Optical Whispering Gallery Mode Resonators Research Articles

Optical whispering gallery mode resonators: analysing thermo-optic tuning in a silicon sphere

ABSTRACT In this work, we discuss and experimentally investigate the whispering gallery modes (WGMs) in a 500 μm radius silicon sphere. We begin by reviewing the basics of WGM resonators, followed by simulations and experimental results obtained with the aforementioned silicon spherical WGM resonator. The recorded WGM signatures in the transmission and scattering spectra excited, in the near-infrared (near-IR) region, agree well with the simulations. Thermo-optic tuning of these WGMs is achieved by introducing a pump laser in the violet-blue region. Red-shifts in wavelength are observed, which increase with the increase in the pump power. A shift of 0.63 nm is observed at a pump power of 6.32 mW. We also study switching by analysing the transient response of the WGM spectra. The measured WGMs exhibit high quality factors ( Q ∼ 10 5 ), which along with the aforesaid tunability, demonstrates the potential of spherical silicon resonator as a platform for photonic applications, e.g. sensing and communication. In summary, this work contributes towards the understanding of the fundamental physics of WGMs and provides insights into silicon WGM resonators.

Enhanced chiral sensing by optical whispering gallery mode microresonator.

Chiral sensing is essential in biochemistry, biomedicine, and food industries. Traditional chiral sensing mainly focuses on enhancing the chiral near fields, where the quality factor of the resonator has largely been ignored. Here, we propose a whispering gallery mode (WGM) optical microresonator to enhance the chiral signal by exploiting its high quality factor. The enhancement factor in the WGM resonator is linearly dependent on the quality factor of the microresonator and exponentially increases with the decreasing thickness of the chiral analyte. Based on this, we achieve a chiral enhancement factor of more than two orders of magnitude, which features a background-free chiroptical signal. Our results open a route to attain highly sensitive chiral sensing based on the WGM optical resonator, which promotes the realization of chiral applications in integrated optics.

Annular rogue waves in whispering gallery mode optical resonators

Based on the variable-coefficient Lugiato-Lefever equation, nonlinear dynamics of rogue waves in whispering gallery mode optical resonators are studied. By means of the analytical solution in the absence of pump, dynamical behaviors of the rogue waves in the periodic system are discussed. Firstly, the first-order rogue wave under the condition of no pumping has obvious periodic nature, and the periodicity of width and amplitude of rogue wave can be consistent by controlling the wavefront curvature, and the energy of rogue wave remains stable under the small gain parameter, and the inverse diffraction effect makes the center position of rogue wave produce a small offset. In addition, the amplitudes of second-order rogue waves have more complicated periodicity and longer period lengths both with the good stability. At last, the dynamical behaviors of rogue wave after adding external excitation are investigated. Under the periodical external excitation with the lower intensity, rogue wave can achieve a longer stable transmission time, while the smaller wavefront curvature can effectively prolong the stable transmission time of rogue wave. Moreover, we investigate the influence of different forms of external excitation on rogue wave, and the periodical external excitation makes rogue waves more stable than the constant external excitation. These results extend the application of higher-order rogue waves in (2 + 1)-dimensional nonlinear systems, and have implications for the development of high-energy optical pulses.

Simultaneous determination of AC and DC magnetic field by a whispering gallery mode optical resonator

Alternating current (AC) and direct current (DC) magnetic fields are monitored by an optical whispering gallery mode (WGM) microcapillary resonator filled with magnetostrictive material. The DC magnetic field is demodulated with the signal-to-noise ratio of an AC-modulated magnetic field at a particular frequency and a sensitivity of 2.1 dB/mT is obtained across a detection range of 0.9 mT. An AC field sensitivity of 2.08 μT/ H z at 346 kHz is obtained with the same resonator. Introducing a biased DC field of 90Gs provides a four-fold improvement in AC field sensitivity. The AC and DC fields may be used to improve the corresponding DC and AC sensing. Therefore, this scheme may play a role monitoring magnetic fields in defence, biomedical analysis, and consumer electronics.

