**5. Conclusions**

This research compares the sensitivity of a single FPI and the Vernier effect performed with two FPIs for the same number of fringes. The results reveal that the Vernier effect envelope, where 60 ± 1 nm/μm is achieved, has twice the sensitivity of the single FPI, 29.2 ± 0.3 nm/ μm. Moreover, with the optimization of the system, an envelope sensitivity of 254 ± 6 nm/μm with *M*-factor of 1071 and a *MVernier* of 1.9 was obtained and corresponded to a resolution of 79 pm (considering the spectral resolution of 0.02 nm). In addition, the sensitivity of the Vernier effect envelope depends only on the relationship between interferometers, which allows problems in the production of ultra-sensitive sensors to be overcome as well as the same sensitivity for any distance of the fibers to the reflecting surface to be observed. This new displacement sensor architecture allows the implementation of giant-sensitive non evasive measurements with high relevance in areas such as micro and nano manufacturing and biology.

**Author Contributions:** P.R. performed the experimental work, analyzed the data, and wrote the article; O.F. supervised and reviewed. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is financed by National Funds through the Portuguese funding agency, FCT—Fundação para a Ciência e a Tecnologia, within project UIDB/50014/2020.

**Data Availability Statement:** Data available on request due to restrictions privacy. The data presented in this study are available on request from the corresponding author. The data are not publicly available due to it is from the Barcelona supercomputer.

**Acknowledgments:** Paulo Robalinho acknowledges the support of the Foundation for Science and Technology (FCT), Portugal through the Grant 2020.04562.BD.

**Conflicts of Interest:** The authors declare no conflict of interest
