Polarisation

Question 1

What is birefringence?

Answer 1

The term “birefringence” is used to describe the situation in which the two polarisation
modes exhibit different propagation constants or effective refractive indices.

Question 2

What causes stress on the core and cladding?

Answer 2

Depositing a cladding layer on top of the waveguides can introduce stress & strain to the waveguide core and cladding.

Question 3

What effect does stress have on the core and cladding?

Answer 3

Introducing stress can create small changes to not only the waveguide geometry, but also to the refractive index of the silicon, by causing changes to its crystalline structure.

The stress-induced refractive index changes are likely to be highly polarisation dependent, and will thus create an additional source of birefringence (i.e. “stress-induced birefringence”). It is possible to take into account both the geometrical birefringence and stress-induced birefringence when designing a waveguide if the fabrication process parameters (i.e. top cladding stress) are well known.

Question 4

Is it possible to achieve zero birefringence?

Answer 4

Yes, this state is defined as:

Δn_eff = n_eff^TM - n_eff^TE= 0

This is the “zero birefringence condition” (ZBC), which can be achieved for certain combinations of waveguide dimensions.

In fact, it can be achieved for a range of combinations of dimensions. As an example, in the figure the lines show combinations of waveguide etch depth, height, width, and top cladding thickness.