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Crystals Enable Wavefront Sensor

Photonics Spectra
Feb 2001
Richard Gaughan

Because light interacts with the environment through which it travels, it is used to probe the characteristics of a wide variety of systems. Researchers collect the transmitted light and analyze intensity patterns and spectra, but no matter how advanced standard techniques are, they throw away half the story: the phase information. While there are means to recover this significant information, including interferometry, Shack-Hartmann sensors and holography, they are restricted in scope or difficult to implement.

Now a new method has the potential to obtain complex phase information from straightforward imaging techniques.

Karsten Buse and Marc Luenemann at the University of Osnabrück have created a wavefront sensor using two crossed polarizers and a 0.5-mm-thick lithium niobate crystal. The initial polarizer transmits light with polarization 45° from the ordinary and extraordinary axes of the birefringent crystal, and the light then strikes the lithium niobate.

If the wavefront of the incident radiation is perfectly planar, the light undergoes the same degree of polarization rotation. For a wave with a nonuniform phase front, however, the differently tilted regions of the beam travel through slightly different thicknesses of birefringent crystal.

The degree of polarization rotation depends on the distance that the light travels through the crystal. The final polarizer transmits only the light oriented on its axis, which is set 90° from that of the initial polarizer.

A 384 x 256-pixel CCD camera then images the light, creating an inten-sity map with information on the phase as a function of the X and Y position in the incident beam. The birefringent crystal rotates slightly around the X- and Y-axes, and additional exposures enable the user to specify the wavefront.

Larger dynamic range

The demonstration system, which the researchers described in the Oct. 16, 2000, issue of Physical Review Letters, displayed a 15-µm sensitivity to wavefront deviations and a dynamic range of 3 mm. "We have less sensitivity than interferometry," Buse said, "but we have a much larger dynamic range." The thickness and type of birefringent crystal determined the sensitivity and effective dynamic range.

Buse, who is now at the University of Bonn in Germany, plans to improve the sensor's performance, including its depth resolution, dynamic range and spatial resolution. He also will begin to investigate specific practical applications.

"It would be great if the phase sensor could be applied for in situ measurements of the shape of the eyeball during laser vision correction," he suggested. He has not started such a project, but he believes the potential is high. "The sensor is just that simple."

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