# High-Speed Photodetectors for Microwave Photonics
## 3 Integrated Photodiode-Antenna Emitters
### Photonic Emitter at 100 GHz  
For the W-band integrated photodiode-antenna emitter, a high-power MUTC PD with a bandwidth of 110 GHz was used [12]. Due to the high attenuation of millimeter-wave radiation in the atmosphere, an antenna with high directional gain is desired. Commercial horn antennas and Si lenses are typically used for this purpose. However, they are bulky and incompatible with large system integration. In ref. [23], a planar Vivaldi antenna with high directional gain of 5.5 dBi was used to be integrated with the MUTC PD.  
In order to achieve high radiation power, the transition between the PD and the antenna needs to be carefully designed. It has been shown that impedance matching has a significant effect on the radiation power of integrated photonic emitters [24] and that maximum RF power can be extracted from the PD by conjugate impedance matching. In [25], a matching network was used to achieve conjugate matching between the MUTC PD and the Vivaldi antenna.  
Figure 7 shows the integrated photonic emitter after the PD die was flip-chip bonded onto the antenna with a matching network on AlN.  
Figure 5: Experimental setup for measuring the radiation power at V-band. EDFA: erbium doped fiber amplifier, Att.: optical attenuator.  
Figure 6: (**a**) Measured and simulated received RF power from the photonic emitter at 5 V bias voltage and 15 mA photocurrent. The inset shows the circuit model used to simulate the RF power. (**b**) RF received power versus photocurrent at 5 V bias and 60 GHz frequency.  
In order to suppress distortions of the radiation pattern due to substrate modes, an AlN superstrate was placed on top of the antenna [25]. The scattering parameter S11 of 5-, 6-, and 14-\(\upmu\)m-diameter PDs were measured and are shown in Figure 8 after de-embedding the RF pads. Also shown in Figure 8 is the S11 of the Vivaldi antenna with a matching network. Owing to the optimized matching network, the input impedance of the Vivaldi antenna is transformed close to the conjugate impedance of the 5- and 6-\(\upmu\)m-diameter PDs, while it is far from the conjugate matching condition of the 14-\(\upmu\)m-diameter PD.  
The E-plane radiation power of the integrated photonic antenna was characterized in the far field using the setup shown in Figure 9.  
Devices with 5-, 6-, and 14-\(\upmu\)m-diameter PDs were measured and their EIRP from 95 to 110 GHz are shown in Figure 10a. Photonic emitters with 5- and 6-\(\upmu\)m-diameter PDs  
Figure 8: Scattering parameters of 5-, 6-, and 14-\(\upmu\)m-diameter PDs and the Vivaldi antenna with a matching network.  
Figure 7: (**a**) Optical image and (**b**) 3-D schematic of the integrated photonic emitter.  
Figure 9: Experimental setup for measuring the radiation power of the photonic emitter at W-band.  
Figure 8: Scattering parameters of 5-, 6-, and 14-\(\upmu\)m-diameter PDs and the Vivaldi antenna with a matching network.  
the photonic emitter with the 14-\(\upmu\)m-diameter PD, which can be explained by the larger PD junction capacitance and poor impedance matching. The photonic emitter with 5-\(\upmu\)m-diameter PD reached 5 dBm at 110 GHz, and the -6-dB bandwidth for all three photonic emitters was 10 GHz. The fact that the EIRP varies across the measured frequency range might result from the varying impedance matching condition. The EIRP at 100 GHz as a function of photocurrent is shown in Figure 10b.