Understanding Spectrometers (ACE-FTS)

One of the ACE mission's primary objectives is to study and monitor stratospheric ozone.

At the heart of the SCISAT-1 Atmospheric Chemistry Experiment (ACE) mission is a Fourier Transform Spectrometer (FTS). The task of the ACE-FTS is to observe the infrared spectrum of the sun before and after sunlight has passed through the Earth's atmosphere.

Analysis of the spectra will give the ACE scientific team information about various processes involved in the chemical dynamics of the Earth's upper atmosphere. Because the heart of this experiment uses a MichelsonFootnote 1 Interferometer it is extremely good at detecting the motion of gases (winds) in the upper atmosphere, as well monitoring the chemical dynamics of the atmosphere.

The great advantage of the interferometer is its ability to provide very high resolution spectra.


Footnote 1

A. A. Michelson (1852-1931): Nobel prize in Physics in 1907 for his work in optics.

Return to footnote 1 referrer

The Fourier Transform Spectrometer

The Fourier Transform Spectrometer
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A Fourier Transform Spectrometer consists of three major parts

  1. A small telescope which collects light from a luminous source. The light is focused into a thin beam using a narrow slit and a set of specially designed lenses.
  2. An interferometer which splits the beam of light into two separate beams and then recombines them back into a single beam.
  3. A detector, such as a photocell, to measure the intensity of the light in the recombined beam as it emerges from the interferometer.

Parts Of The Fourier Transform Spectrometer
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The Michelson Interferometer replaces the traditional diffraction grating used in conventional spectrometers.

The key to the interferometer is its ability to change the path-length of one of the split beams before it recombines with the second beam. A movable mirror accomplishes this.

The beam splitter is usually a mirror with 50% surface reflectivity and 50% transparency. One-half of the light goes straight through the beam splitter to the movable mirror and one-half of the light is reflected upwards to the fixed mirror.

These two half-intensity beams are then reflected back to the beam-splitter where once again each is split into two beams which are 25% the intensity of the incident beam.

Two of these 25%-intensity beams recombine and emerge from the interferometer in the direction of the detector, and two in the direction of the original incident beam.

Interferometer Basics

An interferometer takes advantage of the fact that light exhibits wave properties.

Constructive Interference
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When two light waves of the same wavelength (colour) combine exactly in phase (in step) their amplitudes add to produce a large (brighter) wave of maximum intensity. This is known as constructive interference.

The illustration above shows the results of two waves, (a) and (b), merging when their amplitudes are in phase with each other. The result is shown as wave (c).

By adjusting the movable mirror of the interferometer, a specific wavelength can be selected so that the merging waves of that wavelength are exactly in phase. This will produce a beam of maximum brightness of that wavelength (colour).

Destructive Interference
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If the light waves combine out of phase (out of step) their combined amplitudes are less, and may even totally cancel each other! This is known as destructive interference.

The illustration above shows total destructive interference. When Waves (a) and (b), which are out of phase, are combined; the result is shown as (c).

Of course, the light entering the interferometer is of many wavelengths, and because they all have different wavelengths they do not all merge exactly in phase. In fact, most waves will experience either total or partial destructive interference which suppresses their brightness.

Spectra Produced by Interference

Have you ever noticed the rainbows that are produced by a few drops of oil or gasoline on the surface of a puddle or on the surface of a soap bubble?

These rainbows are a result of light being reflected back to your eye from the inside and outside surfaces of the oil (or soap) film. The light beam to your eye is experiencing interference. One colour's intensity is amplified, all other colours are suppressed.

You observe different colours because the thickness of the film is not uniform. Strong constructive interference occurs at only one "favoured" wavelength and this "favoured" wavelength gradually changes as the thickness of the film changes. All other wavelengths are suppressed by destructive wave interference.

As a result we see a rainbow of colours across the surface of the oil film. The same effect is seen in soap bubbles.

Optical Interference from a Non-Uniform Thin Film

Incident Sunlight
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The slightly wedge shape of the oil film means that the reflected waves from the bottom surface each have slightly different path lengths to travel before recombining with the reflected waves from the top surface. The result is that some colours will recombine in-phase at some places, producing regions bright in that colour, while at other places that same colour will be suppressed due to destructive interference.

The main difference between a thin film (interference pattern) and that produced by the Michelson interferometer is that the path difference (and hence the observed colour) can be selected in the Michelson interferometer. By varying the position of the movable mirror it is possible to "tune" across the spectrum of the incident light beam. The effect is similar to the changing thickness of the thin film.

FTS Output

Ozone Absoption
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As the position of the moveable mirror changes, the output from the FTS spectrometer is not simply a single spectrum, instead it is a series of multiple overlapping spectra of alternating coloured lines called "fringes."

To sort out a single "best spectrum" the raw data is processed using computer programs which perform operations on the data called Fourier transforms.

Data from the FTS requires considerable processing before it yields a "clean" spectrum as shown above.