There are extensive studies to date on optical nonlinearities in microcavities at the near and mid-IR wavelengths. Pushing this research into the visible region is equally valuable. Here, we demonstrate a directly pumped, blue band Kerr frequency comb and stimulated Raman scattering (SRS) at 462 nm in a silica nanofiber-coupled whispering gallery microcavity system. Notably, due to the high optical intensities achieved, photodarkening is unavoidable and can quickly degrade the optical quality of both the coupling optical nanofiber and the microcavity, even at very low pump powers. Nonetheless, stable hyperparametric oscillation and SRS are demonstrated in the presence of photodarkening by taking advantage of in-situ thermal bleaching. This work highlights the challenges of silica-based, short wavelength nonlinear optics in high-quality, small mode volume devices and gives an effective method to overcome this apparent limitation, thus providing a baseline for optics research in the blue region for any optical devices fabricated from silica.

We propose and demonstrate a microbubble resonator combined with a fiber Bragg grating (FBG) for simultaneous measurement of gas pressure and temperature. The approach used allows for very close proximity (<200 μm) between the FBG and the microbubble resonator. The microbubble resonator is fabricated in a section of hollow-core fiber (HCF), spliced to a single-mode fiber (SMF) with a pre-inscribed FBG to form a single-ended open structure. Fresnel reflection from the HCF cleaved end means that broadband light is reflected backwards in the same direction as the Bragg wavelength from the FBG. A tapered fiber couples the reflected broadband light to the microbubble, and both the Bragg peak and whispering gallery modes (WGM) are distinguishable in the signal available for interrogation. The experimental results show that this sensor structure consisting of a microbubble resonator combined with an FBG has the ability to simultaneously measure gas pressure and temperature, while also eliminating temperature crosstalk in gas pressure measurement and providing a useful platform for high accuracy gas pressure detection.

In this work, an Yb3+-Ho3+ codoped sol-gel silica microsphere lasing at around 2.0 μm is reported. The gain microsphere is fabricated by overlaying the 1.0 mol% Yb3+-0.2 mol% Ho3+ codoped sol-gel solution on the surface of a pure silica microsphere and is then heated using a CO2 laser. Using a traditional fiber taper-microsphere coupling method, we observe the single- and multi-mode microsphere laser outputs around 2.0 μm using a 980 nm laser diode as a pump source, with a low threshold pumping power of 14.7 mW. The ability to fabricate sol-gel codoped silica glass microlasers represents a new generation of low-threshold and compact mid-infrared laser sources for use as miniaturized photonic components for a wide range of applications including gas sensing and medical surgery.

Directly pumped single-mode and multimode Ho3+-doped sol-gel silica microspheres lasing at 2.0 mum are observed when an 1150 nm fiber Raman laser pump is used in a conventional microfiber-microsphere coupling method. Experimental results show single-mode output at 2 μm as well as the characteristic whispering-gallery-mode (WGM). By adjusting the taper contact position on the microsphere, we achieved switching between single-mode and multimode laser operations.

Up-conversion luminescence and C-band microsphere laser output is reported for an $\text{Er}^{3+}$ -doped $\text{ZrF}_4 \text{-BaF}_2 \text{-YF}_3 \text{-AlF}_3$ (ZBYA) fluorozirconate glass microsphere. The microsphere was fabricated by heating a ZBYA glass filament using a $\text{CO}_2$ laser beam. The fabrication process accurately and repeatably produces microspheres of 68 μm diameter. The input and output laser light was coupled to the microsphere using a tapered optical fiber. The coupling position between the tapered fiber and microsphere was adjusted using a sophisticated three-dimensional translation stage. The up-conversion luminescence emission, single-mode and multi-mode laser at C-band (1530 to 1565 nm) were observed when pumped using a 980 nm laser.

Whispering-gallery-mode (WGM) microcavities, with their ultra-high quality factors and deeply confined mode volumes, provide strong light–matter interaction and underpin a broad range of emerging photonic technologies. Their capabilities now span high-sensitivity sensing, ultra-low-noise microwave and frequency-comb generation, integrated quantum light sources, narrow-linewidth microlasers, and efficient nonlinear frequency conversion. As WGM devices advance toward greater practicality and integration, this paper reviews the research progress of WGM microcavity lasers across the visible to mid-infrared spectrum, which represents a key focus area, and discusses the challenges hindering their broader application.