Page 711 - Introduction to Information Optics

12.2. Optical Network Elements 695
channel wavelength information is also available, in addition to optical power
level.
With increases in the channel count and transmission distance of DWDM
systems, demand for optical layer information increases too. Channel monitor
has become an attractive product recently because it measures the optical
spectrum of the DWDM composite signal, giving valuable information such as
individual channel optical signal-to-noise ratio (OSNR) and real-time channel
wavelength value. A channel monitor is basically an optical spectrum analyzer
(OSA) with a reduced feature set. But unlike OSA as a costly testing
instrument, channel monitors are integrated into DWDM system equipment.
They must be small in size and cost effective. While OSA is based on tilting a
diffraction grating to disperse the spectral content of optical signals, channel
monitors can be cataloged into two types: one is based on a photodiode array;
the other on a tunable Fabry-Perot (FP) filter. In the first case, the
optical signal is dispersed into spatially separated beams per their wavelength/
frequency content, by either diffraction grating or fiber Bragg grating. A
photodiode (PD) array then detects these single-frequency beams and the full
signal spectrum information is obtained when signals from all photodiodes
are combined. In the second case, the cavity length of a Fabry-Perot
filter, consisting of two reflection facets with an air or glass gap in between, is
tuned by a voltage-driven piezoelectric (PZT) actuator. This tunable FP filter
acts as a bandpass filter, which allows the DWDM channel to pass through
sequentially.
The key issues regarding channel monitors are channel resolution, number
of channels, wavelength accuracy, OSNR accuracy, dynamic range, and sensi-
tivity. For channel monitors based on a photodiode array, the number of pixels
of PD array posts an intrinsic limit to the number of channels a monitor can
resolve. For a channel monitor using 256 PD array, there are only about 3
spectral points to represent each channel when the incoming DWDM signal
has 80 channels. To monitor today's DWDM systems with > 100 channels, a
521 or bigger PD array is definitely needed. However, the sensitivity of the PD
array may suffer as pixel number increases. For channel monitors based on
tunable FP filters, PZT as a moving part posts concerns about its long-term
reliability.
Channel monitors provide channel power, OSNR, and wavelength informa-
tion that is valuable for optical layer networking. This information is not
adequate for network management to diagnose the health of each transmitted
optical channel, though, Chromatic dispersion, polarization mode dispersion
(PMD), and nonlinearities and their interplay in transmission fiber all have an
impact on signal quality. To this date, bit-error rate (BER) in the electrical
domain remains the most trusted measure of the health of optical transmission.
There is still quite a long way to go for optical monitoring to overtake BER,
thus enabling all-optical networking.