Wavelength Division Multiplexing Research Articles

Coherent beam combining of wavelength-division multiplexed telecom signals: toward high-power and high-bit-rate free-space optical telecommunications

This work presents a coherent beam combining system achieving constructive interferences of 8 arms seeded by 4 wavelength-division multiplexed (WDM) channels modulated at 10 Gbit/s each. The whole spectrum spans over 2.4 THz (19 nm) in the C-band. Phase-locking of one channel, combined with an accurate equalization process of the optical path differences (OPD) of the arms, allows us to maximize the combining efficiency of all the channels simultaneously. Each channel achieves a combining efficiency higher than 95%. Supported by an analytical description of the system, tolerancing rules underline the need for precise OPD equalization accuracy: a 5 µm OPD mismatch results in a 1% combining efficiency loss. Bit error rate and eye diagram analysis demonstrate the reliability of the system to transmit data without penalties. This marks a step toward the development of innovative WDM laser sources for high-power, high-bit-rate free-space optical telecommunications.

A CROW/MZI-based switch for silicon photonics applications

In this work, the authors propose the design of an electro-optical silicon-photonics hitless switch that follows wavelength-division multiplexing (WDM) conditions in terms of channel bandwidth, shape factor, and free spectral range. Through a careful step-by-step design, the authors study a switch device composed of a coupled-resonator optical waveguide (CROW) structure merged with a Mach-Zehnder interferometer (MZI) modulator. The MZI modulator controls the coupling to the CROW and hence turns the switch on and off. The traditional matrix analysis of the CROW structure is modified to accommodate the MZI modulator section. The proposed switch requires a chip area of less than 0.01225 mm2. The switch transmission satisfies WDM requirements with a channel-free spectral range of 100 GHz, channel flat-top response with a passband for the through/drop ports of 64.6 GHz/33 GHz, and a shape factor of 0.76/0.6. Besides, the switch transmission features an insertion loss for the through/drop ports of 0.23/1 dB, a switching speed of 100 GHz, and a competitive switch-on/off ratio of 33 dB.

Enhancing Free Space Optical System Performance through Fog and Atmospheric Turbulence using Power Optimization

Free Space Optical (FSO) communication is gaining traction as a pivotal technology for next-generation communication systems, offering extremely high data rates, unlicensed bandwidth, and rapid transmission capabilities. However, its performance is significantly hindered by atmospheric factors such as turbulence and fog. This paper presents a comprehensive model for FSO communication designed to optimize performance under various atmospheric conditions. We assess channel capacity across a spectrum of weak to strong atmospheric turbulence using a Gamma-Gamma channel distribution. To mitigate channel losses, our system employs Wavelength Division Multiplexing (WDM) and multi-beam Multiple Input Multiple Output (MIMO) technologies. Results indicate that the integration of diversity techniques and WDM substantially enhances system performance in adverse weather conditions. Furthermore, power optimization is achieved through the implementation of optical amplifiers and feedback mechanisms from the receiver to the transmitter to adjust the transmitter power in accordance with received Bit Error Rate (BER). The proposed power-optimized WDM MIMO system demonstrates a remarkable BER of 1.48884e-15, while extending the transmission link distance to 2500 meters with a Q factor of 21.5 even under strong atmospheric turbulence conditions.

On-Chip Cascaded Metasurfaces for Visible Wavelength Division Multiplexing and Color-Routing Meta-Display.

Integrating metasurfaces on-chip offers a promising strategy for modulating and extracting guided waves, suggesting tremendous applications in compact wearable devices. However, despite the full acquisition of on-chip manipulation of optical parameters, including phase, amplitude, and polarization, the functionality of on-chip metasurfaces remains limited by the lack of wavelength selectivity. Here, an on-chip approach to differentiate wavelength components is proposed in the visible regime for wavelength division multiplexing (WDM). Through horizontally cascading on-chip meta-atoms with structural dimension variation and optimization, different wavelength components propagating along the waveguide would be selectively extracted, realizing meta-demultiplexing functionality. More intriguingly, color nanoprinting images or holographic displays can be correspondingly enabled. This approach surpasses conventional free-space meta-devices in terms of exhibiting improved wavelength-selective allocation and eliminating the energy waste caused by spatial multiplexing. We envision that such an on-chip cascading strategy paves the way for next-generation WDM devices in photonic integrated circuits and wearable miniature meta-displays.

