Abstract
The effects of substrate temperature on laser focal position and tear-drop morphology in laser internal modification of glass are investigated. A model is derived to predict the shift of the tear-drop at high substrate temperature. Femtosecond laser pulses are scanned inside borosilicate glass at room temperature, 150 °C, and 200 °C using a pulse energy of 4.5–18 µJ, a scanning speed of 5–20 mm/s, and a distance between lens and glass of 9.56–10.76 mm. Temperature effects are characterized by defining a height (width) gain ratio as the ratio between the tear-drop height (width) measured at high temperature to that measured at room temperature. Thermal expansion is simulated using a profile temperature acquired by a thermal camera and image processing. Results show that substrate temperature has a significant effect on self-focusing, and modifications at 200 °C show a relaxed discoloration compared to 150 °C and room temperature. Analytical predictions match the measurements of focal position in the distance of 9.56–9.96 mm at 200 °C where self-focusing is not significant while underestimating the measurements for the distance of 10.76 mm and 10.36 mm by 30–50 µm. At 200 °C, the tear-drop's gain ratio is increased when the pulse energy is increased in the range of 4.5–18 µJ. Within this pulse energy range and at 200 °C, the maximum width gain is 10–100% higher compared to the maximum height gain.
