M.Sc Thesis

M.Sc StudentLaredo Gilad
SubjectDFB Fiber-End Laser with Near Surface Coupling and
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Baruch Fischer
Full Thesis textFull thesis text - English Version



The basic structure of a “classic laser”, includes two mirrors forming a Fabri-Perot cavity and a gain medium. The Fabri-Perot cavity allows, under its boundary conditions, the formation of a finite number of longitudinal modes to exist. Each longitudinal mode, has a different wavelength (and frequency) and all together they provides a frequency comb.

In DFB (Distributed Feedback) lasers, the coupling (or the feedback) between the two opposite waves is distributed along the cavity, resulting for example from a uniform refractive index grating. Interestingly, a DFB laser does not usually oscillate at the Bragg wavelength but at two detuned wavelengths. For application in which one lasing mode is required there is a need to insert a quarter wavelength phase shift in the DFB grating. This phase shift changes the confinement conditions of the EM field inside the cavity and gives preference to single lasing mode operation at the Bragg wavelength.

In our study we use the fact that a small disruption (as phase shift) in the grating changes the lasing characteristics. We folded a DFB fiber tip laser (the grating located at the fiber end) by using reflective mirror (or a reflective sample) at high proximity to the fiber end. In this case, the free space distance between the fiber tip and the mirror creates a phase shift that changes the lasing modes spectral location.

A numerical solution based on an existed mathematical model, shows two interesting phenomena for the longitudinal modes allowed in the system, in their spectral location and threshold gain. The first phenomenon is “mode crunching” where two longitudinal modes share the same threshold gain, and has almost the same frequency. The second phenomenon is “double gain threshold” in which two longitudinal modes with different gain thresholds share the same frequency, creating a double gain threshold mode.

In this study we also examine experimentally, the system vertical resolution. We show that under certain conditions, the system has a vertical resolution below the order of  ,without using lock-in amplifier or phase locked loop (PLL). This resolution shows that our system can be used as a high sensitive proximity sensor for various applications as atomic force microscope (AFM),near field scanning optical microscope (NSOM), micro electro mechanical system (MEMS) fabrication, nano-scale meteorology and more.