Saddle-Shape Whispering Gallery Mode Microresonators

Optical Whispering Gallery Mode (WGM) resonators are an ideal platform for highly sensitive sensors and nonlinear interactions. They exhibit extremely high quality (Q) factors, providing a sensing platform with low detection limit to environmental changes and optical confinement for low threshold nonlinear stimulation. Here we demonstrate a new type of WGM resonator we coin as a Saddle-Shape Microresonator (SSM). The unique modal properties of the SSM's WGMs significantly differs them from spheres, disks, toroids, and other well-known microresonator types. We show that the SSM offers, in addition to the many attractive attributes that are found in "fiber-tip" sphere microresonators, the added characteristic of structural stability of a taper coupled resonator. The combination of these traits make it ideal for strain-based sensing, cavity quantum electrodynamics, and for real life applications in which mechanical tunability of the microresonator's WGMs is essential. The distinctive structure of SSMs – allowing exceptional high mechanical stability as well as mechanical tunability – opens a route to manufacture miniature packaged fiber-coupled WGM microresonators-based devices.

Fabrication and packaging of whispering gallery mode rod resonators with adjustable coupling

Optical Whispering Gallery Mode (WGM) resonators are an ideal platform for high sensitivity sensors. They exhibit extremely high Q factors - on the order of 106–109 – providing a sensing platform with low detection limit. For some sensing applications, the micrometric diameters of microresonators may be undesirable, and resonators of large diameters are required. Here we present the fabrication and packaging of a cm-scale silica rod resonator with a Q factor of ∼ 7.5·108. The demonstrated packaging provides – for the first time to the best of our knowledge – the ability to tune and adjust the coupling efficiency on demand. The coupling adjustment makes it possible to modulate the transmission power of the tapered fiber used to couple light into the rod resonator, as well as to modulate the loaded Q factor of the resonator.

Whispering Gallery Mode Resonator Temperature Compensation and Refractive Index Sensing in Glucose Droplets.

Among the different types of photonic sensor devices, optical whispering gallery mode resonators (WGMRs) have attracted interest due to their high level of sensitivity, small size, and ability to perform real-time temperature measurements. Here we demonstrate the applicability of temperature measurements using WGMR in both air and liquid environments. We also show that WGMR allowed measurements of the refractive index variations in an evaporating glucose–water solution droplet. The thermal tuning of WGMR can be reduced by coating WGMRs with a thin layer of polymethyl methacrylate (PMMA). Dip-coating the silica microsphere multiple times significantly reduced the resonance shift, partially compensating for the positive thermo-optical coefficient of silica. The shift direction changed the sign eventually.

Analytical description of resonances in Fabry–Perot and whispering gallery mode resonators

Whispering gallery mode optical resonators have attracted attention due to their simplicity and applicability for sensing. In this paper, analytical formulas are provided that describe resonance conditions in optical resonators. Basic terms (resonance wavelengths and frequencies, free spectral range,Q-factor, summation principle ofQ-factors of various processes, finesse, etc.) are introduced. A description of interference of an infinite number of waves of progressively smaller amplitudes and equal phase differences is given. A description of a Fabry–Perot resonator with nonequal reflection coefficients is also given as well as analysis of all-pass and add-drop optical filters. The presented description of resonators will help to analyze the effects of optical resonators, interpret the results of experiments, and guide the development of novel applications of microresonators.

Dispersion effects on the performance of whispering gallery mode based optoelectronic oscillators

Optoelectronic oscillators (OEOs) based on optical whispering gallery mode resonators (WGMRs) are amongst state-of-the-art technologies to generate ultra-low phase noise microwave signals. In this paper, a WGMR-based OEO is designed and effects of dispersion on its performance, such as phase noise, amplitude, and frequency of oscillation are theoretically investigated which are verified by simulations. Dispersion causes each mode existing inside a WGMR to experience a distinctive optical delay depending on its frequency. Considering the material and geometric dispersion of the WGMR used as the storage element of the OEO, we show that dispersion does not mainly affect the amplitude of oscillation while it affects the frequency of oscillation and phase noise of the output microwave signal. Indeed, through calculating the group delay and the group delay dispersion, we show that laser frequency noise (LFN) is converted into the phase noise of the generated microwave signal via the dispersion of the optical WGMR. Theoretical calculations are verified by the results obtained from two other previously published methods, i.e. the Langevin approach and the conversion matrix approach.

Devices with Whispering Gallery Mode Optical Resonators: Current State of Research and Prospects for their Application in Time and Frequency Metrology

The development of highly stable, narrow-band laser sources and oscillators producing sets of equidistant narrow optical spectral lines (so-called optical frequency combs) constitutes one of the most urgent tasks in the metrological support of time and frequency measurements. These devices are necessary for the transmission of frequency reference signals, as well as for use in mobile sources of time and frequency reference signals in optical and microwave spectral ranges. Such devices are also used in precision, ultra-high resolution spectroscopy. A promising direction for the development of highly stable, narrow-band lasers and generation of optical frequency combs (OFCs) is the use of miniature whispering gallery mode (WGM) optical resonators. Recently, the properties of such resonators have been actively studied along with technologies for their production. This article presents a review of the current state of research into the use of WGM optical resonators in time and frequency metrology. The types, advantages, and disadvantages of WGM resonators, as well as methods for generating various signals, specifically microwave signals, are described. Examples of existing laboratory setups used by Russian and foreign scientific groups are given. The authors examine prospects for the further use of WGM resonators.

