Applications of Hollow Waveguide in Spectroscopic Gas Sensing

Hollow Waveguides (HWGs) are used in spectroscopic gas sensing devices as both a optical waveguide and gas transmission cell that provide an extended optical pathlength yielding high sensitivity measurements. Their advantages over traditional multipass gas cells are that they have a much smaller sample volume and can provide a faster response time as well as stable and flexible light path with lower cost. The studies recommend three main types of HWGs commonly used for gas sensing, namely Ag/AgI-coated hollow waveguides (Ag/AgI-HWG) photonic bandgaps hollow waveguides (PBG-HWG) and substrate-integrated hollow waveguides (iHWG), respectively. Research and development of spectroscopic gas sensing with HWG modules have been reviewed for the past few years. In addition, the application modes and fields have also been summarized. The studies show that HWGs have been coupled to FIIR, laser absorption spectroscopy (LAS) and Raman spectroscopy to replace conventional multipass gas cells. Now, a series of achievements were made and relevant applications of HWGs based gas sensing included atmospheric environment monitoring, breath diagnostics and industrial process monitoring. Particularly, the HWG sensors based on mid-infrared (MIR) LAS offer a number of advantages over other spectroscopic methods, i. e., relative simple composition, low cost, superior compatibility with various of HWGs and strong adaptability to environment, which are fairly attractive for practical application. To summarize, with the development of laser technology, optical waveguide technology and spectral technology, HWG based spectroscopic gas sensing is developing rapidly, and gradually operated from the laboratory to the field application.