The laser demo was delivered yesterday.
I replaced the fiber without moving the collimator.
I realigned the beam moving the last lens before the sample, and I maximized the signal on the surface reference.
In order to have a DC of 2V on the imaging unit PD, I set the laser power to 428uW (the laser display shows 0.7mW).
Then I noticed that I had a better signal moving the PD further on the IU, and the DC dropped to 1.3V.
Then I increased the power to 980uW (the laser display shows 1.55mW) to have 3V of DC on the PD.
I attach the last calibration scan.
The pump power without the sample measured with the new power meter is 33mW
I measured the noise on the reference sample with and without chopper and pump
| chopper OFF | chopper ON | |
| pump OFF | 2.77 ppm*W | 3.13 ppm*W |
| pump ON | / | 28ppm*W |
When everything is off, the noise is 2uV (0.14ppm*W), which is the dark noise of the PD plus the environmental light.
The noise of the new laser is 3ppm*W, without any filter, nor control loop. Much better that the previous laser, but still above the expected signal level.
It could be worth to try a control loop and see if the noise reduces more.
