A 128-channel linear array photodetector, with 25 μm pixel pitch, connected to a readout integrated circuit (ROIC) was developed for mid-infrared spectroscopic applications that use a wavelength-variable quantum cascade laser (QCL). The detector is composed of III-V semiconductor InAsSb, has sensitivity in the mid-infrared region, and is operated at room temperature.
The photovoltaic type of MIR detectors has a low shunt resistance, which causes high dark current. Therefore, a multi-series detector, which has a high total shunt resistance, is generally used for uncooled operation. However, in our newly developed detector array, a single element is employed as each channel’s detector to achieve high signal sensitivity. Also, a DC feedback (DCFB) mechanism is applied to the ROIC to draw out the detector’s high dark current.
The detector’s performance is evaluated using a pulsed QCL with an emission wavelength of 7-10 μm at room temperature. A reverse voltage is applied to the detector to improve the detector’s characteristics and allow it to respond to a QCL’s pulse width of 100 nsec. Although the reverse voltage increases the detector’s dark current, the DC feedback draws out the dark current up to 1 mA. The detector’s sensitivity is 1.5 A/W at 7 μm, the TIA’s gain is designed to be 1k ohm, so the total trans gain obtained is 1.5 V/mW. The detector’s noise input equivalent power is 200 nW. Therefore, a high signal-to-noise ratio can be achieved because the pulsed QCL can output a peak power higher than several tens of milliwatts.
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.
A MEMS-FTIR engine has been developed as a key device for the Fourier-Transform Infrared Spectrometer, which consists of a Michelson interferometer including an electro-static actuator to control a moving mirror, an optical fiber groove for incident light and a photodetector. All these elements except for the photodetector are monolithically fabricated in Silicon using MEMS technology. The optical elements such as a beam splitter, a fixed mirror and a moving mirror are formed and aligned simultaneously with high degree of precision by Deep Reactive Ion Etching (DRIE). The vertical side walls are utilized as optical planes so that the incident light path is located in parallel with the Silicon substrate. The moving mirror is driven by an electro-static MEMS actuator. The photodetector is placed above an angled mirror, which is formed by alkaline wet etching exposing the Silicon crystal plane at the end position of light path. All the elements including the photodetector are hermetically covered by a lid of Silicon in the vacuum chamber by using a surface activate bonding technology. In order to reduce the cost, wafer level process and separation of each chip by a laser dicer after all assembly processes are introduced. The realized MEMS-FTIR is 10×17×1 mm in size and a signal noise ratio (SNR) of better than 35dB, which comes from a good verticality of less than 0.2 degree in the vertical side walls as optical planes by managing the DRIE etching conditions.
A single-mode-fiber-compatible SC-type optical module (SC module) is developed. This module realizes low optical coupling loss and low cost by means of plastic molding technology and the developemnt of automatic fabrication equipment. The SC module consist of a plastic adapter to be compatible with a SC-mode-fiber connector and a plastic housing for optical components: a GRIN rod lens and an optical transmitting/receiving device. The housing is made from a plastic with an extremely low thermal expansion coefficient, and is designed as acylindrical structure for thermal stability. It has a mount for an optical device, a lens holder, and a fiber connection. A zirconia ceramic sleeve, installed in the housing by insert molding technology, is employed to obtain a high plug coupling strength and the optimum optical power coupling. The optical axis between the zirconia sleeve and GRIN rod lens is adjusted automatically and accurately by insert molding technology: the lens holder is formed by the same plastic mold that holds the zirconia sleeve along it. In order to lower the cost, the optical module is fabricated using automatic fabrication equipment, which carried out the processes from supplying the optical device to fixing it with UV-cured resin. The following characteristics of the SC module are obtained from experimental results where total optical loss was 6dB. The portion of 5.5dB of the total loss is contributed by the GRIN rod lens. The remainder is the excess loss of 0.5dB due to assembly. The thermal stability is found to be within 0.1dB in the temperature range from -10 degrees to 65 degrees C.
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