PhD defence of Md Samiul Alam – Subsystems and Systems for Direct Detect Optical Fiber Transmission

Abstract

In this thesis, we explore several direct detect system architectures and necessary digital signal processing (DSP) algorithm to either extend the transmission speed and reach or lower the cost and complexity. This thesis can be divided into three parts. In the first part, we focus on high throughput data transmission over 500 m to 2 km of standard single-mode fiber (SSMF) enabled by Mach Zehnder modulators (MZMs). Utilizing high order pulse amplitude modulation (PAM) format and linear and nonlinear DSP algorithm, we demonstrate a record net 300 Gbps/λ IM/DD transmission with an O-band silicon photonic (SiP) Mach-Zehnder modulator (MZM). Although SiP offer a large-scale, low-cost solution, further performance improvement is gated by the inherent bandwidth and phase shifting efficiency limitations of these modulators. To push this capacity limit, we employ a TFLN MZM that enables net 350 Gbps/λ transmission with PAM8 signaling.

Transmission in the C-band with direct detection suffers from chromatic dispersion (CD) induced power fading, which limits the transmission reach for high symbol rate signals. In the second part of the thesis, we explore optical domain solutions to tackle this CD. We demonstrate the transmission of 60 Gbaud PAM-4 signal with polarization multiplexing (net 224 Gbps) over 10 km of SSMF in the C-band using Stokes vector receiver and an integrated ring resonator based optical dispersion compensator (ODC). We use a chirp free LiNbO₃ MZM to test the performance of the ODC. As compared to externally modulated MZM, directly modulated lasers (DML) show strong frequency chirp but are attractive for their cost and power efficiency. We study the chirp-CD interaction of DML analytically over positive and negative dispersion regime and through extensive simulation show the benefit and choice of optical filtering to enhance the transmission performance. As a next step, we experimentally verify the advantages of optical filtering in a DSP free DML/DD system and transmit 32 Gbps PAM4 signal over a CD range of -170 ps/nm to +340 ps/nm. With the aid of a proper optical filter and optimized DSP, we further extend the symbol rate to 35 Gbaud over 20 km of SSMF.

In the last part of the thesis, we try to push the transmission reach to 40 km in the C-band by adopting single-sideband (SSB) scheme with MZM, which is theoretically immune to power fading. However, creating an ideal SSB signal is complex and costly; and efficient solution is needed. We demonstrate that a skew between the differential driving signals in a dual-drive (DD) MZM based SSB transmitter can significantly relax the optical filtering requirement, reducing the cost. This enables net 200 Gbps/λ signal transmission over 40 km with a second order super gaussian optical filter. Overall, the proposed SSB scheme provides an innovative solution to enable high speed data transmission over 40 km of SSMF with direct detection in the C-band.