Transmitter and receiver solutions for VCSEL exploitation in access and metro networks

Internet-generated traffic growth is forcing the development of new low-cost solutions in metropolitan area networks (MANs), in particular in the transceiver and network node architectures. Semiconductor optical amplifier (SOA)-based wavelength blockers can be used as fundamental building blocks to add and drop optical channels in the node architecture of several network hierarchical levels. Even if its employment is advantageous in terms of costs and amplification bandwidth, the SOA can operate in a nonlinear regime. This work analyzes the impact of SOA-based node crossing on high-capacity discrete multitone (DMT) signals. In order to properly evaluate the interplay between optical channels and SOA self-gain modulation, both external and direct modulations are considered. Dual-sideband (DSB) and single-sideband (SSB) DMT variants are taken into account, showing no significant difference in the impact of SOA crossing for external modulation of a DFB laser. On the other hand, an important effect of subcarrier suppression arises on DSB DMT direct modulation of a vertical cavity surface emitting laser (VCSEL), less consistent for SSB DMT direct modulation. The analysis allowed to properly choose the bias current for the SOA employed in an experimental setup used to evaluate the transmission performance in a MAN scenario including add-drop lossless switch nodes. The experimental results demonstrate that a target capacity of more than 50 Gb/s per channel can be achieved in a 116-km MAN network composed of an SOA-based metro-access node and two metro-core aggregators, considering the transmission of three 25-GHzspaced DMT channels.

Network infrastructure plays a key role in the success of added services and in user satisfaction. It is widely accepted that Passive Optical Networks (PON) are the most promising, cost-effective, and high-performance access network solutions. Access networks are also commonly referred to as either ‘the last mile’ by the operators, or ‘the first mile of the network’ in IEEE terminology. The term ’mile’ is often related to the path portion that is used to reach the user from a network node, however access networks go as far as 20 km depending on the technology used. An access network comprises connections between different subscribers and a Central Office (CO), which is attached to the metro or core network. The wired technology deployed varies significantly from one country to another, i.e. Digital Subscriber line (xDSL), based on copper wires; Hybrid Fiber-coax (HFC), and optical fiber. The trends for Next Generation Access Networks (NGA) based on PON are: Wavelength Division Multiplexing (WDM), 10 Gb/s or more, and longer reach/higher splits. NGA must be able to cope with challenges, such as delivering diverse broadband services and facilitating the integration of different technologies. Moreover, NGA should provide higher bandwidths or further reduce the cost of existing delivering services, and serve as backhaul of wireless access networks (WiFi, WiMAX). The last one constitutes a relevant issue for the convergence between wireless and wired technologies.

The panorama of the “Last mile” is rapidly evolving (or has recently evolved) in many countries under the pressure of demand for broadband Internet and new services requested both by the residential and the business subscribers. In the convergence towards ubiquitous IP and Ethernet in access and metro networks, legacy fixed-infrastructure access-network technology is transforming from xDSL to fibered access (in particular, passive optical networks), also thanks to new low-impact deployment techniques. On the other hand, alternative technological solutions, competing with fibre, offer less bandwidth but in change require lower investments (e.g. HFC, PLC) and/or ensure user mobility (e.g. WiMax, LTE, LEO/MEO satellites). This short tutorial paper provides a quick bird-eye overview of the evolution of the access network, from present to future technical solutions and technologies for implementing the “Last mile”. A comprehensive picture looking at the entire panorama of broadband access, and not just at a single technology or only at the latest findings, is of key importance, in our view, for understanding the changes and for adapting to the field reality when providing broadband to private users.

With the mass availability of Ethernet in local area networks (LANs) deployed globally and connecting LANs to metropolitan area networks (MANs) and wide area networks (WANs) via the current transport technologies there is a need to resolve bandwidth gap between access networks and transport networks. One of the proposals to overcome the famous last mile issue is to use a passive optical network (PON) for this networking segment. Varieties of access methods are under development to increase the offered potential transport capacity of passive optical networks (PONs). With the emergence of Ethernet as the convergence layer in access networks, a new breed of PON, namely Ethernet PON (EPON) has evolved. Ethernet passive optical network is one of the potential solutions, offering to solve the bottleneck issue of access networks and adaptability of carrying Ethernet frames from subscriber to service provider. In this study we will review Ethernet Passive Optical Networks (EPONs) and then we propose a delay-based provisioning algorithm to be able to offer differentiated services and QoS in EPON. Mathematical analysis and simulation results will be presented also.

