What is the proper way of calculating loss for waveguide bends if I only have knowledge on the propagation loss for a foundry-specific straight waveguide section? For example, how would one account for an s-bend? Is it like approximating the s-bend as three-straight waveguide sections forming a right triangle? Or there is a way in terms of the bend’s offset? Thank you
There are basically three loss factors in a bent waveguide:
1- material loss
2- sidewall (roughness) loss
3- radiation loss
The first two are also present in a straight guide. The extra loss in the bend is both an increase in (2), due to a higher field strength at the waveguide edge in a bend and (3). Both increase with smaller radii. To obtain (3) you need a complex (2D) modesolver that applies a conformal transformation to cylindrical coordinates. The complex index of the solution provides the bend loss. Hence, you need 2D bend solver, of which there are many, commercial ones like in VPI photonics, Lumerical or FIMwave, or open source solvers like in wgms3d.
To get the loss in the bent guide a good approximation is to add the radiation loss to the known straight waveguide loss.