Microresonator-based solitons for massively parallel coherent optical communications

Journal name:
Nature
Volume:
546,
Pages:
274–279
Date published:
DOI:
doi:10.1038/nature22387
Received
Accepted
Published online

Solitons are waveforms that preserve their shape while propagating, as a result of a balance of dispersion and nonlinearity1, 2. Soliton-based data transmission schemes were investigated in the 1980s and showed promise as a way of overcoming the limitations imposed by dispersion of optical fibres. However, these approaches were later abandoned in favour of wavelength-division multiplexing schemes, which are easier to implement and offer improved scalability to higher data rates. Here we show that solitons could make a comeback in optical communications, not as a competitor but as a key element of massively parallel wavelength-division multiplexing. Instead of encoding data on the soliton pulse train itself, we use continuous-wave tones of the associated frequency comb as carriers for communication. Dissipative Kerr solitons (DKSs)3, 4 (solitons that rely on a double balance of parametric gain and cavity loss, as well as dispersion and nonlinearity) are generated as continuously circulating pulses in an integrated silicon nitride microresonator5 via four-photon interactions mediated by the Kerr nonlinearity, leading to low-noise, spectrally smooth, broadband optical frequency combs6. We use two interleaved DKS frequency combs to transmit a data stream of more than 50 terabits per second on 179 individual optical carriers that span the entire telecommunication C and L bands (centred around infrared telecommunication wavelengths of 1.55 micrometres). We also demonstrate coherent detection of a wavelength-division multiplexing data stream by using a pair of DKS frequency combs—one as a multi-wavelength light source at the transmitter and the other as the corresponding local oscillator at the receiver. This approach exploits the scalability of microresonator-based DKS frequency comb sources for massively parallel optical communications at both the transmitter and the receiver. Our results demonstrate the potential of these sources to replace the arrays of continuous-wave lasers that are currently used in high-speed communications. In combination with advanced spatial multiplexing schemes7, 8 and highly integrated silicon photonic circuits9, DKS frequency combs could bring chip-scale petabit-per-second transceivers into reach.

At a glance

Figures

  1. Generation of broadband frequency combs using DKSs in high-Q silicon nitride (Si3N4) microresonators.
    Figure 1: Generation of broadband frequency combs using DKSs in high-Q silicon nitride (Si3N4) microresonators.

    a, Principle of soliton frequency comb generation. The integrated photonic microresonator is pumped by a tunable continuous-wave laser that is amplified by an erbium-doped fibre amplifier (EDFA). Lensed fibres are used to couple light to the chip. A fibre polarization controller is used to optimize the power in the waveguide. After the microresonator, a notch filter suppresses the remaining pump light. The insets show scanning electron microscopy images of a Si3N4 microresonator with a radius of 240 μm. Right inset, top view. The checker-board pattern results from the photonic Damascene fabrication process29 (Methods). Left insets, cross-sections of the resonator waveguide (indicated by the dashed red arrows; dimensions, 0.8 μm × 1.65 μm) at the coupling point (top) and at the tapered section (bottom; dimensions, 0.8 μm × 0.6 μm). The tapered section is used for suppressing higher-order modes33 while preserving a high optical quality factor (Q ≈ 106; see Methods). b, Pump tuning method for soliton generation in an optical microresonator, showing the evolution of the comb power that is generated versus the wavelength of the pump laser: (I) the pump laser is tuned over the cavity resonance from the blue-detuned regime (blue shading), in which high-noise modulation-instability combs are observed, to the red-detuned regime (red shading), in which bistability of the cavity enables the formation of soliton states (here a multiple-soliton state); (II) after a multiple-soliton state is generated, the pump laser is tuned backwards to reduce the initial number of solitons down to a single one30. The schematics on the right show the corresponding intracavity waveforms in different states (modulation-instability, multiple-soliton and single-soliton states). c, Measured spectrum of a single-soliton frequency comb after suppression of residual pump light. The frequency comb features a smooth spectral envelope with a 3-dB bandwidth of 6 THz comprising hundreds of optical carriers extending beyond the telecommunication C and L bands (blue and red, respectively).

  2. Data transmission using microresonator dissipative Kerr-soliton combs for massively parallel WDM.
    Figure 2: Data transmission using microresonator dissipative Kerr-soliton combs for massively parallel WDM.

