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Fourier-transform spectroscopy

Fourier-transform spectroscopy (FTS) is a measurement technique whereby spectra are collected based on measurements of the coherence of a radiative source using time- or space-domain data, followed by application of the mathematical Fourier transform to obtain the spectrum. In the optical domain, it obtains the infrared, visible, or ultraviolet spectrum of a substance by measuring the interference pattern, or interferogram, produced when light passes through a two-beam interferometer, such as a Michelson interferometer, and then reconstructing the spectrum via the inverse Fourier transform.^[1] In this optical method, broadband radiation from a source is divided into two beams by a beam splitter; one beam travels to a fixed mirror and the other to a moving mirror, after which the beams recombine to form an interferogram that encodes all wavelengths simultaneously as a function of the path difference between the mirrors.^[1] The resulting interferogram is digitized and processed by a computer using the fast Fourier transform algorithm to yield the intensity as a function of wavenumber or frequency, providing a complete spectrum in a single measurement.^[2] The origins of FTS trace back to the late 19th century with Albert A. Michelson's development of the interferometer for measuring light wavelengths and visibility functions, laying the groundwork for interferometric spectroscopy.^[2] Significant advancements occurred in the mid-20th century: Peter Fellgett proposed the multiplex principle in 1951, recognizing that interferometers could measure multiple wavelengths at once to improve signal-to-noise ratio for weak sources, an idea known as the Fellgett advantage.^[2] Pierre Jacquinot further developed the concept in 1958 by emphasizing the high throughput or étendue of interferometers compared to dispersive slits, termed the Jacquinot advantage, which allows greater light collection efficiency.^[2] Janine and Pierre Connes advanced practical implementations in the 1960s, achieving high-resolution planetary spectra and demonstrating wavelength accuracy through internal laser referencing, known as the Connes advantage; their work was pivotal in transitioning FTS from theoretical to routine use.^[2] The 1965 introduction of the Cooley-Tukey fast Fourier transform algorithm revolutionized computational feasibility, enabling rapid processing of interferograms and widespread adoption of FTS.^[2] Compared to traditional dispersive spectroscopy using prisms or gratings, optical FTS offers several key advantages, including faster acquisition times due to simultaneous measurement of all frequencies (multiplex advantage), higher sensitivity for low-light applications, superior resolution limited only by the maximum path difference in the interferometer, and reduced stray light interference.^[1] These benefits stem from the interferometer's ability to use large apertures without slits, achieving up to 200 times greater light-gathering power in the infrared region, and its inherent photometric accuracy.^[1] In Fourier-transform infrared (FTIR) spectroscopy, a common variant, these features have made it the preferred method for routine analysis, scanning full spectra in seconds rather than minutes.^[3] FTS finds broad applications across scientific fields, including molecular identification through absorption or emission spectra in chemistry and materials science, where it serves as a "fingerprint" for compounds by quantifying components in mixtures and assessing sample quality.^[3] FTS also encompasses time-domain methods in nuclear magnetic resonance (NMR) and other pulsed spectroscopies, enabling high-resolution studies in chemistry and physics. In atmospheric and environmental research, it analyzes trace gases like water vapor and pollutants with high precision.^[1] Astronomical observations benefit from its sensitivity to weak sources, as demonstrated in early high-resolution studies of planetary atmospheres, such as Venus's CO₂ bands.^[2] Additionally, FTS extends to biological processes, bioenergy production, and medical diagnostics via FTIR for non-invasive tissue analysis.^[4] Today, it remains a cornerstone technique due to its versatility, with ongoing developments in portable and high-resolution instruments enhancing its impact.^[5]

Introduction

Definition and Overview

Fourier-transform spectroscopy (FTS) is a measurement technique that records the interferogram—an interference pattern generated by combining light waves with varying path lengths—and applies the Fourier transform to convert this time- or path-domain signal into a frequency-domain spectrum, revealing the intensity distribution across different wavelengths or frequencies. This approach enables the simultaneous acquisition of data across the entire spectral range, offering advantages such as improved signal-to-noise ratio (known as the Fellgett or multiplex advantage) and enhanced light-gathering power compared to traditional dispersive methods that scan wavelengths sequentially. FTS is widely applied in fields like infrared, ultraviolet, visible, and microwave spectroscopy for analyzing molecular structures, atmospheric composition, and material properties.^[6]^[1] At its core, an FTS system consists of a broadband light source, a beam splitter that divides the incoming radiation into two beams, a fixed mirror and a moving mirror to introduce a controllable optical path difference, and a detector that captures the intensity of the recombined beams as the mirror translates. In a typical schematic, light from the source strikes the beam splitter, which transmits one portion to the fixed mirror and reflects the other to the moving mirror; the beams then recombine at the splitter, pass through or interact with the sample, and reach the detector to form the interferogram, which is digitally processed to produce the spectrum. This configuration leverages interferometry principles, where constructive and destructive interference encodes spectral information.^[3]^[6] Prerequisite to understanding FTS is the distinction between spectral representations in wavelength (typically in nanometers or micrometers) and frequency (often expressed as wavenumber in cm⁻¹, the reciprocal of wavelength), with FTS inherently yielding results in the frequency domain for direct interpretation of vibrational or electronic transitions. The technique supports both emission modes, where spectra arise from light emitted by excited samples such as gases or plasmas, and absorption modes, where transmitted light through a sample reveals absorbed frequencies corresponding to molecular bonds. Although the term "Fourier-transform spectroscopy" emerged in the 1950s amid advancements in computing that enabled practical Fourier analysis, its foundational interferometric principles originated in 19th-century work by Albert A. Michelson.^[6]^[1]

