Pengbo and Yuhang

We measured shot noise spectrum with 5, 15, 26mW seperately. For each power, we measured also with different beam size ranging from to.

The set-up is using the second segement of QPD2. We checked DC voltage with oscillscope. We also checked RF channel after an amplification of 32dB with spectrum analyzer operates in 1MHz RBW(this time we average for 10 times so that the noise vurve is smooth). The noise spectrum of RF signal is plot for each case.

1. 5mW case(attached as the first picture): the noise floor is the same for all the beam size.

2. 15mW case(attached as the second picture): the noise floor reaches maximum when the power density is below ~25 mW/mm2

3. 26mW case(attached as the third picture): the noise floor reaches maximum when the power density is below ~22mW/mm2

We didn't measure the changing point for 5mW, however, from the beam density we measred, all the measurement we did for 5mW has low power density relative to the threshold (roughly between 22 and 25mW/mm2). We should see the noise floor decrease when the beam size is smaller than 500um.

Conclusion: The QPD response will saturate and decrease if the power density exceeds around 23mW/mm2.

(Notice: the beam size in this entry is diameter)