Reliability and maintenance considerations in the optical layer
The portable OSA can play an essential role in ensuring proper DWDM system operation as part of a comprehensive maintenance program.
John Marsh
GN Nettest Fiber Optics Div.
The delivery of reliable high-bandwidth services to both businesses and consumers has emerged as a fundamental goal for long-haul and access providers. In this quest for bandwidth, market forces have quickened the pace of technological innovation in both products and services. At no time has technological change been more rapid.
Fiber optics plays a central role in high-bandwidth systems, and dense wavelength-division multiplexing (DWDM) has emerged as a key enabling technology. The portable optical spectrum analyzer (OSA) is an essential instrument in any plan for network reliability when DWDM is present. Desirable features in a portable OSA include ruggedness, high absolute wavelength accuracy, the ability to work continuously for long periods without mechanical wear, and software that turns long-term monitoring into a powerful analysis tool.
With DWDM, multiple high-bandwidth signals propagate through a single optical fiber. DWDM can increase the capacity of a single fiber by as much as two orders of magnitude Long-haul providers are attracted to the savings realized when the optical amplifiers associated with DWDM replace expensive electro-optic regeneration. Meanwhile, metro and access providers are taking the first evolutionary steps toward a new network architecture built on optical crossconnects and active wavelength conversion. Thus, DWDM forms the basis for the emerging optical layer that works in conjunction with the time-division multiplexed (TDM) electrical layer.
A large amount of revenue may be riding on a DWDM optical link due to the tremendous bandwidth (up to 80 Gbits/sec) the technology provides. Thus, network reliability is of paramount concern for network operators. Each layer of the network must have, as part of its structure, a mechanism for identifying and responding to failure. The operational support system (OSS) acts as the repository for status information from all network layers.
Synchronous Optical Network (SONET) systems often ensure protection in the electrical layer with redundant point-to-point links or self-healing ring structures. However, mechanisms for protection in the optical layer are less fully developed, especially when DWDM systems are in place. Optical protection may be based on embedded OSA monitoring of channel center wavelength and power. For example, a built-in OSA monitor could send an alarm to the OSS due to loss of signal in a particular optical channel, triggering a reroute to a redundant link. But even with optical protection, network operators must be prepared to troubleshoot and repair faults in a timely manner. A carefully designed program of optical-layer maintenance is necessary. DWDM systems are sufficiently new that a widely accepted set of procedures does not currently exist.
The integration of optical-layer monitoring with other system monitoring may result in alarm redundancy, where a failure in one layer of the system produces simultaneous alarm conditions in higher layers. The goal is to use all available monitoring systems to isolate the root cause of failure and implement repairs. Unfortunately, built-in monitoring, although able to identify a failure and initiate signal rerouting, generally provides incomplete information for a thorough root-cause analysis. This shortcoming is due in large part to the fact that built-in monitoring devices are fixed in place, resulting in a limited ability to isolate faults. The portable OSA is thus the instrument of choice for detailed troubleshooting in the event of a DWDM system failure, because signal analysis at any location along the link can be performed.
Figure 1 illustrates the troubleshooting process and shows a typical DWDM system, including transmitters, multiplexer, fiber, optical amplifier, demultiplexer, and receivers. Many details are omitted for simplicity, including dispersion compensation modules, gain-flattening filters, optical booster amplifiers, and optical preamplifiers. Below each part of the system is a list of some associated hardware that can be the source of failure in that part of the system. In general, there are power supplies and temperature control systems associated with each electro-optic component. A failure in any of these systems can lead to failure in one or more channels. For example, if a single channel fails, it could be the result of the transmitter going dark. Alternatively, it may be the result of transmitter wavelength simply drifting beyond the system passband for that channel. Only by probing the signal with an OSA at the transmitter, both before and after the multiplexer, are these situations distinguishable. Clearly, fault isolation may require access at various locations in the network, and portability of the OSA is essential.
OSA in DWDM system turn-up
Bringing up a DWDM system is no simple task. The system must be tested at each step during installation and turn-up. Portable OSA monitoring during provisioning can immediately identify simple problems such as incorrect connections. More importantly, it can help troubleshoot more subtle problems with the system.
Despite extensive laboratory testing of individual components, the behavior of the installed system is unpredictable. One reason for this unpredictability is the difference between the fiber spools used in the laboratory and the buried fiber used in the actual link. Environmental factors also play a role and include periodic temperature fluctuations as well as temperature gradients from one node to the next. In general, stability of the system is a major concern, and portable OSA monitoring at various points along the link is a useful tool for system verification.
Continuous monitoring during a period of weeks can provide the needed proof that the system is functioning properly. During such monitoring, it is desirable to have the OSA software automatically calculate and save statistical information about the channel table. This statistical profile of system behavior can then serve as a baseline for future installations or for future troubleshooting of the same link. Monitoring drift in laser wavelengths or verifying multiplexer/demultiplexer passband alignment may require 50-pm absolute wavelength measurement accuracy.
OSA in preventive maintenance
System failures are generally unavoidable, but their impact can be minimized if anticipated with appropriate preparations. Instability in the output of a transmitter laser is a prime example of a sign that failure may be imminent.
Long-term monitoring is a powerful tool for anticipating system failure and forms the cornerstone of a periodic preventive maintenance program. Instabilities not apparent on the time scale of an hour may become apparent when monitoring on a time scale of hundreds of hours. Just as in monitoring during system turn-up, an important feature of the portable OSA for this purpose is the ability to enter a long-term monitoring mode where a detailed statistical profile of the system is generated and saved. The ability to compare the statistical profile of the channel table to an earlier saved profile can highlight subtle system degradations not otherwise apparent. A program of periodic long-term monitoring (e.g., one week of monitoring every six months) may be used to keep a valuable log of system performance over a number of years.
