The dissertation titled “Mach-Zehnder Interference with Symmetrical Balls based on the Optical Vernier Phenomenon for Multiple Wavelength Generation from Fiber Optic Lasers and Sensing Applications” by Saif Aqeel Mohammad was deliberated at the Institute of laser for Postgraduate Studies, University of Baghdad.
This research was conducted under the supervision of Professor Dr. Abdel Hadi Mutasher Abd and Assistant Professor Dr. Sarah Kazem Al-Hayali.
The primary objective of this dissertation was to pioneer a novel study in constructing a laser system capable of producing multiple wavelengths from fiber optic lasers and facilitating sensing applications. This was achieved by leveraging the Mach-Zehnder interference technique with symmetrical balls based on the optical Vernier phenomenon. A multifunctional comb filter was ingeniously designed and implemented within the cavity of an erbium-doped fiber laser (EDFL) to enable multi-wavelength laser generation, wavelength selection, control, and refractive index sensing applications.
The optical Vernier effect, akin to its mechanical counterpart, is attained through the amalgamation of two optical interferometers—a sensor interferometer and a reference interferometer. The integration of these interferometers results in the modulation of Vernier, facilitating precise wavelength control and selection.
By integrating the proposed comb-shaped filter into the cavity of a ring-cavity erbium-doped optical fiber laser, a stable dual-wavelength laser cantered at 1533.2 and 1558.2 nm was successfully produced. Additionally, a parallel-coupled Mach-Zehnder interferometer, in conjunction with step-change polarization controllers, enabled switchable erbium-doped optical fiber laser generation with tunable wavelength spacing in a selective manner.
The Mach-Zehnder interferometer (MZI) filters, designed as two symmetrical balls, served as spectral filters for wavelength selection and tuning. They facilitated simultaneous generation of dual wavelengths in the EDFL, while also functioning as a refractive index sensing unit. This was demonstrated by immersing the MZI filter in sodium chloride/water solutions with varying refractive index concentrations.
Moreover, the integration of MZI-3 into the sensing arm of a parallel-coupled Mach-Zehnder interferometer facilitated triple switchable wavelength emission in the erbium-doped optical fiber laser. This broadened the superimposed spectrum, enabling wavelength tuning and selection.
The outcomes of this research underscore the reliability, operational simplicity, and cost-effectiveness of the proposed Mach-Zehnder interferometer structure based on the Vernier effect. Furthermore, this innovative filter structure presents a promising avenue for generating tunable dual- and multi-wavelength optical fiber lasers, particularly for refractive index sensing applications.

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