Abstract
The dew point is the temperature at which vapour begins to condense out of the gaseous phase. The deterministic relationship between the dew point and humidity is the basis for the industry-standard “chilled-mirror” dew point hygrometers used for highly accurate humidity measurements, which are essential for a broad range of industrial and metrological applications. However, these instruments have several limitations, such as high cost, large size and slow response. In this report, we demonstrate a compact, integrated photonic dew point sensor (DPS) that features high accuracy, a small footprint, and fast response. The fundamental component of this DPS is a partially exposed photonic micro-ring resonator, which serves two functions simultaneously: 1) sensing the condensed water droplets via evanescent fields and 2) functioning as a highly accurate, in situ temperature sensor based on the thermo-optic effect (TOE). This device virtually eliminates most of the temperature-related errors that affect conventional “chilled-mirror” hygrometers. Moreover, this DPS outperforms conventional “chilled-mirror” hygrometers with respect to size, cost and response time, paving the way for on-chip dew point detection and extension to applications for which the conventional technology is unsuitable because of size, cost, and other constraints.
Highlights
The precise, real-time measurement of relative humidity (RH) is highly important for applications ranging from appropriately correcting for the density of the air in standardized conditions for aero engine acoustic testing to accurate natural gas metering[1,2,3,4,5,6,7,8]
The temperature of the device was controlled by a Peltier system controller (Model: Newport ILX Lightwave LDC-3700C), and the optical response was measured in the stationary state
Because the dew point is identified at the very onset of condensation, the Q-factor change is estimated to be less than 15%. This 15% Q-factor decease will cause the full width at half maximum (FWHM) to increase by 15%; the resolution changes from 1 pm to 1.15 pm, and the dew point uncertainty is 0.004 °C (0.15 pm/ (37 pm/°C))
Summary
The precise, real-time measurement of relative humidity (RH) is highly important for applications ranging from appropriately correcting for the density of the air in standardized conditions for aero engine acoustic testing to accurate natural gas metering[1,2,3,4,5,6,7,8]. The common, cost-effective miniaturized solutions utilize a solid-state sensing film that interacts with water vapour and converts the water vapour concentration into an electrical signal[10,11,12,13] These traditional electronic humidity sensors have the universal shortcomings of drift and contamination-induced inaccuracy, which typically limits their accuracy in the range of +/−1% to +/−5%; their accuracy is often even poorer in low RH (0~20%) and high RH (80~100%) environments[11]. Compared to “chilled-mirror” hygrometers, this device features high accuracy, a small footprint, and fast response This cost-effective, mass-manufacturable photonic dew point sensor (DPS) eliminates the cost/performance trade-off that affects both electrical thin-film humidity sensors and “chilled-mirror” hygrometers
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