Order Whispering Gallery Modes Research Articles

On-chip non-uniformly spaced multi-channel microwave photonic signal processor based on an ultrahigh-Q multimode micro-disk resonator.

Multi-channel microwave photonic (MWP) signal processing can simultaneously perform different task operations on multiple signals carried by multiple wavelengths, which holds great potential for ultrafast signal processing and characterization in a wavelength-division-multiplexed (WDM) network. As emerging telecommunication services create more data, an elastic optical network, which has a flexible and non-uniform spectrum channel spacing, is an alternative architecture to meet the ever-increasing data transfer need. Here, for the multi-channel ultra-fast signal processing in the elastic optical network, we propose and demonstrate an on-chip non-uniformly spaced multi-channel microwave photonic signal processor based on an ultrahigh-Q multimode micro-disk resonator (MDR). In the proposed signal processor, an MDR supporting multiple different order whispering-gallery modes (WGMs) with an ultrahigh Q-factor is specifically designed. Benefiting from the large and different free spectral ranges (FSRs) provided by the different order WGMs, a non-uniformly spaced multi-channel microwave photonic signal processor is realized, and various processing functions are experimentally demonstrated including bandpass filtering with a narrow passband of 103 MHz, a rejection ratio of 22.3 dB and a frequency tuning range from 1 to 30 GHz, multiple frequency measurement with a frequency measurement range from 1 to 30 GHz, a frequency resolution better than 200 MHz and a measurement accuracy of 91.3 MHz, and phase shifting with a phase tuning range from -170°∼160°, an operational bandwidth of 26 GHz from 6 GHz to 32 GHz and a small power variation of 0.43 dB. Thanks to the coexistence of different order WGMs supported by the MDR, non-uniformly spaced multi-channel signal processing is enabled with the key advantages including a broad operation bandwidth, an ultra-narrow frequency selectivity, and a large phase tuning range with a small power variation. The proposed signal processor is promising to be widely used in future elastic optical networks with flexible spectrum grids.

Loss mechanisms of multi-frequency whispering gallery mode RF-MEMS resonators

This work investigates the dominant loss mechanisms of the multi-frequency whispering gallery mode (WGM) resonators theoretically and experimentally, which provides a deep understanding about the working principles of the devices and guidance on their high-end applications. For the first time, the distinct Akhiezer damping (AKE) effects on various order WGMs were comprehensively clarified. At atmosphere, the air damping, which is conventionally expected to be negligible in high-order bulk acoustic wave (BAW) modes, is nonnegligible for all the WGMs. In vacuum, substantial Q enhancements more than two times were obtained. The high-order WGMs benefit more f × Q improvement from the high vacuum than the low-order ones, achieving a promising f × Q product of 1.16 × 1013. What’s more, the Q dynamics over a wide temperature range were characterized, which matched well with the theoretical analysis. The AKE is comparable with anchor loss in low-order modes, while it becomes dominated in the high-order ones. Therefore, the high-order WGMs exhibit higher Q enhancements at cryogenic temperatures. An excellent f × Q product up to 5.91 × 1013 was implemented at 7 K.

Observation of ultraviolet whispering gallery modes in ZnMgO microrods

Abstract We report on the fabrication of ZnO and Zn0.9Mg0.1O microrods using nanoparticle assisted pulsed laser deposition (NAPLD). Micro-photoluminescence studies reveal that Zn0.9Mg0.1O microrods exhibited a blue shift in the ultraviolet (UV) emission relative to unintentionally doped ZnO that was attributed to the increase in bandgap due to Mg doping. High-resolution micro-photoluminescence clearly indicated the presence of whispering gallery modes (WGMs) for both ZnO and Zn0.9Mg0.1O. Transverse electric (TE) and transverse magnetic (TM) optical WGMs were identified using the existing plane-wave model. The observation of lower order WGM modes in Zn0.9Mg0.1O microrods may be of great interest for future UV WGM lasing.

Controllable and selective single-mode lasing in polymer microbottle resonator.

We report a single-mode dye-doped polymer microbottle resonator (MBR) laser. The selective single-mode lasing from different order whispering gallery modes is achieved by precisely controlling the axial and radial coupling position between a tapered nanofiber and an MBR, respectively. The side-mode suppression ratio is above 20 dB. By doping different fluorescence dyes into the MBR, single-mode lasers at various colors are demonstrated.

