Learning Forum

[Then Elango]

For   a mode to be guided into the waveguide, the effective index neff should be larger than the refractive index of the cladding. In the structure shown below, what is the cladding, (is the subtrate below or is it the air above )

 

So when I run to calculate modes, 

So, is only the first two modes supported here, since the substrate is silica(r.index ~1.45) and when effective index is greater than this, only the first two are selected. 

 

Or, since the waveguide is surrounded by air(r. index=1), is other modes below are suported in the waveguide

[Amrita Pati]

Hi Then,

The first two modes will be the guided modes of the structure and the remaining will be leaky modes. Since as you said, their effective indices are lower than the substrate refractive index.

Regards,
Amrita

[Then Elango]

Thanks Amrita for the reply. 

But, when I simulate for bent waveguide, there are higher modes generated for the same dimension, which are higher than the effective indices of the substrate. 

For eg, a rectangular waveguide hads, 2 guided modes, now i enable bend waveguide anaylsis for  calculating modes in circular bend, and I get more than 2 modes which are much higher effective index than straight waveguide. 

 

 

also, based on the radius, the effective index values differ and the first order modes look like this:  

 

 

As the radius is increased, there is optimum effective index with 2  modes. 

What could this be?

 

Thanks

[Amrita Pati]

Hi Then,

Bends can often excite higher order modes. The smaller the radius of the bend is, the quicker the waveguide changes exciting more modes. You can find a lot of exisiting articles to see how the bending radius influnences the effective index and higher-order modes. We also have an example that shows the impact of bends, but it is in FEEM (https://optics.ansys.com/hc/en-us/articles/4409707153811-Bent-Waveguide-FEEM-).

Regards,
Amrita

[Then Elango]

Thanks a lot!