Researchers have implemented a creative approach to reduce stray light using spatial locking technology based on periodic shadows

Reducing stray light is one of the main challenges in combustion experiments using laser beams (such as Raman spectroscopy) for detection. By using a combination of ultrafast laser pulses and gated ICCD or emICCD cameras, a time filter can be effectively used to remove bright and constant flame backgrounds. When the signal reaches the detector, these cameras can open electronic shutters within the time range of ps to ns. However, stray light from other sources still interferes with available signals, especially for low-frequency measurements.

Researchers around Andreas Ehn at Lund University have implemented a creative method to reduce stray light based on a spatial locking technique called periodic shadows. Although the concept of this measurement technique has been demonstrated before, Lund's researchers described in a recent paper that they "improved its feasibility, strength, and robustness" by implementing this method using fiber optic probes.

The experiment used a bundle of 19 optical fibers, which were arranged in a dense circular pattern on the signal collection side of the fiber, but in a linear pattern on the spectrometer side. Each fiber carrying the signal is followed by a dark fiber to generate appropriate fiber spacing along the entrance slit of the Isoplane SCT 320 spectrograph. The fiber pattern is approximated by a square wave function. At the exit of the spectrograph, use a fast gated PI-MAX4 ICCD camera to collect spectral images. The Isoplane spectrograph is an aberration correction system that can maintain high signal accuracy without distortion even in a large sensor area, which helps to reconstruct signals from collected images. The periodic structure of the analyzed signal is calculated column by column through Fourier transform, multiplication with the reference signal, and bandpass filter, which leads to a reduction in DC like offset like the stray light component on the detector.

The researchers further demonstrated the application of this technology in gas and premixed flame Raman spectroscopy. They not only indicate that the reconstructed signal provides a quantitative and accurate measurement of species mole fraction and temperature in the flame, but also that stray light is suppressed by almost two orders of magnitude (with a factor of 80). Their conclusion is that their concept is very valuable for accurate spectral measurements in experiments with limited optical channels.

Source: Sohu