Next generation access networks are leveraging on the efficiency of fiber access networks to deliver higher data rates in a sustainable manner balancing network deployment costs and the network energy footprint. Towards this direction Passive Optical Networks have been attracting a lot of interest since they can deliver an optimal balance of these two critical factors. However, the transition to next generation optical networks is still an issue under research since no mature technology has yet prevailed. Beyond the cost of the equipment that is used to develop the access systems the overall network architecture must be carefully studied and engineered in order to optimize overall system costs, taking power consumption into account. In this paper we demonstrate how a hybrid fiber access network architecture can combine mature TDM PON technologies and WDM multiplexing to exploit the benefits of an ultra long reach access network leading to a collapsed access and metro network architecture. We show that this combination can lead to lower overall power consumption mainly due to efficient traffic aggregation, node consolidation and simplification of metro and core network design.

Real-time wavelength switching and routing at key access network nodes is an emerging fundamental functionality requirement for transparent content resolution and wavelength assignment towards better utilization of network resources under dynamic traffic patterns. In this paper, we experimentally demonstrate the first vertical cavity surface emitting laser (VCSEL)-based broadband switch for ultra-wide wavelength data traffic routing in Datacom. An 850 nm multimode VCSEL is directly modulated with 8.5 Gbps data and successfully transmitted error free over 100.3 m of OM3 multimode fibre link. At the integration node, the received data signal is used to drive a second VCSEL at 1550 nm, and forwarded over a second network link over 24.7 km of single mode fibre. We therefore achieve the first reported all-optical, real-time, inter-band wavelength switch to C-band. By exploiting wavelength tuneability, the 1550 nm VCSEL-based forwarding node is tuned to over 3.2 nm spectral range, for ultra-wide data routing over the second network link. A receiver sensitivity of -14.28 dBm of the converted signal is achieved at back-to-back analysis at the integration node. An additional penalty of 2.72 dB is introduced over the 24.7 km of single mode fibre transmission link. This work offers a viable enabling development technology for broadband wavelength converters for application real-time wavelength routing in the access network, to address content resolution and wavelength assignment problem for current and future Datacom.

FTTH development is slow but steady in China. The phenomenon is rooted from some major challenges, such as cost, limited bandwidth in metro networks, bandwidth consumption behaviours. To address above challenges, FTTH architectures and bandwidth consumption behaviours are studied in the paper. To remove the bandwidth bottleneck between access and Metro networks and reduce cost, LR-PON (Long-Reach Passive Optical Network) is suggested to be applied in two ways in China. For resolving the challenge of bandwidth exhaustion in Metro networks caused by high-capacity consumption in access networks, a new bandwidth scheduling, Bandwidth on Schedule (BoS) is proposed to improve efficiency of bandwidth utilization. In the paper, the proposed solutions to challenges will be helpful to increase the penetration rate of FTTH deployment in China.

Vertical cavity surface emitting lasers (VCSELs) are low cost and reliable light sources for high-speed local area and storage area network (LAN/SAN) optical fiber data communication systems and short-reach computer interconnects. The continuing rapid increase of serial transmission data rates driven by multi-core microprocessor's bandwidth upgrades cannot be sustained via conventional copper-based links as bit rates move beyond 10 Gbit/s and distances greater than 1 m. The intrinsic limitation of copper at high single-channel data rates facilitates the need to transition to optical fiberbased links at ever shorter distances. For LAN/SAN applications the 850 nm wavelength is standard. This same wavelength is also the standard for several other evolving short-reach application areas including Fibre Channel, CEI, USB, InfiniBand, and HDMI optical link systems. Herein we present our recent results on 850 nm oxide-confined VCSELs operating at data bit rates up to 40 Gbit/s. The low operational current density in the range of ~10 kA/cm2 ensures viable device reliability and long-term stability based on well-known industry certification specifications. Key VCSEL device parameters including the relaxation resonance frequency, damping, and parasitic cut-off frequency are determined for VCSELs with oxide-confined apertures of various diameters. We find that a parasitic cut-off frequency of 24-28 GHz limits the VCSEL's high speed operation at the highest optical modulation rates. We believe that with some effort the device parasitics can be further reduced such that current modulated VCSELs can be realized with larger than 30 GHz optical modulation bandwidth and reliable and practical operation beyond 40 Gbit/s.

