A real-time hole depth diagnostic based on coherent imaging with plasma amendment during femtosecondlaser hole-drilling

TL;DR

This paper presents a real-time coherent imaging diagnostic system for measuring hole depth during femtosecond laser drilling, incorporating plasma density evaluation to improve accuracy in deep hole measurements.

Contribution

It introduces a novel in-process optical diagnostic method that accounts for plasma effects, enhancing depth measurement precision during femtosecond laser drilling.

Findings

Achieved 12 μm depth resolution using super-luminescent diode.

Implemented interference-based detection for real-time depth measurement.

Demonstrated plasma density's impact on optical path length and measurement accuracy.

Abstract

An in-process coherent imaging diagnostic has been developed to real-time measure the hole depth during air-film hole drilling by a femtosecond laser. A super-luminescent diode with a wavelength of 830~13 nm is chosen as the coherent light source which determines a depth resolution of 12 {\mu}m. The drilled hole is coupled as a part of the sample arm and the depth variation can be extracted from the length variation of the optical path. Interference is realized in the detection part and a code has been written to discriminate the interference fringes. Density of plasma in the hole is diagnosed to evaluate its amendment to the optical path length and the depth measurement error induced by plasma is non-ignorable when drilling deep holes.