Thanks to the flat magnitude and ph

Thanks to the flat magnitude and phase response of optical devices, it is quite easy to realize broadband signal phase shifting and power manipulating with very small frequen-cy-dependent variations. One example is illustrated in Fig. 4 (c) and (d), showing the frequency responses of a commercial optical 90 hybrid. The power and phase variations are 0.1 dB and 1 over a 6-THz frequency range, which is much better than its electrical counterpart. The excellent amplitude-phase con-sistency would enable wideband noise and interference can-cellation in the optical domain. Previously, coherent cancellation is explicitly or implicitly used in many microwave photonic systems, such as high-linearity analog optical links [141-145], image-reject mixers [10, 146-152], co-site interference cancellation [153-159], and frequency multipliers [160-162] to suppress the noise, undesirable nonlinear components, interference and image frequencies. For example, a linearized analog optical link with the third-order intermodulation distortion (IMD3) component suppressed by 40 dB was built in [142]; an im-age-reject mixer with an image-rejection ratio of 25 dB for a 1.2-GHz instantaneous bandwidth linearly frequen-cy-modulated (LFM) signal was realized in [150] (as a com-parison, the instantaneous bandwidth of an electrical im-age-reject mixer is less than 160 MHz [146]); a 30-dB co-site interference cancellation ratio over 9.5 GHz frequency range was obtained in [156] (while for electrical method the maxi-mum reported bandwidth is only 120 MHz [163]); and an op-tical link with the common-mode noise suppressed by 15 dB over an 18-GHz frequency range was implemented in [164].