In this work, we demonstrate an ultra-narrow linewidth self-injection locked distributed feedback (DFB) diode laser emitting at 452 nm, achieving an intrinsic linewidth of 170 Hz with a fiber output power of 11 mW. The linewidth reduction of the DFB laser is obtained thanks to the coupling with an external cavity based on a fiber Bragg grating (FBG). Experimental results demonstrate excellent agreement with our theoretical modeling of the self-injection dynamics. Furthermore, the tuning capabilities of the system are characterized, with tunability achieved via current modulation, yielding a tuning efficiency of 300 MHz/mA over a continuous mode-hop-free range of 600 MHz. This architecture offers a robust pathway toward an integration within a compact package. Ultimately, such compact, stable, and frequency-tunable visible light sources are key for integrated optical atomic clocks and underwater lidar application.

Compact, low-noise coherent light sources in the visible remain challenging due to limited gain platforms and inefficient pumping. We report a new route to visible microlasing based on direct, one-photon blue pumping and an amorphous fluoride gain material platform. Dysprosium doped fluoride microspheres are fabricated via plasma-torch-induced, pressureless amorphization of single crystals, enabling compositions beyond conventional glass-forming limits while ensuring ultrasmooth morphology, low phonon energy, and homogeneous dopant distribution. We demonstrate the first fiber-coupled whispering-gallery-mode lasing from an amorphous fluoride microsphere in the yellow (573 nm), with an ultralow threshold of 190 μW despite spin-forbidden Dy^3+ transitions. Lasing is evidenced by characteristic light-light curve indicating a low spontaneous emission factor, narrow-linewidth emission, and relaxation oscillations yielding a loaded quality factor of Q = 3.5 × 10^6. This platform is readily extendable to other rare-earth emitters, enabling entire visible spectral coverage beyond the limitations of upconversion pumping, with prospects for color-tunable and white-light emission. Finally, fiber-based amplification of the WGM signal demonstrates a pathway toward compact, fiber-integrated visible microlasers with controllable noise and linewidth.

Whispering-gallery-mode (WGM) microsphere resonators have emerged as a versatile platform across various photonic applications. Despite significant progress, their performance at short wavelengths is fundamentally limited by scattering-induced optical losses that restrict achievable quality factors (Q-factor). Although surface roughness has long been recognised as the leading cause of these losses, its physical origin has remained unclear, with current understanding attributing it to unavoidable fabrication imperfections. Here, we show that thermally excited capillary waves are the fundamental source of scattering losses in microsphere cavities. Using high-resolution atomic force microscopy (AFM) combined with rigorous statistical analysis, we quantitatively identify the characteristic signatures of frozen capillary fluctuations at the sub-nanometre level. The experimentally extracted roughness parameters show close agreement with theoretical predictions based on capillary wave theory. These findings fundamentally revise the prevailing interpretation of surface scattering losses and establish thermodynamic fluctuations, rather than fabrication defects, as the limiting roughness mechanism. By identifying frozen capillary waves as the limiting factor, this work opens new pathways for engineering ultra-high-Q microsphere resonators through fabrication management strategies, particularly for visible- and ultraviolet-photonic applications where scattering losses are most severe.

Aluminum oxide offers an excellent platform for integrated photonics in the visible and ultraviolet ranges, combining low propagation losses, high index contrast with silica, and strong compatibility with rare earth dopants. This work reports a comprehensive study of 460 nm microring resonators in various configurations. The reported devices, obtained through a commercial multi-project wafer service, exhibit a record intrinsic quality factor of 3.6 × 105 on the aluminum oxide platform. Thermal tuning of the resonances is achieved via integrated metallic heaters, with a tuning efficiency of 1.63 pm/mW over a 500 pm range. These results demonstrate fully integrated photonic functionalities in the visible range, opening new prospects for sensing, spectroscopy, and quantum photonic applications.

Whispering-gallery-mode (WGM) microresonators with a high Q-factor, such as glass microspheres, enable the development of low-threshold and narrow linewidth lasers [1]. So far, the research on such coherent light sources mainly focused on near-infrared emission. Upconversion lasers based on this architecture has also been reported [2]. Recently, there is a constantly growing interest in lasers directly emitting visible light under pumping in the UV and blue spectral ranges, e.g., using GaN-based diode lasers. Dysprosium ions $(\text{Dy}^{3+})$ are appealing because of their yellow emission, Fig. 1(a), and Dy lasers can address the so-called “green gap” problem of semiconductor laser sources [3]. We report on WGM yellow lasing from a $\text{Dy}^{3+}$ -doped silica glass microsphere.

