Fourier Transform Infrared Spectroscopy (FTIR) Data Processing

Introduction:

FTIR is a ratio technique. This means that the spectrum of the sample is ratiod against a background spectrum.

To produce a sample absorbance spectrum, background and sample spectra are needed.

Interferograms

In FT techniques, the background and sample are acquired from the instrument as interferograms.

An interferogram is a signal that is generated when a beam of light is split into two paths, and then recombined. The two beams can interfere constructively or destructively, depending on their phase relationship, which is influenced by the optical path length difference between them.

The heart of an FT spectrometer is the interferometer. There are many interferometer designs, but the Michelson interferometer is the most widely used. In this design, there are two mirrors and a beamsplitter. One mirror is fixed, the other is moving. The beamsplitter is a half-transparent mirror. https://www.britannica.com/technology/Michelson-interferometer

The interferogram is obtained by measuring the intensity of light as a function of the optical path difference (OPD) between the two beams. This measurement is typically done using a Michelson interferometer. The resulting interferogram contains information about the spectral content of the light source. Each point in the interferogram corresponds to a specific optical path difference, capturing the interference pattern of the light waves. The Fourier Transform of the interferogram converts this time-domain signal into a frequency-domain spectrum, revealing the different wavelengths present in the light source.

The Michelson interferometer is a powerful optical instrument used to measure the properties of light and is widely utilized in various fields, including spectroscopy, metrology, and telecommunications. This article will explain how the Michelson interferometer works in simple terms, making it accessible for students and technicians without a technical background.

What is a Michelson Interferometer?

At its core, the Michelson interferometer splits a beam of light into two separate paths and then recombines them to create an interference pattern. This interference pattern can provide valuable information about the light source and the materials being studied.

1. Basic Components

A typical Michelson interferometer consists of the following components:

  • Light Source: This can be a laser or any other coherent light source that emits light waves.
  • Beam Splitter: A partially reflective mirror that divides the incoming light beam into two separate beams.
  • Mirrors: Two mirrors that reflect the split beams back towards the beam splitter. One mirror is stationary, the other mirror is moving. The moving mirror creates an optical path difference when the beams are recombined.
  • Detector: A device (like a camera or photodetector) that captures the combined light from the two beams.

2. Splitting the Light

When the light from the source hits the beam splitter, it is partially reflected and partially transmitted. This creates two beams of light:

  • Beam A: The beam that is transmitted through the beam splitter.
  • Beam B: The beam that is reflected off the beam splitter.

3. Traveling Different Paths

The two beams travel different paths to their respective mirrors because of the moving mirror:

  • Beam A travels to Mirror A.
  • Beam B travels to Mirror B.

4. Reflection and Return

After reaching their mirrors, both beams are reflected back towards the beam splitter. As they return, they recombine at the beam splitter.

5. Creating Interference Patterns

When the two beams recombine, they interfere with each other. This interference can be constructive (where the light waves add together, making the light brighter) or destructive (where the light waves cancel each other out, making the light dimmer). The resulting pattern of light and dark fringes is called an interference pattern.

6. Analyzing the Results

The interference pattern (the interferogram) is captured by the detector. The units of the inteferogram are centimeters, because the inteferogram measures optical path differences. The Fourier Transform converts the centimeter units to frequency, expressed as reciprocal centimeters, cm-1.