High-density WDM Research Articles

Bidirectional WDM Multi-Nodes Analog Radio-Over-Fiber Mobile Fronthaul Link Enhanced by Photonic Integrated Devices

A bidirectional wavelength division multiplexing (WDM) analog radio-over-fiber (A-RoF) mobile fronthaul (MFH) link is enhanced using photonic integrated devices. Two key photonic integrated devices are combined in the A-RoF link: an 8-channel InP directly modulated laser (DML) transmitter and a 32×100-GHz silicon array waveguide grating (AWG). The DML transmitter has 8 parallel monolithically integrated distributed feedback lasers, enabling cooperative and reconfigurable downlink analog transmission. Moreover, the 32×100-GHz AWG is featured by low insertion loss (<4.5-dB) and low crosstalk (<−20.4-dB), to achieve a high-density WDM system. In the distributed field experiments, we have successfully demonstrated a bidirectional A-RoF MFH over 10-km standard single mode fiber, providing an 8×5-Gbit/s 4-quadrature amplitude modulation (QAM) orthogonal frequency division modulation (OFDM) downlink transmission and a 3×12-Gbit/s 16-QAM uplink transmission.

Design of a Graphene-Enabled Dual-Mode Kerr Frequency Comb

On-chip Kerr frequency combs open up unprecedented opportunities for developing compact, low-cost, and high-density wavelength-division multiplexing (WDM) techniques for optical communications and optical interconnects. However, to achieve high-density WDM channels, a large-radius microring could be used to develop a Kerr frequency comb with a small free spectral range (FSR), inevitably increasing the comb's footprint and threshold. Herein, we theoretically study a dual-mode Kerr frequency comb based on a graphene-on-silicon (GOS) microring to overcome the limitation. Theoretical results show that a TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -mode comb with a 50-dB bandwidth of 200 nm and FSR of 170 GHz, and a TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> -mode comb with a 50-dB bandwidth of 130 nm and FSR of 212.4 GHz could be generated in the single GOS microring. Moreover, the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -mode and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> -mode combs could be selectively activated via modulating the Fermi level of the graphene nanoribbons integrated on the silicon microring. The study is expected to provide a useful technique for developing on-chip coherent light sources for high-speed optical communications.

Frequency-Stabilized Links for Coherent WDM Fiber Interconnects in the Datacenter

The continued growth in Hyperscale data center (HDC) deployment is expected to drive world-wide Internet traffic to an astounding 21 zettabytes by 2021. This growth will place increased demands on data center interconnects (DCIs) and drive the capacity of the underlying electronic application specific integrated circuit (ASIC) switch chips that route DCI ethernet traffic, from 12.8 Tbps per chip today to 100 Tbps and beyond in the future. This astounding growth will push the limits of today's incoherent fiber link technologies that connect switches, including power dissipation, density, and practical engineering solutions. To overcome these limits, high capacity coherent WDM, traditionally relegated to the metro and long-haul networks, will need to move into the DCI. However, migrating coherent WDM into the DCI, particularly for link distances less than 2 km, will require elimination of power consuming and costly technologies like the digital signal processor (DSP). Additionally, new photonic integration technologies will be needed to co-locate the coherent optical interfaces directly with switch ASICs to alleviate the bandwidth, power, and density limits. In this article, we introduce a new approach to DSP-free coherent WDM for the DCI called FRESCO: FREquency Stabilized Coherent Optical Links for Low Energy DCIs. FRESCO utilizes spectrally pure, ultra-stable light source technology, normally associated with high-end scientific applications like atomic clocks, to enable high capacity high-order WDM QAM with low bandwidth, low power electronics normally associated with RF links. Terabits per second FRESCO links based on shared, stabilized sources and high-density coherent WDM silicon photonic coherent transceivers that are co-located with the switch ASIC will pave the way to a DSP-free coherent WDM scalable DCI solution.

