  Optical beamforming networks (OBFNs) based on optical true time delay lines
(OTTDLs) are well-known as the promising candidate to solve the bandwidth
limitation of traditional electronic phased array antennas (PAAs) due to beam
squinting. Here we report the first monolithic 1x8 microwave photonic
beamformer based on switchable OTTDLs on the silicon-on-insulator platform. The
chip consists of a modulator, an eight-channel OBFN, and 8 photodetectors,
which includes hundreds of active and passive components in total. It has a
wide operating bandwidth from 8 to 18 GHz, which is almost two orders larger
than that of electronic PAAs. The beam can be steered to 31 distinguishable
angles in the range of -75.51{\deg} to 75.64{\deg} based on the beam pattern
calculation with the measured RF response. The response time for beam steering
is 56 {\mu}s. These results represent a significant step towards the
realization of integrated microwave photonic beamformers that can satisfy
compact size and low power consumption requirements for the future radar and
wireless communication systems.
