Optical 36QAM transmitter based on two tandem IQ modulators with simplified driving electronics

To scale up the capacity and spectrum efficiency in optical transmission systems, high-order quadrature-amplitude modulation (QAM), has raised a lot of research attention. Recently, optical 32 or 36QAM has been deployed to realize high spectrum-efficient Nyquist wavelength-division multiplexing transmission with spectral efficiency of up to 8.37b/s/Hz [1-2]. Usually optical 32 or 36QAM is synthesized using a single in-phase/quadrature (IQ) modulator driven by 6-level driving electronics, which is referred to as 'electrical' approach here. Although the single IQ modulator solution has a simple optical hardware, it requires sophisticated technique for preparing superior-quality 6-level electronics by either combining three binary electrical signals or deploying high-speed digital-to-analog converters (DACs). The operation symbol-rate is restricted by the DACs resolution, linearity of driver amplifiers or bandwidth of electrical components. On the other hand, the tandem-modulator approach, a serial combination of IQ modulators has been utilized for synthesizing various multilevel optical signals such as 8QAM and 16QAM [3], where only binary electronics are deployed to drive each modulator. If extending this scheme to generate higher-order QAM, we can also benefit from the reduced complexity in driving electronics. Instead of preparing superior-quality 6-level electronics for generating 36QAM, just binary and 3-level electronics are required. In contrast to the 'electrical' [1-2] approach, the proposed scheme provides an alternative approach to synthesize high-order QAM with simplified driving electronics.