Optical channel monitoring can ensure quality of service and unlock the benefits of tunable technologies.
By Ulf Olin and Ulf Ohlander
Proximion Fiber Optics AB
Next-generation networks must accommodate the ever-increasing demands for more bandwidth. The common solution is to use more of the potential bandwidth of the optical fibers by increasing the transmission rate of each channel and/or reducing the channel separation. Bandwidth constraints have also prompted the need for greater network flexibility that will transform today's point-to-point and ring structures to more mesh-like topologies. The changing demands of customers imply that networks must be easily reconfigurable.
These trends are driving functionality from tunable optical add/drop multiplexers, tunable lasers, dynamic gain equalizers, etc., to the network. However, to control these new easily reconfigurable networks, it is essential to provide fast and accurate spectral measurement of each channel at locations distributed across the entire network. This article will discuss the importance of optical layer monitoring (OLM) in dynamic optical networks.
The importance of OLM equipment
From a service provider's perspective, larger revenues can be gained with a larger number of transported bits. However, to maximize revenue, service providers need to minimize the operations cost per bit, while still keeping the quality of service (QoS) that customers require. Thus, it is important for network operators to have instrumentation distributed in the network that signals when the network is not operating according to its specifications. This gives critical input for protection switching, equalization, and span monitoring. With a well-monitored network, the service provider can take the required actions before transmission errors start to occur, which is of importance with gradually degrading devices like lasers and amplifiers.
OLM equipment is used to measure the optical power, the center wavelength, and the optical signal-to-noise ratio (OSNR) for each channel of a DWDM network. This instrumentation is distributed in the network to detect, identify, and localize faults and deviations from specifications in the network. If the spectral analysis is performed fast enough, it is also possible to use the same equipment to generate alarms for protection switching.
Figure 1 shows different signal quality measurements and the response time required from the OLM equipment. Since many of the signal measurements are required at the same points in the network, it is cost-effective to integrate as many of the features as possible into a single instrument. This is especially important for the use of OLM instruments in cost-sensitive metro networks.
Transparent optical networks
Optically transparent networks consist of ultra long-haul transmission spans that interconnect optical crossconnects. The cost benefit from these networks is gained by limiting the expensive optoelectronic conversions to the edge of the network. In addition, as indicated by the terminology, these networks are transparent to bit rates and protocols. Thus, it is easy to change traffic patterns and services, which improve provisioning flexibility.
However, removing electrical conversions from transmission spans makes balancing the power of the optical channels more important. In the short term, a power imbalance can be due to new channels that are added or dropped at optical add/drop multiplexer (OADM) or optical crossconnect (OXC) nodes. On a longer time scale, imbalances can be due to degradation of the fiber and the optoelectronic components.
With dynamic spectral equalizers, it is possible to get a uniform channel power distribution. Optical layer monitor data is used to give feedback signals to the equalizers and also to the laser sources. To have stable and uniform channel powers, the OLM equipment and the dynamic equalizers must have very fast response times -- within milliseconds.
A strong driver for OLM is the trend towards increased spectral efficiency in multi-terabit DWDM networks. When the channel bandwidths become wider and the channels are more densely packed, it is of the utmost importance to have accurate control of channel power, OSNR, channel wavelength drift, and line width degradation.
As indicated by Figure 2, OLM can be used for protection switching, if the equipment can signal failing channels within milliseconds. Within that time, the network can correct itself before critical information loss has occurred. Thus, if a fiber cut occurs, service restoration will be transparent to the user.
The OADM and the OXC are switching elements used in ring and meshed networks to manage the traffic. Through the quick reconfiguration of the devices, carriers can provision and protect their networks dynamically. However, there is often a change in the output power as new channels are dropped or inserted in an OADM or OXC node. To prevent disturbances, the network needs to be rebalanced by dynamically equalizing the power of each channel.
Wavelength locking
Lighting up new wavelengths in a DWDM system while maintaining the integrity of existing wavelengths becomes increasingly difficult as the systems grow in size and complexity. The need for detection of changes in channel wavelengths (wavelength drift) calls for multi-channel wavelength locking.
With the help of spectral tuning, laser sources can maintain distinct wavelengths over a long period of time, while accounting for such things as drift of the diode-laser line centers due to thermal and mechanical effects. With optimal source spacing within the channel grid, the interference from adjacent channels can be minimized. Accurate wavelength data should be fed back to the laser source to accomplish a full-spectrum locking of the channels to the nominal grid.
Tunable lasers
Tunable transmitters and reconfigurable optical add/drop units will become increasingly common in next-generation networks. The switching of separate wavelengths improves the speed and flexibility of provisioning. It also creates efficiency in fiber utilization during protection switching. When the switching of individual wavelengths is introduced in networks, it becomes increasingly important to directly monitor the optical layer to control the channel power, OSNR, and channel wavelength drift.
In conclusion, optical layer monitoring equipment is a key enabler for the control of today's and tomorrow's complex dynamic optical networks. The monitoring functionality provides the necessary feedback for the real-time control of channel wavelengths, power, and signal-to-noise ratios. It safeguards that neighboring channels do not interfere with one other, and it can provide alarm signals for protection switching.
OLM makes it possible to build and control reconfigurable networks, with more complex topologies. Thus, operators can maintain a high QoS as well as quickly and cost-efficiently provision more bandwidth or new services upon requests from their customers.
Ulf Olin, Assoc. Prof., is senior scientist at Proximion Fiber Optics AB (Kista, Sweden). Email: [email protected]
Ulf Ohlander, PhD, is senior scientist at Proximion Fiber Optics AB (Kista, Sweden). Email: [email protected]
