What Is Laser Mode Locking?

Laser mode locking is a technique for generating ultrashort optical pulses by forcing the longitudinal cavity modes of a laser to oscillate with a fixed phase relationship. When many modes are locked in phase, they interfere constructively once per round trip of the resonator, producing a brief, intense pulse rather than continuous emission. Depending on the number of modes locked and the resonator length, pulse durations range from tens of picoseconds down to a few femtoseconds, with peak powers orders of magnitude higher than the average power would suggest.

The technique emerged in the 1960s, shortly after the first continuous lasers were demonstrated, and was initially implemented in ruby and Nd:glass systems. It became foundational to ultrafast science when titanium-doped sapphire (Ti:sapphire) lasers, capable of supporting bandwidths exceeding 100 terahertz, enabled sub-10-femtosecond pulse generation in the 1990s. Today, mode-locked fiber lasers and diode-pumped solid-state lasers have brought ultrashort pulses into industrial and clinical settings.

Active and Passive Mode Locking

Two broad approaches exist for establishing phase coherence among cavity modes. In active mode locking, an external signal drives an acousto-optic or electro-optic modulator inside the resonator at a frequency matching the round-trip time, periodically opening and closing the cavity to a pulse that circulates in synchrony. Active schemes offer precise repetition rate control and are used where the pulse timing must be synchronized to an external radio-frequency reference.

Passive mode locking uses a saturable absorber, a material or nonlinear optical element whose absorption decreases as intensity rises, to favor pulse formation spontaneously. A short pulse experiences lower loss than a low-intensity background, so the pulse grows relative to the background on each round trip. Semiconductor saturable absorber mirrors (SESAMs) and nonlinear polarization rotation in fiber are the most widely deployed passive mechanisms. Passive mode locking typically reaches shorter pulses and higher peak powers than active schemes because the saturable absorber responds on a timescale comparable to the pulse itself.

Ultrashort Pulse Characteristics

The duration of a mode-locked pulse is related to the inverse of the bandwidth over which modes are locked. A laser supporting oscillation over a 10-terahertz bandwidth is capable of producing pulses as short as roughly 100 femtoseconds, although dispersion management within the resonator is required to realize this limit. Group-delay dispersion from mirrors, lenses, and the gain medium must be compensated, typically using chirped mirrors or prism pairs, to prevent pulse broadening. Peak intensities in focused femtosecond pulses reach 10^13 to 10^15 watts per square centimeter, enabling nonlinear optical interactions that are impractical with continuous-wave or nanosecond sources. Research published in Science on spatiotemporal mode-locking in multimode fiber lasers demonstrated a new degree of freedom in which spatial and temporal modes lock simultaneously, expanding the accessible pulse energy and waveform parameter space.

Semiconductor and Fiber Mode-Locked Lasers

Beyond bulk solid-state platforms, mode locking has been implemented in optical fiber lasers and semiconductor devices. Fiber-based systems benefit from alignment-free construction and compatibility with fiber-optic infrastructure, and they operate reliably at repetition rates from megahertz to gigahertz. Semiconductor mode-locked lasers monolithically integrate gain and saturable absorber sections on a chip, enabling repetition rates from 5 gigahertz to over 1 terahertz and making them compact enough for photonic integrated circuits. Work on picosecond pulse generation from mode-locked dysprosium fiber lasers has extended mode-locked operation into mid-infrared wavelengths beyond 3 micrometers, which are relevant for molecular spectroscopy.

Applications

Laser mode locking has applications in a range of fields, including:

• Optical frequency combs for metrology, spectroscopy, and atomic clocks
• Two-photon and multiphoton fluorescence microscopy in biomedical imaging
• Ultrafast spectroscopy probing electron and phonon dynamics in materials
• Laser micromachining and corneal surgery with minimal thermal damage
• Optical coherence tomography for high-resolution medical imaging