Thin-disk laser

A thin-disk laser is a solid-state laser whose gain crystal is shaped as a disk far wider than it is thick: typically 0.1–0.3 mm thick and a centimetre or more across, with a pumped spot of a few millimetres to over a centimetre in diameter. The back face carries a highly reflective coating for both pump and laser wavelengths and is soldered or glued to a water-cooled heat sink; the disk thus acts as an active mirror at one end of, or folded into, the laser cavity. The concept was introduced by Giesen and coworkers in 1994, and Yb:YAG pumped at 940 nm or 969 nm by diode lasers remains the standard material. A single disk in multimode continuous operation delivers several kilowatts, and mode-locked thin-disk oscillators reach average powers above 100 W directly, without an amplifier.

Why the geometry scales

In a rod, heat generated in the volume must flow radially to the barrel, so the centre runs hotter than the edge and the transverse temperature gradient forms a strong thermal lens. In a thin disk the heat flows through the thin dimension into the heat sink, parallel to the laser beam. Across the pumped spot the temperature is nearly uniform, so the transverse gradient, and with it the lens and stress birefringence, is small except near the spot edge.

The temperature rise across a disk of thickness $t$ and thermal conductivity $K$, heated uniformly with heat flux $q$ per unit face area and cooled on one face, is

$$\Delta T = \frac{q\,t}{2K}.$$

For a heat flux of 0.4 kW/cm², roughly 10 % of an absorbed pump intensity of 4 kW/cm², a 200 µm disk with $K \approx 7$ W/(m·K), a typical value for 10 % doped Yb:YAG, rises by 57 K from back to front, before adding the drop across the solder and heat sink. Because $\Delta T$ depends on intensity and thickness only, the pumped area can be enlarged at constant intensity and temperature: at 4 kW/cm² a 5 mm spot takes 0.79 kW of pump and a 10 mm spot 3.1 kW. Output power scales accordingly, which is the design principle of the geometry.

Multipass pumping

A disk this thin absorbs only a small fraction of the pump in one pass. The pump module therefore reimages the light onto the disk many times, usually with a parabolic mirror and retroreflecting prisms, so the pump crosses the disk a few tens of times and more than 90 % is absorbed. Doping around 10 % helps. Pumping at 969 nm, the zero-phonon line of Yb:YAG, cuts the quantum defect to 5.9 % from 8.7 % at 940 nm, but the line is narrow and needs wavelength-stabilized pump diodes (see DPSS laser).

Yb:YAG is a quasi-three-level medium: its lower laser level lies only about 612 cm⁻¹ above the ground state and holds about 5 % of the ions at room temperature, so part of the pump serves to bleach this reabsorption (see four-level laser); high pump intensity and good cooling keep the lower-level population small.

Gain, cavities and pulses

The thin gain medium gives a small gain per pass, a few percent, so output couplers transmit only a few percent, and the laser beam may be folded to cross the disk several times per round trip. The low gain and nearly aberration-free disk suit mode locking with a SESAM or Kerr lens, producing pulses of several hundred femtoseconds to a few picoseconds at high average power, and suit regenerative amplifiers delivering millijoule-level picosecond pulses at kilohertz rates. Multimode kilowatt thin-disk lasers compete with fiber lasers for cutting and welding in laser machining; single-mode power per disk is lower, limited by the residual aspheric part of the thermal lens and by disk deformation.

Pitfalls and limits

Amplified spontaneous emission travelling sideways across a large disk depletes the gain before the laser can use it, a limit that grows with spot diameter; undoped caps bonded to the disk reduce it. Mechanical and thermal deformation of the disk changes its curvature with pump power, altering the cavity mode and beam quality. Mounting quality matters: voids in the solder layer produce hot spots that can fracture the crystal.

Common questions

What is the difference between a thin-disk laser and a fiber laser?

Both geometries remove heat through a large surface relative to the pumped volume. A fiber spreads heat along metres of length and confines the beam in a waveguide, which gives excellent beam quality but high peak intensity in a small core; a disk keeps the beam in free space with a large spot, which is better for high pulse energy and peak power.

Why is Yb:YAG used for thin-disk lasers?

It has a small quantum defect, a long upper-state lifetime of about 0.95 ms, and absorption bands at 940 and 969 nm matched to high-power diodes, and it tolerates the high doping that a thin disk needs.

References: A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, H. Opower, Scalable concept for diode-pumped high-power solid-state lasers, Appl. Phys. B 58, 365 (1994); A. Giesen, J. Speiser, Fifteen years of work on thin-disk lasers: results and scaling laws, IEEE J. Sel. Top. Quantum Electron. 13, 598 (2007); W. Koechner, Solid-State Laser Engineering 6th ed. (Springer, 2006).