3-D IMAGING: Kerr gate measures diffusing surfaces

IBARAKI—The National Research Laboratory of Metrology has demonstrated an ultrafast, time-resolved, two-dimensional (2-D) imaging system that uses a femtosecond-amplifying optical Kerr gate for three-dimensional (3-D) shape measurement of a completely diffusing surface.

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IBARAKI—The National Research Laboratory of Metrology has demonstrated an ultrafast, time-resolved, two-dimensional (2-D) imaging system that uses a femtosecond-amplifying optical Kerr gate for three-dimensional (3-D) shape measurement of a completely diffusing surface. The device can measure stepped and spherical surfaces with high spatial resolution and sensitivity because of a short opening time of 459 fs and a maximum transmittance of 185%.

According to researchers Kaoru Minoshima, Hirokazu Matsumoto, and Takeshi Yasui (now at Osaka University), the optical Kerr gate (an ultrafast optical switch that uses the optical Kerr effect) has several advantages as a tool for time-resolved imaging of diffusing surfaces. For example, real-time 2-D imaging and a wide spectral bandwidth are possible because of freedom from the phase-matching condition and high sensitivity in the picosecond region. A move into the femtosecond region, however, can necessitate a trade-off between high sensitivity and fast response, limiting gate performance in time-resolved imaging. Minoshima and colleagues resolved the issue using techniques developed to amplify optical Kerr gates in the picosecond region.1

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Experimental 2-D imaging system relies on a femtosecond amplifying optical Kerr gate to provide ultrafast time-resolved 3-D imaging of a diffusing object. The high peak intensity of femtosecond optical pulses provides an efficient optical nonlinear effect without a thermal effect, as well as an increase of the signal-to-noise ratio.

The experimental imaging system used a light source from an amplified, modelocked Ti:sapphire laser with a central 790-nm wavelength, 100-fs pulsewidth, 1-mJ pulse energy, and 100-Hz repetition rate (see figure on p. 47). The light was divided into two pathways as probe and pump beams, with the white-continuum probe beam incident on the diffusing surface at a 45

The researchers' goal was precision 3-D shape measurement of a diffusing surface without the need for a highly sensitive detection device or complex image processing. They applied the imaging system to shape measurement of a stepped surface, as well as a heart-shaped object. In each case, a 3-D image of the surface was obtained using time-resolved 2-D images. The transverse resolution of the time-resolved imaging system was 70 µm (14.3 line pairs per millimeter) at a contrast of 0.28 (where optical nonlinear effects do not cause deterioration in the quality of the time-resolved image). The positioning accuracy was 5.9 µm, and it was possible to distinguish two separate images on a sample with a depth interval of 100 µm without any image processing.

Paula Noaker Powell

REFERENCE

  1. T. Yasui, K. Minoshima, and H. Matsumoto, Appl. Opt. 39, 65 (Jan.1, 2000).

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