 ## Photonics

Topics related to Lumerical and more

• Nathan Lin
Subscriber

I am trying to calculate the amount of phase accumulated by the waveguide of a certain refractive index, and I am trying out two approaches.

Approach 1): Use the beta propagation constant and the wavelength to determine the accumulated phase based on distance.  Approach 2) Using EME and sweep the group span of the waveguide length, and see that the phase difference of the Input port 1 , 2 (TE, TM) to that of Output port 3, 4 (TE, TM). Bascially seeing how the phase changes between S31 and S42 dependong on the distance. But the accumulated phase calculated is drastially different from approach 1. What is the best way to calculate the accumulated phase with simulation? • Guilin Sun
Ansys Employee

Phase is an interest quantity. In general we only count the principal phase after removing 2pi*n.

When you used the 1st method, it is pure math;

When you use the 2nd method, it unwraps the phase. and the reference point begins from the first length.meaning it does not remove the 2pi*n from this point. This is why you see much larger phase. It removes the 2pi only from the reference point, since it does not know what is the absolute phase.

thus the referace point changes the result will change.

• Nathan Lin
Subscriber

I see, so the first approach would be accurate to find the actual value of the phase accumulated.

But the second approach should work in terms of the incremental phase added to the WG with the increase of WG length, based on your description of the referencing point? For example, the SWG length swept is 4800um, then the total phase accumulated per um should be 166 radian / 4800 um= 0.03458 radian / um

• Guilin Sun
Ansys Employee

Yes, the incremental in the 2nd approach should be correct. But it refers to the begining point and the begining point does not have a zero phase. You have to subtract the phase at reference point. 