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Digital Linearization and Wideband Measurements in Optical Links

  • Author(s): Lam, Daniel
  • Advisor(s): Jalali, Bahram
  • Madni, Asad M
  • et al.
Abstract

Optical fiber networks have been in use for many decades to transport large amounts of data across long distances. Internet traffic grows at an exponential rate and demand for increased bandwidth and faster data rates is higher than ever. Radio frequency over fiber is used for a plethora of applications such as providing wireless access to remote and rural areas, phased array radars, and cable television to name a few. As signals are transmitted over longer distances, nonlinearities are incurred which degrades the performance and sensitivity of the link. Moreover as the data rates increase, it becomes a challenge to measure and monitor the signal integrity.

This dissertation will cover two main topics: digital broadband linearization and performing wideband high speed measurements using time-stretch technology. Over the last few years there has been considerable interest in reducing the intermodulation distortions in optical links. The intermodulation distortions are caused by the nonlinear transfer function of the optical link. To reduce the nonlinearities, linearization of the optical link is performed. A novel digital post-processing algorithm has been developed to suppress nonlinearities and increase the dynamic range of the link. Digital broadband linearization algorithm has been implemented and demonstrated a record 120 dB.Hz2/3 Spurious Free Dynamic Range (SFDR) over 6 GHz of bandwidth and is shown to suppress third order intermodulation products by 35 dB. By reducing the nonlinearities and improving SFDR, we have increased the sensitivity of the receiver. Afterwards, simulation of the real-time implementation of the digital broadband linearization algorithm onto a field-programmable gate array was performed by designing the architecture and translating the code into Verilog HDL. Simulations on collected data show comparable results in both Matlab and iSim which were used to evaluate the performance.

In the second part of this dissertation, two applications using time-stretch are demonstrated: ultra-wideband instantaneous frequency estimation and high speed signal analysis measurements. By combining time-stretch technology and windowing and quadratic interpolation, ultra-wideband frequency measurements with improved frequency estimation are demonstrated. Moreover, multiple signal measurements are performed, and the frequency resolution can be tuned to measure signals close together. Lastly, time-stretch is used for measuring high speed signal integrity parameters such as bit error rate, jitter, and rise and fall times by taking advantage of the high sampling throughput and the ability to generate and analyze eye diagrams. In addition, we were able to integrate this technology into a test-bed for aggregate optical networks and use it for an optical performance monitoring application.

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