Historical Background

The foundations of Fourier-transform spectroscopy (FTS) were laid in the late 19th century through the work of Albert A. Michelson, who developed the Michelson interferometer in the 1880s primarily to measure the wavelength of light with high precision.^[7] Michelson's instrument, initially designed for the Michelson-Morley experiment to detect the luminiferous ether, demonstrated the interference patterns essential for later spectroscopic applications, marking the inception of interferometric techniques in optical measurements.^[8] Advancements in the mid-20th century transformed these early concepts into practical spectroscopy. In 1947, Marcel J. E. Golay invented the Golay cell, a sensitive pneumatic detector for infrared radiation that overcame limitations of previous detectors and enabled the recording of interferograms in the infrared region, facilitating the practical implementation of FTS.^[9] This was complemented by Peter Fellgett's 1951 PhD thesis at the University of Cambridge, where he proposed the multiplex advantage of interferometry—allowing simultaneous measurement of all wavelengths to improve signal-to-noise ratio—and performed the first numerical Fourier transformation of an interferogram to derive a spectrum.^[10] In 1958, Pierre Jacquinot emphasized the high throughput or Jacquinot advantage of interferometers, which allows greater light collection efficiency compared to dispersive instruments with slits. The 1965 introduction of the Cooley-Tukey fast Fourier transform algorithm revolutionized the computational feasibility of processing interferograms. Janine and Pierre Connes advanced practical implementations in the 1960s, achieving high-resolution planetary spectra and demonstrating wavelength accuracy through internal laser referencing, known as the Connes advantage; their work was pivotal in transitioning FTS from theoretical to routine use.^[2] The 1960s saw the adoption of FTS in astronomy, driven by improvements in computing and instrumentation. Pioneering observations by Pierre and Janine Connes using a custom two-beam interferometer yielded the first high-resolution near-infrared FTS spectra of Venus in 1966, revealing detailed carbon dioxide absorption bands in its atmosphere and demonstrating the technique's potential for planetary studies. Concurrently, facilities like the Kitt Peak National Observatory began integrating FTS into solar telescopes in the early 1970s, with early systems supporting atmospheric and stellar spectroscopy.^[11] By the 1970s, the availability of minicomputers such as the PDP-8 enabled routine laboratory use of FTS, with commercial instruments like Digilab's FTS-14 (introduced in 1969) making the technology accessible beyond specialized observatories.^[12] Pulsed FTS emerged prominently in the 1970s through applications in nuclear magnetic resonance (NMR), where Richard R. Ernst advanced Fourier-transform methods to enhance sensitivity and resolution. Ernst's innovations, starting from his 1966 work at Varian Associates and expanding in the early 1970s, allowed rapid acquisition of time-domain signals (free induction decays) followed by Fourier transformation to yield frequency spectra, revolutionizing NMR and earning him the 1991 Nobel Prize in Chemistry.^[13] Recent developments up to 2025 have focused on enhancing resolution and speed through integration with advanced sources and detectors. Synchrotron radiation facilities have enabled synchrotron-based FTS (SR-FTIR) for high-brightness, broadband infrared microspectroscopy, achieving sub-micrometer spatial resolution in applications like biomolecular imaging.^[14] Quantum detectors, such as superconducting nanowire single-photon detectors, have improved sensitivity in the mid- and far-infrared, while post-2020 computational advances in fast Fourier transform algorithms—leveraging GPU acceleration—have enabled real-time spectrum processing for dynamic systems like transient molecular events.^[15]

Theoretical Foundations

Interferometry Basics

Interferometry in Fourier-transform spectroscopy relies on the fundamental principles of wave interference, where light waves from a common source superimpose to produce patterns of reinforcement or cancellation depending on their relative phases. Constructive interference occurs when waves are in phase, resulting in maximum intensity, while destructive interference happens when they are out of phase by π radians, leading to minimum intensity. These effects arise from differences in optical path lengths traveled by the waves, which introduce phase shifts proportional to the path difference δ.^[16]^[17] In a two-beam interferometer, the intensity of the recombined light for a monochromatic source of wavelength λ is given by

I(\delta) = I_0 \left[1 + \cos\left(\frac{2\pi \delta}{\lambda}\right)\right],

where I_0 is the average intensity and δ represents the optical path difference between the beams. This equation describes how the interference pattern oscillates between maximum (constructive) and minimum (destructive) values as δ varies, with the cosine term capturing the phase-dependent modulation. For a polychromatic source containing multiple wavelengths, the total intensity forms a superposition of these individual patterns, encoding the spectral information into a single interferogram I(δ) = ∫ B(ν) [1 + cos(2π ν δ)] dν, where B(ν) is the spectral radiance as a function of wavenumber ν = 1/λ.^[16]^[5]^[17] The interferogram's role in spectroscopy stems from this superposition, which multiplexes the contributions of all wavelengths into the path-difference domain, allowing the entire spectrum to be captured simultaneously. In emission mode, the interferogram directly reflects the intensity distribution of light emitted by the source, while in absorption mode, the sample modulates the incident broadband light, imprinting its transmission spectrum onto the resulting interference pattern. A key feature is the zero-path-difference point, where δ = 0 and the two beams have identical path lengths, yielding the maximum intensity due to perfect constructive interference across all wavelengths and forming the central peak of the interferogram.^[16]^[5]^[17]

Mathematical Basis: Fourier Transform

The mathematical foundation of Fourier-transform spectroscopy relies on the relationship between the interferogram, which records intensity as a function of optical path difference δ, and the spectrum B(σ), representing intensity as a function of wavenumber σ = 1/λ. For a broadband source, the interferogram I(δ) is given by the integral

I(\delta) = \int_0^\infty B(\sigma) \cos(2\pi \sigma \delta) \, d\sigma,

assuming the spectrum is real and even, and neglecting the constant background term that arises from the zeroth-order interference.^[18] This equation describes how the interference pattern encodes the spectral information in the time- or path-difference domain, with the cosine term reflecting the phase difference for each wavenumber component. To recover the spectrum from the measured interferogram, the inverse Fourier transform is applied:

B(\sigma) = 2 \int_0^\infty I(\delta) \cos(2\pi \sigma \delta) \, d\delta,

valid for even interferograms and under ideal conditions without apodization.^[18] This pair establishes the interferogram as the forward cosine transform of the spectrum, enabling reconstruction through inversion. In practice, the integrals are approximated due to finite measurement ranges and noise. For digital implementation, the continuous integrals are discretized into the discrete Fourier transform (DFT), where the interferogram is sampled at N points with interval Δδ, yielding a finite sequence I(n Δδ) for n = 0 to N-1. The spectrum is then computed as

B(m \Delta\sigma) = \sum_{n=0}^{N-1} I(n \Delta\delta) \cos(2\pi m n / N),

with Δσ = 1/(N Δδ), though the full complex form is often used for computational efficiency.^[17] The fast Fourier transform (FFT) algorithm, particularly the Cooley-Tukey radix-2 variant, efficiently computes the DFT in O(N log N) operations when N is a power of 2, revolutionizing digital processing in spectroscopy since its introduction in 1965 and subsequent application to interferogram analysis.^[19] Apodization is essential in digital processing to mitigate artifacts from the finite interferogram length, which otherwise produces Gibbs ringing (sidelobes) in the spectrum due to the sinc-like instrumental lineshape. A window function w(δ) is multiplied by I(δ) before transformation, such as the Boxcar (rectangular) window w(δ) = 1 for |δ| ≤ Δδ_max/2, which offers maximum resolution but high sidelobes (~22% of peak height), or the Hann window w(δ) = 0.5 [1 - cos(2π δ / Δδ_max)] for |δ| ≤ Δδ_max/2, which suppresses sidelobes to ~2.5% at the cost of ~50% resolution broadening.^[20] The choice balances ringing reduction against mainlobe widening, with the transform of the window convolving the true spectrum. Units in spectroscopy typically express path difference δ in cm and wavenumber σ in cm⁻¹, so that the product σδ is dimensionless, aligning with the oscillatory argument in the cosine.^[21] Sampling must satisfy the Nyquist theorem, requiring interval Δδ ≤ 1/(2 σ_max) to avoid aliasing of the highest wavenumber σ_max in the spectrum.^[17] The achievable spectral resolution Δσ, defined as the minimum resolvable wavenumber separation (e.g., via Rayleigh criterion), is limited by the maximum path difference Δδ_max to Δσ ≈ 1 / Δδ_max for unapodized cases, where longer scans enhance resolution but increase acquisition time and noise sensitivity.^[22]

Instrumentation

Continuous-Wave Michelson Spectrometer

The continuous-wave Michelson spectrometer forms the core instrumentation for traditional Fourier-transform spectroscopy (FTS), employing a broadband light source to generate an interferogram through interferometric modulation.^[21] The setup typically includes a collimated broadband source, such as an incandescent lamp for near-infrared wavelengths up to approximately 5 μm or a Nernst glower for mid- to far-infrared up to approximately 25 μm, which provides stable, continuous emission across the spectral range of interest.^[21]^[23] This radiation enters a 50/50 beam splitter, which divides the beam into two paths: one directed to a fixed mirror and the other to a moving mirror mounted on a precision translation stage.^[24] The reflected beams recombine at the splitter, producing interference patterns that are detected by a single-element photodetector, such as a mercury cadmium telluride (MCT) detector optimized for infrared sensitivity and rapid response.^[25] In operation, the moving mirror scans at a constant velocity along the optical path, systematically varying the path difference between the two arms and modulating the interference as a function of time, which is recorded as an interferogram representing detector signal versus path difference.^[22] This scanning ensures uniform sampling of the optical path difference (OPD), with velocity precisely controlled by a motorized drive to maintain consistent increments and avoid distortions in the interferogram.^[24] Beam splitters are selected based on the wavelength range; for mid-infrared applications, potassium bromide (KBr) substrates coated with germanium provide efficient 50/50 splitting up to 25 μm, though KBr's hygroscopic nature requires protective measures, and a compensator plate addresses dispersion-induced OPD asymmetry for balanced transmission and reflection.^[21]^[24] Typical scan lengths range from 1 to 10 cm of maximum OPD, enabling resolutions from 1 cm⁻¹ to 0.1 cm⁻¹, where finer resolution corresponds to longer scans to capture higher-frequency components in the interferogram.^[22] Calibration relies on a helium-neon (HeNe) laser reference beam, which generates evenly spaced fringes during the scan to track mirror position with sub-wavelength accuracy, correcting for any non-linearities and ensuring precise OPD sampling.^[21] This setup leverages basic interferometry principles to encode spectral information into the temporal domain.^[24]

