modelocked

Mode locking, or mode-locked operation, is a technique in laser physics that forces the phases of the laser’s longitudinal modes to lock together. When the modes are phase coherent, they interfere constructively at regular intervals, producing a train of ultrashort pulses that exits the laser. The pulse repetition rate equals the cavity round-trip frequency, and the spectral bandwidth broadens with shorter pulse duration, giving a characteristic wide spectrum.

Mode locking can be achieved in two broad ways: active and passive. In active mode locking, an external modulator (electro-optic or acousto-optic) modulates the intracavity loss or phase at a frequency near the round-trip rate to promote phase coherence among the modes. Passive mode locking relies on a fast saturable absorber or on nonlinear optical effects inside the cavity, such as Kerr lensing or nonlinear polarization rotation, to favor pulsed operation. Saturable absorbers may be semiconductor-based devices or arise from intrinsic nonlinearities in the laser medium or cavity.

Pulses produced by mode-locked lasers typically range from tens of femtoseconds to a few picoseconds, with repetition rates from tens of megahertz to several gigahertz. Achieving stable ultrashort pulses requires dispersion management to balance group-velocity dispersion within the cavity, often using prism pairs, chirped mirrors, or specialized fiber designs. Common mode-locked sources include Ti:sapphire lasers for broad ultrafast spectra, Er-doped fiber lasers for infrared operation, and various solid-state or fiber-based systems for metrology, frequency combs, spectroscopy, and biomedical imaging.

Limitations include sensitivity to environmental changes, alignment complexity, and stabilization requirements for cavity length and dispersion. Mode-locked lasers remain central to ultrafast science and precision measurement due to their ability to generate regular, extremely short light pulses.