The Spectropolarimeter for Planetary EXploration SPEX is designed to measure aerosol and cloud particles in atmospheres of planets within our solar system. The instrument is, among else, able to find the number densities of particles within the atmosphere, to detect the size distribution, and determine chemical properties, such as refractive index. In order to achieve this, SPEX directly measures the (intensity) spectrum and the degree of polarization for visible light.
In order to measure the degree of polarization, SPEX uses a completely new technique, which is developed by members of the iSPEX team (Snik, Karalidi, & Keller(2009)). In stead of temporal modulation or spatial modulation, techniques described in the polarimetry page, SPEX uses the technique which we call spectral modulation. An extensive, technical description on spectral modulation can be found on the SPEX site of SRON.
The modulated spectrum
Whenever the light is unpolarized, SPEX will measure a regular spectrum, as if it was nothing more than a spectrograph. This unmodulated spectrum is shown in the left plot of the image above. The upper band shows a color spectrum, similar to what we would see with our eyes. The graph below plots the light intensity against the color, or wavelength. The wavelengths of the graph roughly correspond to the colors in the upper band.
However, as soon as the light becomes polarized, the spectrum is superimposed with a synusoidal pattern, as shown in the right plot of the above picture. Dark and bright areas interchange one another across the spectrum. We call this a modulated spectrum, where the synsoidal pattern is the modulation.
The amplitude of the synusoidal pattern is an indication of the fraction of light that is polarized. Completely polarized light will thus have a maximum amplitude to the modulation, which can be recognized for its minima reaching the zero intensity line. The opposite, where the light is fully unpolarized shows no modulation at all, i.e. the normal spectrum. Any degree of polarization between 0 and 1 (between completely polarized and unpolarized) yield a modulation with a shallower modulation.
The direction in which the light is polarized can also be determined from the modulated spectrum. Change in this direction will cause a (phase)shift of the synusoidal pattern across the spectrum.
SPEX optical design
The SPEX concept consists of four basic elements, briefly described below.
1: quarter wave plate
The quarter wave plate is not the most fundamental element. Some polarization directions (or angles) are even left completely unaffected. The purpose of this waveplate is to change the linear polarization for specific polarization angles into circular polarization. This is needed to make sure that the (initial linear) polarization angle will be modulated by the next element, the multi-order waveplate.
The effect is that incident light that is polarized in any random direction will always be modulated by the multi-order waveplate.
2: multi order waveplate
This element is primarily responsible for the spectral modulation. Different from the quarter wave plate is that the phase shift (change of polarization) depends on the color (wavelength) of the light.
In other words, the change in polarization angle caused by this waveplate is different for each wavelength. This causes a difference in polarization angle between two separate wavelengths of 90º.
Conceptual drawing of the SPEX optical design. Of the incident horizontally polarized beam (indicated by the arrows), only certain colors keep this horizontal polarization, while the polarization angle of other colors is transformed by the multi order waveplate into vertical polarization. Only vertically polarized colors are transmitted by the polarizer.
Similar to the other polarimetric methods, finally, a polarizer is used.
As we described in the caption of the conceptual drawing above, the polarization angle of one wavelength can be perpendicular to the polarization angle of another wavelength. When the latter angle is exactly aligned with the polarization axis of the polarizer, this color will be fully transmitted, while the first color will be fully blocked by the polarizer, because it’s polarization angle is perpendicular to the polarizer.
Since the modulation of the light beam is spectral by nature, we can only observe it after we look at each color seperately. That is why the SPEX concept needs a spectrograph at the end of the light path. The modulation of the light that is caused by the previous elements can then be seen in the spectrum that is created and measured by this spectrograph.