A grating-based ultra-compact spectral sensor head was developed to overcome obstacles in characteristics, cost, and size, and it was designed for commercialization in large volume applications. More and more compact spectrometers have been brought to market in recent years—for example, those used for food and beverage quality tests based on measuring sugar content—but their spread is still limited because the conventional types don’t fully fulfill the market requirements. The ultra-compact spectral sensor’s characteristics have been improved while reducing cost and size. Its 20 nm wavelength resolution, equivalent to that of conventional compact spectrometers, was made possible by applying multiplex reflection and small collimation. Using nano-imprint technology, a grating was replicated onto a concave surface inside a compact surface-mount device package. On the opposite side of the concave grating, an entrance slit and a mirror that directs light onto the grating were directly formed on a high-sensitivity CMOS linear image sensor that is sensitive to the 640-1050nm range. The 15μm wide entrance slit was formed on the CMOS silicon chip using MEMS technology, and a short-wavelength cut filter was attached to the entrance window. By reducing the number of components, the structure was simplified. An ultra-compact package measuring 11.7mm × 4mm × 3.05mm and weighing about 0.3g is made possible while maintaining good characteristics and reasonable cost.
In recent years, many compact spectrometers for purposes such as environmental monitoring and process quality control
in industrial production have been realized. However, most of them still employ spectrometer mounts with focal lengths
in the range of several cm. Therefore, their size is about that of a palm which is too large for OEM-use in handily sized
optical sensor equipment.
Accordingly, we have developed a thumb-sized, truly miniaturized spectrometer for the spectral range 340nm to 750nm,
which is particularly suited for use inside hand-held or portable color management sensor equipment. The spectrometer
is using a self-imaging, aberration-corrected concave grating with very short focal length and a blazed grating profile for
high diffraction efficiency. The grating is replicated onto the top of a convex glass lens using nano-imprint technology.
Opposite to the concave grating, a dedicated C-MOS image sensor with an in-built on-chip slit is placed. The slit with a
width of 75μm is formed into the silicon chip using MEMS technology. Due to this advanced technology, the distance
between the sensor area and the slit is as small as 1mm. Based on this high level of integration, the number of optical
components could be kept to a minimum and the distance between the concave grating and C-MOS image sensor is
about 8.5mm only.
In summary, we have realized a well-performing miniaturized spectrometer with an extremely small package size of
28mm - 17mm - 13mm and a weight of only about 9g, which is highly suited for integration into optical sensing
equipment.
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