Laser powder bed fusion is a metal 3D printing technology that involves melting and 8 solidifying a metal powder using a process laser or electron beam. The interaction of metal 9 vapors with the melt pool is known to cause manufacturing defects. Here we use absorption 10 spectroscopy of laser light reflected from the liquid metal surface to measure atomic Ti vapor 11 during processing of Ti-6Al-4V inside the vapor cavity forming in the melt pool. The 12 absorption line strength and shape were found to vary significantly with processing parameters . 13 In addition, l aser-induced fluorescence (LIF) was observed, provid ing further opportunities for 14 in situ diagnostics. 15 16 1. Introduction 17 Powder bed fusion (PBF) is a 3D printing process in which parts are built layer by layer by 18 scanning a focused laser (L -PBF) or electron beam (EB -PBF) over a bed of metal powder. The 19 beam selectively melts the powder fusing it to the previous layers before a new layer of powder 20 is applied and the process is repeated. During melting, the metal partially vaporizes, forming a 21 vapor plume that creates a depression or vapor cavity in the liquid metal as the gas pressure 22 exerts force on the melt pool surface. The vapor cavity, which is steep and narrow under optimal 23 processing conditions in L -PBF, is also referred to as a keyhole which oscillates at a stable 24 frequency due to the complex interplay of gas pressure, metal absorptivity, capillary forces, and 25 liquid metal surface tension [1]. Perturbations to this delicate balance cause instabilities that 26 lead to defects significantly affecting the quality of the parts produced, a concern that has gained 27 urgency as additive manufacturing enters safety -critical industries such as aerospace and 28 medical devices [2,3] . 29 Research has linked melt pool instabilities to a range of issues, including part roughness, 30 porosity, cracks, and mechanical performance [3–5]. Consequently, real -time in situ 31 monitoring of melt pool dynamics is crucial for quality control and online parameter 32 adjustment [2,5] . Prior studies have suggested that only a multimodal sensing approach can 33 enhance the fidelity of process monitoring while facilitating a deeper understanding of defect - 34 forming mechanisms [5–7]. A key factor contributing to melt pool instability is the dynamic 35 interaction between the gas in the vapor cavity and the melt pool surface. However, traditional 36 methods for predicting print quality, such as acoustic monitoring and measuring thermal 37 emissions from the melt pool and vapor plume, provide only indirect insights into the gas 38 conditions [4,5] . 39 Here we present a novel approach for process diagnostics in PBF based on laser absorption 40 spectroscopy (LAS) within the vapor cavity . This proof -of-principle study investigates the 41 relationship between the absorption signal of atomic Ti and the processing parameters during 42 the processing of a Ti -6Al-4V sample. A sensor derived from this technique could be integrated 43 into existing commercial L-PBF machines for online process monitoring. 44 45 OP I ICA PUBLISHING GROUP 2. Backscatter absorption spectroscopy 46 2.1 Background 47 The spectral line strength and shape of the elements in the vapor plume depend on the intrinsic 48 and extrinsic conditions of the plume such as its composition, pressure, temperature, and flow 49 velocity. In the past, spectral measurements of the plume have been reported by optical 50 emission spectroscopy (OES), in which the light emitted from the metal vapor plume and the 51 melt pool is collected and analyzed in spectrometers [8,9] or using multiple detectors equipped 52 with spectral filters [10,11] . However, while the OES signal is indicative of plume composition 53 and temperature, it provides only indirect insight into parameters such as pressure and gas 54 velocity. This information is encoded in the shape and precise spectral position of the atomic 55 lines, which typically cannot be resolved by OES. 56 Recently, absorption spectroscopy has been applied to EB-PBF processes [12,13] . Here, the 57 metal vapor was probed by measuring the absorption of laser light through the vapor plume 58 above the build surface . With tunable laser absorption spectroscopy (TLAS) , absorption line 59 shapes can be spectrally resolved allowing by tuning the laser wavelength across an atomic line 60 while measuring the transmission , allowing, e.g., temperature measurements from the Doppler 61 broadening in the near -vacuum conditions in EB-PBF [13]. We have recently demonstrated 62 time-resolved thermometry using TLAS in a single -track EB -PBF experiment targeting a minor 63 alloying element in the metal vapor using a novel vertical cavity surface -emitting laser 64 (VCSEL) [14]. 