Next-generation Optical Networks Research Articles

Experimental demonstration of optical MIMO transmission for SCFDM-PON based on polarization interleaving and direct detection

The OFDM-PON and SCFDM-PON based on powerful digital signal processing (DSP) are promising candidates for next-generation optical access networks. Recently polarization-division-multiplexing (PDM) transmission with direct detection has been proposed for OFDM-PON to effectively reduce the bandwidth requirement of optical and electrical components. However, the PDM scheme has high algorithm complexity. In this paper, we propose a polarization interleaving (PI) approach, which can significantly reduce the bandwidth requirement for components while achieving a similar 2×2 MIMO algorithm with coherently-detected PDM-OFDM scheme. Downstream PI-SCFDM transmission is experimentally demonstrated. The scheme can be easily extended to OFDM-PON.

Cost-Effective Upstream Scheme for the Next-Generation PON Based on Interleaved Frequency-Division Multiple Access

Recently orthogonal frequency-division multiple access (OFDMA) has been proposed in the passive optical network (PON) for downstream and upstream transmissions. Due to its powerful digital signal processing and high flexibility, OFDMA-PON has emerged as a promising candidate for the next-generation optical access networks. In this letter, we propose a cost-effective upstream scheme based on the interleaved frequency-division multiple access (IFDMA), which is a modified form of OFDMA. The IFDMA has lower peak-to-average power ratio and lower computation complexity at the transmitter, which is cost-effective for the optical network unit. Upstream IFDMA transmission is experimentally demonstrated.

4$\,\times\,$4 InAlGaAs/InAlAs Optical-Switch Fabric by Cascading Mach–Zehnder Interferometer-Type Optical Switches With Low-Power and Low-Polarization-Dependent Operation

High-speed optical switches are essential devices to construct next-generation optical networks. We have studied 2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\,\times\,$</tex></formula> 2 InAlGaAs/InAlAs hybrid-waveguide-type Mach–Zehnder interferometer-type optical switches and demonstrated low-polarization-dependent switching operation with crosstalks of less than <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${-}{\rm 20}~{\rm dB}$</tex></formula> , injection currents of about 5 mA, and response times of about 3 ns. In this letter, our conventional 2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\,\times\,$</tex></formula> 2 optical switch was extended to a 4 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\,\times\,$</tex></formula> 4 optical-switch fabric by a cascading approach. As a result, fundamental switching operation was confirmed with low power consumption and low polarization dependence.

Next-Generation Optical Network Architecture and Multidomain Issues

Optical networks continue to play an essential role in scaling network capacity at a dramatically reduced unit bandwidth cost, and the next generation of technology will continue this trend. Automation of optical networks has reduced operations costs and enabled customer control of high-bandwidth services. We anticipate application-driven control of optical networks to appear in the near future. This paper looks to emerging technologies, both at the physical layer and network control layer, with a goal of assessing their impact on next-generation optical network architectures. We look forward to orders of magnitude improvement both to the capacity and complexity of intelligent optical networks.

Evaluation of four-dimensional nonbinary LDPC-coded modulation for next-generation long-haul optical transport networks

Leveraging the advanced coherent optical communication technologies, this paper explores the feasibility of using four-dimensional (4D) nonbinary LDPC-coded modulation (4D-NB-LDPC-CM) schemes for long-haul transmission in future optical transport networks. In contrast to our previous works on 4D-NB-LDPC-CM which considered amplified spontaneous emission (ASE) noise as the dominant impairment, this paper undertakes transmission in a more realistic optical fiber transmission environment, taking into account impairments due to dispersion effects, nonlinear phase noise, Kerr nonlinearities, and stimulated Raman scattering in addition to ASE noise. We first reveal the advantages of using 4D modulation formats in LDPC-coded modulation instead of conventional two-dimensional (2D) modulation formats used with polarization-division multiplexing (PDM). Then we demonstrate that 4D LDPC-coded modulation schemes with nonbinary LDPC component codes significantly outperform not only their conventional PDM-2D counterparts but also the corresponding 4D bit-interleaved LDPC-coded modulation (4D-BI-LDPC-CM) schemes, which employ binary LDPC codes as component codes. We also show that the transmission reach improvement offered by the 4D-NB-LDPC-CM over 4D-BI-LDPC-CM increases as the underlying constellation size and hence the spectral efficiency of transmission increases. Our results suggest that 4D-NB-LDPC-CM can be an excellent candidate for long-haul transmission in next-generation optical networks.

