Dense wavelength-division multiplexing requires channel monitoring

Aug. 1, 1997

Dense wavelength-division multiplexing requires channel monitoring

In our second look at dwdm test requirements, the author argues that channel monitoring equipment provides the best view of network activity.

Adrian Meldrum Queensgate Instruments Ltd.

iber-optic networks for telecommunications are rapidly reaching capacity as demand increases at a greater rate than expected. At peak times, long-distance carriers have used a large percentage of their existing capacity, with the remaining fiber being of lowest quality. Dense wavelength-division multiplexing (dwdm) has emerged as a leading contender to upgrade capacity by expanding the bandwidth of existing installed fiber.

The aim of an optical communications system is to provide transmission of error-free data at a low cost, with a minimal amount of downtime. System downtime resulting from channel failure can cost thousands of dollars per second. It is therefore desirable to monitor individual channel performance in a bid to detect possible problems before actual failure, with the monitor acting as an early warning system.

Unfortunately, commercially available dwdm systems do not currently offer embedded channel monitoring solutions as standard. Furthermore, current test equipment--which relies on measurements of total channel power and is limited to detecting channel failure after the event has occurred--has not, until recently, offered system manufacturers and users the required measurements for channel monitoring. However, recent developments have led to a new generation of channel monitoring equipment that meets the demanding requirements for such systems. The parameters to consider when evaluating such equipment are discussed below.

Channel monitoring requirements

Effective channel monitoring requires determination of the wavelength, power, and signal-to-noise ratio (snr) of each populated channel. For example, measurement of channel wavelength is vital for reliability and to ensure that channel interference does not occur. As channel spacing becomes tighter, the stability of the channel central wavelength becomes increasingly important. Deviation from the central wavelength as a function of time or temperature can cause crosstalk problems between channels, while drift of particular sources toward the edge of their transmission window can cause data transmission errors and eventual channel failure.

Power measurements are again vital in ensuring transmitter operation for each channel for reliability and performance optimization. Power degradation in a channel is seen as the onset of transmitter failure. Power variations that occur due to varying amplifier efficiency across the wavelength range can be accounted for and removed during build and operation of wavelength-division multiplexing systems.

snr measurements of each channel are important in determining the complete transmission spectrum and, therefore, the optical profile of each channel with respect to the noise floor. The snr is often referred to as "cross-channel interference" and can be taken as a measurement of the power present at various wavelength intervals between two adjacent channels. This snr determines individual channel performance with respect to other channels, and multichannel performance with respect to the noise level. The snr is a direct function of the state of the system and can provide details of adjacent cross-channel interference, which can cause channel failure.

Early identification of possible channel failure through these measurements allows for the switching or rerouting of data during repair of faulty components, without loss of time or information. In addition, measurements of channel wavelength, power, and snr provide data on a number of system coefficients, such as gain flatness across the wavelength range and drift, as well as transmitter and amplifier stability. This ensures that systems are both reliable and optimized throughout their operational lifetime.

The measurement accuracies necessary for current dwdm systems depend on the channel spacing required for individual systems. For 100-GHz channel spacing, wavelength measurements over time, temperature, and humidity to better than 25 GHz are required, and power accuracies to a few tenths of a decibel are desirable. Measurement resolution to better than 1 GHz provides the required wavelength data for signal-to-noise measurement and determination of the system`s transmission spectrum. Signal-to-noise measurement accuracies are again required to a few tenths of a decibel. Update rates of a few milliseconds are also desirable in some instances to minimize channel downtime and information loss.

The desired output from a channel monitor is shown in the table. Details of channel number, channel wavelength, channel power, and snr are displayed. This information can be fed back to a dwdm system for optimization or used to monitor the reliability and performance of the system.

Other parameters

The size of an optical-wavelength monitor may be important in some applications. Monitors may take the form of stand-alone instruments for use in construction and test of dwdm systems or may be integrated into a system as an original equipment manufacturer component for operational monitoring. It is vital for applications such as embedded test equipment that the monitors be small enough to be integrated into systems without substantially increasing the size and cost of the system. Portability for use in the field is critical. Traditional optical spectrum analyzers provide some of the required measurements but are limited to laboratory use because of the bulkiness of such devices and their response time. High levels of operator knowledge and effort are also required with spectrum analyzers in configuring them to make channel monitor measurements.

Monitors to be used as embedded test equipment must operate reliably in harsh environments. Ruggedness and reliability over their operational lifetime are therefore essential. Monitors can be exposed to temperatures from -20° to +80°C and all levels of humidity over lifetimes greater than 15 years. Ideally, recalibration should not be required during the lifetime of the equipment, as this usually requires fixed wavelength sources and increases the number of components and cost of a channel monitor. Interruption of operation for recalibration in the field is also costly and time-consuming.

Many system designers require that the components integrated into their systems operate without the use of moving parts. Simplicity in mechanical design and control using electronics is therefore desirable. This again improves reliability and the long-term performance of the channel monitor.

The required environmental performance is demonstrated in the figure, which shows the stability in transmitted wavelength of a channel monitor as a function of temperature and humidity. Rigorous environmental tests can be demanded by system manufacturers for qualification of channel monitors to accepted standards prior to purchase.

As dwdm systems mature and form a major part of telecommunications networks, channel monitors will become a necessity for reliability testing and system optimization (see photo). An effective channel monitor should provide both the measurement accuracies and features detailed above. Incorporation of these monitors into existing and new dwdm systems will provide error-free data transmission with little or no system downtime, thereby reducing cost and maintaining the operation of the system. u

Adrian Meldrum is a product engineer in the Telecom Div. of Queensgate Instruments Ltd., Bracknell, Berkshire, UK.

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