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Understanding Mode Locking of Lasers
Mode locking is a method used to generate ultrashort pulses from lasers, resulting in what is known as mode-locked lasers. In this operational state, a single picosecond or femtosecond pulse circulates in the laser resonator, leading to a regular pulse train emission from the laser.
Active and Passive Mode Locking
Active Mode Locking: Involves the periodic modulation of resonator losses or round-trip phase change, achieved through various methods. This typically results in picosecond pulse durations.
Passive Mode Locking: Utilizes a saturable absorber to generate even shorter femtosecond pulses. The rapid modulation of resonator losses by the absorber allows for faster pulse generation.
Hybrid Mode Locking: Combines active and passive mode locking techniques, offering advantages such as externally controlled pulse repetition rates and reliable mode-locked operation.
Continuous Versus Synchronous Pumping
Most mode-locked lasers are continuously pumped, where the pump source supplies energy to the gain medium while the circulating pulse extracts energy at regular intervals. Synchronous pumping, on the other hand, requires another mode-locked laser as the pump source.
High and Low Pulse Repetition Rates
Harmonic mode locking allows for high repetition rate pulse trains, even in fiber lasers with fundamental round-trip frequencies. For high repetition rates without harmonic mode locking, very short laser resonators are needed.
The Optical Spectrum: Frequency Combs
The optical spectrum of a mode-locked laser consists of discrete lines with a constant spacing, known as frequency combs. Despite non-equidistant resonator modes, these equidistant lines are generated through mode locking mechanisms.
Instabilities and Noise of Mode-locked Lasers
Mode-locked lasers can exhibit various instabilities, leading to bunches of pulses, unstable energy pulses, or noise backgrounds. Different types of noise, including timing jitter and phase noise, can affect the laser output.
Mode Locking Diagnostics
Various methods, such as autocorrelation, nonlinear crystal frequency doubling, and RF spectrum analysis, are used to diagnose proper mode locking in lasers. Optical spectrum analysis and advanced pulse diagnostics can also provide valuable information.
Mode Locking Devices
Optimal mode-locked performance relies on a well-designed laser setup, including the selection of appropriate mode locking devices like modulators or saturable absorbers.
Source: Wise Research Group
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