## 4 Applications in Clock Recovery and Fiber Optic Supported Radio Frequency Communications
### Fiber Optic Supported Radio Frequency Communications  
The demand in the coverage and speed of optical communications networks, together with rapid growth in wireless networks, has motivated an increasing attention into hybrid wireless-photonics communication systems. The integration and transmission of RF signals over fiber, RoF systems, are expected to play an important role in broadband wireless communication systems. Taking advantage of optical fiber low loss and wide bandwidth transmission characteristics, it is possible to distribute broadband data and/or high frequency signals to many wireless subscribers. Among several approaches for realizing fiber optic supported radio frequency communication systems, many efforts has been dedicated to providing low cost and simple radio access points bringing the fixed network bandwidths closer to the mobile users [3,4]. Digital standards based on phase modulation, such as Gaussian minimum shift keying (GMSK), quadrature phase shift keying (QPSK), and quadrature amplitude modulation (QAM) are the basis for communications through the latest generation of mobile telephones and wireless local area networks (LAN). In this subsection, results on the transmission of digital modulated signals are demonstrated using a wireless/photonics arrangement that take advantage of the optical injection locking characteristics of RTD-PD oscillator circuits, and the error vector magnitude (EVM) performance of the RTD-PD for different phase and frequency modulation formats are evaluated, namely phase shift keying (PSK), Gaussian minimum shift keying (GMSK) and frequency shift keying (FSK). The investigation presented here allows us to assess the implementation of RTD-PD-based oscillator transceivers as simple and compact low phase-noise oscillators for optical/electrical signal processing units in fiber optic supported radio frequency communications and cellular network base stations. Although in this work we operate at 1-2 GHz frequencies partly because the signals are easier to analyze and the data rates easier to measure with the equipment available in our laboratories, the principles we present could be replicated at higher frequencies [18]. Figure 9: (**a**) SSB phase noise of free-running and recovered clock signals. (**b**) BER measurements of recovered and direct clocks. The setup of an optical-RoF downlink is shown schematically in Figure 10. The vector signal generator (VSG) signal was used to modulate an optical carrier generated by a tunable laser diode at 1,550 nm using a 10 Gb/s Mach-Zehnder modulator. The modulated optical signal was subsequently amplified by an EDFA, then fed to a variable optical attenuator, and then split in a 90/10 coupler. One of the coupler outputs went to an optical multimeter used for controlling the power level. The other part of the optical signal was coupled to the RTD-PD ridge waveguide, which locks to the carrier modulated on the incoming optical signal and converts it to the electrical domain, with responsivity around 0.25 A/W. Finally, a power supply provides the necessary DC biasing and a spectrum analyzer downconverts the received signal to be demodulated by the Agilent 89600 VSA software. The locking range was measured as a function of the injected optical carrier power. It was found to be in average {2, 4.2, 6.5, 9} MHz, for injected optical powers of {\(-\)0.5, 3.5, 6.5, 9.5} dBm, respectively. The power of the electrical carrier on the VSG was kept at 17 dBm, and we verified that the locking range also depends significantly on this parameter. The results show that the higher the power of the modulating electrical carrier or the injected optical carrier the larger the locking range. The measured electrical output power of the RTD-PD was \(-\)7.9 dBm. The measured performance for different modulation formats is shown in Figure 11, where the EVM was obtained as a function of the power of the electrical carrier modulation, using a data-rate of 2 Msymbols/s, for a fixed injected optical power of 9.5 dBm.