Source: Semantic Scholar
The Science Behind Active Mode Locking in Lasers
Active mode locking is a technique used in lasers to generate ultrashort pulses by modulating the resonator losses or round-trip phase change. This method involves using devices such as acousto-optic or electro-optic modulators to achieve synchronization with the resonator round trips, resulting in the generation of a regular pulse train.
Principle of Active Mode Locking
Active mode locking involves modulating the resonator losses to favor a pulse with the correct timing, allowing it to pass through the modulator at times of minimum losses. This leads to the pulse saturating the laser gain, while other radiation circulating in the resonator dies out due to negative round-trip gain. The modulator also causes slight pulse shortening in each round trip, resulting in the generation of ultrashort pulses.
Behavior with Switched-off Modulator
If the optical modulator is turned off after achieving mode locking, the process typically ends. However, in some cases, passive mode locking mechanisms may sustain the mode locking, especially with optical nonlinearities like Kerr lens mode locking.
Frequency Modulation Mode Locking
Active mode locking can also involve phase modulation using devices like Pockels cells, known as FM mode locking. This technique can lead to chirped pulses and sometimes exhibit instabilities that require correction through dispersive and nonlinear effects.
Synchronization and Regenerative Feedback
For stable operation, precise synchronization between the resonator round-trip time and modulator signal period is crucial. Feedback circuits are often used to automatically adjust modulation frequency or resonator length to maintain synchronization. Regenerative mode locking is a common technique for achieving stable operation and tunable pulse repetition rates.
Active Mode Locking with Higher Pulse Repetition Frequencies
To reach high pulse repetition rates, harmonic mode locking can be employed, where multiple pulses circulate in the resonator with the modulator frequency as an integer multiple of the round-trip frequency. Rational harmonic mode locking is a variation that offers flexibility in modulation frequency.
Comparison with Passive Mode Locking
Active mode locking typically results in longer pulse durations compared to passive mode locking due to the diminishing effectiveness of pulse shortening as pulses get shorter. While active mode locking requires additional components like optical modulators and synchronization means, it is preferred for applications requiring synchronized pulse trains or operation of multiple lasers in synchrony.
Source: ResearchGate
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