Whispering Gallery Mode Resonances in Thermally Poled Borosilicate Glass Hetero-Fibers

Whispering gallery mode (WGM) resonances in radially, thermally poled glass hetero-fibers, are investigated for the first time. Upon radially oriented thermal poling, both TE and TM polarized responses undergoing a spectral “cleaning” process, namely higher-order radial modes are being suppressed, or even fully annihilated, compared to the spectral behavior of the pristine fibers. Second-harmonic generation (SHG) microscopy, Energy Dispersive X-Ray Spectroscopy (EDX) and micro-Raman (μ-Raman) measurements were conducted in order to reveal the spatial profile of the structural and optical changes introduced to the thermally poled hetero-fibers, and correlate them with the WGM spectral measurements. The specific selective, mode cleaning behavior is attributed to refractive index changes generated in proximity to the fiber's circumference, which are associated with radially-symmetric ionic re-distribution. The thermally poled hetero-fiber WGM cavities have been rigorously simulated using the finite element method, the calculated modal eigenstates and transmission spectra, further confirm the specific mode selection mechanism, introduced by the azimuthally symmetric thermal, poling process.