Linearization of analog photonic links
Date
2019
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Analog photonic links are of great importance in future high data-rate telecommunications. However, their performance has been limited by the nonlinearities arising from the link components, such as lasers, optical fibers, modulators, optical amplifiers, and photodetectors. In this dissertation, the nonlinearities of these components, and corresponding linearization techniques are discussed and investigated. ☐ First, an arcsine-predistortion technique is used in a dual-electrode Mach-Zehnder modulator (DE-MZM) to demonstrate a link with suppressed third-order intermodulation distortion (IMD3). As examples, telecommunications industry standard 5 MHz and 20 MHz Long-term Evolution (LTE) signals are digitally predistorted and delivered by the DE-MZM link. The recovered radio frequency (RF) signal on the photodetector maintains high fidelity compared to the original signal, as quantified in terms of adjacent channel leakage ratio (ACLR). Later, this arcsine-predistortion technique is implemented in a photonic transceiver based on tunable optical paired sources (TOPS) and single sideband (SSB) modulation. LTE signals are recovered with high fidelity at a carrier frequency up to 17 GHz. ☐ Besides 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. Compared to a conventional single-MZM link, ACLR 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 single laser is used to drive two parallel MZMs while the two outputs are orthogonally coupled into a polarization-maintaining fiber with the help of a polarization beam combiner (PBC). 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 standard two-tone test method, the link is characterized in terms of conventional key link performance parameters, namely gain, noise figure, third-order output intercept point (OIP3), and spur-free dynamic range (SFDR), and compared to a conventional photonic link based on intensity-modulation direct-detection (IMDD). Again, LTE signals with 10-97 MHz bandwidth are tested, where 11.8~19.6 dB ACLR improvement is achieved at the cost of 2 dB power penalty. While the performance results are comparable to the dual-wavelength approach, this technology uses only single laser and is thus, easier to implement. ☐ In addition to those modulator linearization techniques, various approaches to measure the photodetector nonlinearity are discussed, including a preferred three-tone test scheme using external modulation. The proposed three-tone system is analyzed and simulated, and the predicted IMD3 levels generated by two-tone mixing and three-tone mixing are compared. A three-tone photodetector nonlinearity measurement setup capable of measuring OIP3 up to 20 GHz is built and tested. Lastly, the dependence of photodetector OIP3 on photocurrent, frequency, and bias voltage is investigated.