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Compensating Thermal Effects in Rod Lasers

Photonics Spectra
Jul 2003
Breck Hitz

By forcing an Nd:YAG laser to operate in radial or azimuthal polarization, a team at Soreq Nuclear Research Center in Yavne, Israel, has overcome the thermally induced beam-quality aberrations commonly associated with solid-state rod lasers. The researchers believe the results will lead to better beam quality from future rod-based solid-state lasers.

The first laser was based on a long, circular cylinder of chrome-doped ruby, and during the subsequent four decades, thousands of solid-state lasers have been designed around circular cylinders -- or "rods" -- of rare-earth-doped crystals. But a rod's circular symmetry introduces a fundamental flaw: Because the heat of optical pumping is generated (more or less) uniformly in the rod's volume but the heat is removed peripherally, a radial thermal gradient is necessarily present; i.e., the center of the rod is hotter than the edge. This causes a radial strain in the rod, which in turn induces a cylindrical birefringence. If the fast axis at the 12 o'clock position in the rod is vertical, it is horizontal at the 3 o'clock position.

This position-dependent birefringence has two detrimental effects on laser operation. First, it scrambles linear polarization passing through the rod. Each section of the rod's cross section imposes a different birefringence on the linearly polarized light, coupling light from any linear orientation into the orthogonal orientation. An attempt to force the laser into linear polarization with an intracavity polarizer causes serious intracavity loss and diminished output power. Second, it imposes two back focal distances on light passing through the rod, which degrades the quality of the beam emerging from the laser.

Slab lasers were invented to avoid the cylindrical thermal gradients, but they require large volumes of material and thus often are impractical for crystalline lasers. Other techniques developed over the past 40 years have been at least partially successful at compensating for the cylindrical birefringence. Placing a quarter-wave plate between the rod and the back mirror, for example, causes the worst of the birefringence to be undone when the light passes through the rod for the second time.

resRod1.gif
Figure 1. In a radially polarized beam, the electric field vector is oriented radially.

The Israeli team has demonstrated a technique that, in theory at least, completely compensates for thermal birefringence in laser rods. In a radially polarized beam, the electric-field vector is always oriented in the radial direction (Figure 1). (In the orthogonal polarization, azimuthal polarization, the electric field is always in the azimuthal, or tangential, direction.) These two polarizations match the thermally induced cylindrical birefringence in a laser rod. A radially polarized beam will always "see" the same refractive index, no matter which section of the rod's cross section it passes through. The polarization of a radially polarized beam will not be scrambled in the rod, and the beam will "see" only a single back focal distance.

Two Flat Mirrors
Figure 2. With two flat mirrors, the only focusing element in the resonator is the laser rod. One cylindrical polarization will pass through the aperture with minimal loss, while the other can experience enough loss to extinguish it.

The team took advantage of this fact to force a laser to operate in a pure cylindrical polarization using a new resonator design (Figure 2). Because the resonator depends on thermal focusing for stable operation, the difference between the two back focal distances for the two different cylindrical polarizations is critical. One of the polarizations will besmall enough to pass through the aperture with minimal loss, and the other will experience a much higher diffraction loss at the aperture. If the loss is great enough, the disfavored polarization will not oscillate, and the laser will produce a beam of one, pure cylindrical polarization.

Pump Power Increases
Figure 3. As the pump power into the amplifier increases, the beam quality of the unpolarized beam degrades much more rapidly than that of the partially or completely polarized beam.

To demonstrate the advantage of cylindrical polarization, the Israeli team amplified this laser and an unpolarized laser in a rod amplifier (Figure 3). The results demonstrate that the beam quality of the cylindrically polarized laser, whether partially or completely polarized, is superior to that of the unpolarized one.


GLOSSARY
azimuthal polarization
Polarization that is perpendicular to a cylinder's radial axis. This description of polarization is useful for systems with cylindrical symmetry, such as interactions between focused beams and matter, or with the optomechanical tensor in components with cylindrical geometry, such as laser rods, circular lenses and mirrors. Also known as tangential polarization in the case of thermally induced birefringence.
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