Abstract
There is a deficiency of low-loss microstructured waveguides that can be fabricated with a single laser-pass to minimize stress build-up, which can enable enhanced functionality and higher compactness for integrated optical devices. We demonstrate, for the first time, a series of multi-ring claddings each with a pair of cores in BK7 glass. Each waveguide was fabricated using only a single laser-pass at 1 MHz pulse repetition rate, 5 mm/s translation speed, 250 fs pulse width, over a set of pulse energies. We obtained the lowest-reported propagation loss of 0.062 dB/cm, measured at 1155 nm wavelength from the waveguide written with 340 nJ pulse energy. The maximum observed numerical aperture is 0.020, measured at 1155 nm wavelength from the waveguide written with 620 nJ pulse energy. Such waveguides could be incorporated in integrated Raman laser platforms for biomedical applications.
Highlights
Of particular interest are borosilicate glasses, such as the borosilicate-crown glass from Schott (N-BK7, refractive index is 1.5055 at 1155 nm wavelength10), which exhibit high transmission in the visible and near-infrared, few defects, low density, high chemical-stability, and low processing-cost
The images are oriented such that the writing beam is incident from the top of each image. They exhibit varying sizes, structures and colors depending on the pulse energies used for writing, and different spectral transmission characteristics depending on the illumination aperture size
A common feature of all waveguides is a non-circular geometry with one side of the multiple ring layers featuring a dip/spot, featuring two cores aligned perpendicular to the glass interface seen from the longitudinal perspective
Summary
As pulse energy is increased, the following trends were observed: (a) the waveguides are generally larger; (b) the evolution from one-layer to four-layer RI modifications consisting of lower, higher, lower, higher RI relative to the pristine bulk, judging by the different intensities of the guided light; (c) a change in guidance region geometry, from a pair of spots to a spot and a ring/spot; and (d) a change in wavelength selectivity in the form of color shift from blue, green, yellow, red and back again, obtained using a small illumination aperture. To show that light can be launched into either the central core and/or the offset core adjacent to the ring-shaped guiding region, the 1155 nm laser diode was used in conjunction with lenses to probe the waveguide written with 900 nJ pulse energy. All the guiding regions are not strictly single-mode, while the central core approaches single mode guidance, judging by its circular symmetry
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