Coherent terabit communications with microresonator Kerr frequency combs

Abstract

Optical frequency combs have the potential to revolutionize terabit communications¹. Generation of Kerr combs in nonlinear microresonators² represents a particularly promising option³ enabling line spacings of tens of GHz. However, such combs may exhibit strong phase noise^4-6, which has made high-speed data transmission impossible up to now. Here we demonstrate that systematic adjustment of pump conditions for low phase noise^4,7-9 enables coherent data transmission with advanced modulation formats that pose stringent requirements on the spectral purity of the comb. In a first experiment, we encode a data stream of 392 Gbit/s on a Kerr comb using quadrature phase shift keying (QPSK) and 16-state quadrature amplitude modulation (16QAM). A second experiment demonstrates feedback-stabilization of the comb and transmission of a 1.44 Tbit/s data stream over up to 300 km. The results show that Kerr combs meet the highly demanding requirements of coherent communications and thus offer an attractive solution towards chip-scale terabit/s transceivers.

Fig. 1. Principles of coherent terabit/s communications with Kerr frequency combs

a, Artist’s view of a future chip-scale terabit/s transmitter, leveraging a Kerr frequency comb source. The demonstration of coherent data transmission with Kerr combs is the subject of this work. (DEMUX: De-multiplexer, VOA: Variable optical attenuator, IQ-Mod: IQ-modulator, MUX: Multiplexer) b, Illustration of Kerr comb formation by multi-stage four-wave mixing (FWM). Degenerate FWM (1) converts two photons at the pump frequency to a pair of photons that are up- and downshifted in frequency, whereas cascaded non-degenerate FWM (2) populates the remaining resonances. c, Scanning-electron micrograph of an integrated high-Q SiN microresonator: High index-contrast SiN waveguides enable dense integration. d, Constellation diagrams of QPSK and 16QAM signals: Information is encoded both on the amplitude and the phase of the optical carrier, which can be represented by the in-phase (I, horizontal axis) and the quadrature component (Q, vertical axis) of the complex electrical field amplitude.

Fig. 2. Comb generation setup

The optical pump comprises a tunable laser source (TLS), a polarization controller (PC), and an erbium-doped fibre amplifier (EDFA). Lensed fibres (LF) couple light to and from the microresonator chip. A fibre Bragg grating (FBG 1) serves as a tunable narrowband notch filter to suppress residual pump light. For adjustment of the pump parameters, we monitor the power conversion from the pump to the adjacent lines (PM: Power meter). An electronic spectrum analyser (ESA) is used to measure the RF linewidth in the photocurrent spectrum of the photodetector (PD). A 5 nm-wide spectral section is extracted from the comb spectrum and used for data transmission. Insets: 1, waveguide cross section and mode profile; 2, optical micrograph of the resonator; 3, RF spectrum of a high phase-noise comb state (RBW 10 kHz); 4, RF spectrum of a low phase-noise comb state (RBW 30 kHz); 5, Selected part of the comb spectrum (OSA: Optical spectrum analyser).

Fig. 3. Coherent data transmission using a Kerr microresonator frequency comb

a, Spectrum of modulated carriers for all six data channels, measured at the input of the optical modulating analyser (OMA). b, Constellation diagrams for each channel and for both polarizations along with the corresponding error vector magnitude (EVM). The constellation diagrams show no sign of excessive phase noise, which would result in constellation points that are elongated along the azimuthal direction. For QPSK, the BER of all channels is below 4.5 × 10⁻³, which corresponds to an EVM of 38 %; for channels 4 and 5 the BER is even smaller than 10⁻⁹ (EVM < 16.7 %). The good quality of channel 5 enables transmission of a 16QAM signal with a measured BER of 7.5 × 10⁻⁴.

Fig. 4. Coherent terabit/s data transmission using a feedback-stabilized Kerr frequency comb

a, EVM for all data channels and fibre spans. The carriers are modulated at a symbol rate of 18 GBd using QPSK and Nyquist pulse shaping. Using polarization multiplexing at each of the 20 WDM channels, an aggregate data rate of 1.44 Tbit/s is achieved. The polarizations are distinguished by diamonds and squares, while the different fibre spans (0 km, 75 km, 150 km, 225 km, 300 km) are color-coded and slightly offset in the horizontal direction as indicated by the arrow. The red dashed line indicates an EVM of 38 %, which corresponds to the BER threshold for second-generation FEC. b, Optical spectrum of the 1.44 Tbit/s data stream. The spectrum was flattened prior to modulation.