Source: Ferdinand-Braun-Institut

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The Concept of Laser Threshold

When it comes to lasers, the threshold is a crucial state where the small-signal gain matches the resonator losses, allowing laser emission to commence. This state is determined by a specific pump power (threshold pump power) or threshold current in the case of electrically pumped lasers.

Importance of Operating Above Threshold

To achieve significant power output, high power efficiency, and stable, low-noise performance, it is essential to operate a laser well above its threshold. Typically, lasers are operated between 3 and 10 times above their threshold levels.

Factors Influencing Laser Threshold

The threshold pump power can be calculated based on the condition that the small-signal round-trip gain equals the round-trip power losses at the laser wavelength. Lowering the threshold power necessitates minimizing resonator losses and maximizing gain efficiency, often achieved by using a small laser mode area in an efficient gain medium with limited emission bandwidth.

Optimizing Laser Performance

Optimizing laser performance involves balancing high slope efficiency and low laser threshold. Other considerations include pulse duration in Q-switched lasers, avoiding instabilities in mode-locked lasers, and minimizing thermal effects.

Luminescence and Laser Operation

Even below the laser threshold, the gain medium emits luminescence or fluorescence for optically pumped lasers. Above threshold in continuous-wave operation, luminescence intensity is clamped close to the threshold level. A laser emits some power just below threshold due to amplified spontaneous emission.

Thresholdless Lasers

The laser threshold originates from power loss through luminescence into various spatial modes. Thresholdless lasers can be achieved by suppressing luminescence with a microcavity, leveraging photonic band gaps in photonic crystals.

Operational Considerations

Most lasers operate with a pump power a few times above the threshold, typically in the range of 3–10 times the threshold pump power. This balance is crucial to efficiency and avoiding issues related to high intracavity intensities.

Conclusion

Understanding the laser threshold is fundamental in optimizing laser performance and efficiency. By operating above the threshold and considering various design aspects, lasers can deliver the desired output power with stability and reliability.

Source: Ferdinand-Braun-Institut
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