Advancements in Silicon Photonics - Optical Signal Processing by Silicon Photonics - page 36

fibers are in the form of strand of glass with circular cross sectioned core at the
center surrounded by concentric cladding glass, the fiber core with higher refrac-
tive index as compared to cladding maintains the light to propagate inside the core
by total internal refraction ( Δ n ≈ 1.45 − 1.40 ≈ 0.05).
As the size of the core (micrometers) of a typical optical fiber is much larger
than the core size of SOI planer waveguide (nanometers), the coupling of light
from fiber to waveguide may introduce large loss at the interface. The waveguide
grating couplers and adiabatic tapers are designed with polished surfaces for cou-
pling of light from/to optical fibers and planer waveguides. There exist a number
of techniques to couple light into optical waveguides and/or to collect light from
optical waveguides, e.g., butt coupling, grating couplers, tapered couplers, and the
evanescent coupling. Each of these techniques has drawback of very high coupling
loss, when coupling scheme is applied to wafer scale testing.
Different researchers and manufacturers present their designs for coupling of
light with emphasis on the coupling losses, e.g., Tai Tsuchizawa and Koji Yamada
et al. came with “the inverse taper approach.” Their work shows an efficient fiber-
waveguide coupling (a coupling loss of 0.5 dB per connection was obtained),
although the modal mismatch is too high between the silicon wire waveguide with
effective area ( A eff ≈ 0.1 μ m 2 ) and a single-mode fiber ( A eff ≈ 50 μ m 2 ) [ 6 ].
S. McNab et al. used the technique of gradual expansion of a core guided mode
into a much larger cladding guided mode, which resulted in coupling loss as low
as 0.2 dB from a single-mode fiber to a silicon wire waveguide [ 7 , 8 ]. Another
approach, shown in Fig. 3.4 [ 9 , 10 ] used surface gratings etched onto silicon in
which the coupling loss of 1 dB is obtained (Fig. 4.4 ).
Xia Chen, Chao Li, and Hon Ki Tsang patented their design of optical devices
for coupling of light. This design comprises a planar substrate and an optical
wave-guiding layer disposed on the planer substrate. The optical wave-guiding
layer comprises a grating portion for coupling light between a planar waveguide

Photonic waveguide
circuit

Output Optical
Fiber

Input Optical
Fiber

VVV

Grating Coupler Grating Coupler

Fig. 4.4 Schematic structures of efficient fiber to waveguide coupling via surface gratings [ 26 ]