Graphene Photonics Nested Mach-Zehnder Modulator for Advanced Modulation Formats

Graphene is a 2D gapless material with electrically tunable optical properties that can operate over an ultra-wide optical spectrum. Graphene wafer scale films can be grown through chemical vapor deposition (CVD) and transferred on passive waveguide platforms at low temperature [1]. Photonic integrated devices for efficient and fast electro-absorption and electro-refraction modulation [2] have been demonstrated so far, as well as ultra-fast photodetection [3]. Graphene integrated photonics is an emerging technology with great potential for many photonic applications, from sensing to datacom/telecom [4]. Here, we show the first graphene photonic I/Q modulator based on a nested Mach-Zehnder loaded with $100\ \mu \mathrm{m}$ long graphene EAMs, and demonstrate 40 Gb/s quadrature phase shift keying (QPSK) modulation. The device consists of a passive nested MZI where the child MZIs' arms are equipped with graphene EAMs and thermal phase shifters (TPS), while the parent arms are provided with only TPSs to set the quadrature phase shift $(\pi/2)$. The principle of operation of the nested I/Q MZM consists in the generation of binary phase shift keying (BPSK) modulation at the outputs of the child MZMs obtained by driving the EAMs with differential signals [5], i.e. one with DATA signal and one with inverse DATA signal. By adding a constant $\pi$ shift with the integrated TPS, the output is a perfectly symmetric BPSK. The output of the two child MZIs are then combined in the parent MZI which builds the QPSK signal from the two BPSKs after introducing a $(\pi/2)$ phase shift with the TPS. We used a dual differential channels 100 GS/s DAC to generate the binary signals, properly amplified by two >40GHz dual channel driver amplifiers. Off-line DSP was performed to extract the QPSK constellation diagrams after coherent reception with a real-time oscilloscope.