The design of fiber-optic filters

Fiber-optic filters can be used for processing wideband radio-frequency, mi crowave, and millimetric signals directly in the optical domain avoiding co stly electrooptic and optoelectronic conversions. Because these filters sum signals on an intensity basis, they are bound to be positive and, hence, h eavily restricted in their achievable performance. Nevertheless, such filte rs are the only choice because positivity is a consequence of the underlyin g physical mechanism. In this paper, the authors present a new design metho dology to realize an arbitrary filter as a difference of two positive filte rs, thus overcoming any performance limitation.