Built-in OSAs aid new DWDM operations and maintenance

Raymond Moyer

Optical-power balancing and gain/tilt parameters must be set to optimize performance but can be difficult and expensive to control. Optical-spectrum analyzers reduce operating costs and support transmission over the longest distance with the most bandwidth at the lowest cost per bit.

Additional problems occur if a wavelength is lost for any reason. When a system is turned up, optical-power balancing and gain/tilt parameters must be set to optimize performance. If a wavelength goes away, all other wavelengths change their power levels. Wavelengths react differently—increasing or decreasing in power. Wavelengths that decrease in signal level will not be successfully propagated to the other end of the fiber. This reaction poses a serious problem: when one channel drops out of service, it affects or eliminates the ability of several other channels to reach the other end of the fiber.

Systems using DWDM currently support 160 to 176 channels/ wavelengths. Each wavelength can carry traffic capacity up to OC-192 (10Gbit/s). Every hour that a wavelength is not carrying traffic translates into substantial lost revenue. Thus, power monitoring capability is necessary for DWDM equipment to maximize amplification. Products offer a variety of power monitoring options, but not all of these methods will work in next-generation DWDM systems.

These same technologies can also be used to reduce ongoing maintenance and related downtime. As fiber and other optical components age, their characteristics change, generally resulting in a shift of optical gain/tilt. As a result, many DWDM systems require costly out-of-service optical-power balancing and gain/tilt correction as part of routine maintenance.

THE PROBLEM
A typical DWDM deployment requires two technicians to set the optical power and gain/tilt on a wavelength. Each technician represents a cost of about $200 per hour. They must travel to terminals at opposite ends of the fiber, set up an optical-spectrum analyzer to look at the received signal, and load the information into a computer to calculate the correct attenuator values to level the signal. Further, this process introduces human error.

Some DWDM systems employ external fixed attenuators; others use internal manual attenuators or microprocessor-controlled variable attenuators. Initial settings must be transmitted to the technician at the opposite end of the fiber. In most cases, the technician manually sets these values, waits for the system to stabilize, then checks to ensure the settings provided the correct results. This procedure does not always work the first time; unexpected results may occur as one wavelength's signal strength is increased, others could decrease. Consequently, the process tends to be very iterative, especially if there are intermediate in-line amplifiers that also require a visit and manual adjustment.

One method used to simplify the process requirements uses microprocessor-based variable attenuators. When data from a spectrum analyzer is obtained at the receive end of the fiber, a TL1 command is sent back to the transmit terminal to adjust the variable attenuator. This automatic adjustment reduces the time required for optical-power balancing from several days to one hour; it also compresses necessary manpower needed to a single technician—saving time and money. The output of a spectrum analyzer connected to a terminal at the receive end of the fiber shows that every wavelength initially carries a different signal or power level (see Fig. 1, left).

There is a total power level threshold, above which nonlinear effects generate enough noise to make the signals indistinguishable. If the signals are amplified to the maximum level allowed by the nonlinear threshold, each signal is increased in amplitude by that same percentage. Thus, large signals get larger while the small signals increase in amplitude but remain much smaller than the larger signals. These small signals may not have enough power to reach the other end of the fiber. When enough amplification is applied to the aggregate signal to allow the smaller signals to reach the other end of the fiber, the increased power level of the larger signals could damage the receiver or other optical components.

In the example of an optical-power balancing procedure, attenuators on each of the optical inputs are properly adjusted to within 2 dB (see Fig. 1, right). The signal can be amplified to the nonlinear effects threshold and all of the signals have an equal piece of the aggregate power. This equalization allows all of the signals to be received at the other end of the fiber.

Unfortunately, there is no easy way to set the attenuator on a new signal to ensure that it will work successfully with existing wavelengths. Each wavelength requires a different amount of power to reach a given level, and each wavelength will exhibit a unique reaction when others are changed or added. In most DWDM systems this variability means the entire power-leveling and balancing procedure must be performed whenever a new wavelength is added.

EXISTING SOLUTIONS
Most DWDM systems in operation provide some type of optical-power monitoring capability that prevents the aggregate signal from being amplified into the nonlinear region. Since these systems typically address the aggregate signal level, they cannot make the adjustments required to balance the signal without a technician's help.

The spectrum analyzer digitizes the optical waveforms and determines what, if any, adjustments to make. A spectral gradient and power meter accomplishes a similar function at a lower cost, but also with lower performance and reliability than an optical-spectrum analyzer. The spectral gradient splits an aggregate signal into its individual light components on the power-meter input. The power meter then measures the power of each wavelength simultaneously. This measurement is less accurate because the power meter must be uniform across the input surface or errors will be introduced. If a signal generates high power levels at initial turn-on, or during operation, the power meter can be damaged at that position, completely eliminating readings for that wavelength or causing errors. In this situation, the entire power meter would have to be replaced to correct the problem once the error is isolated.

In the second situation, where a wavelength is suddenly lost or removed, a typical system would require technicians to travel to the system to rebalance optical power and restore wavelength conformity. For efficient operation, the specific problem should be pinpointed and repaired before the power-balancing procedure is performed. Otherwise, excess time is spent performing initial power balancing, then repeating the process when the lost channel is turned back on. Time added to the process by repetitive procedures results in longer system downtime, mounting repair bills, and lost revenue.

