Abstract
Low-loss hollow waveguides with a multiple dielectric layer are designed and fabricated for use in cavity ring-down spectroscopy (CRDS). Calculations of the waveguide design revealed that hollow waveguides for infrared CRDS system need to have transmission loss of 0.1 dB/m or less. We fabricated rectangular hollow waveguides with multiple dielectric layers to obtain low-loss properties. The waveguides are composed of four glass strips each with a dielectric multilayer deposited on the surface in advance. Experimental results show that the waveguides with double dielectric layers have lower losses at a target wavelength in the infrared than those with a single dielectric layer. The measured transmission loss of a 10-cm long waveguide deposited with a multilayer of Si/Al 2 O 3 /Ag for the transverse magnetic wave and Al 2 O 3 /Ag for the transverse electric wave was 0.24 dB at a target wavelength of 5.2 μm. The loss of the waveguide will be reduced when only the lowest-order mode is excited in the waveguide.
Highlights
Exhaled breath contains various gases related to human diseases, so breath analysis that is a noninvasive diagnosis method may be utilized as a complement of blood or urine tests to increase diagnostic accuracy.[1,2] Concentration of these disease-related components in the breath is usually in a parts-per-million order or less, so a gas analysis system with high sensitivity and selectivity is required
Unlike conventional cavity ring-down spectroscopy (CRDS) systems with large volume gas cells,[7] for hollow-waveguide based CRDS systems, transmission loss in the gas cell needs to be considered because the loss influences the intensity of light leaking from the cell
The loss is reduced by lengthening the coupling fiber because extremely high-order modes are eliminated in the fiber and the loss was 0.24 dB with the 0.7-m long coupling fiber. This result shows that the loss of the waveguide will be reduced much more when only the lowest-order mode is excited in the waveguide
Summary
Exhaled breath contains various gases related to human diseases, so breath analysis that is a noninvasive diagnosis method may be utilized as a complement of blood or urine tests to increase diagnostic accuracy.[1,2] Concentration of these disease-related components in the breath is usually in a parts-per-million (ppm) order or less, so a gas analysis system with high sensitivity and selectivity is required. A breath analysis system based on infrared absorption spectroscopy that detects vibrational modes of molecules is a candidate low-cost high-sensitivity system. Previous works have reported measurements using a multipath gas cell with multiple mirrors[3,4] and cavity ring-down spectroscopy (CRDS). In a CRDS system, light is trapped in an optical cavity consisting of a pair of highly reflective mirrors, and decay time of leaking light that correlates with the absorption coefficient of the medium gas in the cavity is measured. Gas detection systems based on CRDS achieve parts-per-billion (ppb) sensitivity.[5,6] the volume of the gas cell is large and this limits the analysis of exhaled breath. We design and fabricate low-loss hollow waveguides as a micro-volume gas cell for highly sensitive infrared CRDS systems
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