Future Optical Access Networks Research Articles

Future optical access network enabled by modularization and softwarization of access and transmission functions [Invited

This paper discusses the future direction of optical access networks and proposes a new optical access architecture that will support the evolution of optical access systems from a fiber-to-the-home infrastructure to a common optical access platform that connects various devices and systems to anywhere they want to reach. We describe two technologies for its implementation. One is “modularization,” which is expected to be realized before 2030, and the other is “full softwarization,” which is the longer-term goal of drastically increasing network flexibility. For each technology, we summarize related technical trends and discuss a reasonable system configuration focusing on the access nodes.

Electro-absorption and Electro-optic Characterization of L-Band InAs/InP Quantum-dash Waveguide

Electro-absorption and electro-optic characteristics of InAs/InP quantum-dash active region-based waveguide, emitting at ∼1600 nm is investigated. Two major peaks were observed in the change of absorption spectrum with a maximum value of 7070 cm−1 at a bias voltage of -8V with an excellent uniform extinction ratio of ∼15 dB across the wavelength range of operation (1460–1620 nm). The effect of temperature on electro-absorption (EA) measurement suggests a strong influence resulting in merging of two major change of absorption spectrum peaks with higher temperature. Furthermore, electro-optic measurements indicate a change in refractive index and its efficiency values of ∼2.9 × 10−4 and ∼0.5 × 10−4 V−1, respectively, hence exhibiting a low chirping factor of 0.9 and 1.5 at bias voltages of −2 V and −4 V, respectively. As a quasi-three-dimensionally confined structure possessing both quantum well- and quantum dot-like features, the quantum dash waveguide showed superior electro-absorption and electro-optic properties compared to quantum dots and close to that of quantum wells, while attaining low chirp and broad wavelength range of operation. This paves a way for potential realization of quantum dash-based EA and electro-optic modulator for future optical access networks, particularly operating in wide C- to L-band region.

Design of optical wavelength conversion based on cross polarization modulation effect of SOA-MZI

In this paper, the all optical up and down wavelength converter based on cross polarization modulation (XPolM) effect in semiconductor optical amplifier Mach–Zehnder Interferometer (SOA-MZI) has been proposed and evaluated for error free operation to show the feasibility of 40 Gbps differential phase shift keying modulated data signal. The wavelength conversion is achieved by rotational state of polarization of pump and probe signal in SOA. Also, investigation of wavelength converter is theoretically and analytically validated for optimum performance and investigation results show good agreement with each other. The impact of variable signal power along with different parameter of SOA on performance of converted wavelength signal in terms of optical received power and Q-factor has been given. The investigations revealed that wavelength converter based on XPolM effect in SOA-MZI can be the promising option to enhance the expandability and accessibility of future hybrid optical access networks.

Considerations on the Use of Digital Signal Processing in Future Optical Access Networks

In this invited article, we address the impacts of the rise of signal processing for increased capacity per wavelength and better receiver sensitivities in next generation optical access networks. We start by recalling the main channel limitations of currently deployed intensity modulated, directly detected, passive optical networks. Then, with the intention of providing a benchmarking of signal processing approaches used in communication systems other than optical access, we provide a historic perspective on how digital signal processing emerged in copper access systems, and we evaluate powerful techniques envisaged for future mobile generation in both air interface and radio access networks. We also assess signal processing in light of multi-vendor interoperability by providing insights on burst mode operation and the needed protocol and monitoring procedures. Interoperability with regard to optical transceivers is also considered. Last but not least, we evaluate power consumption of digital signal processing and forward error correction solutions used in disruptive, coherent transmission approaches.

Comparative Analysis of High Speed 20/20 Gbps OTDM-PON, WDM-PON and TWDM-PON for Long-Reach NG-PON2

Abstract Nowadays, bandwidth demand is enormously increasing, that causes the existing passive optical network (PON) to become the future optical access network. In this paper, next generation passive optical network 2 (NG-PON2) based, optical time division multiplexing passive optical network (OTDM-PON), wavelength division multiplexing passive optical network (WDM-PON) and time & wavelength division multiplexing passive optical network (TWDM-PON) systems with 20 Gbps (8 × 2.5 Gbps) downstream and 20 Gbps (8 × 2.5 Gbps) upstream capacity for eight optical network units has been proposed. The performance has been compared by varying the input power (−6 to 27 dBm) and transmission distance (10–130 km) in terms of Q-factor and optical received power in the presence of fiber noise and non-linearities. It has been observed that TWDM-PON outperforms OTDM-PON and WDM-PON for high input power and data rate (20/20 Gbps). Also, TWDM-PON shows its superiority for long-reach transmission up to 130 km, which is a cost-effective solution for future NG-PON2 applications.

