We show that whispering gallery modes on the surface of optical fibers can be reflected from the optical fiber facet. Due to reflection, a locking effect occurs near the optical fiber facet for the whispering gallery modes with an axial component of the wave vector. We obtained an analytical expression for determining the resonant wavelengths. We also experimentally measured the phase velocity of the optical pulse propagating in whispering gallery modes and reflecting from the fiber facet.
Cylindrical microresonators based on the surface of optical fibers (SNAP structures) appear to be a promising platform for a variety of photonic devices. It turned out that the manufacturing accuracy of standard telecommunication optical fibers and the smoothness of their surface may be high enough to excite high-quality whispering gallery (WGM) modes in their cladding. Here we consider the question of the quality of resonances obtained on the surface of optical fibers, and the possibility of using them to create high-finesse optical filters. We used standard telecommunication fiber SMF-28 with a silica cladding diameter of 125 microns as samples to excite whispering gallery modes. We found that annealing with fire allows to obtain quality factors up to Q∼10<sup>7</sup>. Corresponding decay time in the microcavity was measured to be τ∼15 ns. We also discussed different schemes of optical filters that may be based of cylindrical microresonators.
The 16-channel multi-longitudinal mode fiber laser sensor array is investigated experimentally by the wavelength/frequency division multiplexing technique. In the proposed sensing system, a 4×4 sensor array is established by a few different coupling-ratio couplers, and four different fiber Bragg gratings (FBG) with different center wavelengths are used as four different sensing units. In each sensor unit, four parallel fiber laser sensors have the same operating wavelengths FBGs, but their effective laser cavity lengths slightly different from each other. Every cavity is formed by a fiber Bragg grating (FBG) serving as one reflection mirror, a piece of erbium-doped fiber (EDF) acting as the gain medium, and a Faraday rotator mirror (FRM) serving as other reflection mirror. When the pump power is higher than threshold value, the 16-channel fiber laser sensor array is stimulated stably. The frequency of the beat signal of the fiber laser sensor with different cavity lengths is used to realize frequency division multiplexing, and the wavelength division multiplexing is realized according to the operation wavelength of the fiber laser. The beat frequency signals are generated on a photodetector(PD), and monitored by a frequency spectrum analyzer(FSA). By tracking the shift of the beat frequency, all of the 16 laser sensors can be demodulated and real-time discriminated. The result of the experiment shows that different channels can be demodulated independently. The applications of the sensor array for strain and temperature measurements are also investigated. The strain or temperature information can be extracted from the change of the beat frequency signals according to the wavelength-frequency division multiplexing and the beat signal demodulation. The proposed hybrid multiplexing system can greatly reduce the weight, volume, and cost of the fiber laser sensors system while increasing the amount of the sensors multiplexable, which making it very competitive in some applications fields requiring large scale arrays such as space vehicles, marine infrastructure systems and constructional engineering.