
output has a Gaussian-shaped passband with a constant negative chromatic dis-
persion, the dispersion given by
D
0
=
2Rf
nðaDf Þ
2
, (9.32)
where R is the star coupler radius, n is the refractive index in the slab, a is the
grating waveguide pitch at the star coupler, f is the optical frequency (not focal
length, as it was used for earlier), and Df is the AWG FSR. It is usually best to
make Df equal to the WDM channel spacing. Because this d evice contains an
N N coupler, it can act as a TODC with low intrinsic loss.
To understand why the device has a negative dispersion, consider the lower part
of Figure 9.76. The center lines of the Gaussian beams for ...