In-Loco Optical Spectroscopy through a Multiple Digital Lock-In on a Linear Charge-Coupled Device (CCD) Array

Hugo Fonsêca, Diego Rativa, and Ricardo Lima

Published: 16 August 2023 | University of Pernambuco, Recife, Brazil

Several optical techniques are used to analyze the interaction between light and matter in visible and infrared spectrums, providing valuable information about their optical and structural properties. By studying the reflectance, scattering, transmittance, absorbance, and fluorescence of light intensity spectrums, materials can be characterized more accurately. As such, a spectrometer is an indispensable tool for performing these experiments.

Abstract

Accurate and reliable measurements of optical properties are crucial for a wide range of industrial and commercial applications. However, external illumination fluctuations can often make these measurements challenging to obtain. This work proposes a new technique based on digital lock-in processing that enables the use of CCD spectrometers in optical spectroscopy applications, even in uncontrolled lighting conditions. This approach leverages digital lock-in processing, performed on each pixel of the spectrometer’s CCD simultaneously, to mitigate the impact of external optical interferences.

Figure 3 illustrates the detection and modulation modules used in this paper.

The effectiveness of this method is demonstrated by testing and recovering the spectrum of a yellow LED subjected to other light sources in outdoor conditions, corresponding to a Signal-to-Noise Ratio of −70.45 dB. Additionally, it was possible to demonstrate the method’s applicability for the spectroscopic analysis of gold nanoparticles in outdoor conditions. These results suggest that the proposed technique can be helpful for a wide range of optical measurement techniques, even in challenging lighting conditions.

Figure 5. Optical spectrum: (a) green LED, (b) yellow LED, (c) both LEDs on simultaneously, and (d) after lock-in processing for the yellow LED subjected to noise emulated by a higher intensity green LED. Figure 6. Optical spectrum: (a) high-intensity white light and yellow LED separated, (b) high-intensity white light and yellow LED simultaneous, and (c) after lock-in processing for the yellow LED subjected to noise emulated by high-intensity white light. Figure 7. Results of the optical spectroscopy experiment in outdoor colloidal analysis for the (a) water only, (b) gold colloidal, and (c) after lock-in processing.

…The Detection Module consists of the StellarNet® BLUE -Wave Miniature Spectrometer (StellarNet, EUA, Tampa, FL, USA) (200–1150 nm wavelength range and 2048 element CCD sensor) and a notebook, as shown in Figure 3. The StellarNet obtains the optical spectrum through the SpectraWiz software of the manufacturer. The functionality SpectraWiz for exporting data is called Episodic Data Capture, with a maximum sampling rate of 1 ms, corresponding to 100 spectra captured in 1 s. The exported data is stored in its own encrypted format .EP1 or in a plain text format (.EP1x), posteriorly used by SpectroPlot to perform the lock-in processing…