Mode locking is a technique used in lasers to produce ultrashort pulses—often in the picosecond (10⁻¹² s) or femtosecond (10⁻¹⁵ s) range—by forcing multiple longitudinal modes of the laser cavity to oscillate with a fixed phase relationship.
How It Works:
- A laser cavity supports many resonant frequencies (modes).
- Normally, these modes have random phases, resulting in continuous or noisy output.
- Mode locking synchronizes the phases of these modes.
- When aligned, their interference produces constructive peaks at regular intervals—short, intense pulses of light.
Types of Mode Locking:
- Passive mode locking: Uses a saturable absorber that favors short pulses by transmitting higher intensities more readily.
- Active mode locking: Uses an external modulator (e.g., acousto-optic or electro-optic) to modulate the light at the mode spacing frequency.
Applications:
Mode-locked lasers are essential in fields like ultrafast spectroscopy, high-speed imaging, telecommunications, and medical diagnostics, where precise, extremely short pulses are required.