Laser Ablation Shapes Precision Optics

Aaron J. Hand

Trying to precisely shape optics and achieve low wavefront distortion with conventional polishing methods can be a daunting task, particularly for optical components that are larger than 50 cm or smaller than 1 cm. For both large and small optics, wavefront measurements are difficult.

"It is much more convenient if we can shape and measure the optics on the same stage," said Takahisa Jitsuno, an associate professor at Osaka University. "Of course it is also convenient in medium-size optics, but we have several schemes for dealing with medium-size optics."

Jitsuno and fellow researchers at the university's Institute of Laser Engineering have developed a technique that performs shaping and phase measurement simultaneously, achieving low wavefront aberrations. Called laser ablative shaping, it uses a 193-nm ArF excimer laser to shape optics and a phase-shift interferometer to monitor aberration in situ.

The researchers have tried to shape glass and quartz using this method, Jitsuno said, but debris causes unacceptable surface roughness. With plastics, ablated materials react with O₂ in the atmosphere to make the amount of debris negligible. Bringing together the uniform ablation properties of plastics and the preferred optical properties of glass, the researchers coated a glass substrate with a 50-µm-thick layer of UV-cured resin.

The experimenters used an excimer laser from Lambda Physik GmbH of Göttingen, Germany, to generate flat and spherical shapes in a 5-cm-diameter hybrid glass-plastic substrate. They used a fluence of 45 mJ/cm², which minimized surface roughness.

Laser ablative shaping could reshape laser diode microlenses after assembly to compensate for position errors.

The starting-point wavefront distortion of the flat surface was 3.0 lambda, dropping to 0.17 lambda over 90 percent of the surface after laser ablation. The spherical surface started at 2.5 lambda, and the shaping method produced an aspherical component with a wavefront of less than 0.2 lambda. With the laser operating at 17 Hz, processing took about four hours. A standard polishing machine in good condition would take about six hours to reach the same level with a glass substrate, Jitsuno said.

The accuracy and surface roughness achieved make laser ablative shaping useful for general-purpose optics. The technique could also be used to shape the rear surface of diffractive optics. It may be particularly suited to microlenses for laser diodes or single-mode fibers because it could reshape the surface after assembly to compensate for position errors.

The researchers have experimented in this area since publishing their results in the May 20, 1999, issue of Applied Optics. Because of wavefront errors on the laser diodes, they switched to a Shack-Hartman wavefront sensor, which is more sensitive to the surface ripple generated in polymethyl methacrylate. They recently succeeded in smoothing the surface by ablating with assistance from a new pulsed CO₂ laser, Jitsuno said. The single-mode fibers are proving easier, he said, but surface roughness is not yet acceptable.