40-Gb/s all-optical digital 4-bit priority encoder employing cross-gain modulation in semiconductor optical amplifiers

High-speed all-optical logic circuits have attracted much attention because of their important roles in signal processing in next-generation optical networks. The digital encoder is widely used in binary calculation, multiplexing, demultiplexing, address recognition and data encryption. A priority encoder allows the existence of multiple valid inputs simultaneously, identifies the priority of the request signals and encodes the priority. We propose and experimentally demonstrate an all-optical 4-bit priority encoder for return-to-zero signals at 40 Gbit/s based on cross-gain modulation in semiconductor optical amplifiers. Detuning filters after semiconductor optical amplifiers are employed to improve the output performance. Correct logic bit sequences and clear open eye patterns with extinction ratios exceeding 10 dB are achieved.

Next-generation optical access networks: Dynamic bandwidth allocation, resource use optimization, and QoS improvements [Guest Eeditorial

Rapidly increasing traffic demands of current residential and business applications and newly evolving services require access network solutions that can offer dramatically higher bandwidth. New applications tend to be media-rich such as high-definition television (HDTV), video on demand (VoD), voice over IP (VoIP), and high-speed Internet. Emerging applications include multimedia conferencing, multiplayer online gaming, online content generation, and consumer-oriented cloudcomputing solutions, and are even more demanding and bandwidth-hungry. As a result, future wireline access networks will be faced with the challenge of transporting the ever increasing volume of data-centric traffic with ever tighter timing and quality of service (QoS) requirements. The seven articles in this special issue focus on next generation optical network communications and networking.

OFDM for Next-Generation Optical Access Networks

In this tutorial overview, the principles, advantages, challenges, and practical requirements of optical orthogonal frequency division multiplexing (OFDM)-based optical access are presented, with an emphasis on orthogonal frequency division multiple access (OFDMA) for application in next-generation passive optical networks (PON). General OFDM principles, including orthogonality, cyclic prefix use, frequency-domain equalization, and multiuser OFDMA are summarized, followed by an overview of various optical OFDM(A) transceiver architectures for next-generation PON. Functional requirements are outlined for high-speed digital signal processors (DSP) and data converters in OFDMA-PON. A techno-economic outlook for such a “software-defined,” DSP-based optical access platform is also provided.

Terabit Optical Access Networks Based on WDM-OFDMA-PON

Next-generation optical access networks are envisioned to evolve into a converged, high-speed, multiservice platform supporting residential, business, mobile backhaul, and special purpose applications. Moreover, bandwidth demand projections suggest that terabit aggregate capacity may need to be reached in such next-generation passive optical networks (PON). To satisfy these requirements while leveraging the large investments made in existing fiber plants, a wavelength division multiplexed (WDM)-based long-reach PON architecture combined with a multiple access technology that features a passive last-mile split, large per-λ speeds, and statistical bandwidth multiplexing can be exploited. In this paper, the first terabit PON based on hybrid WDM orthogonal frequency division multiple access (OFDMA) technology is proposed and experimentally verified. To enable high-speed, long-reach transmission with simplified optical network unit (ONU)-side digital signal processing, multiband OFDMA with ONU-side sub-band selectivity is proposed. Design challenges and tradeoffs between analog and digital domain sub-band combining and selection are also discussed. Finally, the experimental setup and results of the first 1.2 Tb/s (1 Tb/s after overhead) symmetric WDM-OFDMA-PON over 90 km straight single-mode fiber and 1:32 passive split, featuring multiband OFDMA, digitally selective ONUs, and a coherent-receiver OLT are presented and analyzed. By supporting up to 800 ONUs with 1.25/10 Gb/s guaranteed/peak rates and exhibiting a record rate-distance product achieved in long-reach PON, the demonstrated architecture may be viewed as promising for future converged terabit optical metro/access.

