Background-Free Absorption Spectroscopy with Undithered Light Sources

Absorption spectroscopy is a chemical analysis tool that measures light absorption of a material over a range of frequencies. In laser absorption spectroscopy, a laser may be directed through the material while the frequency is tuned over the absorption range of that material. The amplitudes of light before and after it passes through the material are compared to produce an absorption spectrum.

One issue in measuring absorption spectra using laser absorption spectroscopy is that the light detector receiving the directed light must be able to discriminate small intensity variations caused by the absorption while the signal from the laser causes the presence of a strong “background” signal from the unabsorbed light. Background signal may be suppressed to some extent through the use of modulation techniques in which the frequency of the laser is “dithered” as it is swept through the range of frequencies. Dithering refers to a slight frequency variation on top of monotonic variation of the swept frequencies, and it serves to encode the absorption bands in the material as a result of interaction of the varied wavelengths with the absorption feature. The frequency encoded absorption signal then can be demodulated to eliminate the background radiation.

Although these modulation techniques effectively reduce the influence of background radiation, they have inherent practical difficulties. Only small frequency ranges of dithering are possible relative to the absorption features of interest, not all laser sources permit modulation with injection current and dithering can be complex and difficult to control. A “background-free” method to detect small intensity variations in absorption spectroscopy without a dithering signal is needed.

UW–Madison researchers have developed a technique for reducing the effect of background radiation in absorption spectroscopy that works with undithered light sources. Versions of the technique utilize a combination of optical delays, a splitter and/or balanced photoreceivers. The simplest implementation combines all three components. Wavelength-swept sensor light is directed through a sample and then split into two paths of different lengths. Each of the paths terminates in one diode of a balanced photoreceiver, which allows direct digitization of the difference in sample response at those two wavelengths. A similar version is based on the split-path delay component but uses a single (not balanced) photoreceiver combined with intensity modulation. Yet another implementation uses a modification of a standard grating spectrometer with a spatial offset in place of the split-path light delay.

All versions of this technique are superior to current background-free techniques because of their inherent stability, simple set-up and ability to work with undithered light sources. By implementing laser swept signal without requiring an additional dithered signal, the processing and recording needed in current modulation techniques are bypassed.

File Number: P09196US