Abstract:In this work, we show that the application of a sol-gel coating renders a microbubble whispering gallery resonator into an active device. During the fabrication of the resonator, a thin layer of erbium-doped sol-gel is applied to a tapered microcapillary, then a microbubble with a wall thickness of 1.3 µm is formed with the rare earth diffused into its walls. The doped microbubble is pumped at 980 nm and lasing in the emission band of the Er 3+ ions with a wavelength of 1535 nm is observed. The laser wavelength can be tuned by aerostatic pressure tuning of the whispering gallery modes of the microbubble. Up to 240 pm tuning is observed with 2 bar of applied pressure. It is shown that the doped microbubble could be used as a compact, tunable laser source. The lasing microbubble can also be used to improve sensing capabilities in optofluidic sensing applications.

Frequency comb generation in microresonators at visible wavelengths has found applications in a variety of areas such as metrology, sensing, and imaging. To achieve Kerr combs based on four-wave mixing in a microresonator, dispersion must be in the anomalous regime. In this Letter, we demonstrate dispersion engineering in a microbubble resonator (MBR) fabricated by a two-CO₂ laser beam technique. By decreasing the wall thickness of the MBR to 1.4 μm, the zero dispersion wavelength shifts to values shorter than 764 nm, making phase matching possible around 765 nm. With the optical Q-factor of the MBR modes being greater than 10⁷, four-wave mixing is observed at 765 nm for a pump power of 3 mW. By increasing the pump power, parametric oscillation is achieved, and a frequency comb with 14 comb lines is generated at visible wavelengths.

It is shown that, in multimode Raman lasers, cascaded light scattering (CLS) not only extends the optical frequency range, but can also modulate the laser dynamics. The origin of this phenomenon lies in the fact that many Raman lasing modes are directly correlated through CLS. The coupled‐mode equations only describe single‐mode cascaded Raman lasers and are insufficient for describing the multimode case. In this work, additional terms are introduced to account for intermodal interaction and, therefrom the physical mechanism behind the mode‐switching phenomenon is revealed. Additionally, mode‐switching controlled solely by a single‐mode pump in a whispering gallery mode (WGM) silica Raman laser is demonstrated. As the intracavity pump power is increased, laser switching happens between two adjacent WGMs in the same mode family.

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generation [1,2], laser stabilization [3][4][5] and sensing [6,7]. The high optical quality factor (Q-factor) and relatively small mode volume of whispering gallery resonators (WGRs) render the modes very sensitive to subtle environmental changes. Until now, WGRs have been used to measure changes in a number of parameters such as refractive index [8,9], temperature [10][11][12], pressure [13,14] and stress [15,16]. Aside from parameter change detection, ultrahigh Q resonators have also been used to detect nanoparticles [17,18] and single viruses [19,20]. The mechanism behind ultrahigh sensitivity sensing in WGRs is based on a reactive (i.e. dispersive) frequency shift of the whispering gallery modes [19] as a result of perturbations that may be present. Alternatively, a perturbation may increase the optical linewidth of the WGM by introducing more dissipation [21,22], or may change the back-scattering strength [23] and subsequent mode splitting if modal coupling is present [17,20]. The optomechanical properties of WGRs can also be used for force [24] or viscosity sensing [25].Currently, in order to retrieve the dispersive, dissipative and mode splitting information, the transmission spectrum of a WGR through an externally coupled waveguide, such as a tapered optical fibre, is usually measured. Light from a tunable laser source is coupled into the tapered fibre and the transmission is monitored. Low powers are used in order to minimise thermal and nonlinear effects on the whispering gallery modes. By sweeping the laser frequency, the transmission spectrum through the fibre can be recorded. Any changes to the frequency, mode splitting or linewidth are used to monitor perturbations induced by the physical parameter that is being sensed. During measurements, the transmission spectrum represents a steady state of the coupled system; therefore, time response of the sensor [26][27][28] is related to the lifetime of the resonator which limits the scanning speed. For a WGR with Abstract In whispering gallery mode resonator sensing applications, the conventional way to detect a change in the parameter to be measured is by observing the steady-state transmission spectrum through the coupling waveguide. Alternatively, sensing based on cavity ring-up spectroscopy, i.e. CRUS, can be achieved transiently. In this work, we investigate CRUS using coupled mode equations and find analytical solutions with a large spectral broadening approximation of the input pulse. The relationships between the frequency detuning, coupling gap and ring-up peak height are determined and experimentally verified using an ultrahigh Q-factor silica microsphere. This work shows that distinctive dispersive and dissipative transient sensing can be realised by simply measuring the peak height of the CRUS signal, which may improve the data collection rate.