Paul J. L. Webster

In situ 24 kHz coherent imaging of morphology change in laser percussion drilling

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.

High speed in situ depth profiling of ultrafast micromachining

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.

Real-time guidance of thermal and ultrashort pulsed laser ablation in hard tissue using inline coherent imaging

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.

Automatic real-time guidance of laser machining with inline coherent imaging

Optical coherence imaging can measure hole depth in real-time (>20 kHz) 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 (2010)]. 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 traditio...

Real time monitoring of laser beam welding keyhole depth by laser interferometry

Abstract 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 μm 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 μm. Real time keyhole depth data are compared with the weld depths measured from the corresponding metallographic cross-sections. 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.

Coaxial real-time metrology and gas assisted laser micromachining: process development, stochastic behavior, and feedback control

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.

Time-gated Fourier-domain optical coherence tomography

A novel optical coherence tomography (OCT) system is presented that combines Fourier-domain OCT with incoherent nonlinear time gating. By processing backscattered light in the optical domain, the user can select a restricted depth field of view for improved contrast and acquisition speed. This technique has the additional advantage that imaging is done in the infrared (approximately 1280 nm) but is detected in the visible(approximately 504 nm).

High-quality percussion drilling of silicon with a CW fiber laser

It has been shown that 30 ns FWHM duration pulses from a MOPA fiber laser (wavelength: 1064 nm) cleanly micromachines silicon with little cracking or heat-affected zone1. In this paper, we show that similar results can be achieved using a 1070 nm quasi-continuous wave laser pulsed with a 6.6 μs duration (average power: 2.8 W) in combination with coaxially delivered nitrogen assist gas. The holes are cut at a 5 kHz repetition rate with a resulting diameter on the order of 15 μm and an etch rate of up to 18 μm/pulse. Hole size is increased for longer pulses and the heat-affected zone broadens to greater than 25 μm with no assist gas. By combining low coherence microscopy with machining, we depth image the machining front and obtain in situ images during and after the drilling process showing rich cut dynamics in real time.

Guidance of hard tissue ablation by forward-viewing optical coherence tomography

A key issue in laser surgery is the inability for the human operator to stop the laser irradiation in time while cutting/ablating delicate tissue layers. In the present work, we forward-image through the laser machining front in complex biological tissue (dense bovine bone) to monitor the incisions approach to subsurface interfaces in real-time (47-312 kHz line rate). Feedback from imaging is used to stop the drilling process within 150 micron of a targeted interface. This is accomplished by combining the high temporal and spatial resolution of infrared optical coherence tomography (OCT) with a robust, turn-key, high brightness fiber laser. The high sensitivity of the imaging system (~100 dB) permit imaging through the rapidly changing beam path even with the additional scattering caused by the thermal cutting process. In spectral-domain OCT, the imaging acquisition period is easily locked to the machining laser exposure. Though motion-induced artifacts reduce interface contrast, they do not introduce incorrect depth measurements as found in other OCT variants. Standard tomography imaging of the tissue (B-scans) is also recorded in situ before and after laser processing to highlight morphology changes.

Real-time depth monitoring and control of laser machining through scanning beam delivery system

Scanning optics enable many laser applications in manufacturing because their low inertia allows rapid movement of the process beam across the sample. We describe our method of inline coherent imaging for real-time (up to 230 kHz) micron-scale (7–8 µm axial resolution) tracking and control of laser machining depth through a scanning galvo-telecentric beam delivery system. For 1 cm trench etching in stainless steel, we collect high speed intrapulse and interpulse morphology which is useful for further understanding underlying mechanisms or comparison with numerical models. We also collect overall sweep-to-sweep depth penetration which can be used for feedback depth control. For trench etching in silicon, we show the relationship of etch rate with average power and scan speed by computer processing of depth information without destructive sample post-processing. We also achieve three-dimensional infrared continuous wave (modulated) laser machining of a 3.96 × 3.96 × 0.5 mm3 (length × width × maximum depth) pattern on steel with depth feedback. To the best of our knowledge, this is the first successful demonstration of direct real-time depth monitoring and control of laser machining with scanning optics.