Both waveguides have traveling waves with average phase constant .
(2)
The amplitude of both waves vary in amplitude as they travel along the waveguides.
(3)
If the two waveguides are dissimilar i.e. if , and if , field amplitude of waveguide A remains almost constant and amplitude of the wave on waveguide B is much less than 1. That is in this situation most of the power remains with waveguide A and a small power is transferred to waveguide B (see Fig).
(4)
If the two waveguides are identical, i.e. if , we get
and the fields on the two waveguide become,
In this case when , i.e. when , . That means the entire power gets transferred from waveguide A to waveguide B.
Beyond this distance the power slowly goes back to waveguide A and when , the entire power goes back to waveguide A.
This process continues indefinitely. The power keeps fluctuating between the two waveguides (see Fig.).
The distance over which the maximum power is transferred to the other waveguide, is called the Coupling Length, . The coupling length is related to the coupling coefficient, as
Using this basic phenomenon of mode coupling a variety of devices can be realized.
. 
, and if 
, field amplitude of waveguide A remains almost constant and amplitude of the wave on waveguide B is much less than 1. That is in this situation most of the power remains with waveguide A and a small power is transferred to waveguide B (see Fig). 
, we get
and the fields on the two waveguide become, 
, i.e. when 
, 
. That means the entire power gets transferred from waveguide A to waveguide B. 
, the entire power goes back to waveguide A. 
. The coupling length is related to the coupling coefficient, 
as 
