We demonstrate that a 2f-to-3f self-referencing interferometer (SRI) becomes a useful tool for stabilizing a carrier-envelope offset frequency of an Er-doped fiber laser. A dual-pitch periodically poled lithium niobate (PPLN) ridge waveguide, consisting of two monolithically integrated segments with different quasi-phase matching pitch sizes, allows us to generate third-harmonic light with high efficiency. By using this device, we obtain a 45-dB signal-to-noise ratio in 100-kHz bandwidth of a heterodyne beat signal and instability of the in-loop f CEO of 8 × 10(-18) at 1 s of averaging time. This result is important for f CEO stabilization of a frequency comb, for which it is difficult to obtain a one-octave supercontinuum spectrum.