Yuhang, Michael

We measured the relative intensity noise of the new Mephisto 2W NdYAG laser to be used at TAMA. The laser was sent into a 75mm lens, then a 50:50 beamsplitter going to a photodetector and power meter sensor. The distance between the laser and detectors is fairly short due to the initial divergence of the laser beam. With the photodetector we took the following spectra, while the power meter was used to measure long term power stability. Some ND filters were placed between the laser and beam splitter to counteract saturation of the PD and prevent back reflection into the laser. 

First PD: Thorlabs PDA36A-EC 

Second PD: Qubic PD-AC100-Si 1-100 MHz

Figure 1 shows the low frequency laser RIN at approximately the minimum current required for the laser to start emitting (about 0.8 A), compared to the PD dark noise. The measurement was passed through the Stanford pre-amp at DC coupling and 100 gain.

Figure 2 shows the RIN at different laser current (power). The DC level on the detector changes on the highest two measurements due to the PD saturating. The gain on the pre-amp was reduced by a factor of half for the highest two current measurements. Due to the PD reading being close to the noise floor, this may be responsible for the jump between the levels of the measurements between different pre-amp gain values.

Figure 3 shows the effect of activating the noise eater. In our first measurement its effect only became noticeable at laser current > 1.55 A.

Figure 4 shows the predicted frequency dependence of RIN from the Coherent Mephisto manual. Ideally the noise eater suppresses the large peak and reduces the 10k-100kHz noise floor by ~15 dB. We don't see much if any of the latter effect though. It seems like the low frequency signal is too close to the detector noise level. The levels are a bit different from the measured results - I didn't apply normalization to the measurements from the spectrum analyzer readings, but the relative values for noise eater on/off are what is important.

To follow up, we tried changing the preamp gain settings and changing the spectrum analyser coupling from DC to AC to see if we could get better SNR. The Stanford pre-amp was close to saturating though.

Figure 5 shows the result of increasing the PD gain and optimising the pre-amp settings. We can get a bit more clearance from the PD dark noise versus figure 1, but still less than desireable.

We then tried to do another measurement using the second PD, but it seemed like it had more dark noise than the Thorlabs one just from inspection on the oscilloscope. The Qubic PD is AC only, but we couldn't see anything on the oscilloscope when applying 1-10 kHz modulation, just ~ 5 mV rms noise.