In this paper two-wave coupling in photorefractive KNbO

3

:Fe crystal is discussed in order to obtain the optimal conditions

under which the two-wave coupling exponential gain r reaches the maximum point.KNbO

3

:Fe crystal is an important photorefractive material.Its advantages are the fast response time and high sensitivity. The exponential gain r depends both on material propertiess

such as electro-optical tensor components rij and concentration of traps N

and on externally controlled parameters

such as the angle between the two incident beams inside the crystal 2θ

the angle between the grating vector and the crystal optical axis β

etc. We have studied two KNbO

3

:Fe crystal specimens with N

1

=1.8×10

16

cm

-8

and N

2

= 1.1×10

16

cm

-3

respectively. And one of them (the second one) is specially cut so that we can obtain β-value near 45° for small 2θvalue.