The aim of this thesis is to analyze the OSNR sensitivity on a 100 Gb/s optical coherent PM-QPSK system against the use of different types of laser sources, looking for the impact of the associated phase noise at different laser linewidth. The drive for higher performance in optical fiber systems has focused its interest in coherent detection, a field revived by advances in digital signal processing (DSP). DSP-based phase and polarization management techniques make coherent detection robust and practical. With coherent detection, the complex field of the received signal is fully recovered, allowing compensation of linear impairments including chromatic dispersion and polarization-mode dispersion using difital filters. In addition, fiber nolinearities can also be compensated quasi-exactly. Coherent PM-QPSK has been attracting considerable attention for 100G long haul optical interfaces because of its OSNR performance (>2dB improvement over direct detection formats), spectral efficiency, and tolerance to inter-symbol interference. This thesis firstly describes basic principles of fiber optic communications and digital modulations in order to introduce the system object of our analysis. Then the use of coherent optical systems is discussed, comparing other kind of system detection and looking for the benefits of its use. Thereafter, the Polarization multiplexing – QPSK modulation is introduced, giving a short evolution on optical modulation and showing the main architecture of a PM-QPSK transmitter. Subsequently performance and characteristics of 100 Gb/s PM-QPSK systems are given focusing on its OSNR sensitivity, filtering tolerance and dispersion. In a second stage of the thesis, we define a model of a coherent PM-QPSK system, explaining the principal parameters and modules that conforms the whole composition. Once our starting point of the analysis, wich is the system, has been introduced, we start the analysis on the sensitivity against different types of laser linewidth, performing several simulations using different values on the range of 0 Hz to 10 MHz and looking for the most interesting results. In conclusion, this thesis presents, defines, describes and analyses a coherent 100 Gb/s PM-QPSK system in order to define its OSNR sensitivity against changes on the laser linewidth between 0 Hz – 10MHz.