What Is Thulium?

Thulium is a rare earth element with atomic number 69 and chemical symbol Tm, belonging to the lanthanide series of the periodic table. It is one of the least abundant naturally occurring lanthanides, found primarily in minerals such as monazite and bastnäsite alongside other rare earth elements. Despite its scarcity, thulium has found important roles in photonics and biomedical engineering, most prominently as a dopant in fiber lasers and optical amplifiers operating in the 1.8 to 2.1 micrometer wavelength range. Its electronic structure, characterized by a partially filled 4f subshell, produces the optical transitions that make it valuable as a gain medium.

Thulium's practical significance in engineering grew substantially from the 1990s onward, as silica fiber fabrication technology matured to the point where thulium ions could be incorporated reliably into fiber cores with low losses, enabling the development of thulium-doped fiber lasers (TDFLs) and amplifiers as alternatives to erbium-doped systems for longer-wavelength applications.

Properties and Electronic Structure

Thulium's optical and spectroscopic properties arise from the shielded 4f electron configuration [Xe] 4f13 6s2, which produces sharp electronic transitions largely insensitive to the host material environment. In crystalline or glass matrices, the energy level structure of Tm3+ supports emission in the 1800 to 2100 nm window through the 3F4 to 3H6 transition, a wavelength range that coincides with strong water absorption bands and falls within the atmospheric transmission window useful for remote sensing. The element's relatively large spin-orbit coupling and the cross-relaxation process between two adjacent thulium ions, in which one ion excited to the 3H4 level transfers half its energy to a neighboring ground-state ion, allows quantum efficiencies exceeding 100% per pump photon under certain doping concentrations. This cross-relaxation mechanism makes thulium-doped fibers particularly efficient when pumped at 793 nm using semiconductor diode lasers, a combination that underpins most commercial TDFL systems.

Thulium-doped Fiber Lasers

Thulium-doped fiber lasers operate primarily near 1940 nm, though the broad emission bandwidth of Tm3+ in silica allows tuning across much of the 1.8 to 2.1 micrometer range. Research on thulium fiber laser technology shows that high-power continuous-wave thulium fiber lasers can achieve optical efficiencies comparable to erbium-based systems while operating at wavelengths where water absorbs energy approximately four times more strongly than at 2100 nm, making them highly effective for laser-tissue interaction and mid-infrared applications. Single-mode continuous-wave TDFLs have been demonstrated at output powers exceeding 400 W, as documented in IEEE work on high-power thulium fiber lasers, through double-clad fiber geometries that confine the pump in the outer cladding while maintaining single-mode propagation in the doped core. Mode-locked thulium lasers produce ultrashort pulses in the two-micrometer band, supporting applications in nonlinear frequency conversion and broadband spectroscopy.

Biomedical and Sensing Applications

The 1940 nm emission wavelength of thulium fiber lasers falls at a strong water absorption peak, which makes them effective surgical tools in urology for stone lithotripsy and soft-tissue ablation, where the laser energy is deposited within a very short penetration depth and produces precise, low-thermal-damage cuts. In comparison to the holmium:YAG laser used for decades in urology, thulium fiber lasers offer more efficient stone ablation at the same delivered energy. Beyond surgery, thulium-doped fiber amplifiers operating in the S-band (1480 to 1530 nm) have been investigated to extend the transmission capacity of fiber optic networks beyond the conventional C and L bands used by erbium-doped amplifiers. NIST spectroscopic data for thulium provides the reference atomic line data used to calibrate thulium-based spectroscopic instruments.

Applications

Thulium has applications in a wide range of fields, including:

• Urological laser surgery for kidney stone fragmentation and prostate treatment
• High-power fiber laser sources for materials processing and plastic welding
• Mid-infrared spectroscopy and chemical sensing
• S-band optical amplification in telecommunications fiber networks
• Portable X-ray sources using thulium-170 as a radioactive gamma emitter