We investigate a novel dispersion compensating technique in double sideband (DSB) modulated and directed-detected (DD) passive optical network (PON) systems using super-Nyquist image induced aliasing. We show that diversity is introduced to the higher frequency components by deliberate aliasing using the super-Nyquist images. We then propose to use fractional sampling and per-subcarrier maximum ratio combining (MRC) to harvest this diversity. We evaluate the performance of conventional orthogonal frequency division multiplexing (OFDM) signals along with discrete Fourier transform spread (DFT-S) OFDM and code-division multiplexing OFDM (CDM-OFDM) signals using the proposed scheme. The results show that the DFT-S OFDM signal has the best performance due to spectrum spreading and its superior peak-to-average power ratio (PAPR). By using the proposed scheme, the reach of a 10-GHz bandwidth QPSK modulated OFDM-PON can be extended to around 90 km. We also experimentally show that the achievable data rate of the OFDM signals can be effectively increased using the proposed scheme when adaptive bit loading is applied, depending on the transmission distance. A 10.5% and 5.2% increase in the achievable bit rate can be obtained for DSB modulated OFDM-PONs in 48.3-km and 83.2-km standard single mode fiber (SSMF) transmission cases, respectively, without any modification on the transmitter. A 40-Gb/s OFDM transmission over 83.2-km SSMF is successfully demonstrated.

High-frequency discrete and distinct mode-beating noise (MBN) spectra are an anomalous phenomenon observed in the related intensity noise spectrum from the vertical-cavity surface-emitting laser (VCSEL), which reveals strong correlation with fine-splitting transverse modes caused by non-circular emission aspect of the VCSEL with imperfect true-roundness. Such a phenomenon is demonstrated for the first time, which seriously degrades the transmission performance of high-speed and broad-band encoding data carried by the multi-mode (MM) VCSEL biased under a large DC bias. In this work, the 4-ary quadrature-amplitude modulation (4-QAM) orthogonal frequency-division multiplexing (OFDM) and the non-return-to-zero on–off keying (NRZ-OOK) transmissions with the direct modulation of different 850-nm MM VCSELs, as suffered from the MBN-induced notches of the signal-to-noise-ratio (SNR) spectra, are characterized under their high-level lasing conditions with multi-mode numbers. The significant SNR degradation on the receiving and decoding performances caused by the cross-correlation between MBN and data spectra is also examined by tuning the lensed fiber along the transverse (X/Y)-axis of the VCSEL (perpendicular to the surface normal) when collimating the output beam, which finds the disorderly shifted MBN spectra, as attributed to the collection of light emitted from the overlapped gain region of different transverse modes. In contrast, the MBN reveals invariant spectral features while the lensed fiber is either back-and-forth tuning along the vertical (Z)-axis (parallel with the surface normal) or tilting its angles from the surface normal under the same horizontal position and distance concerning the MM VCSEL. Such MM VCSEL exhibits several MBN spikes to decay the SNRs of NRZ-OOK and QAM OFDM data encoded around 21 and 34 GHz. For other MM VCSELs with reducing numbers of their transverse lasing modes and deforming roundness of their circular emitting aspects, the corresponding MBN spectral peaks can also be respectively observed at 14/17/24 GHz and 4.5/10.5/13 GHz to degrade the SNRs of the encoded data at such frequencies. Even though pre-emphasized encoding technology is employed, such an MBN effect is strongly correlated with the asymmetric circular aspect of the VCSEL with a degraded roundness ratio, which still affects the data modulation performance of the MM VCSEL for data center communication.

This article describes Ethernet passive optical networks, an emerging local subscriber access architecture that combines low-cost point-to-multipoint fiber infrastructure with Ethernet. EPONs are designed to carry Ethernet frames at standard Ethernet rates. An EPON uses a single trunk fiber that extends from a central office to a passive optical splitter, which then fans out to multiple optical drop fibers connected to subscriber nodes. Other than the end terminating equipment, no component in the network requires electrical power, hence the term passive. Local carriers have long been interested in passive optical networks for the benefits they offer: minimal fiber infrastructure and no powering requirement in the outside plant. With Ethernet now emerging as the protocol of choice for carrying IP traffic in metro and access networks, EPON has emerged as a potential optimized architecture for fiber to the building and fiber to the home.

