Abstract
We report a new method of accurately measuring the mid-infrared absorption of fluid analytes using on-chip waveguides that allows measurements to be referenced for noise and drift reduction whilst using only a single external detector. A silicon-on-insulator waveguide circuit integrated with a microfluidic channel is made in which a pair of switches alternate light between an evanescent absorption sensor waveguide and a reference waveguide at a rate of a few kHz, thus allowing low frequency common noise components to be removed from the measurement, and leading to a reduction in the noise floor by a factor of 11 compared to a simple waveguide absorption sensor. The device was verified by using it to detect an Isopropyl alcohol (IPA) absorption peak at a wavelength of 3.77 µm.
Highlights
A BSORPTION spectroscopy in the mid-infrared (MIR) wavelength region has a variety of potential applications [1] including atmospheric monitoring [2], detection of chemicals and explosives [3], human breath analysis [4], non-invasive glucose monitoring [5], and plant and crop science [6]
We demonstrate the integration and operation of the switching sensor realized in the SOI platform with an integrated Polydimethylsiloxane (PDMS) microfluidic channel, and characterize its performance by using it to measure the absorption of water-Isopropyl alcohol (IPA) solutions at 3.77 μm wavelength
A 2 μm SiO2 cladding layer was deposited on the top surface of the chips by Plasma Enhanced Chemical Vapour Deposition (PECVD) (Oxford Plasmalab System 100) (Fig. 5(c)), the sensing windows were opened by isotropic hydrofluoric acid (HF) wet etching (Fig. 5(d))
Summary
A BSORPTION spectroscopy in the mid-infrared (MIR) wavelength region has a variety of potential applications [1] including atmospheric monitoring [2], detection of chemicals and explosives [3], human breath analysis [4], non-invasive glucose monitoring [5], and plant and crop science [6]. A recent demonstration of on-chip sensing [9] showed that germanium-on-silicon nitride waveguides can be used to measure the concentration of Isopropyl alcohol (IPA) in DI water, using a microfluidic channel. To attach the micro-fluidic channel to an SOI waveguide chip, a cladding layer needs to be deposited onto the sensing device, to act as a bonding layer This cladding layer can be used to ensure that there is no contact between the (potentially optically absorbing) microfluidic channel material and the sensitive waveguide surface, and to define where the sample interacts with the waveguide. We demonstrate the integration and operation of the switching sensor realized in the SOI platform with an integrated Polydimethylsiloxane (PDMS) microfluidic channel, and characterize its performance by using it to measure the absorption of water-IPA solutions at 3.77 μm wavelength. We investigate the noise of the absorption measurement, and show that the switch sensor is able to reduce noise by a factor of 11 compared to a simple waveguide absorption sensor
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