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Infrared (IR) Spectroscopy: Overview01:09

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When electromagnetic radiation passes through a material, atoms or molecules transition from a lower to a higher energy state by absorbing radiation corresponding to the energy difference between the two states. The absorption of infrared (IR) radiation causes transitions between vibrational energy levels in a molecule. Therefore, IR spectroscopy is a useful analytical tool for determining the molecular structure of molecules.
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The non-destructive nature and ability to provide valuable chemical information make IR spectroscopy a versatile technique with broad applications in various scientific and industrial fields. IR spectroscopy is commonly used to identify and characterize organic and inorganic compounds. It provides information about the functional groups present in a molecule and the bonding between atoms. This helps in the structural elucidation of compounds during organic synthesis, pharmaceutical research,...
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IR spectra are divided into two main regions: the diagnostic region and the fingerprint region. The diagnostic region of the spectrum lies above 1500 cm−1. The absorptions resulting from single-bond vibrations of the N–H, C–H, and O–H stretch at higher wavenumbers and appear on the left side of the spectrum. The stretching absorptions of the C≡C and C≡N occur between 2100–2300 cm−1. In contrast, those arising from stretching absorptions of the...
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There are two main infrared (IR) spectrophotometers: dispersive IR spectrometers and Fourier transform infrared (FTIR) spectrometers. In a dispersive IR spectrometer, a beam of infrared radiation produced by a hot wire is divided into two parallel equal-intensity beams using mirrors. One beam passes through the sample, while another is a reference beam. The beams then move through the monochromator, which separates the radiations into a continuous spectrum of different frequencies. The...
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When infrared (IR) radiation passes through a molecule, the bonds stretch or bend by absorbing the radiation. This absorption creates the molecule's absorption spectrum, which is the plot of its percentage transmittance versus wavenumber.
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Broad-Bandwidth Frequency-Domain Near-Infrared Spectroscopy System on a Chip.

Siavash Yazdi1, Saba Mohammadi2, Shashikant Lahade3

  • 1Department of EECS, University of California, Irvine, CA 92697 USA. He is now with NXP Semiconductors, Irvine, CA 92618.

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|December 29, 2025
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Summary
This summary is machine-generated.

A novel integrated circuit (IC) enables precise gain and phase measurements for frequency-domain near-infrared spectroscopy (fd-NIRS). This advancement paves the way for scalable, wearable optical sensing systems.

Keywords:
CMOS processbiomedical optical imagingnear-infrared spectroscopyphantomsphase detectionphotodiode

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Area of Science:

  • Integrated circuit design
  • Optical spectroscopy
  • Biomedical engineering

Background:

  • Frequency-domain near-infrared spectroscopy (fd-NIRS) is crucial for non-invasive tissue analysis.
  • Existing fd-NIRS systems require complex hardware for accurate gain and phase measurements.
  • Broad modulation bandwidth is essential for high-resolution fd-NIRS.

Purpose of the Study:

  • To design and evaluate an integrated circuit (IC) for gain and phase measurements in fd-NIRS.
  • To enable a broad modulation bandwidth for enhanced fd-NIRS performance.
  • To facilitate the development of scalable and wearable fd-NIRS systems.

Main Methods:

  • Developed an IC featuring a voltage-controlled oscillator (50 MHz–1 GHz) and an N-path filter-based phase detector.
  • Integrated the IC with a laser diode and avalanche photodiode (APD) for a complete fd-NIRS system.
  • Characterized tissue-simulating phantoms using modulation frequencies from 105–405 MHz.

Main Results:

  • The IC achieved accurate analog measurements of amplitude and phase.
  • Optical property accuracy for absorption and reduced scattering was within 0.00064 mm-1 and 0.054 mm-1, respectively.
  • System performance was limited at higher frequencies by the APD.

Conclusions:

  • The designed IC significantly advances fd-NIRS system capabilities.
  • The developed IC supports accurate optical property measurements.
  • This work presents a promising foundation for future wearable and scalable optical sensing technologies.