Abstract
In this paper, the transmission characteristics of hollow silica waveguides with bore diameters of 300 and 1000 μm are investigated using a 7.8-μm quantum cascade laser system. We show that the bore diameter, coiling and launch conditions have an impact on the number of supported modes in the waveguide. Experimental verification of theoretical predictions is achieved using a thermal imaging camera to monitor output intensity distributions from waveguides under a range of conditions. The thermal imaging camera allowed for more detailed images than could be obtained with a conventionally used beam profiler. The results show that quasi-single-mode transmission is achievable under certain conditions although guided single-mode transmission in coiled waveguides requires a smaller bore diameter-to-wavelength ratio than is currently available. Assessment of mode population is made by investigating the spatial frequency content of images recorded at the waveguide output using Fourier transform techniques.
Highlights
Hollow waveguides [1] were developed in the 1970s for transmitting infrared radiation at a time when researchers were keen to develop alternatives to chalcogenidebased IR fibres, which exhibit high losses and are brittle
Hollow silica waveguides (HSWs) consist of a silica tube with bore diameters ranging from about 250 μm [2] up to around 1000 μm [3]. They are coated internally with a layer of silver which is exposed to a halogen, which converts the silver surface to a silver halide [4]
A schematic showing the construction of a typical HSW is shown in Fig. 1 with radial thickness values of waveguides supplied by Polymicro TechnologiesTM [6]
Summary
Hollow waveguides [1] were developed in the 1970s for transmitting infrared radiation at a time when researchers were keen to develop alternatives to chalcogenidebased IR fibres, which exhibit high losses and are brittle. HSW gas cells are advantageous in applications where only small volumes are available and/or where fast response times are required [11] Transmission properties such as attenuation characteristics, modal transmission and the influence of launch conditions of hollow silica waveguides are of interest. Multimode transmission through a hollow waveguide gas cell may result in temporal dispersion of the signal, causing degradation of the resolved gas absorption features when using very high-bandwidth detectors. These properties have been investigated previously based on the CO2 laser application wavelength [12] at 10.6 μm, using an established theoretical framework which describes waveguide transmission [13]. We quantify the modal properties using the mean spatial frequency of the resulting images
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