Relative humidity (RH) is an important factor in the field of structural health monitoring, especially during the early stage of corrosion. Many methods have been proposed for humidity sensing, and one of the attractive sensors is fiber-optic long-period grating (LPG) sensors. Unfortunately, the current sensing sensitivity of this kind of sensors is limited. A thin fiber-optic LPG sensor with a self-assembled thin film of PAH + / PAA − is proposed and demonstrated for humidity measurements. The LPG sensor is inscribed in a single-mode fiber using a CO2 laser, and the cladding size is reduced to about 27 μm. It shows that an LPG sensor with a reduced cladding size has an enhanced refractive index sensing characteristic, compared to those with normal cladding size due to the enhancement of the evanescent field. In a next step, selectivity is added to the LPG sensor coated with a film of PAH + / PAA − for functionalization to be sensitive to humidity. The resonance spectral responses of LPGs are experimentally investigated with respect to its sensitivity to a change in humidity that modifies the index of the nanolayer and the cladding, leading to a resonant wavelength shift. The experimental results show that the coated thin LPG has a highly sensitive resonance wavelength shift of −220.75 pm / % RH for an RH variation from 25% to 80%, of which the sensitivity is enhanced thrice compared to those with a normal cladding size. The proposed sensing setup opens LPG structures for a variety of sensing and detection applications.
In this work, we have proposed a model for the ultimate physical limit on the sensitivity of the heterojunction bipolar phototransistors (HPTs). Based on our modeling we have extracted the design criteria for the HPT for high sensitivity application. HPT with the submicron emitter and base area has the potential to be used for the low number photon resolving in near-infrared (NIR) wavelength. However, in practice, the quality of materials, processing, and the passivation plays an important role in the realization of the highly sensitive HPT. For short wave infrared (SWIR) HPTs based on lattice matched InGaAs to InP is studied. For these devices, conditions to reach to the highest possible sensitivity is examined. We have made an HPT based on InGaAs collector and base on the InP substrate. After developing proper processing combination of wet and dry etching and the surface passivation for the device we made an imager with 320x256 pixels based with a 30m pixel pitch. The imager shows the sensitivity less the 30 photons for each pixel with the frame rate more than 1K frames per second.
This article reports the progress on the development of a novel detector with the promise of addressing the needs of extreme AO (ExAO) in the near-IR band (NIR), 0.9-1.7 μm. The camera is based on the electron injection mechanism which resembles how the human eye processes light. The camera design allows high sensitivity operation at TEC reachable temperatures for ExAO at 1-4 kHz frame rates, and at the same time the concept produces sufficient gain to overcome the read noise of the device. Here we present the overall design, test results on Gen-1 (outdated but operable) camera, along with early results of our next generation of detectors.
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