Contents
Source: Nature
Understanding Mode Locking in Lasers
Active Mode Locking with Modulators
Active mode locking in lasers involves using modulators to force the laser into emitting a regular train of ultrashort pulses. An intensity modulator is typically used, operated with a sinusoidal drive signal matching the laser resonator’s round-trip frequency. This modulation helps in achieving shorter pulse durations and maintaining steady-state laser operation.
Modulators for Active Mode Locking
Various types of modulators can be used, such as electro-optic modulators or acousto-optic modulators, depending on the laser type. The modulator’s function is to ensure that the circulating pulse passes through at times of minimum loss, leading to pulse compression and mode locking.
Modulator Drivers
Modulator drivers generate the drive signal for the modulator and maintain synchronism with the resonator’s round-trip time. Feedback loops can involve fine-tuning the resonator length or tuning the drive frequency for optimal performance.
Passive Mode Locking with Saturable Absorbers
Passive mode locking utilizes saturable absorbers in the laser resonator to initiate and maintain mode locking. These absorbers can be semiconductor saturable absorber mirrors (SESAMs) tailored for specific operation parameters like pulse durations, repetition rates, and output power.
Real Absorbers
Saturable absorbers favor the most intense part of the circulating radiation, allowing only a single pulse to survive after multiple resonator round trips. They suppress light circulating at other times, contributing to stable mode locking.
Artificial Saturable Absorbers
Artificial saturable absorbers exhibit decreasing optical losses for higher intensities without real light absorption, offering alternative options for mode locking in lasers.
Optimizing Saturable Absorbers
The parameters of a saturable absorber, such as modulation depth, recovery time, and saturation fluence, play a crucial role in achieving efficient and stable mode locking in lasers. Understanding the laser system and selecting the right absorber parameters are essential for optimal performance.
Conclusion
Mode locking in lasers, whether through active modulation or passive absorption, is a powerful technique for generating ultrashort pulses essential in various applications like spectroscopy, micromachining, and telecommunications. By utilizing modulators and saturable absorbers effectively, researchers and engineers can harness the capabilities of mode-locked lasers for advanced technological advancements.
Source: MDPI
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