65 Typically, TLAS requires a line of sight between the light source and a detector [15,16] 66 limiting the studies above to measurements above the vapor cavity. However, efforts have been 67 made to develop single -ended sensors that can detect light reflected or backscattered from 68 rough surfaces even in harsh environments such as combustion chambers and reacting 69 flows [17–20]. 70 We adopt this configuration to measure the absorption within the vapor cavity in powder bed 71 fusion below the metal surface by measuring the light from a probe laser backscattered from 72 the melt pool surface, as shown in Fig. 1. Here, an input probe beam is focused into the vapor 73 cavity and the resulting backscattered TLAS (BTLAS) signal is recorded after passing through 74 the plume twice. 75 76 Fig. 1. Backscatter absorption spectroscopy applied to laser powder bed fusion. 77 2.2 Theory 78 The attenuation of light traversing through a material is described by Beer’s law (1) where I0 is 79 the initial light intensity, I the intensity after passing through an absorbing medium of thickness 80 L, and S is the line strength. The number density N (particles per unit volume) is proportional 81 to the density, and for an ideal gas, it depends on the gas pressure p, temperature T, and molar 82 Process laser Backscattered probe laser lightProbe laser Metal vapor plume Metal powder Previous layerNew layerMelt pool Vapor cavity OP I ICA PUBLISHING GROUP fraction x of the absorber. The (base -e) absorbance A is defined as the negative natural 83 logarithm of the transmission I/I0. 84 −ln(𝐼 𝐼0)=𝑁(𝑇,𝑝,𝑥)𝜙(𝑇,𝑝,𝜆)𝑆(𝑇)𝐿=𝐴(𝑇,𝑝,𝜆,𝑥) (1) 85 Although spectral lines arising from electronic transitions within the atoms have discrete 86 wavelengths λ, the experimentally observed absorption lines exhibit a finite width owing to 87 various line -broadening mechanisms [21]. Their influence is collectively described by the line - 88 shape function ϕ. 89 While natural broadening, owing to the uncertainty principle, is mostly negligible at 90 elevated temperatures and pressures encountered in laser processing, pressure and Doppler 91 broadening significantly affect the line shape [21]. Pressure broadening results from particle 92 collisions and increases with increasing gas density and temperature, resulting in a Lorentzian 93 line shape. Equation (2) describes its full width at half maximum (FWHM p), where γ is a 94 broadening coefficient defined at a reference temperature Tref and n is a temperature exponent 95 dependent on the collision partner. 96 FWH M𝑝(𝑇,𝑝)=2𝛾(𝑇ref 𝑇)𝑛 𝑝 (2) 97 Doppler broadening occurs from the thermal motion of particles, causing spectral lines to 98 undergo a blueshift or redshift, depending on the motion of the particle relative to the observer. 99 This effect leads to a Gaussian line shape, with its full width at half maximum (FWHM D) 100 described in (3). where M denotes the atomic weight of the absorber. 101 FWH MD(𝑇)=7.16×10−7𝜆√𝑇 𝑀(3) 102 An additional Doppler shift arising from the gas velocity relative to the propagation of light, 103 distinct from thermal Doppler broadening, can induce a spectral shift in the line center. In 104 backscatter measurements, where light enters the vapor cavity against the bulk flow of expelled 105 gases and exits in the same direction, this results in additional line broadening due to the overlap 106 of the red and blue shifts. The nonrelativistic spectral Doppler shift is given below (4). 107 𝜆obs=𝜆√1+𝑣 𝑐 1−𝑣 𝑐 (4) 108 As illustrated in Fig. 2, the observed absorbance depends on several factors including the 109 number density of the absorber, absorption path length, and gas density or pressure. The 110 resulting line shape function ϕ from Doppler and pressure broadening is a convolution of the 111 Gaussian and Lorentzian profiles, commonly referred to as the Voigt profile [21]. While it is 112 challenging to isolate each of these effects for quantitative measurements, especially in the 113 backscatter configuration where strong gradients may exist, the signal is significantly 114 influenced by gas properties, providing a means for process monitoring or control. 