13 Apr 2007
A side effect of fusion splicing is put to good use to make an optical strain sensor only a few centimeters in length.
A short length of microstructured optical fiber (MOF) between two fusion splices can act as a compact interferometer for strain sensing, say a team led by Joel Villatoro at ICFO, Spain. (Optics Express 15 4 1491)
"Longitudinal strain on the MOF shifts the interference pattern to shorter wavelengths, so our interferometer has strong potential for strain-sensing applications," Villatoro told optics.org. "Fabrication is fast and simple, and interrogation with a simple LED and a palm-size spectrometer are sufficient. What's more the devices are robust, operate from approximately 650 to 1600 nm, and can withstand temperatures of 1000 °C or even higher."
Splicing together MOFs with an electrical arc can damage the fiber's interior structure, as the air holes invariably collapse in the vicinity of the splice. This usually causes only minimal loses to the optical signal and can be used to make all-MOF filters or attenuators.
Villatoro's interferometer exploits the significant effect which the collapsed voids have on the fundamental MOF mode, which immediately begins to diffract and broaden when it reaches the first splice. Between splices the fundamental core mode and a higher order mode are both excited, before the two modes are enlarged and recombined by the second slice. This means that the excited modes can behave as the arms of an interferometer, with the splices themselves acting as couplers or splitters.
The team deliberately created two areas of collapsed air holes in an MOF, separated by several centimeters (shown as L in the diagram), using fusion splicing. The MOF was carefully chosen, with an internal structure of five rings of slightly larger than normal air holes to reduce bending losses, but was otherwise conventional.
Each collapsed zone was about 300 microns in length, and losses at each splice were found to be less than 5 dB when a 850 nm source was used. A tapering of the air channels just before the collapsed regions was spotted under the microscope, but the outer diameter of the fiber stayed uniform.
"So far we have fabricated interferometers with a maximum working length of 15 cm, as interferometers with longer lengths exhibit poor fringe contrast." Villatoro said. "For practical applications we've worked with interferometers under 10 cm long."