We report on the development and characterization of a compact visible Brillouin fiber laser. The laser is designed in a short cavity configuration and demonstrates an output power of 30 mW. Detailed analysis reveals an intrinsic linewidth of 6 Hz, indicating exceptional spectral purity. Comprehensive gain characterization was performed as a function of pump power, providing critical insight into the laser's performance and limitations. These findings contribute to the advancement of compact, highly stable visible fiber lasers for quantum technologies and microwave photonics.

Dy3+-doped silica glass whispering gallery mode microspheres are fabricated by fiber fusion splicing. They present an almost ideal spherical morphology with a radius ranging from 60 to 67 µm, as determined by confocal laser microscopy. The host composition of the microsphere is close to that of the fiber core. The dopant Dy3+ ions are uniformly distributed across the microsphere, as evidenced by µ-luminescence studies, and present a luminescence lifetime of the 4F9/2 state of 514.7 ± 1.1 µs, indicating a weak luminescence quenching. The Dy3+-doped glass microspheres were excited via evanescent field coupling using a half-tapered fiber and a blue 450-nm GaN laser diode (direct pumping scheme). The yellow fluorescence of Dy3+ ions (the 4F9/2 → 6H13/2 transition) is filtered by the whispering gallery modes (free spectral range: 0.5 nm for 67-µm radius microsphere). The onset of stimulated emission is further observed, highlighting the potential of such microresonators for narrow-linewidth light sources directly emitting visible light.

The development of Al2O3-based integrated components in the blue/near-UV range is reported. An MMI splitting ratio of 3.20±0.34 dB/port at 405 nm and microring resonators quality factors of 2.1*105 measured at 460 nm are demonstrated.

Dy 3+ -doped amorphous fluoride microspheres are fabricated from Dy:LiYF 4 crystals by the plasma torch method. Yellow lasing at 573 nm is achieved under blue GaN-diode pumping at 454 nm with a threshold of 190 μW.

Research on integrated blue and near-ultraviolet photonics has been increasingly investigated in recent years. To enable the development of photonic integrated circuits in this wavelength range, one of the challenges is to identify a transparent platform that can provide a variety of integrated components. Aluminum oxide (Al2O3) was demonstrated to exhibit low propagation losses for wavelengths below 450 nm, making it a very promising platform to operate at short wavelengths. MMIs are very convenient integrated components for splitting or combining signals and can be used for many applications such as on-chip spectrometry or microscopy. The development of a 50/50 coupler at 405 nm is reported based on symmetrical multimode interferometer (MMI) presenting a splitting ratio of 3.20 +/- 0.34 dB/MMI. Characterizations at 375, 420, and 454 nm are also presented.

This study presents a comprehensive analysis of a whispering gallery mode (WGM) microsphere optical properties in the near ultraviolet spectrum, and its practical implementation for laser linewidth reduction via frequency locking. The light coupling is achieved thanks to the utilization of a robust angle-polished fiber, enabling exploration of various coupling behaviors. The intrinsic Q(0)-factor, measured at 2.2x10(8), along with a finesse of 7.3x10(4), is reported at 420 nm. Physical mechanisms contributing to the Q(0)-factor are discussed and routes to improve the performances are drawn. Through the implementation of frequency locking onto a high-Q resonance of the WGM microsphere, the reduction of the linewidth of an external cavity diode laser from 887 kHz to 91 kHz has been obtained. The study of these outcomes brings to performance assessment, enabling a thorough understanding of limitations and identifying potential pathways for enhancing noise reduction. Such high Q-factor and high finesse are key ingredients to ease the study of photonic devices based on WGM microresonators.

We present a butt-coupled InGaN fiber Bragg grating (FBG) semiconductor laser diode operating below 400 nm in the single-mode emission regime. This compact coherent laser source exhibits an intrinsic linewidth of 14 kHz in the near-UV range and a side-mode suppression ratio reaching up to 40 dB accompanied by almost 2 mW output power. Furthermore, the properties of the FBG, including its central wavelength, bandwidth, and reflectivity, can be readily customized to fulfill specific requirements. As a result, the small footprint design of this laser is compatible with integration into a standard butterfly package to ease the lab-to-market technology transfer. The combination of low-frequency noise and fibered output signal positions these FBG laser systems as strong candidates for hybridization with integrated photonic platforms tailored for quantum information processing and metrology.