Since new modulation schemes have become a great center of interest, Mach-Zehnder fiber fused interferometers (MZI) became necessary to ensure the direct detection (DD) from phase modulated signals, like DPSK (Differential Phase Shift Keying) or DQPSK. As these modulation schemes are very interesting in optical transmissions due to their capabilities, MZIs cannot be overlooked in the future optical links. In this paper, we review the base principles for using MZI as DPSK demodulators, and the major characteristics associated to these devices. In order to evaluate the minimal characteristics required for efficient modulation format conversion, we propose some numerical simulation results. Based on these results, we depicted a new fabrication process for the realization of MZIs by adiabatic stretching of one of the interferometer's arm. This is carried out by a CO2 laser, and the fabrication major points that must be respected are discussed. After review the MZI thermal dependency mechanism, the last part presents the experimental results obtained for the quantification of MZI temperature control accuracy needed for its utilization as an 43 Gbit/s PSBT (Phase Shaped Binary Transmission) encoder.