Beyond the ultralow loss across a broadband spectrum, the Ge-silica PICs have three more key advantages in dispersion engineering, acoustic confinement and thermal noise mitigation. To demonstrate them, soliton microcombs, stimulated Brillouin lasing (SBL) and SIL laser experiments are performed using the new platform. As these applications do not require monolithic or heterogeneous integration with other materials, annealed Ge:silica devices are used.
Single-ring soliton generation
Soliton microcombs represent the main new application area of microcavities and enable transfer of large-scale frequency comb technology to an integrated photonic chip. The Q factor of the microcavity determines the microcomb pumping power required as well as its coherence. However, at this time, the only ultrahigh-Q integrated platform—thin Si 3 N 4 —has a limited dispersion-engineering ability because of its thin waveguide thickness. This platform exhibits only normal dispersion and requires coupled-ring structures to generate soliton microcombs25. Here, an integrated Ge-silica resonator is used to demonstrate a new ability for an integrated design: soliton generation in a single ultrahigh-Q microring with anomalous dispersion. The microring resonator was designed to have anomalous dispersion and single-mode transmission as required for soliton generation (Supplementary Figs. 3 and 4). And the soliton mode family dispersion is characterized by measuring the frequency of all modes between 1,520 nm and 1,630 nm using an external cavity laser calibrated by a Mach–Zehnder interferometer (Fig. 3a). There is no observable mode-crossing-induced distortion of the mode family, therefore making the mode family well suited for soliton formation.
Fig. 3: Demonstration applications using the germano-silicate platform. a, Dispersion of a Ge-silica single-ring resonator, D int = ω μ − ω 0 − D 1 μ, where μ = 0 is assigned to the mode at 1,550 nm, D 1 /2π is the mean free spectral range and D 2 is the group velocity dispersion (GVD) (positive for anomalous GVD). A fitting gives D 1 /2π = 21.2 GHz and D 2 /2π = 8.9 kHz. Inset, soliton step in transmission. b, Optical spectrum of generated soliton microcomb (left) and radiofrequency spectrum of microcomb beatnote (right). RBW, resolution bandwidth. c, Measured SBS gain spectrum of a fully cladded Ge-silica waveguide. The simulation result is overlaid on the measurement response. Insets, electric-field mode profile of the fundamental TE mode (left) and Brillouin-scattering induced displacement response (right). d, Optical spectrum of stimulated Brillouin laser (left) and radiofrequency spectrum of SBL beatnote (right). a.u., arbitrary units. Full size image
Soliton triggering and stabilization are performed using the frequency kick and capture-lock26 technique. A measured soliton spectrum generated using a 3-mm-diameter ring is shown in Fig. 3b (for Q characterization, see Supplementary Fig. 6). The spectral envelope exhibits a well-defined sech2 envelope. To confirm a stable repetition rate, the soliton pulse stream was detected and analysed using an electrical spectrum analyser. The electrical spectrum in Fig. 3b (right) gives a repetition rate near 21.2 GHz, and the resolution bandwidth of 1 kHz confirms pulse stream stability.
Acoustic confinement and Brillouin lasing
Apart from soliton microcombs, the stimulated Brillouin laser (SBL) is another device that has attracted considerable interest27,28. A key challenge for SBL operation is achieving simultaneous optical and acoustic waveguiding so as to enhance photon–phonon interactions—a feat hindered in conventional platforms by the low acoustic impedance of the silica cladding. Ge-silica PICs overcome this barrier through GeO 2 doping, which reduces the longitudinal acoustic velocity of the waveguide core relative to the silica cladding, thereby enabling a fully transverse confined acoustic mode.
In the experiment, an approximately 25-mm-long waveguide is used to characterize the stimulated Brillouin scattering (SBS) gain spectrum and verify acoustical confinement by comparison to simulation. The waveguide core is 4 μm × 6 μm Ge-silica, with 15-μm thermal silicon oxide bottom cladding and 14-μm P 2 O 5 -doped silica upper cladding (the same cladding as shown in Fig. 2b, bottom right). Simulations of the normalized electric field and mechanical displacement distributions, shown in Fig. 3c, demonstrate simultaneous optical and acoustic wave confinement. Applying a pump–probe method (Methods), the SBS gain spectrum is obtained and agrees well with the simulation. The gain peak at 9.55 GHz has a full width at half maximum of 44.7 MHz, corresponding to a mechanical quality factor of about 210.
As a device demonstration, integrated Ge-silica resonators are used to generate a high-coherence Brillouin laser. To achieve phase matching for the Brillouin process near 1,550 nm, air-clad devices with diameters of approximately 20 mm were fabricated (for Q characterization, see Supplementary Fig. 7). Figure 3d (left) shows the optical spectrum of the lasing Stokes wave. The weaker pump signal peak in the spectrum arises from the need to collect the lasing Stokes wave in the propagation direction opposite to the pumping direction. Its strength is determined by residual reflection and backscattering in the measurement. The Brillouin lasing frequency shift is 9.68 GHz, which is lower than the typical 10.9 GHz shift observed in silica resonators18. Figure 3d (right) shows the microwave beatnote between the pump and Stokes waves, revealing its high coherence, as indicated by the high signal-to-noise ratio. This synergy of ultralow optical loss and engineered acoustic confinement unlocks low-noise Brillouin lasers for high-performance on-chip gyroscopes13, integrated microwave photonics and temperature/strain sensors.
LMA-enhanced hybrid-integrated low-noise laser
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