Search

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.

This paper describes an all-holographic straight-line scanner consisting only of a holographic disk and a holographic lens. Scanning beam aberration correction was extensively analyzed using diffraction theory. A new technique for simultaneously optimizing the phase transfer functions of these two holograms is proposed, and a method to construct these two holograms using holographic recording is discussed. This technique led to a compact, high resolution holographic line scanner with a 1/e(2) scanning beam spot size of 100-120 microm for a scanning width of 252 mm. The radius of the disk at the center of illumination is only 28 mm.

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.

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.

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.