Analysis of the unbalanced sidebands of phase modulation

Phase modulation was always used to prevent stimulated Brillouin scattering (SBS) which could damage the optics such as lenses in high power laser systems. Band pass filters such as fiber Bragg gratings were usually used in the optical fiber amplifiers to restrain the amplified spontaneous emission (ASE). When the phase modulated laser passes the filter, unbalanced sidebands will be caused by the wavelength mismatch between the laser and filter. The result is fast amplitude modulation on the top of the waveform. The problem is discussed and simulated with a simulation software developed by ourselves in this paper. It is obtained that the fast amplitude modulation is worsened with narrower bandwidth and larger wavelength mismatch. With 0.05 nm mismatch for 0.2 nm bandwidth filter, the modulation ratio is about 24.3%. The ratio is optimized to 4.4% by replacing the 0.2 nm filter by a 0.4 nm filter. The results are well used to direct the development of ICF driver front-end system.