Intracavity frequency-doubled neodymium lasers are attractive sources of green power, which is useful in many materials-processing applications and as a pump for other lasers. However, the so-called “green problem” often introduces deleterious fluctuations in the output-power level, limiting the utility of intracavity frequency doubling.Figure 1. The two Nd:YVO4 pump cavities in Figure 2 are in the lower left, and upper center, of this photo (at the ends of the blue cables). The intracavity LBO doubling crystal is in the white enclosure at the top of the photo.Scientists at Technische Universität Kaiserslautern in Germany have demonstrated an internally doubled Nd:YVO4 laser that avoids the green problem and generates what they believe is the highest 532-nm power reported in a diffraction-limited beam: 62 W in a beam whose M2 value is 1.05.Because the conversion efficiency of second-harmonic generation is proportional to the fundamental power, the high peak power of pulsed lasers makes it easy to double their frequency efficiently. It is tougher with CW lasers. A common trick is to put the nonlinear crystal inside the resonator, where the fundamental power is significantly greater than in the output beam.The green problem arises because the intracavity nonlinear crystal generates not only second-harmonic light but also light at the sum frequencies of the laser’s different longitudinal modes. This sum-frequency generation couples the competing longitudinal modes and gives rise to chaotic behavior as the modes gain and lose oscillating strength, causing an undesirable fluctuation in output power.Numerous solutions to the green problem have been proposed over the years. Some, although effective, require awkward placement of the components inside the resonator. One straightforward technique is to force the laser to oscillate in only a single longitudinal mode, so that there is no mode competition — hence, no green problem. If the single mode laser is configured as a one-directional ring, there are no standing waves and no loss of power from spatial hole burning. A commercial thin-disk laser from Elektronik Laser System GmbH in Gross-Bieberau, Germany, uses this approach and is specified to produce more than 50 W of green power.Figure 2. The long resonator (M1-M8 ) had a longitudinal-mode spacing of 100 MHz, so ~120 modes could oscillate, averaging out the fluctuations of the “green problem.” The scientists also operated the laser in a shorter configuration (M1-M5 ), observing greater noise in the green power because fewer modes were oscillating. Mirror M1 was reflective at both 1064 and 532 nm, so the second harmonic generated in both directions was combined into a single output beam that emerged through mirror M2. The scientists adjusted the spacing between M1 and the lithium triborate (LBO) crystal to ensure that the dispersion of air did not result in destructive interference when the green beams were combined. Reprinted with permission of Optics Letters.An alternative approach is to allow a great many longitudinal modes to oscillate so that the fluctuations are averaged over many modes. This was the approach taken by the scientists in Kaiserslautern. They pulled the resonator mirrors more than a meter apart so that the longitudinal-mode spacing was reduced to 100 MHz and some 120 modes could oscillate (Figure 2).They initially operated the laser in Figure 2 in its short configuration, with mirror M5 in place. Without heating the lithium triborate (LBO) crystal to phase-matching temperature, they observed 55.5 W of 1.06-μm power through an optimal coupling, 35-percent-transmitting output mirror. When they replaced the output mirror with one of higher reflectivity and heated the LBO to phasematch temperature, they observed 34 W of green power.Figure 3. The periodicity of the resonator meant that the intracavity beam maintained the same geometry, whether the resonator operated in its short or long configuration (i.e., with or without mirror M5 ). Reprinted with permission of Optics Letters.They replaced mirror M5 with a second Nd:YVO4 rod in a periodic configuration (Figure 3). Before heating the LBO to phase-matching, and with a 65-percent-transmitting optimal output coupler, they observed 115 W of fundamental output power. Again replacing the output coupler with a mirror of higher reflectivity and heating the crystal to phasematch generated 62 W at the second harmonic. The total diode power at 888 nm pumping both Nd:YVO4 rods was 211 W, so the overall optical efficiency, from diode pump power to green output, was 29 percent.Figure 4. The green power generated by the short resonator configuration in Figure 2 (a) was much noisier than the green power from the long configuration (b). Reprinted with permission of Optics Letters.The green power from the long resonator configuration had significantly less noise than that from the short resonator (Figure 4). Using a Fabry-Perot interferometer to observe the optical spectra of the laser from the short to the long configurations, the scientists saw the oscillating bandwidth increase from 7 to 12 GHz and the mode separation decrease from 175 to 100 MHz. They concluded that the mode count increased from ∼40 to ∼120 and that the many more oscillating modes accounted for the reduced noise.Optics Letters, April 1, 2007, pp. 802-804.