Contents
Source: Nagwa
Laser Gain Medium and Reabsorption Effects
In a laser gain medium, both stimulated and spontaneous emission bring laser-active ions or atoms into the lower laser level. If a significant population accumulates in that level because it is not quickly depleted, laser radiation can be reabsorbed, reducing the laser gain. This reabsorption process increases the threshold pump power and decreases power efficiency.
Cases with Moderately Large Lower-state Lifetime
In many solid-state laser gain media, the lower laser level’s population remains low due to its distance from the ground state and quick equilibrium establishment through multi-phonon transitions. The lower-state lifetime typically ranges from picoseconds to tens of nanoseconds, while pumping and lasing occur on microsecond to millisecond time scales.
For continuous-wave lasers, the short lower-state lifetime effectively suppresses reabsorption on the laser transition. However, in Q-switched lasers and ultrafast pulse amplifiers, rapid energy extraction can lead to reabsorption effects, reducing amplifier gain and efficiency for short pulse durations. Selecting a host crystal with a smaller lower-state lifetime or using chirped-pulse amplification can mitigate this issue.
Cases with Long Lower-state Lifetime
Some solid-state gain media have long lower-state lifetimes, such as several milliseconds, leading to self-terminating laser transitions. Cooperative lasing can be utilized to prevent this termination in such cases.
Conclusion
Understanding the dynamics of lower-state lifetimes in laser gain media is crucial for optimizing laser performance and efficiency. By managing reabsorption effects through appropriate techniques, such as crystal selection and pulse amplification methods, laser systems can operate more effectively and reliably.
Source: arXiv
Feel free to comment your thoughts.
