Abstract: An apparatus and method of controlling the frequency of laser outputs are provided. The apparatus includes an optical resonator having an optical length that supports a plurality of resonant modes dependent thereon. The optical resonator includes a modulation device for modulating the optical length in response to a modulation signal. A first and second laser having a respective first and second optical output at a respective first and second laser frequencies are independently locked to different resonant modes of the optical resonator using known locking mechanisms to produce first and second laser outputs having narrow linewidths. A beatnote generator generates a beatnote signal representative of the difference between the first and second frequencies of the first and second laser outputs. A negative feedback system having a reference signal and the beatnote signal as inputs generates the modulation signal representative of a difference between the reference and beatnote signals.