Spectrum Acquisition and Processing

In Fourier-transform spectroscopy, spectrum acquisition begins with the digitization of the interferogram signal detected by the interferometer. The analog signal from the detector undergoes analog-to-digital conversion, typically triggered by the zero-crossings of a reference helium-neon (HeNe) laser beam to ensure uniform sampling. A common sampling rate is one or two points per HeNe fringe (corresponding to OPD intervals of approximately 0.316 μm or 0.158 μm), which satisfies the Nyquist criterion and prevents aliasing of high-frequency components up to ~15800 cm⁻¹ or higher for the mid-infrared range.^[26]^[27] Zero-filling is often applied during data collection by appending zeros to the interferogram array, typically doubling or quadrupling the data points to a power of two, which facilitates interpolation and improves the apparent resolution in the final spectrum without altering the true optical resolution.^[28] Pre-processing of the digitized interferogram addresses instrumental imperfections to ensure accurate transformation. Phase correction compensates for misalignment in the interferometer mirrors or detector response delays, which introduce phase errors manifesting as imaginary components in the complex Fourier transform; this is achieved using methods like the Mertz correction, where a low-resolution phase spectrum from a central portion of the interferogram is applied multiplicatively to the full dataset.^[26]^[28] DC offset removal subtracts the mean value of the interferogram to eliminate baseline drifts caused by electronic noise or detector bias, preventing artifacts in the spectral baseline.^[28] These steps are typically implemented in dedicated software packages, such as those integrated with commercial FTIR systems from vendors like Thermo Fisher or Bruker, or general platforms like LabVIEW for custom setups.^[22] The core of spectrum processing involves applying the fast Fourier transform (FFT) to convert the time-domain interferogram into a frequency-domain spectrum, leveraging the mathematical Fourier transform to recover intensity as a function of wavenumber.^[26] Interferograms are classified as double-sided (symmetric around the zero-path-difference point) or single-sided (recorded from zero path difference to maximum only); double-sided data naturally yield real-valued spectra with minimal phase errors, while single-sided interferograms require additional phase correction to avoid distortions, though they reduce acquisition time.^[26] FFT algorithms, such as the Cooley-Tukey method, efficiently handle these computations on arrays up to millions of points, enabling real-time processing in modern instruments.^[22] Common artifacts in processed spectra include ghosting from aperture reflections or multireflections in the sample, which appear as spurious peaks, and aliasing due to undersampling beyond the Nyquist wavenumber (half the maximum sampling frequency).^[29]^[26] Atmospheric absorption lines from CO₂ (around 2350 cm⁻¹) and H₂O (broad bands near 3700–3600 cm⁻¹ and 1600 cm⁻¹) are subtracted by ratioing the sample spectrum against a background interferogram recorded under identical conditions, often automated in software to yield a clean transmittance spectrum.^[30] Apodization is applied prior to the FFT to mitigate spectral leakage from abrupt truncation of the interferogram, which causes ringing or sidelobes in sharp spectral features. The Happ-Genzel function, a trapezoidal apodization with continuous first derivatives, is widely used in infrared applications for its balance between suppressing sidelobes (to about 1–2% of peak height) and preserving resolution (slight broadening to 55–60% of the unapodized value).^[26]^[31] For absorption spectra, baseline correction follows the transform, using polynomial fitting or rubberband algorithms to remove sloping baselines from scattering or instrumental drift, ensuring accurate peak quantification.^[28] The final output is a spectrum in transmittance or absorbance units versus wavenumber (cm⁻¹). Wavenumber calibration relies on the precise HeNe laser reference, mapping path difference to spectral axis with accuracy better than 0.01 cm⁻¹; apparent absorbance, calculated as -log₁₀(T), may overestimate true values due to uncompensated reflections or scattering, necessitating corrections like Kubelka-Munk transformation for quantitative analysis in diffuse reflectance modes.^[26]^[32]

Pulsed Fourier-Transform Spectroscopy

Operational Principles

In optical implementations of pulsed Fourier-transform spectroscopy (FTS), ultrashort laser pulses, typically in the femtosecond to picosecond range, generated by mode-locked lasers such as Ti:sapphire oscillators, are employed rather than steady broadband sources.^[33]^[34] Analogous principles apply in other domains, such as nuclear magnetic resonance with radiofrequency pulses in the microsecond range. These pulses, often 50–100 fs in duration with repetition rates of 80–100 MHz, are split into a pump beam for excitation and a probe beam for detection, enabling the study of ultrafast transient phenomena.^[33] This pulsed operation facilitates direct measurement in the time domain, where the broadband spectral content of the pulses allows simultaneous probing across a wide range of frequencies.^[33] The operational core involves time-domain interferometry, in which the short pulses excite transient coherences that produce a decaying signal, such as an electric field waveform, which is captured over time scales of picoseconds.^[33] This signal is sampled at high temporal resolution, equivalent to terahertz rates (e.g., ~150 THz for 6.6 fs steps), using an optical delay line to incrementally vary the probe pulse timing relative to the signal.^[33] The resulting time-domain interferogram is then Fourier-transformed to yield the frequency-domain spectrum, leveraging the inverse relationship between time and frequency domains.^[33] Triggering and synchronization rely on precise alignment of the pulse train, with the laser repetition rate governing the frequency coverage in advanced configurations like asynchronous optical sampling, where differential repetition rates (e.g., 1 GHz) define the spectral resolution and range up to the pulse bandwidth. Phase-stable referencing, often via common optical paths or electronic stabilization, preserves the coherence of the measurement throughout the delay scan. The coherence time of the transient signal inherently limits the spectral resolution, as longer decay times enable finer frequency discrimination (Δν ≈ 1/τ_coherence).^[33] Detection employs time-resolved techniques, including photoconductive antennas that convert the THz or IR field into measurable photocurrents via femtosecond-gated carriers, or electro-optic sampling in nonlinear crystals where the probe pulse polarization is modulated by the signal field.^[33] For higher speeds, streak cameras can capture the temporal evolution directly. Signal-to-noise ratio is enhanced by averaging multiple pulse train acquisitions, typically thousands of scans, often with lock-in amplification to isolate the coherent signal from noise.^[33]