Introducing a radial absorbing boundary condition for the calculation of quality factor of whispering gallery resonators

ABSTRACTA one dimensional radial absorbing boundary condition (ABC) is introduced for the analysis of low order Whispering Gallery Mode (WGM) microresonators. Because the radiation of WGM is mostly in the form of radially outgoing waves, the employed ABC assumes that the absorbing material is anisotropic only in the radial direction. This formulation enables us to precisely calculate both the resonance frequency and quality factor of the WGMs. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1065–1068, 2016

Multi-mode technique for the determination of the biaxial Y2SiO5 permittivity tensor from 300 to 6 K

The Y2SiO5 (YSO) crystal is a dielectric material with biaxial anisotropy with known values of refractive index at optical frequencies. It is a well-known rare-earth (RE) host material for optical research and more recently has shown promising performance for quantum-engineered devices. In this paper, we report the first microwave characterization of the real permittivity tensor of a bulk YSO sample, as well as an investigation of the temperature dependence of the tensor components from 296 K down to 6 K. Estimated uncertainties were below 0.26%, limited by the precision of machining the cylindrical dielectric. Also, the electrical Q-factors of a few electromagnetic modes were recorded as a way to provide some information about the crystal losses over the temperature range. To solve the tensor components necessary for a biaxial crystal, we developed the multi-mode technique, which uses simultaneous measurement of low order whispering gallery modes. Knowledge of the permittivity tensor offers important data, essential for the design of technologies involving YSO, such as microwave coupling to electron and hyperfine transitions in RE doped samples at low temperatures.

Controlled excitation of higher radial order whispering gallery modes with metallic diffraction grating.

Metallic diffraction grating coupler is investigated for controlled excitation of whispering gallery modes (WGMs) of different radial orders. Based on effective mode index calculations and finite difference time domain method, it is found that higher radial order WGMs can be separated from the fundamental modes by sending them into the opposite propagation direction. By phase-matching designs, the metallic diffraction grating provides extra freedom to switch propagation directions, and is able to selectively enhance or suppress different radial-order WGMs. Such structure offers a simple and practical configuration for various WGM applications including liquid sensing, band pass filtering and fiber lasers.

Packaged chalcogenide microsphere resonator with high Q-factor

The fabrication and characterization of a packaged As2S3 microsphere resonator coupled to a tapered fiber using a low refractive index UV-curable polymer are reported. Embedding provides an efficient means to remove the highest order whispering gallery modes in the microsphere resonator, thus cleaning the resonator spectrum. At wavelengths near 1549.5 nm, high-Q modes up to 1.8 × 105 can be efficiently excited in a 110 μm diameter chalcogenide microsphere via evanescent coupling from a 2 μm diameter tapered silica fiber. The device photosensitivity, useful for tuning, is still present and useable after the packaging process.

Coupling single NV-centres to high-Q whispering gallery modes of a preselected frequency-matched microresonator

In this paper, we report the controlled coupling of fluorescence from a single NV-centre in a single nanodiamond to the high-Q modes of a preselected microsphere. Microspheres from an ensemble with a finite size distribution can be characterized precisely via white light Mie-scattering. The mode spectrum of individual spheres can be determined with high precision. A sphere with an appropriate spectrum can be selected, and a nanodiamond containing a single NV-centre can be coupled to it. The spectral position of the calculated lowest order whispering gallery modes are found to be in very good agreement with the experimentally observed resonances of the coupled fluorescence from the single NV-centre.

Mode coupling between first- and second-order whispering-gallery modes in coupled microdisks

The mode characteristics for two coupled microdisks are investigated by the finite-difference time-domain technique. In the two coupled micodisks, mode coupling between the same order whispering-gallery modes (WGMs) results in coupled WGMs with split mode wavelengths. The numerical results show that the split mode wavelengths of the coupled first- and second-order WGMs can have a crossing point in some cases, which can induce anticrossing mode coupling between them and greatly reduce the mode Q factor of the coupled first-order WGMs. The time variation of mode field pattern shows the transformation between the coupled first- and second-order WGMs.

Reduction of mode Q-factor due to coupling between fundamental and first order whispering-gallery modes in coupled microdisks

The mode characteristics for twin coupled microdisks are investigated by finite-difference time-domain technique. In the coupled microdisks, the same order whispering-gallery (WG) modes can form coupled modes with split mode wavelengths. We find that the coupled fundamental and first order WG modes can have anticrossing mode coupling as their wavelengths approach the same value in some case, which prevents the cross of the coupled mode wavelengths. The anticrossing mode coupling greatly reduces the coupled mode Q-factor, because the coupled mode field distribution transfers between the fundamental and the first order WG modes.