Coupling

Question 1

What is a grating coupler?

Answer 1

Grating couplers are periodic lines that are etched into
the top of the waveguide, which re‐direct the light from
the optical fibre into the waveguide.

Question 2

What is an edge coupler?

Answer 2

An edge coupler is a device that couples light from fibre to the chip using inplane/butt/ edge coupling.

Tapering is a vital component for these devices.

Question 3

Advantages of edge coupling

Answer 3

Edge coupling, in essence, is the inverse taper method.

• Broad wavelength range
• Low facet reflection
• Easy to fabricate (if you can make tips)

Question 4

Advantages of grating couplers

Answer 4

• No facet preparation necessary
• Can be fabricated using lithography and etching at the wafer scale, so cheap
• Do not need to be placed at the edge of the chip
• The shape can be matched well to the shape of the fibre mode

Question 5

Disadvantages of general edge couplers

Answer 5

Relatively incompatible with wafer-level testing

Requires high-quality facets on the chip

Question 6

Disadvantages of Grating Couplers

Answer 6

• More wavelength-selective than edge couplers
• Fibre must approach chip from the surface (possibly a packaging problem)

Question 7

Chip Facet Considerations in edge coupling

Answer 7

Chip facet: the chip facet must be made very smooth, to reduce unwanted scattering. This can be done by polishing, cleaving, or dicing…all of which can be expensive processes.

Question 8

Describe Regular Planar Taper

Answer 8

• Widen the waveguide, with no change in height.
• Still inefficient, due to mode mismatch in height.

Question 9

Describe 3D Taper

Answer 9

• Gradually widen the waveguide in width & height, for better mode matching
• This is very difficult to fabricate – a greyscale lithography mask can be used (unusual), and the fabrication process must be well calibrated

Question 10

Describe 3D Taper Etch Steps

Answer 10

• Increase the height in steps, requiring only standard lithography steps.
• More lithography steps are needed (more expensive)
• If starting with thick layer and etching down to form waveguide, more scattering loss.

Question 11

Edge Coupler Consideration

Answer 11

Even if you match the mode shape of the waveguide to the mode shape of the fibre well, if the waveguide effective index is greater than that of the silica glass that the fibre is made of (n=1.45), there will be a reflection at the interface.

Question 12

Give Equation for Fresnel Reflection

Answer 12

Question 13

What is an anti-reflection coating?

Answer 13

Thin-film coating, which has a refractive index between that of two materials.

Used to reduce the reflection at the interface of the two optical materials.

Question 14

How does Rayleigh’s film anti-reflection coating reduce reflection?

Answer 14

To minimise the reflection, the refractive index of the
intermediate layer should be the geometric mean of the
refractive indices of the two outer layers:

Reflection can be further suppressed by including more
layers between waveguides of different widths.

Question 15

Describe interference Coatings

Answer 15

which also involves inserting a thin film at the interface.
There will be a reflection from the first and second boundaries of
that thin film, which will then interfere with each other. The
thickness of the thin film can be chosen such that they interfere
destructively, minimising the reflections.
For this there should be a 𝜋 phase shift (Δ𝜃) between the two
reflected beams:

Δ𝜃 = (2πn₁L)/𝜆

Hence π = (2πn₁L)/𝜆

Rearrange, L = 𝜆/2n₁

For Normal Incidence, Layer thickness

t = L/2

Therefore t =𝜆/4n₁