UV 20W-class single-mode nanosecond pulse delivery using a vacuum-free/ambient air inhibited-coupling hollow-core fiber

Silica fibers have been successfully used to deliver high-power high-energy laser beam in the near-infrared and visible range but suffer from high absorption and color-center formation in the UV spectrum which disqualify them for such wavelength. Recently, the advances of inhibited-coupling hollow-core photonic crystal fibers have made them an unique tool to transport UV radiation thanks to an ultralow overlap with the silica cladding and record losses down to 10 dB/km. By further optimizing such fiber, we report in this letter a record single-mode delivery of 23.3 W (155 μJ) with 92% transmission from a 343 nm, 10 ns, 150 kHz laser source, corresponding to an improvement of the current state-of-the-art by two orders of magnitude.


Introduction
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 highenergy and high-peak-power pulses. In the UV spectrum in particular, silica is subject to a stronger absorption, multiphotons absorption [7] and a prominent photodarkening. The creation of color-centers formation requires a solarizationresistant 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 and 93% [11] whereas the kW level was achieved in the CW regime [12,13]. However, in the UV spectrum, results remain in the 10-200 mW range [14][15][16]. 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 [15] or simply exposing the fiber in ambient air [14,16] as achieved in this present article.
In this study, a specific IC-HCPCF was optimized from [17] 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 [18], UV ultrafast pulse-compression [19], resonance Raman sensing [20], and potentially microdissection of biological materials [21].
In recent years, an extension of the negative-core contour concept [22] has emerged based on a single-ring tubular lattice HCPCF [23]. The design encompasses a single ring of non-touching tubes which delimits a fiber core with a hypocycloid form and no connecting nodes. Indeed, to enhance the light guidance performances in such fibers, strong reduction in the coupling between the cladding mode continuum and the core-mode is required. This can be done either by having little spatial intersection between the fields of the core mode and the cladding modes or by having a strong mismatch in their respective transverse spatial phases. A direct consequence is to use a negative curvature of the core contour with no connecting tubes, i.e., tubular lattice fiber design because (i) the LP 01 mode field spatial overlap with silica is reduced to the tangent sections of the inner cups of the negative contour, (ii) the larger perimeter of the negative contour results in a higher azimuthal-like number in the silica core-surround modes and hence, stronger transverse phase-mismatch with the core-mode, (iii) and finally the spatial overlap between the core-mode with the unwanted connecting nodes supporting low azimuthal number modes is here canceled. 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 [24] and of 13.8 dB/km at 539 nm [25]. Such performances of few dB/km can also theoretically be achieved in the UV domain. However in this short wavelength range, the transmission will be limited by the surface scattering loss (SSL) [26] and improvement of the core surface quality will be required. In a recent 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 [27]. This technique is based on the concept that shear flow significantly attenuates the unwanted capillary waves in the fiber drawing process which directly reduce the impact of the SSL contribution in the total loss at such short wavelengths. A first set of those new generation of IC-HCPCF was used in 2022 to demonstrate near-and middle-ultraviolet reconfigurable Raman source [17]. Here, we slightly further optimized the opto-geometrical parameters of the fibers (i.e., mainly the silica thickness of the tubes) 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 hollowcore with a diameter of 27 μm (core size a bit smaller than usual to mitigate the impact of high order mode at the short  Fig. 1 a Optical image of the HCPCF fabricated; b corresponding loss spectrum of the fiber in the UV region. The near field pattern recorded at the output of the fiber by a camera and lens at 343 nm is also indicated wavelengths). As shown by Fig. 1b, a low-loss value around 27 dB/km at 343 nm is measured by the cut-back technique from 100 to 5 m pieces of fibers set in loops of 1 m-bending radius obtained with a laser-driven plasma source (ENER-GETIQEQ-99X-FC) and a spectrum analyzer (Ocean Optics HR4000CG-UV-NIR). All the details on the fiber measurement protocols can be found in the section Methods of our recent publication [27]. Notice that the input light coupling has been meticulously adjusted using free-space optics to achieve a fundamental mode-dominated modal content. On the laser side, a source named CAREX 30-343 was developed by BLOOM Lasers [28]. This system presents a 343 nm single-mode signal (M 2 = 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 h. This fiber laser is able to deliver programmable pulse shapes from 2 to 20 ns, and from 100 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.
To properly coupled the laser (TEM 00 spatial mode with a beam diameter measured to 3.5 mm ± 0.35 mm) into the HCPCF, two standard fused-silica plano-convex lenses of 300 mm and 100 mm focal length (with a total losses at 355 nm measured to be less than 0.1%) was used to shape the beam size down to 19 µm, the mode-field diameter of the fiber LP 01 mode (estimated value from 27 µm core diameter × 70% [29]), see Fig. 2. The fiber for this study was 1-m-long used in an almost straight configuration at the beginning. 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 dilatationinduced 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 quasi 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. Figure 3a shows that the maximum of 92% transmission was maintained at the output of the HCPCF with no significant degradation over 45 h time. The weak reduction of power was due to the mechanical stability of the translation stages used to couple the UV light into the fiber as previously observed on Fig. 2.
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 using calibrated mandrels to spool the fiber, with diameters ranging from 20 cm down to 4 cm. Care was taken during the study to maintain straight fiber tips at the input and output, with a length at each fiber ends kept straight < 10 cm. The input UV power launched was fixed to 200 mW. For a bending radius greater than 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 the quasi gaussian fundamental mode is maintained for all the configurations. More information about

Conclusion
To conclude, the recent advances in IC-HCPCF and laser systems were used to demonstrate a record 23.3 W (155 µJ) single-mode nanosecond pulse delivery at 343 nm. This figure corresponds to an improvement of 2 orders of magnitude on the power handling of fibers in the UV range. All these results confirmed the potential of IC-HCPCF solution for UV transport in industrial applications. In this context, future development will include the increase of the fiber length to few tens of meters and the average UV power of the laser source above 100 W. This upgraded source will present similar beam quality and pointing stability to achieve efficient coupling in the UV fiber delivery system. Such a highpower fiber-coupled system would represent a significant improvement in industrial markets, combining high level of performance with simplified integration and maintenance.
Author contributions FL did experimental work, JD and JS conceived the laser, FG made the fiber and JB supervized all this work.
Funding This research was funded by PIA program (Grant 4F) and the Conseil Régional de Nouvelle Aquitaine.