I have a question regarding bidirectional coupler cosine dependence on wavelength.
I created a system in OptiSystem which consists of continuous wave laser and bidirectional coupler in order to measure wavelength dependence of bidirectional coupler. I expected that for operating wavelength of 1550nm coupling ratio 1-1 and coupling ratio 1-2 curves intersect at this wavelength. But, I get curves that never intersect, only touch each other at operating wavelength (I tried for coupler bandwidth of 20nm and 150nm). Is there any way to make these curves intersect at 1550nm?
Also, why there is a limit of 200 nm on bidirectional coupler bandwidth? I expected that coupling ratio 1-1 and 1-2, and also power meter 1 and 2 ratios will behave like in photo in upper right corner..
.