The GMPLS Controlled Optical Networks as Industry Communication Platform

In this paper, Generalized Multiprotocol Label Switching (GMPLS) controlled optical networks are considered as industry communication network. GMPLS is a Next-Generation Network technology which will be used to build new Internet backbone. Thanks to the features of a new public network, Internet can become a platform for industrial communication including critical data, transactional flow, and real-time data. The benefits of such a network for industry are explained in this paper. Next, the idea and architecture of intelligent, next-generation optical transport networks is discussed and explained. One of the challenges of GMPLS technology is a reliability as the main factor which influences service quality. Therefore, the method of improving reliability of GMPLS is proposed. This method bases on the implementing of redundancy in the control plane. Using Monte Carlo simulation and proposed reliability analysis method quantity improvement of GMPLS network reliability is shown and the merits of using this method are confirmed. Achieved simulation results and analysis confirm also that this emerging optical transport network will offer industry long distance communication with transmission condition compared to what is provided by dedicated WAN links. Simultaneously using public infrastructure can reduce cost of long distance communication.

Back Cover: Thin film optimization design of organic–inorganic hybrids for waveguide high‐rejection optical filters (Phys. Status Solidi RRL 8/2011)

Carlos M. S. Vicente et al. (see pp. 280–282) have fabricated high-rejection optical filters based on first-order Bragg gratings which were inscribed in channel waveguides through UV-laser writing in transparent, low surface roughness (<0.1 nm), and low-loss (1.5 ± 0.2 dB cm–1, 532 nm) thin films of organic–inorganic hybrids with controlled refractive index values stable under aging. The high-rejection figure of merit (∼24 dB) of these optical filters is the best value found until now for organic–inorganic hybrids, reinforcing the potential of sol–gel technology in the fabrication of cost-effective integrated optics devices for the next generation of optical networks.

Invited Paper: Challenges in next-generation optical access networks: addressing reach extension and security weaknesses

Recently, broadband optical access networks were being deployed worldwide to deliver large bandwidth to end users. Currently, the operators have been mainly deploying time-division multiplexing (TDM) passive optical networks (PON) in their networks. As bandwidth demand continues to increase, the industry further selected several next generation optical access (NGA) options to upgrade from current TDM-PON architectures. Besides increasing bandwidth requirement, current TDM-PONs suffers from limited reach and split-ratio. To further reduce the capital and operational expenditure requirements of current TDM-PON, NGA architectures must consolidate the number of central office sites. Long-reach optical access (LROA) architectures are promising solutions that employ optical amplifiers to increase optical budget to support longer reach and higher split-ratio in the distribution network. As more end users are supported over shared physical medium in LROA, access network security is of greater importance. The first part of this article reviews important LROA architectures and their enabling technologies. Moreover, few works in current literature address security weaknesses in LROA. The second part of this article reviews these security weaknesses as well as monitoring and control techniques employed in current TDM-PONs. In addition, a novel class of quasi-passive and reconfigurable devices is presented that addresses aforementioned security weaknesses and is suitable for LROA.

