Tsuguhiro Korenaga

Diffraction light analysis method for a diffraction grating imaging lens

We have developed a new method to analyze the amount and distribution of diffraction light for a diffraction grating lens. We have found that diffraction light includes each-order diffraction light and striped diffraction light. In this paper, we describe characteristics of striped diffraction light and suggest a way to analyze diffraction light. Our analysis method, which considers the structure of diffraction grating steps, can simulate the aberrations of an optical system, each-order diffraction light, and striped diffraction light simultaneously with high accuracy. A comparison between the simulation and experimental results is presented, and we also show how our analysis method can be used to optimize a diffraction grating lens with low flare light.

Diffraction grating lens array

We have proposed a new type of camera module with a thin structure and distance-detection capability. This camera module has a four-lens-array with diffraction gratings (one for blue, one for red, and two for green). The diffraction gratings on the mold are formed mechanically, and the plastic lens array is fabricated by injection molding. The two green images are compared to detect parallax, and parallax-corrected blue, red and green images are then composed to generate a color image. We have developed new design software and molding technologies for the grating lenses. The depth and period of blazed gratings and the shapes of aspheric lenses are optimized; and blue, red and two green aspheric lenses with gratings are molded as a single four-lens-array. The diffraction gratings on both surfaces of each lens act to improve field curvature and realize wide-angle imaging. However, blazed gratings sometimes cause unnecessary diffraction lights that impede the formation ofhigh-resolution images. We have developed a new method to measure necessary first-order diffraction lights and unnecessary diffraction lights separately. Use of this method allows the relationship between molding conditions and necessary/unnecessary diffraction lights to be shown. Unnecessary diffraction lights can be diminished by employing the optimal molding processes, allowing our grating lenses to be used for image capture.

New camera module with thin structure, high resolution and distance-detection capability

Our new type of camera module has a four-lens-array and an imaging sensor. The imaging sensor is divided to four regions, and these four regions are aligned in one-to-one correspondence with the four lenses. Four color filters are placed over the four imaging regions. First region has a blue filter, second has a red, and the other two have green filters, and two regions with green filters are aligned diagonally. Diffraction gratings are formed on aspheric surfaces of the four lenses, and MTF characteristics of these lenses are improved. The four images taken through the different lenses have parallax, but these parallaxes can be calculated by comparison of the two green images. Pixel shifts of blue, red and green images are realized by rotating the four-lens-array slightly with respect to the imaging sensor. After correcting the parallaxes, the green image, the parallax-corrected blue image and the parallax-corrected red image are composed to generate the resultant color image with high resolution. Distances between objects and the four-lens-array are detected by use of the above parallaxes, and measurement error is less than 2.5% for near objects. With above configuration and functions, our camera module has realized smaller height, higher image resolution and distance-detection capability, and will be applied for cellular phones and automobile vehicles.

Phase function design of a diffraction grating lens for an optical imaging system from a Fraunhofer diffraction perspective

The potential exists to apply diffraction gratings to optical imaging systems to improve camera resolution and shorten optical length. However, we have noted the generation of striped flare lights, which differ from unnecessary-order diffraction lights, under intense lighting. We have elucidated the generation principle of these new striped lights and have discovered that they are caused by narrow diffraction grating rings. In this paper, using an analysis based on Fraunhofer diffraction, we suggest a way of minimizing them by designing an appropriate phase function structure, and test the efficacy of this design using our own manufactured prototype.

Explication of diffraction lights on an optical imaging system from a Fraunhofer diffraction perspective

Low-height camera modules are demanded for such applications as cellular phones and vehicles. For designing optical lens, it has widely been recognized that a trade-off exists between reducing the number of lenses and camera resolution. The optical performance of imaging lenses has been improved by diffraction gratings, which have a peculiar inverse dispersion in the wavelength and exhibit the efficacy of correction for chromatic aberration. We can simultaneously reduce the number of lenses and maintain optical resolution using diffraction gratings. However, we have found a generation of striped flare lights under intense light sources that differ from unnecessary order diffraction lights. In this paper, we reveal the generation mechanism of these new striped diffraction lights and suggest a novel structure of diffraction gratings that can decrease them.