Recent advancement in technology have seen tremendous global development in communications and information sharing. The main goal has been to develop transmission systems with remarkable and efficient performance to meet the rising demand and growth in various field such as metro works and data center interconnections. This has specifically attracted significant attention of numerous researchers.
For instance, coherent detectors have dominated the haul transmission systems owing to their excellent performance and high spectral efficiency. Unfortunately, coherent receivers, on the other hand, require more complex hardware which increase their initial cost. To this end, researchers have been looking for alternatives and have identified direct detection as a promising solution especially in reducing the complexity and overall cost of the detectors and receivers. Single sideband modulation based direct detection avoids the effects of power fading by removing one of the sidebands. Therefore, optical and digital virtual carriers have been employed to utilize the full bandwidth of the digital-to-analog converter.
However, single sideband modulation is associated with the signal-signal beat interference that compromises its applications. This has led to the development of several digital linearization methods to overcome the challenge. For example, Kramers-Kronig detection technique has successfully controlled intra-polarization signal-signal beat interference thus enabling the reconstruction of the single sideband signal electric field.
Recently, polarization division multiplexing was introduced into single sideband systems to double the system capacity and the spectral efficiency. For single sideband based direct detection, optical carriers are normally added to the transmitter after which it undergoes random polarization rotation. On the other hand, the main challenge with direct detection systems based on the polarization division multiplexing and single sidebands is the carrier fading. Up to now, several methods have been put forward to end the carrier fading problem. One of the methods is using stokes vector receiver in polarization division multiplexed single sideband systems, which, however, has complex optical frontend comparable to that of coherent receiver.
Recently, Pekin University researchers Yixiao Zhu, Mingxuan Jiang, and Professor Fan Zhang designed and developed a novel polarized division multiplexed single sideband direct detection system with two orthogonal carriers located at the opposite sides of the X/Y polarizations. They removed the unwanted carriers using optical filters at the receiver side to obtain the polarization separation. Compared with Stokes space detection, the proposed receiver detects polarization division multiplexed signals in Jones space, which is simpler and inherent polarization rotation-invariant. Eventually, the system was validated with a 40 Gbaud polarization division multiplexing single sideband 16-ary quadrature amplitude modulation experiment. Their work is published in Journal, Optics Express.
The experiment was based on 80 km standard single-mode fiber transmission from which the authors observed that the bit error rates achieved 20% forward error correction at a threshold of approximately 0.015. Consequently, the performance of Kramers-Kronig detection, iterative signal-signal beat interference cancellation, and joint iterative signal-signal beat interference cancellation were compared at different carrier-to-signal power ratio. The simulations results showed that joint iterative signal-signal beat interference cancelation could compensate for both inter and intra polarization thus exhibiting better performance as compared to the others.
The Professor Fan Zhang and his research team successfully developed an optical filter-based polarization division multiplexed single sideband direct detection scheme. If a multi-input-multi-output equalization was incorporated, it suppressed the residual linear inter-polarization crosstalk thus allowing effective utilization of optical filters on the smooth edges. Therefore, owing to the feasibility and practicability of the study, Pekin University scientists are optimistic that it will advance various fields such as metro applications thus meeting the ever-increasing demand due to technological advancement.
Zhu, Y., Jiang, M., & Zhang, F. (2018). Direct detection of polarization multiplexed single sideband signals with orthogonal offset carriers. Optics Express, 26(12), 15887.Go To Optics Express