Continuously variable delay line

Abstract

Coherent light is incident upon an acousto-optical cell through which bulk acoustic waves are propagating. These acoustic waves correspond to a signal to be delayed and result from the application of the signal to a piezoelectric transducer of the cell. The waves cause periodic variations in the refractive index of the cell which interact with the coherent light. Acoustic frequency components of the propagating waves correspond to the signal frequency components and cause the cell to diffract the beam of incident coherent light and to frequency shift the diffracted beam. The relative phases of the acoustic frequency components of the acoustic waves also correspond to the phases of the frequency components of the signal to be delayed and are imparted to the respective diffracted light beams. The diffracted light is combined with undiffracted coherent light so that optical heterodyning takes place. The recombined light is focused by a Fourier transform lens onto a photomixer which detects the frequency difference between the diffracted and undiffracted light beams. This detected frequency difference corresponds to the signal to be delayed. The delay time is the time required to convert the signal to corresponding acoustic waves and to optically detect the waves. The delay time is equal to the time required for the acoustic waves to propagate from the transducer to the point of incidence of the coherent light on the cell. Deflecting the coherent light incident upon the acousto-optical cell varies the point of incidence on the cell and, therefore, the delay time.