115 116 Fig. 2. Effects of gas properties on the spectral profile of atomic transitions under conditions 117 relevant to metal PBF. 118 OP I ICA PUBLISHING GROUP 3. Experiment 119 3.1 Laser melting testbed 120 The testbed shown in Fig. 3a is designed to provide optical access to a stationary melt pool and 121 vapor plume under conditions similar to L -PBF and EB -PBF. It consists of a process laser 122 mounted on top of a vacuum chamber. While the process beam is static, the sample, in the form 123 of a solid Ti -6Al-4V metal disk, is mounted on a rotating and translating motion stage to 124 simulate the motion of the process beam scanning over the powder bed. The chamber is 125 connected to a vacuum roughing pump and an Ar gas cylinder allowing the chamber pressure 126 to be adjusted between 0.1 –760 Torr. The vacuum chamber has various ports that can be 127 equipped with optical windows. 128 129 Fig. 3. (a) The optical testbed and (b) the optical arrangement for the BTLAS approach. 130 As shown in Fig. 3b, the process laser, a Yb:YAG fiber laser (IPG Photonics YLR, 131 λp = 1070 nm, 500 W, beam quality Mq2 = 5.9; IPG P30 collimator: f = 40 mm, d0 = 20 mm exit 132 beam diameter), was focused on the sample disk with a focal length ff = 500 mm lens. The 1/ e2 133 spot size calculated using (5) is ds ~ 200 µm. 134 𝑑𝑠=4𝑓𝑓𝜆𝑝𝑀𝑞2 𝜋𝑑0(5) 135 The top flange of the chamber has viewports that provide optical access nearly coaxial with 136 the process laser, with an offset of ~ 7.2° from the process laser axis. This angle was the smallest 137 achievable considering the space required for the windows and optics on the top flange. One 138 window is used for the probe laser, which is delivered via a single -mode optical fiber (see 139 Section 3.2 for details) and focused on the melt pool using an f = 11 mm aspheric lens with 140 focus adjustment (Thorlabs CFC11A -A), resulting in a spot size of ~ 200 µm similar to the 141 process laser. Adjacent to this is a detector (variable -gain Si photodiode, Thorlabs PDA100A2) 142 that captures the backscattered probe laser light through an f = 40 mm lens. There is a 90° offset 143 between the illumination and detection ports to reduce the amount of specular reflected light 144 reaching the detector and causing intermittent saturation. Both viewports are equipped with 145 fused silica windows with a 30 -arcmin wedge to prevent etalon effects caused by interference 146 from light partially reflected within the glass. Short -pass filters (Thorlabs FESH1000, 1000 nm 147 cut-off wavelength) are used to protect the fiber and detector from backscattered process laser 148 light. In front of the detector, a narrow bandpass filter matching the probe laser wavelength (see 149 Section 3.3 for details) further suppresses the process laser light and blocks blackbody radiation 150 OP I ICA PUBLISHING GROUP from the melt pool and the vapor plume. With a combined optical density of 13.5 at 1070 nm, 151 both filters effectively suppress the backscattered process laser light. All lenses and windows 152 are anti -reflective coated for the appropriate wavelength. Unless otherwise stated, the optics 153 are made of BK7 glass. 154 A camera (35 µm/pixel magnification ) mounted on one of the 45° ports ( Fig. 3a) was used 155 for process monitoring and alignment of the laser beams. To align the beams, the process laser 156 is first used to mark a spot on the metal disk. The probe beam is then adjusted until it aligns 157 with the mark using knobs on a kinematic mount that holds the fiber and optics to the top flange 158 of the vacuum chamber. 159 3.2 Probe laser 160 The probe laser used in this experiment was a commercial visible laser configured in a sum 161 frequency generation (SFG) arrangement (M Squared SolsTiS and EMM, 515 –661 nm). A 162 narrow -linewidth Ti:sapphire lasing cavity first produces light between 700 –1000 nm, which 163 is then coaligned with a 1950 -nm fiber laser and mixed inside a periodically -poled lithium 164 niobate (PPLN) crystal. This setup allows for the modulation of the output wavelength over 165 picometers. The generated visible light was coupled into a single mode optical fiber that 166 delivered it to the optical test bed. The optical power at the fiber output was measured to be 167 approximately 10 mW (Thorlabs PM160 power meter). For all absorption measurements in this 168 paper, the tuning bandwidth was set to 20 GHz, corresponding to a tuning range of 18.2 pm at 169 522 nm. The tuning rate was set to 10 Hz, i.e., the frequency at which the wavelength is 170 modulated from the lowest to the highest wavelength and back (Fig. 4). 171 172 Fig. 4. Long -term spectral measurement characterizing the laser scanning method. The full trace 173 (a) shows the overall stability and consistency despite undersampling, as demonstrated by cuts 174 in the scan starting (b) and stopping (c) periods. 