Owing to the absence of moving mechanical components, dual-comb spectroscopy (DCS) can attain greater spectrum sensitivity and a low acquisition duration (in contrast to Fourier-transform spectroscopy). This article discusses the basics of dual-comb spectroscopy, its challenges, and the recent advances in this particular field.
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Optical Frequency Comb (OFC)- The Foundation of DCS
With the advancement of laser technology, ultrashort pulsating lasers give a new technological technique for highly precise spectroscopic measurement.
To precisely monitor wavelengths, the optical frequency comb (OFC) was designed, which is an equally spaced ultrashort laser pulsing pattern in the time domain or a persistent and evenly distributed spectroscopic band with the same spacing. In the past few decades, dual-comb spectroscopy (DCS) has made extensive use of two OFCs with a small frequency variation.
Difference Between Fourier Transform Spectroscopy (FTS) and DCS
Fourier transform spectroscopy (FTS) involves the integration of the wave interference pattern diagram of two fixed phase beams of Michelson interference, termed the moving arm and the static arm.
The DCS process involves two distinct OFC beams with varying replication intervals used to substitute the moving and static arm pulse of the coherent beams. Due to the core reason of varying repetition frequency, the pulsing impulses of the dual OFC will "cross through each other" in the temporal domain to produce connected wave graphs.
Types of DCS Modules
DCS may be classified into two categories based on the variation in detecting light. The first kind of DCS can instantly surpass the impulses of the two OFC to identify the specimen whose output was balanced, and after Fourier transformation, only the intensity output can be retrieved.
The other type of DCS allows one OFC to penetrate through the specimen as the probing laser and surpass the other standard OFC to acquire the magnitude output of the specimen and the phase profile after the Fourier transformation.
What Are the Challenges Faced by DCS?
Recently, DCS has received considerable attention for its ultrafast scanning speed, remarkable signal-to-noise (SNR), outstanding resolving power, and reliability, but realizing high-repetition and high-precision DCS is still challenging due to the difficulties in realizing two OFCs with extraordinarily high correlation.
The core reason for it is that the carrier wave pulse was measured in femtoseconds, a slight temporal spike fluctuation or phase variation between the 2 OFCs would cause the interference pattern to deform. And such distortion would be difficult to eradicate by future data processing.
Application of DCS Near-Infrared Absorption Spectroscopy
The latest article in the journal Chinese Chemical Letters provides a thorough background and advances in dual-comb spectroscopy. It has mentioned the applications of DCS for near-infrared absorption spectroscopy.
The variable and repeating amplitude maxima of chemical intrinsic assimilation of groups in the mid-infrared domain are involved in the spectroscopic absorbent spectrum in the near-infrared range. However, only the DCS of gas particles with good transparency, a large absorption point, and a small absorption peak breadth in the near-infrared spectrum has been studied for shortcomings of present technology limits.
The rotational-vibrational band of H13C14N was monitored, encompassing THz with 120 SNR for the absorption bands using a stable DCS setup with a detectability of 1495 nm to 1620 nm and periodic increments of 155,000 Hz.
Can DCS be Used in Mid-Infrared Absorption Spectroscopy?
As per the research, the highest spectrum for the vibrational assimilation of practically all sorts of particles can be found in the mid-infrared range, which is roughly 1-2 orders of magnitude bigger than that in the near-infrared region.
Because high precision was thought to be more suited for chemical monitoring, notably gas particles, several near-infrared DCS technologies were successfully translated into the mid-infrared range using difference frequency generation (DFG) techniques.
Ultraviolet Dual-Comb Spectroscopy
An article published in Optics Express discusses the essential steps taken in the development of the first UV dual-comb spectrometer. In the UV region, a number of compound families have crowded multi-line spectra. Nitrous oxides, the bulk of carbon emissions, and astrophysically significant gases, in particular, exhibit distinctive reflectance spectra in the UV with substantial absorbance cross-sections.
The researchers proposed an appropriate prototype model for UV-DCS based on optimum performance novel technology. V-DCS has the ability to deliver a comparative precision of at least 10-9. UV-DCS allowed for extensive ultra-broadband investigations of molecular and atomic vapors in a spectrum region including element-specific valence electronic transition, which were frequently overlaid with vibrational resonance.
What is Compressive Dual Comb Spectroscopy?
Novel research published in Science Reports provides a review regarding Compressive Dual Comb Spectroscopy. Compressive sensing (CS) is a processing approach that permits signal regeneration from a drastically decreased number of points by exploiting a signal's invariance.
To solve the data size issue of high-speed, bandwidth, and high-resolution dual-comb spectrometry, the researchers devised compressive dual-comb spectroscopy (C-DCS). The quantitative simulation of C-DCS of two molecular species (N2O and CO) revealed a decrease of more than a factor of two in the needed data points within the permissible mole fraction uncertainty of 3%.
It has been discovered that the CS is particularly beneficial for DCS because it creates a big amount of data with a broad spectral frequency and multispectral, ensuring the greatest sparseness and hence effective data contraction. Furthermore, the fast scan rate of DCS creates a high-speed data stream, which might lead to data transit and storage issues.
In short, Dual comb spectroscopy has vast advantages but research is needed to overcome the problems that it faces for its rapid commercialization.
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References and Further Reading
Kieu, K., 2022. DUAL COMB SPECTROSCOPY. [Online]
Available at: https://wp.optics.arizona.edu/kkieu/dual-comb-spectroscopy/
Schuster, V. et. al. (2021). Ultraviolet dual comb spectroscopy: a roadmap. Optics Express, 29(14). 21859-21875. Available at: https://doi.org/10.1364/OE.424940
Kawai, A. et al. (2021). Compressive dual-comb spectroscopy. Sci Rep 11, 13494. Available at: https://doi.org/10.1038/s41598-021-93005-1
Wei, Z. et. al. (2022). The development and application of dual-comb spectroscopy in analytical chemistry. Chinese Chemical Letters. Available at: https://doi.org/10.1016/j.cclet.2022.02.05