  The dependence of the Josephson plasma frequency omega_p in Bi_2Sr_2CaCu_2O_8
on a tilted field H is reported. Measurements over a large range of B and tilt
angle theta allow a detailed comparison with a recent calculation by Koshelev.
With a slight modification of the model, close agreement is obtained. From the
fits, we find values for the in-plane correlation length and the zero-field
critical current density J_{c0} (4,600 A/cm^2 at 30 K). An analogy to Bragg
diffraction is described, as well as a picture for the fractional-exponent
behavior of omega_p versus H.
