W-band radio transmission enabled by a wideband reconfigurable photonic oscillator based on two phase-locked lasers

The increasing requirements in terms of data rate for radio communication links are driving a technological migration towards the use of mm-wave frequency carriers. Very high carrier frequencies up to the sub-THz range can be efficiently obtained through microwave-photonics techniques, avoiding the use of high-frequency and high-performance electrical oscillators, together with the additional functionalities of frequency reconfigurability and electromagnetic interference-free and remote distribution of the signal through either fiber-optics or free-space optics links. Here, the suitability of the performance of a wideband reconfigurable photonic oscillator (WRPO), i.e., spectral purity and stability of its output signal, has been verified for supporting analog up-conversion of GHz-rate complex data signals in a W-band radio transmission testbed. The mm-band oscillator output can be derived by heterodyning into a photodiode two mutually phase-locked narrow-linewidth lasers, with such phase locking obtained through an electronic locking system. Due to the limited WRPO operation frequency (≤ 40 GHz), the device output frequency has been multiplied through electro-optic modulation to reach the target W-band carrier frequency (88 GHz in this demonstration). By adding a second electro-optic modulation step, up-conversion of complex data has been implemented, and both phase noise and error vector magnitude (EVM) measurements attest to the suitability of the solution for next generation 6G mobile networks, demonstrating that better performance is achieved for lower frequency multiplication factor, as expected by theory. An additional performance comparison with respect to a previously demonstrated optoelectronic solution based on the use of a lower-frequency (∼ 20 GHz) RF local oscillator and consequent optical frequency comb generation is made and discussed, based on EVM measurement.