What is the physical dimension of transduction coefficient in photonics?

[jojoe]

I am studying Tunable couplers with Programmable Integrated photonics by Jose Capmany.
In this textbook, what is the physical meaning of signal s1 and s2? Is it an electric field intensity or the phase of electromagnetic wave?
And what is the physical dimension of transduction coefficient here?

 

[anuttarasammyak]

From the formula you referred I observe physical dimension of $\gamma$ is an inverse of physical dimension of s which is not clear to me who has no idea on the context.

 

[Ibix]

It seems to be talking about signal phase, so the $s_i$ have to be amplitudes, surely?