Demonstration of Deep UV Fiber Delivery Patchcord Down to 213 nm

The ultraviolet (UV) spectral range represents today as a new frontier to be conquered for advancing next generation of photonics. Recent developments have centered on the field of nanophotonics [1] and fiber photonics [2]. In the latter, the traditional limitations of solid-phase materials, such as bandgap limited bandwidth and low laser damage threshold, have been lifted through air guidance enabled by inhibited-coupling guiding hollow-core photonic crystal fibers (IC-HCPCF) with minimal optical overlap with the cladding silica. Within this context, an innovative fabrication method involving the application of shear stress to improve the core surface quality has resulted in record-low attenuations in the short-wavelength range, with values inferior to 50 dB/km in the UV [3]. This advancement has enabled efficient and solarization-free fiber delivery at industrial UV laser wavelengths of 355/343 nm [4] and 266/257 nm [5]. To drive further progress, however, improving fiber bend insensitivity and understanding the interplay between the bend loss (BL) and surface scattering loss (SSL) remain a critical objective. This paper reports on the design, fabrication and characterization of a tubular lattice cladding IC-HCPCF with reduced core to lower bending sensitivity, and fabricated drawn using the counter-directional gas-flow technique to minimize core surface-roughness [3]. Micrographs of 3 fibers with varying inner core diameters (ID) are shown in Fig. 1a, with core sizes ranging from 27 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}$</tex> down to 16 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu\mathrm{m}$</tex>, significantly smaller than those reported in [3]. Surface roughness measurements of the fiber cores yielded an average RMS value of 0.19 nm (Fig. 1 b), which is below the thermodynamic limit and consistent with the results in [3]. Fig. 1(c) shows the 180–700 nm spectra of a plasma lamp source, transmitted through 70 m long fiber with the 22 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}-\text{ID}$</tex> the measured (red solid curve) and the calculated transmission loss (red dashed curve). The latter includes the confinement loss (CL), SSL and BL for a bend radius of 25 cm, reflecting the experimental conditions. Moreover, the fiber transmission loss was measured using two different lasers with wavelength of 355 nm (UV) and 213 nm (DUV) respectively. Loss figures lower than 150 dB/km were achieved over the whole UV-DUV range. Specifically, representative UV and DUV laser wavelengths yielded losses of 65 dB/km at 343/355 nm, 85 dB/km at 257/266 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{nm}$</tex> and 135 dB/km at 213 nm (see Fig. 1c, red curve). Using a laser source, lower losses were observed, such as 30 dB/km at 355 nm and 130 dB/km at 213 nm. This discrepancy is attributed to differences in coupling conditions and modal content. The measured losses consistently exceeded the simulated total losses, likely due to uncertainties in the empirical formula for SSL. To evaluate bending sensitivity, transmission loss at 355 nm was measured for different bending radii and compared with a fiber featuring a larger core (Fig. 1d). The results highlight the advantage of a smaller core for bending radii lower than 15 cm, though a persistent offset remains between the calculated and measured losses. Finally, the suitability of the IC-HCPCF for DUV beam delivery was demonstrated using a 1 m-long patchcord to transport the beam of a 213 nm pulsed laser developed by Teem Photonics. The laser delivered 3 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu\mathrm{J}$</tex> pulses with nanosecond durations. The bottom of Fig. 1d illustrates the patchcord's endurance, with stable transmission observed over 48 hours, indicating no photodarkening or solarization effects.