Free Induction Decay

In pulsed Fourier-transform spectroscopy, the free induction decay (FID) is the transient time-domain signal that arises after an excitation pulse, capturing the relaxation of coherences or transverse magnetization in the system. This signal encodes the frequency components of the spectrum through the precession of excited states at their characteristic frequencies, modulated by decay processes. The FID is mathematically described as I(t) = \sum_k A_k \exp(i \omega_k t) \exp(-t / T_2), where A_k represents the amplitude of the k-th component, \omega_k is its angular frequency, and T_2 denotes the transverse relaxation time governing the exponential decay. Applying the Fourier transform to this FID produces the frequency-domain spectrum, featuring absorption lines with Lorentzian lineshapes whose full width at half maximum is approximately $1 / (\pi T_2). In ultrafast Fourier-transform infrared (FTIR) spectroscopy, the FID emerges from vibrational coherences induced by a pump-probe sequence, where short infrared pulses excite molecular vibrations, leading to a decaying polarization that reveals dephasing dynamics. Similarly, in nuclear magnetic resonance (NMR) spectroscopy, a radiofrequency pulse perturbs the spin magnetization into the transverse plane, generating an FID that encodes chemical shifts and scalar couplings through the phase evolution of individual spin isochromats. To acquire the FID comprehensively, quadrature detection is employed, utilizing two phase-shifted receiver channels to simultaneously record real and imaginary components, thereby preserving phase information and avoiding spectral folding artifacts. From the acquired FID, the transverse relaxation time T_2 can be determined by fitting the signal envelope to an exponential decay model, providing insights into molecular mobility and interactions. Processing the FID prior to Fourier transformation often involves apodization with functions like the shifted sine-bell, which tapers the signal to reduce sidelobes from truncation while optimizing resolution for the inherent exponential decay, particularly beneficial in noisy or limited-duration acquisitions.

Specific Examples

Fourier-transform nuclear magnetic resonance (FT-NMR) exemplifies pulsed FTS in the radio-frequency domain, where a 90° radiofrequency pulse is applied to tip the net magnetization into the transverse plane, generating a free induction decay (FID) signal that is acquired over 1-100 ms to cover spectral ranges from 1-800 MHz depending on the magnetic field strength. This setup requires precise shimming to achieve magnetic field homogeneity, minimizing linewidth broadening and enabling high-resolution spectra of molecular structures.^[35] In two-dimensional FT spectroscopy, coherent multidimensional pulses create correlation spectra that reveal internuclear connectivities and dynamics, with techniques like photon echo methods developed in the 2000s enhancing resolution for complex systems such as biomolecules or excitons. These implementations use sequences of phase-coherent pulses to map couplings in two frequency dimensions, providing insights into energy transfer processes that one-dimensional methods cannot resolve. Ultrafast Fourier-transform infrared (FTIR) spectroscopy employs mid-infrared pulses generated from optical parametric oscillators (OPOs) to probe vibrational dynamics with 100 fs temporal resolution, capturing transient molecular responses in condensed phases. This approach allows real-time observation of bond breaking and solvation changes in photochemical reactions, leveraging the broadband nature of OPO outputs for simultaneous multi-mode tracking. A distinctive application involves time-resolved FTS for plasma diagnostics in laser-induced breakdown spectroscopy (LIBS), where mid-infrared dual-comb spectrometers acquire broadband spectra of species like methane and ethane in laser-generated plasmas during the 2020s.^[36] These setups enable quantitative analysis of transient emission and absorption lines, aiding in the characterization of reaction kinetics and composition in high-temperature environments.^[36] Addressing data challenges in pulsed FTS, oversampling the FID at rates exceeding the Nyquist limit captures rapid initial decays without truncation artifacts, preserving high-frequency components in the transformed spectrum.^[37] Noise reduction is further achieved through phase cycling of excitation pulses, which suppresses coherent artifacts and solvent signals, improving signal-to-noise ratios in multidimensional acquisitions.

Alternative Configurations

Stationary Interferometers

Stationary interferometers in Fourier-transform spectroscopy (FTS) employ fixed optical components to generate interferograms through spatial rather than temporal modulation of the optical path difference. Unlike scanning designs, these systems produce interference fringes across a spatial dimension, where the position on a detector array corresponds to varying path lengths, enabling the interferogram to be captured in a single exposure without mechanical motion. This approach relies on elements such as wedged beam splitters or tilted plates to create a linear variation in path difference, forming spatial fringes that encode the spectral information. For instance, a wedged beam splitter introduces a gradual thickness change, resulting in path differences proportional to the lateral position, which are then "scanned" by the pixels of a detector array or by relative movement of the source.^[38] A notable configuration is the birefringent wedge interferometer, where a tilted birefringent plate exploits the difference in refractive indices for ordinary and extraordinary rays to produce the required spatial path variation. In this setup, the input beam is split into two orthogonally polarized components that propagate with a position-dependent phase delay, yielding an interferogram along one dimension of the detector. This design eliminates the need for a traditional beam splitter and moving mirrors, simplifying construction while maintaining the core FTS principle of Fourier inversion to recover the spectrum. Early implementations in the 1960s explored stationary designs for compact, stable setups, though modern variants prioritize spatial encoding for broader applicability.^[39]^[7] Spatial FTS with focal plane arrays extends this concept to imaging applications, particularly hyperspectral imaging, where the entire interferogram is recorded simultaneously across a two-dimensional detector. Here, the spatial position directly maps to path difference, allowing reconstruction of spectral cubes that combine spatial and spectral information without scanning. Detector arrays, such as CMOS or focal plane arrays in the infrared, sample the fringes at high spatial resolution, enabling broadband spectral recovery; for example, channel-dispersed designs use fixed prisms or gratings to separate wavelengths into parallel interferograms, boosting efficiency and resolution. These systems achieve resolving powers up to 1600 in the near-infrared, corresponding to spectral resolutions of several cm⁻¹ (e.g., ~8 cm⁻¹ at 800 nm).^[40]^[41]^[42] The absence of moving parts in stationary interferometers provides inherent insensitivity to vibrations, making them robust for environments where mechanical stability is challenging, such as field deployments. Resolution is fundamentally set by the maximum path difference across the sampled field, which for wedged or tilted elements scales with the wedge angle or tilt; a 1° tilt over a typical 1 cm aperture can yield fringes supporting resolutions of tens of cm⁻¹ (e.g., ~30–50 cm⁻¹) in optimized infrared setups by maximizing the number of interference cycles. This vibration resistance and compactness facilitate portable devices for on-site analysis, including handheld units for environmental monitoring and remote sensing, where rapid, single-shot acquisitions are essential.^[42]^[38]^[43]

