Tetsuo Yoshizawa

Simultaneous three-dimensional step-height measurement and high-resolution tomographic imaging with a spectral interferometric microscope.

A noncontact, nonmechanical scanning, wide-field spectral interference microscope is developed for simultaneous measurement of three-dimensional step-height of discontinuous objects and tomographic imaging. A superluminescent diode (SLD) is used as a broadband light source and a liquid-crystal Fabry-Perot interferometer (LC-FPI) as a frequency-scanning device. By means of changing the injection current to the SLD, the spectral profile of the SLD is equalized, and a constant light input to the interferometer is achieved over the entire frequency-scan range. The Fourier-transform technique is used to determine both the amplitude and the phase of spectral fringe signals. Three-dimensional height distribution of a discontinuous object is obtained from the phase information, whereas optically sectioned images of the object are obtained either from the amplitude information alone or from the combination of both the amplitude and phase information. Experimental results with submicrometer resolution are presented for both step-height measurement and tomographic sectioning.

Spectral interferometric microscope with tandem liquid-crystal Fabry-Perot interferometers for extension of the dynamic range in three-dimensional step-height measurement

The maximum measurable range of a spectral interference microscope depends on the coherence length of the light transmitted by its tunable spectral filter. To achieve a large range in step-height measurement we have developed a new tunable spectral filter that uses tandem liquid-crystal Fabry-Perot interferometers (LC-FPIs), which can simultaneously attain both a high spectral resolution and a large tuning range. Fringe visibility measurements were carried out, and it was found that the coherence length of the light transmitted through tandem LC-FPIs is two times larger than that transmitted through a single LC-FPI. Using this novel tunable spectral filter, we developed a new spectral interference microscope for the measurement of three-dimensional shapes of discontinuous objects. Experimental results of step-height measurements both with a single LC-FPI and with tandem LC-FPIs are presented for a combination of standard steel gauge block sets with 1-, 99-, and 100-microm steps. A large range (1-100 microm) of measurement with submicrometer resolution was achieved with tandem LC-FPIs that was not possible with our previous system in which a single LC-FPI was used.

Polarization-Independent Tunable Liquid-Crystal Fabry-Perot Interferometer Filters

Two methods are studied for reducing the polarization dependence of a tunable liquid-crystal Fabry-Perot interferometer filter (LC-FPI). One uses a twisted nematic (TN) liquid-crystal in a Fabry-Perot cavity. We have theoretically explained that the twist angle should be π/2+mπ (m=0, 1, 2,) to eliminate the polarization dependency. An approximate 5 nm polarization-independent tuning with a π/2 TN LC-FPI cell was achieved. The other uses a polarization diversity configuration. We have fabricated a polarization-independent LC-FPI filter module with a Faraday rotator using polarization diversity double-pass configuration. This filter module has a 3 dB bandwidth of less than 0.5 nm, a 15 dB bandwidth of 1.7 nm, and a polarization-dependent loss (PDL) of 0.8 dB. The filter module was utilized in 10 Gbit/s transmission experiments to confirm its feasibility.