A simple and compact photonic crystal fiber (PCF) interferometer that operates in reflection mode is proposed for
refractive index (RI) sensing. The device consists of a ~12mm-long stub of commercially available PCF (LMA-10)
fusion spliced to standard optical fiber (SMF-28). The device reflection spectrum exhibits interference patterns with
fringe contrast up to 40 dB. One of the excited modes in the PCF is sensitive to external RI therefore the device can
be useful for refractrometry. The shift of the interference pattern can be monitored as a function of the external
index. In the operating range, from 1.33 to 1.43, the maximum shift is less than the interferometer period, so there is
no-ambiguity in the measurements. The maximum sensitivity and resolution achieved were 735 nm per RI units and
7×10-5, respectively. Another approach to measure the external RI consists of monitoring the reflection power
located at the quadrature point of the inference pattern in a properly selected wavelength. Consequently the
measuring range is narrower but the resolution is higher, up ~7×10-6, thanks to the high fringe contrast.
We report on mode interferometers built with photonic crystal fibers (PCFs) and optical micro/nano fibers (MNFs). This
type of mode interferometers exploit the beating between two modes, are very compact and highly stable over time for
which they are suitable for a myriad of sensing applications. Moreover, their fabrication is simple since it can be carried
out by means of cleaving and splicing or tapering techniques. The transmission spectrum of these interferometers
typically exhibits truly sinusoidal interference patterns which simplifies their analysis. PCF-based mode interferometers
may have niche applications since they are capable of operating at extreme temperatures (up to 1000ºC). To make these
interferometers functional and competitive, our group has placed emphasis on the design of the PCF microstructure,
minimizing the insertion losses, and on the elaboration of ad-hoc packaging for both harsh environment and biosensing
applications. MNF-based interferometers, on the other hand, are extremely compact, require minimal amount of sample
and can be combined with microfluidics for which they may be adequate for refractometric or biosensing applications.
Adequate protection of the MNFs and ad-hoc microfluidics are being implemented to make MNF interferometers
practical.
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