Direct detection of modulated coherent optical signals by means of a structure exhibiting Fano resonance
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.
1. An optical receiver for detecting a modulated coherent optical signal, comprising:
a waveguide-coupled cavity structure comprising an optical waveguide having an input port for receiving an optical signal and an output port for providing an output optical signal, wherein the waveguide-coupled cavity structure is configured to exhibit Fano resonance and wherein a transmission spectrum of the Fano resonance overlaps with a spectrum of a received modulated coherent optical signal to suppress transmission of at least one sideband of the modulated coherent optical signal through the waveguide-coupled cavity structure, the sideband suppression being asymmetrical with respect to a carrier frequency of the modulated coherent optical signal; and
a photodetector configured to receive the output optical signal from the output port.
2. The optical receiver according to claim 1 , wherein a minimum in the transmission spectrum of the Fano resonance overlaps with a sideband of the modulated coherent optical signal.
3. The optical receiver according to claim 2 , wherein the minimum in the transmission spectrum overlaps with a sideband of the modulated coherent optical signal having largest amplitude.
4. The optical receiver according to claim 1 , wherein the waveguide-coupled cavity structure exhibits a blue or a red parity Fano resonance.
5. The optical receiver according to claim 1 , wherein the optical waveguide supports a continuum of modes and, wherein the waveguide-coupled cavity structure comprises an optical cavity supporting a discrete mode with resonance frequency, ω 0 , with the optical waveguide being arranged within an optical distance of the optical cavity to allow evanescent excitation of the discrete cavity mode from the continuum of modes.
6. The optical receiver according to claim 5 , further comprising a heating element configured to tune the resonance frequency of the optical cavity by thermo-optic effects.
7. The optical receiver according to claim 1 , wherein the optical waveguide comprises a partially transmitting element (PTE).
8. The optical receiver according to claim 1 , wherein the waveguide-coupled cavity structure is a ring resonator.
9. The optical receiver according to claim 8 , further comprising an additional optical waveguide evanescently coupled to the waveguide-coupled cavity structure to provide a loss channel for radiation out of the ring resonator.
10. The optical receiver according to claim 1 , wherein the waveguide-coupled cavity structure is a first waveguide-coupled cavity structure, which is configured to exhibit red-parity Fano resonance, the optical receiver further comprising:
a second waveguide-coupled cavity structure comprising an optical waveguide having an input port for receiving an optical signal and an output port for providing an output optical signal, wherein the second waveguide-coupled cavity structure is configured to exhibit blue-parity Fano resonance and wherein a transmission spectrum of the blue-parity Fano resonance overlaps with a spectrum of a received modulated coherent optical signal to suppress transmission of at least one sideband of the modulated coherent optical signal through the second waveguide-coupled cavity structure, the sideband suppression being asymmetrical with respect to a carrier frequency of the modulated coherent optical signal;
a first splitter for splitting the modulated coherent optical signal between the input ports of the first and second waveguide-coupled cavity structures; and
a balanced photodiode comprising the photodetector and being arranged at the outputs of the first and second waveguide-coupled cavity structures.
11. The optical receiver according to claim 10 , further comprising:
a second splitter for splitting the modulated coherent optical signal between a phase detection branch and an amplitude detection branch, the phase detection branch comprising the first splitter arranged to receive the modulated coherent optical signal from the second splitter, the amplitude detection branch comprising a photodetector arranged to receive an optical signal from the second splitter.
12. An optical communication kit comprising:
an optical transmitter comprising a coherent light source and at least one optical phase modulator for encoding data onto light from the coherent light source to form a modulated coherent optical signal; and
an optical receiver according to claim 1 for receiving and detecting the modulated coherent optical signal.
13. The optical communication kit according to claim 12 , wherein the optical transmitter comprises only one or more optical phase modulators for encoding data.
14. The optical communication kit according to claim 12 , wherein the optical transmitter further comprises at least one optical intensity modulator for encoding data.
15. A method for detecting a modulated coherent optical signal, comprising:
coupling the coherent optical signal into an optical waveguide of a waveguide-coupled cavity structure configured to exhibit Fano resonance, wherein a transmission spectrum of the Fano resonance overlaps with a spectrum of the coherent optical signal to suppress transmission of at least one sideband of the coherent optical signal through the optical waveguide, the sideband suppression being asymmetrical with respect to a carrier frequency of the coherent optical signal; and
detecting an intensity modulation of an output from the waveguide-coupled cavity structure.
16. An optical format converter for converting a phase modulation format of a modulated coherent optical signal into an intensity modulation format, comprising a waveguide-coupled cavity structure comprising an optical waveguide having an input port for receiving an optical signal and an output port for providing a converted output optical signal, wherein the waveguide-coupled cavity structure is configured to exhibit Fano resonance and wherein a transmission spectrum of the Fano resonance overlaps with a spectrum of a received modulated coherent optical signal to suppress transmission of at least one sideband of the modulated coherent optical signal through the waveguide-coupled cavity structure, the sideband suppression being asymmetrical with respect to a carrier frequency of the modulated coherent optical signal.
17. The optical format converter according to claim 16 , wherein a minimum in the transmission spectrum of the Fano resonance overlaps with a sideband of the modulated coherent optical signal.
18. The optical format converter according to claim 16 , wherein the minimum in the transmission spectrum overlaps with a sideband of the modulated coherent optical signal having largest amplitude.
19. The optical format converter according to claim 16 , wherein the waveguide-coupled cavity structure exhibits a blue or a red parity Fano resonance.
20. The optical format converter according claim 16 , wherein the optical waveguide supports a continuum of modes and wherein the waveguide-coupled cavity structure comprises an optical cavity supporting a discrete mode with resonance frequency, @0, with the optical waveguide being arranged within an optical distance of the optical cavity to allow evanescent excitation of the discrete cavity mode from the continuum of modes.
21. The optical format converter according to claim 20 , further comprising a heating element configured to tune the resonance frequency of the optical cavity by thermo-optic effects.
22. The optical format converter according to claim 16 , wherein the optical waveguide comprises a partially transmitting element (PTE).
23. The optical format converter according to claim 16 , wherein the waveguide-coupled cavity structure is a ring resonator.
24. The optical format converter according to claim 23 , further comprising an additional optical waveguide evanescently coupled to the waveguide-coupled cavity structure to provide a loss channel for radiation out of the ring resonator.
25. A method for converting a phase-modulation on a coherent optical signal into an intensity-modulation of the optical signal, comprising coupling the coherent optical signal into an optical waveguide of a waveguide-coupled cavity structure configured to exhibit Fano resonance, wherein a transmission spectrum of the Fano resonance overlaps with a spectrum of the coherent optical signal to suppress transmission of at least one sideband of the coherent optical signal through the optical waveguide, the sideband suppression being asymmetrical with respect to a carrier frequency of the coherent optical signal.