Guided light propagation in planar dielectric films

The simulations intend to illustrate some basic principles of confined light propagation in dielectric waveguides. The waveguide and coupler structures are specified by the following common parameters: background refractive index nb = 1.5, refractive index of the core regions ng = 2.0, film thickness t = 0.5 µm. All calculations are meant for a vacuum wavelength of 1.5 µm and for TE polarization in a spatial 2D setting, i.e. Ey is the single nonvanishing component of the electrical field.

Single mode waveguide

Symmetric single layer slab waveguide

A forward propagating guided field, superimposed with a backward traveling mode of the same amplitude, leads to a standing wave inside the film. Not that much exciting. But if the waveguide ends somewhere, the optical field spreads rapidly once it has passed the facet ...

Waveguide coupling

A coupler segment

If a second identical waveguide is placed close to the first core, the optical power starts to couple periodically forth and back between the two parallel waveguides. The characteristic length for this transfer depends strongly on the distance g between the cores (and besides that on all other geometrical and material parameters): While for a "large" gap g = 0.5 µm it takes a "long" propagation distance of Lc = 28.2 µm until the entire input power is found in the opposite core, for gap widths of g = 0.3 µm and g = 0.1 µm the optical power oscillates more rapidly with coupling lengths of Lc = 11.9 µm and Lc = 5.05 µm.