A Y-branch integrated dual wavelength laser diode is fabricated for optical microwave generation based on the principle of sideband injection locking. The device integrates a master laser and a slave laser with associated Y-branch coupler. By directly modulating the master laser near its relaxation resonance frequency, multiple sidebands are generated due to enhanced modulation nonlinearity. Beat signal with high spectral purity is obtained by injection locking the slave laser to one of the modulation sidebands. A millimeter-wave carrier of 42-GHz with a phase noise of -94.6 dBc/Hz at 10 kHz offset is demonstrated.