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:
where is an integer ( – for visible/near-IR combs), is the comb spacing (the laser pulse repetition rate), and is the carrier–envelope offset frequency (the rate at which the optical carrier phase shifts relative to the pulse envelope).
When both and 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 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 | 10 MHz – 1 GHz | C-band (octave-spanning to 1000 – 2000 nm) |
| Yb-doped fiber mode-locked | Ps to fs FS | 10 MHz – 1 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 | 30 nm (limited by modulator bandwidth) |
Self-referencing. Measuring and stabilizing requires an -to- interferometer: a comb tooth at frequency is frequency-doubled to , then beaten against the comb tooth at . The beat frequency is exactly . 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 at the few cm/s level
- 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
Half of the 2005 Nobel Prize in Physics went to John Hall and Theodor Hänsch for laser-based precision spectroscopy, including the optical frequency comb technique; the other half went to Roy Glauber. Microresonator-based Kerr combs (developed since ) brought combs from tabletop mode-locked laser systems down to chip-scale photonic integrated circuits, enabling compact deployable applications.