High-power fiber delivery for laser manufacturing provides flexibility in comparison with free-space set-ups: improved space management with the reduction of the beam path, direct delivery to the processing area with integration in 5-axis robots for instance, and better serviceability. And ultraviolet (UV) part of the spectrum is no exception. The absorption of certain metals such has copper, silver, gold and tantalum is greater by one order of magnitude in the UV than in the visible and near-infrared [1]. Also, the higher photon energy allows the photolytic “cold” process, the breaking of chemical bonds, benefitting to organic materials processing such as wood and plastic but also highly transparent materials. The nanosecond regime is appreciated for its shorter processing times and applications as varied as laser processing of carbon fiber-reinforced plastic [2], ablation of silicon nitride layers [3] grooving of semiconductor devices [4], patterning of indium tin oxide [5], or sapphire micromachining [6] were reported in that regime. Most applications require near diffraction limited beams outputs.
Nonlinearities and silica damage threshold in traditional solid-core fibers limit the preservation and delivery of high-energy and high-peak-power pulses. In the UV spectrum in particular, silica is subject to a stronger absorption, multi-photons absorption [7] and a prominent photodarkening. The creation of color centers requires a solarization-resistant treatment such as hydrogen passivation [8]. These limitations associated to the medium of propagation can be circumvented with hollow-core photonic crystal fibers (HCPCFs) left in ambient air, filled with gas, or vacuumed. Indeed, HCPCFs that guide via inhibited-coupling (IC) are very attractive because they exhibit a guided field that overlaps extremely weakly with the glass cladding material in the order of 10− 6 [9–10]. Consequently, silica material absorption losses and undesirable solarization effects are here negligible making those fibers ideal for UV applications such as high-power high-energy laser beam delivery.
On the laser system side, up to 200 W average power was reported in the near-infrared ultrashort pulse regime with transmission efficiencies between 85 to 93% [11] whereas the kW level was achieved in the CW regime [12]. However, in the UV spectrum, results remain in the 10–200 mW range [13–15].
Table 1 summarizes the current state-of-the-art of UV fiber delivery. Notice that all IC-HCPCF used show a negative-core contour with different numbers of tubes, inner tube radius over core radius ratios, and silica wall thicknesses. For all, light was injected with a free-space configuration either using vacuum cells on both ends of the fiber [14] or simply exposing the fiber in ambient air [13,15] as achieved in this present article.
Table 1. State of the art of maximum average power transmitted through hollow-core fibers in the UV spectrum.
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References
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[13]
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[14]
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[15]
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Wavelength
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266 nm
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343 nm
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355 nm
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Pulse duration
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17 ns
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350 fs
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20 ps
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Fiber length
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2 m
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5 m
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1 m
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Transmitted average power
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13.7 mW
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180 mW
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106 mW
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Coupling/transmission efficiency
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40%
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52%
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66%
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In this study, a specific IC-HCPCF was optimized from [16] to demonstrate record UV guidance at 343 nm. By using a novel nanosecond UV pulse laser, 20 W average power corresponding to 150 µJ was successfully delivered over the fiber without any degradation versus time, improving by two orders of magnitude the current state-of-the-art. This result opens the way to a wide range of applications such as generation of deep UV light [17], UV ultrafast pulse-compression [18], resonance Raman sensing [19], and potentially microdissection of biological materials [20].
