CN1141664C - Fingerprint image sensor based on waveguide holography technology and method for collecting fingerprint images - Google Patents

Abstract

The invention belongs to the field of photoelectric image sensors, comprising: a semiconductor laser linear array is illuminated by collimated coherent light emitted by a microlens linear array; an optical waveguide utilizes a total reflection mechanism to form a closed optical path system; a waveguide holographic grating element is used to couple an incident beam out of a waveguide substrate , use the total reflection mechanism to obtain the fingerprint image, and reconstruct the wavefront of the outgoing beam; use the fiber cluster as the optical transmission and imaging element, use the CMOS imaging chip to collect the fingerprint image, and transmit it to the computer in digital video format. Its core optical component, waveguide holographic grating, is fabricated using Ar ⁺ laser with interference fringe exposure method.

Description

Fingerprint image sensor based on waveguide holography technology and method for collecting fingerprint images

Technical Field The present invention belongs to the field of photoelectric image sensors, in particular to the preparation and application of waveguide holographic micro-optical elements for detecting the surface topography of human fingerprints.

Background Art Fingerprint identification technology is a widely used high-reliability identification technology. Using a fingerprint image sensor to obtain a high-resolution, high-contrast, and low-distortion fingerprint image is a prerequisite for accurate fingerprint recognition. At present, most of the widely used fingerprint collection devices are photoelectric fingerprint image sensors of the total reflection prism type, which generally have large volume, weight, power consumption and high manufacturing cost. Therefore, the development of a new type of high-performance, low-cost integrated micro-photoelectric fingerprint image sensor has been paid more and more attention by experts at home and abroad.

At present, fingerprint image sensors mainly include photoelectric, capacitive array, and thermal array. Among them, the working principle of the most well-developed and most widely used photoelectric fingerprint image sensor of total reflection prism is shown in Figure 1 and Figure 2, and is described as follows:

The collimated parallel incident light beam 2 emitted by the extended light source 1 for illumination enters the prism from the right angle side of the total reflection prism 3 . The picked finger 4 is pressed on the hypotenuse of the prism. The fingerprint groove part 5 is not in contact with the prism surface, and the incident light beam 2 is totally reflected here; the fingerprint ridge 6 part contacts the prism surface, destroying the total reflection condition, and the incident light beam 2 will be scattered. Thus, the incident light beam 2 is modulated by the fingerprint pattern to form the outgoing light beam 7 carrying the fingerprint image, which enters the image acquisition system 8, and the image of the fingerprint can be obtained after further processing.

In the above-mentioned method, since each optical element is separated and an open space optical path is adopted, the structure is complex and the volume is large, and it is easily disturbed by background light noise.

SUMMARY OF THE INVENTION The purpose of the present invention is to overcome the deficiencies of the prior art, and propose a fingerprint image sensor based on waveguide holographic technology, which uses waveguide holographic optical elements and adopts a micro-integrated closed optical path design, so that it has a small size and is not easily affected. The background light noise interferes, the preparation cost is low, and the characteristics are suitable for mass production.

The present invention proposes a fingerprint image sensor based on waveguide holography technology, which is characterized in that it includes

One side is inclined to form an optical waveguide plate that reconstructs the incident angle of light and a waveguide hologram attached to its upper surface. The inclined end of the optical waveguide is equipped with a semiconductor laser line array and a microlens array. The operating wavelength of the semiconductor laser is the same as the said The reconstructed light incident angle of the optical waveguide plate is matched, the lower surface of the waveguide plate is closely attached to the large end of the fiber cluster, and the small end of the optical fiber cluster is closely attached to the CMOS imaging chip. The CMOS imaging chip is installed in the signal conditioning and interface board.

The waveguide holographic sheet of the present invention consists of two parts: an optical glass base plate with a thickness of 1.8-2.5 mm and a holographic grating on one side of the plate made on a silver halide photosensitive emulsion layer with a thickness of 5-15 μm.

According to the holographic diffraction grating theory and the waveguide holographic theory, the refractive index of the photosensitive emulsion layer on the waveguide hologram is slightly smaller than that of the optical waveguide plate, so the holographic grating formed forms a volume holographic structure.

The working wavelength of the semiconductor laser line array is 650nm red light, and the reconstructed light incident angle of the optical waveguide plate is 70°.

