Abstract
We demonstrated low-loss and single-mode laser beam delivery through hollow-core waveguides (HCWs) operating in the 3.7–7.6 μm spectral range. The employed HCWs have a circular cross section with a bore diameter of 200 μm and metallic/dielectric internal coatings deposited inside a glass capillary tube. The internal coatings have been produced to enhance the spectral response of the HCWs in the range 3.5–12 µm. We demonstrated Gaussian-like outputs throughout the 4.5–7.6 µm spectral range. A quasi single-mode output beam with only small beam distortions was achieved when the wavelength was reduced to 3.7 μm. With a 15-cm-long HCW and optimized coupling conditions, we measured coupling efficiencies of >88% and transmission losses of <1 dB in the investigated infrared spectral range.
Highlights
Many imaging and sensing applications require a good laser beam quality with circular symmetry, as close as possible to the fundamental transverse TEM00 mode, and essentially diffraction-limited low beam divergence with M2 values in the range 1.0–1.3 [1,2,3]
We investigated the beam-guiding performances of hollow-core waveguides (HCWs) fabricated by OptoKnoweldge Systems Inc. (OKSI) with a bore size of 200 μm and internal coatings realized to enhance the spectral response at 3.5 μm
Single-mode laser beam delivery in the mid-infrared range was demonstrated at λ ≤ 5.5 μm with solid core fibers [7,25] and at λ ≥ 5.1 μm by using HCWs [18]
Summary
Many imaging and sensing applications require a good laser beam quality with circular symmetry, as close as possible to the fundamental transverse TEM00 mode, and essentially diffraction-limited low beam divergence with M2 values in the range 1.0–1.3 [1,2,3]. Chalcogenide glass fibers are the most used single-mode solid-core optical waveguides in the infrared spectral range. HCWs are optical fibers capable of single-mode mid-IR beam delivery [12,13] and are composed of a cylindrical glass capillary tube and metallic/dielectric layers deposited inside a hollow core. A minimum fiber length will exist that allows only the fundamental mode transmission through the fiber [12,13,14] In this limit, the output beam exiting from the HCW will have an approximate Gaussian intensity distribution. Minimum loss condition and single-mode output beam can be achieved by optimizing the laser beam coupling into the HCW. HCWs with bore diameters d ~ 30 times the wavelength have been shown to provide mid-IR laser beam delivery with low losses and single-mode output. We demonstrated that, using a HCW with a 300 μm bore size, it is not possible to achieve single-mode operation at wavelength λ ď 7.6 μm [17]
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