High-speed transmission system using directly modulated laser and multi-level modulation format

Abstract

As the data traffic keeps increasing in an exponential manner, fiber-optic transmission systems are evolving in a way that accommodates more wavelength-division-multiplexed channels as well as the increased data rate per channel. Current short- and intermediate-haul fiber-optic transmission systems are largely dominated by an intensity modulation/direct detection system operating at 10 or 25 Gb/s per lane or channel due to its simplicity and cost-effectiveness (in comparison with, for example, the coherent system). The transmitters of these systems are implemented using external modulation or direct modulation of laser diode. However, it is highly desirable to utilize directly modulated lasers (DMLs) because they can provide high output power, low power consumption, small footprint, and low implementation cost. Major technical challenges associated with the use of DMLs for high-speed transmission include the limited modulation bandwidth of DMLs and the waveform distortions arising from the DML’s frequency chirp when it is combined with fiber chromatic dispersion. In this thesis, I explore the possibility of 28-Gb/s transmission utiliz-ing a 1.55-μm DML developed for 10G applications. In order to accommodate 28-Gb/s signals over the limited modulation bandwidth of DML, I attempt to exploit the bandwidth-efficient multi-level modulation formats including the duobinary and 4-level pulse amplitude modulation (PAM-4). I optimize the signal’s extinction ratio and employ the post-detection electrical equalization to maximize the transmission distance without using costly optical dispersion compensation modules. I first compare through experiment two different types of duobinary encoders, a low-pass filter (LPF) and a delay-and-add circuit. I find out that the LPF-based duobinary encoder outperforms the delay-and-add circuit in terms of receiver sensitivity, implementation complexity, and loss. Then I carry out the 28-Gb/s duobinary transmission experiment by using the DML and LPF-based duobinary encoder. A PIN receiver developed for 10G application is utilized to further enhance the cost-effectiveness of the system. I optimize the extinction ratio of the 28-Gb/s signal to maximize the transmission distance limited by the waveform distortions caused by the interplay between the laser’s chirp and fiber dispersion. Also I employ the electrical equalization at the receiver for the compensation of these waveform distortions. The results show that I can transmit the signal over 50-km standard single-mode fiber (SSMF) using a 14-tap feed-forward equalizer (FFE) followed by a 5-tap decision-feedback equalizer (DFE). I also transmit the 28-Gb/s PAM-4 signals by using the DML and PIN receiver. Similar to our previous experimental results obtained by using the duobinary signal, I can transmit the 28-Gb/s PAM-4 signal over 50-km SSMF using the FFE and DFE. Our experimental comparison between the duobinary and PAM-4 transmissions using the DML shows that the PAM-4 system marginally outperforms the duobinary system in terms of receiver sensitivity. However, the PAM-4 encoder, which is typically implemented using either a 2-bit digital-to-analog converter or a power combiner with one of its input 6-dB attenuated, would be more complicated and lossy than the duobinary encoder (i.e., LPF). Thus, the duobinary transmitter based on the DML would be better suited for the cost-effective implementation of 28-Gb/s transmission systems for the applications of optical access net-works, inter-datacenter networks, and mobile front/backhaul networks.