Multilevel Modulation and Transmission in VCSEL-based Short-range Fiber Optic Links

As the demand for ever higher throughput short-range optical links is growing, research and industry associations have shown increased interest in multilevel modulation formats, such as the four leveled pulse amplitude modulation, referred to as 4-PAM. As on-off keying (OOK) persists to be the choice for low latency applications, for example high performance computing, datacenter operators see 4-PAM as the next format to succeed current OOK-based optical interconnects. Throughput can be increased in many ways: parallel links can be deployed, multicore fibers can be used or more efficient modulation formats with digital signal processing is an alternative. Therefore, to improve link data rates, the introduction of new modulation formats and pre-emphasis are primarily considered in this thesis. In a bandwidth-limited link, turning towards spectrally efficient formats is one of the methods to\ua0 overcome the bandwidth requirements of OOK. Such are the considerations when opting for 3-PAM or 4-PAM schemes. Both require lower bandwidth than OOK and are potential candidates in such scenarios. 4-PAM provides double spectral efficiency and double data rate at the same symbol rate as on-off keying, but, as with any technology transition, new challenges emerge, such as a higher SNR requirement, a lower tolerance to VCSEL nonlinearities and skewing of the signal in the time domain. 3-PAM could potentially be an in-between solution, as it requires 33% less bandwidth than OOK and is less sensitive to VCSEL dynamics which could impair the transmission. A study is presented where 3-PAM has outperformed both OOK and 4-PAM in the same link. Detailed investigation of legacy 25G class VCSELs has shown that devices with moderate damping are suitable for the transition to 4-PAM. The pre-emphasis of signals is a powerful tool to increase link bandwidth at the cost of modulation amplitude. This has been investigated in this thesis for on-offkeying and has shown 9% and 27% increase in bit rate for error-free operation with two pre-emphasis approaches. Similarly, pre-emphasis of a 4-PAM electrical signals has enabled 71.8 Gbps transmission back-to-back with lightweight forward error correction and 94 Gbps net data rate was achieved with the same pre-emphasis and post-processing using an offline least-mean-square equalizer

Optoelectronics Enabled Dense Patch Antenna Array for Future 5G Cellular Applications

The interconnection between densely-spaced antenna array elements to separated signal processors is a challenge in practical systems of future 5G applications. We present an interconnect concept based on optoelectronic link and a proof-of-concept experiment demonstrates successful 6-Gbps 64-QAM data transmission

1060 nm VCSELs for long-reach optical interconnects

Reach extension of high capacity optical interconnects based on vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers (MMFs), as needed for large-scale data centers, would benefit from high-speed GaAs-based VCSELs at 1060 nm. At this wavelength, the chromatic dispersion and attenuation of the optical fiber are much reduced in comparison with 850 nm. We present single and multimode 1060 nm VCSELs based on designs derived partly from our high-speed 850 nm VCSEL designs. The single-mode VCSEL, with a modulation bandwidth exceeding 22 GHz, supports back-to-back data rates up to 50 Gbps at 25 \ub0C and 40 Gbps at 85 \ub0C under binary NRZ (OOK) modulation. Using mode-selective launch, we demonstrate error-free 25 Gbps transmission over 1000 m of 1060 nm optimized MMF. Higher data rates and/or longer distances will be possible with equalization, forward-error-correction, and/or multilevel modulation

Short-range Optical Communications using 4-PAM

As the demand for ever higher throughput short-range optical links is growing, research and industry has shown increased interest in multilevel modulation formats, such as the four leveled pulse amplitude modulation, referred to as 4-PAM. As on-off keying persists to be the choice for low latency applications, such as high performance computing, datacenter operators see 4-PAM as the next format to succeed current OOK-based optical interconnects. Throughput can be increased in many ways: parallel links can be deployed, multicore fibers can be used or more efficient modulation formats with digital signal processing is an alternative. Cost- and power efficiency and available physical volume are the main aspects considered when designing and building a new datacenter. Therefore, to improve link data rates, the introduction of new modulation formats are primarily considered for this task. 4-PAM provides double spectral efficiency and double data rate at the same symbol rate as on-off keying, but, as with any technology transition, new challenges emerge.Considerations for this transition is the topic of this thesis. Since the vast ma\ua0jority of optical interconnects use the vertical cavity surface emitting laser as the transmitter and multimode fiber as the transmission medium, simulated and experimental work relied on links with these devices. Many techniques are presented in this thesis to improve and analyze such links.The pre-emphasis of signals is a powerful tool to increase link bandwidth at the cost of modulation amplitude. This has been investigated in this thesis for on-off keying and has shown 9% and 27% increase in bit rate for error-free operation with two pre-emphasis approaches. Similarly, pre-emphasis of a 4-PAM electrical signals has enabled 71.8 Gbps transmission back-to-back with lightweight forward error cor rection and 94 Gbps net data rate was achieved with the same pre-emphasis and post-processing using an offline least-mean-square equalizer.Links using 850 nm vertical cavity surface emitting lasers still dominate today’s 25 Gbps lane rate datacenter interconnect links. Introducing 4-PAM in these links creates new challenges and it is important to know what design considerations are needed during this transition. Detailed investigation of legacy 25G class VCSELs has shown that devices with moderate damping are suitable for 4-PAM operation

Malware Collection and Analysis via Hardware Virtualization

Malware is one of the biggest security threat today and deploying effective defensive solutions requires the collection and rapid analysis of a continuously increasing number of samples. The collection and analysis is greatly complicated by the proliferation of metamorphic malware as the efficacy of signature-based static analysis systems is greatly reduced. While honeypots and dynamic malware analysis has been effectively deployed to combat the problem, significant challenges remain. The rapidly increasing number of malware samples poses a particular challenge as it greatly inflates the cost of the hardware required to process the influx. As modern malware also deploys anti-debugging and obfuscation techniques, the time it takes to formulate effective solutions is further exacerbated. There is a clear need for effective scalability in automated malware collection and analysis. At the same time, modern malware can both detect the monitoring environment and hide in unmonitored corners of the system. It has also been observed that malware modifies its run-time behavior to lead the analysis system astray when it detects a monitoring environment. Consequently, it is critical to create a stealthy environment to hide the presence of the data collection from the external attacker. Such systems also need to isolate critical system components from the executing malware sample while keeping the concurrent collection and analysis sessions separate. Furthermore, the fidelity of the collected data is essential for effective dynamic analysis. As rootkits now employ a variety of techniques to hide their presence on a system, the broader the scope of data collection, the more likely the analysis will reveal useful features. Over the last decade hardware virtualization has been proposed to develop such tools with promising results. In this dissertation we present a systematic evaluation of hardware virtualization as an underlying technology to construct effective malware collection and analysis systems. The evaluation is realized via the combination of four distinct objectives such systems need to fulfill: scalability, stealth, fidelity and isolation

Demonstration of a 71.8 Gbps 4-PAM 850 nm VCSEL-based Link with a Pre-emphasizing Passive Filter

We present 71.8 Gbps 4-PAM back-to-back transmission in an 850 nm VCSEL-based link using a passive pre-emphasis filter and FEC with no post-equalization. Bit error rates below 10 -8 are demonstrated with 72 Gbps uncoded data

Demonstration of a 71.8 Gbps 4-PAM 850 nm VCSEL-based Link with a Pre-emphasizing Passive Filter

We present 71.8 Gbps 4-PAM back-to-back transmission in an 850 nm VCSEL-based link using a passive pre-emphasis filter and FEC with no post-equalization. Bit error rates below 10 -8 are demonstrated with 72 Gbps uncoded data