In-Line Single-Mode Optical Fiber Interferometric Refractive Index Sensors

Novel in-line single-mode fiber interferometers- Mach-Zehnder and Michelson-have been designed, fabricated, and tested as refractive index (RI) sensors. Abrupt tapers and connector-offset attenuators are proposed as alternatives to long period gratings (LPGs) as mode-coupling mechanisms to transfer optical power between core and cladding modes in optical fiber. The coupling coefficients between core and cladding modes in the proposed designs were calculated using numerical packages and the devices were subsequently implemented using commercially available fusion splicer. For an abrupt taper, most coupling occurs between the LP
₀₁
and LP
₀

m
modes, with the first ten modes accounting for 98% of the incident mode energy. For a connector-offset attenuator, coupling mainly occurs between the LP
₀₁
and LP
₁

m
modes, with the first ten LP
₀

m
modes and first ten LP
₁

m
modes accounting for 92% of the incident mode energy. In particular, in the case of connector-offset attenuator, the relative direction between the two connector-offsets was found to be very important to the interference performance. Interference patterns were realized in simulation for the interferometers using both mode-coupling mechanisms. Three interferometers were realized in the experiment using abrupt taper-Mach-Zehnder and Michelson-and connector-offset attenuator-Michelson. They showed large extinction ratios (up to 23 dB) and small insertion losses (smaller than 3 dB). Although it is difficult to make Mach-Zehnder interferometers using connector-offset attenuator pair due to the lack of polarization control in the fusion splicer, some evidence of constructive interference was observed in the experiment. The interferometers were tested as RI sensors using the maximum attenuation wavelength shift. Given that the minimum resolution of optical spectrum analyzer is 10 pm, ~ 10
^-4
difference of RI can be detected by the proposed interferometric sensors, providing similar performance as LPG-based interferometers at a lower cost and simpler fabrication process.