The quaternary alloy (AlxGa1-x) y In1-yP is an attractive material due to its bandgap tunability while being lattice-matched to GaAs. Although this direct gap quaternary material is being extensively used for the visible lasers and LEDs due to its unique light absorption/emission properties, the applications in visible photodetectors remain unexplored. In this paper, A symmetric mesa AlInP/AlGaInP/AlInP double-heterostructure has been successfully designed based on the quaternary alloy for visible photodetection. The composition of (AlxGa1-x) y In1-yP was investigated with the help of MATLAB simulation and energy band discontinuity theory giving the values of y=0.48 and x=0.46. By using the first-order approximate data processing method and analogy with other III-V main group quaternary alloy compounds, the simulated values of bandwidth and responsivity are obtained to be 10 GHz and 0.85 A/W respectively at wavelength 560 nm. Meanwhile, by analyzing the influence of doping concentration and thickness of each layer on bandwidth and responsivity, the optimal doping concentration and thickness are obtained to be 2×1018cm-3 and 0.5μm of P layer, 5×1015cm-3 and 1μm of absorption layer, 5×1017cm-3 and 0.5μm of N layer respectively. By selecting optimized parameters, the device exhibits a detectivity of 108 cm√HzW-1 and a noise equivalent power of 10-11W. With high responsivity, high speed, and large bandwidth, the designed photodetector has a good application prospect.
Salinity is one of the most important characteristic parameters of seawater. The absolute salinity 𝑆𝐴 is defined to be the “Total amount of dissolved material in grams in one kilogram of seawater”, and it is directly related to the seawater density. However, the absolute salinity is difficult to be measured in practice. At present, practical salinity 𝑆𝑃 is calculated by using conductivity, temperature and pressure measured by CTD sensors. The conductivity of seawater is dependent on the ions concentrations. However, the non-ionic components cannot be detected, though they may have significant influence on density. The optical refractive index is sensible to all dissolutions of seawater, therefore it is a good proxy of the concepts of salinity/density. In this paper, urea was used to study the influence of urea on the seawater refractive index based on optical refractive index method. The changes of seawater refractive index versus the urea concentration were studied by a Vgroove refractometer. This method provides a new method for measuring urea in seawater.
Liquid refractive index (RI) is one of the important optical parameters closely related to density, concentration, and temperature. The measurement of liquid RI is widely used in chemistry, medicine, physics, and biology research. A Fabry- Perot interferometer-based liquid RI sensor by using PMMA polymer material is proposed and demonstrated experimentally. The influence of cavity length and reflectivity on the sensor sensitivity is theoretically investigated. A long cavity length gives rise to a small free space range. In the experiment, high reflectivity of F-P cavity is made by evaporating metal film (aluminum film). An open F-P interferometer is realized by punching a hole in the PMMA plate, which is convenient for liquid exchange and in situ measurement. Experimental results show that the FSR the F-P cavity is about 5 nm with a fringe contrast of 3 dB. The designed sensor is compact, low-cost, easy fabrication and flexible replacement. Keywords: Fabry-Perot interferometer, Refractive index sensor, polymethyl methacrylate.
A new sensor is proposed in this paper for measuring the refractive index of seawater based on optical refraction. Different from the traditional method of measuring the angle of refraction, the optical path is more complex. An aspheric lens is placed between the V-shaped groove and PSD to increase sensitivity. And a down-collimater is used to reduce the size of spot. The influence of aspheric lens on sensitivity and nolinearity is studied. Theoretically, the sensitivity of the refractive index can reach up to 6329.45mm/RIU.
Salinity of seawater is one of the most important ocean parameters. Salinity of seawater is mainly obtained by conductivity measurement using CTD (Conductivity-Temperature-Depth). Conversion accuracy between conductivity and salinity relies on the assumption that components of seawater are fixed, as well as high accuracy and synchronism measurements of conductivity, temperature and pressure. The study of seawater salinity based on the V-block optical refractive index method provides a total different principle for salinity measurement. Achieving high resolution seawater optical refractive index measurements could help to study factors affecting the accuracy of salinity measurement. In this paper, the various instrument parameters that affect the accuracy of seawater refractive index measurement are analyzed and the optical refractometer is optimized based on the components on the shelf. This paper systematically analyzed the resolution and tolerance of refractive index measurement on the parameters of V-block refractometer, such as incident angle, external environment and prism refractive index, etc. The optical refractometer with an air film layer on both sides of the V-block was proposed for seawater salinity measurement. With such an optimization, the measurement accuracy is further improved and the tolerance is increased. The theoretical resolution to the seawater refractive index and salinity are 1.8×10-6 and 0.01‰, respectively. Experimentally, we have achieved 3.9×10-6 and 0.021‰ respectively, and a good linearity. The difference between theoretical and experimental results are analyzed.
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