Compact silica dual-band wavelength demultiplexer based on asymmetric-defined multimode interference coupler

Limited refractive index difference of silica waveguide brings great size challenges in wavelength demultiplexing. A silica dual-band wavelength demultiplexer (DBWD) based on an asymmetric multimode interference coupler (MMI) is demonstrated. The selective separation and output of target dual bands can be implemented using the proposed design method based on asymmetric-defined MMI couplers. The beam propagation method is adopted to verify the proposed design principle. Standard CMOS fabrication is used in demultiplexer preparation. At λ=1350 nm, insertion loss (IL) and crosstalk (CT) at port Output1 are 1.94 dB and −24.92 dB, respectively. At λ=1550 nm, IL and CT at port Output2 are 2.44 dB and −29.00 dB, respectively. The 3-dB bandwidth (BW3 dB) for Output1 and Output2 are 82 nm (1297-1379 nm) and 87 nm (1492-1579 nm), respectively. The corresponding CT for Output1 and Output2 are < −7.16 dB and < −12.9 dB, respectively. Due to the introduction of asymmetric MMI coupler, the footprint is reduced to 0.1 mm2 (25 µm × 4000 µm). Because of the periodic characteristic, the wavelength demultiplexing can be extended from O/C bands to E/L bands by a reshaped asymmetric-MMI coupler. Even by combining more asymmetric-MMI couplers with different etched sections, all 18 channels of coarse wavelength division multiplexing (CWDM) transmission (1270-1610 nm) can be covered. The favorable manufacturing tolerance facilitates large-scale integration and mass production.

273.6 Tbit/s real-time S + C + L-band same-wavelength bidirectional wavelength division multiplexing transmission in anti-resonant hollow-core fiber.

Based on its ultra-low backscattering characteristic, anti-resonant hollow-core fiber is a good medium for bidirectional transmission, which means that the wavelengths of the transmission channels are the same in both directions. In this Letter, we present the first, to the best of our knowledge, demonstration of S + C + L-band same-wavelength bidirectional transmission in an anti-resonant hollow-core fiber. By using a commercial real-time coherent optical transceivers and amplified spontaneous emission (ASE) source, a total net throughput of 273.6 Tbit/s is achieved over 1.4 km nested anti-resonant nodeless fiber (NANF) with 19.65 THz bandwidth and bidirectional wavelength division multiplexing (WDM) transmission. Commercial 400 G, 1 T, and 800 G optical modules are employed in the S-band, C-band, and L-band, respectively. We experimentally evaluate the same-wavelength bidirectional WDM transmission performance by verifying 2 × 102 channels of 68 Gbaud probabilistically constellation-shaped (PCS) 16 quadrature amplitude modulation (QAM) in the S-band with 75 GHz grid, 2 × 48 channels of 118 Gbaud PCS-64QAM in a C-band with 125 GHz grid and 2 × 60 channels of 91.6 Gbaud PCS-64QAM in the L-band with 100 GHz grid. All 420 test channels could satisfy the bit error ratio (BER) requirements. The result shows great potential for same-wavelength bidirectional WDM transmission in an anti-resonant hollow-core fiber.

Self-homodyne 2-OTDM for doubling the baud rate in low-energy optical interconnect.

