Mode-resolved optical frequency comb fixed point localization via dual-comb interferometry.

We describe improved methods for locating the fixed point of an optical frequency comb. Two continuous-wave lasers are locked to a reference frequency comb and track the optical phase of a second comb-under-test (CUT) at two points separated by approximately 1.6 THz. Carrier-envelope and optical phase tracking (OPT) yields a precise fixed point measurement across a range of pump modulation frequencies (400 Hz-250 kHz). Sub-nanometer shifts of the fixed point are observed. The fixed point is also determined with high precision using dual-comb interferometry (DCI), and the value closely matches the calculation from the dual-point tracking method.