## 4 Applications in Clock Recovery and Fiber Optic Supported Radio Frequency Communications
### Clock Recovery  
The ability to recover the clock signal from incoming random data is a fundamental operation in digital communications. Clock recovery consists in deriving a clock signal from the received signal which is synchronized in both frequency and phase, avoiding its transmission along with the data, and providing a timing basis to sample the received signal. In this context, the ability to perform clock recovery using optical techniques is an important issue in the development of circuits for novel high speed optical communication systems. RTD-PD oscillators are candidates for that purpose since they combine the functions of microwave oscillation and photo-detection, while offering intrinsic gain and high responsivities, being suitable for injection locking with incoming data in both optical and electrical domains. In what follows we present results on a clock recovery experiment using optical injection (O/E conversion). Clock recovery via optical injection is important because it enables the clock of a high-speed data signal in a fiber-optic system to be directly recovered without first converting the data to electrical pulses. In this experiment the free running RTD-PD oscillator is tuned to oscillate at a frequency sufficiently close to the incoming signal. The locking range is typically on the order of tens of MHz, and depends on the power of the injected clock frequency. The incoming optical signal is injected into the oscillator and the recovered signal is accessed at the electrical output port. Figure 7 shows the experimental schematic for timing extraction and bit-error-rate (BER) measurements. The performance of the recovered clock was assessed from timing jitter, phase noise and locking bandwidth. Additionally, BER measurements were carried out using the recovered clock as a time base for sampling of the received data. Clock recovery and error free timing extraction results are demonstrated using return-to-zero (RZ) format data signals that optically injection locks the free-running RTD-PD oscillator. A 1.24 Gb/s RZ signal generated from a bit-error-rate test (BERT) transmitter, whose spectrum together with the respective eye diagram are shown in Figure 7b, was used to modulate a tunable laser diode optical carrier at 1.550 nm using a 10 Gb/s modulator that then fed an EDFA which output was subsequently split in a 50/50 coupler. One of the coupler outputs went to a commercial photo-receiver whose output  
Figure 7: **(a)** Experimental schematic for timing extraction and BER measurements via optical injection locking. **(b)** RZ data stream input of 1.24 Gb/s with 100 mV amplitude (V\({}_{\text{P-P}}\)) and 50% duty cycle using a PRBS of \(2^{7}-1\); (top): eye diagram of RZ data; (bottom): RZ data signal measured in the frequency domain. was connected to the error analyzer. The other part of the optical signal was coupled to the RTD-PD optical input, Figure 7a. Figure 8a shows the frequency spectrum of the data after clock recovery via optical injection of a 3 mW in fiber optical power signal, showing a large frequency component at the clock frequency. Figure 8b presents the power spectra of the free-running oscillation slightly above 1.24 GHz and the injection locked output at 1.24 GHz. The output spectrum was clearly pulled to 1.24 GHz in the presence of the optical injected signal. The inset of Figure 8b presents also the injection locked RTD-PD output signal in time domain. A phase noise reduction of around 24 dB at 10 kHz offset from the carrier frequency was achieved for an optical injection power level in fiber of around 6 mW, Figure 8a. The minimum optical power in the fiber to obtain locking was around 1.1 mW with a locking range of 0.3 MHz. The locking range and the phase noise reduction were strongly dependent on the power of the optically injected signal. The phase noise was considerably reduced as shown in Figure 9a. The RMS jitter measured by the oscilloscope was 2.16 ps, which also matches well with that from phase noise measurement.