The Impact of Local Oscillator Frequency Jitter and Laser Linewidth to Ultra High Baud Rate Coherent Systems

Abstract

Through theoretical analysis and simulation, we investigate the system impact due to a sinusoidal jitter tone and the resultant local oscillator (LO) laser linewidth requirement in ultra-high baud rate and long distance coherent optical systems. We also carried out experiments in 64 Gbaud, dual-polarization (DP)-16 QAM systems to verify the theoretical analysis and simulation. We have also obtained a jitter interference tolerance mask to qualify LO lasers. A jitter tone with a frequency lower than ∼1 MHz has a higher tolerance since it generally causes constant frequency or phase shift, which can be tracked by a receiver DSP. For a jitter tone with a frequency higher than ∼1 MHz, the tolerance becomes much tighter since the tone will affect laser lineshape and induce equalizer-enhanced phase noise (EEPN). Consequently, a jitter tone in the higher frequency region could severely affect the system performance. Theoretical analysis and numerical result illustrate that EVM2 due to the effect of laser linewidth and a sinusoidal jitter tone is proportional to the weighted sum of $[ \Delta \nu \times B_{s}\times \text{L}] $ and $[ \Delta f_{pp}\times B_{s}\times \text{L}] ^{2}$ , where Δν is the laser linewidth, $B_{s}$ is the baud rate, $\Delta f_{pp}$ is the laser peak-to-peak frequency deviation due to a sinusoidal jitter tone, and L is the fiber transmission length. This result is applicable for all orders of QAM constellations. The implication to future 100 Gbaud and beyond systems is delineated.