Other Interferometric Designs

In addition to the conventional Michelson design, several alternative interferometric configurations have been developed for Fourier-transform spectroscopy (FTS), offering enhanced stability, compactness, or suitability for specific wavelength regimes through stationary or hybrid approaches. The Sagnac interferometer represents a circular common-path design that achieves path differences by splitting and recombining counter-propagating beams in a loop configuration, often modified for FTS by replacing one mirror with a transmission grating to generate wavefront tilts via diffraction. This setup produces Fizeau fringes that are imaged onto a detector, with the spectrum recovered via Fourier transformation of the interferogram; heterodyning around a selected wavelength enables high resolution without moving parts in static variants. Such systems provide exceptional vibration insensitivity due to the common-path geometry and have been applied in fiber-optic FTS for telecom wavelengths around 1550 nm, where tunable gratings allow self-calibration over broad ranges.^[44] Mach-Zehnder-based interferometers adapt the dual-beam splitting architecture into a modified common-path form, where light is divided into parallel paths using beam splitters and mirrors, then recombined to form a spatial interferogram detected by an array sensor. This configuration rejects common-mode vibrations effectively, as perturbations affect both arms equally, enabling robust operation in unstable environments without mechanical scanning. Applications in visible and UV spectroscopy benefit from the design's high throughput and adaptability to array detectors, achieving resolutions suitable for high-speed spectral imaging in the 400–700 nm range.^[45] Fabry-Pérot etalons operated in FT mode leverage multiple reflections between two parallel high-reflectivity mirrors to achieve high finesse, enhancing interference contrast and effective optical path length. The etalon is quasi-stationary, with path differences introduced by piezo-tuning one mirror over distances up to 60 μm, generating an interferogram from the modulated output intensity that is Fourier-transformed to yield the spectrum. This yields resolutions around 2 nm at 532 nm, limited by the scan range, and supports compact imaging spectrometers with high luminosity for visible to near-IR applications.^[46] Lamellar grating interferometers, featuring movable parallel plates or gratings that divide the wavefront into multiple segments, extend FTS to challenging regimes like the UV by using all-mirror designs to minimize dispersion and absorption. These wavefront-division systems produce interferograms through adjustable overlaps of beam segments, enabling broadband operation from UV (down to ~200 nm) to far-IR with high étendue preservation. In UV applications, they facilitate high-resolution spectroscopy of solids and gases, with knife-edge prisms allowing split-ratio tuning for optimized signal-to-noise. Birefringent FTS configurations, often based on Savart or Wollaston prisms, introduce path differences via polarization-dependent refractive index variations, making them ideal for simultaneous spectropolarimetry. High-order retarders encode polarization states into channeled interferograms, recovered via Fourier analysis to yield both spectral and polarimetric data in a single snapshot. In 2010s astronomical instruments, such as compact imaging spectropolarimeters, these designs enabled wavelength-dependent Stokes parameter measurements for studying magnetic fields in stars and planets, achieving resolutions of ~10 nm in the visible with robustness to alignment errors.^[47] These designs trade off throughput against complexity: while common-path and birefringent variants enhance stability and étendue (often >50% light utilization), they require precise polarization control or custom gratings, increasing fabrication challenges compared to simple two-beam systems. Resolution in multiple-reflection schemes, such as Fabry-Pérot, scales as \Delta \lambda / \lambda = 1/(2N) where N is the effective number of reflections, balancing finesse gains against reduced field of view from higher reflectivity.^[46]

Advantages and Limitations

Multiplex and Throughput Advantages

Fourier-transform spectroscopy (FTS) offers the Fellgett advantage, also known as the multiplex advantage, which arises because all wavelengths across the spectrum are detected simultaneously by the interferometer, encoding the entire spectral information into a single interferogram.^[48] This multiplexed measurement improves the signal-to-noise ratio (SNR) compared to dispersive spectrometers, where wavelengths are scanned sequentially. In the detector-noise-limited regime, the SNR gain is approximately \sqrt{M}, where M is the number of spectral channels, as the noise from the detector is shared across all channels while the signal is coherently reconstructed via the Fourier transform.^[48] More precisely, the SNR in FTS exceeds that of a scanning dispersive instrument by a factor of \sqrt{N/8}, with N representing the number of spectral elements.^[48] The Fellgett advantage applies primarily in detector-noise-limited or source-limited (signal photon-noise-limited) regimes, where the multiplexed detection efficiently utilizes the available signal without amplifying uncorrelated noise.^[1] However, it diminishes or reverses in background-limited cases, such as when thermal or sky background photon noise dominates, as this noise is also multiplexed, leading to an SNR penalty of up to \sqrt{M}.^[49] For broadband infrared spectroscopy, typical SNR improvements from the Fellgett advantage range from 10 to 30 times over dispersive methods, depending on the number of resolved elements (e.g., \sqrt{M} \approx 50 for scanning 1 to 20 \mum at moderate resolution).^[1] Complementing the Fellgett advantage is the Jacquinot advantage, or throughput advantage, which stems from the absence of entrance and exit slits in FTS, allowing a larger etendue—the product of aperture area A and solid angle \Omega—to pass through the instrument without sacrificing resolution.^[50] In dispersive spectrometers, slits restrict the etendue to maintain spectral resolution, limiting light collection. For a circular aperture in FTS, the etendue is given by A \Omega = \pi (D/2)^2 \sin^2 [\theta](/page/Theta), where D is the aperture diameter and \theta is the half-angle of acceptance, enabling up to 100 times higher throughput than equivalent grating-based systems.^[50] This results in practical throughput ratios of 100 to 200 for FT-IR versus grating instruments of similar resolution.^[50] The combined Fellgett and Jacquinot advantages yield substantial SNR gains in infrared FTS, often 10 to 30 times higher in broadband measurements compared to dispersive techniques, particularly when detector noise predominates.^[1] These benefits were first predicted by Peter Fellgett in his 1951 PhD thesis at the University of Cambridge, where he introduced the multiplex principle and demonstrated the first numerically Fourier-transformed interferogram.^[7] The prediction was validated in astronomical applications during the 1960s, as interferometric techniques enabled high-resolution infrared observations that outperformed traditional dispersive methods.^[7]

