Void formation in bulk silica glass by fiber fuse induced with a focused laser beam

Glass is transparent to infrared beams at room temperature, but absorbs these beams at high temperatures. When the optical glass fiber is heated, the glass absorbs the beam and emits a bright light, and the emission zone propagates toward the beam source. This phenomenon is called fiber fuse, with some voids observed in the track after fiber fuse. The authors have proposed a method to induce fiber fuse deliberately inside bulk glass and have discovered that void formation might be applied for channel fabrication in glass. However, void formation has not been adequately elucidated. In this study, we investigated the void formation induced by fiber fuse with a focused beam. We conducted in situ observations of the propagating emission zone and microscopic observations of samples fabricated at various illumination times. The void in the high-temperature area enlarges as it progresses, following emission zone propagation. Moreover, void splitting creates vacancies in the track near the beam focus. The track voids were bullet-shaped, but when the beam was tightly focused, needle-shaped voids formed. Preventing void movement because of a reduced beam diameter would cause the difference. Also, we demonstrated control of the track-void formation by changing the beam focus and power, with a void length of 1350 µm and width of 30 µm. These findings of fiber fuse induced by the focused beam are common to fiber fuse in the optical fiber.