Review of optical fiber Mach–Zehnder interferometers with micro-cavity fabricated by femtosecond laser and sensing applications

In recent years, optical fiber sensors have become most popular due to their unique advantages. A review of optical fiber Mach–Zehnder interferometers (MZIs) with micro-cavity fabricated by femtosecond (fs) laser was presented. The sizes of most cavities are in the range of several micrometers to tens of micrometers. Different kinds of structures of MZIs and their sensing applications were summarized and discussed in detail. Firstly, the processing mechanism of fs laser was explained and the MZI sensing principle was described with a simplest example. Secondly, five types of micro-fabricated MZIs were summarized, including those based on a trench, internal air cavity, splicing with various special fibers, optical fiber tapers and other hybrid structures. Subsequently, performances of these structures were compared, and advantages and disadvantages were analyzed. Finally, the prospect and future development of optical fiber MZIs were addressed.