Resolution, Sensitivity, and Limitations

The spectral resolution in Fourier-transform spectroscopy (FTS) is fundamentally determined by the maximum optical path difference, Δδ, over which the interferogram is recorded, with the resolution Δσ expressed as Δσ = 1/Δδ, where Δσ is in wavenumbers (cm⁻¹). Achieving higher resolution requires longer path differences, but practical limits arise from mechanical stability and scan duration. Apodization functions are commonly applied to the interferogram to suppress sidelobes and ringing artifacts inherent to the sinc instrumental line shape, though this broadens the effective line width and trades off some resolution for improved spectral fidelity.^[20] Sensitivity in FTS depends on the detector's noise equivalent power (NEP), which quantifies the minimum detectable signal, and the total scan time, as extended acquisitions enable noise averaging to enhance signal-to-noise ratio (SNR). FTS particularly excels in low-light environments by simultaneously detecting all wavelengths, leveraging its throughput advantage for superior photon collection efficiency compared to dispersive methods. However, long scans necessary for high resolution are prone to 1/f noise from detector dark current and electronics, which degrades low-frequency components and limits overall sensitivity. Key limitations include sampling errors that cause aliasing if the interferogram sampling rate is less than twice the maximum wavenumber σ_max, causing high-wavenumber components (> σ_max) to fold back into the lower-wavenumber range, leading to spectral distortions and ghost peaks. Phase instabilities, often due to optical misalignment or thermal drifts during scanning, introduce errors in the complex Fourier transform, manifesting as asymmetric lineshapes or baseline tilts. High-resolution spectra impose significant computational demands; for instance, a resolution of 0.01 cm⁻¹ requires a 100 cm path difference, generating interferograms with millions of data points that necessitate efficient fast Fourier transform algorithms and substantial processing resources. In open-path FTS configurations, atmospheric interference from variable absorption by water vapor and other gases complicates quantification, while non-common-path designs exhibit heightened sensitivity to mechanical vibrations, which induce phase fluctuations and reduce measurement accuracy.^[51]^[52]^[53]^[54] Mitigations for these challenges include adaptive sampling strategies, such as nonuniform or compressed sensing approaches, to reduce data volume while preserving resolution against aliasing. Post-2020 advances in AI-based denoising, employing unsupervised machine learning techniques like singular value decomposition (SVD) and non-negative matrix factorization (NMF), have effectively suppressed noise and phase errors in FTS data, improving SNR without excessive computational overhead.^[55]

Applications

Mid-Infrared and Far-Infrared Spectroscopy

Fourier-transform mid-infrared (MIR) spectroscopy, operating in the 400–4000 cm⁻¹ range, is widely employed for analyzing vibrational spectra of molecular species in gas and liquid phases using specialized transmission cells. These cells, often constructed with potassium bromide (KBr) windows that transmit effectively in the MIR region, allow for the interrogation of samples such as organic compounds to identify characteristic functional groups like carbonyls (C=O around 1700 cm⁻¹) and hydroxyls (O-H around 3400 cm⁻¹).^[56] Gas cells with path lengths up to 10 cm enable detection of trace volatiles, while liquid cells with thin spacers (typically 0.025–0.1 mm) accommodate neat liquids or solutions without dilution.^[57] To mitigate interference from atmospheric water vapor, which absorbs strongly at 1600–1800 cm⁻¹ and 3700–3900 cm⁻¹, instruments are routinely purged with dry nitrogen or air, significantly reducing baseline noise and improving signal-to-noise ratios.^[58] In far-infrared (FIR) Fourier-transform spectroscopy, covering 10–400 cm⁻¹, the technique targets low-energy rotational transitions, particularly in gaseous samples near the terahertz boundary. Polyethylene windows, offering high transmission (>80%) from 16 to 2500 μm (~4 to 625 cm⁻¹) with minimal absorption, are standard for FIR cells and beam splitters to avoid material-induced artifacts.^[59] Bolometer detectors, such as superconducting transition-edge sensors cooled to 4.2 K, are essential for FIR due to their sensitivity to weak thermal emissions and broad spectral response, achieving a noise equivalent power of 1.2 × 10^{-13} W/√Hz.^[60] Purging remains critical here as well, since water vapor rotations overlap with sample signals around 100–200 cm⁻¹, potentially obscuring transitions in molecules like phosphine (PH₃) at 267 GHz (8.9 cm⁻¹).^[61]^[58] MIR-FTIR has been integral to environmental monitoring since the 1990s, with U.S. Environmental Protection Agency (EPA) methods like TO-16 (open-path FTIR for fenceline pollutants) and 320 (extractive FTIR for vapor-phase organics and inorganics such as benzene and HCl) enabling real-time detection of hazardous air pollutants at parts-per-billion levels.^[62]^[63] In pharmaceutical and forensic analyses, FTIR adoption surged in the 1980s, leveraging its non-destructive nature to confirm substances via fingerprint spectra in under 1 minute.^[64] By 2025, handheld MIR-FTS devices, such as the Agilent 4300 series weighing ~2 kg, have advanced on-site forensics by identifying narcotics and explosives directly at scenes without sample preparation, integrating libraries of over 13,000 spectra.^[65]

