Recent
works have demonstrated that daytime radiative cooling under
direct sunlight can be achieved using multilayer thin films designed
to emit in the infrared atmospheric transparency window while reflecting
visible light. Here, we demonstrate that a polymer-coated fused silica
mirror, as a near-ideal blackbody in the mid-infrared and near-ideal
reflector in the solar spectrum, achieves radiative cooling below
ambient air temperature under direct sunlight (8.2 °C) and at
night (8.4 °C). Its performance exceeds that of a multilayer
thin film stack fabricated using vacuum deposition methods by nearly
3 °C. Furthermore, we estimate the cooler has an average net
cooling power of about 127 W m–2 during daytime
at ambient temperature even considering the significant influence
of external conduction and convection, more than twice that reported
previously. Our work demonstrates that abundant materials and straightforward
fabrication can be used to achieve daytime radiative cooling, advancing
applications such as dry cooling of thermal power plants.
We present an ultra-small all-silica high temperature sensor based on a reflective Fabry-Perot modal interferometer (FPMI). Our FPMI is made of a micro-cavity (approximately 4.4 microm) directly fabricated into a fiber taper probe less than 10 mum in diameter. Its sensing head is a miniaturized single mode-multimode fiber configuration without splicing. The sensing mechanism of FPMI is the interference among reflected fundamental mode and excited high-order modes at the end-faces. Its temperature sensitivity is approximately 20 pm/degrees C near the wavelength of 1550 nm. This kind of sensor can work in harsh environments with ultra-large temperature gradient, but takes up little space because of its unique geometry and small size.
We experimentally demonstrate an all-silica first-order fiber Bragg grating (FBG) for high temperature sensing by focused ion beam (FIB) machining in a fiber probe tapered to a point. This 61-period FBG is compact (~36.6 μm long and ~6.5 μm in diameter) with 200-nm-deep shallow grooves. We have tested the sensor from room temperature to around 500 °C and it shows a temperature sensitivity of nearly 20 pm/°C near the resonant wavelength of 1550 nm. This kind of sensor takes up little space because of its unique geometry and small size and may be integrated in devices that work in harsh environment or for detecting small objects.
Microfiber-based Bragg gratings (MFBGs) are an emerging concept in ultra-small optical fiber sensors. They have attracted great attention among researchers in the fiber sensing area because of their large evanescent field and compactness. In this review, the basic techniques for the fabrication of MFBGs are introduced first. Then, the sensing properties and applications of MFBGs are discussed, including measurement of refractive index (RI), temperature, and strain/force. Finally a summary of selected MFBG sensing elements from previous literature are tabulated.
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