Abstract
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 ${\rm
LP}_{01}$ and ${\rm
LP}_{0{m}}$ modes, with the first ten modes accounting for
98% of the incident mode energy. For a connector-offset attenuator, coupling
mainly occurs between the ${\rm
LP}_{01}$ and ${\rm
LP}_{1{ m}}$ modes, with the first ten ${\rm LP}_{0{ m}}$ modes and first ten ${\rm LP}_{1{ m}}$ 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, ${\sim}
{\hbox {10}}^{ - 4}$ 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.
© 2009 IEEE
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