Troubleshooting DWDM: Compensating for potential problems

Robert McMahon

Implementing dense wavelength-division multiplexing (DWDM) systems involves maximizing the rate of transmitted information, while minimizing the limitations of the existing physical network. Accounting for potential problems in the design and installation of DWDM systems is necessary because of the many performance imperfections of current optical components.

Monochromatic light being emitted by the laser is the source of optical power. Too much optical power can cause nonlinear effects to distort the signals, while too little optical power limits the distance of signal transmission. The transmitted optical power should always be at the maximum rated optical power per channel to maximize the integrity and minimize the distortion of the signal.

When the optical power source is digitally modulated, the optical power changes continuously with time. Peak optical power and the average optical power are two measures of optical power. Peak optical power is the highest level of power that the optical signal obtains. Average optical power is the average optical power received over a length of time (seconds).

Digital systems modulate with roughly 50% duty cycle, which implies that the peak power should be approximately two times the average power for a nice, clean wave. An optical time-domain reflectometer (OTDR) can launch 1W of optical pulses for a microsecond pulsewidth every millisecond, which implies that the average power will be about 1/1000 of the peak power. When measuring optical power, one must be aware of how many lasers are connected to that point because this affects optical power.

Wavelength, frequency, and linewidth

The center wavelengths of the optical power sources should be evenly spaced to avoid elevated crosstalk between two light sources. The spacing partly depends on how many optical channels are in the DWDM fiber. Having the optical sources spaced as far as possible from each other minimizes interference from adjacent channels.

Every wavelength of optical power has its own frequency, from the equation:

Frequency = c/(wavelength) = (299.7925E6 (m/s)) / wavelength

Linewidth measurements acquired when the optical power sources are being modulated at the highest frequency should have minimal interference with the other wavelengths in order to minimize noise. Crosstalk noise exists at different magnitudes for all frequencies (see Fig. 1). The most dominant crosstalk noise has the highest amplitude. The maximum amplitude of the crosstalk noise is the difference between the crossover and the peak signal. Increasing the rate of modulation or spacing the channels closer, will increase the magnitude of the crossover, thereby increasing the amplitude of the crosstalk noise.

When the light source is modulated over a frequency range, the spectrum-called linewidth or channel width-is the center frequency ± modulation frequency. The modulation frequency is approximately one half the bits-per-second rate (for example, 10-GHz modulation frequency is approximately equal to 20 Gbit/s). This is approximate because of the spectrum roll-off.

Bandwidths of optical components have a frequency roll-off-when one signal is in the next signal`s spectrum-similar to electronic bandpass filters. This fact indicates that at its highest frequency, the modulation is not a digital square wave (or immediately jumps from 100% transmission to 100% attenuation), but has a waveform more like a sine wave.

Spectral response with nonlinearities

When many channels are transmitted in a DWDM system, four-wave mixing (FWM), stimulated Brillouin scattering (SBS), and stimulated Raman scattering (SRS) are the nonlinear effects that increase when the optical power is increased.

Brillouin scattering occurs at low optical power, enough to cause acoustic vibrations in the glass of the optical fiber. The density of the optical fiber changes with the acoustic waves and this changes the refractive index. The difference in the refractive index in the optical fiber can cause stimulated Brillouin scattering to occur.

Raman scattering scatters the light and changes its wavelength. An atom in a crystalline lattice absorbs the light, and then it emits a photon with the energy equal to the original photon, plus or minus the energy vibration on the atom. Stimulated Raman scattering requires more optical power than Brillouin scattering. Raman scattering travels in the forward and backward direction of the optical fiber and can cause crosstalk between channels.

If the transmitted optical power is above its rated limit, nonlinear phenomena such as four-wave mixing (FWM) can increase the noise floor and cause crosstalk in another channel. As coupled power increases, so do the nonlinear effects.

Four-wave mixing of optical channels occurs in optical fiber when the channels are operating close to zero-dispersion wavelengths. In some fiber, the zero-dispersion parameter is moved outside the wavelength operating range of the optical fiber. This optical fiber is referred to as nonzero-dispersion-shifted fiber (NZDSF).

When four-wave mixing occurs, three wavelengths interact to cause a fourth wavelength:

F_mix=F1+F2+F3

The F_mix can be one of the wavelengths transmitted, causing noise and crosstalk at the F_mix frequency.

Dispersion in optical fiber

Dispersion is the broadening of a light pulse as it travels through the optical fiber. The pulse width of the light increases the further it travels in the fiber. Dispersion, therefore, can limit a fiber`s bandwidth as a function of the length of the fiber.

The total dispersion that occurs in an optical fiber is a combination of several types of dispersion, including modal, chromatic, polarization, waveguide, and material dispersion. Modal dispersion, which can be the largest contributor to total fiber dispersion, is not present in single-mode applications. The two types of dispersion that most contribute to signal distortion in single-mode fiber are chromatic-mode dispersion (CMD) and polarization-mode dispersion (PMD).