OSA hardware choices
The OSA gives a graphical display of the optical power as a function of wavelength (see Fig. 2). Important parameters associated with this signal include the number of channels, the center wavelength, signal strength, and the optical signal-to-noise ratio (OSNR) of each channel. The OSA extracts this information automatically by software and displays it in a channel table.
The four most important OSA performance parameters are the absolute wavelength accuracy, the power-level accuracy, the resolution bandwidth of the instrument, and the ranges for wavelength and power measurement. Absolute wavelength accuracy is important because adjacent channels may be separated by as little as 50 GHz, or 0.4 nm. The precise alignment of corresponding passbands of the multiplexer and demultiplexer imply that very small wavelength variations, on the order of 10% of the channel spacing (as small as 40 pm), can result in large variations in system throughput. Thus, absolute wavelength accuracy of approximately 50 pm is needed.
Optical power in a single DWDM channel can range from as little as -30 to as much as +10 dBm. Optical power per channel varies with location in the system, being largest immediately after amplification. Because nonlinear effects place limits on total system power (i.e., the sum of power in all channels), the maximum optical power per channel also depends on the number of active channels. DWDM system designers must deal with the tradeoff between large channel count and the enhanced reliability (reduced bit-error rate) associated with larger channel power. The resolution bandwidth of an OSA indicates how rapidly the impulse response of the instrument falls off away from the center wavelength. This parameter determines the interchannel rejection between adjacent DWDM channels, which in turn determines the minimum measurable OSNR of the DWDM system channels. The OSNR needed to ensure an acceptable bit-error rate of, say 1䁾-12, is a system-dependent quantity, generally in the range of 15 to 20 dB.
Spectral-analysis instruments rely on one of three possible hardware technologies, each with different strengths and weaknesses. The first and most mature technology is the diffraction grating OSA. Originally used as laboratory instruments, portable grating-based OSAs have recently emerged. A diffraction grating disperses the incoming light, an arrangement that allows only a narrow band of wavelengths to reach the detector. Grating-based OSAs have good wavelength accuracy and excellent OSNR measurement capabilities. Unfortunately, the wavelength resolution of grating-based OSAs is typically about 100 pm, which is marginally acceptable at best when making measurements on channels with 50-GHz spacing (400 pm).
The second technology is the Michelson Interferometer. An OSA based on this technology is called a multiwavelength meter. Using interferometric comparison of an incoming signal wavelength with that of a reference signal, the multiwave meter has the highest available absolute wavelength accuracy. Disadvantages of the multiwave meter include cost as well as a somewhat reduced ability to measure OSNR.
The third technology for spectral analysis is the Fabry-Perot tunable filter (see Fig. 3). Scanning the narrow passband of a tunable filter through its tuning range allows reconstruction of the entire spectrum. The tunable filter instrument has specifications that give it the "best of both worlds" specifications: absolute wavelength accuracy approaching that of the multiwave meter, yet with the OSNR measurement capability needed to ensure system performance.
The tunable filter technology is comparable in price to grating-based instruments, both of which are significantly less expensive than wavelength meters. A further advantage of the tunable filter is the lack of moving parts. Both the grating-based and interferometer-based instruments have moving parts that are subject to mechanical wear and shock and may require maintenance if run for extended periods of time or subject to rough treatment.
The tunable filter, on the other hand, has only a micro-mechanical motion supplied by a piezoelectric actuator, and is therefore extremely rugged and can run continuously without mechanical wear.
Excellent specifications, cost-effectiveness, and long-term monitoring ability all point to the tunable filter technology as the best choice for a portable OSA (see Table).
OSA software for testing
The engineer or technician operating the instrument is perhaps more concerned with the software user interface and feature set than with hardware specifications. Indeed, an entirely new set of functionality has become "essential" to portable OSA instrumentation.
In addition to a real-time display of both the spectrum and channel table, robust tools for long-time monitoring are vital. It is clear that long-term monitoring functions are useful for both DWDM system turn-up and periodic preventive maintenance. Software that specifically addresses this need lets the user monitor the statistics of the channel table easily, which enables later comparison to previously saved channel-table profiles. Another desirable feature is the ability to see a graphical display of system parameters as they evolve over time.
Finally, the user interface must be clean and simple. Too many features or modes may be cumbersome rather than useful. For example, many of the functions that laboratory OSAs use for DWDM component characterization are of relatively little use in the field and would only serve as a distraction from the normal uses of the instrument.
The state of the art
Operating and maintaining a network with DWDM systems in place is an immense challenge. The large bandwidth carried by even a single fiber mandates a rigorous program of preventive monitoring and careful plans for troubleshooting.
The portable OSA is an essential instrument in all aspects of this program, including turn-up, maintenance, and troubleshooting. Long-term monitoring is a crucial feature of the instrument, and wavelength resolution of 50 pm is needed when dealing with 50-GHz channel spacing.
Various portable OSA technologies are available, but only the tunable filter meets the requirements of ruggedness, long-term monitoring, and high wavelength resolution. Software for the portable OSA must be designed for ease of use with long-term monitoring and comparison capabilities.
The art of DWDM system testing and maintenance is still in its infancy and awaits some years of experience with a large deployed base of DWDM systems. Nevertheless, with the proper tools and plans, network operators can guarantee the highest possible network reliability and the best possible responsiveness when troubleshooting failures. u
John Marsh is optical networking product manager at GN Nettest Fiber Optics Div. (Utica, NY).