Design of all optical contention detection circuit based on HNLF at the data rate of 120 Gbps

Abstract This paper presents the optical contention detection circuit for all-optical network operating at data rate of 120 Gbps with enhancement of four wave mixing (FWM) intereaction between the co-propagating signals in highly nonlinear fibe (HNLF). The performance of contention detection circuit has been evaluated for different input signal power levels, effective nonlinear length and other parameters of HNLF in terms of Q-factor. The proposed optical contention detection circuit can be further utilized for future optical access network on chip for detection of optical centended packets and wavelength continuity.

Power budget of 25-Gb/s digital orthogonal filter multiple access PONs with 10G-class optics

Abstract Power budget is a key factor that should be considered in intensity modulation and direct detection (IM/DD) passive optical networks (PONs). Based on the low-cost 10G-class electro-absorption modulated laser (EML) operating at C-band, we not only experimentally and theoretically investigate the interplay between the chromatic dispersion, the chirp of EML, and the self-phase modulation (SPM), but also explore the influences of the resolution and timing jitter of the analog to digital converter (ADC). Both theoretical and experimental results show that high power budget can be effectively achieved by making full use of the high launch power induced SPM effect and EML-induced frequency chirp. Based on the 10-G transmitter optical sub-assembly (TOSA) and receiver optical sub-assembly (ROSA), we successfully realize 25 Gb/s signal transmission at C-band over 25 km standard single mode fiber (SSMF) in IM/DD PON system. Even the worst performing bands can still achieve 25 dB power budget. The proposed scheme in this work has a potential application in future cost-effective optical access networks.

Wavelength reused bidirectional reflective coherent-PON based on cascaded SOA/RSOA ONUs

Abstract This paper demonstrates wavelength reused bidirectional transmission for a reflective coherent passive optical network (R-Co-PON). By reusing the downstream optical signal as the upstream optical carrier, the proposed R-Co-PON enhances the optical spectral efficiency. In an experimental demonstration, the proposed R-Co-PON achieved a symmetrical bidirectional transmission rate of 12 Gb/s using a 73-km-long fiber loopback architecture. The experiments confirmed the potential applicability of the proposed system in future optical access networks, particularly those using wavelength-division multiplexing.

100 Gb/s SDM-PON Using Polarization-Diversity Silicon Micro-Ring Resonator Enhanced DML

We experimentally demonstrate a bidirectional space-division multiplexing passive optical network system by using a commercial directly modulated laser modulated by a 25 Gb/s NRZ-OOK signal, and followed by a polarization-diversity silicon micro-ring resonator (PolD-MRR) for enhancing the modulation extinction ratio. A multi-core fiber with negligible inter-core crosstalk is used for bidirectional transmission, not only increasing the aggregated capacity but also simplifying the splitting of upstream and downstream and enabling colorless optical network units. The capacity of 100 Gb/s for downstream and 75 Gb/s for upstream has been achieved, respectively. In addition, transmission capacity could be further increased by wavelength-division multiplexing according to PolD-MRR periodical feature, which might be beneficial for future large-capacity optical access networks.

Comparison of Low Complexity Coherent Receivers for UDWDM-PONs ($\lambda$-to-the-User)

It is predicted that demand in future optical access networks will reach multigigabit/s per user. However, the limited performance of the direct detection receiver technology currently used in the optical network units at the customers’ premises restricts data rates per user. Therefore, the concept of coherent-enabled access networks has attracted attention in recent years, as this technology offers high receiver sensitivity, inherent frequency selectivity, and linear field detection enabling the full compensation of linear channel impairments. However, the complexity of conventional (dual-polarization digital) coherent receivers has so far prevented their introduction into access networks. Thus, to exploit the benefits of coherent technology in access networks, low complexity coherent receivers, suitable for implementation in ONUs, are needed. In this paper, the recently proposed low complexity coherent (i.e., polarization-independent Alamouti-coding heterodyne) receiver is, for the first time, compared in terms of its minimum receiver sensitivity with five previously reported receiver designs, including a detailed discussion on their advantages and limitations. It is shown that, of all the configurations considered, the Alamouti-coding based receiver approach allows the lowest number of photons per bit (PPB) transmitted (with a lower bound of 15.5 PPB in an ideal implementation of the system), while requiring the lowest optical receiver hardware complexity (in terms of the optical component count). It also exhibits comparable complexity to the currently deployed direct-detection receivers, which typically require over 1000 PPB. Finally, a comparison of experimentally achieved receiver sensitivities and transmission distances using these receivers is presented. The highest spectral efficiency and longest transmission distance at the highest bit rate (10 Gb/s) was reported using the Alamouti-coding receiver, which is also the only one, to date, to have been demonstrated in a full system bidirectional transmission.

