Focus Supplement: Why the Frequency Comb Is So Uniform

The frequency comb that Hall and Hänsch used for their Nobel-winning research is emitted by lasers that create trains of light pulses, each as short as a few femtoseconds, or 10^-15 seconds, just a few oscillations of the electromagnetic field.

These pulses form as a disk of light bounces back and forth between the two mirrors that define a laser cavity. On each round trip, a "lasing" material in the cavity maintains the pulse as a short, intense burst of light. Some of this light escapes through one mirror to form the emitted pulse every time that mirror is struck. The time period between pulses, typically 10 nanoseconds (10^-8 seconds), is roughly the time a pulse takes for a round trip between the mirrors.

It is these extremely narrow and reproducible pulses that leads to such a uniform comb. But before 1978, most physicists had not thought much about the frequency spectrum that short, repetitive pulses would generate. And the frequencies that emerge from a laser cavity--the "modes," which resemble those of a guitar string or organ pipe--usually depend on the details of the cavity. However, when the pulses are repetitive, the laser materials can adapt by introducing slight variations in the speed of light for different modes. The resulting frequencies must be precisely the right ones that allow the electric fields from millions of modes to line up at the moment each pulse is emitted. This view of the pulse as composed of many "locked" modes is exactly equivalent to the simpler, "time domain" view of a single pulse bouncing back and forth in the cavity.

In 1999 Hänsch and his colleagues showed that the spacing of the teeth was uniform to 3 parts in 10¹⁷ [1]. This astonishing uniformity is what makes the comb so useful for precise measurements.

References:

[1] Th. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, "Accurate Measurement of Large Optical Frequency Differences with a Mode-Locked Laser," Optics Letters, 24, 881 (1999).