Source: Nature

The Versatile Applications of Thulium in Laser Technology

Introduction to Thulium

Thulium (Tm) is a rare earth metal commonly used in laser technology as a laser-active dopant in various host materials such as crystals, glasses, and ceramics. Thulium-doped media are utilized in high-power lasers, wavelength-tunable solid-state lasers, and ultrafast amplifiers, offering a wide range of applications in fields like medical treatments, LIDAR, and optical data transmission.

Laser Transitions of Thulium-doped Gain Media

Thulium ions have multiple excited energy levels that play a crucial role in their operation as a laser gain medium. The most common transition is the 2-μm transition, which offers a substantial gain bandwidth and is typically pumped using high-power laser diodes emitting around 790 nm.

2-μm Transition

The 2-μm transition is the primary laser/amplifier transition of thulium ions, providing emission in the 2-μm spectral region. In-band pumping at around 1.6 μm to 1.8 μm is an efficient way of optical pumping for this transition. Tandem pumping is often used for achieving the highest output powers.

1.5-μm Transition

Thulium-doped fiber amplifiers can utilize the 1.5-μm transition to amplify optical signals in the S-band, complementing erbium-doped fiber amplifiers commonly used in the C-band. These amplifiers are typically pumped at about 1.4 μm.

2.3-μm Transition

The 2.3-μm transition involves pumping into the upper laser level around 790 nm, with the lower laser level quickly depopulated by a multiphonon transition. Tellurite fibers are often used for this transition due to their better mid-IR transparency.

0.8-μm and 0.48-μm Transitions

Emissions around 0.8 μm and 0.48 μm are achieved through specific transitions, each requiring different pumping schemes and host materials to optimize laser efficiency.

Influences of the Host Medium

The host medium significantly impacts the operation of thulium-doped lasers and amplifiers. Factors such as transition cross-sections, level lifetimes, polarization dependence, thermo-optic properties, and doping concentration limits are influenced by the choice of host material.

Conclusion

Thulium-doped gain media offer a versatile platform for various laser applications, ranging from medical treatments to scientific research. Understanding the different laser transitions and pumping schemes is essential for optimizing the performance of thulium-doped lasers and amplifiers in different spectral regions.

Source: Chin. Phys. B
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