Mode-locking is a technique used in lasers to produce ultrashort pulses of light with durations on the order of picoseconds, femtoseconds, or even attoseconds. This method synchronizes the phases of different longitudinal modes within the laser cavity, causing them to interfere constructively and generate a train of short pulses.
Here is a basic overview of mode-locked operation in a laser:
Longitudinal modes: Lasers support different longitudinal modes, each corresponding to a specific wavelength or frequency. These modes can interact constructively or destructively, affecting the overall output of the laser.
Mode-locking: In mode-locking, the phases of different longitudinal modes are synchronized or "locked" to each other. This is achieved by introducing a mechanism, such as an external modulator or a saturable absorber, into the laser cavity.
Constructive interference: As the modes become synchronized, their electric fields add constructively, resulting in a short and intense pulse of light. This process is known as a "mode-locked pulse."
Ultrashort pulses: The constructive interference of modes produces a train of ultrashort pulses, each separated by the round-trip time of the laser cavity.
High repetition rate: Mode-locked lasers can have high repetition rates, with pulses occurring at frequencies ranging from tens of megahertz to terahertz.
Key Points:
Ultrashort pulses: Mode-locked lasers can generate pulses with extremely short durations, making them valuable for applications requiring high precision and resolution, such as in scientific research, telecommunications, and ultrafast spectroscopy.
Saturable absorber: One common method for achieving mode-locking is using a saturable absorber, a device that becomes less absorbing as the intensity of the light increases. This allows for the selective amplification of short pulses.
Applications: Mode-locked lasers are used in various fields, including optical communications, frequency metrology, nonlinear optics, and the study of ultrafast phenomena in physics and chemistry.
Ti:sapphire lasers: Titanium-doped sapphire lasers are a common type of mode-locked laser, known for their tunability and ability to generate ultrashort pulses.
Mode-locked lasers play a crucial role in numerous applications where ultrashort pulse durations are essential for achieving high precision and resolving rapid events in time. The development and optimization of mode-locked lasers have significantly advanced the field of ultrafast optics and laser technology.