- Thermal Compensator Polarizes Nd:YAG Laser
Most of today's conference presentations and journal articles dealing with solid-state lasers are concerned with exciting new technologies such as fiber lasers, photonic crystals and thin-disk lasers. But out in the global marketplace, the traditional Nd:YAG laser still has the lion's share of the action. That's why improved designs for these rod-based lasers, such as one developed during the past several years at the University of Bern in Switzerland, are so important.
Nd:YAG lasers must endure a heavy thermal load because neodymium's quantum defect is large. The waste heat dissipated in the rod induces a thermal stress, which has two deleterious effects: thermal focusing and thermal birefringence. The birefringence is not in rectangular coordinates as it is in, say, a quarter-wave plate; rather, because the rod and its thermal gradients are circularly symmetric, the birefringence is in polar coordinates, with a radial and a tangential component. Because these two polarizations experience different refractive indices in the strained Nd:YAG rod, the effective focal length of the rod is different for the two.
Over the years, numerous techniques have been developed to compensate for these thermal effects. A second intracavity laser rod, separated from the first by a half-wave plate, can compensate for some of the thermal birefringence. Concave ends on the laser rod, or other intracavity negative-focusing components, can compensate for thermal focusing -- but only at a specific pump power. The technique developed at the University of Bern offers a dynamic compensation for thermal focusing and can force the laser into a single, albeit radial, polarization state.
Figure 1. A thin layer of elastomer, sandwiched between two glass rods, provided negative focusing proportional to the circulating power that compensated the positive focusing of the laser rod. Images ©OSA.
The approach uses an intracavity negative-focusing device whose strength increases with intracavity circulating power (Figure 1). The device is simply a thin layer of gel or liquid placed within the resonator such that its image is relayed to the center of the laser rod. The researchers have found several materials that work well, but Sylgard 184, a silicone elastomer manufactured by Dow Corning Corp., performs well and also does not degrade at high laser powers. In their experiment, the researches sandwiched a 1-mm-thick layer of the elastomer between two 15-mm-long BG7 glass rods.
The scientists calculated and plotted the negative-focusing strength required of the elastomer as a function of pump power in a resonator with two flat mirrors (Figure 2). Logically, as the pump power increases, so does the requisite negative focusing needed to keep the laser stable. But because the radial and tangential polarizations experience different positive focusing strengths in the Nd:YAG rod, they require different negative-focusing strengths to compensate. The regions of stability for the two polarizations are identified in Figure 2.
Figure 2. The required negative-focusing power of the elastomer was calculated as a function of pump power. Because the radial (r) and tangential (Φ) polarizations experience different positive focusing in the laser rod, they require different negative-focusing strengths in the elastomer.
Where the laser operates (within the stable region shown in Figure 2) is a function of the thickness of the elastomer layer. Increasing the thickness pushes the laser higher in the chart. The researchers overcompensated their laser by using a layer of elastomer slightly thicker than required to force the laser to the top of the stable region, following the path indicated by the heavy arrow in the figure. Along this path, a stable equilibrium exists that locks the laser to the top of the stability region. Any perturbation that increases the compensation drives the laser into the unstable region, where diffraction losses decrease the circulating power. But because the focusing compensation depends on the circulating power, the decreased circulating power causes the compensation to decrease, driving the laser back into the stable region.
Figure 3. The near-field intensity distribution of the laser beam was photographed with no polarizer (a), a vertical polarizer (b), a diagonal polarizer (c) and a horizontal polarizer (d). Only light aligned with both the radial direction in the laser rod and the orientation of the linear polarizer was transmitted to the camera.
As indicated in Figure 2, only the radial polarization is stable along the path followed by the overcompensated laser. Thus, the device will operate in a radial polarization. The researchers confirmed this by photographing the near field of the output beam through a linear polarizer (Figure 3). As they rotated the polarizer, only light consistent with both the radial direction in the laser rod and the orientation of the linear polarizer was transmitted. The imperfections in the patterns of transmitted light indicated that the beam was not perfectly polarized. The researchers estimated that it was at least 80 percent polarized, the lack of perfect polarization probably due to irregularities from perfect circular symmetry in the rod's thermal gradients.
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