High-Bandwidth Graded-Index Plastic Optical Fiber With Low-Attenuation, High-Bending Ability, and High-Thermal Stability for Home-Networks

The graded-index plastic optical fiber (GI-POF) is expected to be a communication medium for the next-generation optical home network because of its simple-to-use connection, installation, and high bandwidth. In spite of the expectation, we had a problem that a typical GI-POF using poly (methyl methacrylate) (PMMA) had high transmission loss in the expected communication wavelength band (VCSEL: 670-680 nm) for home networks; the required values of being below 200 dB/km could not be achieved. We, therefore, propose poly (2, 2, 2-trichloroethyl methacrylate) (PTCEMA) as a base material for the GI-POF. A PTCEMA-based GI-POF was fabricated, and its characteristics were evaluated. The PTCEMA is a prominent material in terms of its transparency and heat-resistant property. Our results demonstrated that the fabricated fiber surpassed the desired characteristics for the home network pertaining to attenuation and heat resistance. Specifically, the attenuation in the wavelength band (670-680 nm) was 104-136 dB/km, and the glass transition temperature (Tg) was 102°C in the core center where the Tg was at its lowest. Moreover, we confirmed that our PTCEMA-based GI-POF had sufficient mechanical strength and low bending loss. These results indicate that our novel GI-POF can be a candidate for home networks.

Preprocessing-Free All-Optical Clock Recovery from NRZ and NRZ-DPSK Signals Using an FP-SOA Based Active Filter

We demonstrate a simple scheme to perform all-optical clock recovery from the input nonreturn-to-zero (NRZ) and nonreturn-to-zero differential phase shifted keying (NRZ-DPSK) data, which are avoided using any preprocessing measures. A multi-quantum-well Fabry-Pérot semiconductor optical amplifier plays the dual role of the data format converter and the clock recovery device. Using this scheme, a stable and low jitter 35.80-GHz optical clock pulse sequence is directly extracted out from the input NRZ or NRZ-DPSK data. This scheme has some distinct advantages such as simple device fabrication, transparence to data format, multiwavelength operation, free preprocessing and convenient tuning. Potential powerful adaptability of this scheme is very important for next-generation optical networks, in which there exist various modulation formats and the used devices are required to be transparent to data formats.

25.78-Gb/s Operation of RSOA for Next-Generation Optical Access Networks

We report the 25.78-Gb/s operation of the reflective semiconductor optical amplifier (RSOA) for the next-generation optical access network. For this purpose, we develop a butterfly-packaged RSOA and minimize the electrical parasitics. As a result, the modulation bandwidth of RSOA is improved from 2.2 to 3.2 GHz (which is the fundamental limit imposed by the carrier lifetime). In addition, the slope of the RSOA's frequency response curve is enhanced from -40 to -20 dB/decade. Using this butterfly-packaged RSOA, we have demonstrated the 25.78-Gb/s operation. The receiver sensitivity is measured to be -11 dBm with the help of the electronic equalization and forward-error-correction (FEC) techniques. To evaluate the possibility of implementing the 100-Gb/s passive optical network (PON) by using this RSOA and the coarse wavelength-division-multiplexing (CWDM) technique, we evaluate the BER performance at four different wavelengths in the C-band. The results show that the error-free transmission can be achieved in the wavelength range of 20 nm with a penalty less than 2 dB. Thus, we can realize the 100-Gb/s PON cost-effectively by utilizing the directly modulated RSOAs operating at 25 Gb/s.

Joint port-cost and power-consumption savings in hybrid hierarchical optical networks

In optical networks, with the number of wavelengths in fibers increasing, the size and cost of Optical Cross-Connects (OXC) have been increased. Waveband merging technology, which groups several wavelength routes into a single waveband tunnel with at least two hops, could perform port-cost savings by reducing the size and cost of OXC since switching one waveband in the All Optical (OOO) domain only consumes two ports at each bypass node. On the other hand, the absence of wavelength conversion restricts the capability of OOO–OXC. Therefore, designing a hybrid hierarchical node to transmit wavebands in an OOO switch and add/drop local requests or perform wavelength conversion in an Optical-Electrical-Optical (OEO) switch has been one of the promising solutions for next-generation optical networks. However, due to the increasing effects of greenhouse gases and the drastic consumption of power, port-cost and power-consumption savings are becoming the new and twin objectives to further develop hybrid hierarchical optical networks. In this paper, we jointly consider port-cost and power-consumption in a hybrid hierarchical optical network and propose efficient approaches including Integer Linear Programming (ILP) models and heuristics. Simulation results of two heuristic approaches, a Cost and Power savings algorithm based on an End-to-End waveband Merging strategy (CPEEM) as well as a Cost and Power savings algorithm based on a Sub-Path waveband Merging strategy (CPSPM), show that the cost of Transmitting Ports (TPs) used in an OOO switch is in contrast to the use of the power consumed by the OEO part of a hybrid hierarchical node. Moreover, comparing with CPEEM, CPSPM has a better performance for port-cost savings but the performance improvement is at the cost of higher power consumption in an OEO switch. On the contrary, CPEEM has a better performance for power-consumption savings but the performance improvement is achieved at the expense of a higher cost of transmitting ports comparing with CPSPM. Based on the parameter, Boundary of Consumed Power (BCP), a New Integrated Algorithm (NIA) that could effectively choose between CPEEM or CPSPM according to the current network state is proposed to obtain the trade-off performances.

