Something Old, Something New: German Lab Marries Ring and Disk
One of the prime patents on the nonplanar ring oscillator (NPRO) expired last month, an occasion its inventors celebrated with a small party. On the other hand, the thin-disk laser is just beginning its career in the commercial marketplace. Scientists at the University of Applied Sciences Münster in Steinfurt, Germany, are trying to achieve the best of both worlds -- and to generate up to 7 W of single-frequency power -- by diffusion-bonding a thin Nd:YAG disk to an undoped YAG NPRO.
The NPRO, developed at Stanford University in California in the 1980s by Robert L. Byer and Thomas J. Kane, is a small chunk of YAG cut at odd angles so that a beam of light is totally internally reflected around a nonplanar ring inside it. Doped with an appropriate rare-earth element such as neodymium, the NPRO becomes a ring laser, and unidirectional oscillation can be forced by placing it in a magnetic field, where YAG's natural Verdet constant removes the degeneracy between the two directions. One of the internal reflections is less than total and serves as the laser's output coupler.
Unidirectional ring lasers have a significant advantage over conventional two-mirror lasers as sources of single-frequency output because the single-frequency standing wave in a two-mirror laser cannot access the entire population inversion. Instead, it burns spatial holes in the inversion at the peaks of the standing wave.
Stable and reliable
The traveling wave in a unidirectional ring, on the other hand, readily accesses the entire inversion. NPROs have become widely accepted during the past two decades as very stable, reliable sources of single-frequency output, with powers up to several watts. At higher powers, however, thermal birefringence in the YAG overwhelms the polarization effect that restricts oscillation to a single direction around the ring.
The thin-disk laser, developed in the laboratory of Adolf Giesen at the University of Stuttgart in Germany, is based on a small piece of YAG or other host whose thickness is typically 100 µm. Bonded to a heat sink on one side and optically pumped from the other, the disk develops thermal gradients that are nearly perfect planes parallel to its surface. The planar gradients dramatically reduce thermal lensing and birefringence so that very high thermal loading can be applied to the disk without introducing serious optical aberrations.
Figure 1. (a) The intracavity power in the NPRO circulates around the ring ABCD, with total internal reflections at B, C and D, and polarization-dependent output coupling at A. (b) An end-view shows the nonplanar aspect of the ring. These drawings show the 808-nm pump beam reflected for multiple passes through the disk, but experiments to date have utilized only one double-pass through the disk. (c) To ensure perfect total internal reflection at point C, the scientists placed a thin layer of SiO2 between the undoped YAG and the pump reflector.
Many watts of single-transverse-mode power and a kilowatt of multimode power have been obtained from tiny, 100-µm-thick disks. Because the pump absorption in a single pass through the disk is small, these high-power lasers rely on mechanically complex arrangements to reflect the pump light back and forth through the disk many times.
In marrying the thin-disk and NPRO technologies, the scientists in Steinfurt sought to remove thermal issues from the NPRO and place them in the thin disk, which is better suited to handle them. They bonded a heat sink to one side of the thin disk and an undoped NPRO to the other, and optically pumped the disk through the NPRO (Figure 1). In initial experiments, they passed the pump light through the disk only twice, resulting in an absorbed pump power up to 8 W. At this pump power, they observed 2.6 W of output in a TEM00 beam, but with more than a single longitudinal mode oscillating (Figure 2). They obtained single-frequency oscillation only at pump powers below 6 W, with corresponding output powers below 1.6 W.
Figure 2. The compound thin-disk/NPRO laser generated up to 1.6 W in a single longitudinal mode.
Avenues to explore
In future work, the scientists want to explore replacing the Gaussian pump beam with a flattop profile to reduce the pump-induced aberrations, and to adjust the NPRO geometry and the coating on its output mirror to increase the ring's discrimination between oscillation directions. By implementing these improvements, they hope to obtain up to 7 W of single-frequency output.
Another factor that the scientists will want to consider is the trade-off between their high-power oscillator and a low-power NPRO with a power amplifier. The latter configuration routinely generates 10 W of single-frequency output in gravitational-wave applications.
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