Hollow Glass Waveguides Maintain Polarization of CO2 Laser Beam

Breck Hitz

Hollow glass waveguides, whose inside surfaces are coated to reflect 10.6-µm radiation, often are employed to deliver the beam from an industrial CO₂ laser to the workpiece. In many cases, such as for metal welding and cutting, a polarized beam is more efficient than an unpolarized beam.

Unfortunately, hollow waveguides of circular cross section scramble the polarization. Investigators have studied square and rectangular metal waveguides, which maintain the polarization of the radiation passing through them, but found that they are difficult to fabricate and relatively inflexible.

Now engineers at Rutgers University in Piscataway, N.J., have fabricated hollow glass waveguides that have square and rectangular cross sections. They say that these waveguides provide good polarization-maintenance properties for use with 10.6-µm radiation from CO₂ lasers.

Figure 1. The inner dimension, a, of the square waveguide was 500 µm. The inner dimensions of the rectangular waveguide, a X b, were 225 X 1250 µm. The inside corners were not as sharp as depicted here, but were somewhat rounded as a result of the drawing process.

The researchers fabricated their waveguides from borosilicate glass tubing (Figure 1). They deposited a silver film on the inside surfaces of the waveguides and, using an iodization process, converted some of the silver to AgI to form a uniform dielectric layer. They measured the waveguides' attenuation to be 0.8 dB/m for the square waveguide and 2.2 dB/m for the rectangular one. These are higher than the values for round hollow glass waveguides, which typically are 0.5 dB/m, but they are consistent with the values that other investigators have reported for square and rectangular metal waveguides.

Figure 2. The mode patterns of the radiation emerging from the square and rectangular waveguides were inferior to that from a round waveguide.

The mode quality of the light emerging from the square and rectangular waveguides was inferior to that from a round one (Figure 2). The mode that emerged from the square waveguide showed fringes of energy near the corners, and the mode that emerged from the rectangular one showed a series of fringes or higher-order modes in the direction of the longer axis.

Both waveguides showed very good polarization-maintaining characteristics (Figure 3). The researchers measured the polarization ratio, Polarization = (P₍₍- P_'/ (P₍₍ + P_'), for 125-cm-long fibers.

Figure 3. The polar-coordinate figures represent the optical power measured through a polarizer oriented at the indicated angle.

In the figure, (a) is the polarization from the CO₂ laser that entered the fiber under test; it was 100 percent polarized. The light emerging from the square fiber was 69 percent polarized when the input polarization was parallel to the waveguide edge (b) and 54 percent polarized when the input polarization was at 45° to the edge (c). A round fiber failed to maintain the polarization significantly, and the emerging radiation was only 26 percent polarized (d). The rectangular fiber maintained the polarization better than 90 percent for input polarization aligned along both the long (e) and short (f) waveguide dimensions.

Figure 4. The rectangular waveguide was twisted 90°, and the radiation emerging from it still was highly polarized.

The rectangular waveguide maintained its polarization even when it was twisted. The polarization patterns, before and after the waveguide was twisted 90°, are shown in Figure 4. The 90 percent polarization ratio was maintained despite the twist in the waveguide.