Hello everyone,

I have problem with bending losses and converging my results to reasonable values.

My structure is given in attached file with parameters: r_core = 25 um, n1=1.45245, n2=1.450567055.

LMA_50_um_step_index.lms (244.0 KB)

Here is my converging test for simulation span region for 500 mesh and bend radius 5 cm.

Bend radius is quite small so I expected large losses. What is more for span 120 and 130 there is no more fundamental mode propagating. I prevented this by putting complex refractive index in “search” in FDE. But then there were two polarizations of fundamental mode with losses x dB/m and -x dB/m for TM and TE polarization respectively. It is similar behavior for other modes as well. Why is that happening and which mode is correct? What does negative value mean? And how can I choose correct simulation span for various structures (I am going to use script which will be sweeping core index and calculate bend losses)?

And last question: What is the best procedure for calculating bending losses? Sweeping with straight mode in d-card and coupling with bent waveguide (PCF LMA example on Lumerical site)? Or just setting bending radius and finding propagation loss of mode is enough? I noticed that the both methods give the same values, but I’d like to make sure.

I am aware that core size is quite big and maybe the problem comes from physics itself.

I would be grateful for any help.