Monolithically integrable optical single sideband transmitter for low-cost, high-density optical interconnects

Current high-speed optical interconnects are composed of cost-effective direct-detection (DD) systems. However, the continuous and exponential growth of data traffic is pushing new technologies to be adopted for optical interconnections. One promising technological candidate is the coherent optics. This technology facilitates the use of multi-level modulation format and narrow channel spacing. Despite the supreme performance of coherent optics technology, this technology is still regarded as too expensive to be used for cost-sensitive and very-short-reach applications. Thus, it is anticipated that DD systems would still dominate the market in the near future. Optical single sideband (SSB) modulation is an effective way to double the spectral efficiency of conventional double sideband-based DD systems. The narrow spectral width of the signal can be used to increase the number of wavelength-division-multiplexed (WDM) channels or to relax the requirements of WDM filters. This signal can be readily generated by using an in-phase/quadrature modulator (IQM) driven by the Hilbert transform pair. However, the IQM has a high insertion loss, requires a couple of automatic bias controllers for its stable operation, and occupies a large footprint. In this paper, we present our recent research activities on the dual modulation of directly modulated laser and electro-absorption modulator for the generation of optical SSB signals. This scheme can be implemented into a monolithically integrated semiconductor device in a highly cost-effective manner, just like electro-absorption modulated lasers. We present the modulation conditions of this dual modulation scheme for high-speed optical SSB signals and experimental results.