Whispering-gallery-mode excitation in a microsphere by use of an etched cavity on a multimode fiber end

Whispering gallery mode microsphere resonator based on cylindrical air cavity coupling

In this Letter, an optical fiber whispering gallery mode microsphere resonator based on cylindrical air cavity coupling is proposed and demonstrated. The cylindrical air cavity is vertical to the axis of a single-mode fiber and in touch with the fiber core, fabricated by using femtosecond laser micromachining together with hydrofluoric acid etching. A microsphere is inserted into the cylindrical air cavity and in tangential contact with the inner cavity wall, which is in touch with or inside the fiber core. The light traveling in the fiber core is coupled into the microsphere via an evanescent wave when the light path is tangential to the contacting point of the microsphere and the inner cavity wall, resulting in whispering gallery mode resonance when the phase-matching condition is satisfied. Such a device is highly integrated, robust in structure, low in cost, stable in operation, and has a good quality factor (Q) of 1.44 × 10⁴.

Whispering gallery mode excitation using exposed-core fiber

Whispering gallery modes (WGMs) in micro-resonators are of interest due to their high Q-factors. Ultra-thin fiber tapers are widely deployed to couple light into micro-resonators but achieving stable and practical coupling for out-of-lab use remains challenging. Here, a new WGM coupling scheme using an exposed-core silica fiber (ECF) is proposed, which overcomes the challenge of using fragile fiber tapers. Microspheres are deposited onto the exposed channel for excitation via the evanescent field of the fiber's guided modes. The outer jacket of the ECF partially encapsulates the microspheres, protecting them from external physical disturbance. By varying the mode launching conditions in this few-mode ECF, in combination with a Fano resonance effect, we demonstrate a high degree of tunability in the reflection spectrum. Furthermore, we show multi-particle WGM excitation, which could be controlled to occur either simultaneously or separately through controlling the ECF mode launching conditions. This work can bring value towards applications such as optical switches and modulators, multiplexed/distributed biosensing, and multi-point lasing, integrated in a single optical fiber device that avoids fiber post-processing.

Hollow-glass-microsphere-assisted half-circle interference for hydrostatic pressure measurement with high sensitivity

We propose and demonstrate a half-circle interferometer using a hollow glass microsphere (HGM) resonator. The half-circle interference is induced by a mismatch between the fundamental mode in the HGM and the modes in the capillary wall. The theoretical model is verified by comparing the simulated and experimental results. The variation in capillary length induced by the axial pressure contributes the most to the half-circle interference, which features a device with a high hydrostatic pressure sensitivity of -1.099 nm/kPa. This device shows potential as a hydrostatic pressure sensor owing to its stability, high sensitivity, and robustness.

Packaged whispering gallery resonator device based on an optical nanoantenna coupler

In this work, we present a packaged whispering gallery mode (WGM) device based on an optical nanoantenna as the coupler and a glass microsphere as the resonator. The microspheres were fabricated from either SiO₂ fiber or Er³⁺-doped fiber, the latter creating a WGM laser with a threshold of 93 µW at 1531 nm. The coupler-resonator WGM device was packaged in a glass capillary. The performance of the packaged microlaser was characterized, with lasing emission both excited in and collected from the WGM cavity via the nanoantenna. The packaged system provides isolation from environmental contamination, a small size, and unidirectional coupling while maintaining a high quality (Q-) factor (∼10⁸).

Excitation of whispering gallery modes with a "point-and-play," fiber-based, optical nano-antenna

We demonstrate the excitation and detection of whispering gallery modes in optical microresonators using a "point-and-play," fiber-based, optical nano-antenna. The coupling mechanism is based on cavity-enhanced Rayleigh scattering. Collected spectra exhibit Lorentzian dips, Fano shapes, or Lorentzian peaks, with a coupling efficiency around 13%. The spectra are characterized by the coupling gap, polarization, and fiber tip position. The coupling method is simple, low-cost and, most importantly, the Q-factor can be maintained at 10⁸ over a wide coupling range, thereby making it suitable for metrology, sensing, or cavity quantum electrodynamics experiments.