Frequency-Doubled Nd:YAG Generates 200 W

Breck Hitz

A high-power green Nd:YAG laser has been developed at Mitsubishi Electric Corp.'s Advanced Technology R&D Center in Amagasaki, Japan, and is being studied for the crystallization of amorphous silicon films into polycrystalline silicon films during the fabrication of thin-film transistor display screens. The laser's 200-W second-harmonic average output power apparently sets a world record and is at least a factor of two greater than any commercial competitor.

Figure 1. A wedge lens coupled the pump power from the diode stacks into the Nd:YAG laser rods.

Tetsuo Kojima, who described the laser at CLEO in San Francisco in May, provided additional details in a discussion after the conference. He explained that each Nd:YAG rod in the laser is pumped by six individual diode stacks and that the diodes' output is coupled into the rods with a wedge lens (Figure 1). The laser comprises four rods, with a 90° polarization rotator between each pair of rods to alleviate thermal birefringence (Figure 2).

Figure 2. The ~1 kW of 1.06-µm output of the four-rod Nd:YAG laser was frequency-doubled in an external crystal aligned for Type II phase-matching.

The team first developed a burst-mode version of the laser pumped by 400-µs pulses from the diodes at a 250-Hz repetition frequency (i.e., a 4-ms period). An acousto-optic modulator Q-switched the laser at a repetition frequency of 70 kHz. The laser's output beam, containing ~1 kW of average 1.06-µm power, had an M² of approximately 9. It was frequency-doubled in a 15-mm-long LiB₃O₅ crystal aligned for Type II phase-matching, to produce 200 W of 532-nm power.

Figure 3. The 200-W green output of the frequency-doubled laser was converted into a top-hat intensity distribution for annealing thin-film transistor display screens.

The burst-mode behavior -- with a 10 percent duty cycle -- was not optimal for the application, so the engineers refined the design for continuous pumping. The laser was Q-switched at a significantly lower rate, 4 kHz. Although it produced lower infrared power, the laser's conversion efficiency was greater because of its higher peak power, so it again produced 200 W of average green power. This second-harmonic output was stable for many hours, and the beam was shaped into a top-hat intensity distribution for annealing thin-film transistor screens (Figure 3).

Although laser annealing of these display screens has traditionally relied on ultraviolet excimer lasers, Kojima and his co-authors conclude that frequency-doubled Nd:YAG is a viable alternative that requires less daily maintenance.