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Fibre Optic Calibrator for KAT


Background

The Karoo Array Telescope (KAT) is a radio telescope being build in the Karoo, South Africa. You can read more about the KAT telescope at www.kat.ac.za.

Radio telescopes are designed to capture the Electro Magnetic waves of Radio frequency wavelengths (??MHz to ??GHz) emitted by the cosmos. By definition, it differs from optical, infrared, xray, etc. telescopes in the frequency band which is being observed..

The high level view of the KAT telescope is shown in the figure below.





The parabolic reflector dish will focus the electromagnetic energy onto the antenna feeds. Immediately after the feeds (and as close as possible), there will be a Low Noise amplifier, bandpass filtering, and further amplification and filtering. At the end of this initial chain of signal conditioning, the RF signal will be modulated onto an optical carrier, and transmitted across a fibre optic cable, of about 3km length, to the control room. Inside the control room the optical signal will be converted back to a RF signal, and then filtered and amplified to a sufficient level to drive an Analogue to Digital converter (ADC)  for digitisation.   

In the ideal scenario, we would like to have a scaled, but exact replica of the input signal at our bandwidth of interest, at the input of the ADC. In reality however, we find that the amplification, filtering and transport mechanism introduces amplitude, phase and intermodulation degradation of this signal of interest. This degradation should be minimised, firstly by good hardware design, and subsequently by software calibration.

Note that this information signal might contain undesirable RF sources, such as man-made RF interferences, etc which have already corrupted the astrological signal before it was captured by the dish. The point is just that ideally the means of  "capturing", which is our amplifying-optical chain described above, should not add any distortion / degradation to our signal of interest.


My Thesis

My thesis focusses on the fibre optic links. And more specifically, how the phase of the optical signal changes over long time (more than a hour), temperature and aging. Also the problem of quantifying the amount of phase change, and possibly how to correct for this change in phase.


my final-year undergraduate thesis