NAOJ GW Elog Logbook 3.2
To have an idea about how PD responses (at RF frequency) to green light. We did this test.
The test set-up is as shown in the attached figure 1 and 2. We put a periscope after the first green FI. It brings the green beam up to the second layer of the bench. At the second layer, we put a lens and a steering mirror to make sure the light is small enough and hit in the first quarter of QPD2. We connected the corresponding DC channel to the oscilloscope to check the output voltage DC. We also connect the corresponding RF channel to a 32dB amplifier and then a spectrum analyzer(Keysight N9320B) to check its noise spectrum.
To avoid any modulation and even RF cross-talk, we turned off the DDS system. The green power is changed by moving the offset of high voltage driver(connect to the PZT of SHG). We checked this offset is stable enough so that the green power doesn't change larger than 4% within the time of one measurement.
Within one measurement, we measured the light power/output voltage DC/output current(measured by using the 'putty')/RF channel spectrum.
| Output voltage DC(V) | Light power(mW) | Output current(mA) |
Calculated current(mA) (photosensitivity is 0.2A/W according to specification) |
power loss (%) (inferred from measured and calculated current) |
|---|---|---|---|---|
| 5.6 | 34.5 | 5.3 | 6.9 | 23.2 |
| 7.7 | 53.5 | 7.4 | 10.7 | 30.8 |
| 8.8 | 70 | 8.7 | 14 | 37.9 |
| 10 | 90 | 10 | 18 | 44 |
The relation between power and voltage/current is attached in figure 4.
I also calculated the shot noise by using this formula: shot noise = 10*log((sqrt(2*1.6e-19*I)*sqrt(RBW))**2*35/1mW)+32+22, then I got the calculated shot noise level
| Output Voltage DC(V) | Calculated shot noise(dBm) |
|---|---|
| 5.6 | -48.3 |
| 7.7 | -46.8 |
| 8.8 | -46.1 |
| 10 | -45.5 |
While the measured shot noise is attached in figure 3, which shows -60dBm for all different light power.
