We present a novel method to phase lock two lasers with a controllable frequency difference. A microwave frequency-modulated vertical-cavity surface-emitting laser is used to phase connect two diode lasers by a two-step injection locking. The phase fluctuations of the two lasers are measured to be , corresponding to 99.94% phase coherence. The frequency difference of the two lasers is tunable up to tens of gigahertz. The sideband suppression of the slave laser is more than at seed power. A narrow linewidth spectrum of coherent population trapping in rubidium is achieved using such beams.
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