Integrated Optics - Applied Optics Fundamentals and Device Applications - page 243

6 µm

80 µm

FIGURE 5.18
Butt coupling of fiber to integrated optic waveguide.

waveguide is the end-fire or butt coupling technique in which the two are
aligned end to end and butted into contact (see Figure 5.18). The key fac-
tors affecting butt coupling between a fiber and a waveguide are area mis-
match between the waveguide cross-sectional area and that of the fiber core,
misalignment of the waveguide and fiber axes, and numerical aperture loss
(caused by that part of the waveguide light output profile that lies outside
of the fiber's acceptance core or vice versa). In the case of butt coupling of
the semiconductor waveguide and a glass laser, reflection at the glass-air-
semiconductor interface are also important.
In considering coupling of light from the fiber into the waveguide, the
problems of area mismatch and numerical aperture loss are limiting fac-
tors because the core diameter of a single mode fiber is approximately
4-10 μm and the rectangular waveguide dimensions are 1-5 μm. Exact
calculations of the coupling efficiency from fiber to waveguide cannot be
done until a particular waveguide and optical fiber have been selected and
their characteristics known; however, a typical experimentally observed
coupling efficiency for this situation would be about 10%. Robertson et al.
[32] have performed a theoretical analysis of butt coupling between a semi-
conductor rib waveguide and a single mode fiber, and have designed an
optimized waveguide structure with a large (single) mode size for which
they calculate a coupling efficiency of 86%. However, no experimental data
is available.
One approach to the problem of coupling fibers to multiple, closely spaced
waveguides is to fabricate a “v-groove chuck” with etched grooves to posi-
tion and hold the fibers in place, as shown in Figure 5.19. Due to the precision
of photolithographic techniques used to etch the grooves and the dimen-
sional precision of standard optical fibers, this method provides a highly
accurate means of aligning the fibers and waveguides.
In principle, grating couplers can be used to avoid the problem of area
mismatch and numerical aperture loss in coupling between a fiber and a
waveguide by providing a coherent distributed coupling over a mutually
coupled length of fiber and waveguide. For example, Hammer et al. [33]
demonstrated that a grating can be used to couple between a low-index fiber