As a low-energy method to increase the data rate of optical links in data centers, we propose self-homodyne Nyquist optical time division multiplexing (OTDM). In Nyquist OTDM, spectrally efficient high-baud rate signals can be generated exceeding the limit of electronic signal processing. However, full integration of OTDM systems has not been reported, mainly because of the complicated signal detection scheme, which involves demultiplexing and clock recovery. In our proposal, the Nyquist pulse train is transmitted to the receiver as a local oscillator (LO) to leverage self-homodyne detection, which allows using large linewidth lasers and a simplified digital signal processing (DSP) algorithm. As the transmitted pulse train serves as an optical clock, demultiplexing and detection of the OTDM signal can be performed without using power-intensive high-bandwidth electronics and DSP. In this method, the LO pulse train needs to enter the coherent detector in exact synchronization with the OTDM signal for detecting the individual tributary correctly. For this purpose, we present a pulse delay control method suitable for photonic integration. A Nyquist pulse train with m carriers enables m-time multiplexing of optical signals. We explain and demonstrate the proposed concept in the case of m = 2, as it is the most feasible implementation. In the O-band where the chromatic dispersion (CD) is negligible, DSP-free operation can be achieved using the QPSK format. At the band edges where CD is non-negligible, it can be compensated by the DSP as in the conventional coherent detection. We verify this numerically and in an experiment involving the transmission of a 64-Gbaud QPSK signal at 1550 nm over a single-mode fiber. In terms of low energy, self-homodyne Nyquist OTDM is advantageous in wavelength division multiplexing (WDM). Taking it into consideration, we perform 4-channel WDM transmission of the 64-Gbaud QPSK signal over a 1-km dispersion shifted fiber without CD compensation. The results demonstrate a data rate of 512 Gb/s with a BER of <1 × 10-10.

Multi-wavelength DFB laser based on a sidewall third-order four phase-shifted sampled Bragg grating with uniform wavelength spacing.

We present the first, to our knowledge, demonstration of a 1550 nm multi-wavelength distributed feedback (MW-DFB) laser employing a third-order, four-phase-shifted sampled sidewall grating. By utilizing linearly chirped sampled gratings and incorporating multiple true π-phase shifts within a cavity, we achieved and experimentally validated a four-wavelength laser with a channel spacing of 0.4 nm. The device operates stably and uniformly across a wide range of injection currents from 280 mA to 350 mA. The average wavelength spacing was measured at 0.401 nm with a standard deviation of 0.0081 nm. Additionally, we demonstrated a 0.3 nm MW-DFB laser with a seven-channel output, achieving a wavelength spacing of 0.274 nm and a standard deviation of 0.0055 nm. This MW-DFB laser features a ridge waveguide with sidewall gratings, requiring only one metalorganic vapor-phase epitaxy (MOVPE) step and a single III-V material etching process. This streamlined fabrication approach simplifies device manufacturing and is well-suited for dense wavelength division multiplexing (DWDM) systems.

Tailoring Spectral Response of Grating-Assisted Co-Directional Couplers with Weighting Techniques and Rational Transfer Functions: Theory and Experiment

This work addresses the tailoring spectral response of grating-assisted co-directional couplers (GADCs) in the context of wavelength filtering for fiber-to-the-home (FTTH) applications. Design methods for spectral response engineering by means of coupling profile apodization-type weighting techniques and also more advanced rational transfer functions fitting a predefined spectral window template are presented. Modeling results based on coupled mode theory are then applied for the design and experimental fabrication of InGaAsP/InP GADCs targeting 1.3+/1.3− µm diplexer application in FTTH access networks. The experimental results are found to be in good agreement with the modeling predictions. The design tools presented are quite general and can be easily adapted to other technology platforms, such as silicon photonics for the use of GADCs as add-drop wavelength division multiplexers. The field of parity–time symmetry is another avenue where these types of gain–loss-assisted GADCs as active components are of interest for switching applications, and the design methods presented here may find utility.

All-polarization-maintaining dual-comb Er-fiber laser with combined figure-8 and figure-9 architectures.

