The present invention relates to frequency rulers. At least one embodiment includes a mode locked pump source operated at pulse repetition rate, and a pump output having a pump carrier envelope offset frequency. A nonlinear optical system outputs a frequency ruler spectrum comprising individual frequency modes. The frequency modes may be characterized by a frequency spacing which is an integer multiple of the repetition rate and by distinct ruler carrier envelope offset frequencies which exhibit at least one discontinuity across the frequency output. The ruler carrier envelope offset frequencies are substantially locked to the carrier envelope offset frequency of the pump laser. One preferred embodiment includes a frequency doubled, doubly resonant, non-degenerate OPO (DNOPO), a supercontinuum generation (SC) stage and at least one reference laser arranged downstream from a Tm fiber-based pump source. A plurality of beat signals generated therefrom provide for stabilization of the system.