Long Period Grating-Based Fiber Coupling to WGM Microresonators

Long-period D-fiber grating based robust and efficient bidirectional coupler for whispering gallery mode excitation

A robust and efficient bidirectional coupler for whispering gallery mode (WGM) excitation based on a long-period grating (LPG) inscribed in D-fiber is theoretically and experimentally demonstrated. The LPG coupling the fundamental core mode to the forward propagating cladding modes according to the phase-matching condition not only enhances the evanescent field of the fiber but also selectively excites the WGM in a wavelength band of interest. Experimental results show that a maximum resonance contrast as high as 10.5 dB and a quality factor (Q-factor) on the order of 10⁴ can be achieved in an LPG coupled spherical silica WGM resonator with a diameter of 242 µm, where the LPG with a pitch of 680 µm is fabricated by arc-discharging in a side-polished D-fiber with a maximum polishing depth of 56 µm. In addition to high robustness and efficiency, such an LPG-based WGM coupler also demonstrates bidirectionality, i.e., it is independent of the injection direction of the input light, which provides a reliable and flexible fiber coupler for the WGM resonator based practical applications.

Fiber Optic Refractive Index Sensors Based on a Ball Resonator and Optical Backscatter Interrogation

In this work, we introduced fabrication and interrogation of simple and highly sensitive fiber-optic refractive index (RI) sensors based on ball resonators built on the tip of single-mode fibers. The probes have been fabricated through a CO₂ fiber splicer, with a fast (~600 s) and repeatable method. The ball resonator acted as a weak interferometer with a return loss below −50 dB and was interrogated with an optical backscatter reflectometer measuring the reflection spectrum. The ball resonators behaved as weak interferometers with a shallow fringe and a spectrum that appeared close to a random signal, and RI sensitivity could be measured either through wavelength shift or amplitude change. In this work, we reported four samples having sensitivity ranges 48.9–403.3 nm/RIU and 256.0–566.2 dB/RIU (RIU = refractive index unit). Ball resonators appeared as a sensitive and robust platform for RI sensing in liquid and can be further functionalized for biosensing.

Probing a chiral drug using long period fiber gratings

The electromagnetic field theory for a step-index fiber geometry is developed to sense a surrounding chiral drug via long-period fiber gratings (LPFGs). This theory employs Debye potentials and electromagnetic fields for cladding modes in the LPFGs by introducing constitutive relations for a chiral drug. The fields in the chiral drug are transformed and decomposed into right- and left-hand circularly polarized components to account for the magnetoelectric coupling due to the chirality. The solving process for complex propagation constants is given. Numerical results show that responses of the LPFGs to refractive index and chirality changes are different. The two minimum transmissions of a coated LPFG are very sensitive to the variation of the complex chirality. On the other hand, the two resonance wavelengths keep invariant as real and imaginary parts of the comparatively small chirality change. This work enriches the electromagnetic field theory for better design of LPFGs against the highly sensitive chirality detection.

Analysis of Kerr Noise in Angular-Rate Sensing Based on Mode Splitting in a Whispering-Gallery-Mode Microresonator

Whispering-gallery-mode (WGM) microresonators have shown their potential in high-precision gyroscopes because of their small volume and high-quality factors. However, Kerr noise can always be the limit of accuracy. Angular-rate sensing based on mode splitting treats backscattering as a measured signal, which can induce mode splitting, while it is considered as a main source of noise in conventional resonator optical gyroscopes. Meanwhile, mode splitting also provides superior noise suppression owing to its self-reference scheme. Kerr noise in this scheme has not been defined and solved yet. Here, the mechanism of the Kerr noise in the measurement is analyzed and the mathematical expressions are derived, indicating the relationship between the Kerr noise and the output of the system. The influence caused by Kerr noise on the output is simulated and discussed. Simulations show that the deviation of the splitting caused by Kerr noise is 1.913 × 10⁻⁵ Hz at an angular rate of 5 × 10⁶ °/s and the corresponding deviation of the angular rate is 9.26 × 10⁻⁹ °/s. It has been proven that angular-rate sensing based on mode splitting offers good suppression of Kerr noise.

Editorial for the Special Issue on Glassy Materials Based Microdevices

Glassy materials, i.e., glasses and most polymers, play a very important role in microtechnologies and photonics[...].