  Breaking the reciprocity of light propagation in photonic integrated circuits
(PIC) - especially in the CMOS-compatible silicon-on-insulator platform - is a
topic of intense research. However, a practical solution for monolithic
integrating of optical isolators and circulators remains elusive. Here, we
propose and analyze a new non-reciprocal photonic architecture operating with
standard single-mode waveguides (or optical fibers). Our design exploits
cascaded phase modulators separated by optical delay lines and suitably driven
by time shifted waveforms. Because it is based on fully balanced
interferometers and does not involve resonant structures, our scheme is also
intrinsically broadband. Using realistic parameters we calculate an extinction
ratio superior to 20 dB and insertion loss below -3 dB.
