Simultaneous multi-analyte sensing using a 2D quad-beam diffraction smartphone imaging spectrometer

Abstract

A multichannel smartphone spectrometer exploiting multiple identical diffraction orders of a customized 2D diffractive element is reported. The instrument utilizes diffraction grating holographic imaging to converge and capture the multiple diffraction orders simultaneously within the limited field-of-view of the smartphone’s camera detector. This eliminates the requirement of convergence optics as used in conventional spectrometer systems. Each diffraction order can be utilized as a single optical channel for spectroscopic analysis. As proof of concept, a customized 2D grating composed of two identical thin-film 1D gratings orthogonally stacked together is utilized to diffract the light rays emanating from a broadband visible source and produce four identical 1st orders dispersive diffraction in two orthogonal axes. The smartphone camera captures the diffraction orders within its solid angle – a quad-channel spectrometer exploiting the 2D properties of a CMOS detector is demonstrated. Mindful of losses, further increases in channel numbers is possible by simply increasing the number of gratings (N) within the stack spatially separated by an angle θN=180°/N between successive grating axes. The instrument offers an ultra-low-cost, lightweight, high throughput, rugged and small form factor multichannel spectrometer ideal for field use as well as a laboratory benchtop alternative. To evaluate the performance of the instrument, three chemically and biologically significant parameters are measured simultaneously: - (a) iron concentration [Fe], (b) chlorine concentration [Cl], and (c) pH of the drinking water.