I acquired 3 minutes of the AC noise in the following conditions;
- pump off
- probe on
- front door closed
- various lock in integration time constants;
As shown in the plot , the higher the time constant, the lower the noise.
But we have to choose it depending on the total acquisition time, for example we cannot spend a week to acquire a surface map.
I report the values:
|
timeconstant (ms) |
Noise std (microV) |
| 10 | 47 |
| 30 | 14 |
| 100 | 18 |
| 300 | 9.5 |
| 1000 | 5.2 |
We remind the specification of the system says between 5 and 25 microV at time constant: 100ms; and chopper frequency: 380Hz.
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Then we switch off the HeNe probe and measure the noise of the photodetector with 100ms time constant.
The photodetector noise standard deviation is 0.8 microV (see the histogram below) and it includes the detector noise and the lockin noise
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For absorption of 22% we have about 350mV AC signal at 30mW pump power;
The signal should be at least 2 or 3 times the noise (18microV * 3);
For 12ppm we expect 19microV of AC signal at the same pump power (30mW)
This means that a pump power of 100mW might be sufficient to see a good signal for 12ppm. It follows that we need 10W to see absorptions of 0.1ppm