175 To evaluate the wavelength tuning stability, the output of the laser system was monitored 176 with wavemeter (HighFinesse WS6 -600, 600 MHz accuracy) over a time span of 30 min while 177 the laser was tuned ( Fig. 4). Since the sampling rate of the wavemeter was limited to 25 Hz, the 178 tuning rate and range were determined by fitting a symmetrical sawtooth function to the under - 179 sampled data for small time windows of 10 wavelength sweeps each, as shown in Figs. 4a and 180 4b. The resulting tuning rate from the individual fits averaged 10.000 Hz over the whole dataset 181 with a standard deviation of 0.0016 Hz, while the tuning range was 17.3 pm with a standard 182 0 200 400 600 800 1000 1200 1400 1600 1800522.110522.115522.120522.125522.1165 nm center wavelength, 10 Hz tuning rate, 20 GHz bandwidthWavelength (nm) Time (s) (a) (b) (c)0.0 0.2 0.4 0.6 0.8 1.0522.105522.110522.115522.120522.125522.130 Data FitStartWavelength (nm) Time (s)1920.0 1920.2 1920.4 1920.6 1920.8 1921.0522.105522.110522.115522.120522.125522.130StopWavelength (nm) Time (s) OP I ICA PUBLISHING GROUP deviation of 0.32 pm, slightly less than the target tuning range. However, the tuning rate was 183 also observed to vary between measurement days. Since the experimental setup did not allow 184 simultaneous monitoring of the laser wavelength with the wavemeter during the experiments, 185 we decided not to derive a wavelength axis from the time trace. Consequently, all signals in 186 Section 4 are presented as a function of time instead of wavelength. 187 It is important to note that the probe laser power is more than four orders of magnitude 188 lower than the process laser powers used in this work, so the influence of the probe beam on 189 the melt pool is negligible. 190 3.3 Line selection 191 Using the gas temperature reported in previous studies [13,14] , spectral simulations were 192 performed ( Fig. 5) to identify candidate Ti absorption lines within the spectral range accessible 193 by the probe laser (see [14] for details on the simulations). Since pressure, absorption path 194 length, and Ti concentration are largely unknown at this point and were therefore arbitrarily 195 chosen for the simulations, the optical bandpass (Thorlabs FBH520 -10, 520 nm central 196 wavelength, 10 nm bandwidth) was chosen to accommodate multiple Ti lines with significantly 197 different absorption (dashed green line in Fig. 5). 198 Initial tests (not shown) showed that the absorption of the stronger lines was too strong, 199 causing the plume to be completely opaque. Therefore, the weakest line within this range at 200 522.155 nm (3d24s2 a 3F 3 → 3d2(3F)4s4p(3P°) z 3F° 2 [22], red arrow in Fig. 5) was selected 201 for all absorption measurements. 202 203 Fig. 5. Simulation of Ti absorption assuming a temperature of 3000 K at quasi -vacuum pressure 204 for an absorption path length of 200 µm (see [14] for details) along with the bandpass 205 transmission curve. The red arrow marks the line selected for the experiments presented in 206 Section 4 unless stated otherwise. 207 4. Results 208 For each measurement sequence, the Ti -6Al-4V disk was accelerated to reach the desired speed 209 (400 mm/s unless stated otherwise) at the process beam location. The process laser was then 210 activated for the full rotation of the sample disk. During the entire run -up and measurement 211 sequence, the probe laser was active and tuned over the target absorption line, while data 212 acquisition (National Instruments PXIe -1062Q and PXI -5105) was triggered just before the 213 process laser was activated. The hatch distance between two consecutive scans was 0.2 mm. 214 Figure 6 shows the raw signal recorded during a measurement sequence. As soon as the 215 process laser is triggered, the signal level increases significantly. This is consistent with the 216 backscattered light becoming more directional: as the vapor cavity forms, the beam angle is 217 expected to become smaller because the light escapes mainly upward through the aperture after 218 multiple reflections from the walls of the vapor cavity [23]. As a result, more light reaches the 219 nearly coaxial detector. 220 OP I ICA PUBLISHING GROUP The intensity and shape of the recorded signal has been found to depend significantly on 221 the process parameters applied to the signal, as described in the following sections. 