Mitigation of Fiber Nonlinearity Effects in Ultra High-Dense WDM System by Using Fractional Fourier Transform for 32 Channel System

The optical signal is severely degraded when transmitted in the fiber due to both the linear and nonlinear distortions. It is well known that, the linear distortions in single mode fiber include chromatic dispersion (CD) and polarization-mode dispersion (PMD) while self-phase modulation (SPM), cross phase modulation (XPM) and four-wave mixing (FWM) are considered the most among nonlinear distortions. A method called Fractional Fourier Transform (FRFT) has been applied to mitigate fiber nonlinearity impairments. In this work, 10 Gb/s optical fiber system with five spans, each of length 60 km, for three types of modulation formats (Return-to-Zero Differential Phase Shift Keying (RZ-DPSK), Return-to-Zero Differential Quaternary Phase Shift Keying (RZ-DQPSK) and Carrier-Suppressed Return-to-Zero (CSRZ)), single and multi-channel transmission systems are designed and simulated using “OptiSystem (2011) version 10.0” software package. The BER of thirty two channel systems (channel No.16) at bit rate 10 Gb/s per channel, 300 km link length of CSRZ modulation format with channel spacing of 50 GHz at SNR 10 dB without FRFT is 10-4.6 and 10-6.4 with FRFT. The BER of thirty two channel system (channel No.16) at bit rate 10 Gb/s per channel, 300 km link length of CSRZ modulation format with channel spacing of 25 GHz at SNR 10 dB without FRFT is 10- 2.56 and 10-3.24 with FRFT.

Demonstration of an 8-dimensional modulation format with reduced inter-channel nonlinearities in a polarization multiplexed coherent system.

We demonstrate a polarization-managed 8-dimensional modulation format that is time domain coded to reduce inter-channel nonlinearity. Simulation results show a 2.3 dB improvement in maximum net system margin (NSM) relative to polarization multiplexed (PM)-BPSK, and a 1.0 dB improvement relative to time interleaved return-to-zero (RZ)-PM-BPSK, for five WDM channels propagating over 1600 km ELEAF with 90% inline optical dispersion compensation. In contrast to the other modulations considered, the new 8-dimensional format has negligible sensitivity to the polarization states of the neighboring WDM channels. High-density WDM (HD-WDM) measurements on a 5000 km dispersion-managed link show a 1.0 dB improvement in net system margin relative to PM-BPSK.

Characterization of Strongly Coupled Si-Wire Waveguides for High-Density Optical WDM and Sensing Applications

This paper presents theoretical and experimental study of ultra-compact Si-wire Optical Directional Couplers (ODCs) on Silicon-on-Insulator wafer for optical signal processing. The presence of the controllable evanescent light strongly confined in the region bounded by the Si nano-wires has a large impact on the optical power coupling between waveguides. The characteristics of coupling length and power transmission in ODCs based on separation, wavelength, light field propagation distance and geometry of waveguides are described in detail by the coupled mode theory, 3-D finite-difference time-domain analysis and beam propagation method, and are confirmed by experiments. The exponential dependency of coupling length on the separation of coupled waveguides and wavelength shows interesting high-sensitivity optical sensing, switching and multiplexing properties. Custom spectral properties can be achieved by the configuration of coupled nano-wire waveguides based on their separation and lengths. We show that optimization of ODCs based on the physics of the coupled waveguides will lead to short optical devices which can be integrated as building blocks within high-density photonic circuits with the desired spectral characteristics. In the end, two new systems based on Mach-Zehnder structure and Micro-Ring Resonators are proposed in which ODCs are implemented as embedded tunable devices resulting in more functional optical sensing and signal processing devices.

Low Latency and Energy Efficient Optical Network-on-Chip Using Wavelength Assignment

Optical network-on-chip (ONoC) is a promising technology for high-performance and energy-efficient intra-chip interconnection. Many existing proposals resort to high-density wavelength division multiplexing (WDM) to increase bandwidth and to mitigate contention. Unfortunately, high-density WDM increases the cost of optical devices and the difficulties of integration. We propose WANoC, a novel ONoC design with careful wavelength assignment aimed at more effective reuse. A novel WDM optical router is also designed with simple layout, which needs fewer waveguides and has fewer crossings. The simulation results show that the saturation load is improved more than 2.7 times, while the energy consumption is decreased by 33% compared with prior circuit-switched ONoC.

