Optical Modulator With a Near-Linear Field Response

The principle and characteristics of a linear optical modulator (LOM), which has a high field-response linearity, are discussed. A conventional Mach-Zehnder modulator (MZM) shows a nonlinear (sinusoidal) field response. This nonlinearity causes some problems in advanced digital coherent optical transmission systems that employ advanced multilevel modulation or pulse-shaping technologies. To solve this, we devised the LOM, which has a two-stage asymmetric lattice configuration with two MZMs driven with a pair of complementary voltage signals. The key idea of the LOM is to utilize the complementary output from the first MZM for generating the compensation signal. This enables us to improve the linearity without losing much optical power. Theoretical characteristics of the LOM are discussed in detail for the first time. To verify the LOM concept, we fabricated a linear in-phase-and-quadrature modulator (LIQM) with a hybrid configuration of silica planar lightwave circuits and a LiNbO₃ chip with a high-speed phase-modulator array. Using the LIQM, we experimentally proved that the LOM provides a better loss-linearity trade-off than the conventional MZM does. The LIQM was also tested for 12.5-GBd 16-QAM modulation, and it generated a clear constellation where the nonlinearity of the MZM was compensated well.