Background and Objective: During tissue ablation, laser light can be delivered with high precision in the transverse dimensions but final incision depth can be difficult to control. We monitor incision depth as it progresses, providing feedback to ensure that material removal occurs within a localized target volume, reducing the possibility of undesirable damage to tissues below the incision. Materials and Methods: Ex vivo cortical and cancellous bone was ablated using pulsed lasers with center wavelengths of 1,064 and 1,070 nm, while being imaged in real-time using inline coherent imaging (ICI) at rates of up to 300 kHz and axial resolution of $6 mm. With realtime feedback, laser exposure was terminated before perforating into natural inclusions of the cancellous bone and verified by brightfield microscopy of the crater crosssections accessed via side-polishing.Results: ICI provides direct information about incision penetration even in the presence of intense backscatter from the pulsed laser and plasma emissions. In this study, ICI is able to anticipate structures 176 AE 8 mm below the ablation front with signal intensity 9 AE 2 dB above the noise floor. As a result, the operator is able to terminate exposure of the laser sparing a 50 mm thick layer of bone between the bottom of the incision to a natural inclusion in the cancellous bone. Versatility of the ICI system was demonstrated over a wide range of light-tissue interactions from thermal regime to direct solid-plasma transition. Conclusions: ICI can be used as non-contact real-time feedback to monitor the depth of an incision created by laser ablation, especially in heterogeneous tissue where ablation rate is less predictable. Furthermore, ICI can image below the ablation front making it possible to stop laser exposure to limit unintentional damage to subsurface structures such as blood vessels or nervous tissue.

Optical coherence imaging can measure hole depth in real-time ͑Ͼ20 kHz͒ during laser drilling without being blinded by intense machining light or incoherent plasma emissions. Rapid measurement of etch rate and stochastic melt relaxation makes these images useful for process development and quality control in a variety of materials including metals, semiconductors, and dielectrics. The ability to image through the ablation crater in materials transparent to imaging light allows the guidance of blind hole cutting even with limited a priori knowledge of the sample. Significant improvement in hole depth accuracy with the application of manual feedback from this imaging has been previously demonstrated ͓P. J. L. Webster et al., Opt. Lett. 35, 646 ͑2010͔͒. However, the large quantity of raw data and computing overhead are obstacles for the application of coherent imaging as a truly automatic feedback mechanism. Additionally, the high performance components of coherent imaging systems designed for their traditional application in biological imaging are costly and may be unnecessary for materials processing. In this work, we present a coherent imaging system design that costs less than a fifth of comparable commercial products. We also demonstrate streamlined image processing suited for automated feedback that increases processing speed by two orders of magnitude.

We observe sample morphology changes in real time (24 kHz) during and between percussion drilling pulses by integrating a low-coherence microscope into a laser micromachining platform. Nonuniform cut speed and sidewall evolution in stainless steel are observed to strongly depend on assist gas. Interpulse morphology relaxation such as hole refill is directly imaged, showing dramatic differences in the material removal process dependent on pulse duration/peak power (micros/0.1 kW, ps/20 MW) and material (steel, lead zirconate titanate PZT). Blind hole depth precision is improved by over 1 order of magnitude using in situ feedback from the imaging system.

The stochastic effects of assist gas in QCW and pulsed laser machining (percussion drilling) in steel are measured with a novel in situ high speed low coherence imaging system. Real-time imaging is delivered coaxially with machining energy and assist gas revealing relaxation and melt flow dynamics over microsecond timescales and millimeter length scales with ~10 micrometer resolution. Direct measurement of cut rate and repeatability avoids post cut analysis and iterative process development. Feedback from the imaging system can be used to overcome variations in relaxation and guides blind hole cutting.

Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX Though new affordable high power laser technologies make possible many processing applications in science and industry, depth control remains a serious technical challenge. Here we show that inline coherent imaging, with line rates up to 312 kHz and microsecond-duration capture times, is capable of directly measuring laser penetration depth in a process as violent as kWclass keyhole welding. We exploit ICI's high speed, high dynamic range and robustness to interference from other optical sources to achieve fully automatic, adaptive control of laser welding as well as ablation, achieving micron-scale sculpting in vastly different heterogeneous biological materials.

We demonstrate real-time depth profiling of ultrafast micromachining of stainless steel at scan rates of 46 kHz. The broad bandwidth and high power of the light source allows for simultaneous machining and coaxial Fourier-domain interferometric imaging of the ablation surface with depth resolutions of 6 mum. Since the same light is used to machine as to probe, spatial and temporal synchronization are automatic.

The utility of a new laser interferometric technique, inline coherent imaging, for real time keyhole depth measurement during laser welding is demonstrated on five important engineering alloys. The keyhole depth was measured at 200 kHz with a spatial resolution of 22 mm using a probe beam, which enters the keyhole coaxially with the process beam. Keyhole fluctuations limited average weld depth determination to a resolution on the order of 100 mm. Real time keyhole depth data are compared with the weld depths measured from the corresponding metallographic crosssections. With the exception of an aluminium alloy, the technique accurately measured the average weld depth with differences of less than 5%. The keyhole depth growth rates at the start of welding are measured and compare well with order of magnitude calculations. The method described here is recommended for the real time measurement and control of keyhole depth in at least five different alloys.

We achieve high aspect-ratio laser ablation of silicon with a strong nonlinear dependence on pulse duration while using a power density 10(6) times less than the threshold for typical multiphoton-mediated ablation. This is especially counter-intuitive as silicon is nominally transparent to the modulated continuous wave Yb:fiber laser used in the experiments. We perform time-domain finite-element simulations of thermal dynamics to investigate thermo-optical coupling and link the observed machining to an intensity-thresholded runaway thermo-optically nonlinear process. This effect, cascaded absorption, is qualitatively different from ablation observed using nanosecond-duration pulses and is general enough to potentially facilitate high-quality, high aspect-ratio, and economical processing of many materials.