A portable laser absorption sensor for quantitative measurement of ambient temperature and humidity

A portable and sensitive laser absorption sensor is developed for quantitative measurement of ambient temperature and humidity. The H2O absorption lines in the combination and overtone bands were comprehensively investigated and a promising line pair (7181.16 cm^-1 and 7185.60 cm^-1) was selected to ensure sensitive measurement with high accuracy and low uncertainty within the temperature range of 0-30 °C. A single distributed feedback laser near 1392 nm was used to access the selected line pair. Scanned wavelength direct absorption spectroscopy was used for temperature and humidity measurement. A Multi-line Vogit-fitting strategy was adopted to reduce the fractional fitting residual. The sensor performance was firstly evaluated numerically at different noise levels (0-10%) and then demonstrated experimentally under the temperature range of 0-30 °C and relative humidity range of 10-50 %. The temperature and humidity as measured with the laser sensor were in good agreement with those from the negative temperature coefficient (NTC) thermistor and commercial hygrometer, respectively. The sensor was then applied to perform in situ and real-time measurements of ambient temperature and humidity outside the lab. With a time response of 1s, the sensor demonstrated a precision (uncertainty) of 0.150.2 °C (0.09-0.2 °C) and 0.5-1 % (0.25-0.8%) for temperature and relative humidity, respectively. The sensor is expected to have potential applications in the vehicle wind tunnel and environmental monitoring.

I. Introduction

Temperature and humidity are two important parameters that require accurate monitoring and precise control in scenarios such as micro-electro-mechanical systems [1], wind tunnel tests [2], indoor air quality [3] and products storage [4]. For the manufacturing of microchips, an excessive humidity would undermine the cleanliness of the photomask and wafer surface. Long time exposure to a high humidity environment would cause haze on the surface, which in turn would reduce the yield. In a wind tunnel, the range of temperature and humidity should be well controlled to ensure accurate calculation of Reynolds and Mach numbers which are essential experimental parameters. For indoor air quality, temperature and humidity are strongly related to viral and bacterial infections. For products storages, precise optoelectronic instruments are sensitive to the variation of temperature and humidity, and drugs are easily contaminated by water vapor. Therefore, accurate knowledge of temperature and humidity is significant for industrial production, environmental monitoring and fundamental research.