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.
More and more small spectrometers have been brought to market in recent years. Compactness is certainly a necessary feature for spectrometers used in the field, but keeping the detection capability sufficiently high is also key to fulfilling application requirements. Using MOEMS technologies, we developed an FTIR engine for field use that is both compact and has good detection capability. The FTIR engine measures 59mm x 28mm x 35mm and weighs about 130g. Although compact, it has a signal-to-noise ratio (SNR) higher than 40 dB, a light efficiency higher than 15%, and sensitivity from 4000 to 8500 cm-1. This new FTIR engine has a φ3mm MEMS movable mirror formed on a Si wafer surface to enhance light utilization. The MEMS actuator is driven with an amplitude of 125 micrometers orthogonal to the surface. A fixed mirror was formed on the 4.4mm thick compensating plate through metal deposition. A miniature MEMS mirror device measuring 21mm x 14mm x 4.8mm was fabricated, which integrates the movable mirror and fixed mirror by direct bonding. A 4.4mm thick trapezoidal prism beam splitter was placed on the MEMS device, and it combines two light arms reflected in the same direction to obtain interferograms with an InGaAs PIN photodiode. In addition, a VCSEL was integrated in the housing of the FTIR engine to monitor the driving mirror position.
We developed a compact Yb:YAG ceramic regenerative amplification system. A rectangular glass block is used to
elongate the cavity. A pulse to be amplified is propagated in a long distance in the glass block by being reflected
repetitively at the end faces of the glass under a condition of total internal reflection. Furthermore, we produced
transmission gratings with a diffraction efficiency of more than 95%. The floor area of the entire amplification system is
reduced to less than 2,000 cm2. In 20-kHz operation, the system generates 1.0-ps compressed pulses of 4.5-W average
power, i.e., 0.225-mJ energy.
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.
CCDs feature low amplifier noise, high sensitivity and wide dynamic range. Therefore, CCDs have been used in such diverse applications as general purpose imaging, industrial inspection, medical diagnostics and astrophysics. For each application, a suitable image area, optical characteristics, electrical characteristics and even appropriate package must be required. Among these, back illuminated CCDs have enhanced sensitivity for UV light, soft x-rays and electrons. Hamamatsu Photonics KK has developed FFT, MPP mode back illuminated CCDs optimized for spectroscopy. For spectroscopic applications, CCDs should have responsivity from the UV to near-IR as well as having low dark current and wide dynamic range in the binning mode. Stability of the spectral response curve is also important to achieve high precision measurements.
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