Control Traffic Flows and Differing Quality of Service with Reservation of Bandwidth in Optical Network

In this paper, a traffic flow control scheme is presented. Wavelength division multiplexing (WDM) technology have significantly increased the transmission capacity of today’s transport networks, and played an extremely important role in high-speed network. Since high bandwidth wavelength channels will be filled up by many low-speed traffic streams, efficiently provisioning customer connections with such diverse bandwidth needs is a very important problem and is also known as the traffic-grooming problem. Traffic grooming is an extremely important issue for next-generation optical WDM networks.

A Novel MAMSK-OFDM Technology for Next-Generation Optical Access Networks

We have proposed a novel multiamplitude minimum shift keying orthogonal frequency-division-multiplexing (MAMSK-OFDM) technology for the optical access network. In our experiment, the 5-Gb/s 2AMSK-OFDM signal and 2.5-Gb/s upstream on-off keying (OOK) signal are transmitted over 20-km single-mode fiber (SMF) successfully. We further compare the MAMSK-OFDM signal with 4-quadrature amplitude modulation (4QAM)/8QAM-OFDM and MSK-OFDM signals, and the results show the prospect of this technology in future optical access networks.

Design of broadband erbium-doped fiber amplifier using very high-speed integrated circuit hardware description language for next-generation optical network

We mainly concentrate on the problem of gain broadening and flattening of erbium-doped fiber amplifier (EDFA) using dual forward-pumping configuration in which we use pumping both at 980 and 1480 nm wavelengths. The gain cascading of two EDFAs pumped separately at 980 and 1480 nm combines the advantage of high gain characteristics of 980 nm pumping and broad bandwidth characteristics of 1480 nm pumping. The use of increased number of channels in the present dense wavelength division multiplexing (DWDM) optical network requires a flat gain spectrum across the whole usable bandwidth. The use of very high-speed integrated circuit hardware description language (VHDL-AMS) in lightwave system modeling has been explored. Its application in designing wideband EDFA has not been reported thus far. This work facilitates to design broadband EDFA operating in both the C-band and L-band regions with VHDL-AMS. The amplifier design is made of a dual stage, copumped at 980 and 1480 nm, which delivers an average output power of -16 dBm over the entire 3-dB bandwidth of 80 nm.

Subcarrier multiplexed optical label swapping networks

A promising approach is presented for routing and forwarding optical packets with 10 Gbit s rate based on the information of a label conveyed by a subcarrier multiplexed (SCM) signal around 18 GHz. Although the application of an SCM label for routing optical packets has already been demonstrated, there are still many open problems. Therefore a complete routing node was developed and its functionalities were investigated. Six parallel channels were operated in a 50 GHz dense wavelength division multiplexing (WDM) system without any noticeable interference between the channels containing the SCM labels as well. The degradation of signal transmission over many cascaded hops was also investigated. Only 1 dB penalty was obtained for 16 cascaded hops at 10 -12 bit error rates. A novel method was developed for routing packets with variable length which is important for the transmission of IP signals. The experimental results show the feasibility of the presented approach for the next-generation optical internet networks.