Strain Sensor Based On Thin Core/multimode Fiber

Compared with the traditional fiber Bragg grating,fiber optic sensor based on modal interference has many advantage of high sensitivity.In this paper,based on the theory of in-line Mach-Zehnder interferometer(MZI),the thin core fiber(TCF)and Multi-mode fiber(MMF)based strain sensors with core-offset and tapered structure were proposed by arc-discharge splicing technique.The strain responses were studied and the strain sensitivity was enhanced.Additionally,the tapered-TCF structure was packaged and tested in terms of strain and temperature,and the possible crosstalk was suppressed by difference compensation.The main contents are as follows:(1)The principles of in-line MZI with respects to interference and sensing of strain and temperature were studied theoretically.Through beam progation method,the core-offset based TCF and MMF structures are simulated.The interfered spectra and modes were also studied with different offset values.(2)The core-offset based TCF and MMF structures were fabricated.The quantitative relations between offset values and fringe visibility were studied,and the strain and temperature sensitivity tests were conducted.The experimental results showed that,similar to Single mode fiber,the sensitivity of core-offset TCF structures was about 1 pm/??.Comparatively,the maximum strain sensitivity in the core-offset MMF structure was-2.53 pm/??,due to the excited higher modes.The temperature sensitivities of the core-offset based TCF and MMF structures were 0.081 nm/? and0.185 nm/?,respectively.(3)In order to improve the sensitivity,the tapered TCF and MMF structure was proposed and investigated.The light energy distribution and interference modes of tapered structures were simulated and analyzed,and their strain and temperature response were studied experimentally.The experimental results showed that the maximum sensitivity was up to 4.52 pm/?? in tapered MMF structure,but with a worsened transmission spectrum.Inversely,for tapered TCF structure,its wavelength sensitivity is increased with the reduced waist diameter.When waist diameter was 30.13?m,the maximum wavelength sensitivity was ?3.74 pm/??.Morevoer,the strain response has an obvious intensity feature with the reduction of TCF length.The maximum sensitivity was 0.11 dB/?? when the TCF length was reduced to 1.5-cm,and the corresponding detection resolution was 0.09 ??.