222 223 Fig. 6. (a) the raw signal versus time curve recorded at a laser power of 500 W at a scan speed 224 of 400 mm/s in a 100 -torr Ar atmosphere and (b) the same curve zoomed in on a single 225 wavelength sweep of the probe laser across the Ti spectral line. 226 4.1 Signal intensity 227 The intensity of the backscattered signal depends on the attenuation of the probe laser light due 228 to absorption at the melt surface and the angle at which it is reflected depending on the shape 229 of the vapor cavity, which depends on the processing parameters. During the experiments, the 230 process laser power and the test chamber Ar pressure were varied. Figure 7 shows the signal 231 ensemble -averaged over six scans for different process laser powers (a) and pressures (b). Here, 232 the x-axis represents the time relative to the time at which the peak absorbance of each of the 233 six scans is observed. 234 The probe laser signal level increases with process laser power at a constant pressure of 235 100 Torr. This can be explained by the increasing depth of the vapor cavity with increasing 236 laser power, resulting in more directed backscatter [23], and, therefore, more light reaching the 237 detector. 238 With increasing pressure, however, first an increase of the signal level between 0.1 – 239 100 Torr can be observed before it drops between 100 –700 Torr. This is consistent with the 240 simulations by Li et al. [24] who observed a shallow and round vapor cavity at 10 mbar as the 241 vapor jet is expelled at a large divergence angle in the low -pressure environment. With 242 increasing chamber pressure, their simulations show that the vapor cavity becomes initially 243 steeper and deeper before it becomes shallower again at significantly higher pressure. This 244 could be caused by the reduced recoil pressure as the pressure difference between the vapor 245 cavity pressure and the chamber pressure decreases. 246 Another explanation for the drop at higher pressure could be plasma shielding from partial 247 ionization of the metal vapor, since the temperature in the vapor cavity increases with chamber 248 pressure due to the higher boiling point of the metals at elevated pressure. However, as pointed 249 out by Hanemann et al. [25], the laser fluence in L-PBF is probably insufficient for ionizing the 250 gases in the plume . We calculated the ionization fraction using the Saha equation [26] (not 251 shown) to be on the order of 0.1% near the boiling point of Ti ( 3558 K) at ambient pressure. 252 Although the ionization fraction is small, it could have some influence on the delicate balance 253 of forces in the vapor cavity. However , we did not observe any optical emission from ionized 254 Ti during this study (see Fig. 12 for emission spectra) and never observed any ion emission 255 during initial testing of the optical testbed when measuring the emission between 300–500 nm 256 with a high -resolution spectrometer (not shown) at near vacuum pressure . However, while the 257 ionization fraction increases with temperature, it also decreases with pressure . Therefore, there 258 OP I ICA PUBLISHING GROUP may be conditions in the pressure and temperature range in L -PBF where the influence of 259 plasma shielding is relevant . 260 261 Fig. 7. BTLAS signal at 400 mm/s scan speed for varying process laser power at 100 Torr (a) 262 and test chamber pressure at 500 W (b). The plots shown were averaged over six consecutive 263 laser sweeps to reduce noise. 264 4.2 Absorbance 265 The absorbance is calculated using (1) using the averaged signal ( Fig. 7) as I, whereas I0 is 266 determined by fitting a horizontal baseline to the data trace (dashed line in Fig. 7b). This 267 baseline correction makes the calculation of the absorbance independent of the overall signal 268 intensity. As shown in Fig. 8a, the absorbance increases with increasing laser power. This trend 269 is consistent with previous findings, where mass loss rates increased with the process beam 270 power [27]. For the pressure sweep ( Fig. 8b), the absorbance first increases w ith pressure before 271 it decreases again. [24] 272 273 Fig. 8. Absorbance derived from the intensity data in Fig. 7 showing a significant dependence 274 on (a) the process laser power at a constant chamber pressure of 100 Torr and (b) chamber 275 pressure at a constant process laser power of 500 W, both at a scan speed of 400 mm/s. 276 We hypothesize that two competing effects explain this behavior : 277 (a) Increasing pressure increases the boiling point which reduces the mass loss rate (i.e., 278 the amount of vaporized material) [28] leading to reduced absorption due to a lower 279 absorber number density. Figure 9 shows the boiling point of Ti as a function of the 280 pressure calculated using data from Ref. [29]. The boiling point increases rapidly from 281 2205 –3123 K between 0.1 –100 Torr before approaching an asymptote at higher 282 pressure. 