## 3 Regimes of Operation of DBQW RTD Photo-Detectors
### Oscillating Regime and Optical Control of Microwave Signals  
Optical injection locking of an electrical oscillator can synchronize the frequency and phase of a free-running oscillator to the modulated optical signal. This, is very attractive in applications such as local oscillators embedded in radio base station cells and in optical transmission systems, e.g., optoelectronic clock recovery circuits, because it can lead to much simpler systems and is more suitable for higher bit-rates than a fully electrical circuit. There are two types of optically injection locked oscillators (OILOs): the direct and indirect type. A direct OILO, whose active oscillator device itself is directly illuminated for synchronization is preferred because is much simpler and is more suitable for monolithic integration than an indirect one which needs an external photo-detector. In this section, we investigate the injection locking phenomena of an RTD-PD circuit showing self-sustained oscillations up to 1.41 GHz. The influence of optical injection locking on the quality of the microwave carriers is evaluated and we achieved phase noise quality of the signals that are of interest for many microwave photonics applications, as discussed in detail in Section 4. The RTD-PD based oscillator circuit utilized in the optical injection locking experiments was realized using RTD components attached directly onto the surface of a microwave substrate containing a 50 \(\Omega\) copper microstrip transmission line laminated onto a non-conductive substrate. The transmission lines were realized in microstrip hybrid technology using Rogers substrate with a dielectric constant of 3.48 and a thickness of 0.762 mm. At the end terminals of the 50 \(\Omega\) microstrip line were high frequency SMA connectors which acted as the RF output port. Gold bond wires were employed to connect the RTD-PD emitter and collector to the 50 \(\Omega\) transmission line and to the ground, respectively. The RTD-PD devices were attached to the substrate with a high conductivity adhesive. The orientation of the RTD's optical waveguide was taken into account in the layout and in the chip bonding to PCB to ensure good alignment to an optical fiber. In the oscillator circuit, a 10 \(\Omega\) thin-film resistor, Figure 5, was utilized as part of the bias circuit of the oscillator to suppress the bias circuit spurious oscillations for the RF signals generated by the RTD [14]. The operation frequency of the RTD-PD is essentially determined by the RTD-PD's parallel capacitance and the series equivalent inductance of the wire bonding connections (the wire length was around 4 mm, giving an equivalent inductance around 3.6 nH), which for the circuit reported here gives an operation frequency up to 1.4 GHz. High-frequency characterization of these devices was previously investigated [29] providing detailed analysis of RTD impedance _versus_ frequency dependence for various bias voltage values. For a 45 MHz to 18 GHz frequency range the samples with negative resistance \(R\sim-15\)\(\Omega\) (estimated from the NDR region) showed RTD equivalent capacitance of RTD around 1.5 pF. For these circuit parameters, one can estimate the typical maximum frequency of operation of RTD devices, \(f_{max}\sim\)14.15 GHz. Therefore, the RTD-PD hybrid circuit oscillators employed in our experiments could operate up to 10 GHz reducing circuit parasitics. The circuit diagram and experimental setup are presented schematically in Figure 5 showing the optical transmitter and measurement setups. Optical modulated signals at communications wavelengths were generated by a 10 Gb/s Mach-Zehnder modulator that encode the light (typically \(\lambda\) = 1,550 nm) from a tunable single mode laser. The output from the Mach-Zehnder modulator was amplified by an erbium-doped fiber amplifier (EDFA), and then filtered to eliminate spontaneous emission noise from the EDFA. Then, optical modulated signals illuminated the RTD-PD oscillator waveguide input facet through single mode lensed fiber.