We demonstrate a technology to extend the bandwidth of optical transmitters based on optical time interleaving. An optical time-interleaving in-phase-and-quadrature (IQ) modulator (TI-IQM), which consists of a 1-to-2 optical selector followed by two IQMs, generates optical signals with a bandwidth of each side of the optical carrier that is up to twice that of the electronic sub-signals sent to the IQMs. The sub-signals are generated by a digital spectral weaver followed by digital-to-analog converters and tailored so that the targeted extended-bandwidth signal is obtained as the final optical output from the TI-IQM. We verified the concept in a transmission experiment with 150-GBaud 4, 8, and 16-level quadrature amplitude modulation. We successfully generated optical signals with a bandwidth of each side of optical carrier of around 76 GHz, which is almost twice that of the electronic sub-signals.