283 OP I ICA PUBLISHING GROUP (b) Increasing pressure linearly increases the total gas number density, assuming an ideal 284 gas, resulting in increased absorption. 285 286 Fig. 9: Boiling point of Ti calculated using the Clausius -Clapeyron equation referencing the 287 boiling point (3558 K) at ambient pressure (760 Torr) [29]. 288 The boiling point increase is logarithmic with pressure whereas the density increase is linear 289 with pressure, making a pronounced minimum in measured absorbance plausible. However, 290 the boiling point of Ti6Al4V (3315 K at ambient conditions) differs significantly from that of 291 pure Ti and modeling the mass loss rate as a function of the melt pool temperature is not 292 trivial [28]. Therefore, further studies are needed to confirm that the two mechanisms described 293 above are responsible for the observations. 294 4.3 Line shape 295 Variations in line shape are evident when examining the normalized absorbance ( Fig. 10). 296 During the power sweep at 100 Torr ( Fig. 10a), the line shape remained invariant, suggesting 297 that the gas temperature remains constant, indicating that evaporation takes place under 298 equilibrium conditions [13]. 299 During the pressure variation ( Fig. 10b), the Ti line width initially narrows with increasing 300 pressure up to 100 Torr and then broadens again to 700 Torr. This behavior is attributed to the 301 interplay of line -broadening effects: The broadening between 100 –700 Torr is consistent with 302 the increased pressure broadening due to the increased chamber pressure. Additionally, 303 increased Doppler broadening occurs due to the rising plume temperature because of the 304 increased boiling point at higher pressures. 305 306 Fig. 10. Normalized absorbance during (a) the process laser power sweep at 100 Torr and (b) 307 the pressure sweep at 500 W at a scan speed of 400 mm/s showing differences in line shape 308 depending on the process parameters. 309 OP I ICA PUBLISHING GROUP The initial narrowing between 0 and 100 Torr can be attributed to Doppler shifts (not to be 310 confused with Doppler broadening) because of the increased velocity at which the gas is 311 expelled from the vapor cavity in vacuum: As the probe beam passes through the vapor, first 312 against and then in the direction of flow, the signal undergoes a redshift and a blueshift, 313 respectively. Using the gas velocity of 1500 m/s observed by Li et al. [24] at 10 mbar, the 314 resulting Doppler shift was approximately 2.6 pm (4), which is similar in magnitude to the 315 expected linewidth in vacuum [14]. Since the gas velocity decreases with chamber pressure, 316 the influence of this effect would explain an apparent line narrowing over the 0 –100 Torr range. 317 The observed line shapes were found to b e near-Gaussian at all investigated conditions as 318 shown in Fig. 11. At low pressure, it has been shown that Doppler broadening is the only 319 dominant broadening mechanism [13] explaining the Gaussian shape at 0.1 Tor r. This might 320 also be the case for the higher -pressure cases, since pressure broadening would lead to a more 321 Lorentzian line shape. Without wavelength calibration and without published broadening 322 coefficients for Ti -Ti and Ti -Ar collisional broadening, we cannot distinguish between self - 323 broadening (resonance broadening) and the influence of the bath gas on the line width at this 324 point. 325 326 Fig. 11: Line shapes measured during the pressure and power sweep with Gaussian, Lorentzian, 327 and Voigt line shape fits (wherever the fit converged) and corresponding coefficients of 328 determination R2. The Gaussian fits generally approximate the line shapes better with also the 329 Voigt fit converging to a Gaussian line shape. 330 It is important to emphasize that the analysis presented in this section is largely qualitative, 331 with some speculative elements. To fully discern the interplay of mechanisms affecting line 332 shape and strength, alternative or supplementary measurements and a more comprehensive 333 understanding of the dynamics between gases in the vapor cavity and the melt pool are needed. 334 Nevertheless, the pronounced variability in the observed signals based on processing 335 parameters highlights the potential for advanced process monitoring and closed -loop control, 336 potentially leveraging techniques such as machine learning. 337 4.4 Laser -induced fluorescence 338 The optical emission from the plume was also measured by replacing the detector with an 339 optical fiber (Ocean Optics XSR, 450 µm, 180 –800 nm) delivering light to a spectrometer 340 (Optosky ATP2000P, 2.1 nm resolution, 2 ms exposure time). The bandpass filter on the 341 detection side was replaced with a neutral density filter (Thorlabs ND10) to capture light in the 342 visible range without overexposing the spectrometer. 