Glossary entry

Frequency comb

An optical spectrum consisting of equally-spaced narrow lines. Provides a calibrated frequency ruler linking optical to microwave frequencies — the enabling technology of optical clocks and broadband precision spectroscopy.

A frequency comb is an optical spectrum composed of evenly-spaced narrow frequency lines:

f_n \;=\; n \, f_\text{rep} + f_\text{CEO},

where $n$ is an integer ( $\sim 10^5$ – $10^6$ for visible/near-IR combs), $f_\text{rep}$ is the comb spacing (the laser pulse repetition rate), and $f_\text{CEO}$ is the carrier–envelope offset frequency (the rate at which the optical carrier phase shifts relative to the pulse envelope).

When both $f_\text{rep}$ and $f_\text{CEO}$ are phase-locked to a stable RF reference (typically a hydrogen maser or GPS-disciplined oscillator), every optical frequency in the comb is known to the absolute precision of the RF reference — typically a fractional uncertainty of $10^{-15}$ or better. The comb serves as a precision frequency ruler bridging the optical and RF/microwave domains.

Generation methods:

Method	Source	Comb spacing	Coverage
Ti:Sapphire mode-locked laser	Femtosecond pulse train	80 MHz – 1 GHz	500 – 1100 nm (octave-spanning with nonlinear broadening)
Er-doped fiber mode-locked laser	Soliton or stretched-pulse FS	100 MHz – 10 GHz	C-band (octave-spanning to 1000 – 2000 nm)
Yb-doped fiber mode-locked	Ps to fs FS	100 MHz – 10 GHz	1000 – 1100 nm
Microresonator Kerr comb	Continuous-wave pumped high-Q ring	10 – 1000 GHz	Octave with anomalous dispersion design
Electro-optic comb	CW laser + cascaded modulators	1 – 100 GHz	$\sim$ 30 nm (limited by modulator bandwidth)

Self-referencing. Measuring and stabilizing $f_\text{CEO}$ requires an $f$ -to- $2f$ interferometer: a comb tooth at frequency $f_n$ is frequency-doubled to $2 f_n$ , then beaten against the comb tooth at $f_{2n}$ . The beat frequency is exactly $f_\text{CEO}$ . This requires the comb to span at least one octave — hence the importance of supercontinuum generation in early frequency comb development.

Applications:

Optical atomic clocks: provide the gear train converting an optical transition frequency to a countable RF output (foundational to next-generation time standards)
Astronomical spectrograph calibration: laser frequency comb generators ("astrocombs") calibrate exoplanet-search spectrographs to better than 1 cm/s
Dual-comb spectroscopy: two slightly mismatched combs heterodyne to map an entire optical spectrum to a measurable RF spectrum in microseconds
Optical frequency synthesis: select any line for use as a stable reference at any wavelength in the comb's range
High-bit-rate WDM transmitter sources: a single comb provides hundreds of phase-coherent channels

The 2005 Nobel Prize in Physics was awarded to Hänsch and Hall for the development of optical frequency comb technology. Microresonator-based Kerr combs (developed since $\sim 2007$ ) brought combs from kilometer-scale fiber laser systems down to chip-scale photonic integrated circuits, enabling compact deployable applications.