Wavefront Splitting Fiber Mach–Zehnder Interferometer Modulated by the White Light Fraunhofer Diffraction

Stability, robustness, sensitivity, and reproducibility are important parameters in the interferometry. We propose a wavefront splitting fiber Mach-Zehnder interferometer (MZI) to achieve such performances. A polyethylene terephthalate (PET) film splits the wavefront of a Gaussian beam emitted from the first fiber collimator. Then, the Gaussian beam is focused into a single mode fiber by another collimator, constituting a stable and robust fiber MZI. The effective spectral width is larger than 390 nm, which is wide enough for constructing a sensitive fiber MZI. The COMSOL multiphysics simulation and the experimental results demonstrate that this MZI is based on the multi-beam interference. Thanks to the low insertion loss of the fiber MZI, two types of stable and repeatable interference fringes are obtained when the sampling accuracy of the optical spectrum analyzer is as high as 0.1 nm. It ensures that the fiber sensor based on this MZI has a high resolution. For the first type of interference fringe, the two adjacent resonant wavelengths, respectively, shift -0.78 and -0.81 nm when salinity increases one part per thousand, showing high sensitivities. The low order interference fringe has a larger sensitivity. However, its visibility may be small due to the modulation of the second type of interference fringe that has small sensitivity to the refractive index of the sample. Since the PET film has uniform thickness, the fiber MZI can ensure good reproducibility. It can be widely utilized in detecting physical, chemical, or biological quantities.