<p>Upcoming metro networks are required to transport huge and highly dynamic data traffic whilst resulting cost and power efficient. Thus, novel metro network architectures are being investigated to increase the transport capacity (e.g., leveraging dense photonic integrated technologies), and provide effective SDN-based network programmability. The EU-H2020 PASSION project targets such a metro network design exploiting: i) programmable, modular, cost-efficient and low-power sliceable bandwidth variable transceivers (S-BVTs) formed by a pool of vertical cavity surface emitting lasers (VCSEL) and Coherent Receivers (CO-Rx); ii) hierarchical switching nodes in a flexi-grid network to attain all-optical transport and aggregation. The SDN-based control functions considers the features of the adopted PASSION transport layer. First, we detail the defined interfaces (APIs) for the automatic transport network programmability. Such (RESTful-based) interfaces are validated within the ADRENALINE testbed. Next, upon dynamic connection requests with heterogenous data rate demands, an on-line Routing and Spectrum Assignment (RSA) algorithm based on a modified K shortest path (K-SP) is proposed. To accommodate a connection request, a set of optical flows having their own S-BVT VCSEL and CO-Rx devices at the endpoints and path's frequency slots are selected and configured. Each optical flow is subject to the spectral continuity and contiguity constraints. The performance of the RSA algorithm for several traffic loads is experimentally evaluated through different figures of merit (e.g., blocked bandwidth ratio, average used of S-BVT devices, etc.) and diverse RSA's K-SP settings.</p>

This paper presents for the first time the transmission of LTE signal based-on Radio-over-Fiber (RoF) scheme by using SiGe Hetero-junction Photo-Transistor as photo-receiver. The photo-transistor is fabricated based-on the commercial bipolar transistor technology. An 850 nm single mode Vertical Cavity Surface Emitting Lasers (VCSELs) and Standard Single Mode Fiber (SSMF) are used to complete the link so that makes the total cost of the system very low. The transmission of 20 MHz bandwidth LTE signal with high modulation format is achieved. The performance of the system is evaluated through the measurement of Error Vector Magnitude (EVM) under different operation conditions of the photo-transistor (in photodiode mode, photo-transistor mode and two terminals mode). We reach an EVM below 8% for the input power of -25 dBm to -2 dBm when the photo-transistor operates under two terminals condition. This EVM value fulfills the requirements of the LTE standard for 64-QAM modulation. The stability of the system with regard temperature variation and modal noise is also presented.

The ever-increasing demand for improved speeds, expanded bandwidth, and reduced latency in emerging applications has not only driven the recent advanced algorithm breakthroughs but also fostered closer integration of metro and access networks. This work proposes a novel all-optical metro-access integration network (MAIN) enabled by coherent digital subcarrier multiplexing technology. This specially designed architecture can effectively eliminate the latency and jitters caused by the optical–electrical–optical conversion in the conventional scheme and save wavelength resources at the same time. For experimental validation, we successfully demonstrate 400G bidirectional coherent transmission within our proposed architecture, involving three nodes in the metro network and the access network as an example. Also, the related problems during the transmission are extensively discussed with the experimental results. Finally, we reach an aggregation rate of 4×100Gbps using a DP-16QAM signal. By integrating a SOA into the transmitter of the ONU, a power budget of 32.6 dB with a 29 dB dynamic range is achieved. In addition, the experimental results also show that the proposed all-optical MAIN architecture can be smoothly combined with the TFDM scheme to further enhance the flexibility, which also paves the way for further research on the next-generation coherent metro and access network.

This paper presents the advances and challenges of design, integration, and performance evaluation of a space-division multiplexing passive optical network (SDM-PON) system based on newly developed 7-core weakly-coupled multicore fiber (WC-MCF). A long-wavelength vertical cavity surface emitting laser (VCSEL) array, customized for O-band operation, was developed and experimentally characterized. To address optical alignment challenges, both expanded beam coupling and adaptive coupling using spatial light modulators were examined. Meanwhile, experimental results under low-crosstalk conditions demonstrated that simplified single-input-single-output (SISO) digital signal processing (DSP) offers comparable performance to multiple-input-multiple-output (MIMO) DSP in the proposed SDM transmission system, enabling reduced complexity and power consumption. Our proposed SDM-PON architecture also delivers performance comparable to standard single-mode fiber (SMF) configurations, with the potential for increased capacity and reduced fiber deployment costs. The scope is then broadened to investigate the scalability of general WC-MCF-based SDM links toward higher core counts. The study also mentioned key design trade-offs and technical challenges at the device and system levels, including limitations in VCSEL bandwidth, coupling efficiency and laser chirp. Overall, the proposed SDM-PON architecture demonstrates strong potential for cost-effective and energy-efficient deployment in future access networks.