Photonic W-Band E-Field Sensor and Down-Conversion System

We demonstrate the measurement of W-band radio-frequency (RF) electric fields using a sensor integrated on an electrically-passive fiber-coupled thin-film lithium niobate platform. The sensor consists of three cascaded bow-tie antennas that directly phase-modulate the optical signal. By using a two-tone optical probe, the high-frequency RF signal is down-converted to a much lower intermediate frequency. The measured intermediate frequency of the photonic down-conversion system can be tuned by changing the separation of the two optical tones. In our case, the two optical tones are generated by driving an external phase modulator with a frequency approximately half of the RF signal to be measured, allowing the measurement of RF E-fields from 78-89 GHz while driving the phase modulator at < 44 GHz and keeping the resulting intermediate frequencies < 2 GHz. The E-field measured sensitivity of 6 mV/m/√Hz is limited in practice by the low (-19 dBm) optical power exiting the sensor, which is largely due to the high (32 dB) optical insertion loss of the device. This loss is technical in nature and not due to a fundamental limitation. The experimental results are consistent with derived expressions for the E-field sensitivity.