Step index mode solver

[ovanvincq]

Hi,

The computation of a step-index fiber modes seems incorrect.
As an example, we can study a step-index fiber with the parameters below:

• core radius : 2 µm
• core index : 1.47
• cladding index : 1.45

At a wavelength of 1µm, V=3.04 and only the modes HE11 (equivalent to LP01), TE01, TM01 and HE21 (equivalent to LP11) must exist.

However:

• StepIndexFibre.findneff(2,2*pi,1.47,1.45,0,1,:TM) returns 1.4574901533331723
• StepIndexFibre.findneff(2,2*pi,1.47,1.45,0,1,:TE) returns 1.4574901533331723
• StepIndexFibre.findneff(2,2*pi,1.47,1.45,0,2,:TM) returns 1.4538242972546838
• StepIndexFibre.findneff(2,2*pi,1.47,1.45,0,2,:TE) returns 1.4537675924407851

There are two problems:
1/ Roots.jl detects both zeros and discontinuities. As the char_TE, char_TM, char_HE and char_EH functions present a discontinuity between two consecutive zeros, it is necessary to check if the result given by Roots.jl is a zero or not. In this case, the effective indices of the TE01 and TM01 modes correspond to discontinuities.

2/ I think there is an inversion between TE and TM.

Regards,

Olivier

[jtravs]

Hi Olivier!,

You have found a part of the code that is relatively less well tested (we 99% use hollow core), and also I mostly use HE11, so this could well be the case. I will take a look at this when I get a chance, but given my current teaching/research/vacation load/plans, this will likely be several weeks, so please be patient.

Even better, as I think at first glance you are correct about this, you could always submit a PR! I'll still need to review it carefully when I get time, but it would be very welcome.

Best wishes,
John