Fiber Laser Generates 60-W, High-Quality Green Beam

Breck Hitz

Fiber lasers are proving capable of many tasks as the technology matures, and they are supplanting other lasers in an increasing number of applications. Because they have an enormous surface-to-volume ratio, they can dissipate waste heat efficiently and are not subject to the thermal beam distortions of other solid-state lasers.

This absence of thermal distortions has enabled a fiber laser constructed by scientists at Aculight Corp. in Bothell, Wash., to generate 60 W of second-harmonic power at 540 nm in a nearly diffraction-limited beam and with an overall electrical efficiency (wall plug to green light) of 10 percent.

Efficient, high-power visible lasers with good beam quality have countless applications in industry, medicine and research. Although diode-pumped solid-state lasers have generated second-harmonic powers well in excess of 60 W, the thermal distortions in the lasers have resulted in relatively poor beam quality. The Aculight result appears to be the first instance of green, second-harmonic power in excess of 10 W with good beam quality; that is, an M² of less than 1.5.

The fiber laser that produced the fundamental-wavelength light was a three-stage system comprising an oscillator, preamplifier and power amplifier (Figure 1). A pair of fiber Bragg gratings provided feedback for the ytterbium doped fiber oscillator and kept its spectral width to less than 20 pm, a measurement limited by the resolution of the experimenters' optical spectrum analyzer.

Figure 1. The three-stage, Yb-doped fiber oscillator/amplifier produced as much as 120 W of average power in a near-diffraction-limited beam. A slightly lower-power output beam was frequency-doubled in the pair of LBO crystals to produce 60 W of second-harmonic power at 540 nm.

A polarizer and an amplitude modulator polarized the CW, 1080-nm output from the oscillator and broke it into a stream of pulses. The modulator was capable of pulse widths from microseconds to nanoseconds, and repetition frequencies from hundreds of kilohertz to hundreds of megahertz. To generate the 60 W of second-harmonic power, the modulator was set to produce 5-ns pulses at 10 MHz, with an average power of 1 mW.

These pulses then passed through a single-mode, Yb-doped fiber preamplifier, which boosted the average power to as much as 200 mW. The experimenters generated peak powers of tens of watts in the preamplifier without encountering nonlinear effects.

After emerging from the preamplifier, the pulses were focused into a polarization-maintaining, double-clad, large-mode-area fiber with an optical system that matched the beam from the single-mode preamplifier to the fundamental mode of the large-mode-area fiber. This 10-m-long, Yb-doped fiber amplifier was capable of as much as 120 W of average power with the 5-ns, 10-MHz pulses. At this power level, the spectral width was still within the instrumentally limited 20 pm, and the scientists observed no nonlinear effects. They measured the output beam's quality to have an M²of 1.1 and the polarization ratio to be better than 95 percent.

Figure 2. The 60-W second-harmonic output was generated from 110 W of fundamental power. The beam scan measurements (inset) were made at a second-harmonic power of 50 W.

They obtained the 60-W second-harmonic power when 110 W from the amplifier was frequency-doubled in a pair of 25-mm-long lithium triborate (LBO) crystals oriented for noncritical phase-matching at 1080 nm. They installed the crystals in an oven maintained at the correct phase-matching temperature, and a 15-cm lens focused the incoming pulses into a 56-µm spot. The second-harmonic power scaled linearly with the fundamental power (Figure 2), and no adjustment of the oven temperature was required over the power-tuning range. The M² value for the green output was 1.33.