We propose and demonstrate, for the first time to the best of our knowledge, an all-polarization-maintaining (all-PM) dual-comb Er-fiber laser based on combined figure-8 and figure-9 architectures. The opposite signs of the non-reciprocal phase shifts required for figure-8 and figure-9 architectures in the shared nonlinear amplifying loop mirror (NALM) are achieved using a single non-reciprocal phase shifter (NRPS) that operates in two orthogonal polarizations. The capability of common mode noise cancellation, environmental stability, long-term reliability, and the tunable range of the repetition rate difference Δf rep between two combs has been investigated and characterized. In addition, the dual-comb spectroscopy measurement with our dual-comb fiber laser is demonstrated to resolve the transmission spectrum of a flat-top DWDM (dense wavelength division multiplexer) demultiplexer, with a passband of ∼50 GHz at 1564.5 nm. Our experimental results indicate that the combined-architecture all-PM fiber laser, with its large tunable range of 100 kHz in Δf rep , shows significant potential for dual-comb applications in precise and rapid metrology outside the laboratory.

Influence of variation in indium concentration and temperature on the threshold current density in In x Ga1-x As/GaAs QD laser diodes.

The influence of variations in indium concentration and temperature on threshold current density (Jth) in In x Ga1-x As/GaAs (x=0, 0.8 and 0.16) quantum dot (QD) laser diodes - synthesized via molecular beam epitaxy (MBE) with three distinct indium concentrations on GaAs (001) substrates - was meticulously examined. An X-ray diffraction (XRD) analysis revealed that increasing indium concentrations in InAs/InGaAs 'dot in a well' (DWELL) structures caused significant structural changes, including peak broadening and shifting, indicating increased lattice strain and potential defect formation. An investigation of light-current (L-I) characteristics revealed a super-linear increase in Jth up to the temperature of 323K for all three lasers. Above 323K, Jth exhibited an exponential increase, which is indicative of carrier leakage into the barriers from the quantum well. Additionally, Jth displayed a decreasing trend with increasing indium concentrations, suggesting enhanced QD symmetry and size from a higher indium content. A significant reduction in the characteristic temperature at which the threshold current occurred was observed as the indium concentration increased in the 288-363K temperature range. Photoluminescence (PL) and electroluminescence (EL) measurements revealed a redshift of the lasing wavelength in correlation with higher indium concentrations. This redshift, attributable to variations in bandgap energy that decrease as indium content increases, highlights QD laser instability. These findings emphasize how versatile adjusting indium concentrations to tailor emission wavelengths can be and demonstrate potential applications in wavelength division multiplexing (WDM) systems and optical communications.

Performance analysis of a multi-user outdoor visible light communication system using wavelength division multiplexing (WDM) with RZ-OOK and NRZ-OOK modulations for V2X communications

Performance analysis of a multi-user outdoor visible light communication system using wavelength division multiplexing (WDM) with RZ-OOK and NRZ-OOK modulations for V2X communications

Reconfigurable inverse-designed phase-change photonics

Chalcogenide phase-change materials (PCMs) offer a promising approach to programmable photonics thanks to their nonvolatile, reversible phase transitions and high refractive index contrast. However, conventional designs are limited by global phase control over entire PCM thin films between fully amorphous and fully crystalline states, which restricts device functionality and confines design flexibility and programmability. In this work, we present a novel approach that leverages pixel-level control of PCM in inverse-designed photonic devices, enabling highly reconfigurable, multi-functional operations. We integrate low-loss Sb2Se3 onto a multi-mode interferometer and achieve precise, localized phase manipulation through direct laser writing. This technique allows for flexible programming of the photonic device by adjusting the PCM phase pattern rather than relying on global phase states, thereby enhancing device adaptability. As a proof of concept, we programmed the device as a wavelength-division multiplexer and subsequently reconfigured it into a mode-division multiplexer. Our results underscore the potential of combining inverse design with pixel-wise tuning for next-generation programmable phase-change photonic systems.

Abrupt lateral beam shifts from terahertz quasi-bound states in the continuum.

