Fast and sensitive air-coupled ultrasound detection is essential for many applications such as radar, ultrasound imaging, and defect detection. Here we present a novel approach based on a digital optical frequency comb (DOFC) technique combined with high- Q optical microbubble resonators (MBRs). DOFC enables precise spectroscopy on resonators that can trace the ultrasound pressure with its resonant frequency shift with femtometer resolution and sub-microsecond response time. The noise equivalent pressure of air-coupled ultrasound as low as 4.4 mPa / √ Hz is achieved by combining a high- Q ( ∼ 3 × 10 7 ) MBR with the DOFC method. Moreover, it can observe multi-resonance peaks from multiple MBRs to directly monitor the precise spatial location of the ultrasonic source. This approach has a potential to be applied in 3D air-coupled photoacoustic and ultrasonic imaging.

In this work, we report experimental characterizations of stimulated Brillouin scattering (SBS) in low-loss Ge ₂₅ Sb ₁₀ S ₆₅ (GeSbS) chalcogenide waveguides. A 7-cm-long spiral waveguide with a propagation loss as low as 0.2 dB/cm and a microring resonator with a high loaded quality factor of 1.34×10 ⁶ are demonstrated. In addition, we use a high-resolution pump-probe measurement technique to investigate the SBS characteristics in the GeSbS waveguide. The measured Brillouin frequency shift and the intrinsic Brillouin linewidth are 7.443 GHz and 47.8 MHz, respectively. A Brillouin-gain coefficient of 338 m ^-1 W ^-1 is obtained, which corresponds to an intrinsic Brillouin gain of 0.524×10 ^-9 m/W, comparable to that of As ₂ S ₃ chalcogenide. Moreover, a 17.6-dB probe gain is achieved with a continuous-wave pump of 200 mW, while nonlinear losses are not observed. The large Brillouin gain, together with the low linear propagation loss and the negligible nonlinear loss, make the GeSbS chalcogenide waveguides become a very promising arsenic-free integrated platform for on-chip Brillouin applications.

Chalcogenide glass (ChG) is an attractive material for highly efficient nonlinear photonics, which can cover an ultrabroadband wavelength window from the near-visible to the footprint infrared region. However, it remains a challenge to implement highly-efficient and low-threshold optical parametric processes in chip-scale ChG devices due to thermal and light-induced instabilities as well as a high-loss factor in ChG films. Here, we develop a systematic fabrication process for high-performance photonic-chip-integrated ChG devices, by which planar-integrated ChG microresonators with an intrinsic quality ( Q ) factor above 1 million are demonstrated. In particular, an in situ light-induced annealing method is introduced to overcome the longstanding instability underlying ChG film. In high- Q ChG microresonators, optical parametric oscillations with threshold power as low as 5.4 mW are demonstrated for the first time, to our best knowledge. Our results would contribute to efforts of making efficient and low-threshold optical microcombs not only in the near-infrared as presented but more promisingly in the midinfrared range.

Compared to glass walls with a positive curvature, those with a negative curvature have been proven to have stronger confinement of light. Therefore, we change the multi-layered air holes in a photonic crystal fiber into several negative curvature tubes. As a result, the confinement medium is shifted from a low-index cladding material into a special structure. The theoretical analysis shows that each vector eigenmode has a corresponding threshold value for the outer tube thickness. It means that we can confine the target modes and filter the unnecessary modes by shifting the outer tube thickness. After substantial investigation on this fiber, we obtain the appropriate values for each structural parameter and then fabricate this negative curvature ring-core fiber under the guidance of the simulation results. Firstly, we draw the central cane under vacuum condition, then stack the cane and six capillaries to form the preform, and finally draw the ring-core fiber by using vacuumization method. The fiber test experiment indicates that the fiber length should be at least 15 m∼20 m to form the donut facula, and the tested losses of OAM_+1,1, OAM_+2,1, OAM_+3,1, and OAM_+4,1 are 0.30 dB/m, 0.36 dB/m, 0.37 dB/m, and 0.42 dB/m, respectively.