Takefumi Inagaki

Real-time fingerprint sensor using a hologram

A holographic fingerprint sensor has been developed for a system that identifies a person by his or her fingerprints. The sensor uses a laser as its light source and consists of a light-conducting plate, which is a transparent glass plate with a plain grating-type hologram, and a focusing lens system just under the hologram. Since the sensor uses a plane-parallel plate, all the optical paths from each point of a fingerprint to the hologram are equal, and a bright fingerprint can be created without the trapezoidal distortion that is inherent in conventional prism-type sensors.

Wavelength independent grating lens system.

Grating lenses are small, light, and easily mass-produced. However, wavelength variation in the light source causes aberration and changes the focal length of the grating lens. Therefore, it has been difficult to use grating lenses in high-precision focusing optical systems that use a light source with wavelength variations (e.g., a diode laser). To solve this problem, we designed a grating lens system of two grating lenses, which substantially suppresses aberration and keeps the focal length constant at several tens of nanometers of variation. Each grating is made as a concentric circle. These lenses are arranged so that their centers are collinear. Diffraction angle changes due to wavlength variations are compensated for by the second grating. Our calculations confirmed that the allowable wavelength range was +/-15 nm or more for a numerical aperture (N.A.) of 0.5. We made a prototype of this grating lens system by electron beam lithography and confirmed that this lens system was not affected by limited wavelength variations.

Holographic Fingerprint Sensor

The fingerprint sensor that we developed uses a hologram. Two requirements are important for actual use; laser safety and high-contrast images. The illumination method we developed uses total reflection and a new type of detection. For safety, total-reflection lighting ensures that laser beams cannot enter an operators eyes. To obtain high-contrast images, signal and noise light were separated.

Aberration corrections for a POS hologram scanner

Use of an optically generated IZP (interferometric zone plate) hologram scanner enables highly accurate reading in a supermarket point-of-sale (POS) label reader with a simple optical arrangement. The laser beam spots on the scanning plane which construct the lattice scan pattern designed for the POS label reader cause severe aberrations when a conventional IZP hologram is used. A simple and effective method for removing the aberration is discussed from both the theoretical and experimental aspects, and the feasibility of the method is demonstrated. This method employs oblique-angle coherent plane-wave illumination in the IZP hologram recording process.

Polarization and frequency control of a semiconductor laser with a new external cavity structure

A technique using a new external cavity structure to cause degeneration at the frequencies of two orthogonally polarized modes and to control the polarization and the frequency in a semiconductor laser has been devised. The phase difference between the polarizations in the gain medium can be canceled in a round trip through quarter‐wave plates set in the cavity. By rotating one of the quarter‐wave plates, orthogonally polarized laser beams can theoretically be made to oscillate at slightly different frequencies. Experimentally, two orthogonally polarized oscillations with rather large frequency differences occurred. The difference between the two frequencies alternated according to the quarter‐wave plate rotation. This phenomenon could be explained theoretically.

Automatic opitcal through hole inspection method for printed wiring boards using leakage light detection

New automatic through hole inspection techniques for printed wiring boards (PWBs) were developed. The new method called Leakage Light Detection can detect defects such as cracks and breaks in plating of through holes with aspect ratio as large as ten. A prototype system for automatic through hole inspection is constructed, based on these techniques. This system detects defects in a 250 mm × 250 mm area of a PWB in 8 minutes.

Automatic Mask Pattern Inspection For Printed Circuits Based On Pattern Width Measurement

This is a new mask Inspection technique based on pattern width measurement that can check the fatality of defects. Mask quality depends on pattern width and defects in a fatal area where a normal pattern is seriously damage. In this technique, pattern width measurement and fatal defect detection with a laser beam are used for inspection of masks of PC boards. In order to determine the direction of measurement, diffracted laser light at the pattern edge and spatially divided photodetectors are used. The new inspection system (l) measures pattern width, (2)does not require the original data for comparison, (3)detects fatal defects, (4)has no special alignment and (5)detects defects as small as 10μm can be isolated with 100% accuracy.

New holographic technology for a compact POS scanner.

A new holographic technique has been used to make a compact, accurate, and reliable point-of-sale scanner. Our holo-window technique is capable of changing the scan direction, collecting the signal light, and equalizing the scan velocity. At present, compact scanners tend to sacrifice read operation accuracy, speed, and reliability for size. Our technique permits the miniaturization of the optical system of a scanner while preserving performance. Using the holo-window, we have developed a new scanner that has a letter-size footprint and is only 8 cm high.

Straight-line scanning analysis of an all holographic scanner.

The all holographic straight-line scanner we developed for laser diode printers consists of only a holographic disk and a holographic lens. This simple scanner meets all scanning requirements for printers such as straight-line scanning, low scan line placement error, and beam focusing. It also overcomes the deterioration in scanning characteristics caused by the individual wavelength variations among laser diodes. We extensively analyzed how to obtain straight-line scanning based on different wavelengths and the generalized concept of virtual wavelength ratio enabling flexible scanner design.

Quantitative Voltage Measurement By A Software Closed Loop Technique In Electron Beam Testing

A software closed loop technique was devised for quantitative voltage measurement in electron beam testing for LSIs. A retarding voltage of an energy analyzer is controlled iteratively by a computer to reduce difference between a slice level and a secondary electron signal to zero. The voltage is determined by the retarding voltage at the cross point of the slice level and the energy distribution curve. Using this technique, the waveform of 256 sampling phases with more than 5 V amplitude can be measured in about 30 s with 200 mV voltage resolution and 100 ps time resolution.