Bound states in the continuum (BICs) are notable in photonics for their infinite Q factors. Perturbed BICs, or quasi-BICs (QBICs), have finite but ultra-high Q factors, enabling external coupling. So far, most studies have focused on the momentum-space properties of BICs and QBICs, with few discussions on their properties in real space. Here, we experimentally demonstrate that QBICs can induce abrupt lateral beam shifts. By applying Brillouin zone folding to a compound grating waveguide, we form a QBIC band where all states become QBICs. When excited at specific incident angles, these QBICs produce sudden lateral beam shifts, rapidly disappearing as frequencies deviate from the QBIC band. Using terahertz imaging, we capture these beam shifts at different incident angles, characterizing the QBIC band. This work offers alternative insights into QBIC behaviors and supports the development of advanced sensors and wavelength division (de) multiplexers.

Heterogeneously integrated silicon-conductive oxide MOSCAP microring modulator array

In pursuit of energy-efficient optical interconnect, the silicon microring modulator (Si-MRM) has emerged as a pivotal device offering an ultra-compact footprint and capability of on-chip wavelength division multiplexing (WDM). This paper presents a 1×4 metal-oxide-semiconductor capacitor (MOSCAP) Si-MRM array gated by high-mobility titanium-doped indium oxide (ITiO), which was fabricated by combining Intel’s high-volume manufacturing process and the transparent conductive oxide (TCO) patterning with the university facility. The 1×4 Si-MRM array exhibits a high electro-optic (E-O) efficiency with V π ·L of 0.12 V·cm and achieves a modulation rate of (3×25+1×15) Gb/s with a measured bandwidth of 14 GHz. Additionally, it can perform on-chip WDM modulation at four equally spaced wavelengths without using thermal heaters. The process compatibility between silicon photonics and TCO materials is verified by such an industry-university co-fabrication approach for the MOSCAP Si-MRM array and demonstrated enhanced performance from heterogeneous integration.

Design and optimization of a low-loss waveguide taper for a narrow-band O-band multiplexer

The fast deployment of high-performance data centers accelerates the development of O-band wavelength division multiplexing (WDM) and its devices. To minimize insertion loss in WDM devices, it is essential to maximize coupling efficiency between optical fibers and waveguides at micro- and nano-scale. In this paper, we propose a low-loss waveguide tapered coupler and its optimization around 1310 nm. The coupler has a vertically placed double-layer rib taper as an intermediate waveguide, and the inverted taper is in a tapered tandem structure. Rib tapers are introduced in the intermediate waveguide and the fiber is coupled to the rib tapers, which are adiabatically reduced in width along the direction of the coupler to vertically reduce the mode size. To minimize footprint, a cascaded inverted taper is introduced. The maximum taper angle per stage corresponding to the long adiabatic taper is first determined. Subsequently, the long tapers are replaced with new shorter tapers. Several parameters are optimized using frequency domain time division (FDTD), including the output waveguide and input surface dimension, the height ratio of rib tapers, the length of inverted tapers, the length of rib tapers, and the tapered profile of tandem structure. The optimization results show that the coupler has an insertion loss of only 0.06 dB at 1310 nm with a device length of 513.4 µm. The two-stage in-plane coupler operates in O-band narrow-band with low insertion loss, offering an efficient solution for coupling fiber to on-chip waveguides.

Spatial-division multiplexing self-homodyne coherent transmission over 19-/24-core fibers assisted by homemade fan-in/fan-out 3D photonic devices.

The self-homodyne detection (SHD) is a promising solution to achieve low-cost and low-power-consumption fiber-optic communications. In this work, we propose and demonstrate a high-capacity spatial-division multiplexing (SDM) system with SHD technology by employing single-mode multi-core fibers (SM-MCFs), where the fan-in/fan-out (FIFO) 3D photonic devices are designed and fabricated based on the femtosecond laser direct writing technique, enabling high-efficiency coupling between single-mode fibers (SMFs) and SM-MCFs. The FIFO 3D photonic devices, serving as the SDM (de)multiplexer, facilitate superior performance of low insertion loss and low inter-channel crosstalk. Using a low-cost MHz linewidth distributed feedback (DFB) laser, we experimentally demonstrate the SDM-SHD transmission system of 102-Gbaud dual-polarization (DP) 16-ary quadrature amplitude modulation (16QAM) signals per channel over a 2.7-km 24-core SM-MCF and 22-Gbaud DP-64QAM signals per channel over a 1-km 19-core SM-MCF. Without using wavelength-division multiplexing (WDM), the total data rate per channel reaches 816 Gbit/s, with an aggregate data throughput of 18.7 Tbit/s. The obtained results indicate that, by extending SHD technology to SDM communication systems, one can achieve low-cost, low-power-consumption, and scalable high-capacity data transmission in next-generation fiber-optic communication networks.

