Interferometer Yields Large Gratings

Perry J. Greenbaum

A European research team has demonstrated a low-cost solution to the manufacture of large diffraction gratings using a variant of an interferometer developed more than a decade ago. The work promises to yield better gratings for chirped pulse amplification lasers.

The team, led by Ian N. Ross of Rutherford Appleton Laboratory's central laser facility in Didcot, UK, has developed an interferometer that produces high-quality gratings using low-coherence laser light sources and small optical lenses. The system is based on an interferometer proposed in 1990 by Robert R. Hershey and Emmett N. Leith that transmits the recording beam through a grating with n lines per millimeter and then through a pair of gratings with 2n lines per millimeter to record a 2n-lines-per-millimeter element. The team instead used reflection gratings in its interferometer.

An interferometer based on a design by Hershey and Leith enables the low-cost production of large diffraction gratings.

Traditional interferometric methods for the production of gratings rely on well-collimated beams. This necessitates high-quality -- and, therefore, expensive -- optics whose size exceeds that of the grating being recorded, or produced.

The new technique relaxes these requirements. In experiments using 458-nm light from a 1-W argon-ion laser with a <1-cm coherence length, the group recorded 1200-lines-permillimeter gratings up to 200 mm in size (the largest blanks that were available to the team) with an rms error of l/12.

Even larger gratings are possible, Ross said, because the process can be repeated as many times as desired, each time magnifying by a factor of at least two. "All this means is a low-cost, low-tech method for making gratings of arbitrary size," he said, "which could be afforded and set up within most laboratories."

Ross expects that the research will lead to commercial ventures but said that the team itself has not taken steps to do so.

"Although we are not currently talking to any companies, we would be happy to set up licensing agreements."