Researchers at Université Laval in Quebec City have shown that, in some situations, one arm is better than two. They have demonstrated a Bessel beam interferometer that does not require a stable reference arm, promising a simpler, less-expensive and less-sensitive-to-vibration technique than other interferometric instruments.Most interferometers split a light beam into a reference arm and into an arm that probes the object being characterized. When the two are recombined, they interfere and reveal subwavelength details about the object. The problem, noted graduate physics student Mathieu Fortin, is that changes in the reference arm can cloud any conclusions. As a result, he said, it is crucial that the reference arm be absolutely stable against any type of vibration.Professors Michel Piché and Ermanno F. Borra, along with Fortin, have shown that Bessel beam interferometry solves this problem. In their demonstration, they produced optical beams whose shape was described by Bessel functions, solutions of the free-space wave equation in circular cylindrical coordinates. The output of a 632.8-nm HeNe laser from Newport Corp. of Irvine, Calif., passed through a mask featuring two 16-µm-wide concentric annular slits. The mask produced a superposition of two Bessel beams that interfered with each other. After traveling through an optical setup, the interferometric pattern produced was captured with a 512 × 480-pixel CCD camera from Hitachi Ltd. of Tokyo.The researchers used the interferometer to probe the surface of a 1.4-m-diameter liquid mirror made out of a rotating bowl of mercury. By adjusting the rotation, they could change the parabolic curvature of the mirror. Using Bessel beam interferometry to extract the mirror's parameters, they measured its curvature to a precision of approximately λ/20 and its tilt to approximately 1 µrad.Fortin noted that Bessel beam interferometry complements such tools as Shack-Hartmann wavefront sensors, which are capable of accuracies better than λ/100. He thinks it is possible to achieve comparable results with improved Bessel beam technology. One enhancement could involve the use of three or more Bessel beams. Another would be to perform better image analysis.Some real-world applications for such technology could be in measuring deformable surfaces or in characterizing optical systems. Commercial Bessel beam interferometers, according to Fortin, would not be expensive.