Knowledge distillation aided by probability information enabled efficient joint monitoring of SNRL and SNRNL for WDM systems.

For wavelength division multiplexing (WDM) systems, excessive linear and nonlinear noise will seriously decrease the quality of optical signals, and the effective joint monitoring scheme can prevent the degradation of system performance due to noise accumulation. In this paper, we propose a probability information assisted knowledge distillation (PIAKD) scheme that achieves intelligent joint monitoring for linear signal-to-noise ratio (SNRL) and nonlinear signal-to-noise ratio (SNRNL) in WDM systems. Under the condition of multi-task regression, outputs are independent and continuous, PIAKD addresses the longstanding challenge that the student model fails to effectively learn knowledge from the teacher model by introducing probability information into the loss function. The effectiveness of the scheme is verified by WDM simulation and experiment system which has a symbol rate of 28 GBaud per channel. The simulation results demonstrate that the overall mean absolute error (MAE) for jointing SNRL and SNRNL monitoring of the student model after PIAKD is reduced by 0.08 dB and 0.09 dB, corresponding to 32% and 34% error reductions respectively. Furthermore, when compared with the KD scheme without probability information, our scheme also reduces the overall MAE of SNRL and SNRNL by 16% and 11%, respectively. The results of the experiment reveal that the estimated MAE could be reduced by 0.13 dB and 0.16 dB, respectively, corresponding to error reductions of 17% and 18%. Moreover, the floating-point operations (FLOPs) and parameters (Params) of the student model is only 3.30 M and 0.0015 M, respectively, which is both significantly lower than the complexity of existing joint monitoring schemes for SNRL and SNRNL.

Dynamic control, routing, and resource assignment in multi-granular optical node topologies combining wavelength, waveband, and spatial switching for 6G transport networks [Invited

Effective management of end-to-end 6G network services is crucial, with peak capacity requirements for 6G transport connections expected to exceed 1 Tb/s. As demand for high bandwidth rises, there is a growing necessity for high-capacity optical fiber links, including ultra-wideband (UWB) and multiple fiber links within the network. Scaling up to accommodate these demands, designing wavelength-selective switches (WSSs) for such networks significantly increases the port count. To tackle this issue, we propose various multi-granular optical node (MG-ON) architectures utilizing heterogeneous wavelength, waveband, and spatial switching. We evaluate these architectures’ performance against high-capacity wavelength division multiplexed (WDM) networks through various simulation parameters.

Dynamically controllable two-color electromagnetically induced grating via spatially modulated inelastic two-wave mixing

A scheme for controlling two-color electromagnetically induced grating (TCEIG) in a coherently prepared cold atomic ensemble with a four-level inverted Y-type configuration is proposed via exploiting inelastic two-wave mixing process. By adding an intensity mask behind the auxiliary control field, the transmission functions of the incident probe field and the generated signal field are modulated periodically, thereby leading to the formation of TCEIG. Using experimentally achievable parameters, both the probe and signal fields with different frequencies can be simultaneously diffracted into high-order diffractions. It is found that the diffraction efficiencies of TCEIG, especially the first-order diffraction efficiency, can be significantly improved via adjusting the detunings of the probe and signal fields. Furthermore, it is also demonstrated that the diffraction of TCEIG can be manipulated via tuning the intensity and detuning of the control field. Finally, we investigate the influence the phase mismatch on the diffraction of TCEIG. It is shown that the diffraction pattern of the probe field is rather robust against the phase mismatch, while the diffraction intensities of the signal field are suppressed by the phase mismatch. Our scheme may provide a possibility for the all-optical control of optical switch and wavelength division multiplexing.