In this manuscript, we report on a 1 × 4 optical beam forming network (OBFN) chip using optical phase shifters (OPSs) based on thermo-optically controlled optical ring resonators (ORRs) for 1D beam steering at 0.3 THz. The 1 × 4 OBFN chip consists of four OPSs and is fabricated using TriPleX technology. Each of the four OPSs is realized by two cascaded identical ORRs, to reach a phase shift of 2π. To allow transfer of the optical phase shift to the THz domain by optical heterodyning in high-frequency 1.55 μm modified uni-travelling carrier photodiodes, the ORRs are designed such that one carrier of the optical heterodyne signal is at the ORR’s resonance frequency, whereas the
second optical heterodyne signal is at its off-resonance. By adjusting the resonance frequencies of the two ORRs in each OPS synchronously, a relative phase variation between two optical heterodyne carriers of up to 2πwith a tuning efficiency of 0.058 W/π, is experimentally demonstrated. Due to the dispersive power transmission loss of the ORRs, phase tuning leads to a power variation of the optical heterodyne-generated signals up to 3.8 dB, which is experimentally characterized at 0.295 THz. It is shown numerically that this power variation only has a minor impact on the steering performance of a 1 × 4 phased array. The determined beam direction deviation and maximum absolute radiation power change are smaller than 1◦ and 2 dB, respectively. By sweeping the phase difference between two adjacent THz antennas in the 1 × 4 phased array, from −120◦ to 120◦, a beam steering range of ~62◦ is demonstrated numerically at 0.295 THz.