Advancements in Silicon Photonics - Optical Signal Processing by Silicon Photonics - page 34

4.3 Types of SOI Waveguides Used in Photonics

Numerous types of silicon waveguides with different structures and light guid-
ing techniques have been presented by various researchers and scientist. These
waveguide structures are strictly application-oriented and manufactured consider-
ing assorted parameters like signal wavelength of interest, refractive index con-
trast ( Δ n ), bending radius, optical absorption and scattering losses, coupling to
the input and output fiber, loss, and so on. Some of the basic types of waveguides
which are not application-oriented are introduced as under:
The four basic types of SOI waveguides are rib, channel, slot (vertical and hori-
zontal), and photonic crystal (PhCs) waveguides. The most common waveguides
being used for silicon photonics are rib and channel waveguides which are shown
in Fig. 4.2 b, d. In rib waveguides, guiding layer consists of a slab with a dielectric
rib on top embedded between two low-index-of-refraction layers. The dielectric
rib creates an effective-high-index region just below the rib, with a slightly higher
effective index than the index of the slab, while in channel waveguides the guiding
layer is completely surrounded by a cladding layer. In both the channel and the
rib waveguides, light is confined due to total internal reflection between the high
and low index-of-refraction region [ 1 ]. Photonic crystal waveguides, Fig. 4.2 c, are
the dielectric structures which are periodic using alternating regions of high and
low refraction indices. In case, the period is of the wavelength scale, high reflec-
tivity is produced with forming of complete photonic bandgap crystal. Silicon is
the suitable material to create single dimensional (1D), two dimensional (2D), and
three dimensional (3D) photonic crystal (PhCs) waveguides [ 2 ]. The latter can be
engineered to yield devices with unique properties, such as negative-refraction

(a) (b)

n1

n1

n2

n2

(c) (d)

n1

n1

n2

n2

Fig. 4.2 Four configurations of waveguides in silicon. a Channel waveguide. b Rib waveguide. c
Photonic-crystal waveguide. d Slot waveguide [ 1 ]