Electrically tunable whispering gallery mode microresonator based on a grapefruit-microstructured optical fiber infiltrated with nematic liquid crystals

Spectrally tunable liquid resonator based on electrowetting

We present a tunable on-chip liquid resonator in conjunction with a tapered fiber coupling scheme. The resonator consists of a glycerol droplet submerged within an immiscible liquid bath, which mitigates the effects of environmental fluctuations. The platform is fabricated using standard semiconductor techniques, which enable the future integration of photonic components for an on-chip liquid resonator device. The liquid resonator maintains its high Q-factor on chip (10⁵) due to surface tension forming an atomically smooth liquid-liquid interface. Higher Q-factor resonance modes experienced linewidth broadening due to the random excitation of thermal capillary vibrations. Spectral tuning is demonstrated using the electrowetting effect, increasing the surface's wettability and an expansion in the droplet diameter. A maximum spectral tuning of 1.44 nm ± 5 pm is observed by applying 35 V. The tuning range is twice the free spectral range (FSR) of 0.679 nm measured at a pumping wavelength range of 770-775 nm. A 2D axisymmetric finite-element simulation shows resonance modes in good agreement with experimentally measured spectra and with predicted tuning speeds of 20 nm/s.

Optothermal dynamics in whispering-gallery microresonators

Optical whispering-gallery-mode microresonators with ultrahigh quality factors and small mode volumes have played an important role in modern physics. They have been demonstrated as a diverse platform for a wide range of applications in photonics, such as nonlinear optics, optomechanics, quantum optics, and information processing. Thermal behaviors induced by power build-up in the resonators or environmental perturbations are ubiquitous in high-quality-factor whispering-gallery-mode resonators and have played an important role in their operation for various applications. In this review, we discuss the mechanisms of laser-field-induced thermal nonlinear effects, including thermal bistability and thermal oscillation. With the help of the thermal bistability effect, optothermal spectroscopy and optical nonreciprocity have been demonstrated. By tuning the temperature of the environment, the resonant mode frequency will shift, which can also be used for thermal sensing/tuning applications. The thermal locking technique and thermal imaging mechanisms are discussed briefly. Finally, we review some techniques employed to achieve thermal stability in a high-quality-factor resonator system.

Liquid-Crystal-Filled Side-hole Fiber for High-Sensitivity Temperature and Electric Field Measurement

We propose a highly sensitive sensor based on a nematic liquid-crystal-filled side-hole fiber. The liquid crystal is precisely filled into an air hole of the optical fiber using a method of manually gluing in the fusion splicer. Due to the coupling between the liquid crystal waveguide and the fiber core, multiple response dips appear in the transmission spectrum of the device. When an external temperature or electric field variation is applied to the liquid crystal and its refractive index changes, the transmission spectrum of this device will shift accordingly. Temperature and electric field response tests were performed on the device in the experiment, and the obtained temperature and electric field sensitivities were as high as −1.5 nm/°C and 3.88 nm/V_pp, respectively. For the exhibited advantages of being easy to manufacture, low cost, and high sensitivity, the proposed sensor is very promising for actual application in temperature or weak electric field monitoring.

Tunable Whispering Gallery Mode Photonic Device Based on Microstructured Optical Fiber with Internal Electrodes

We propose and experimentally demonstrate the first tunable whispering gallery mode (WGM) photonic device based on side-hole microstructured optical fiber (SH-MOF) with internal electrodes, in which the WGM quality factors do not decrease significantly during the tuning process. The resonant modes are redshifted simply by increasing the temperature. A description of the thermal tuning properties of the WGMs in SH-MOF with internal electrodes is performed by using a two-stage computational methodology, where the effects of metal filling process are considered. SH-MOF devices with internal electrodes are tested and the experimental results show excellent agreement with the theory. A linear relationship between the shift rate of the WGM modes and temperature is observed. The tunable SH-MOF microresonator with internal electrodes is anticipated to find potential applications in optical filtering, optical switching, and highly integrated tunable photonic devices.

Electrically tuned whispering gallery mode microresonator based on Kagomé photonic crystal fibers infiltrated with nematic liquid crystals

In this study, an electrically tunable whispering gallery mode (WGM) microresonator based on Kagomé hollow-core photonic crystal fiber (HC-PCF) is proposed by infiltrating the cross-sectional air holes with nematic liquid crystals (NLCs). Experimental results indicate that the LC molecules turn out in planar alignment. As the applied electric field strength gradually increases, the WGM resonance wavelengths shift toward the longer-wavelength region. The Freedericksz transition threshold of the proposed Kagomé HC-PCF microresonator is experimentally proved to be around 1.2  V μm^-1. The electrically tunable microresonator, integrated with NLCs, is anticipated to find potential applications in optical filtering, all-optical switching, and electrically controlled micro-optics devices.

Strain-based tunable optical microresonator with an in-fiber rectangular air bubble

We demonstrate a strain-based fully tunable, near-lossless, whispering gallery mode (WGM) resonator made of an in-fiber rectangular air bubble, which is fabricated by splicing two segments of standard single-mode fibers. Such a resonator, with a 39 μm order radius and 1 μm order wall thickness, contributes to a high quality factor exceeding 10⁶. The tuning in resonant wavelength is achieved by applying tensile strain to the resonator, and the voltage-tuning rate of the WGM resonance peaks is about 31.96 pm/V (strain-tuning rate ∼14.12  pm/μϵ), and the corresponding tuning accuracy is better than 0.03 pm. Since the tensile strain applied on the resonator can reach 1000  μϵ, the achievable total tunable bandwidth of ∼14.12  nm is more than two times that of its azimuthal free spectral range.

Optical bio-chemical sensors based on whispering gallery mode resonators

Whispering gallery mode (WGM) resonators have attracted extensive attention and their unique characteristics have led to some remarkable achievements. In particular, when combined with optical sensing technology, the WGM reonator-based sensor offers the advantages of small size, high sensitivity and a real-time dynamic response. At present, this type of sensor is widely applied in the bio-chemical sensing field. In this paper, we briefly review the sensing principle, the structures and the sensing applications of optical bio-chemical sensors based on the WGM resonator, with particular focuses on their sensing properties and their advantages and disadvantages. In addition, the existing problems and future development trends of WGM resonator-based optical bio-chemical sensors are discussed.