Automatic Alignment Method for Controlled Free-Space Excitation of Whispering-Gallery Resonances

Abstract

Whispering-gallery mode microresonators have gained wide popularity as experimental platforms for different applications, ranging from biosensing to nonlinear optics. Typically, the resonant modes of dielectric microresonators are stimulated via evanescent wave coupling, facilitated using tapered optical fibers or coupling prisms. However, this method poses serious shortcomings due to fabrication and access-related limitations, which could be elegantly overcome by implementing a free-space coupling approach; although additional alignment procedures are needed in this case. To address this issue, we have developed a new algorithm to excite the microresonator automatically. Here, we show the working mechanism and the preliminary results of our experimental method applied to a home-made silica microsphere, using a visible laser beam with a spatial light modulator and a software control.

Keywords: Mie scattering; automatic alignment; free-space coupling; spatial light modulator; whispering-gallery mode resonator.

Figure 1

Experimental setup. A grating-like phase mask is imaged onto the SLM panel to tilt the laser beam by acting on the reflection angle r (shown in turquoise). Light scattered from the resonator is imaged using a CCD camera through a 1:1 telescope. The telescope, and a second photodiode (PD) for backscattering detection, are omitted for sake of clarity.

Figure 2

Images recorded with the CCD camera when a WGM is excited (a) or not (b). On the corresponding lower panels (c,d), we report a 1 D integration of the total number of counts. We ruled out via software the contribution of the two stray light spots, visible in both upper panels, so as to obtain a robust selection criterion to distinguish among the system’s available alignment conditions.

Figure 3

Microresonator’s self-aligned modes, normalized for ease of visualization. In each plot, the tilt angle and the number of iterations are also reported. For N = 100 iterations of the algorithm, the system tends to align on the mode in the lower panel, while for lower N values, the system finds local maxima of the scattered light that correspond to lower-Q whispering-gallery modes (from the top, panels 1 and 2).

Figure 4

Microresonator’s self-aligned modes in the presence of dust particles (normalized). Split modes are visible in the spectra, revealing the presence of scatterers on the surface of the WGMR.