Formaldehyde is the most abundant aldehyde in the troposphere and a primary indoor pollutant, classified as a human carcinogen. High-sensitivity on-line measurement of formaldehyde is critical for monitoring atmospheric environments and indoor pollution. High-resolution and accurate formaldehyde spectrum are essential for developing high-sensitivity detection instruments and improving spectral inversion accuracy. Therefore, it is of great significance to obtain a high-quality formaldehyde spectrum for formaldehyde research. Fourier transform infrared (FTIR) spectroscopy is a commonly used infrared spectral technique. However, the traditional FTIR spectrometer with an incoherent thermal light source suffers from low sensitivity and requires long averaging time to improve sensitivity. The optical frequency comb is essentially a pulsed laser with the advantages of wide spectrum, high brightness, and good collimation. It can replace the thermal light source in traditional FTIR spectrometer and improve the detection sensitivity by combining a multi-pass cell or an optical resonant cavity. Therefore, we built a Fourier transform spectrometer based on an optical frequency comb to measure the broadband spectrum of formaldehyde and have a quantitative analysis, including measurements in the presence of water interference.

Leveraging the advantages of the optical frequency comb and the FTIR in detecting wide-range molecular absorption spectra, an FC-FTIR spectrometer is built to measure the formaldehyde spectrum near 3.5 μm. The comb source used is FC1500-250-WG and mid-IR optical frequency comb, generating an infrared laser with a center wavelength of 3200 nm by difference frequency conversion. The laser is collimated into the Herriott-type optical multi-pass cell. The optical base length of the cell is only 1.2 m, and an effective absorption path of 60 m can be obtained after multiple reflections. The comb beam exiting the cell is then focused into the FTIR spectrometer for formaldehyde spectrum detection.

The constructed FC-FTIR spectrometer successfully measures the broadband infrared spectrum of formaldehyde in the 2730-2970 cm^-1 band (Fig. 3). The sensitivity of the instrument reaches 3×10^-8 cm^-1·Hz^-1/2 (Fig. 4), and the detection limit of formaldehyde is 414×10^-9. The wavenumber accuracy is better than 150 MHz, allowing for precise quantitative analysis of formaldehyde concentration with an uncertainty of approximately 9%-11% (Fig. 5). At the same time, the absorption spectra of formaldehyde in the presence of a large amount of water are measured (Fig. 6), confirming that the device’s spectral range and resolution are adequate for detecting various species.

In this paper, an FC-FTIR spectrometer is built. The broadband spectra of formaldehyde in the 3.5 μm spectral range are obtained at low pressure and room temperature. The sensitivity of the system is 3×10^-8 cm^-1·Hz^-1/2, corresponding to the formaldehyde detection sensitivity of 414×10^-9. At the same time, the formaldehyde absorption spectrum under water signal interference is measured with prominent band characteristics, and the concentration of formaldehyde can be accurately obtained. The mid-infrared FC-FTIR device can not only detect formaldehyde but also detect greenhouse gases and pollution gases in the atmosphere, which is expected to take an advantage in the field of multi-species dynamic concentration monitoring. This device combines the high-resolution, wide-spectrum measurement and quantitative analysis benefits of traditional FTIR spectrometer with enhanced system sensitivity, reduced response time, and improved frequency accuracy and precision. In addition, the sharp spectral bands of the molecular species obtained by the measurement make it possible to use all the peaks with complete shapes for multi-line fitting in data processing. With its capability for fast acquisition and high-sensitivity broadband spectral data in molecular fingerprint regions, the mid-infrared FC-FTIR spectrometer is expected to gradually replace traditional FTIR spectrometer in molecular spectroscopy.