KEYWORDS: Mid infrared, Linear regression, Near infrared, Bioalcohols, Spectroscopy, Statistical modeling, Near infrared spectroscopy, Light absorption, Laser spectroscopy, Data modeling, Calibration, Attenuated total reflectance
The strong absorption of water in the mid-infrared (MIR) causes difficulties in performing combined near-infrared (NIR) and MIR spectroscopy on aqueous samples using a single instrument. Combining spectra from different instruments can result in unwanted spectral variations, which can influence the prediction models and mitigate the advantages of the combination approaches. In this work, absorption spectra were collected in the NIR (1300nm-2500nm) and MIR (2500nm-3800nm) region by combining a single high-brightness broadband supercontinuum (SC) laser spanning from ~1-4.0 μm with attenuated total reflectance (ATR) and a transmission cuvette in a single-path configuration to provide a uniform spectral response across the NIR and MIR regions. The measured NIR- and MIR- spectra were assessed based on their ability to predict varying concentrations of ethanol, sucrose, and L-proline in aqueous solutions. The NIR-based partial least square regression (PLSR) model gave higher prediction accuracy for sucrose (R2 = 0.95) as compared to both ethanol and L-proline (R2 = 0.75 and R2 = 0.57 respectively). On the other hand, the MIR-based model enhances the prediction accuracy of ethanol (R2 = 1.00) and L-proline (R2 = 0.62) while demonstrating no significant change in prediction accuracy for sucrose (R2 = 0.96). The prediction models based on the combined NIR-MIR spectra performed similar but slightly worse than the MIR-only models for ethanol and L-proline (R2 = 0.97 and R2 = 0.54 respectively), while for sucrose, it was slightly improved (R2 = 0.99).
We demonstrate a high repetition rate (3 MHz) Mid-IR supercontinuum (SC) source spanning whose spectrum spanning 1000-4200 nm using a cascade of different nonlinear fibers. Multi-tone absorption spectroscopy measurements are subsequently carried out using this source and a scanning spectrometer probing various concentrations and a combination of different analytes. We further explore a novel algorithm for rearranging the absorption in the IR-region and the NIR region for three-dimensional modeling. We show this method of analyzing the data is robust, that is being able to predict newly added samples of slightly different nature without having to the recalibrate the model.
Supercontinuum (SC) lasers are of high interest for applications like multispectral photoacoustic imaging (MSPAI), where the wide optical bandwidth of the SC laser system facilitates functional investigations on top of the structural information of various endogenous agents inside the human body. The current work addresses a promising attempt at devising high pulse energy SC laser source using telecom-range diode laser systems and few meters of standard single-mode fibers for various MSPAI applications in near-infrared (NIR) and extended-NIR wavelength regions.
An all-fiberized tunable repetition rate (50kHz-10MHz) SC source for photoacoustic microscopy (PAM) and optical coherence tomography (OCT) is developed. OCT is a scattering based imaging technique, requiring low spectral noise. Noise level requirement in OCT is usually mitigated using high repetition rate (MHz) laser sources. On the other hand, PAM is a hybrid imaging modality based on optical absorption that requires high pulse energies, thus, sources operating at lower repetition rates (kHz) are preferred. Nevertheless, it is always important to quantify and understand the RIN dynamics of the SC sources for good quality PAM and OCT images.
We demonstrate a robust photoacoustic medium for measuring the concentration of ammonia in an aqueous solution. We target the near-infrared (NIR) overtone absorption band (~1540 nm) of ammonia with a supercontinuum (SC) laser-based excitation system and an immersion-based acoustic transducer as the detection system. We further present how such a simple system can be used to perform effective in-situ measurements of ammonia over a range of concentrations with a sensitivity of parts per million (ppm) by volume and linearity of <96%. We demonstrate how the sensing system can be readily tailored to monitor the concentrations of other miscible gases in the aqueous solution.
Concatenating fibers to extend supercontinuum generation beyond the multiphonon absorption edge of silica to access longer wavelengths in the mid infrared region has received much attention due to the various molecular fingerprints that exist in this region. Thulium doped (Tm) fiber has been used as an intermediate fiber to get above the multiphonon absorption edge of silica through absorption (900 and 1600 nm) and emission processes between (1800 – 2100 nm) and (2200 - 2600 nm). The advantages of the Tm-doped fiber is not only limited to generating SC above the multiphonon edge of silica but also reducing the high peak power which easily damages the facet of soft glass fibers whose transmission window extends well into the longer mid infrared region. The mechanisms governing the generation of SC in active fibers is quite different from passive fibers as there is interplay of the nonlinear processes that are commonly observed in passive fibers as well as energy transition contributions by the dopants in the fiber.
SC generation and its application are hindered by pulse-to-pulse fluctuations as the generation is initiated by noise seeded processes especially in the long pumped regime (> 1ps). In this work we have experimentally studied the pulse fluctuations in thulium gain fiber whose SC spans 1550 – 2700 nm at three different repetition rates (10 KHz , 100 KHz, 200 KHz). We have illustrated the relative intensity noise across the whole spectrum most importantly within the absorption and emission regions of the Tm-doped fiber.
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