Before using the Corning PANDA PM980 fibers in FOL, we must characterize the type of noise we expect to see it introduce into the system, so that it may be accounted for when taking measurements. There are essentially three types of noise that we must test for in the fibers.
Polarization Extinction Ration
While the PM980 fibers are supposed to be "Polarization Maintaining," single-mode fibers, nothing's perfect. We plan to test this by designing an optical setup that first modulates and measures the polarization of 1064nm NPRO light. The light is then coupled into the fibers. The light exiting the fibers is collimated, and then we measure the polarization of the remaining light. By further controlling the input polarization, we will be able to test the polarization maintaining properties of the fibers. This will allow us to determine the ratio of S vs P polarization that we expect out of the fibers, for a given input polarization configuration.
Frequency Noise
As this system is designed to measure frequencies, another type of noise that we must know about in order to make more accurate measurements is frequency noise. That is to say, we want to know what kind of noise is introduced to the system at different frequencies. In order to do this, we want to measure the transfer function and power spectral density (PSD) of the fibers in order to:
a) Measure the relative modulation of the input signal by the fibers over a range of different frequencies (transfer function)
b) Measure the amount of power "contained" within each frequency, or frequency range, within the signal (PSD)
These measurements will allow us to know the level of certainty we may have in a reading given by the FOL beat note setup at a given frequency, and adjust the system response accordingly.
Temperature Effects
Heating or cooling the any medium relative to itself changes its index of refraction. Naturally, we want to know the response of our system to these types of heat effects, and changing index of refraction (N) within the fibers can introduce frequency noise.
In order to measure this, we can compare two fibers coupled to the same NPRO, which has had its beam split. One fiber may be heated, within the range of temperature that we expect to see in the 40m lab (that is to say, we needn't test extremes of temperature) and the other held at a constant temperature.
We may then compare the outputs of the fibers, measuring frequency of each beam before and after being in the fibers, which will give us a plot of relative temperature response vs temperature, so that we may know the level of noise introduced into the system by temperature effects.