We measured shot noise level with white light source. The noise level is slightly off from shot noise calculation in the noise budget.
After identifying electronic noise in the detection part, we characterized shot noise in RCAV and ACAV systems. We shined a white light source on the RFPD, and adjusted its power so that V_DC level was the same when the cavity was locked. Then measured the noise at mixer out (out1 for RCAV system). This voltage noise has contribution from shot noise and electronic noise. It can be converted to frequency noise by multiplying with the slope of error signal.
| |
ACAV |
RCAV |
| Vdc [V] |
0.4 |
0.1 |
| Vnoise(flat) [nV/rtHz] |
49.4 |
40.7 |
| Slope [kHz/V] |
81.7 |
40.3 |
| f Noise [mHz/rtHz] |
4.0 |
1.64
|
The frequency noise calculated in the last row has contribution from shot noise and electronic noise (from previous entry). By subtracting(quadraturely) the electronic noise, we should obtain the contribution from shot noise only. Electronic noise (from previous entry) are 1.8 and 1 mHz/rtHz for ACAV and RCAV, respectively. This gives shot noise level = 3.6/1.3 mHz/rtHz for ACAV/RCAV. The calculation in the noise budget gives 1.6/0.9 mHz/rtHz. The measured and calculated results do not match. I think because the error in modulation index, finesse, cavity mode matching are too large. The results are summarized in the table below. I will use the measured shot noise/ electronic noise in the noise budget for now.
| |
ACAV |
RCAV |
| Electronic noise(measured)[mHz/rtHz] |
1.8 |
1 |
| Shot+electronic noise [mHz/rtHz] |
4 |
1.64 |
| Shot noise [mHz/rtHz] |
3.6 |
1.3 |
| Calculated shot noise* [mHz/rtHz] |
1.6 |
0.9 |
* The calculated shot noise use 1mW power into each cavity. Modulation index = 0.14. The following numbers are in [ACAV/RCAV] format. Finesse = (7500/9700), Visibility = (90%/80%) |