  We present the Belavkin filtering equation for the intense balanced
heterodyne detection in a unitary model of an indirect observation. The
measuring apparatus modelled by a Bose field is initially prepared in a
coherent state and the observed process is a diffusion one. We prove that this
filtering equation is relaxing: any initial square-integrable function tends
asymptotically to a coherent state with an amplitude depending on the coupling
constant and the initial state of the apparatus. The time-development of a
squeezed coherent state is studied and compared with the previous results
obtained for the measuring apparatus prepared initially in the vacuum state.
