A study on image recognition of three-dimensional objects by undestroyed photorefractive optical memory using two wavelengths

光折变晶体是一种非常有前途的全息存储介质，因为它不需要像普通照相板那样的显影过程，而且它通过三维存储目标波的信息而具有大容量的存储能力。然而，当使用像LiNbO_3(LiNbO_3)这样的光折变晶体长期存储存储器时，写入过程需要很长的时间。通常，使用与写入过程相同的波长通过读取过程逐渐擦除信息。因此，人们期待一种既能快速写入又能长期读出的新方法。在这项研究中，我们提出了在写入过程中使用不同波长的激光作为一种无损读出的方法，并对其有效性进行了基本的实验。此外，我们还将光折变记忆应用于三维物体的图像识别。一种光折变钛酸钡晶体(BaTiO_3…)此外，利用在可见光光谱中具有强烈依赖于波长的透过性的)，进行了不擦除存储信息的读出实验。写入过程使用YAG激光器(532 Nm)作为光源，读取过程使用红外LD(785 Nm)。这两个波长分别对应于BaTiO3的高敏感区和不敏感区。实验表明，用较短的波长从激光器读出的信息可以用较长的波长从激光器中读出，读出时间大约是原来较短波长读出的10倍。然而，由于较短和较长波长之间的布拉格角的差异，存储的信息的完整图像无法重现。利用柱面透镜扩大读出光的入射角范围，可以有效地改善再现图像的质量。测量了依赖于记忆图像和输入图像之间的相似性的衍射束强度。一项使用四张二维图像作为对象的初步实验表明，通过比较衍射束强度，可以从其他图像中识别出特定的图像。为了识别三维物体，使用了三枚硬币作为物体。由于较近的布拉格条件，仅有衍射束的存在就可以清楚地识别三维物体。无损读出和使用光折变晶体的三维识别的组合可以应用于物体识别、位置传感等。较少

Photorefractive crystal is very promising as a medium of hologram memory because it does not require a process of development unlike an usual photographic plate and it has a large capacity of memory by three-dimensional storage of the information of an objective wave. However, when using a photorefractive crystal like a lithium niobate (LiNbO3) with a long-term storage of memory, it needs long time for writing process. In general, the use of the same wavelength as the writing process gradually erases the information through the reading process. Therefore, a new method that enables both quick writing and long-term readout has been expected. In this study, as a method for undestroyed readout, we propose the use of laser beam with different wavelength from the writing process and carry out fundamental experiments for its effectiveness. In addition, we apply the photorefractive memory to the image recognition of three-dimensional objects.A photorefractive crystal of barium titanate (BaTiO3 … More ), which has a transmissibility strongly dependent on the wavelength in the visible spectrum, was employed for the experiments of readout without erasing the stored information. A YAG laser (532 nm) was used as a light source for the writing process and an infrared LD (785 nm) for the reading process. The two wavelengths correspond to the high sensitive and insensitive regions of BaTiO3, respectively. The experiments made clear that the information stored quickly by the shorter wavelength from the YAG laser can be read out by the longer wavelength from the LD for about 10 times longer-term than readout by the original shorter wavelength. However, a whole image of the stored information could not be reproduced because of discrepancy of the Bragg angles between the shorter and longer wavelengths. Broadening the range of incident angle of the readout light by use of a cylindrical lens turned out to be effective for improvement of the reproduced image.A LiNbO3 crystal was used in the experiments for image recognition. The diffractive beam intensity, which depends on the similarity between the memorized image and the input image, was measured. A preliminary experiment using four two-dimensional images as the objects made clear that it was possible to recognize a specific image among other images by comparison of diffractive beam intensities. For recognition of three-dimensional objects, three coins were used as the objects. The three-dimensional objects could be distinctly recognized by the existence of the diffractive beam alone owing to the closer Bragg condition. The combination of the undestroyed readout and the three-dimensional recognition using a photorefractive crystal can be applied to identification of an object, position sensing, and so on. Less