O-band wavelength-division multiplexed (WDM) transmission. A pair of in-house built bismuth-doped fiber amplifiers (BDFAs) and the use of Kramers-Kronig detection-assisted single-sideband transmission are adopted to overcome the fiber loss and chromatic dispersion, respectively, in a reach-extended O-band coarse WDM (CWDM) system with a channel spacing of ∼10 nm. Through experiments on an amplified 4×50-Gb/s/λ direct-detection system based on booster and pre-amp BDFAs, we show the superior performance of single-sideband transmission in terms of both optical signal-to-noise ratio sensitivity and uniformity in performance amongst CWDM channels relative to double-sideband transmission after both 75-km and 100-km lengths of single-mode fiber. As a result, up to 100-km reach with comparable performance at all 50-Gb/s channels was achieved without the need for in-line optical amplification.

ML-Assisted Equalization for 50-Gb/s/λ O-Band CWDM Transmission Over 100-km SMF

We propose and demonstrate a bidirectional Vanilla recurrent neural network (Vanilla-RNN) based equalization scheme for O-band coarse wavelength division multiplexed (CWDM) transmission. Based on a 4 × 50-Gb/s intensity modulation and direct detection (IM/DD) system, we demonstrate the significantly better bit error rate (BER) performance of the Vanilla-RNN scheme over the conventional decision feedback equalizer (DFE) for both Nyquist on-off keying (OOK) and Nyquist 4-ary pulse amplitude modulation (PAM4) formats. It is shown that the Vanilla-RNN equalizer is capable of compensating for both linear and nonlinear impairments induced by the transceiver and the single-mode fiber (SMF). As a result, up to 100-km and 75-km SMF transmission can be achieved for OOK and PAM4 transmission, respectively. Furthermore, through the comparison with other equalization schemes, including the linear equalizer, 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> -order Volterra equalizer, and Volterra+DFE, it is demonstrated that the Vanilla-RNN equalizer achieves the best BER performance. In the meantime, it also exhibits lower implementation complexity when compared to Volterra-based schemes. Our results show that the Vanilla-RNN scheme is a viable solution for realizing simple and effective equalization. This work serves as an exploration and offers useful insights for future implementations of reach-extended O-band CWDM IM/DD systems.

Experimental demonstration of robust nanophotonic devices optimized by topological inverse design with energy constraint

In this paper, we present the experimental results for integrated photonic devices optimized with an energy-constrained inverse design method. When this constraint is applied, optimizations are directed to solutions that contain the optical field inside the waveguide core medium, leading to more robust designs with relatively larger minimum feature size. We optimize three components: a mode converter (MC), a 1310 nm/1550 nm wavelength duplexer, and a three-channel C-band wavelength demultiplexer for coarse wavelength division multiplexing (CWDM) application with 50 nm channel spacing. The energy constraint leads to nearly binarized structures without applying independent binarization stage. It also reduces the appearance of small features. In the MC, well-binarized design, improved insertion loss, and cross talk are obtained as a result. Furthermore, the proposed constraint improves the robustness to fabrication imperfections as shown in the duplexer design. With energy constraint optimization, the corresponding spectrum shifts for the duplexer under ± 10 nm dimensional variations are reduced from 105 nm to 55 nm and from 72 nm to 60 nm for the 1310 nm and 1550 nm channel, respectively. In the CWDM demultiplexer, robustness toward ± 10 nm fabrication error is improved by a factor of 2. The introduction of the energy constraint into topological optimization demonstrates computational gain with better-performing designs.

Silicon-Based Metastructure Optical Scattering Multiply–Accumulate Computation Chip

Optical neural networks (ONN) have become the most promising solution to replacing electronic neural networks, which have the advantages of large bandwidth, low energy consumption, strong parallel processing ability, and super high speed. Silicon-based micro-nano integrated photonic platforms have demonstrated good compatibility with complementary metal oxide semiconductor (CMOS) processing. Therefore, without completely changing the existing silicon-based fabrication technology, optoelectronic hybrid devices or all-optical devices of better performance can be achieved on such platforms. To meet the requirements of smaller size and higher integration for silicon photonic computing, the topology of a four-channel coarse wavelength division multiplexer (CWDM) and an optical scattering unit (OSU) are inversely designed and optimized by Lumerical software. Due to the random optical power splitting ratio and incoherency, the intensities of different input signals from CWDM can be weighted and summed directly by the subsequent OSU to accomplish arbitrary multiply–accumulate (MAC) operations, therefore supplying the core foundation for scattering ONN architecture.