Vernier-effect was firstly employed by vernier caliper and barometers to enhance the accuracy of length and air pressure measurement. Recently, Vernier-effect has also been employed to fiber-optic sensing for the magnification of spectrum shift. The optical sensors consist of two cascaded two-beam interferometers with a small fringe interval difference. Sensitivity can be amplified by an order of magnitude through implementing wavelength interrogation with a curve fitting method of the superimposed spectrum envelope. Recently, fiber-optic in-line Mach-Zehnder interferometer (MZI) sensors have attracted great attention due to their advantages of simple, compact structure, good stability, and low cost compared to traditional MZIs. Large FSR and multi-mode interference are the characteristics of in-line MZI sensors compared to other interferometric fiber-optic sensors. Arising from the spectrum characteristics of inline MZIs, no obvious envelope is observed from the superimposed spectrum of cascaded in-line MZIs. So, Vernier-effect is scarcely applied to in-line MZI sensors to the best of our knowledge.
In this paper, a novel sensitivity amplification method for fiber-optic in-line Mach-Zehnder interferometer (MZI) sensors has been proposed and demonstrated. The sensitivity magnification is achieved through a modified Vernier-effect. Two cascaded in-line MZIs based on offset splicing of single mode fiber (SMF) have been used to verify the effect of sensitivity amplification. Frequency component corresponding to the envelope of the superimposed spectrum is extracted to take Inverse Fast Fourier Transform (IFFT). Thus we can obtain the envelope precisely from the messy superimposed spectrum. Experimental results show that temperature and curvature sensitivity of the sensing MZI is 45.36pm/℃ and -4.55nm/m-1, respectively. As a result of the modified Vernier-effect, the sensitivity can be amplified to 397.36pm/℃ and -36.26nm/m-1. A maximum sensitivity amplification factor of nearly 9 is realized. The proposed sensitivity amplification method is universal for the vast majority of in-line MZIs.
This work, named “Sensitivity amplification of fiber-optic in-line Mach–Zehnder Interferometer sensors with modified Vernier-effect” has been published in Optics Express (Vol.25, No.22, 26898-26909, 2017) at October 16th 2017. This work is supported by National Natural Science Foundation of China (NSFC) (No. 61775070, 61290315, 61290311)