    a, Principle of data transmission using a single DKS comb generator as the optical source at the transmitter. A de-multiplexer (DEMUX) separates the comb lines and routes them to individual dual-polarization in-phase/quadrature modulators (‘IQ-mod’), which encode independent data on each polarization. The data channels are then recombined into a single-mode fibre using a multiplexer (MUX) and boosted by an EDFA before being transmitted. At the receiver, the wavelength channels are separated by a second de-multiplexer and detected using digital coherent receivers (‘Coh. Rx’) along with individual continuous-wave lasers as local oscillators (‘CW LO’). In our laboratory experiments, we emulate WDM transmission by independent modulation of even and odd carriers using two in-phase/quadrature modulators; see Supplementary Information section 2 for more details. We use 16QAM at a symbol rate of 40 GBd per channel. CW, continuous-wave laser; FCG, frequency comb generator. b, Section of the optical spectrum of the WDM data stream. Nyquist pulse shaping leads to approximately 40-GHz-wide rectangular power spectra for each of the carriers, which are spaced by approximately 100 GHz. c, Principle of data transmission using a pair of interleaved DKS combs at the transmitter. The resulting comb features a carrier spacing of approximately 50 GHz, which enables dense spectral packing of WDM channels and hence high spectral efficiency. At the receiver, this scheme still relies on individual continuous-wave lasers as local oscillators for coherent detection; see Supplementary Information section 2 for details. d, Section of the optical spectrum of the WDM data stream. e, Measured bit error ratios (BERs; 106 bits compared) of the transmitted channels for the single-comb (red triangles) and interleaved-comb (blue diamonds) experiments, along with the BER thresholds31 for error-free propagation when applying FEC schemes with 7% overhead (4.5 × 10−3, dashed orange line) and 20% overhead (1.5 × 10−2, dashed blue line); see Methods for details. For the interleaved-comb experiment, the outer 14 lines at the low-frequency edge of the L band (purple shaded region) were modulated with quadrature phase-shift keying (QPSK) signals rather than 16QAM owing to the low optical signal-to-noise ratio of these carriers. f, Measured BER versus optical signal-to-noise ratio of three different channels, centred at frequencies of 196.34 THz, 193.56 THz and 192.06 THz, and derived from a DKS frequency comb (blue) and a high-quality ECL (red), all with 16QAM signalling at 40 GBd. A total of 106 bits were compared. The comb lines do not show any additional penalty compared to the ECL tones. The black solid line (‘Theory’) indicates the theoretical dependence of the BER on the optical signal-to-noise ratio for an ideal transmission system; see Supplementary Information, section 3. Similar results were obtained at other symbol rates, including 28 GBd, 32 GBd and 42.8 GBd. g, Constellation diagrams obtained for an ECL and DKS comb tone at a carrier frequency of 193.56 THz. The colour indicates the relative density of symbols detected in the complex plane, with red indicating a higher density and blue a lower density. The optical signal-to-noise ratio is approximately 35 dB.

  3. Coherent data transmission using microresonator dissipative Kerr-soliton combs at both the transmitter and the receiver.
    Figure 3: Coherent data transmission using microresonator dissipative Kerr-soliton combs at both the transmitter and the receiver.

    a, Massively parallel WDM data transmission scheme using DKS frequency combs as both multi-wavelength source at the transmitter and multi-wavelength local oscillator (LO) at the receiver. In contrast to Fig. 2a, a single optical source provides all of the local-oscillator tones that are required for coherent detection. An extra de-multiplexer is used to route each local-oscillator tone to the respective coherent receiver. b, Section of the spectrum of the transmitted channels. c, Corresponding section of the spectrum of the local-oscillator frequency comb. Note that the comparatively large width of the depicted spectral lines is caused by the resolution bandwidth of the spectrometer (0.1 nm) and does not reflect the sub-100-kHz optical linewidth of the local-oscillator tones. d, Measured BERs for each data channel. Blue squares show the results obtained when using a DKS comb as the multi-wavelength local oscillator and red triangles correspond to a reference measurement using a high-quality ECL. Dashed lines mark the BER thresholds of 4.5 × 10−3 (1.5 × 10−2) for hard-decision (soft-decision) FEC with 7% (20%) overhead. Black circles show the channels with BERs above the threshold for 7% FEC and specify the reasons for low signal quality: a low optical carrier-to-noise ratio of the carriers from the local-oscillator comb (‘Local oscillator’) and the signal comb (‘Signal’), and bandwidth limitations of the C-band EDFA (‘EDFA’); see Methods for more details.

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Author information

  1. These authors contributed equally to this work.

    • Pablo Marin-Palomo,
    • Juned N. Kemal &
    • Maxim Karpov

Affiliations

  1. Institute of Photonics and Quantum Electronics (IPQ), Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany.

    • Pablo Marin-Palomo,
    • Juned N. Kemal,
    • Joerg Pfeifle,
    • Philipp Trocha,
    • Stefan Wolf,
    • Ralf Rosenberger,
    • Kovendhan Vijayan,
    • Wolfgang Freude &
    • Christian Koos
  2. Laboratory of Photonics and Quantum Measurements (LPQM), École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland.

    • Maxim Karpov,
    • Arne Kordts,
    • Joerg Pfeifle,
    • Martin H. P. Pfeiffer,
    • Victor Brasch,
    • Miles H. Anderson &
    • Tobias J. Kippenberg
  3. Institute of Microstructure Technology (IMT), Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany.

    • Wolfgang Freude &
    • Christian Koos

Contributions

P.M.-P., J.N.K. and J.P. built the system for data transmission, supervised by C.K. A.K. and M.H.P.P. designed and fabricated the Si3N4 microresonators, supervised by T.J.K. P.M.-P., M.K. and A.K. characterized and selected the samples. M.K., V.B., A.K. and M.H.P.P. developed the soliton generation technique, supervised by T.J.K. J.P., P. M.-P. and K.V. performed the OSNR characterization of the optical source. P.M.-P., J.P., P.T., S.W., J.N.K., K.V. and R.R. carried out the data transmission experiments, supervised by C.K. M.K. and M.H.A. theoretically investigated the optimization of the power conversion efficiency of DKS sources. The project was initiated and supervised by W.F., T.J.K. and C.K. All authors discussed the data. The manuscript was written by P.M.-P., J.N.K., M.K., T.J.K. and C.K.

Competing financial interests

T.J.K. is a co-founder and shareholder of LiGenTec SA, a start-up company that is engaged in making Si3N4 nonlinear photonic chips available via foundry service and is aiming to commercialize frequency channel generators. T.J.K. is a co-inventor of patents owned by Max Planck Society and EPFL in the technical field of the publication.

Corresponding authors

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Reviewer Information Nature thanks V. Torres-Company and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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