Feature Articles: Optical Node and Switch Technologies for Implementing Flexible and Economical Networks

Optical Node and Switch Technologies for Flexible and Economical Networks

Mitsunori Fukutoku, Yasuhiro Sato, and Senichi Suzuki


Network traffic is increasing rapidly, and high-performance optical nodes are needed to build networks that can handle it with flexibility and efficiency. This article introduces the latest technical trends in this area, including reconfigurable optical add/drop multiplexer (ROADM) technology, which is used to implement advanced optical nodes, and optical switching technology, which is the basis for the primary devices used for ROADM.

Keywords: ROADM, optical switch, optical node


1. Introduction

The Internet and various other network services were developed based on their use via personal computers (PCs). However, with the spread of smartphones and tablet PCs, such services have become even more common and have permeated broadly into our lives. NTT’s research laboratories have continuously engaged in research and development (R&D) of optical networks as a base technology for providing network services. Domestic and international Internet traffic figures are shown in Figs. 1 and 2 [1], [2]. As network services have expanded, traffic has increased by a factor of approximately 1.4 each year. In the future, the use of smartphones and tablet PCs will continue to spread, but various other items in our daily lives will also become networked devices, or in other words, devices that are always connected to the network and that generate data without human intervention, and communicate machine-to-machine (M2M). When such an environment is realized, the data traffic passing through optical networks will surely increase further still. It has been estimated that global traffic on the Internet will triple over the five-year period from 2012 to 2017 [2]. Such rapid increases in network traffic will require optical network systems, especially long-haul and metropolitan area systems that concentrate data traffic, to accommodate the yearly increases with flexibility, efficiency, and economy.

Fig. 1. Internet traffic in Japan.

Fig. 2. Global Internet traffic.

The latest optical transport technologies achieve ultrahigh-speed communication at 100 Gbit/s on a single channel using digital coherent technology by combining coherent transmission, which uses wave properties widely used in wireless networking, and digital signal processing (DSP) technology [3]. In addition to conventional high-density optical wavelength division multiplexing (WDM), which uses fixed frequency intervals, research has begun on the use of optical frequencies with higher information density for high-capacity, ultrahigh-speed optical network systems [4].

2. Advanced optical node technology for optical networks

2.1 WDM technology

Similarly, optical cross-connect technology uses high-density WDM technology to achieve flexible and economical optical networking systems. The changes in optical network system architectures over the years are shown in Fig. 3. These networks began with point-to-point connections, which realized high capacity through optical signal WDM, and progressed through single-ring configurations using reconfigurable optical add/drop multiplexers (ROADM) able to add and drop individual optical signals, to the current economical optical network systems in multi-ring configurations using multi-degree ROADM.

Fig. 3. Optical Network Development.

An illustration of ROADM is shown in Fig. 4. Input optical signals can be dropped or added, and paths can be selected without converting optical signals to electrical signals. This allows ring network systems to be constructed economically. An example of a basic two-path ROADM configuration is shown in Fig. 4(a). Optical multiplexers and demultiplexers separate multiple wavelengths, and an optical switch adds or drops signals according to the wavelength. Signals can be added and dropped without converting them from optical to electrical, enabling high-capacity optical paths to be provided economically. The optical switches used for ROADM use quartz planar lightwave circuit (PLC) technology, which was first used in components such as optical multiplexers, demultiplexers, and splitters, and is very reliable since there are no moving mechanical parts [5].

Fig. 4. ROADM configuration examples.

2.2 CDC technology

An example of a multi-port ROADM configuration implementing a multi-ring mesh network is illustrated in Fig. 4(b). As shown in the figure, with multi-port ROADM, signals carried by multi-path fibers are not converted to electrical signals, and paths can be changed, dropped, or added (optical cross-connect operations) according to wavelength. Currently, in addition to path changes, drops, and adds, even more advanced optical cross-connect operations using ROADM are being studied. With ROADM thus far, the wavelength or path has been fixed for each trans­ponder (the optical transmit/receive interface). Colorless, directionless, and contentionless (CDC) functions are being studied that will enable implementation of flexible and economical network systems in which wavelengths and paths can be configured freely, faults can be switched out by wavelength, and optical paths can be configured remotely. ROADM using CDC functions is called CDC-ROADM. Here, colorless refers to functionality that allows input and output wavelengths on ports to be allocated freely through the addition of a variable wavelength function to the demultiplexing and multiplexing filters. The transponder wavelengths can be changed without having to physically switch connections. Directionless refers to functionality whereby the directions of input and output paths can be configured freely rather than being fixed by creating switches comprised of transponders. Contentionless refers to functionality in optical nodes implementing the previous two functions whereby any wavelength can be allocated to other paths with no restrictions on wavelength configuration. These functions enable signals to be configured freely. CDC-ROADM is described in detail with configuration examples implementing this functionality in the Feature Article entitled, “Next-generation Optical Switch Technologies for Realizing ROADM with More Flexible Functions [6].”

