Part 8: WDM SYSTEM

Multiplexing Schemes Multiplexing is a term that is used to describe the concept of bringing together several (and sometimes many) independent communications streams (such as telephone calls or computer data streams) into single stream. One of the major benefits of optical communication systems is that they can carry data at extremely high bit rates. Commercial systems operating at 10Gigabits/second are available. This means that many thousands of voice channels can be simultaneously carried. Despite this, even higher system data rates are demanded, due to the explosion of services such as video links, Internet, computer networking, etc. As the required system capacity increases, it is economically advantageous to bring together many lower data rate signals and combine them into a higher data rate system. There are three main multiplexing schemes used in optical communications : (i) Optical Time division multiplexing (OTDM) (ii) Optical frequency division multiplexing (OFDM) or Wavelength division multiplexing (WDM) (iii) Sub carrier multiplexing (SCM) The decision to use either one of the multiplexing schemes is determined by the business needs of the company deploying the system.

Time Division Multiplexing (TDM) The high speed channel probably will be a combination of many lower-speed signals, since very few individual applications today utilize this high bandwidth. These lower-speed channels are multiplexed together in time to form a higher-speed channel. This TDM can be accomplished in the electrical or optical domain, with each lower-speed channel transmitting a bit (or allocation of bits known as a packet) in a given time slot and the waiting its turn to transmit another bit (or packet) after all the other channels have had their opportunity to transmit. TDM is quite popular with today’s electrical networks, and is fairly straightforward to implement in an optical network at < 100-Gbps speeds.

Time-Division Multiplexing (TDM) In time-division multiplexing, time on the information channel, or fiber, is shared among the many data sources. The multiplexer MUX can be described as a type of “rotary switch,” which rotates at a very high speed, individually connecting each input to the communication channel for a fixed period of time. The process is reversed on the output with a device known as a demultiplexer, or DEMUX. After each channel has been sequentially connected, the process repeats itself. One complete cycle is known as a frame. To ensure that each channel on the input is connected to its corresponding channel on the output, start and stop frames are added to synchronize the input with the output. TDM systems may send information using any of the digital modulation schemes described (analog multiplexing systems also exist). This is illustrated in the figure.

WDM In WDM, several baseband-modulated channels are transmitted along a single fiber but with each channel located at a different wavelength. Each of N different wavelength laser is operating at the slower Gbps speed, but the aggregate system is transmitting at N times the individual laser speed, providing a significant capacity enhancement. The WDM channels are separated in wavelength to avoid cross-talk when they are (de)multiplexed or individually recovered by WSCs. Each WDM channel may contain a set of even slower time-multiplexed channels.

Development of WDM Technology Early WDM began in the late 1980s using the two widely spaced wavelengths in the 1310 nm and 1550 nm (or 850 nm and 1310 nm) regions, sometimes called wideband WDM. See figure 1 The early 1990s saw a second generation of WDM, sometimes called narrowband WDM, in which two to eight channels were used. These channels were now spaced at an interval of about 400 GHz in the 1550-nm window. By the mid-1990s, dense WDM (DWDM) systems were emerging with 16 to 40 channels and spacing from 100 to 200 GHz. By the late 1990s DWDM systems had evolved to the point where they were capable of 64 to 160 parallel channels, densely packed at 50 or even 25 GHz intervals.

Evolution of DWDM

DWDM system The most important components of any DWDM system are transmitters, receivers, Erbium-doped fiber Amplifiers, DWDM multiplexors and DWDM demultiplexors. The figure gives the structure of a typical DWDM system.

