Design, fabrication, and characterization of dual-wavelength inhibited-coupling guiding hollow-core fibers

The present paper reports on the design, fabrication, and characterization of an 8-tube inhibited-coupling guiding hollow-core photonic crystal fiber (IC-HCPCF) capable of guiding both the beam emitted from an Yb:YAG laser at the fundamental wavelength of \(\lambda =1030 \text{nm}\) and its second harmonic at \(\lambda =515 \text{nm}\). By controlling the strut thickness of the glass capillaries to approximately \(362 \text{nm}\), the transmission of laser radiation at both wavelengths was possible with low losses. Optimizing the outer diameter of the glass capillaries mitigates the bending-induced increase of the confinement loss at the wavelength of \(515 \text{nm}\) without compromising the optical performance of the fiber at the wavelength of \(1030 \text{nm}\). Experimental results confirm the near to diffraction-limited beam quality \(\left({M}^{2}<1.15\right)\) of the laser beams exiting the fiber at both operational wavelengths. Operating in the first transmission band at the wavelength of \(1030 \text{nm}\), the calculated chromatic dispersion is \(1.02 \text{ps}/(\text{nm}\bullet \text{km})\), despite a diameter of the hollow core of \(40 \mu \text{m}\). At the wavelength of \(515 \text{nm}\) this value amounts to \(0.62 \text{ps}/(\text{nm}\bullet \text{km})\). The measured losses are \(27.5\pm 0.3 \text{dB}/\text{km}\) at the wavelength of \(515\text{ nm}\) and \(25.7\pm 0.7 \text{dB}/\text{km}\) at the wavelength of \(1030\text{ nm}\), which is comparable to the loss of state-of-the-art IC-HCPCFs with tubular cladding structures. The measured bending-induced increase of the confinement losses confirms the potential of the proposed approach for flexible, low-loss guiding of ultrashort laser pulses at the two wavelengths using a single fiber. This gained flexibility can significantly enhance the options for wavelength selection in laser material processing applications.