When laser light bounces between mirrors, it can form a train of very short pulses. The two laureates realized that these mode-locked pulses represent a frequency comb with evenly spaced teeth. By locking the comb spacing to a microwave atomic clock, optical frequencies can be counted directly. This enabled the redefinition of the metre and provides a foundation for future optical clocks and gravitational-wave detectors.

An optical frequency comb is fully specified by its repetition rate f_r and carrier-envelope offset f_0. Hänsch achieved an octave-spanning spectrum using a Ti:sapphire oscillator and photonic crystal fiber self-phase modulation, allowing f_0 to be detected and stabilised via the 2f–f scheme. Hall, working at NIST, provided ultra-stable microwave references to control f_r and pioneered beat-note measurements for determining unknown laser frequencies. Together these advances delivered 10^{-18} fractional uncertainty in the visible-NIR range, enabling high-precision tests of fundamental constants and searches for spacetime variation.

Comb generation hinges on maintaining ultrashort pulse duration through Kerr non-linearity and dispersion management. Hänsch implemented carrier-envelope phase stabilisation via a PLL, demonstrating long-term stability of Δf_0 < 1 mHz. Hall introduced zero-dead-time counters and MCMC beat-analysis, calibrating unknown sources at sub-mHz levels by simultaneous fitting of multiple comb lines. These techniques now underpin optical lattice clocks, molecular spectroscopy (H$_2^+$, HD$^+$), and extensions of combs into the XUV via high-harmonic generation.