Sparse-fast-Fourier-transform-based quick synchronization for optical direct detection orthogonal frequency division multiplexing systems.

Orthogonal frequency division multiplexing (OFDM) has always been a promising candidate for optical access networks. However, it is very sensitive to synchronization errors and requires complex digital signal processing to eliminate this influence, thus increasing the computation complexity, delay, and system cost, which hampers its applications in cost-sensitive and low-latency scenarios of future optical access networks. To deal with this issue, a sparse-fast-Fourier-transform (FFT)-based quick synchronization algorithm for optical direct detection OFDM systems is proposed and demonstrated with greatly reduced computation complexity. Detailed simulations and experimental verifications along 50km standard single-mode-fiber transmission prove the efficiency, accuracy, and feasibility of the sparse-FFT-based synchronization technique in cost- and delay-sensitive applications for next-generation optical access networks.

Utilization of spatial ternary coding scheme in optical CDMA system

In this paper, we propose a spatial ternary coding scheme for optical code-division multiple access systems. Since the multiple access interference (MAI) and beat noise leads to the error floor effect, using error correction mechanism is a preferable manner to further enhance the system performance. The proposed spatial ternary coding scheme possesses larger free distance, which is an effective way to relieve such deteriorating effect. It is expected that more users accommodated in the network can obtain higher throughput, while the side effects of MAI and beat noise will turn out to be more serious. To further reduce such adverse impact, the optical hard-limiter is also proposed. Once the channel condition is improved, it will give rise to the more powerful utilization of error correction code. The countermeasures utilized in the proposed system can effectively fight against the MAI and beat noise, and can allow more simultaneous users accommodated in the network. Therefore, it is a promising way for future high-speed optical access networks.

Outage probability of WDM free‐space optical systems affected by turbulence‐accentuated interchannel crosstalk

Massive deployment of free space optical (FSO) communication systems in future optical access networks will use wavelength division multiplexing (WDM) technology to leverage extended reach and enhanced user capacity. An analytical framework for the estimation of outage probability in a turbulence-affected WDM FSO system is presented in this study. The authors derive simplified expressions of the probability of outage in the presence of turbulence-accentuated interchannel crosstalk, and show how design parameters such as the FSO link length, demultiplexer adjacent channel rejection and receiver aperture diameter affect the performance of the system. Numerical results show that the outage probability due to irradiance fluctuation is worsened in WDM FSO systems by the effects of turbulence-affected interchannel crosstalk signals. Specifically, the average crosstalk power and the degree of irradiance fluctuations on the crosstalk set a limit on the outage probability performance of the system which appears in the form of outage floors.

Energy-Efficient Scheme Based on Sub-Band Grouping and Allocating for Digital Filter Multiple Access Adopted PON

An energy-efficient optical access network scheme based on sub-band grouping and allocating is proposed and studied for the digital filter multiple access adopted passive optical network (DFMA-PON). In this scheme, sub-bands generated in digital sdomain are grouped as band groups and properly allocated for optical network unit (ONU) transmissions to reduce the transmission time. Based on this, longer sleeping time can be provided to each ONU, and some more energy can be saved. Numerical evaluations indicate that comparing with basic DFMA-PON, significant energy is saved by the proposed scheme, and a promoted energy saving performance can be achieved by properly arranging the number of sub-bands assigned in a band group. Furthermore, the technology improvement and its impact on the performance of the scheme are also investigated. It is demonstrated that comparing with the conventional network, the proposed scheme possesses prominent energy saving potential. It can be considered as a potential solution for future energy-efficient optical access network.

