Disk Laser Technology

A disk laser or active mirror is a type of solid-state laser characterized by a heat sink and laser output that are realized on opposite sides of a thin layer of active gain medium. It was introduced in the 1990s by the group of Adolf Giesen at the University of Stuttgart, Germany.

The gain medium of a thin-disk laser is a laser crystal (often Yb:YAG) in the form of a disk with a thickness of 100-200 µm, which is fixed on a water-cooled heat sink. The cooled end face has a dielectric coating that reflects both the laser radiation and the pump radiation.

The heat is extracted dominantly through the cooled end face, and because the disk thickness is considerably smaller than the laser beam diameter, the heat flow is largely in the direction of the beam, rather than in a transverse direction, as for a laser rod. As a consequence, thermal lensing is weak. The figure 1 illustrates the difference between the two types. Hence, the beam quality achievable with Disk Lasers can be much higher than that of a rod system, improving the Beam Parameter Product (BPP) up to 6 times.

The small disk thickness, as required to limit the heating, leads to incomplete pump absorption in a double pass. Therefore, one usually uses some multipass pumping scheme, which can be realized with very compact optics.

Due to improvements in the area of semiconductor pumping diodes the potential of Disk Lasers is not exhausted. While the first generation "only" extracted 1kW of laser power out of one disk, today's generation already generates 2kW out of one disk crystal. Still, the potential for this technology is not limited and expected to increase to 4kW per disk towards the end of 2008. Further, by combining several individual disk cavities, as illustrated in Figure 2, the total available laser power of a Disk Laser is virtually unlimited. The pumping beam from diode pumping stacks is reflected multi-fold via mirrors inside the cavity to pass up to 20 times through the disk. The disk "converts" the optical pumping light into a laser beam for processing. Based on an existing 4-cavity design, a laser power of 16kW will soon be available. The beauty of this Disk Laser principle over the fiber laser principle is that there are no losses in beam quality when scaling up laser power. These improvements in beam quality and power also lead to significant advantages for the design of processing optics and allowed the development of high-power scanner optics.

It is hardly necessary to mention that indispensable features known from conventional lamp-pumped lasers have not changed: Disk Lasers offer closed-loop power control, are insensitive against back reflections returning from the workpiece, their availability (uptime) is greater than 99 per cent and due to their modular construction all components can be replaced and maintained in the field. Last, but not least, for users of Disk Laser this means that not only the performance of such devices improves, but prices for say a 4 kW Disk Laser are falling because less cavities are required to generate the same laser power.

Q switching is possible with high pulse energies but not with very short pulses because the laser gain is quite limited.

Two Photon Absorption

The amount of light absorbed by a substance under normal single photon conditions is given by Beer’s Law, in which the amount of absorbed light is (in the weak absorption limit) proportional to the absorption cross-section of the molecule, s, the pathlength, l, and the concentration of absorbing species, C. In Two Photon Absorption (2PA), the absorption is proportional to the square of the light intensity. As a consequence, 2PA occurs only for very intense light, which, in common application, occurs at the focus of a laser beam. The photo at right shows fluorescence of a dye following 1PA and 2PA. The laser at the top of the cuvette is exciting the dye by 1PA causing yellow fluorescence emission. The emission can be seen clearly along the whole focussed laser path. The laser at the bottom of the cuvette is exciting the dye by 2PA, which causes the same yellow fluorescence. This time emission only occurs at the focal point of the laser because of the (intensity)2 dependence of the 2PA.

Simulated effects of excitation wavelength and numerical aperture on the dimensions of the 2PA volume. a, Normalized distributions of laser intensity-squared in the x-y and x-z plane for three different numerical apertures of water-immersion objective lenses at an excitation wavelength of 850 nm. Intensities-squared at lateral (x,y,0) and axial (x,0,z) positions were calculated using an ellipsoidal Gaussian approximation to the diffraction limited focus7,11 and expressed as the fraction of the intensity-squared at the focal point [I(0,0,0)2=1]. Color-coded contour plots depict isointensity lines in the x-z and x-y plane at the levels of 0.1, 0.3, 0.5, 0.7, and 0.9 of I(0,0,0)2. Note different scales for each panel. b, Dependence of 2PA volume on numerical aperture of the objective lens and illumination wavelength. Values were obtained by approximating the intensity-squared distribution as a three-dimensional Gaussian volume.12 For all calculations, it is assumed that the objective lens is uniformly illuminated (overfilled) and that no saturation of the fluorescence excitation process occurs. NA indicates numerical aperture.

Figure right provides a simplified illustration of the difference between single photon and two-photon activated processing. A material is polymerized along the trace of the moving laser focus, thus enabling fabrication of any desired polymeric 3D pattern by direct “recording” into the volume of photosensitive material. In a subsequent processing step the material, which was not exposed to the laser radiation, and therefore, stayed unpolymerized, is removed and the fabricated structure is revealed. The material sensitive in the UV range (λUV) can be polymerized by irradiation with the infra-red light of approximately double wavelength (λIR=2λUV), under the condition that the intensity of the radiation is high enough to initiate two-photon absorption.