High-Power Nd:YAG Laser Pumped Directly to Upper Level

Breck Hitz

A quarter century ago, Nd:YAG lasers were capable of optical efficiencies of only a few percent at best. Pump photons, produced by high-power arc lamps, were lost to glass flow tubes, and the water in them was lost to the walls of the pump cavity and to reabsorption by the lamp itself.

Some photons eventually managed to pump the laser rod, but only half their energy was converted to laser energy (Figure 1, orange arrow).

Figure 1. Lamp-pumped Nd:YAG lasers are inefficient, requiring pump photons with about twice the energy in the resulting laser photon (orange arrow). Diode-pumped lasers are much more efficient, requiring pump photons only slightly more energetic than laser photons (blue arrow). But the greatest quantum efficiency is achieved by pumping directly to the upper laser level (green arrow), as reported here.

Diode-pumped lasers developed during the intervening decades improved the situation significantly because more pump photons reached the laser rod and because a much larger portion of their energy was converted to laser output (Figure 1, blue arrow). (Nonetheless, the high cost of diodes relative to pump lamps will continue to drive a robust commercial market for lamp-pumped lasers into the foreseeable future.)

During the past decade, several investigators have reported Nd:YAG lasers operating on the most efficient scheme of all, pumping directly to the upper laser level (Figure 1, green arrow). And now scientists at Laser Zentrum Hannover eV in Germany have demonstrated what they believe is the highest-power laser yet developed on this pumping scheme — an end-pumped laser generating up to 250 W of output with an optical efficiency of 57 percent with respect to absorbed pump power.

Figure 2. Pump photons from the diode stack, at 885 nm, pumped the neodymium directly to its upper laser level. Reprinted with permission of Optics Letters.

They pumped their laser with an 885-nm, 500-W diode laser stack from nLight Photonics of Vancouver, Wash. (Figure 2). The resonator was formed by two plane mirrors, one to transmit the pump light to the laser rod, and the other, a 10 percent output coupler. The rod itself, doped with 0.48 atomic percent neodymium, measured 5 mm in diameter and 62 mm long, with two 6-mm, undoped end caps to reduce thermal effects.

Figure 3. The laser’s power-transfer function showed no sign of rollover at the upper end, so the scientists concluded that even higher powers should be possible. Reprinted with permission of Optics Letters.

When they pumped the laser with the stack’s maximum power, 500 W, the scientists observed a multimode (M² <60) output of 250 W, corresponding to a 57 percent optical efficiency with respect to absorbed pump power (Figure 3). They calculated that thermally induced effects in the rod, such as lensing and birefringence, were significantly reduced by pumping at 885 nm, as compared with conventional diode pumping at 808 nm.

The transfer function in Figure 3 shows no sign of rolling over at the upper end, so they concluded that even higher powers are readily achievable with higher-power pumping, and that 885-nm pumping may be suitable for many applications where less efficient, 808-nm-pumped lasers are now used.

Optics Letters, Dec. 15, 2006, pp. 3618-3619.