Terence S.-Y. Chen
Latitude Design Systems
Introduction
Global data traffic is growing rapidly, correspondingly increasing the demand for optical transceivers used in data centers (DCs). Currently, the application of coherent optical communication technologies has shifted from backbone networks to metropolitan area networks, and then to DCs. Coherent fiber optic communications were extensively studied in the 1980s, primarily because the high sensitivity of coherent receivers can increase uncompensated transmission distances. However, since the 1990s, related research and development has been interrupted by the rapid growth of high-capacity wavelength division multiplexing (WDM) systems that employ traditional intensity modulation and direct detection (IM-DD) schemes along with newly developed erbium-doped fiber amplifiers (EDFAs) to compensate for span losses. In 2005, the demonstration of digital carrier phase estimation in coherent receivers generated tremendous, renewed interest in coherent optical communications. This is because digital coherent receivers allow us to employ various spectrally efficient modulation formats, such as M-ary phase shift keying (PSK) and quadrature amplitude modulation (QAM).
Relying on stable carrier phase estimation in the digital domain. In addition, since the phase information is preserved after detection, we can digitally compensate for linear transmission impairments such as fiber chromatic dispersion (CD) and polarization mode dispersion (PMD) using digital signal processing (DSP). These advantages of digital coherent receivers offer considerable potential for innovating existing optical communication systems. Recently, 100-Gb/s systems employing orthogonal PSK (QPSK) modulation, polarization multiplexing, and coherent detection with high-speed DSP at a symbol rate of 25 GBd have been developed and introduced into commercial networks.
This white paper demonstrates the concept design of a coherent optical transceiver using the pSim photonic IC simulator in the PIC Studio tool suite to perform link-level simulations including transmission quality analysis with fiber.
Link Construction Explanation
For link theory, working principles, and device parameter settings, please refer to Latitude Design Automation’s tutorial slides: "SiPh IC Design with pSim". As shown in the figure, a pseudo-random bit sequence (PRBS_1) is connected to FORK_1 to split an electrical signal from one port to two ports. Then, the two ports are connected to two non-return-to-zero (NRZ_1, NRZ_2) pulse generators to produce on-off keying (OOK) signals. The two ports are fed into a Mach-Zehnder modulator (MZM_1).
In the same structure, PRBS_2 is connected to FORK_2 to split a signal into two ports. Then the two ports are connected to two NRZ pulse generators (NRZ_3, NRZ_4) to generate OOK signals. According to the figure, pLogic schematic editor is leveraged to easily place components from the pSim component library, as well as measurement instruments, to set up the coherent transmitter (Tx) side and coherent receiver (Rx) side.
Link Practice & Simulation: A Simple Three-Step Process
Step 1: According to Latitude Design Automation’s tutorial slides "SiPh IC Design with pSim" or the Chinese version "Silicon Photonics IC Design - Using pSim", set up simulation parameters such as temperature = 300K, wavelength = 1.552524 um, etc.
Step 2: In the schematic editor, select and drag components, and configure parameters according to tables in the slides, such as PRBS_1 sample_rate = 3.2 THz, NRZ_1 amplitude = -2, etc.
Step 3: After connecting components, run the simulation, click on the EYE_1 icon, then right-click to open the viewer. As shown in the pSim simulation results, after 5km fiber transmission, the vertical eye opening is about 265 uW and the horizontal eye opening is about 23 ps.
Conclusion
As the field of optical communications continues to evolve, the increasing global data traffic has highlighted the importance of coherent optical transceivers. As the history of coherent fiber optics shows, although the rapid rise of WDM systems in the late 20th century temporarily redirected the technology momentum, the advent of digital coherent receivers reignited broad interest in this area.
The highlights of this paper are the introduction and application of the PIC Studio tool suite, with particular emphasis on its two powerful tools: pLogic and pSim. The pLogic photonic IC schematic editor enables efficient design and layout of complex optical communication systems while providing an intuitive, user-friendly interface. This editor delivers visual simplicity while retaining the depth required for sophisticated optical communications designs. Users can seamlessly place, integrate, and modify opto-electronic devices, enhancing the overall design experience and optimization of coherent transceiver systems.
The pSim photonic IC circuit simulator is indispensable during the verification and optimization phases. It accurately replicates real-world conditions of these coherent systems and also significantly speeds up analysis turnaround times. Referencing the Latitude Design Automation’s slides, the transmission analysis after fiber link integration demonstrates the accuracy and capabilities of the simulation tool.
The seamless integration of pLogic and pSim in the PIC Studio toolkit enhances the coherent optical communications design process. As researchers and engineers work to meet the ever-growing demands for global data transmission, tools like pLogic and pSim will undoubtedly be at the forefront to enable rapid prototyping, testing, and implementation of next-generation optical communication systems. The benefits of the PIC Studio tool suite are not just incremental but transformative, opening new windows of exploration for optical communications research and development.
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