Waveguide Loss

Question 1

What are the three main loss mechanisms in optical waveguides?

Answer 1

Three main mechanisms are scattering, absorption and radiation.

Of those for most waveguides, the primary source of loss is scattering this is divided between interface scattering and volume scattering. Volume scattering is normally negligible for well-established techniques, however, should always be considered. Interface scattering is most common source of loss.

Question 2

Which loss mechanism is dominant in silicon waveguides?

Answer 2

Scattering as that is far more dependent on how the silicon waveguide is built compared to the other mechanisms. Loss due to radiation would be negligible as we are assuming a well-designed waveguide, as a silicon waveguide is passive and we use low-energy photons, interband absorption isn’t a significant concern. Which leaves the scattering of light due to defects. These defects can’t be completely removed and is thus the largest source of loss for a silicon waveguide.

Question 3

What is propagation loss?

Answer 3

Propagation loss is the loss associated with propagation in
the waveguide alone – i.e. measurement of loss coefficient.

Question 4

How do you calculate propagation loss?

Answer 4

Using the cut-back method, propagation loss is defined as:

I₁/I₂ = exp[−α(L1 − L2)]

so that

α = (1/(L₁ -L₂))ln(I₂/I₁)

Question 5

What experimental techniques are associated with waveguide measurement?

Answer 5

Cut-back method, Fabry-Perot resonance method and the scattered light measurement method.