A Prototype Stationary Fourier Transform Spectrometer for Near-Infrared Absorption Spectroscopy

Abstract

A prototype stationary Fourier transform spectrometer (FTS) was constructed with a fiber-coupled lithium niobate (LiNbO3) waveguide Mach-Zehnder interferometer (MZI) for the purpose of rapid on-site spectroscopy of biological and chemical measurands. The MZI contains push-pull electrodes for electro-optic modulation, and its interferogram as a plot of intensity against voltage was obtained by scanning the modulating voltage from -60 to +60 V in 50 ms. The power spectrum of input signal was retrieved by Fourier transform processing of the interferogram combined with the wavelength dispersion of half-wave voltage determined for the MZI used. The prototype FTS operates in the single-mode wavelength range from 1200 to 1700 nm and allows for reproducible spectroscopy. A linear concentration dependence of the absorbance at λmax = 1451 nm for water in ethanolic solution was obtained using the prototype FTS. The near-infrared spectroscopy of solid samples was also implemented, and the different spectra obtained with different materials evidenced the chemical recognition capability of the prototype FTS. To make this prototype FTS practically applicable, work on improving its spectral resolution by increasing the maximum optical path length difference is in progress.

Miniature Fourier Transform Spectrometer Based on Thin-Film Lithium Niobate

“…It is worth noting that due to the presence of a breakdown electric field (~10 V/μm) in LN, the range of modulating voltage is limited, then Equation (8) can be rewritten as where V max is the maximum modulating voltage, and A 0 ( g ) is the spectral intensity distribution with g. Since the collected data of the modulating voltage V ( t ) and the interferometric light intensity I are discrete sequences, the discrete Fourier transform is used, which is expressed as [ 20 ] …”

Algorithmic Enhancement of Spectral Resolution of a Lithium Niobate (LiNbO3) Waveguide-Based Miniature Fourier Transform Spectrometer

“…21 With the introduction of the resolution enhancement method to the FTS, more detailed information appeared in curve (b) comparing to the absorption spectrum in curve (a), which had a similar shape with the curve (c) obtained using the commercial spectrometer, confirming that the prototype FTS with SVD-FBLP combined method indeed had a higher spectrum resolution than the one reported earlier. 12 It is worth noting that curves (a) and (b) were obtained without apodization for a higher resolution, and consequently led to the upturned ends of the absorption spectrum.

Figure 6.(a, b) Absorption spectra of PET at infrared wavelength range from 1300 nm to 1500 nm obtained by the prototype FTS without and with use of the SVD-FBLP combined method; (c) The absorption spectrum of the same sample measured with a commercial spectrometer.

…”

“…This conflict was also found during our preparation of the electro-optic modulator (EOM)-based FTS. [9][10][11][12][13] The EOMbased FTS is a miniature, stationary, and low-power FTS in which the OPD is adjusted by means of the electro-optic effect of LiNbO 3 (LN). The EOM-based FTS without containing a moving mirror overcomes the mirror-tilting induced disadvantages that are often encountered using conventional FTS.…”

“…The recent work demonstrated that the prototype FTS can be used for real-time absorption spectroscopy of liquid and solid samples. 12 However, the prototype FTS cannot be used for detection of the detailed information about molecular fingerprints due to its low resolution. Even in the case of increasing the maximum OPD of the MZI by two times using the end-face reflection structure, 13 the spectral resolution of the FTS is still insufficient.…”

“…The EOM-based FTS without containing a moving mirror overcomes the mirror-tilting induced disadvantages that are often encountered using conventional FTS. References 9-13 contain a more detailed description about Bentini 9 and Li's work [10][11][12][13] on EOMbased FTS. In this paper, an algorithm method for spectral resolution enhancement was applied to an EOM-based miniature FTS, leading to at least a three-fold increase in resolution without distortion of reconstructed spectra.…”