KAIST and MIT develop compact Raman platform

10 Apr 2024

Swept-source design and new signal processing meet "tremendous need for portable Raman system."

Raman spectrometers tend to be bulky and expensive, hindering their use in several applications where their ability to spectroscopically detect specific chemical species could be valuable.

A project from South Korea's KAIST research institute and MIT in the US has now investigated how a compact Raman platform might be designed, and published its findings in Journal of Biomedical Optics.

"Dispersive Raman spectroscopy requires a bulky spectrometer, which limits its field applicability," commented the project. "Therefore, there has been a tremendous need to develop a portable and sensitive Raman system."

The team's solution was to base a platform around swept-source Raman (SS-Raman), as an alternative to the fixed-wavelength light source used to excite a sample and induce Raman scattering in traditional architectures.

"The wavelength-tunable swept-source laser has emerged as a critical component in biomedical imaging, with swept-source OCT applied in various biomedical applications such as intravascular imaging, and two-photon microscopy demonstrated using a swept-source laser," said the project in its JBO paper.

To apply the same principle in Raman spectroscopy the project employed swept-source excitation light from 822 to 842 nanometers, focused onto a sample after filtering through a short-pass filter present to help eliminate background noise. The scattered light is collected by a lens and filtered by a bandpass filter to isolate the desired Raman-shifted wavelength range, and the filtered light is then detected by a silicon photoreceiver.

The SS-Raman platform was tested by applying it to chemical samples, including phenylalanine, hydroxyapatite, glucose, and acetaminophen; and also to cross-sections of swine belly tissues, to assess its use for biological sample.

Faster Raman measurements for a broader range of samples

Raman spectra obtained from the SS-Raman spectroscopy system "closely resembled those obtained from traditional dispersive Raman spectroscopy with correlation coefficients ranging from 0.73 to 0.91, indicating its feasibility for identifying both types of samples," noted the project.

One particular expense in Raman spectroscopy stems from the need for high-quality filters and light sources in order to address the inherent background noise in a Raman operation and the broad peaks in Raman spectra, arising due to the necessary bandpass filter.

Solving this issue led the project to apply a signal processing method to its SS-Raman platform. Gaussian filters eliminated the ripple noise introduced by the unstable laser output, and a deconvolution analysis sharpened the peaks in the Raman spectra and improved their resolution. Polynomial background removal tackled the background noise caused by the low optical density of the filters.

Future work will include improving the sample acquisition time, which takes over 40 seconds in the project's initial SS-Raman implementation. The team is developing a multichannel system with multiple detectors and bandpass filters, with the goal of reducing measurement time to under one second and making the platform suitable for as broad a range of sample types as possible.

"The proposed system sets the stage for future developments in miniaturizing Raman spectroscopy for both chemical and biological analysis," concluded the project.