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A silicon Brillouin laser

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Science  08 Jun 2018:
Vol. 360, Issue 6393, pp. 1113-1116
DOI: 10.1126/science.aar6113
  • Fig. 1 Schematic of laser cavity and basic operation.

    (A) The Brillouin laser consists of a multimode racetrack cavity with two Brillouin-active regions (dark gray). Pump light (blue) is coupled into the antisymmetric spatial mode of the racetrack resonator. Intermodal Brillouin scattering mediates energy transfer from the pump wave (antisymmetric) to the Stokes wave (symmetric; red). L, length. (B) The idealized transmission spectrum for the racetrack cavity. Narrower and broader resonant features correspond to the symmetric and antisymmetric resonances, respectively. Brillouin lasing occurs when the resonance conditions for the pump (antisymmetric) and Stokes (symmetric) waves are simultaneously satisfied. (C and D) Cross sections of the suspended Brillouin-active region and the racetrack bend, respectively. (E) (i) Dimensions of the Brillouin-active waveguide. (ii) Strain profile εxx(x, y) of the 6-GHz Lamb-like acoustic mode that mediates intermodal scattering. (iii and iv) x-directed electric field profiles (Ex) of the TE-like symmetric and antisymmetric optical modes, respectively. Red and blue represent the respective positive and negative values of the electric field and strain profiles.

  • Fig. 2 Experimental apparatus and laser threshold behavior.

    (A) Apparatus used for heterodyne spectroscopy. Continuous-wave pump light (Agilent 81600B, linewidth = 13 kHz) used to initiate Brillouin lasing is amplified by an erbium-doped fiber amplifier (EDFA) and coupled on-chip by grating couplers. Laser light is frequency shifted (+44 MHz) by an acousto-optic modulator (AOM) in a reference arm and combined with the output Stokes light for heterodyne detection. RF, radio frequency. (B) Theory and experiment for the output laser power versus the input pump power. Intracavity pump powers are estimated by using the transmitted pump power and the detuning from resonance, and intracavity Stokes power is determined from the measured bus Stokes power and comparison with the theoretical model (supplementary materials 4.1) (23). (C) Heterodyne spectra of spontaneously scattered Stokes light from a linear waveguide (multiplied by 107) and the linewidth-narrowed intracavity laser spectrum above the laser threshold.

  • Fig. 3 Linewidth measurements.

    (A) Standard heterodyne spectroscopy apparatus to measure the pump-Stokes excess phase noise (or phonon) linewidth (Embedded Image), given by the FWHM of the heterodyne spectrum. (B) Subcoherence self-heterodyne apparatus used to probe the phonon dynamics at higher-output Stokes powers (supplementary materials 2) (23). The phonon linewidth is determined by measuring the fringe contrast or coherence between output Stokes and pump waves (C). τd, delay line transit time. (C) Experimental and theoretical comparison of phonon linewidths (Embedded Image) as a function of peak spectral density. Below the threshold, we use standard heterodyne spectroscopy (A) (red data points). At higher powers, this measurement becomes resolution bandwidth limited (supplementary materials 4.2) (23). For this reason, we use the subcoherence self-heterodyne technique (B), yielding the blue data points (error bars represent the 95% confidence interval of fits to data) (supplementary materials 4.2) (23).

Supplementary Materials

  • A silicon Brillouin laser

    Nils T. Otterstrom, Ryan O. Behunin, Eric A. Kittlaus, Zheng Wang, Peter T. Rakich

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods 
    • Supplementary Text
    • Figs. S1 to S14
    • Tables S1 to S3
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