Master Oscillator Power Amplifiers (MOPAs) are III-V semiconductor based monolithically integrated optoelectronic sources, that are studied as application for Light Detection and Ranging and Free Space Optical communications. MOPAs enable the possibility to have a modulated signal, together with a high output power. A study on the mode confinement inside the Slab Coupled Optical Waveguide structure is presented, in order to maximize the device output power; together with it a theoretical and numerical study on the impact of optical nonlinearities on gain and phase dynamics and on power and gain along the length of the Semiconductor Optical Amplifier output stage is developed, to address the possibility to have amplitude and phase modulation from the SOA output stage.