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. 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, 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” approach, the proposed scheme provides an alternative approach to synthesize high-order QAM with simplified driving electronics.