Evaluation of Band-Pass Filters Influence on NRZ Signal in HDWDM Systems

In this paper we demonstrate performance of three different optical filter transfer functions: Lorentzian, Raised Cosine and Supergaussian, and their influence on non-return-to-zero (NRZ) coded optical signal in high density WDM (HDWDM) systems. The effect of filter bandwidth at -3 dB influence on NRZ and spectral narrowing on signal quality has been studied extensively. We foresee that for 10 Gbit/s data transmission speed NRZ coded optical signal optimal filter bandwidth at -3 dB level referred to full width half maximum (FWHM) value must be greater than 0.1 nm or 12.5 GHz. Ill. 9, bibl. 9 (in English; abstracts in English, Russian and Lithuanian).

Realization of Optimal FBG Band–Pass Filters for High Speed HDWDM

The influence on FBG filters with different apodization profiles on WDM systems is evaluated numerically with OptSim 5.0 and FBGS 2.6 simulation programs at data transmission rate of 2.5 Gbit/s and 10.52 Gbit/s using 40 km of optical fiber. The realization of low insertion loss optical filter is one of key elements to ensure high speed dense WDM system and FBG filter with different apodization profiles influence on high speed dense WDM is enormous. Ill. 8, bibl. 5 (in English; summaries in English, Russian and Lithuanian).

Investigation of thermo-optic effect and multi-reflector tunable filter/multiplexer in SOI waveguides

The paper presents an analysis of thermo-optic phase shifters in silicon-on-insulator (SOI) waveguide structures. It gives recommendations to provide high tuning characteristics at minimum power requirements. Then, this analysis is applied to the description of a novel type of reconfigurable optical add/drop multiplexer (ROADM) utilizing multi-reflector (MR) beam expanders and thermo-optic tuning in SOI structures. It is intended for use in high dense wavelength-division-multiplexing (HDWDM) flexible fiber-optic networks having multi-hundreds wavelength channels and advanced ITU grids (12.5 GHz, 25 GHz, 50 GHz).

Achieving long repeaterless sections in high-density metropolitan WDM networks

We experimentally study limitations imposed by fiber nonlinearities to achieve long repeaterless sections in a 25-GHz spaced metropolitan WDM network. We show that in a high spectral density transmission system using nonzero dispersion-shifted fiber (NZ-DSF) as transmission medium, stimulated Brillouin scattering (SBS) is not the major limiting nonlinearity. Thus, the choice of transmitter is not dictated by its linewidth and high performance DFB fiber lasers should be seriously taken into consideration. We experimentally verify that in metropolitan areas distributed Raman amplification is necessary to lengthen the repeaterless sections.

Comparision of a Kr-stabilized Laser and a Holographic Filter for use as an Optical Frequency Control Element for High Dense WDM Systems

Article Comparision of a Kr-stabilized Laser and a Holographic Filter for use as an Optical Frequency Control Element for High Dense WDM Systems was published on February 1, 2000 in the journal Journal of Optical Communications (volume 21, issue 1).

Integrated Planar Waveguide Demultiplexers for High-Density WDM Applications

A review is presented of state-of-the-art optical planar waveguide demultiplexers (DEMUXes), that is, phasars (arrayed waveguide) and etched grating devices designed for high-density (narrow channel spacing) wavelength division multiplexing (WDM) systems. Their advantages and disadvantages are discussed in comparison with other techniques for demultiplexing WDM signals. The advances made in their performance are reviewed, and ultimate limitations, set by technological constraints, are proposed. Finally, a comparison is made of the advantages and disadvantages of etched grating as opposed to phasar planar waveguide DEMUXes.

An OXC Architecture Suitable for High Density WDM Wavelength Routed Networks

A novel optical crossconnect architecture, capable for tens of terabit throughput and allowing the integration of wavelength and space switching, is presented for both WP and VWP networks. The architecture offers significant advantages in terms of crosstalk performance and node complexity. In the architecture, there are no crossovers and crosspoints. Therefore, the signal-to-crosstalk ratio per wavelength channel can be very high since each channel is processed independently from the others. This will also allow to introduce other all-optical techniques like adding/dropping of OTDM channels as well as optical regeneration. The proposed architecture is wavelength and link modular and particularly suitable for crossconnects with a very large number of wavelength channels. Performance aspects of a single as well as of a cascade of crossconnects have been simulated using a simulation tool.

