PROGRAM | Electrical & Computer Engineering

By: Fuquan Wang Chair: Dennis Prather

ABSTRACT

Analog photonic links are of great importance in future high data-rate telecommunications. However, their performance is limited by the nonlinearities arising from the link components, such as lasers, optical fibers, modulator, optical amplifiers, and photodiodes. In this dissertation, the nonlinearities of these components and corresponding linearization techniques are discussed and investigated.

Firstly, an arcsine-predistortion technique is used in a dual-electrode Mach-Zehnder modulator (DE-MZM) to demonstrate a third-order distortion (IMD3) suppressed link. 5 MHz and 20 MHz Long-term Evolution (LTE) signals are digitally predistorted and delivered by the DE-MZM link. The recovered radio frequency signal on the photodiode maintains high fidelity compared to the original signal in terms of adjacent channel leakage ratio (ACLR). Later, this arcsine-predistortion technique is used in a photonic transceiver where tunable optical paired sources and single sideband modulation are used. LTE signals are recovered on the photodiode with high fidelity at a carrier frequency up to 17 GHz.

Besides the digital predistortion, optically coherent and incoherent modulator linearization techniques are also investigated. Two optically incoherent modulator linearization techniques are proposed: dual-wavelength linearization and dual-polarization linearization.

In the dual-wavelength linearization approach, two lasers are used to drive two parallel MZMs. A primary MZM is used to deliver data while a secondary MZM is used to cancel the IMD3 generated by the primary MZM. The overall IMD3 can be fully suppressed by tuning RF/optical power ratio and maintaining 180-degree phase between the two parallel MZMs. LTE signals of various bandwidths are delivered using this link, with the LTE signals recovered at a modified uni-traveling carrier (MUTC) photodiode. Compared to a conventional single-MZM link, ACLR1 is improved by 9-19 dB in this linearized link for LTE signals with 10-97 MHz bandwidth at the cost of 2 dB power penalty.

In the dual-polarization approach, a laser is used to drive two parallel MZMs while the two outputs are orthogonally coupled into a polarization-maintain fiber with the help of a polarization beam combiner. Similar to the two-wavelength linearization technique, the IMD3 can be fully suppressed by controlling RF/optical power ratio and maintaining 180-degree phase between the two parallel MZMs. Using a two-tone test method, the link is characterized in terms of gain, noise figure, third-order intercept point (OIP3), and spurious free dynamic range (SFDR) which are compared to a conventional intensity-modulation direct-detection (IMDD) link. Again, LTE signals with 10-97 MHz bandwidth are tested, where 11.8~19.6 dB ACLR1 improvement is achieved at the cost of 2 dB power penalty.

In addition to those modulator linearization techniques, different approaches to measure the photodiode nonlinearity are discussed, including the most accurate three-tone test using external modulation scheme. The proposed three-tone system is analyzed and simulated, and the IMD3s generated by two-tone mix and three-tone mix are compared. A three-tone photodiode nonlinearity measurement setup capable of measure OIP3 up to 20 GHz is built and tested. Its accuracy is verified by testing the same photodiodes on two three-tone systems. Lastly, the dependencies of photodiode OIP3 on photocurrent, frequency, and bias voltage is investigated.

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