Systems with built-in optical-spectrum analyzers or spectral gradients and power meters can automatically correct changes in power levels caused by a lost signal (see Fig. 2). When a signal disappears, the optical-spectrum analyzer or spectral gradient and power meter detect the resulting power changes. The equipment then corrects the incoming power levels by sending a signal to the variable attenuators. Using a closed feedback control loop between the analyzer and the variable attenuators, a power balance can be restored within seconds. Data corruption is minimized and system downtime is reduced from days to milliseconds. This automatic correction effectively monitors and controls, and eliminates the need for the repair technicians to manually balance the system. Technicians can focus their efforts on restoring the signal.

Since the analyzer hardware will automatically rebalance the power when the wavelength is regained, the system will not require an out-of-service condition, which could last for several days, to effect a manual rebalance. The capability of the system is therefore maintained throughout a limited catastrophic failure situation.

As a DWDM system ages and weather conditions change, the equipment will not pass light with the same efficiency or characteristics that it did at initial turn-up. This continual degradation requires that wavelength optical power be readjusted at regular intervals to optimize system performance. Many systems require full out-of-service optical power balancing to address the problem, which is expensive and time consuming.

A built-in optical spectrum analyzer or spectral gradient and power meter continuously performs the optical-power adjustments necessary to maintain optimum performance and does not require outside intervention. This capability greatly reduces the maintenance requirements, and eliminates the effects of slowly degrading system performance.

NEXT-GENERATION SOLUTION
As DWDM systems become more complex, the number of channels increases and the optical signal-to-noise ratio (OSNR) becomes a larger issue. Since the aggregate power level threshold for nonlinear effects stays constant as the channel count increases, the amount of power available to each signal is reduced by lowering the amplitude of each signal. When the amplitude of the signal is reduced, the OSNR is also reduced.

Larger numbers of in-line amplifiers are required with longer total distances. Each amplifier introduces noise into the composite signal and amplifies all of the noise already present. Noise increases in direct proportion to the distance a signal travels. The longer the distance a signal must travel results in more noise being injected into the composite signal. This increase in noise level acts to reduce the OSNR, making it more difficult to distinguish the signal from the noise.

Because the signal amplitude is reduced as the number of channels increases, the importance of maintaining a correct optical-power balance grows. In a DWDM system with a good optical-power balance, it is possible to amplify all the signals to their maximum amplitude at each amplifier stage. Since each amplifier introduces noise, fewer amplifiers operating at greater amplification can push the signals over long distances, and generate less noise through the system.

In high-channel-count systems, spectral-gradient and power-meter measurement hardware does not work as well. This equipment must monitor and make adjustments to as many as 160 channels instead of the 32-channel systems it was designed to monitor. The increased number of channels means that spectral sensitivity must be uniform over a range that is now five times larger, and the meter must perform five times the number of readings at the same time.

The optical-power balancing process can be fully automated by connecting a built-in optical-spectrum analyzer to both the transmit and receive fibers (see Fig. 3). Therefore, instead of analyzing the power only at the transmitting terminal, the spectrum analyzer also analyzes the optical power from the receive end of the fiber, thus eliminating the need for an external spectrum analyzer at the receive end of the fiber, and the need for manpower to set up test equipment at either terminal. A simple push of a button and system software sets up its own optical-power balancing at turn up. Up to 50% of the in-service channels can be added or removed at once without repeating the optical-power balancing procedure. And, as the system and fibers age or change characteristics, the spectrum analyzer automatically makes the needed corrections.

Setting the correct signal tilt becomes even more critical as the number of channels, data rates, and distances increase. If all signals are equally balanced to maximize amplification at the transmit terminal, the signal at the receive end will not be optimized. As the composite signal travels down the fiber, nonlinearities cause individual wavelengths to move at slightly different speeds, creating a sloped signal instead of a flat one. To maximize the amplification across the fiber, the initial power balance should be set with the opposite slope so the signal will become flat about half-way down the fiber, then increase after that point.

When the optical-spectrum analyzer is added to the amplifier sites, balancing and tilting functions can access additional information to optimize system performance. This information also allows maximum amplification based on the tilt of the signal at intermediate sites. Since the tilt must be set by adjusting the individual channels, the amplifier sites cannot influence the tilt, only the aggregate signal.

The latest generation systems with built-in optical-spectrum analyzers automatically optimize the critical signal-to-noise ratio for maximum performance. The spectrum analyzer enables more signals with correspondingly smaller amplitude signals to be transmitted farther with less human intervention.

Built-in optical-spectrum analyzers enable a larger number of signals of proportionately smaller amplitudes to be transmitted, resulting in greater bandwidth per fiber. Longer transmission distances offer engineering flexibility and decrease the number of expensive regeneration sites, resulting in a reduction of the total system cost. Optical-spectrum analyzers reduce the ongoing expense of a system by reducing the need for highly skilled field technicians and engineers, and therefore fit the long-haul carriers' requirement for transmitting information over the longest distance with the most bandwidth at the lowest cost per bit.

With the energy business emerging as a new entrant in the long-haul market, bandwidth has become a trading commodity. This new market segment produces a need for faster provisioning, more bandwidth, and, most of all, lower costs. The explosion of data and video traffic has generated a growing demand for more bandwidth to carry more data at higher speeds using more wavelengths. Built-in spectrum analyzers will continue to play an important role in system optimization and cost reduction as this immense growth continues.

Raymond Moyer is senior product manager of the long-haul DWDM department at Fujitsu Network Communications, 2801 Telecom Parkway, Richardson, TX 75082. He can be reached at 972-479-4118 or [email protected].