## 3 Regimes of Operation of DBQW RTD Photo-Detectors
### Oscillating Regime and Optical Control of Microwave Signals
The free running oscillation frequency of the RTD-PD oscillator was controlled by adjusting the bias voltage provided by a power supply and through an external bias tee. Optically injection locked signals were measured in the electrical output of the circuit using a high frequency oscilloscope and spectrum analyzer. In the following, we study the injection locking phenomena of an RTD-PD circuit showing an oscillation ranging from 1.04 GHz to 1.41 GHz as the DC bias is swept across the NDC region with voltage range \(\sim\)0.4 V. The coupling of optical modulated signals to the RTD-PD waveguide locks the self-oscillations to the optically injected RF sub-carrier for in-fiber optical powers above a given threshold that depends on the waveguide characteristics, on the frequency and the RF power of the modulated light, and on the RF power of the unperturbed self-oscillations. To investigate the influence of optical injection locking on the quality of the RF carriers, signals were observed at the output RF port of the RTD-PD oscillator. The oscillator was considered injection-locked when its output spectrum matched the optical modulation reference source frequency with stable oscillation and minimum noise. The optical injection locking investigation included measurements of a RTD-PD circuit self-oscillating at around 1.405 GHz. The circuit showed a substantial improved performance in terms of injection locking and spectral purity. The results are summarized in Figure 6. Figure 6a presents the typical broad spectrum of free-running oscillation (light off) around 1.405 GHz. When the light is on, with an average optical power level in fiber of 1 mW and employing a modulation extinction ratio close to 10 dB, the free-running signal is pulled and the spectrum collapses showing a substantial phase noise reduction. The inset of Figure 6a (top) presents the recorded spectrum surveillance confirming the spectral long-term stability of the optical injection locking. Injection locking for optical power levels in fiber as low as 0.2 mW was observed. The locking range was 23.8 MHz for 4.9 mW optical power in fiber. This corresponds to an exceptional wide value with a fractional locking  
Figure 5: RTD-PD oscillator circuit schematic, and setup of the optical injection locking experiment. bandwidth of 1.7%. The observed locking range corresponds to the largest value reported in the literature for RTD-based devices at communication wavelengths ([12] and references therein). Since injection-locking leads to a significant phase-noise and jitter reduction the respective oscillator performance has been further investigated. Figure 6b presents the phase noise measurements showing phase noise of \(-112.4\) dB/Hz at 10 kHz offset of the carrier frequency, a very close match to the RF source (Agilent E8257D) signal for the moderate optical power levels employed (below 5 mW), which results in a noise suppression of more than 40 dB when compared with the free-running oscillator without illumination. For the infiber optical power locking threshold the phase noise was \(-87.1\) dBc/Hz at 10 kHz offset of the carrier frequency. The results, although obtained using devices with small responsivities when compared with commercial available photo-detectors, show the RTD-PD can operate as a stable optically controlled microwave photonics oscillator to obtain a highly efficient O/E conversion. The RTD-PD phase noise characteristics should be sufficient for many current microwave photonic applications that use phase modulated schemes, e.g., phase shift keying (PSK) data transmission (as discussed in the next section), as long as the locking conditions are achieved and maintained.