Abstract
We experimentally demonstrate tunable modulation format conversion of quadrature phase shift keying (QPSK) to four-level pulse amplitude modulation (PAM4) using nonlinear wave mixing in periodically-poled Lithium-Niobate (PPLN) waveguides. The conversion operation is accomplished by: (a) rotating the data constellation by applying a phase offset, and (b) offsetting the data constellation in the radial direction by adding a constant bias optical power. In this manner, the constellation data points can be tailored to have different amplitude levels, thus realizing the PAM4 format. In a tunable fashion, we convert 20- and 30-Gbit/s QPSK channels to PAM4, and open eye diagrams and low bit-error-rates (BER) are obtained at the receiver. We investigate the impact of phase rotation on the eye diagram of the received signal, and we determine that a phase rotation of ∼71°provides substantially good performance. Wavelength tunability of the output PAM4 is demonstrated by tuning the wavelength of the pump. In addition, open eye is also obtained when a high phase noise laser is used for modulating QPSK signal.