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Analog and digital radio-over-fiber (RoF) systems find a wide variety of applications in civilian, aerospace and tactical defense communication systems. Fiber as an information transport medium provides access to an unregulated spectrum, large information carrying capacity, information security and immunity to radio frequency (RF) interference. RoF is also considered as one of the potential backbone solutions in 5G wireless deployments.
Intermodulation distortion (IMD) arising from nonlinearities in transmitter is one of the key factors adversely affecting performance of radio-over-fiber (RoF) microwave photonic links. A number of studies and experimental results have been reported in literature to alleviate this problem [1-3]. Simulations is a key to technology selection.
This application case study is based on the transmitter architecture proposed in . The architecture is based on using dual-drive, dual-parallel Mach-Zehnder modulator (MZM) in OptSim. With appropriate bias, third-order intermodulation distortion (IMD3), a key impairment in RoF systems, is eliminated. For comparison, a design with traditional, MZM-based transmitter is also simulated.
The OptSim schematic is shown in Figure 1.
Figure 1. Schematic of an RoF transmitter using traditional (upper) and proposed in Ref.  (lower) approaches.
The topology comprises of a two-tone analog transmitter with individual ones at 12GHz and 11.9GHz. The upper part of the schematic uses a traditional, MZM-based transmitter biased at quadrature. The lower part of the schematic implements a dual-drive dual-parallel pair of MZM, one biased at null, and the other at quadrature as described in . A direct-detection back-to-back receiver is used at each transmitter to observe RF spectra and IMD components.
The mathematical details on the phase control circuits and biasing setup, although not too complex, are beyond the scope of this application note, and the user is encouraged to refer to  for more information.
Figure 2 shows outputs at the MZM biased at null (left) and at quadrature (right).
Figure 2. Output spectra at the modulator biased at null (left) and at quadrature (right).
The combiner combines outputs from both the MZMs and transmits the combined signal. The photodetector at the receiving end converts optical signal to electrical signal adding electrical noise to the mix arising from photodetection process. The RF tones are observed at the receivers for both the conventional transmission and for the transmission based in .
As shown in Figure 3, the third-order intermodulation (IMD3) is present in case of conventional transmission (top), while it is eliminated in the proposed architecture (bottom).
Figure 3. RF spectra and detected tones for (a) conventional transmitter and (b) proposed transmitter.
The case study demonstrated implementation of a dual-drive, dual-parallel Mach-Zehnder modulator (MZM) based transmitter proposed in . A comparison was also made with conventional transmitter. The detected RF spectra showed suppression of IMD3 for the former consistent with the analyses of . For more information and to request a demo, please contact firstname.lastname@example.org
Jigesh K. Patel, Product Manager
1. C. Middleton and R. DeSalvo, “Improved microwave photonic link performance through optical carrier suppression and balanced coherent heterodyne detection,” Proceedings of SPIE - Enabling Photonics Technologies for Defense, Security, and Aerospace Applications VI, vol. 7700, p. 7700-08, 2010
2. Tae-Sik Cho, at al., “Effect of third-order intermodulation on radio-over-fiber systems by a dual-electrode Mach-Zehnder modulator with ODSB and OSSB signals,” Journal of Lightwave Technology, vo. 23, no. 5, May 2006, pg. 2052-2058
3. Caiqin We, Xiupu Zhang, “Impact of nonlinear distortion in radio over fiber systems with single-sideband and tandem single-sideband subcarrier modulations,” Journal of Lightwave Technology, vo. 24, no. 5, May 2006, pg. 2076-2090
4. Jian Li, et al., “Third-order intermodulation distortion elimination of microwave photonic link based on integrated dual-drive dual-parallel Mach-Zehnder modulator,” Optics Express, vo. 38, no. 21, Nov. 2013, pg. 4285-4287