Components of DWDM system DWDM is a core technology in an optical transport network. The essential components of DWDM can be classified by their place in the system as follows: On the transmit side, lasers with precise, stable wavelengths On the link, optical fiber that exhibits low loss and transmission performance in the relevant wavelength spectra, in addition to flat-gain optical amplifiers to boost the signal on longer spans On the receive side, photodetectors and optical demultiplexers using thin film filters or diffractive elements Optical add/drop multiplexers and optical cross-connect components

Components of DWDM system Transmitter – multiwavelength laser Receiver – APD Mux/demux – AWG and MIF Add/Drop Module Switch - OXC Dispersion compensator

Technical Challenges of Implementing WDM Stabilising Wavelengths Almost all wavelength specific optical devices are sensitive to changes in temperature. In addition, some devices such as lasers can shift in wavelength for other reasons (such as carrier density or material ageing). Within any optical networking system the stability of all wavelength sensitive devices is critical to system operation. Wavelength Conversion For large circuit-switched nodal optical networks it will be necessary to change the wavelength of individual channels as the signal is switched. Wavelength conversion technology is available but as yet not well developed. Cascading Filters In its transit through an optical network a signal passes through a number of devices which have filtering characteristics. The bandwidth, shape and alignment (of the centre wavelength) of these filters is critical to satisfactory system operation. EDFA Gain Flatness and Tilt Throughout all but the simplest optical networks EDFAs will be extensively used. Equalising the gain between different amplified channels and making sure it doesn't vary with the power of any given channel is a significant challenge.

Dispersion Compensation Dispersion compensation is a significant issue for all WDM communication. Wavelength Tunable Lasers Tunable lasers are needed in many roles. In the switched nodal optical network, the laser must switch wavelength when a channel is allocated for a particular connection. In many situations this switching may not need to be very fast (a few hundred microseconds would be more than adequate). These lasers need to be very stable and select the desired wavelength very accurately in order for the signal to stay in-band Equalising Signal Power Perhaps the biggest engineering challenge in optical networks is in equalising the signal power between channels through a complex (and changing) series of components and processes. For example, in an add/drop multiplexer the added channel should exit the device with about the same level of power as the other channels passing through it. Optical Crossconnects and Switching Elements First generation crossconnects and digital optical switches are now commercially available but there is a lot of room for improvement before this can be considered a stable

WDM Multiplexing and Demultiplexing The mixing and separation of signals is an important issue in any WDM system. Devices with excellent characteristics are commercially available. However available devices are considered very high in cost and must reduce significantly if mass usage of the technology is to become a reality. Research in this area is focusing on cost reduction (especially in the area of AWGs). It is expected that the cost (of manufacture) of AWG devices will reduce by a factor of about 20 within the next few years.

Demultiplexing the Light There are three generic approaches to demultiplexing: i. Split the mixed light up into many mixed outputs (one per required output port) and then filter each port individually. ii. Split off a single channel at a time. iii. Demultiplex the whole bundle of optical channels in one operation. 3 dB Splitter Array with Fabry-Perot Filters In this configuration cascaded 3 dB splitters are used to divide the mixed signal up into as many equal outputs as necessary. It is then necessary to separate each individual signal from the others The figure below shows an 8-port configuration. In this case the separation is achieved with separate Fabry-Perot filters.

Circulators with FBGs Input consisting of many mixed wavelengths arrives (from the left in the figure). It enters the first circulator and is output into the first FBG The FBG reflects the selected wavelength back to the circulator but allows all other wavelengths to pass. With many types of circulators this operation can involve an attenuation of 1 dB or less. The selected wavelength travels around the circulator to Port 3 where it is output. All other wavelengths continue through the FBG to the next circulator where the process is repeated for l2 Individual wavelengths are demultiplexed as we proceed from one stage to another. It can be very selective and separate very narrow channels. Cost is linear with the number of ports so it is a very competitive technology if the number of ports is small.

Multilayer Interference Filters By positioning filters, consisting of thin films, in the optical path, wavelengths can be sorted out (demultiplexed). The property of each filter is such that it transmits one wavelength while reflecting others. By cascading these devices, many wavelengths can be demultiplexed

Array Waveguide Grating (AWG) An AWG device, sometimes called an optical waveguide router or waveguide grating router, consists of an array of curved-channel waveguides with a fixed difference in the path length between adjacent channels (see Figure). The waveguides are connected to cavities at the input and output. When the light enters the input cavity, it is diffracted and enters the waveguide array. There the optical length difference of each waveguide introduces phase delays in the output cavity, where an array of fibers is coupled. The process results in different wavelengths having maximal interference at different locations, which correspond to the output ports.

