Ceramic Nd:YAG Laser Generates 144 W

Breck Hitz

Ceramic laser materials may enable the development of significantly less expensive lasers because the noncrystalline hosts can be fabricated much more quickly and simply than traditional crystalline laser materials. Moreover, because a ceramic laser rod can be doped and shaped to arbitrary geometries, these lasers offer the possibility of optical and physical characteristics not obtainable with conventional lasers.

Recently, researchers in Germany demonstrated what they believe is the highest efficiency achieved from a ceramic laser, generating 144 W of 1.06-μm output power from 226 W of absorbed pump power.

The scientists, based at Laser Zentrum Hannover eV and at Physikalisch Technische Bundesanstalt in Braunschweig, used a ceramic Nd:YAG laser rod 41 mm in length and 3 mm in diameter. A doped cylinder 1.5 mm in diameter and 27 mm in length was centered in the otherwise undoped rod. They end-pumped the rod with up to 300 W of 808-nm radiation from 10 fiber-coupled laser diodes (Figure 1). The outer surface of the ceramic rod was highly polished and acted as a waveguide to keep the pump radiation inside the laser rod.

Figure 1. The ceramic laser rod comprises a 0.3 percent atomic doped central section surrounded by undoped material. In the experiments, it was end-pumped by up to 300 W of 808-nm radiation. Images ©OSA.

Using commercial ray-tracing software from Zemax Development Corp. of Bellevue, Wash., the researchers calculated the distribution of pump radiation within the ceramic rod. They found that the beams from the 10 fibers were clearly discernible 11 mm from the rod’s pump face and that the beams spread out as they propagated down the rod (Figure 2). Their calculation predicted that 79 percent of the pump radiation would be absorbed in the rod, a figure with which a subsequent measurement closely agreed, indicating that 78 percent was absorbed.

Figure 2. A ray-tracing calculation shows the distribution of pump radiation within the rod’s cross section at distances of 11 (left), 21 (middle) and 34 mm from the pumped face.

When they measured the laser’s output with output couplers of 20, 25 and 30 percent, they observed the highest power with the 25 percent coupler, indicating that the laser was very nearly optimally coupled with that coupler (Figure 3). The laser’s maximum output of 144 W was obtained with an optical-to-optical efficiency of 64 percent with respect to absorbed pump power.

Figure 3. Optimal coupling for the laser is approximately 25 percent. The investigators believe that the 64 percent efficiency with respect to absorbed pump power is the highest reported for a ceramic laser.

The scientists also investigated the thermal lensing of the laser by translating the output mirror with respect to the laser rod. Both the output coupler and the back mirror were flats, so by pulling the mirrors apart until the resonator was no longer stable, they could calculate the rod’s effective back focal distance using standard ABCD matrix techniques. They concluded that the refractive power of the rod’s thermal lens increased from 6.2 per meter at 65 W of absorbed pump power to 21.3 per meter at 223 W.

Optics Express, April 3, 2006, pp. 2690-2694.