Hyug-Gyo Rhee

Correlation of Topography Measurements of NIST SRM 2460 Standard Bullets by Four Techniques

Three optical instruments including an interferometric microscope, a Nipkow disc confocal microscope and a laser scanning confocal microscope and a stylus instrument are used for the measurements of bullet profile signatures of a National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 2460 standard bullet. The two-dimensional profile signatures are compared with the virtual bullet standard signature established by the same stylus instrument. The bullet signature differences are quantified by the maximum cross-correlation function CCFmax. If the compared signatures were exactly the same, CCFmax would be 100%. Comparison results show close agreement among the four techniques for bullet profile signature measurements. The average CCFmax values are higher than 90%. This supports the possibility of using surface topography techniques for ballistic identifications as an alternative to the current technology based on image comparisons.

Absolute distance measurement by two-point-diffraction interferometry

We present a point-diffraction interferometer that has been specially devised to perform absolute distance measurements in three dimensions. It is composed of two main parts: One is a target that moves in three dimensions, and the other is a stationary two-dimensional array of photodetectors. The target is made of point-diffraction sources that emit two spherical wave fronts, whose interference is monitored by the photodetectors. Application of a phase-shifting technique allows the phase values of the photodetectors to be precisely measured, which are then fitted to a geometric model of multilateration so as to determine the xyz location of the target by minimization of least-squares errors. Experimental results show that the proposed diffraction interferometer is capable of measuring the xyz coordinates of the target with a volumetric uncertainty of less than 1.0 microm over a working volume of a 100-mm side.

Realization and performance evaluation of high speed autofocusing for direct laser lithography

The autofocusing is one of the important parts in the automated vision inspection or measurement using optical microscopes. Moreover, laser micromachining or laser lithography requires a high speed and precision autofocusing. In this paper, we propose and realize an autofocusing system using two cylindrical lenses, which is the enhanced version of the previous astigmatism method. It shows very good performances, especially very high speed and the largest range in comparison with the previous astigmatic methods. The performance of our autofocusing system was evaluated by tracing the linear stage whose position was monitored by a commercial laser interferometer. Then we applied the autofocusing to the direct laser lithographic system, and successfully fabricated circular symmetry patterns on a 300 mm diameter surface with the resolution of less than 1 microm within the defocusing range of +/-50 microm. The speed of the autofocusing was 150 Hz.

3D surface mapping of freeform optics using wavelength scanning lateral shearing interferometry

Freeform optics have emerged as promising components in diverse applications due to the potential for superior optical performance. There are many research fields in the area ranging from fabrication to measurement, with metrology being one of the most challenging tasks. In this paper, we describe a new variant of lateral shearing interferometer with a tunable laser source that enables 3D surface profile measurements of freeform optics with high speed, high vertical resolution, large departure, and large field-of-view. We have verified the proposed technique by comparing our measurement result with that of an existing technique and measuring a representative freeform optic.

Laser output power stabilization for direct laser writing system by using an acousto-optic modulator

We present experimental results on the output power stabilization of an Ar(+) laser for a direct laser writing system (LWS). Instability of the laser output power in the LWS cause resolution fluctuations of being fabricated diffractive optical elements or computer-generated holograms. For the purpose of reducing the power fluctuations, we have constituted a feedback loop with an acousto-optic modulator, a photodetector, and a servo controller. In this system, we have achieved the stability of +/-0.20% for 12 min and the relative intensity noise level of 2.1 x 10(-7) Hz(-12) at 100 Hz. In addition, we applied our system to a 2 mW internal mirror He-Ne laser. As a consequence, we achieved the output power stability of +/-0.12% for 25 min.

Improvement of linewidth in laser beam lithographed computer generated hologram

We propose a new laser lithographic technique with enhanced resolution. A calcite wave plate is introduced in our system to separate an input lithographic beam into two orthogonally polarized beams. After going through an imaging lens, these two beams meet again on the focal point, and generate a small interferogram that sharpens the shape of the focused beam spot. Using this phenomenon, we can overcome the diffraction limit of the imaging lens and achieve a 486-nm-linewidth.

Absolute three-dimensional coordinate measurement by the two-point diffraction interferometry.

We describe a method of absolute xyz-coordinates measurement based on the two-point diffraction interferometer. In this paper we use a new optimization algorithm to the interferometer. Experimental results show that the systematic error of the interferometer is less than 1 mum (peak-to- valley value) within a 60 mm by 60 mm by 20 mm working volume. To extract the systematic error and verify the absolute performance of the interferometer we applied the Fourier self-calibration concept.

Correction of rotational inaccuracy in lateral shearing interferometry for freeform measurement

A lateral shearing interferometer has an advantage over previous wavefront measuring interferometers since it requires no reference. Therefore the lateral shearing interferometer can be a powerful solution to measure a freeform surface. It, however, has some issues to be resolved before it can be implemented. One of them is the orthogonality problem between two shearing directions in LSI. Previous wavefront reconstruction algorithms assume that the shearing directions are perfectly orthogonal to each other and lateral shear is obtained simultaneously in the sagittal and tangential directions. For practical LSI, however, there is no way to guarantee perfect orthogonality between two shearing directions. Motivated by this, we propose a new algorithm that is able to compensate the rotational inaccuracy. The mathematical model is derived in this paper. Computer simulations and experiments are also displayed to verify our algorithm.

Azimuthal position error correction algorithm for absolute test of large optical surfaces.

Absolute test needs test part rotation to separate errors of the interferometer itself from errors due to the test surfaces. At this time, previous absolute test algorithms assume no azimuthal position error during part rotation. For large optics whose diameters are 0.6 m and over, however, exact rotations are physically difficult. Motivated by this, we propose a new algorithm that adopts least squares technique to determine the true azimuthal positions of part rotation and consequently eliminates testing errors caused by rotation inaccuracy.

Metrological atomic force microscopy integrated with a modified two-point diffraction interferometer

We propose a new concept for a metrological atomic force microscope integrated with a modified two-point diffraction interferometer that can determine the xyz-coordinates in an absolute manner. The interferometer consists of two single-mode optical fibers; one fiber is fixed to be stationary and the other is attached on an xyz-stage that moves with the measuring sample. The two fibers generate two separate spherical wavefronts that interfere with each other. Phase information can be obtained from this interference fringe pattern by applying well-known phase shifting techniques. This phase information is very sensitive to the relative coordinates between the two fiber ends, and thus the xyz-coordinates of the stage can be measured with respect to the fixed machine structure. A 100 μm × 100 μm × 20 μm working volume is available in the proposed system.