343 Figure 12 shows the plume emission spectrum both without the probe laser and with the 344 probe laser tuned to the stronger 519.442 nm line shown in Fig. 5. In the absence of the probe 345 laser (black curve), no emission lines were visible. However, when the probe laser was on (red 346 OP I ICA PUBLISHING GROUP curve), Ti lines became clearly visible, suggesting that these lines are due to laser -induced 347 fluorescence (LIF), which is the spontaneous emission of light upon stimulated excitation by 348 absorption of laser light. 349 350 Fig. 12. (a) Emission spectrum without probe laser and with probe laser tuned to the stronger 351 519.4415 -nm absorption line showing laser -induced fluorescence (LIF) and (b) LIF signal for 352 varying chamber pressures at a scan speed of 1000 mm/s. 353 While there is likely some non -stimulated emission from hot Ti, it is not detectable with the 354 low-resolution spectrometer used in our study. This instrument under -resolves atomic lines by 355 about three orders of magnitude, causing the low -resolution lines to blend into the background 356 signal and become undetectable. 357 The fluorescence signal varied significantly with chamber pressure, peaking at 20 Torr. 358 This correlates with the observations from the absorbance measurements, where the signal level 359 and absorbance are the highest, while the line width is the lowest at intermediate and low 360 pressures. 361 5. Discussion 362 5.1 Speckle noise 363 The relatively high noise level scales with the signal intensity, indicating that it is caused by 364 speckle noise, which is expected and largely unavoidable. When coherent light is reflected from 365 rough surfaces, speckle patterns, as shown in Fig. 13a, can be observed owing to the 366 interference of the laser light scattered from the high and low points of the rough surface of the 367 illuminated area. The interference pattern changes with the surface speckle, thus changing the 368 signal reaching the detector and resulting in transient noise. 369 Figure 13b shows the Fourier transform power spectrum of the backscattered signal for the 370 stationary sample disk, the moving sample disk, and during an absorption measurement. When 371 comparing the power spectra of the stationary disk with the moving disk, it becomes evident 372 that the noise caused by motion extends to about 6 kHz. This is similar in magnitude to the 373 speckle noise frequency of 2.5 –25 kHz observed by Wang and Sanders [19] from a rough 374 surface moving at 300 –3000 mm/s. The noise recorded during the absorption measurement 375 extends beyond the measurement range though also dropping at 6 kHz. This is consistent with 376 recent research suggesting that keyhole oscillations occur at 40 kHz or faster [30], with the 377 surface significantly changing several times during each oscillation cycle. 378 To reduce the influence of speckle noise, the laser must be tuned faster than the rate at which 379 the surface of the melt pool changes. Here, increasing the tuning rate to values well above 6 kHz 380 largely will eliminate the influence of the sample disk motion. To further reduce the influence 381 of melt pool oscillations, the required tuning rate is likely to be in the range of hundreds of 382 kilohertz. Such rates have been routinely achieved using, for example, diode lasers [31,32] . 383 OP I ICA PUBLISHING GROUP 384 Fig. 13. (a) Speckle pattern generated by HeNe laser beam scattered off the sample disk surface; 385 (b) Power spectrum obtained through Fourier transformation of the measured signal, comparing 386 a stationary disk to a spinning sample disk, both with and without the process laser. 387 Transient speckle noise can also result from tuning the wavelength of the probe beam. 388 Assuming a typical root mean square (rms) roughness for the powder bed PBF of hrms = 20 µm 389 (Ti-6Al-4V, D50 = 32 µm) [33], the wavelength change needed to alter the speckle pattern is 390 Δλ = ~ 1.5 nm (6) [19]. Since the probe laser tuning range needed for atomic line measurements 391 is about 2 orders of magnitude lower (16 pm in this work, see Section 3.2), speckle noise 392 originating from wavelength tuning can be neglected. 393 𝛥𝜆=𝜆2 𝜆+2√2𝜋ℎ𝑟𝑚𝑠(6) 394 5.2 Representativeness 395 The conditions of the metal vapor in this simplified experiment differ from the still largely 396 unknown conditions in the vapor cavity in L -PBF. The main differences are the larger spot size 397 and slower scan speed compared to most production machines, use of a solid sample instead of 398 metal powder, and the absence of a shielding gas flow. Further work is required to determine 399 whether the technique is applicable to real -world conditions (see Section 6). 