8.2. WDM COMPONENTS 343
terferometers, such as theSagnacandMichelson interferometers, can also be used to
realize transmission filters. Figure 8.8(c) shows an example of the Michelson interfer-
ometer made by using a 3-dB fiber coupler and two fiber gratings acting as mirrors for
the two arms of the Michelson interferometer [49]. Most of these schemes can also be
implemented in the form of a planar lightwave circuit by forming silica waveguides on
a silicon substrate.
Many other grating-based filters have been developed for WDM systems [50]–[54].
In one scheme, borrowed from the DFB-laser technology, the InGaAsP/InP material
system is used to form planar waveguides functioning near 1.55μm. The wavelength
selectivity is provided by a built-in grating whose Bragg wavelength is tuned elec-
trically through electrorefraction [50]. A phase-control section, similar to that used
for multisegment DFB lasers, have also been used to tune distributed Bragg reflector
(DBR) filters. Multiple gratings, each tunable independently, can also be used to make
tunable filters [51]. Such filters can be tuned quickly (in a few nanoseconds) and can
be designed to provide net gain since one or more amplifiers can be integrated with the
filter. They can also be integrated with the receiver, as they use the same semiconductor
material. These two properties of InGaAsP/InP filters make them quite attractive for
WDM applications.
In another class of tunable filters, the grating is formed dynamically by using acous-
tic waves. Such filters, calledacousto-optic filters, exhibit a wide tuning range (>
100 nm) and are quite suitable for WDM applications [55]–[58]. The physical mech-
anism behind the operation of acousto-optic filters is thephotoelastic effectthrough
which an acoustic wave propagating through an acousto-optic material creates peri-
odic changes in the refractive index (corresponding to the regions of local compression
and rarefaction). In effect, the acoustic wave creates a periodic index grating that can
diffract an optical beam. The wavelength selectivity stems from this acoustically in-
duced grating. When a transverse electric (TE) wave with the propagation vectork
is diffracted from this grating, its polarization can be changed from TE to transverse
magnetic (TM) if thephase-matching conditionk′=k±Kais satisfied, wherek′and
Kaare the wave vectors associated with the TM and acoustic waves, respectively.
Acousto-optic tunable filters can be made by using bulk components as well as
waveguides, and both kinds are available commercially. For WDM applications, the
LiNbO 3 waveguide technology is often used since it can produce compact, polarization-
independent, acousto-optic filters with a bandwidth of about 1 nm and a tuning range
over 100 nm [56]. The basic design, shown schematically in Fig. 8.8(d), uses two po-
larization beam splitters, two LiNbO 3 waveguides, a surface-acoustic-wave transducer,
all integrated on the same substrate. The incident WDM signal is split into its orthog-
onally polarized components by the first beam splitter. The channel whose wavelength
λsatisfies the Bragg conditionλ=(∆n)Λais directed to a different output port by
the second beam splitter because of an acoustically induced change in its polarization
direction; all other channels go to the other output port. The TE–TM index difference
∆nis about 0.07 in LiNbO 3. Nearλ= 1. 55 μm, the acoustic wavelengthΛashould
be about 22μm. This value corresponds to a frequency of about 170 MHz if we use
the acoustic velocity of 3.75 km/s for LiNbO 3. Such a frequency can be easily applied.
Moreover, its exact value can be changed electronically to change the wavelength that
satisfies the Bragg condition. Tuning is relatively fast because of its electronic nature