In this paper, a systematic study of the effect of inter-structural gap between a series of in-line Mach-Zehnder interferometers (ILMZIs) is presented. Two similar ILMZIs, composed of a pair of fiber micro-bottles 3.5 cm apart, were fabricated and spliced in series by performing fusion splicing of a standard single-mode fiber using a commercial machine. The quality of the collective transmission spectrum of this serial formation is affected by the remnant cladding mode transmitted from preceding ILMZI. This study shows that there is a plain minimum structural gap of approximately 40 cm between the cascaded ILMIs to achieve a ripple-free resultant spectrum while retaining a broad free spectral range (FSR) of ~20 nm. It is also discovered that the minimum structural gap can be reduced by half or more before ripple began to deform by simply bending the connecting fiber for inducing sufficient loss to eliminate the remnant cladding mode, demonstrated with a curvature radius around 1.25 cm over only a short length of ~2.3 cm. This finding allows construction of a more compact architecture of serial ILMZIs based on fiber micro-bottles, with pristine interference spectrum, broad FSR and a high extinction ratio, which can be exploited as a new approach for spectrum shaping, multi-parameter sensing and wide dynamic range sensing.
