Optical attenuation of interferers in a radio frequency signal has been accomplished by modulating a beam of coherent light with the signal, removing the unwanted interference spectral components from the modulated beam, and then down-converting the beam to provide a radio signal which is free of these interference components.
Such a technique is shown in U.S. Pat. Nos. 4,699,466 and 4,522,466, assigned to the present assignee. The teachings of these patents are meant to be incorporated herein by way of reference.
The aformentioned technique filters the modulated radiation through an optical Fourier transform lens, a spatial filter and an inverse Fourier transform lens.
In particular, U.S. Pat. No. 4,522,466 teaches that the filter attenuation can be maximized by utilizing recursive methods. The coherent light is passed through the optical filtering system a number of times to provide extremely high notching attenuation on the order of 40 dB, or greater.
It would be desirable, however, to increase the notch depth to 80 dB, or more, or possibly notch the signal to the noise floor to remove all EMI.
The invention has as one of its objectives to provide an improved system and method of optically processing radio frequency signals to remove unwanted EMI, wherein a greater notch depth can be achieved than has been heretofore accomplished.
Another object of this invention is to provide a technique and system which reduces the number of filtering elements in a cascaded, multiple channel optical array.