KEYWORDS: Laser welding, Coherence imaging, Laser processing, Image processing, Temperature metrology, Control systems, Manufacturing, Black bodies, In situ metrology, Backscatter
Directly measuring morphology and temperature changes during laser processing (such as in keyhole welding and selective laser melting) can help us to understand, optimize, and control on-the-fly the manufacturing process. Even with such great potential, the technical requirements for such an in situ metrology are high due to the fast nature of the highly localized dynamics, all the while in the presence of bright backscatter and blackbody radiation, and possible obstructions such as molten ejecta and plumes. We have demonstrated that by exploiting coherent imaging through a single-mode fiber inline with the processing lens, we can image morphology at line rates up to 312 kHz, with sufficient robustness to achieve closed loop control of the manufacturing process. Applied to metal additive manufacturing, inline coherent imaging can directly measure powder layer thickness and uniformity, and formed track roughness including the onset of balling. Inline coherent imaging measures morphology dynamics but that is only part of the story. Temperature is also key to final part quality. Standard thermal imaging exploits blackbody radiation but are plagued by the highly variable emissivity of the region of interest, making quantitative measurement challenging. We were able to exploit the same apparatus used for coherent imaging to collect surface temperature profiles. Since we spectrally resolve a wide signature, we have overcome the emissivity problem to measure absolute temperature on the micron scale during laser processing.
KEYWORDS: Optical amplifiers, Crystals, Absorption, Near infrared, Nonlinear crystals, Thermal effects, High power fiber amplifiers, Finite element methods, Solids, Thermography
Laser amplifiers at high repetition rate are critical for many applications in the chemical, physical and biological sciences. A variety of laser sources from XUV to THz can be derived from Ti:Sapphire laser amplifiers at moderate to low conversion efficiencies. High repetition rate applications require NIR and IR sources based on optical parametric chirped pulse amplifier (OPCPA) to drive these sources, offsetting the conversion efficiency losses with an even higher average power beam to drive the frequency conversion processes. We use these technologies at next generation free-electron laser (FEL) facilities. The Linac Coherent Light Source (LCLS), LCLS-II upgrade, will provide sub-femtosecond and femtosecond X-ray pulses at 100 kHz, and later up to 1 MHz repetition rate. The higher repetition rate benefits pump-probe experiments for weakly scattering samples and serves a variety of experiments which require attenuation to avoid perturbation and damage of the sample by the X-ray probe. A millijoule R&D laser amplifier was developed to test experimental conditions for optical laser beam delivery at LCLS-II. The laser can be operated at two distinct wavelength ranges. At 800 nm center wavelength we use the second harmonic of an Yb:YAG amplifier system to pump an OPCPA in a BBO crystal. A second tunable version operates between 1.45-2 m center wavelength using the fundamental Yb:YAG beam to pump a KTA OPCPA with average output powers in excess of 100 W. Currently the amplifier is operated 24 hours, 7 days a week. It is based on a simple and robust design, which ensures long term stability with good output beam quality.
In laser welding and additive manufacturing, melt pool behavior is directly related to final part quality as instability can lead to pore formation and ejected spatter. In transition and keyhole mode welding of metals, the dynamic balance of forces working to open and close the resulting vapor cavity gives rise to melt pool surface oscillations at frequencies on the order of kHz. Existing process monitoring techniques, such as high-speed video, have been extremely useful to image melt pool boundaries, but are limited in their ability to quantifiably track oscillation amplitudes and monitor high-aspect ratio features.
We exploit inline coherent imaging (a through-the-lens technique based on low coherence interferometry) to directly measure melt oscillations through the transition zone (from conduction to keyhole welding). An Yb:fiber laser is used to perform 10ms spot welds on 316 stainless steel with varying laser powers (120-630W). Morphology is measured in situ at a rate of 170kHz. Oscillations are observed starting at the onset of the transition zone (irradiance 0.63MW/cm²) with a frequency of 10.0 ± 0.3kHz. At higher power (keyhole mode, irradiance 1.8MW/cm²), the bottom of the keyhole oscillates with a frequency of 2.5 ± 0.5kHz around a maximum depth of 1mm. This trend agrees with analytic modelling dependent on melt surface tension, density and diameter, as well as complementary experiments that track total laser absorptance using an integrating sphere.
Precision depth control of bone resection is necessary for safe surgical procedures in the spine. In this paper, we compare the control and quality of cutting bovine tail bone, as an ex vivo model of laminectomy and bony resection simulating spinal surgery, planned with micro-CT data and executed using two approaches: (a) mechanical milling guided by optical topographical imaging (OTI) and (b) optical milling using closed-loop inline coherent imaging (ICI) to monitor and control the incision depth of a high-power 1070 nm fiber laser in situ. OTI provides the in situ topology of the 2-dimensional surface of the bone orientation in the mechanical mill which is registered with the treatment plan derived from the micro-CT data. The coregistration allows the plan to be programmed into the mill which is then used as a benchmark of current surgical techniques. For laser cutting, 3D optical land marking with coaxial camera vision and the ICI system is used to coregister the treatment plan. The unstable, carbonization-mediated ablation behaviour of 1070 nm light and the unknown initial geometry of bone leads to unpredictable ablation which substantially limits the depth accuracy of open-loop cutting. However, even with such a non-ideal cutting laser, we demonstrate that ICI provides in situ high-speed feedback that automatically and accurately limits the laser’s cut depth to effectively create an all-optical analogue to the mechanical mill.
In applications ranging from noncontact microsurgery to semiconductor blind hole drilling, precise depth control of
laser processing is essential. Even a priori characterization cannot compensate for material heterogeneity and
stochasticity inherent to the material modification process. We image along the machining beam axis at high speeds
(up to 312 kHz) to provide real-time feedback, even in high aspect ratio holes. The in situ metrology is based on
broadband coherent imaging (similar to the medical imaging modality optical coherence tomography) and is
practical for a wide-range of light sources and machining processes (e.g., thermal cutting using a quasi-continuous
wave fiber laser, or nonlinear ablation achieved with ultrafast pulses). Coherent imaging has high dynamic range (>
60 dB) and strongly rejects incoherent signals allowing weak features to be observed in the presence of intense
machining light and bright plasmas. High axial resolution (~5 μm) is achieved with broadband imaging light but
center wavelength can be chosen appropriate to the application. Infrared (wavelength: 1320±35 nm) allow
simultaneous monitoring of both surface and subsurface interfaces in nonabsorbing materials like tissue and
semiconductors. Silicon based detector technology can be used with near infrared imaging light (804 ± 30 nm)
enabling high speed acquisition (>300 kHz) or low cost implementation (total imaging system <10k$). Machining
with an appropriate broadband ultrafast laser allows machining and imaging to be done with the same light source.
Ultrafast technology also enables nonlinear optical processing of the imaging light, opening the door to improved
imaging modalities.
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 incision's 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.
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.
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.
Optically excited single-walled carbon nanotube exciton dynamics are predicted using a stochastic model including
linear and nonlinear decay pathways. Results are fitted to the photoluminescence behavior of a single 4-μm long airsuspended
(9,8) single-walled carbon nanotube. Hard photoluminescence saturation is observed even with only a few
excitons are present in the single-walled carbon nanotube. The absorption coefficient is estimated to be 0.01 - 0.04, and
photoluminescence quantum efficiency is 13%. Femtosecond excitation correlation spectroscopy dynamics are consistent
with linear and nonlinear (exciton-exciton annihilation) lifetimes of 80 ps and 0.8 ps respectively.
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