  Based on constructive interference in Sagnac waveguide loop, an efficient
scheme is proposed for selective frequency conversion and multifrequency modes
W entanglement via input-output formalism. We can adjust the probability
amplitudes of output photons by choosing parameter values properly. The tunable
probability amplitude will lead to the generation of output photon with a
selectable frequency and W photonic entanglement of different frequencies modes
in a wide range of parameter values. Our calculations show the present scheme
is robust to the deviation of parameters and spontaneous decay.