The present invention also proposes a method for collecting fingerprint images by a fingerprint image sensor, which is characterized in that an optical waveguide plate formed by tilting one side to form a reconstructed light incident angle, a waveguide hologram closely attached to its upper surface, and a semiconductor laser line array, The fingerprint image sensor composed of CMOS imaging chip, signal conditioning and interface circuit board, the collection of fingerprint images specifically includes the following steps:

1) The fingerprint image sensor is connected to the upper computer, and the finger to be collected is pressed flat on the waveguide hologram;

2) The fingerprint image sensor lights up the lighting source, and emits an incident light beam that passes through the base optical flat glass and enters the waveguide hologram. According to the principle of total reflection to collect fingerprint images, the outgoing light beam carries the fingerprint image, and when passing through the waveguide holographic grating, diffraction occurs and leading to wavefront reconstruction;

3) The outgoing beam after the wavefront reconstruction is converged and imaged on the CMOS imaging chip through the fiber cluster;

4) The computer controls the CMOS imaging chip to obtain the fingerprint image in a snapshot mode;

5) The fingerprint image sensor transmits the fingerprint in digital format to the computer through the transmission cable.

The principle of the present invention to obtain the fingerprint image is: the semiconductor laser light source emits an incident light beam with a central wavelength within the red light range, and enters the obliquely split base optical plate glass at an incident angle matching the central wavelength of the incident light. The incident light beam enters the waveguide hologram through the base optical flat glass, and is totally reflected on its upper surface. The outgoing beam will be amplitude modulated by the fingerprint, carrying the fingerprint image. When the outgoing beam passes through the holographic grating layer on the lower surface of the waveguide hologram, it will diffract and cause the wavefront to be reconstructed, thereby changing the propagation direction, entering the fiber cluster directly below, and converging on the photosensitive unit array of the CMOS imaging chip. The light intensity signal is sent to the computer or other central controllers in digital video format after conditioning and interface circuit conditioning.

Features of the present invention include:

1. Using optical waveguide, using the total reflection mechanism to form a closed optical path;

2. Use semiconductor laser line array and microlens line array to form collimated coherent beam illumination;

3. Use the waveguide holographic grating element to couple the incident beam out of the waveguide, form total reflection to obtain the fingerprint image, and reconstruct the wavefront of the outgoing beam;

The advantages of the present invention are:

1. The closed optical path design can effectively shield the noise introduced by ambient stray light;

2. Effectively integrate the use of integrated optics and fiber optic components, the system is compact and small;

3. The core waveguide holographic element has low manufacturing cost and is suitable for mass production.

Description of drawings

Figure 1 is the working principle of the current total reflection prism photoelectric fingerprint image sensor.

Figure 2 is the principle of using the total reflection method to extract fingerprint images.

Fig. 3 is a device diagram of the waveguide holographic integrated fingerprint image sensor system of the present invention.

Fig. 4 is the working principle of the waveguide holographic element of the present invention.

Fig. 5 is a diagram of a manufacturing device for a waveguide holographic element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of a fingerprint image sensor based on waveguide holography technology of the present invention is described in detail as follows in conjunction with the accompanying drawings:

The fingerprint image sensor structure embodiment that the present invention proposes is shown in Figure 3, Figure 4:

The core optical elements of the fingerprint image sensor are an optical waveguide 11 with one side being slanted at 70° and a waveguide hologram 12 close to its upper surface. Between the waveguide holographic sheet 12 and the optical waveguide 11, a refractive index matching member 13 is used to ensure the close contact between the two. The end of the optical waveguide that is slanted is installed with a semiconductor laser line array (center wavelength 650nm) 9, and the laser beam is collimated by the microlens line array to form an illumination beam. The lower surface of the waveguide plate is closely attached to and installed with the large end of the fiber cluster 16 , and the small end of the optical fiber cluster is closely attached to and installed with the CMOS imaging chip 17 . The CMOS imaging chip is installed on the signal conditioning and interface circuit board 18 . The core optical element of the present invention is the waveguide hologram 12, which consists of two parts: an optical glass base plate with a thickness of 2 mm and a holographic grating layer with a thickness of 6 μm located on its lower surface. According to the holographic diffraction grating theory and the waveguide holographic theory, the holographic grating on the element forms a volume holographic structure, that is, the refractive index of the photosensitive emulsion is slightly smaller than that of the waveguide plate. When the working wavelength is 650nm red light, the incident angle of reconstructed light is 70°, and the incident angle of object light is 0°, the fringe spacing of holographic grating 15 is:

The spatial frequency corresponding to the holographic grating 15 is $f=\frac{1}{d}\≈1800m{m}^{-1}---\left(2\right)$

In order to obtain a high-contrast and high-resolution fingerprint image, the transmittance of the incident light beam 10 passing through the holographic grating 15 should be similar to the diffraction rate of the outgoing light beam 14 passing through the holographic grating 15, so the corresponding diffraction efficiency of the holographic grating 15 should be 0.4~ 0.6.

