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A closed-form expression for differential phase-error variance is derived for the phase noise averaged over a finite time window. Using this expression, a method of laser linewidth characterization and in-service monitoring is proposed in presence of additive white noise applicable to both single-carrier and multi-carrier systems. The method is further substantiated by experiments of 107-Gb/s coherent optical OFDM transmission over 960-km SSMF fiber. Additionally, a novel technique of digitally tuning laser linewidth is proposed and demonstrated which may be useful to systematically study the laser phase noise impact.