Fourier Transform Spectroscopy

Fourier transform spectroscopy (FTS) first came to life in the mind of Albert A. Michelson, inventor of the interferometer that bears his name. Michelson recognized the potential of his instrument to analyze the spectra of various light sources, but he and his contemporaries lacked the sensitive instruments and mathematical algorithms that are now the basis of FTS. One practical advantage of FT spectrometers is that they are much more efficient than conventional diffraction-grating spectrometers, which waste a large percentage of the source intensity on narrow entrance and exit slits. A Michelson interferometer consists of two mirrors orthogonal to one another and a beam splitter at a 45-degree angle to each arm. The dark and light spots within the resulting interference pattern are due to the constructive and destructive interference of light. To use the instrument as a spectrometer, one mirror is smoothly translated while recording the changing light intensities; the resulting interferogram is the Fourier transform of the conventional spectrum of intensity versus wavelength.

For this study a Michelson interferometer was constructed using various first surface mirrors, standard Thorlabs mounts, a 50/50 cube beamsplitter, a translating stage, and a He-Ne laser. So far the setup has been successfully aligned and used to observe various types of fringes at a fixed mirror position. The next step, now underway, is to record fringe changes with a photodetector as a mirror is translated. If the distance the mirror moves is large enough it should be possible to observe "beating" effects related to the spectral distribution of the laser light. In the future we hope to study more exotic low-coherence sources such as xenon high-intensity-discharge headlamps, LEDs and superluminescent diodes (SLDs), and even bioluminescent sources like fireflies.

This research was motivated by an interest in optical coherence tomography (OCT), a non-invasive imaging technique closely related to FTS that has interesting applications in biology. In OCT the light source is intentionally a broad-band one, with a very short coherence length. (The coherence length is the distance over which the various light waves stay in step.) This short coherence length allows a Michelson interferometer in the OCT instrument to select one depth at a time when imaging light scattered in tissue. Three-dimensional images of the sample are created by scanning in x and y in addition to depth z. In some cases, such as retinal scans, even one-dimensional OCT scans can give useful information. Our eventual goal is to create a simple working OCT device that could create such 1-D "images."

This research was supported by the Simons Foundation and the Laser Teaching Center.