Ar⁺激光器20的光束首先通过一个由显微物镜21和针孔22组成的针孔滤波器滤去高频噪声，然后通过球面凸透镜23形成准直光束24，由分束器25分成物光26和参考光27两束，经平面镜调整至夹角为53.9°，并使用柱面镜29调整使物光和参考光光斑覆盖面积大致重合。全息干板19充分曝光后，经显影、定影、烘干后可得到符合参数要求的振幅式波导全息片。An embodiment of the method for preparing the core optical element waveguide hologram 12 in the present invention is shown in FIG. 5 . In this method, a commercial holographic dry plate 19 (emulsion layer thickness 6 μm) coated with a uniform photosensitive emulsion coating is used to expose and fabricate waveguide holographic interference patterns. An Ar ⁺ laser 20 with a wavelength of λ'=514 nm was used as light source. In order to satisfy the parameter d in (1), when making a waveguide hologram, the angle between the object light and the reference light should be:

The light beam of the Ar ⁺ laser 20 first passes through a pinhole filter composed of a microscopic objective lens 21 and a pinhole 22 to filter out high-frequency noise, then passes through a spherical convex lens 23 to form a collimated beam 24, and is divided into object light 26 by a beam splitter 25 The two beams of the reference light and the reference light 27 are adjusted to an angle of 53.9° by a plane mirror, and the cylindrical mirror 29 is used to adjust the spot coverage of the object light and the reference light to roughly coincide. After the holographic dry plate 19 is fully exposed, an amplitude waveguide holographic sheet that meets the parameter requirements can be obtained after developing, fixing, and drying.

In order to obtain high-contrast and high-resolution fingerprint images, it is necessary to bleach the holographic grating to form a phase holographic grating in post-production, so as to improve the diffraction efficiency of the holographic grating.

Using the method of the present invention to extract human fingerprints can use the red light-emitting diode line array with a central wavelength of 650nm as the light source, the fiber cluster can use Edmund Scientific Co.’s product Part No.52362, and the CMOS imaging chip and its subsidiary signal conditioning circuit can use OmniVision’s OV7710 black and white The CMOS image sensing chip and its signal interface processing board are connected to the computer via USB (Universal Serial Bus). The finger to be tested is pressed flat on the upper surface of the waveguide hologram, and the computer gives instructions to extract fingerprints. The CMOS image sensor chip can transmit the acquired fingerprint images to the computer at a sampling rate of 15 frames per second through the signal interface processing board and USB bus. The collection can be stopped when the computer gives an instruction to stop the collection. When the present invention uses an optical fiber cluster with an area ratio of both ends of 3:1 as an imaging system, and uses an area array CMOS imaging chip with a photosensitive unit number greater than 640×480 and a digital quantization depth greater than 8 bits, the resolution can be greater than 600dpi, and the contrast ratio is excellent. At 250:1, the distortion rate is less than 1% of the fingerprint image.

Claims (5)

1. A fingerprint image sensor based on waveguide holography technology, characterized in that it includes: an optical waveguide plate that is inclined to form a reconstructed light incident angle on one side and a waveguide hologram that is close to its upper surface, and one end of the optical waveguide that is inclined A linear array of semiconductor lasers is installed, and the working wavelength of the semiconductor laser matches the incident angle of the reconstructed light of the optical waveguide plate. It is closely attached to the CMOS imaging chip, and the CMOS imaging chip is installed on the signal conditioning and interface circuit board. 2. The fingerprint image sensor according to claim 1, characterized in that said waveguide hologram consists of an optical glass base plate with a thickness of 1.8-2.5 mm and a photosensitive emulsion layer with a thickness of 5-15 μm on one side of the plate. The holographic grating above consists of two parts. 3. The fingerprint image sensor according to claim 1, characterized in that the refractive index of the photosensitive emulsion layer on the waveguide holographic sheet is slightly smaller than that of the optical waveguide plate, so that the fabricated holographic grating forms a volume holographic structure. 4. The fingerprint image sensor according to claim 1, characterized in that, the operating wavelength of the semiconductor laser array is in the red light range, and the reconstructed light incident angle of the optical waveguide plate matches it 5. A method for collecting fingerprint images by a fingerprint image sensor, characterized in that the optical waveguide plate formed by inclination on one side to reconstruct the incident light angle and the waveguide hologram attached to its upper surface, semiconductor laser line array, and CMOS imaging The fingerprint image sensor composed of chips, signal conditioning and interface circuit boards, the collection of fingerprint images specifically includes the following steps: 1) The fingerprint image sensor is connected to the upper computer, and the finger to be collected is pressed flat on the waveguide hologram; 2) The fingerprint image sensor lights up the lighting source, and emits an incident light beam that passes through the base optical flat glass and enters the waveguide hologram. According to the principle of total reflection to collect fingerprint images, the outgoing light beam carries the fingerprint image, and when passing through the waveguide holographic grating, diffraction occurs and leading to wavefront reconstruction; 3) The outgoing beam after the wavefront reconstruction is converged and imaged on the CMOS imaging chip through the fiber cluster; 4) The computer controls the CMOS imaging chip to obtain the fingerprint image in a snapshot mode; 5) The fingerprint image sensor transmits the fingerprint in digital format to the computer through the transmission cable.

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