Frequency Domain Analysis and Determination for Spectrum Characteristics of Laser With External Phase Modulation

Frequency noise of laser causes a broadening of spectral lineshape and introducing noise through external phase modulation can achieve spectrum broadening without destroying the stability of center frequency. Although the relationship between the frequency noise spectrum and lineshape has been studied, there is almost no analysis aiming at the impact on broadened lineshape for laser when phase noise is Gaussian white noise (GWN). In this paper, spectra with GWN phase modulation of different power spectral densities are numerically simulated and analyzed in frequency domain. The carrier suppression and broadening component increase with the improvement of GWN bandwidth and power. The circumstance of amplifier saturation is also taken into account. The external phase modulation system with bandwidth of 10 GHz is built based on electro-optic modulator (EOM) and tested in optical spectrum analyzer (OSA) and interferometric fiber optic gyroscope (IFOG). The intrinsic carrier with the linewidth of 0.1 pm is broadened to 44 pm. The lineshape of modulated spectra and performance of IFOG driven by modulated laser are consistent with the theory and simulation. In addition, the broadened-laser-driven IFOG realizes navigation-grade drift performance. This article provides guidance on phase modulation and spectral broadening in the field of laser.