In recent years, an extension of the negative-core contour concept [21] has emerged based on a single-ring tubular lattice HCPCF [22] The design encompasses a single ring of non-touching tubes which delimits a fiber core with a hypocycloid form and no connecting nodes. These characteristics benefit IC guidance and allow the accomplishment of ultralow loss figures in IR and visible range with values respectively of 7.7 dB/km at 780 nm [23] and of 13.8 dB/km at 539 nm [24]. Such performances of few dB/km can also theoretically been achieved in the UV domain. However in this short wavelength range, the transmission will be limited by the surface scattering loss (SSL) [25] and improvement of the core surface quality will be required. In a forthcoming publication, our group report a first milestone on this long-term work by implementing a new fabrication methods to demonstrate strong reduction on the loss level down to 10 dB/km in the UV range [26]. A first set of those new generation of IC-HCPCF was used in 2022 to demonstrate near- and middle-ultraviolet reconfigurable Raman source [16]. Here, we slightly further optimized the opto-geometrical parameters of the fibers to center the UV transmission band at 343 nm, corresponding to the operating wavelength of the UV laser used in the following delivery experiments. The inset in Fig. 1a shows a micrograph of the fabricated IC-HCPCF made by the stack-and-draw technique at GLOphotonics. The fiber cladding is composed of 8 non-touching tubes with a diameter of 11 µm and a thickness of 595 nm. The tubes are arranged to form the surround of a hollow-core with a diameter of 27 µm. As shown by Fig. 1b, a low-loss value around 27 dB/km at 343 nm is measured by the cut-back technique obtained with a laser-driven plasma source (ENERGETIQEQ-99X-FC) and a spectrum analyser.
On the laser side, a source named CAREX 30–343 was developed by BLOOM Lasers. This system presents a 343 nm single-mode signal (M2 = 1.04), with a circularity of 99% and a residual astigmatism of 4%. The long-term beam pointing stability of this source is evaluated to be below 25 µrad full-angle over 8 hours. This fiber laser is able to deliver programmable pulse shapes from 2 ns to 20 ns, and from 100 kHz to 800 kHz. For our experiment, it was set to deliver up to 26 W of UV power with 10 ns pulses at 150 kHz, corresponding to a pulse energy of 180 µJ per pulse.
In order to properly coupled the laser into the HCPCF, two standard fused-silica plano-convex lenses of 300 mm and 100 mm focal length was used to shape the beam size down to 19 µm, the mode-field diameter of the fiber LP01 mode, see Fig. 2. The fiber for this study was 1m-long. The input end coating was stripped over 1 cm length and covered by graphite sheet to strip out any uncoupled light.
The fiber power handling is then presented on the right of Fig. 2. The delivery of up to 23.3 W is reported with more than 92% average combined coupling and transmission efficiencies but notably peaking at 95.6% at 5.63 W. Any change from this maximum transmission value with power is attributed to slight beam pointing variation and dilatation-induced movement of the fiber end due to local heat generated by the cladding light stripping. At 18.7 W input, a delicate realignment readjusted the transmission efficiency to its initial value after a steady decrease. This rules out both the contribution of photodarkening and atmospheric Raman lines absorption since no protective gas chamber nor vacuum cells were implemented. Each power measurement lasted a few minutes with the final measurement lasting ~ 10 min. No damage of the fiber nor mode quality degradation were observed over that timescale. The fiber output mode was imaged onto a CMOS camera using a + 50 mm-focal-length lens, four tilted wedges and neutral density filters. The single-mode output maintains over the power range as reported in Fig. 2.
On the strength of these results, the stability of the UV delivery and the resilience against solarization were further study at 23 W input UV power. The Fig. 3a show that the maximum of 92% transmission was maintained at the output of the HCPCF with no significant degradation over 45 hours time.
Finally, the bend sensitivity was also evaluated. The results are reported in Fig. 3b. The fiber was coiled in a loop with different radius of curvature ranging from 20 cm down to 4 cm. Care was taken during the study to maintain straight the fiber tips at the input and output. The input UV power launched was fixed to 200 mW. For a bending radius superior to 7 cm, no transmission change was observed. For a stronger curvature, the coefficient of transmission decreases with a critical bending radius of 5 cm measured at 3 dB additional losses. The near field recorded at the output of the fiber reveals a good maintain of the quasi gaussian fundamental mode for all the configurations. All these results confirmed the potential of IC-HCPCF solution for UV transport in industrial applications.