Terahertz and Microwave Spectroscopy

Fourier-transform spectroscopy (FTS) in the terahertz (THz) regime, spanning 0.1 to 10 THz, employs sources such as backward-wave oscillators (BWOs) and photoconductive antennas to generate and detect broadband THz radiation for interferometric measurements. BWOs provide tunable, monochromatic coherent radiation with high output power and stability, enabling phase-sensitive THz spectroscopy where the complex reflection coefficient is measured to derive absorption spectra. Photoconductive antennas, excited by femtosecond lasers, produce ultrafast THz pulses whose time-domain waveforms are Fourier-transformed to yield frequency-dependent spectra, facilitating applications like non-destructive imaging in security screening where THz waves penetrate non-conductive materials without ionizing radiation. These techniques exploit the non-ionizing nature of THz waves for safe, high-resolution imaging of concealed objects. In the microwave regime (GHz frequencies), Fourier-transform microwave spectroscopy (FTMW) utilizes horn antennas for efficient coupling of chirped microwave pulses to gaseous samples, often in supersonic jets generated by pulsed nozzles to cool molecules and reduce rotational congestion. FTMW captures time-domain free induction decay (FID) signals from polarized molecular ensembles, which are Fourier-transformed to produce high-resolution rotational spectra, aiding the identification of interstellar molecules in laboratory simulations for astronomical searches. The broadband capability of chirped-pulse FTMW allows simultaneous detection of multiple species across 7-18 GHz or wider bands, enhancing its utility in molecular structure determination. Fourier-transform nuclear magnetic resonance (FT-NMR) spectroscopy operates as a form of FTS in the radiofrequency (microwave) domain, where high-field superconducting magnets—reaching up to 30.5 T (1.3 GHz for ¹H) as of 2025—enhance spectral resolution by increasing chemical shift dispersion.^[66] In FT-NMR, the FID from spin coherences is Fourier-transformed to generate one-dimensional spectra, which extend to multi-dimensional variants (e.g., 2D, 3D) that resolve complex couplings and correlations essential for determining protein structures in solution or solid states. These multi-dimensional spectra map internuclear distances and dynamics, enabling high-throughput structural biology applications. A notable advancement is cavity-enhanced FTMW spectroscopy, which integrates cryogenic superradiant cavities to amplify weak signals from trace gases, achieving detection sensitivities down to parts-per-billion (ppb) levels for atmospheric monitoring and environmental analysis. This enhancement boosts the effective path length and signal-to-noise ratio, allowing ultra-sensitive detection of transient species without extensive averaging. Challenges in THz FTS include the need for cryogenic detectors, such as bolometers, to achieve sufficient sensitivity due to low photon energies, though recent cryogen-free pyroelectric receivers are addressing speed and cooling limitations. In FT-NMR, spin relaxation—particularly T2 (transverse) relaxation—poses challenges by broadening lines and limiting acquisition times, necessitating optimized pulse sequences to mitigate decoherence in high-field environments.

Astronomical and Remote Sensing Uses

In astronomy, Fourier-transform spectroscopy (FTS) enables high-resolution infrared observations of faint celestial objects, leveraging its broadband coverage and multiplex advantage to detect molecular signatures in exoplanet atmospheres. Imaging FTS instruments, such as the prototype developed for direct detection and characterization of exoplanets, combine interferometric techniques with array detectors to achieve simultaneous spectral and spatial resolution, allowing the isolation of planetary signals from stellar glare through cross-correlation with molecular templates.^[67]^[68] The Fellgett advantage, which improves signal-to-noise ratio by a factor of approximately √M (where M is the number of spectral resolution elements) in detector-noise-limited regimes, is particularly critical for these faint sources, enabling efficient use of limited telescope time on ground-based facilities.^[7]^[21] Hybrid echelle-FTS designs further enhance this capability by integrating dispersive elements for order separation with interferometric multiplexing, as demonstrated in broadband spectral interferometers that bridge the gap between traditional echelle spectrographs and full FTS for mid-infrared astronomical applications.^[69] For instance, the SITELLE instrument on the Canada-France-Hawaii Telescope employs an imaging FTS to produce datacubes with resolutions up to R=5000 over 300–900 nm, facilitating studies of resolved stellar populations and galactic dynamics through integral field spectroscopy.^[70] In remote sensing, open-path FTS systems provide path-integrated measurements of atmospheric greenhouse gases, offering high precision for monitoring urban and regional emissions without sample collection. The Total Carbon Column Observing Network (TCCON), operational since 2004, utilizes ground-based FTS instruments viewing direct solar spectra in the near-infrared to retrieve column-averaged dry-air mole fractions of CO₂ and CH₄ with a spectral resolution of 0.02 cm⁻¹, serving as a global reference for validating satellite data.^[71]^[72] These systems integrate with differential optical absorption spectroscopy (DOAS) techniques to enhance trace gas detection in complex environments, such as urban boundary layers, by combining broadband FTS with targeted absorption modeling.^[73] An example is the open-path FTS observatory in Toronto, which measures CO₂ and CH₄ along kilometer-scale paths with precisions of 0.2 ppm and 5 ppb, respectively, aiding flux quantification in heterogeneous terrains.^[74]^[75] Satellite-based FTS has revolutionized global CO₂ profiling, with the Greenhouse gases Observing SATellite (GOSAT), launched in 2009, employing the Thermal and Near-infrared Sensor for carbon Observation (TANSO-FTS) to achieve column-averaged CO₂ accuracies of ~0.3% from sun-glint and nadir observations.^[76]^[77] Spatial FTS variants enable hyperspectral Earth imaging by generating datacubes where each pixel encodes a full interferogram, convertible to spectra; static imaging FTS designs on small satellites produce gigapixel-scale volumes for vegetation and atmospheric analysis, benefiting from the throughput advantage for broadband remote sensing.^[78]^[79] Recent advancements, including machine learning for cloud contamination rejection in GOSAT-2 data processing (launched 2018), have improved retrieval biases to <0.5 ppm in partially cloudy scenes, enhancing the utility of these datasets for carbon cycle modeling.^[80]

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