Dense WDM multiplexer and demultiplexer with 0.4nm channel spacing

The authors report high performance dense WDM multiplexers and demultiplexers with a 0.4 nm channel spacing. Special apodised FBGs and low loss bandpass WDM filters are utilised. The demonstrated cost effective eight-channel dense WDM devices have a high channel isolation of &gt; 25 dB and an insertion loss of ~7.5 dB. The WDM devices can be expanded to 32 or 64 channels in the same structures.

Analysis of wavelength-switched high-density WDM networks employing wavelength conversion by four-wave-mixing in semiconductor optical amplifiers

In this paper transmission through an optical transparent network layer in which very high capacity data streams are processed without optical to electrical conversion is considered. Highly-dense WDM is assumed. The optical layer is superimposed to an electrical network layer in which the high capacity streams are processed at lower digital transmission hierarchies. Photonic switching is realized inside the optical layer by means of wavelength translators based on four-wave-mixing in traveling wave semiconductor optical amplifiers. The transmission performance of the network is theoretically analyzed exploiting an accurate model of the wavelength converter. ASE and phase noise accumulation along the signal path through the network, linear crosstalk and nonlinear effects during fiber propagation are taken into account. Due to the adopted method to perform wavelength conversion, chirping/dispersion induced penalties result to be greatly reduced and homo-wavelength crosstalk inside the network nodes is almost eliminated. This allows very good transmission performances to be attained even for very high capacity networks, as demonstrated by the reported theoretical results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A thermally stable Fabry-Perot tunable filter for 1 AA-spaced high-density WDM systems

A new tunable filter using two nonidentical Fabry-Perot etalons arranged on a single rotation stage has been demonstrated. The 3-dB passband, tuning range, and insertion loss were 0.2 AA, 1.2 nm, and 1.5 to 3 dB, respectively. The thermally induced wavelength deviation of the output was within 0.02 AA in the range of 0 to 50 degrees C. The sharp wavelength characteristics and the good temperature stability promised to operate FSK-DD stably where the wavelength spacing of FSK signals was 1 AA. >

Integrated grating demultiplexer and pin array for high-density wavelength division multiplexed detection at 1.5 μm

An integrated wavelength demultiplexer and pin detector array is reported for high density WDM detection at 1.5 μm. Signals spaced at 1 nm over the 1.48–1.55 μm wavelength range are separated and detected; channel passbands are 0.6–0.8 nm. Grown on a semi-insulating substrate, and providing a pseudoplanar device surface, the HD-WDM detector array is suitable for monolithic integration with high-speed multichannel receiver circuitry.

Low-threshold four-wavelength DFB laser array for multigigabit/s high-density WDM systems applications

A high-speed four-wavelength distributed-feedback (DFB) laser array with narrow wavelength spacings ( approximately 3 nm), very low-threshold currents ( approximately 5 mA), and all wavelengths, and operating (1540-1550 nm) within the erbium-doped fiber amplifier bandwidth has been fabricated. Electrical crosstalk of less than -25 dB up to 5 GHz was measured and a small wavelength drift caused by heating effects was observed for adjacent lasers. However, neither the electrical crosstalk nor the thermal effect causes any sensitivity penalties in a high-speed wavelength-division multiplexing transmission system. >

Extending transmission distance of high-density WDM systems using post transmitter fiber Raman amplifiers

The authors investigate the behavior of multichannel signals in a high-density wavelength division multiplexing (WDM) system with the consideration of stimulated Raman scattering. A parameter to evaluate the depth of Raman crosstalk is provided, and the transmission distance limitation imposed by Raman crosstalk is analyzed. To extend the system transmission distance, a post-transmitter fiber Raman amplifier is used. The allocation of signal channels on the pump gain profile is discussed. It is found that there exists an optimum pump power which results in a maximum amplifier gain. The example shows that the transmission distance limitation imposed by Raman crosstalk can be extended by 20 km for a 30-channel system with 0.25-dB/km fiber loss.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>