Thulium fiber laser
A fiber laser whose gain medium is thulium-doped silica fiber, emitting in the 2 µm region, typically 1.9–2.05 µm. Pumped near 793 nm, cross-relaxation between thulium ions lets one pump photon produce up to two laser photons, and output reaches the kilowatt class.
A thulium fiber laser is a fiber laser in which Tm³⁺ ions in the core of a silica fiber provide gain on the ³F₄ → ³H₆ transition, near 2 µm. The gain band is broad, and practical lasers operate anywhere from about 1.8 to 2.1 µm, most often between 1.9 and 2.05 µm; a 1940 nm photon carries 0.64 eV. Thulium fiber lasers are pumped either by diode lasers near 793 nm through a double-clad fiber, or in-band by erbium fiber lasers near 1.55–1.6 µm. Continuous output runs from watts in laboratory sources to the kilowatt class in research systems, and pulsed versions are widely used in surgery.
Pumping and cross-relaxation
A 793 nm pump photon lifts a Tm³⁺ ion to the ³H₄ level, well above the ³F₄ upper laser level. If nothing else happened, the best possible photon conversion would be the Stokes ratio
and 59% of the absorbed pump power would become heat. At thulium concentrations of a few weight percent, however, neighbouring ions are close enough for cross-relaxation: an ion in ³H₄ gives part of its energy to a ground-state neighbour, leaving both ions in ³F₄. In the ideal case each pump photon then yields two excited ions, doubling the quantum efficiency limit to about 82%. Real lasers fall short of this; slope efficiencies around 60% with respect to absorbed pump power have been reported from well-designed fibers, well above the 41% Stokes limit, which is possible only through cross-relaxation.
In-band pumping at around 1570 nm excites ³F₄ directly, giving a Stokes efficiency near 81% for 1940 nm output without relying on ion pairs. It needs an erbium fiber laser as the pump, which costs more than diodes, and is used where low heat load or low noise matters. The general principles of optical pumping and cladding-pumped fiber design are shared with ytterbium-doped fiber lasers.
Heat and scaling
Even with cross-relaxation, a thulium laser deposits more heat per watt of output than an ytterbium laser. For 100 W of output at 1940 nm, the pump power needed is 245 W at the bare Stokes limit and 122 W at the two-for-one limit, before any other losses. Because the wavelength is about 1.8 times longer than ytterbium's 1.07 µm, the mode area for single-mode operation can be larger at the same numerical aperture, which raises the threshold for nonlinear effects such as stimulated Brillouin and Raman scattering and partly offsets the thermal disadvantage.
Applications
Water absorbs strongly around 1.94 µm, several times more than at the 2.1 µm wavelength of holmium:YAG lasers, so a thulium beam deposits its energy in a thin layer of wet tissue. This makes thulium fiber lasers useful for cutting and coagulating soft tissue, and pulsed thulium fiber lasers are now used for laser lithotripsy, fragmenting kidney stones into fine dust through small, flexible fibers.
Other uses include welding and cutting of transparent polymers, which absorb better near 2 µm than at 1 µm (see laser machining); pumping holmium lasers and zinc germanium phosphide optical parametric oscillators for the mid-infrared; lidar and free-space links; and, as thulium-doped fiber amplifiers, gain for experimental optical communication near 2 µm, beyond the bands of the erbium-doped fiber amplifier.
Eye safety and atmosphere
Wavelengths beyond about 1.4 µm are absorbed in the cornea and aqueous humour before reaching the retina, so the permitted exposure is much higher than at 1 µm and the 2 µm region is described as eye-safe. At high power the beam can still cause thermal injury to the cornea and skin. Water vapour in air has strong absorption bands in the infrared near 1.85–1.95 µm, so free-space and lidar systems choose lines near or beyond 2 µm, where transmission is better.
Pitfalls
Silica's intrinsic absorption rises steeply beyond about 2 µm, so fibers and passive components near 2.1 µm show more loss and heating than at 1.55 µm. Components designed for the telecom bands, such as couplers and isolators, generally do not work near 2 µm and must be specified for it. Atmospheric water absorption in an unpurged free-space cavity or measurement path can distort spectra and power readings.
Common questions
What wavelength does a thulium fiber laser emit?
Most emit between 1.9 and 2.05 µm, with 1940 nm common in medical lasers; tuning over about 1.8–2.1 µm has been shown with suitable fibers and wavelength-selective feedback.
Why is a thulium fiber laser used for kidney stones?
Its wavelength sits near a water absorption peak, so each pulse is absorbed close to the stone surface, and the fiber laser can deliver low-energy pulses at high repetition rate through thin fibers, which breaks stones into fine fragments.
How does cross-relaxation raise efficiency?
It converts one excitation at ³H₄ into two at ³F₄, so a single 793 nm pump photon can yield up to two 2 µm laser photons, pushing the quantum efficiency limit from about 41% toward 82%.
References: S. D. Jackson, Nat. Photonics 6, 423 (2012); A. E. Siegman, Lasers (University Science Books, 1986); O. Svelto, Principles of Lasers, 5th ed. (Springer, 2010).