Let me make my point more clear, first regarding Beta_2, its equation is:

The unit of Beta(w) is (1/m) where m is (meter) and units of omega is (1/s) where s is seconds, so the units of Beta_2 from the previous equation is (s^2/m)

The defintion of Delta_Beta as you said is inverse of difference of group velocities :

Which makes its unit (s/m)

So you can’t add Beta_2 (s^2/m) with delta_beta (s/m).

I have checked the first reference “Nonlinear Fiber Optics, 4^{th} ed.” and could not find this equation, actually the equation I found supports my argument and here it is:

In the formulation provided in Lumerical, the term Beta_1 (which accounts for the simple fiber delay) is omitted to reduced simulation time, while Delta_Beta is embedded inside Beta1_x & Beta1_y and it is not related to Beta_2.

I have also checked the second reference and it does not address the polarization dispersion in its formulation, so kindly tell me where is this equation in the paper?

Maybe the simulation uses different equation than the one described in the model link.

I have other question posted somehow related to polarization mode dispersion model in : /forum/forums/topic/lumerical-interconnect-fiber-pmd-effects/

kindly have a look and answer it.

Thanks