Modal dispersion

Modal dispersion(ns/km*nm) occurs in multimode optical fiber. This phenomenon is linear; that is, if the distance of the optical fiber doubles, the pulse spreading also doubles. To eliminate modal dispersion, switch to single-mode fiber.

Chromatic-mode dispersion

In free space, all wavelengths of light travel at roughly 300E3 km/s. When chromatic-mode dispersion (ps/km*nm) is present, however, different wavelengths of light travel at different velocities because the index of refraction (n), which determines the velocity of light in the fiber, varies with the wavelength of optical power.

When a high-bandwidth signal, such as a pulse of laser light, is sent down a nonzero-dispersion-shifted fiber, pulse spreading occurs because of chromatic dispersion. The pulse spreading limits the bandwidth with respect to the distance traveled on the optical fiber. The bandwidth of the optical fiber can be improved by dispersion compensation, the most common method of which is to insert an optical fiber with the opposite chromatic dispersion to cancel the original dispersion.

Polarization-mode dispersion

Polarization is caused by the optical signal interacting with silica molecules in the fiber. Optical fibers can be manufactured for reduced polarization, but bending and physical stress on the fiber makes it more susceptible to polarization.

The degree of polarization in a fiber depends on factors such as ambient temperature, and bending and stretching of the fiber. For example, when optical cable is installed beside a railroad track, the vibration on the fiber causes the polarization to change. Polarization of the optical power is responsible for polarization-mode dispersion (ps/km^^0.5), polarization-dependent loss (PDL), and polarization dependence of the center wavelength (PDC). If the fiber polarization is changing, these parameters can cause random modulation of the optical signal.

Polarization-mode dispersion is caused by the slightly different values of index of refraction on the vertical and horizontal axes at the same point on the optical fiber. One axis can have a slightly different speed from the other axis. The index of refraction of the axis depends on the polarization of the photons.

Polarization-dependent loss is due to coupling optical power through some of the couplers. Measuring the polarization-dependent loss is accomplished by shining a polarized light source into one axis, measuring the loss, and repeating for each axis. As the polarization of optical power changes axes, polarization-dependent loss is measured as the minimum power loss subtracted from the maximum power loss. In couplers, the PDL ranges from -0.05 dB to -0.5 dB. The cumulative PDL can run to many dBs in an entire DWDM system. PDL can cause major signal modulation if the polarity is changing.

Variations in polarization can cause the center wavelength at the receiver to change by broadening or narrowing the signal, and shifting it left or right on the frequency axis. This phenomenon is known as the polarization dependence of the center wavelength.

Amplifiers

Erbium-doped fiber amplifiers (EDFAs) normally amplify the wavelengths of optical power between 1530 and 1560 nm. The EDFA is pumped with a 980- or 1480-nm laser diode to stimulate the erbium electrons, which can emit identical photons, thus amplifying the optical power. The amplification is a factor of approximately 10 to 10,000 or approximately10 to 40 dB. The EDFA is capable of amplifying many laser channels simultaneously.

The gain curve does not have equal amounts of gain per wavelength, so a filter must be added to ensure that all channels have the same amplitude. When a filter is mounted on the input signal, the signal-to-noise ratio (SNR) is affected; when the filter is placed on the output, too much amplitude in the signal can be emitted (see Fig. 2).

Receivers

The maximum analog bandwidth, or the digital bit rate at the receiver, is related to the total dispersion in the DWDM system. If a DWDM system has a small dispersion measurement, then the data-transmission rate can be very high. Dispersion operates similarly to an electronic low-pass filter. If a pulse is applied at the input to the filter, it broadens on the output.

A large value of the signal-to-noise ratio, or a high-value signal with low-value noise, indicates that the DWDM system has very good transmission quality. To achieve a good signal-to-noise ratio, the maximum noise floor on each of the optical components must be included when designing the DWDM system.

The bit-error rate (BER) decreases with an increase in the signal-to-noise ratio, or in other words, as the noise floor decreases, so do the errors. The bit-error rate is usually 10-9 for telephone voice transmission, and 10-12 for data transmission. The digital waveform can be displayed with an oscilloscope for an eye-pattern analysis (see Fig. 3). Both the digital signal and the noise are displayed on the oscilloscope. A major noise component involved with a data transmission system is the bandwidth. Because of the reduction in bandwidth and noise, the digital signal does not appear as a square wave, it looks more like a sine wave. On the waveform, the pulse timing, rise time, fall time, pulse duration, repetition rate, jitter, and space between the pulses can be observed.

Test equipment is specifically made for taking measurements in a laboratory, manufacturing environment, or the field. Next month, "Mastering WDM" will review portable field test equipment used by DWDM installers: the optical spectrum analyzer, optical power meter, and optical time-domain reflectometer.

FIGURE 3. The trace (top) shows the different components of a typical eye pattern detected by an oscilliscope (bottom).