  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.
