Interferometer - The heart of FT-IR spectrometers

First, let us look at a simplified diagram of an FT-IR spectrophotometer:

In an FT-IR spectrophotometer, there is no light dispersion, which means that the light beam emitted from the source, passing through the sample compartment and subsequently reaching the detector is always polychromatic. Therefore, the raw data obtained is NOT the IR spectrum that most of us are familiar with (Figure 1). Instead, what is recorded is an Interferogram, as shown in figure 2. A mathematical processing called Fourier transform (performed by computer) helps converting this data into the IR spectrum.

Figure 1. Example of an IR spectrum

Figure 2. Example of an interferogram

Obviously, the difference of the intensity between the light beam that passes through the sample and the one that does not (baseline measurement) is determined. However, that is not enough data for the Fourier transform. In fact, the IR radiation must be altered by the Interferometer, in such a way that make it carries the suitable information for the process.

A basic design of the interferometer is shown in figure 3. This is what is called a Michelson interferometer.

Figure 3. Simplified diagram of Michelson interferometer

The system includes a Fixed mirror, a Moving mirror and a Beam splitter. The beam splitter has a special feature: it allows 50% of the incident light to go through, the other 50% to be reflected. Thanks to this property, the beam coming from the source is split into 2 identical parts. Each part is directed to a mirror (either fixed or moving) and then completely reflected. The reflected beams (R1 and R2) meet at the beam splitter and then then combines (interfered) on their way towards the detector.  

The optical length difference (D) between R1 and R2 changes as the moving mirror travels back and forth. When D = 0, all the waves in R1 and R2 are in phase, therefore, they are all constructively interfered. When D ≠ 0, corresponding to each position of the moving mirror, a certain component (i.e. light wave) in R1 and R2 is at phase reversal (i.e., phase difference = 180o), resulting in a destructively interference. Thus, such component disappears on the combined beam. Consequently, when the moving mirror complete one cycle, the system completes one scanning through the wavelength range.

Figure 4. Constructive interference and Destructive interference

In order to obtain a good interference, R1 and R2 must be parallel after passing through the beam splitter. This has always been an important consideration. In a traditional design, the mirrors are flat, which means that when one is tilted, the reflectance angle will change, the reflected light will no longer be aligned. As a result, the quality of the interferogram will be affected.

One approach to address this problem is to use a technique called dynamic alignment. In a dynamically aligned interferometer, either the moving or fixed mirror is equipped with piezo transducers which tilts one mirror after a positional error is detected on the other. The disadvantage of this technique is that the error must first occur and then be detected before the correction can be made. Another disadvantage of such systems is the use of either an air bearing which is expensive, or a mechanical bearing which is prone to wear. Note that even if a design specifies a drive system that is referred to as a frictionless electromagnetic drive (i.e. voice coil), the bearing itself may in fact be a contact mechanical type prone to wear. Hence, many FT-IR systems are vibration-sensitive, and the Interferometer is usually considered fragile.  

Over the years, different improvements have been made. Among which, cube corner mirror interferometers are in wide use for laboratory and process applications and have some unique characteristics. Unlike flat mirrors, cube corners are practically immune to mirror tilt (i.e. angular movement of the mirror).

The RockSolidTM interferometer incorporates dual retroreflecting cube corner mirrors in an inverted double pendulum arrangement. A wear-free pivot mechanism is located at the center of mass. This patented design optically eliminates mirror tilt and mechanically prevents mirror shear. It is also resistant to vibration and thermal effects. The wear-free nature of the bearing in the RockSolidTM interferometer ensures exceptional stability and reliability even in harsh environments.

With this design, Bruker offers an extensive guarantee time for its IR spectroscopy products, delivers high quality and low maintenance cost.

References:

www.newport.com/n/introduction-to-ftir-spectroscopy

www.brukeroptics.com