Surface magneto-plasmons in magnetic multilayers - Walther ...

Section 3.2
Calibration 43
I in
-
+
R
Figure 3.7: The circuit diagram shows an operation amplifier operated as a current-tovoltage
converter
via
U out
Iout = SλP. (3.4)
The sensitivity is limited by dark noise which is equal to the noise when no light
illuminates the photodiode. To quantify the dark noise, a cover is mounted in front
of the detector, so that no light can reach the photodiode. Figure 3.8 shows the
corresponding output voltage of the photo-detector as a function of time. The graph
shows a background signal of Uout = 74.9 µV with a standard deviation of ∆Uout =
±3.20 µV. Additional to the above mentioned noise sources the power supply with
Urms = 350 µV [76] and the voltmeter with Urms = 1 µV [77] will contributed to the
background signal. The noise of the power supply will have an effect on both the
measured noise floor and the background signal as the power supply is connected to
the voltage inputs of the amplifier and to the photodiode. The noise of the voltmeter
can be neglected, since it is much smaller than the background signal.
To detect a change in laser power this change must yield an output signal greater
than the noise floor (Uout > ±3.20µV). With Eq. (3.3) and R = 1 MΩ it follows that
the change in input current must be Iin > ∓3.20 × 10 −12 A. Assuming that the input
current is equal to the output current of the photodiode, the minimal change in laser
power that can be detected is P > ±0.008 nW, considering that Sλ = 0.42 A/W (Tab.
3.3).
With the smallest detectable change in laser power of P > ±0.008 nW a noise density
for the setup can be calculated. With the bandwidth of the SIM 970 of BW = 3.6 Hz,