State of the art and perspectives of the devices with the optical WGM resonators in time and frequency metrology

One of the most important tasks in the metrological support of time and frequency measurements is a development of an extremely stable narrow-bandwidth laser sources and oscillators of the sets of the equidistant narrow optical spectral lines (the so-called “optical combs”). These devices are necessary for the transmission of the reference frequency signals and for use as part of mobile sources of time and frequency reference signals in optical and microwave spectral ranges. They are also required for precision ultra-high resolution spectroscopy. A promising direction for the creation of highly stable narrow-band lasers and optical combs is the use of miniature optical whispering gallery modes resonators. Recently, research has been actively performed on the properties and manufacturing techniques of such resonators, as well as devices using them. The article provides a review of current research into applications of optical whispering gallery mode resonators in time and frequency metrology. Main advantages and disadvantages of such devices and prospects for their further use are discussed. The authors review in detail some examples of existing laboratory setups of Russian and foreign scientific groups.

A study of relative humidity sensor on micro-ball resonator

We investigate the use of a class of whispering gallery mode optical resonators, namely, optical micro-ball resonator (MBR) as a relative humidity (RH) sensor. The MBR is fabricated using fusion splicer at the tip of standard optical fibers; single-mode fibers (SMF-28). The MBR is then optically excited by using an 8?? optical microfiber and was found to have a Q-factor of >105. The MBR was then employed as a humidity sensor with an RH range of between 40% to 100%. The MBR RH sensor was found to have a sensitivity 0.284 dB/%, linearity >90% and is superior to the bare microfiber in all measured parameters. The MBR RH sensor was also found to have good repeatability and stability compared to bare microfiber.

The performance of CROW structures on detection of virus nanoparticles

Coupled resonator optical waveguide (CROW) structures were investigated as a whispering gallery mode optical resonator for biosensing applications. A single mode optical fiber taper was used to evanescently couple light into the microspheres. We have simulated the theoretical model of the crow structure with two and three microspheres. The coupled-resonator-induced transparency (CRIT) phenomenon was observed in the crow structures. In this paper, two crow structures and a single microsphere resonator are compared with each other. We have reported the high sensitivity of the crow structures compared to a single microsphere. We have demonstrated apparent shifts of CRIT transparent peak when the virus nanoparticles are placed on the surface of the microsphere. As a result, these crow structures can be used as a biosensing sensor with high sensitivity. The theoretical simulation was carried out at the operating wavelength of 1550 nm.

Whispering gallery modes on optical micro-bottle resonator for humidity sensor application

We investigate the use of a class of whispering gallery mode optical resonators, namely, optical microbottle resonators (MBR) as a relative humidity (RH) sensor. The MBR is fabricated via the so-called “soften-and-compress” method to create a bulge (bottle) structure with a bottle diameter Db=190⁡μm, stem diameter of Ds=125⁡μm and bottle length of Lb=182⁡μm. The MBR is then optically excited by using a 2 μm optical microfiber and was found to have a Q-factor of >104. The MBR was then employed as a humidity sensor with a RH range of between 40%–80% and the performance is compared with a no-MBR microfiber. The MBR RH sensor was found to have a sensitivity 0.0487 dB/%, linearity >90% and P-value >105 and is superior to the no-MBR microfiber in all measured parameters. The MBR RH sensor was also found to have good repeatability and stability over a period of 60 s. The wavelength shifted for the MBR is 0.03 nm greater than no-MBR.

On Stiffness of Optical Self-Injection Locking

Spectrally pure semiconductor lasers produced via self-injection locking to high quality factor monolithic optical resonators demonstrate sub-kHz instantaneous linewidth. The lasers are used in photonic sensor systems and microwave photonic oscillators benefitting from the improved spectral purity, the stability and the reduced environmental sensitivity of the lasers. The laser frequency stability is defined by both the optical resonator and the optical path of the entire system comprising the laser, the resonator, and the miscellaneous optical components. The impacts of the various destabilization factors are usually convoluted, and it is hardly possible to separate them. In this paper, we report on an experimental study of an influence of the variations of the optical path on the laser frequency stability. We have created a whispering gallery mode optical resonator having the record small thermal sensitivity, on the order of 0.1 ppm/ ∘ C, and demonstrated a self-injection locked laser based on this resonator. The measured laser stability is characterized with 1 s Allan deviation of 10 − 12 , limited by the thermal sensitivity of the optical path between the laser and the resonator. The thermal stabilization on the order of 10 μ K at 1 s is achieved using a standard thermo-electric element. The long term drift of the laser frequency is determined by both the fluctuations of the atmospheric pressure in the laboratory impacting the monolithic resonator and by the optical path instability.

