Lightweight Raman spectroscope using time-correlated photon-counting detection

Swept-source Raman spectroscopy of chemical and biological materials

Significance

Raman spectroscopy has been used as a powerful tool for chemical analysis, enabling the noninvasive acquisition of molecular fingerprints from various samples. Raman spectroscopy has proven to be valuable in numerous fields, including pharmaceutical, materials science, and biomedicine. Active research and development efforts are currently underway to bring this analytical instrument into the field, enabling in situ Raman measurements for a wider range of applications. Dispersive Raman spectroscopy using a fixed, narrowband source is a common method for acquiring Raman spectra. However, dispersive Raman spectroscopy requires a bulky spectrometer, which limits its field applicability. Therefore, there has been a tremendous need to develop a portable and sensitive Raman system.

Aim

We developed a compact swept-source Raman (SS-Raman) spectroscopy system and proposed a signal processing method to mitigate hardware limitations. We demonstrated the capabilities of the SS-Raman spectroscopy by acquiring Raman spectra from both chemical and biological samples. These spectra were then compared with Raman spectra obtained using a conventional dispersive Raman spectroscopy system.

Approach

The SS-Raman spectroscopy system used a wavelength-swept source laser (822 to 842 nm), a bandpass filter with a bandwidth of 1.5 nm, and a low-noise silicon photoreceiver. Raman spectra were acquired from various chemical samples, including phenylalanine, hydroxyapatite, glucose, and acetaminophen. A comparative analysis with the conventional dispersive Raman spectroscopy was conducted by calculating the correlation coefficients between the spectra from the SS-Raman spectroscopy and those from the conventional system. Furthermore, Raman mapping was obtained from cross-sections of swine tissue, demonstrating the applicability of the SS-Raman spectroscopy in biological samples.

Results

We developed a compact SS-Raman system and validated its performance by acquiring Raman spectra from both chemical and biological materials. Our straightforward signal processing method enhanced the quality of the Raman spectra without incurring high costs. Raman spectra in the range of 900 to 1200    cm - 1 were observed for phenylalanine, hydroxyapatite, glucose, and acetaminophen. The results were validated with correlation coefficients of 0.88, 0.84, 0.87, and 0.73, respectively, compared with those obtained from dispersive Raman spectroscopy. Furthermore, we performed scans across the cross-section of swine tissue to generate a biological tissue mapping plot, providing information about the composition of swine tissue.

Conclusions

We demonstrate the capabilities of the proposed compact SS-Raman spectroscopy system by obtaining Raman spectra of chemical and biological materials, utilizing straightforward signal processing. We anticipate that the SS-Raman spectroscopy will be utilized in various fields, including biomedical and chemical applications.

Time-resolved vibrational dynamics: Novel opportunities for sensing and imaging

The evolution of time-resolved spectroscopies has resulted in significant advancements across numerous scientific disciplines, particularly those concerned with molecular electronic states. However, the intricacy of molecular vibrational spectroscopies, which provide comprehensive molecular-level information within complex structures, has presented considerable challenges due to the ultrashort dephasing time. Over recent decades, an increasing focus has been placed on exploring the temporal progression of bond vibrations, thereby facilitating an improved understanding of energy redistribution within and between molecules. This review article focuses on an array of time-resolved detection methodologies, each distinguished by unique technological attributes that offer exclusive capabilities for investigating the physical phenomena propelled by molecular vibrational dynamics. In summary, time-resolved vibrational spectroscopy emerges as a potent instrument for deciphering the dynamic behavior of molecules. Its potential for driving future progress across fields as diverse as biology and materials science is substantial, marking a promising future for this innovative tool.

Visible to near-infrared single pixel microspectrometer using electrothermal MEMS grating

Compact spectrometers facilitate non-destructive and point-of-care spectral analysis. Here we report a single-pixel microspectrometer (SPM) for visible to near-infrared (VIS-NIR) spectroscopy using MEMS diffraction grating. The SPM consists of slits, electrothermally rotating diffraction grating, spherical mirror, and photodiode. The spherical mirror collimates an incident beam and focuses the beam on the exit slit. The photodiode detects spectral signals dispersed by electrothermally rotating diffraction grating. The SPM was fully packaged within 1.7 cm³ and provides a spectral response range of 405 nm to 810 nm with an average 2.2 nm spectral resolution. This optical module provides an opportunity for diverse mobile spectroscopic applications such as healthcare monitoring, product screening, or non-destructive inspection.

Wavelength-time coding for multispectral 3D imaging using single-photon LiDAR

Single-photon multispectral light detection and ranging (LiDAR) approaches have emerged as a route to color reconstruction and enhanced target identification in photon-starved imaging scenarios. In this paper, we present a three-dimensional imaging system based on a time-of-flight approach which is capable of simultaneous multispectral measurements using only one single-photon detector. Unlike other techniques, this approach does not require a wavelength router in the receiver channel. By observing multiple wavelengths at each spatial location, or per pixel (four discrete visible wavelengths are used in this work), we can obtain a single waveform with wavelength-to-time mapped peaks. The time-mapped peaks are created by the known chromatic group delay dispersion in the laser source's optical fiber, resulting in temporal separations between these peaks being in the region of 200 to 1000 ps, in this case. A multispectral single waveform algorithm was proposed to fit these multiple peaked LiDAR waveforms, and then reconstruct the color (spectral response) and depth profiles for the entire image. To the best of our knowledge, this is the first dedicated computational method operating in the photon-starved regime capable of discriminating multiple peaks associated with different wavelengths in a single pixel waveform and reconstructing spectral responses and depth.

Surface-enhanced Raman spectroscopy: bottlenecks and future directions

This feature article discusses developmental bottleneck issues in surface Raman spectroscopy in its early stages and surface-enhanced Raman spectroscopy (SERS) in the past four decades and future perspectives.

Multiplexed single-mode wavelength-to-time mapping of multimode light

When an optical pulse propagates along an optical fibre, different wavelengths travel at different group velocities. As a result, wavelength information is converted into arrival-time information, a process known as wavelength-to-time mapping. This phenomenon is most cleanly observed using a single-mode fibre transmission line, where spatial mode dispersion is not present, but the use of such fibres restricts possible applications. Here we demonstrate that photonic lanterns based on tapered single-mode multicore fibres provide an efficient way to couple multimode light to an array of single-photon avalanche detectors, each of which has its own time-to-digital converter for time-correlated single-photon counting. Exploiting this capability, we demonstrate the multiplexed single-mode wavelength-to-time mapping of multimode light using a multicore fibre photonic lantern with 121 single-mode cores, coupled to 121 detectors on a 32 × 32 detector array. This work paves the way to efficient multimode wavelength-to-time mapping systems with the spectral performance of single-mode systems.

Photonic lanterns are made by merging several single-mode cores into one multimode core. Here, the authors show this type of structure can both perform wavelength-to-time mapping of multimode states of light and couple such light to an array of single-photon avalanche detectors.