Page 696 - Introduction to Information Optics

12. Networking with Optics
spacing from a reference frequency of 193.1 THz (ITU G.692 Draft).
Channel center frequencies are selected from the "ITU Grid."
2. Insertion loss: The insertion loss is the input-to-output power loss of the
multiplexer. Lower component insertion loss is better for the system,
3. X-dB Passband (where X can be any number): Passband is the spectrum
region around the center wavelength of the multiplexer. For example,
1-dB passband is the bandwidth at insertion loss 1 dB down from the top
of the multiplexer spectrum transfer function. Wider and flatter passband
makes the system more tolerable to wavelength drift of the laser
transmitter.
4. Cross talk: In a DWDM system, there is usually energy leaking from
adjacent and nonadjacent channels. This energy leakage is defined as
cross talk. If the transfer function of the multiplexer has a sharp cutoff,
it can better suppress unwanted cross talk and result in high isolation.

In addition to the above, a high-performance DWDM multiplexer should
have small polarization-dependent loss (PDL), low polarization mode disper-
sion (PMD), and the same performance under hostile environment tempera-
ture.

12.2.3.2. Dielectric Thin-Film Filter
The dielectric thin-film filter is one of the most popular technologies used
to make the DWDM multiplexer. A single-cavity thin-film filter consists of two
dielectric stacks separated by a cavity. It behaves as a bandpass filter passing
through a particular wavelength (or wavelength band) and rejecting all the
other wavelengths. For DWDM applications a multicavity thin-film filter is
used since it results in a filter transfer function with a flatter and wider
passband as well as a sharper cutoff. The characteristics of single-cavity and
multicavity filters are compared in Fig. 12.7.
One configuration of the eight-channel DWDM demultiplexer utilizing the
dielectric thin-film filter is depicted in Fig. 12.8. A wide bandpass filter first
separates the incoming eight wavelengths into two groups. For each group,
three cascaded narrow bandpass filters separate multiwavelength light into
four individual wavelengths.
The dielectric thin-film filter has been the dominant DWDM multiplexer/
demultiplexer technology because of its high isolation, low PDL and PMD,
and acceptable insertion loss. Its performance is extremely stable with regard
to temperature variations. However, it is very expensive to manufacture
interference filters with less than 100-GHz bandwidth due to extremely low
manufacturing yield.
Numerous references, including [14, 15, 16], address the principles and
design of dielectric thin-film filters.