# A Photonically-Excited Leaky-Wave Antenna Array at E-Band for 1-D Beam Steering
## 4 Measurements and Results
### Photonic Measurements  
The antenna integrated with the UTC-PD, as described in Section 3, was used in a wireless link, as well as to measure the output power, using one active UTC-PD. The link set-up is shown in Figure 9a. A standard photonic approach [2; 26] was used to generate and modulate two optical tones of two distributed feedback lasers (Toptica Photonics DFB pro BFY), with a spacing in frequency equal to that of the mm-wave carrier. After modulation using on-off keying (OOK), the signal was amplified (EDFA), monitored in an optical spectrum analyzer (OSA), and coupled to the UTC-PD input optical dielectric waveguide by a lensed fiber (OZ Optics). Subsequently, a QQ system is employed to collimate the radiation. Note that the antenna radiates perpendicularly to the PCB plane (Figure 9b). Hence, it is placed at the focal point of a parabolic mirror that collimates radiation horizontally, as is most convenient for the QQ link. On the receiving side, a 10 cm diameter Teflon lens focuses the radiation onto a Zero bias Schottky Barrier Diode detector (ZSBD) [27]. After the mm-wave carrier is detected, the signal, in baseband, is filtered and amplified (LNA amplifier + limiting amplifier). Finally, the detected signal is compared to the original using a BER tester (Anritsu MP2100A). The link parameters are summarized in Table 1. Figure 9c shows the corresponding eye-diagram for error-free transmission at 2.15 Gbps. At higher data-rates, variations of the antenna phase center with frequency create jitter, which in turn distorts the communication. Finally, Figure 9d shows a photograph of the assembled prototype. \begin{table}
\begin{tabular}{l c} \hline
**Parameter** & **Value** \\ \hline Data rate & 2.15 Gbps \\ RF carrier & 80 GHz \\ Modulation & OOK \\ Link distance & 25 cm \\ Bit error rate & \(<\)10\({}^{-12}\) \\ Photocurrent & 7.0 mA \\ \hline \end{tabular}
\end{table} 
Table 1: Wireless link with one active PD: summary of main parameters. Figure 8: (**a**) Normalized co-polar directivity at H-plane at different frequencies for the two-row prototype adapted for probe measurements. Continuous line: simulation, dashed line: measurements. (**b**) Simulated and measured peak gain. Power measurements of the photonic prototype with a UTC-PD excited were performed on a similar set-up as the transmission link. The Teflon lens, ZSBD, and receiving chain were replaced by a THz absolute power meter (Thomas Keating Instruments). Additionally, the modulator was substituted by a fiber-coupled free-space collimator plus a chopper to enable lock-in detection. Since the broadband detector presented a large window in which pressure variations due to thermal heating were measured, external stimuli could alter measurements. Therefore, the dependence \(P\propto l^{2}\) was checked before power measurements, \(P\) being the detected power and \(I\) the DC photocurrent. This is of utmost importance if an end-fire antenna is measured. In this case, some light, which was chopped at the same lock-in frequency, could couple and destroy the measurements. In this case, however, the previous relationship would be linear and it could be easily noted. Figure 10 shows the frequency dependence  
Figure 9: (**a**) Schematic of the set-up utilized for the wireless link (PC= polarization controller, BP= bandpass, LNA= low noise amplifier). (**b**) Simplified QO system, (**c**) eye diagram for the error-free transmission at 2.15 Gbps, and (**d**) photograph of the prototype. of the detected power. It featured a 3 dB bandwidth of 11 GHz (74 GHz to 85 GHz), with a maximum of 120 uW at 80.5 GHz, for 45 mW input optical power at -2.5 V bias voltage (9.8 mA photocurrent). This was in agreement with the photodiode output power [25] when bonding losses (4-5 dB) and antenna radiation efficiency (77%) were accounted for. For frequencies out of the operation band, the mirror did not capture all the emitted radiation, emitted largely off-boresight, and the detected power decreased.