Abstract
In this paper, we report a capillary-based Mach–Zehnder (M–Z) interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The sensing mechanism is quite straightforward. Cladding and core modes of a capillary are simultaneously excited by coupling coherent laser beams to the capillary cladding and core, respectively. An interferogram would be generated as the light transmitted from the core interferes with the light transmitted from the cladding. Variations in the refractive index of the air filling the core lead to variations in the phase difference between the core and cladding modes, thus shifting the interference fringes. Using a photodiode together with a narrow slit, we could interrogate the fringe shifts. The resolution of the sensor was found to be ~5.7 × 10−8 RIU (refractive index unit), which is comparable to the highest resolution obtained by other interferometric sensors reported in previous studies. Finally, we also analyze the temperature cross sensitivity of the sensor. The main goal of this paper is to demonstrate that the ultra-sensitive sensing of gas refractive index could be realized by simply using a single capillary fiber rather than some complex fiber-optic devices such as photonic crystal fibers or other fiber-optic devices fabricated via tricky fiber processing techniques. This capillary sensor, while featuring an ultrahigh resolution, has many other advantages such as simple structure, ease of fabrication, straightforward sensing principle, and low cost.
Highlights
Research and development of fiber-optic gas refractometers has drawn tremendous attention due to its potential applications in both scientific and industrial fields, such as gas composition detection, gaseous lasers, environmental protection, homeland security, and atmosphere pressure monitoring [1,2].Generally, variations in gas refractive index induced by pressure or temperature changes are in a narrow range of 10−6 to 10−4 refractive index unit (RIU)
Variations in gas refractive index induced by pressure or temperature changes are in a narrow range of 10−6 to 10−4 refractive index unit (RIU)
We first pump out the air in the capillary to lower the internal air pressure to 400 hPa, and shut down the mechanical pump to let the air back stream slowly till the inner air pressure elevates to surrounding atmosphere pressure (1012.0 hPa)
Summary
Research and development of fiber-optic gas refractometers has drawn tremendous attention due to its potential applications in both scientific and industrial fields, such as gas composition detection, gaseous lasers, environmental protection, homeland security, and atmosphere pressure monitoring [1,2].Generally, variations in gas refractive index induced by pressure or temperature changes are in a narrow range of 10−6 to 10−4 refractive index unit (RIU). The majority of fiber-optic gas refractometers typically adopt interferometric sensing schemes due to their relatively high sensitivities. A number of Fabry–Pérot (F-P) or Mach–Zehnder (M-Z) fiber-optic sensors have been proposed and experimentally demonstrated for sensing of gas refractive index. Quan et al demonstrated a F-P fiber sensor fabricated by cascading a PCF to a capillary fiber fusion-spliced with a section of SMF [12]. This sensor operated based on the Vernier effect and reported an extremely high sensitivity of ~30,899 nm/RIU
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