Stability of liquid crystal micro-droplets based optical microresonators

ABSTRACTLiquid crystal (LC) based tuneable optical microresonators are potential for being used as crucial components in photonic devices. In this article, we report experimental studies on LC micro-droplets dispersed in several dispersing media. We find that the size of the micro-droplets formed in a low refractive index and optically transparent perfluoropolymer are most stable with time than commonly used dispersing media. Using a negative dielectric anisotropy nematic liquid crystal, we show that the whispering gallery mode optical resonance properties such as the quality factor and the free spectral range of stable micro-droplets are independent of the strength of the applied electric field. The optical resonance properties under applied field are significantly different than that of the liquid crystals with positive dielectric anisotropy and are explained based on the elastic deformation of the micro-droplets.

Highly efficient coupling of crystalline microresonators to integrated photonic waveguides.

Crystalline optical whispering gallery mode resonators made from alkaline earth fluorides can achieve exceptionally large optical finesse, and are used in a variety of applications, from frequency stabilization and narrow linewidth lasers, to low-noise microwave generation or soliton Kerr frequency combs. Here we demonstrate an efficient coupling method to resonators of these materials, which employs photonic integrated waveguides on a chip based on silicon nitride. By converting a mode from silicon nitride to a free-hanging silica waveguide on a silicon chip, coupling to a crystalline resonator is achieved with a high extinction, while preserving a quality factor exceeding 200million. This compact, heterogeneous integration of ultra-high Q-factor crystalline resonators with photonic waveguides provides a proof of concept for wafer scale integration and robust, compact packaging for a wide range of applications.

Calcium fluoride whispering gallery mode optical resonator with reduced thermal sensitivity

We demonstrate a crystalline CaF2 resonator with thermal sensitivity of the optical modes approaching zero. The resonator is made by laminating a calcium fluoride layer forming an optical monolithic cavity with ceramic compensation layers. The ceramics is characterized with negative thermal expansion coefficient achievable in a certain temperature range. The thermally compensated resonator has a potential application for laser frequency stabilization.

Label-Free Biological and Chemical Sensing Using Whispering Gallery Mode Optical Resonators: Past, Present, and Future.

Sensitive and rapid label-free biological and chemical sensors are needed for a wide variety of applications including early disease diagnosis and prognosis, the monitoring of food and water quality, as well as the detection of bacteria and viruses for public health concerns and chemical threat sensing. Whispering gallery mode optical resonator based sensing is a rapidly developing field due to the high sensitivity and speed of these devices as well as their label-free nature. Here, we describe the history of whispering gallery mode optical resonator sensors, the principles behind detection, the latest developments in the fields of biological and chemical sensing, current challenges toward widespread adoption of these devices, and an outlook for the future. In addition, we evaluate the performance capabilities of these sensors across three key parameters: sensitivity, selectivity, and speed.

Tunable Autler–Townes Splitting Observation in Coupled Whispering Gallery Mode Resonators

In this paper, we experimentally demonstrated an Autler–Townes splitting (ATS) effect in a cascaded optical whispering gallery mode resonator system. Although electromagnetically induced transparency (EIT) has been investigated as an interesting Fano resonance in the recent decades, the critical resonance matched condition of a high-quality factor resonator and a low-quality factor resonator makes EIT difficult to realize and hardly tunable. In contrast, ATS requires relatively simple system configurations, and the effect can be preserved over a large tuning range. In our approach, a theoretical model based on coupled mode theory is developed, and the methodology of creating and controlling the ATS modes is demonstrated. In our experiment, the optical resonators have a loaded quality factor of about 3 × 105 and cascaded in a strong coupling condition. For the experimental demonstration, a thermo-optical mechanism is utilized to control the splitting modes in our system. The asymmetric shape of a splitting resonances shift has a perfect fit with the theoretical calculations and numerical modeling results. Utilizing the precise tenability of splitting modes, the ATS has an extra degree of freedom to perform resonance tuning, which has become an ideal method to overcome the current detection limit.