Wavelength Division Multiplexing Research Articles

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.

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.

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.

Synergizing intelligent signal processing with wavelength-division multiplexing for enhanced efficiency and speed in photonic network communications

Abstract The explosive growth of worldwide mobile data traffic seeks innovations in communication technology to cater to the mounting need for rapid connectivity, high-capacity connections. The mainstreaming of 5G technologies for communication is a dramatic step towards meeting the aforementioned goals, with the ability for reshaping IoT (Internet of things), D2D (device-to-Device) communications, and the smart grids. This work conveys an in-depth study of the fundamental innovations that underlie 5G, including full-duplex distribution, huge multiple-input-multiple-output, ultra-dense connections, the phenomenon of beamforming and millimeter-wave approaches. A special emphasis is focused on the integration of photonic technologies, or microwave photonics, which serves as a critical multidisciplinary study topic. Optical fibers, with their tremendous bandwidth and capacity, have been determined as the best medium for backhaul and fronthaul amenities, outpacing conventional copper cables to accommodate tiny cells and next-generation networks. The synergy between optical and wireless access technologies is analyzed with the emphasis on the central role of wavelength-division multiplexing (WDM) for improving network efficiency and speed. The investigation additionally explores the possibility of intelligent signal processing methods combined with WDM to optimize photonic network communications. The mingling of these technologies anticipates producing unrivaled levels of performance, rupturing the path for an additional intelligent, interconnected era.

Intensity noise reduction in quantum dot comb laser by lower external carrier fluctuations.

This work investigates the impact of carrier noise induced by an external current source on the linewidth enhancement factor (LEF) and relative intensity noise (RIN) of a 100 GHz quantum dot fourth-order colliding-pulse mode-locked laser (MLL), driven by a normal pump with Gaussian-distributed carrier sequences and a quiet pump with sub-Poissonian-distributed carrier sequences. The results indicate that under a normal pump, the LEFs are approximately zero for reverse saturable absorber (SA) bias voltages ranging from 0 to 2.5 V, and the laser achieves a RIN as low as -156 dB/Hz. When using a quiet pump, both the LEF and RIN are reduced across all SA bias conditions, particularly at low reverse SA bias voltages. Specifically, the LEF decreases by up to 0.58 at 0 V, and the average RIN spectrum is reduced by more than 3 dB at the same voltage. This work provides a straightforward approach for the development and optimization of multi-channel light sources for dense wavelength division multiplexing (DWDM) technologies with low optical noise.

Design and performance of WDM system for high-speed optical communication on different modulation formats

Abstract In this paper, the performance analysis of the WDM (wavelength division multiplexing) system on the optical fiber transmission link is proposed. High data transmission is possible by implementing a WDM optical communication system using different modulation formats. Firstly, the WDM optical communication system is designed on an Optiwave simulation tool and distortion in the optical fiber by “eye pattern analysis” is analyzed. Secondly, the optical SNR of the proposed WDM optical communication system is studied. Further, the bit-error rate (BER) plot is studied and optimization techniques are applied to enhance the proposed system for long-haul communication. Furthermore, the BER plots are analyzed by varying the length of the fiber and applying different coding techniques like NRZ (non-return zero), RZ (return to zero), QAM (quadrature amplitude modulation) etc. using the Optiwave simulation tool. The results obtained in the Optiwave simulation tool are validated by theoretical implementation in the MATLAB tool.

Hybrid WDM/MDM (De) multiplexer based on Fabry–Perot resonators with Bragg grating reflectors

The traveling-wave-like Fabry–Perot (TW-like F-P) resonators, utilizing transverse-mode conversion, have been thoroughly investigated as on-chip filters. However, the asymmetric directional coupling (ADC) between the phase shifter and the output waveguide in this structure is not fully utilized, resulting in a rare implementation of hybrid wavelength division multiplexing (WDM) and mode division multiplexing (MDM). In this paper, using the transfer matrix method (TMM), we investigate methods to effectively enhance the quality factor (Q-factor) of TW-like F-P resonators. This is achieved by increasing the phase shifter length and reducing the coupling coefficient between these waveguides, without significantly impacting the channel drop efficiency. MDM can be achieved by adjusting the width of the output waveguides, utilizing the ADC between the phase shifter and the output waveguide. We design nine-channel hybrid WDM-MDM multiplexers based on TW-like F-P resonators. The variational-finite-difference time-domain (varFDTD) method is utilized to analyze the device’s performance, and its single channel extinction ratio (ER) values can reach −20dB. This work paves the way for TW-like F-P-resonator-based large capacity optical communications and interconnections.

