Abstract: A waveguide-coupled cavity structure configured to exhibit Fano resonance (such as asymmetric Fano resonance or symmetric Fano resonance (inverse Lorentzian resonance), is utilized in an optical receiver or a method for direct detection of coherent optical signals by converting a phase-modulation on a coherent optical signal into an intensity-modulation of the optical signal. The waveguide-coupled cavity structure is designed for a transmission spectrum of the Fano resonance to overlap with a spectrum of the In modulated coherent optical signal to suppress transmission of at least one sideband of the modulated coherent optical signal through the structure, the sideband suppression being asymmetrical with respect to the carrier frequency of the modulated coherent optical signal. The invention may be used for direct detection of more advanced coherent modulation formats such as quadrature phase-shift keying (QPSK) signals and high order quadrature amplitude modulation (n-QAM) signals.