400 Furthermore, spatter and ejected powder could be additional sources of scattered light in 401 the L -PBF process. Moreover, the absorption of light by droplets and nanoparticles of 402 condensing metal vapor could be wavelength dependent due to size effects if the particle size 403 is similar to the wavelength of the interrogation laser light. However, these sources of scattered 404 light above the vapor cavity are to some extent out of focus, so that light collection is less 405 efficient, reducing their influence on the overall signal. Further research on the influence of 406 gas-borne particles is needed when applying the BTLAS technique to L -PBF. 407 6. Conclusions and path forward 408 The feasibility of backscatter absorption spectroscopy of gases within the vapor cavity 409 produced during laser processing of solid Ti -6Al-4V was demonstrated. The signal stemming 410 from atomic Ti lines varies significantly with the laser power and build chamber pressure, 411 indicating its potential in process monitoring for quality monitoring or closed -loop feedback 412 control of processing parameters in 3D printing. 413 For a practical impact, the technique should be integrated into existing machines and the 414 time resolution should be increased to capture the dynamics of the melt pool/vapor plume 415 interactions. In addition, attempts to mitigate the effects of speckle noise should be pursued. 416 Application to PBF 417 Before applying the BTLAS technique to production machines, a reasonable next step could be 418 its application to L -PBF or EB -PBF in single -track experiments with metal powders under 419 (a) OP I ICA PUBLISHING GROUP realistic conditions. For this purpose, the probe beam can be fixed in space while the process 420 beam is scanned across the probe beam spot, similar to previous work studying the metal vapor 421 plume with absorption spectroscopy [12–14]. Here, the ability of single -track testbeds to apply 422 complementary measurement techniques such as X -ray imaging [34,35] can be leveraged to 423 test if and how the BTLAS signal correlates with defects such as pore formation. 424 Machine integration 425 Coaxial integration is an obvious solution to equip production L -PBF machines with BTLAS 426 sensors. To this end, a dichroic mirror can be used to combine the probe an d process laser . The 427 backscattered light could either be detected by a sensor inside the build chamber or even though 428 the same optical path as the laser beams. 429 Multi -line measurements 430 Employing multiple probe beams concurrently can facilitate the targeting of absorption lines 431 from different alloying elements. Signal ratios would then offer insights into the individual 432 mass loss rates of each species. Furthermore, targeting absorption lines with variable 433 temperature dependence could allow quantitative temperature measurements. 434 Laser -induced fluorescence 435 It should be further explored whether laser -induced fluorescence has potential as a diagnostic 436 tool in additive manufacturing. While high -energy pulsed lasers have been successfully used 437 for LIF experiments in laser welding [36–38], this work showed that relatively low -powered 438 CW lasers can also be used to significantly boost the emission of the vapor plume compared 439 with optical emission spectroscopy of thermally excited metal vapor. 440 Fundamental research 441 Attention should be paid to determining whether the gas in the vapor cavity comprises neutral 442 atoms or plasma. We did not detect any ionized species in this study. Nonetheless, it remains 443 uncertain whether the observed signal decrease at elevated process laser powers and pressures 444 can be attributed to plasma shielding. 445 446 Funding. U.S. Department of Commerce (grant number 70NANB21H039) ; National Institute of Health (grant 447 number GM -136981); National Defense Science and Engineering Graduate Fellowship Program, sponsored by the Air 448 Force Research Laboratory, the Office of Naval Research , and the Army Research Office (FA9550 -21-F-0003) . 449 Disclosures. A patent application covering the technology described in this manuscript has been filed and is currently 450 pending (application number U.S. 18/231066). There are no other competing interests. 451 Data availability. Data underlying the results presented in this paper are available on Zenodo under DOI: 452 10.5281/zenodo.10602517 . 453 454