Omnidirectional optic fiber shape sensor for submarine landslide monitoring

In-situ submarine landslide monitoring is imperative for ensuring the safety of ocean infrastructures, although it poses greater challenges compared to conventional landslide monitoring methods. In response to this need, we developed an omnidirectional optic fiber shape sensor (OFSS) and a self-contained submarine landslide monitoring system. Wavelength division multiplexing (WDM) and space division multiplexing (SDM) are used to build 10 omnidirectional curvature sensors with 3 fiber Bragg gating (FBG) arrays. Difference curvature demodulation algorithm is introduced to eliminate bending-irrelative effects. The omnidirectional curvature transmission matrix (OCTM) is obtained through calibration process with standard curvature molds. Frenet algorithm is used to recovery the shape. Laboratory experiments demonstrate OFSS’s ability to monitor 3D shape in real time, achieving the deflection error of 0.23 % and the bending direction error of 0.06 rad in the simple bending shape recovery experiment. In the S-shape recovery experiment, the OFSS’s deflection error is 0.77 % and the bending direction error is 0.04 rad. Subsequently, the system underwent testing in shallow water within a ship dock. In this experiment, the OFSS is successfully penetrated into the seabed and remained embedded in the soil for 90 min. The maximal shift observed at the end of the OFSS is 4.61×10-3m with the standard deviation of 9.95×10-4m, while the bending direction exhibited a maximal shift of 3×10-3 rad with the standard deviation of 7.51×10-4rad over the course of the 90-minute duration. These results validate the system’s effectiveness in providing accurate and reliable in-situ monitoring of submarine landslide.

Four-channel graphene optical receiver

Abstract Silicon photonics with the advantages of low power consumption and low fabrication cost is a crucial technology for facilitating high-capacity optical communications and interconnects. The graphene photodetectors (GPDs) featuring broadband operation, high speed, and low integration cost can be good additions to the SiGe photodetectors, supporting high-speed photodetection in wavelength bands beyond 1.6 μm on silicon. Here we realize a silicon-integrated four-channel wavelength division multiplexing (WDM) optical receiver based on a micro-ring resonator (MRR) array and four p-n homojunction GPDs. These photo-thermoelectric (PTE) GPDs exhibit zero-bias responsivities of ∼1.1 V W−1 and set-up limited 3 dB-bandwidth >67 GHz. The GPDs show good consistence benefiting from the compact active region array (0.006 mm2) covered by a single mechanically exfoliated hBN/graphene/hBN stack. Moreover, the WDM graphene optical receiver realized 4 × 16 Gbps non-return-to-zero optical signal transmission. To the best of our knowledge, it is the first GPD-array-based optical receiver using high-quality mechanically exfoliated graphene and edge graphene-metal contacts with low resistances. Apparently, our design is also compatible with CVD-grown graphene. This work sheds light on the large-scale integration of GPDs with high consistency and uniformity, enabling the application of high-quality mechanically exfoliated graphene, and promoting the development of the graphene photonic integrated circuits.

Detailed scrutiny of FWM in holmium-doped fiber amplifier (HOFA) in WDM systems

Abstract Four wave mixing (FWM) in wavelength division multiplexed (WDM) systems is prominent performance degrading nonlinearity and limits the total capacity of the system. FWM is explored in different fiber optical amplifiers such as erbium-doped fiber amplifier (EDFA), Raman fiber amplifier (RFA), and also in semiconductor optical amplifier (SOA). Reported studies in HOFA revealed that nonlinear effects in these amplifiers are low; however, in present study, in this research article WDM-HOFA system has been simulated and studied FWM in detail at different HOFA parameters such as input power, length, core radius, doping radius, numerical aperture (NA), Ho ion density, and effects of co- and counter-pumping. Gain and noise figure (NF) are also investigated at aforementioned parameters, and performance is evaluated in terms of optical signal to noise ratio (OSNR) at 2,030 nm wavelength window. It is observed that least FWM obtained at 3 m Ho fiber length, 2 µm core radius and doping radius, 15.6 × 1023 m−3 Ho ion doping, 0.1 NA, −10 dB input power, 0.8 GHz channel spacing, and 4 WDM channels. However, better gain and NF comes at different values of these parameters, and as a result, there is trade-off between FWM and gain, NF.

A double clad ASE Re-injected hybrid TDFA and HDFA amplifier with ±1.44 dB GF

Abstract Current developments in wireless communications accentuate the exigency of high gain, low noise figure (NF) and minimum gain fluctuations (GF) based optical amplifier in 1.9–2.15 μm eye safe wavelength band. Hybrid thulium (Tm) doped fiber amplifier (TDFA) and holmium (Ho) doped fiber amplifier (HDFA) are the perfect candidates to extend the bandwidth towards 2 μm long wavelengths. In this work, amplifier spontaneous emission (ASE) re-injected double clad TDFA-HDFA hybrid amplifier using Fiber bragg gratings (FBGs) is presented for the amplification of 64 wavelength division multiplexed (WDM) channels covering 63 nm wavelength window. Further, performance of proposed amplifier with and without ASE reinjection is evaluated in terms of Gain, NF, GF and optical signal to noise ratio (OSNR). Effects of single and double clad on the Gain are studied and optimization of Tm and Ho doped fiber length, pump powers, and input power has also been accomplished. The GF as low as ±1.44 dB, Gain >28 dB, NF < 5.08 dB, output power 4.2 W are achieved in ASE re-injected configuration using 785 nm pump at 55 dBm in TDFA and 1,900 nm pump at 20 dBm in HDFA. It is noticed that double clad and ASE reinjection in TDFA and HDFA offer better GF.