Abstract

We discuss two ways to use femtosecond pulsed lasers as a new interferometric light source for enhanced precision surface-profile metrology. First, a train of ultrafast laser pulses yields repeated low temporal coherence, which allows unequal-path scanning interferometry, which is not feasible with white light, to be performed. Second, the high spatial coherence of femtosecond pulsed lasers enables large-sized optics to be tested in nonsymmetric configurations with relatively small-sized reference surfaces. These two advantages are verified experimentally with Fizeau and Twyman–Green type scanning interferometers.

© 2005 Optical Society of America

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References

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Figures (4)

Fig. 1
Fig. 1

Scanning interferometers that use femtosecond laser pulses: (a) unequal-path Fizeau configuration, (b) unequal-path nonsymmetric Twyman–Green configuration. L, surface gap; L c , cavity length; BS, beam splitter; FL, focusing lens; CL, collimating lens; OL, objective lens; a, b, laser pulses.

Fig. 2
Fig. 2

Typical temporal interferogram sampled with scanning from a single pixel of the CCD camera. Inset, spatial interferogram captured instantaneously by use of all the pixels of the CCD camera.

Fig. 3
Fig. 3

Comparison of spatial interferograms obtained from different sources for a spherical mirror: (a) 10.5 fs pulsed laser, (b) He–Ne laser, and halogen lamps filtered by (c) a multimode fiber of 700 μ m core diameter, (d) a multimode fiber of 200 μ m core diameter, and (e) a single-mode fiber of 3 μ m core diameter.

Fig. 4
Fig. 4

Measured surface maps with sectional profiles obtained by (a) envelope peak detection and (b) fringe peak detection. The measured object is the front surface of well-polished glass.

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