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Diffstat (limited to 'faraday_rotation.tex')
| -rw-r--r-- | faraday_rotation.tex | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/faraday_rotation.tex b/faraday_rotation.tex index 23cab82..9745b78 100644 --- a/faraday_rotation.tex +++ b/faraday_rotation.tex @@ -55,7 +55,7 @@ We'll see the Faraday effect by observing changes in the intensity of light as w The phase-locking technique works in the following way. We'll vary the magnetic field periodically with time as a sine wave, and then observe the signal from the photodiode as a function of time. The signal will look -like a large constant with a small wobble on it, along with some random +like a large constant with a small sine wave "wobble" on it, along with some random noise with a similar magnitude to the wobble. However, we can subtract off the non time-varying portion of the signal, using a high pass filter. Then, since we know the period and phase of the magnetic field, we can time our @@ -106,7 +106,7 @@ The experimental setup is shown in Fig.~\ref{fig:setup}. \begin{description} \item[Choice of $\theta$] You need to pick an angle $\theta$, which may seem arbitrary. There is a best choice. Examine Eq.~\ref{eq:Ifinal}. Pick $\theta$ and be sure to tighten the thumbscrew. -\item[Faraday rotation] Plug the photodiode output into the scope, set the scope so its channel is DC coupled, and make sure that the ``probe'' setting is at 1x. Turn the amplifier dial about halfway to the maximum setting you found. Observe the photodiode trace on the scope, perhaps changing the volts/div setting so you can see the trace more clearly. What is the voltage? Record it. The changing magnetic field should be causing a change in the polarization angle of the laser light, which should cause a wobble to the photodiode signal. Can you see any wobble? +\item[Faraday rotation] Plug the photodiode output into the scope, set the scope so its channel is DC coupled, and make sure that the ``probe'' setting is at 1x. Turn the amplifier dial about halfway to the maximum setting you found. Observe the photodiode trace on the scope, perhaps changing the volts/div setting so you can see the trace more clearly. What is the voltage? Record it. The changing magnetic field should be causing a change in the polarization angle of the laser light, which should cause a sinusoidal time dependence to the photodiode signal, referred to as the "wobble." Can you see any wobble? \item[AC coupling] The wobble is riding atop a large constant (DC) signal. The scope can remove the DC signal by ``AC coupling'' the photodiode channel. This essentially directs the scope input |