Design and analysis of all-optical up- and down-wavelength converter based on FWM of SOA-MZI for 60 Gbps RZ data signal

In this paper, the design and analysis of the all-optical up- and down-wavelength converter based on four-wave mixing (FWM) effect of semiconductor optical amplifier Mach–Zehnder interferometer (SOA-MZI) have been presented. The return-to-zero (RZ) modulated data signal at a bit rate of 60 Gbps has been evaluated for error-free operation to show the feasibility of proposed system at different pump wavelength. The converted signal power and quality factor are investigated as the function of variable signal power and pump power. The optimized operating input signal power of −5 dBm with Q-factor of \(\sim \)28 dB for RZ modulated signal by using SOA-MZI structure with enhanced FWM effect. The important contribution of these investigations that it is possible to expand the optical network with limited available channel bandwidth by utilizing the wavelength converter and gives an approach to implement wavelength converter for future hybrid optical access networks.

Coherent Access: A Review

In this paper, we will address the benefits of the coherent detection in future optical access networks. The scarcity of the optical spectrum, the required flexibility, and constant evolution of requirements highlight the effectiveness of coherent techniques toward the future passive optical networks (PON). A set of architectures for coherent optical access networks will be presented and the key attributes of each scenario will be investigated. In addition, as a basis to decrease the cost of the local oscillator (LO) at customer side, we experimentally investigate the possibility of using a low-cost laser as LO with real-time detection of a Nyquist-shaped differential quadrature phase-shift keying (DQPSK) signal using simple 8-bit digital signal processing (DSP) on a field-programmable gate array. Moreover, we experimentally derive a set of optimized parameters and their impact on the network operation for coherent ultradense wavelength-division multiplexing (UDWDM) systems. The balance between the number of channels, power budget, and dynamic power range will be evaluated. Furthermore, we demonstrate a reconfigurable real-time receiver DSP for future flexible UDWDM-PON systems applying the DQPSK and D8PSK modulation formats. By reviewing some of the motivations for this technology, such as flexibility, spectral efficiency, as well as compatibility with software-defined networking, we show that this technology is approaching the required maturity.

Coherent Nyquist UDWDM-PON With Digital Signal Processing in Real Time

Nyquist shaping UDWDM-PON combined with digital coherent optical systems has been recently proposed as a promising technology for future optical access networks. In this paper, we report real-time transmitter and receiver for a future coherent PON with high spectral efficiency and low-cost DSP implementation. Using field-programmable gate array (FPGA)-based 8-bit DSP, we experimentally demonstrate a high performance system based on 16 × 2.5 Gb/s QPSK signals in terms of receiver sensitivity and fiber transmission. An optimal transmitted power of –7 dBm per channel is achieved for 50-km standard single-mode fiber in a 2.5-GHz grid. The experimental results show the feasibility of coherent PON scenarios based on DSP supported by FPGAs.

Adaptive coded spreading OFDM signal for dynamic-λ optical access network

This paper proposes and experimentally demonstrates a novel adaptive coded spreading (ACS) orthogonal frequency division multiplexing (OFDM) signal for dynamic distributed optical ring-based access network. The wavelength can be assigned to different remote nodes (RNs) according to the traffic demand of optical network unit (ONU). The ACS can provide dynamic spreading gain to different signals according to the split ratio or transmission length, which offers flexible power budget for the network. A 10×13.12Gb/s OFDM access with ACS is successfully demonstrated over two RNs and 120km transmission in the experiment. The demonstrated method may be viewed as one promising for future optical metro access network.

Systematic Reconditioning of Fiber Connector Polishing Process by Reducing Statistical Variance of Connector Characteristics

Optical connectors with high quality end-face polishes are crucial in future optical access network, where they must meet standard. Common polishing conditions may not always yield best results in any equipment setup due to high statistical variance in the polished connector characteristics. In this paper we propose a systematic approach to recondition the polishing process where we reduce the variance of the fiber height, radius of curvature and apex offset. The approach is enabled by our pneumatic system modification and improved controller design.

A high‐denseWDMlight source based on mixing‐modulatedFabry–Perot laser diodes

ABSTRACTA scheme of high‐dense wavelength‐division‐multiplexing light source based on external mixing‐modulated Fabry–Perot laser diode (FP‐LD) is investigated in this article. A 10‐GHz spaced 22‐channel optical carrier supply module is achieved by cascading a multiple‐longitudinal mode FP‐LD with two types of external modulators. Then a 110‐channel extendable light source scheme is proposed. This scheme has potential application prospect in future optical access network. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:403–406, 2015