Is it necessary for coherent waves to have same frequency?

[khurram usman]

i mean what more are coherent waves except 'having constant phase differnce'?i mean it is nt necessay for them to be in phase .they can be out of phase but must have a cnsstant phase diff. say pie/2 radian
so what i mean to ask is that is it necessary for coherent waves to be of same frequency. can't they be of different frewuency? bcoz even if they are of different frequency like say one is a multiple of other like twice of it even then the condition of CONSTANT PHASE DIFFERENCE be satisfied?
so shortly is it necessary that the coherent waves have same frequency or can they have frequencies dat are whole multiples of each other(256 , 512)?
also what abt their amplitude?

 

[Drakkith]

See here: http://en.wikipedia.org/wiki/Coherence_(physics )