2.3 WSS technology

Currently, the main device used in multi-degree ROADM as a path-configurable switch is the wavelength selective switch (WSS) utilizing spatial optics technology. Wavelength-multiplexed input signals are demultiplexed using a diffraction grating, and add, drop, and path selection operations are implemented without converting signals from optical to electrical. This is done using spatial light modulators such as microelectromechanical systems (MEMS) mirrors or liquid crystal on silicon (LCOS) devices to configure paths by wavelength. It is now possible to implement multi-path path selection using MEMS mirrors or LCOS, and multi-ring optical networks have been implemented with the development of this optical switching technology. Details of WSS are introduced in the Feature Article entitled, “WSS Module Technology for Advanced ROADM [7].”

Several methods for implementing CDC functionality have been studied. One such method uses multi-cast switches using PLCs; these switches have a simple structure that can be implemented in compact form. The Feature Article entitled, “Multicast Switch Technology that Enhances ROADM Operability [8],” introduces the functions enabled in transponders.

3. Topics covered in Feature Articles

As discussed above, advances in ROADM are necessary in order to implement networks that are able to deal with the increasing traffic flexibly and economically. Currently, NTT Network Innovation Laboratories, NTT Microsystem Integration Laboratories, and NTT Photonics Laboratories are collaborating on optical-switch R&D in order to realize highly advanced ROADM. These Feature Articles introduce the initiatives underway at these three laboratories. First, CDC-ROADM, which is necessary for highly operable and reliable optical networks, is described using practical examples. Then, WSS, which realizes the per-wavelength path configuration function of ROADM, is introduced. MEMS WSS is taken as an example of WSS, and technologies required to implement it are explained, including optical design, implementation design, and control function technologies. Finally, we describe a multicast switch technology that uses PLC technology to implement CDC functions.


[1] FY2012 Information and Communications White Paper by Ministry of Internal Affairs and Communications (in Japanese).
[2] Cisco Visual Networking Index (VNI): Forecast and Methodology: 2012–2017 (in Japanese).
[3] S. Matsuoka, “Ultrahigh-speed Ultrahigh-capacity Transport Network Technology for Cost-effective Core and Metro Networks,” NTT Technical Review, Vol. 9, No. 8, 2011.
[4] M. Jinno, H. Takara, B. Kozicki, Y. Tsukishima, Y. Sone, and S. Matsuoka, “Spectrum-efficient and scalable elastic optical path network: architecture, benefits, and enabling technologies,” IEEE Comm. Mag., Vol. 47, No. 11, pp. 66–73, Nov. 2009.
[5] T. Watanabe, “Silica-based PLC optical switches designed and fabricated for OADM and OXC,” Proc. of the 9th Optoelectronics and Communications Conference/3rd Conference on Optical Internet (OECC/COIN 2004), Paper 13F2-3.
[6] Y. Sakamaki, T. Kawai, and M. Fukutoku, “Next-generation Optical Switch Technologies for Realizing ROADM with More Flexible Functions,” NTT Technical Review, Vol. 12, No. 1, 2014.
[7] Y. Ishii, N. Ooba, A. Sahara, and K. Hadama, “WSS Module Technology for Advanced ROADM,” NTT Technical Review, Vol. 12, No. 1, 2014.
[8] T. Watanabe, K. Suzuki, and T. Takahashi, “Multicast Switch Technology that Enhances ROADM Operability,” NTT Technical Review, Vol. 12, No. 1, 2014.
Mitsunori Fukutoku
Senior Research Engineer, Supervisor, Photonic Transport Network Laboratory, NTT Network Innovation Laboratories.
He received the B.S. and M.S. degrees from Tokushima University in 1989 and 1991, respectively. In 1991, he joined NTT Transmission Systems Laboratories, where he engaged in R&D of optical WDM systems. In 2000, he moved to NTT Communications, where he engaged in WDM network planning. He jointed NTT Network Innovation Laboratories in 2009.
Yasuhiro Sato
Senior Manager, Network Hardware Integration Laboratory, NTT Microsystem Integration Laboratories.
He received the B.S., M.S., and Dr.Sci. degrees in chemistry from the University of Tokyo in 1987, 1989, and 2004, respectively. In 1989, he joined NTT LSI Laboratories, Atsugi. He engaged in research on LSI interconnection technology, ultrathin-film CMOS/SOI process integration for low power applications, low-power communication appliances for ubiquitous services, ultra-high speed millimeter wave wireless communication technology, and MEMS technology for telecommunication applications. He is currently conducting research on wavelength selective switches, silicon photonics, and wearable sensor technology. He is a member of the Japan Society of Applied Physics (JSAP), the Institute of Electronics, Information and Communication Engineers (IEICE), and IEEE.
Senichi Suzuki
Vice President of NTT Photonics Laboratories.
He received the B.E., M.S., and Ph.D. degrees in electrical engineering from Yokohama National University, Kanagawa, in 1984, 1986, and 1995, respectively. In 1986, he joined NTT Electrical Communication Laboratories, Musashino, Tokyo, where he engaged in research on photonic components and guided wave optical research. He subsequently investigated silica-based planar lightwave circuits and their applications to functional and dynamic photonic components and modules. He received the Electronics Society Award in 2010 and the Achievement Award in 2013 from IEICE. He is an IEEE Fellow, a senior member of IEICE and a member of JSAP.