Visualizing a Laser’s Polarization

Breck Hitz

One of the oldest tricks in the book is checking a laser beam’s polarization with a microscope slide: You stick the slide, oriented at Brewster’s angle, into the beam and then rotate it around the beam axis until the reflection disappears. The direction of the disappearing reflection is the direction of the beam’s polarization. A drawback of this timeworn technique is the safety hazard of having stray beams wandering around on the laboratory walls and ceiling.

Figure 1. The pattern of scattered light in the glass sandwich reveals the polarization of the laser beam. The diagram in the lower corner of each photo indicates the polarization of the beam. Reprinted with permission of Applied Physics Letters.

A recent paper in Applied Physics Letters and a product introduced at Photonics West suggest two alternatives to the standard technique. Although neither may be quite as quick and dirty as the venerable microscope slide, both offer interesting and intriguing possibilities. Riccardo Castagna and his colleagues at Università Politecnica delle Marche in Ancona, Italy, sandwiched a thin layer of photosensitive material between two glass plates and illuminated the sandwich with their laser beam. The pattern of scattered light revealed whether the light was linearly polarized and, if so, in which direction it was polarized (Figure 1).

Figure 2. The scattered light is guided by total internal reflection at the glass-air interface. Reprinted with permission of Applied Physics Letters.

The mechanism behind the effect is complex, the scientists say. The incoming laser beam triggers polymerization of the photosensitive material, leading to the growth of microscopic droplets that scatter light from the beam. The classical laws of scattering from small particles prohibit scattering in the direction of the light’s polarization vector, so the light is preferentially scattered out from the beam in a direction perpendicular to the beam’s polarization. Once scattered sideways from the beam, the light is guided by total internal reflection at the glass-air interface (Figure 2). What isn’t clear to the scientists is why the light propagates through the glass only within a limited angular range (i.e., between the two cones illustrated in Figure 2).

Figure 3. Several colored LEDs are mounted in a spinning circuit board. A detector in the middle of the board controls which LEDs are illuminated. Courtesy of Paradigm Lasers Inc.

Whatever the exact physics, the technique is easy and inexpensive to implement, the scientists say, and produces a permanent record of a laser beam’s polarization. For more accurate measurements, they add, CCD cameras or other detectors could record quantitative data.

The product introduced at Photonics West — the o-Tool from Paradigm Lasers of Spokane Valley, Wash. — is a spinning circuit board with embedded colored LEDs. Because the board is spinning, each LED appears to generate a circle of light (Figure 3). The incoming laser beam is directed at a detector in the middle of the circuit board; the more powerful the beam, the greater the number of LEDs that light up.

Figure 4. The pattern displayed by the illuminated LEDs reveals the polarization of the incoming laser beam. Courtesy of Paradigm Lasers Inc.

A simple polarizer positioned in front of the rotating circuit board allows the circuitry to analyze the transmitted intensity as a function of angle and to generate a display that reveals the polarization of the incoming beam (Figure 4).

This instrument has its own set of advantages. It’s sensitive from the visible into the infrared, and it has a submillisecond response time. This latter characteristic makes it useful as an alignment tool: It shows the effect of tweaking a laser’s mirrors even faster than a conventional power meter does. With appropriate beam-expanding optics, it even can display the shape of an incoming beam. And, priced in the same range as an iPod, it’s a useful toy that will not break any budgets.

Applied Physics Letters, Jan. 31, 2008, Vol. 92, 041115.