AWGs are polarization-dependent (which can be compensated), and they exhibit a flat spectral response and low insertion loss. A potential drawback is that they are temperature sensitive such that they may not be practical in all environments. Their big advantage is that they can be designed to perform multiplexing and demultiplexing operations simultaneously. AWGs are also better for large channel counts, where the use of cascaded thin film filters is impractical.

Add/Drop Multiplexer (ADM) An Add/Drop Multiplexer takes a single wavelength from a trunk, pulls the signal out, and allows a new signal at the same wavelength to be inserted into the trunk at (roughly) the same spot. All the other wavelengths pass through the add/drop mux with only a small loss of power (usually a few dB). This concept is especially important when planning WDM networks, and plays an important role in the overall distance a WDM network can span. The Add/Drop Multiplexor as the name suggests, selectively adds/drops wavelengths without having to use any SONET/SDH terminal equipment. There are two types of implementations of the ADM, the Fixed WADM and the Reconfigurable WDM.

There are several devices which may perform this function such as: i. Array waveguide gratings ii. Circulators with FBGs iii. A Cascade of MZIs Array Waveguide Gratings In the figure below, a AWG is configured as an add-drop multiplexer. Wavelength l1 is added to the multiplexed stream on the left of the picture and dropped (demultiplexed) from the input stream on the right of the figure. Multiple channels can be added or dropped in the same operation. However, the signal loss is about 5 dB per pass through the device. So channels that aren't added or dropped experience an insertion loss of around 10 dB. In a system, we need to equalise the signal power between channels (so that the newly added channels are about the same strength as the dropped ones) and probably to amplify the whole stream as well.

Circulators with In-Fibre Bragg Gratings The signal enters at the left of the figure and is routed through the circulator to the FBG. The non-selected wavelengths pass through the FBG to the next circulator. The selected wavelength is reflected by the FBG and then directed out of the next circulator port. The wavelength to be added (which must be the same as the one just dropped) enters through the “add-port” of the rightmost circulator. It travels around to the FBG and is reflected back to the circulator. This process mixes the added channel with the multiplexed stream. This configuration has a relatively low loss of 3 dB for the multiplexed stream. It could be very suitable for operation in a looped metropolitan area network (MAN) where a single fibre loop interconnects many locations within a city area.

Optical Cross Connect (OXC) The Optical CrossConnect acts a crossconnect between n-input ports and n-output ports. It allows the efficient network management of wavelengths at the optical layer. The variety of functions that it provides are signal monitoring, restoration, provisioning and grooming.

Standards for WDM Standards in this area are critical for several reasons: In the early phase of a new technology you need the suppliers of components to supply devices as regular commercial (reasonably priced) components rather than as very expensive make-to-order devices. Standards are required for this to happen. The longer term goal is to allow the building of systems which use different equipment from many different suppliers. Such equipment needs standards to allow interoperation. The ITU draft standard G.692 is entitled “Optical Interfaces for multichannel systems with optical amplifiers”. This is intended to cover long distance point-to-point WDM systems using STM-4, STM-16 and/or STM-64 on 4, 8, 16 or 32 channels. The maximum link distance for a system without amplifiers is 160 km or up to 640 km with optical amplification. The draft standard specifies a wavelength reference grid based on 100 GHz spacings and a reference (centre) frequency of 193.1 THz. This (193.1 THz) approximately equals 1,553.5 nm. Users are free to use any wavelength on the grid in an arbitrary way! Users are also free to select which part of the spectrum they use. Unequally spaced channels are allowed provided the channel wavelengths are situated on the grid.

Early system implementations tend to use: 4 channels with 400 GHz (3.2 nm) spacing 8 channels with 200 GHz (1.6 nm) spacing 16 channels with 200 GHz (1.6 nm) spacing 32 channels with 100 GHz (0.8 nm)spacing For supervisory information an extra channel is needed. There is no standard for the format, bit rate or modulation protocol for this channel. Users are allowed to select any one of the nominated supervisory wavelengths (1310, 1480, 1510 and 1532 nm).