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garaventa, sorrentino - 13:11 Tuesday 14 September 2021 (53143) Print this report
Test of differential reesponsivity of HD photodiodes

Since we recently observed a limited CMRR at the homodyne detector, we investigated the source of the unbalance, which in principle can be a combination of different effects:

1) unequal optical power splitting at the BS

2) different clipping losses at photodiodes

3) different responsivity of the photodiodes

To get rid of clipping losses we align each photodiode with the procedure described here. The residual transverse position error is around 20 um, and the residual axial offset from beam waist is below 400 um. As a result the clipping losses are expected to be lower than 0.1% for both photodiodes.

In principle the responsivity of the two photodiodes should not differ by more than 1%, as they are both quoted with QE>99%. To check whether this is true, we measured the contribution of power splitting. we did repeated measurements of the beam power at BS outputs with a power meter, using as input the BAB and the LO at maximum power (i.e. 2 mW each), in the optical configuration of last CMRR measurement. The resulting splitting ratio is 1.04+-0.01, i.e. the optical power towards PD2 is higher than the optical power towards PD1 by (4+-1)%.

With the BAB well centered on both photodiodes, we measure a residual beat note amplitude resulting from imperfect substraction. To compare this with power unbalance, we measured how much the power ratio on PDs must change in order to recover perfect balancement. To this purpose we artificially lower the detected power on PD2 by movind a bit the photodiode to induce a small clipping. We found that the power on PD2 must be lowered by (3.5+-1)% in order to extinguish the 4 MHz beat and balance the HD, see attached picture. This confirms that the residual unbalance of a few % is dominated by the BS power splitting. In order to remove the uncertainty from the measurement of splitting ratio, we swapped the position of the two photodiodes.

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sorrentino - 15:34 Wednesday 15 September 2021 (53153) Print this report

After swapping the two HD photodiodes and checking again the beam centering to minimize clipping losses, I measured again the HD unbalance by observing the CC_BAB 4 MHz beat magnitude.

Fig 1: when illuminating a single photodiode the 4 MHz magnitude is 2.76+-0.01 mV with PD2 and 2.86+-0.01 mV with PD1. The difference is (4.0+-0.5)% of the average signal.

Fig 2: when illuminating both photodiodes the 4 MHz magnitude is 0.08 mV. In order to minimize it I had to reduce the power on PD1 by about 3.5% (by moving the position of PD1 to induce a small clipping). The second plot shows the ratio of DC signals PD1/PD2.

This result is symmetric w.r.t. before the swapping, confirming that the CMRR is dominated by power umbalance, which is measured to be (4+-1)% with a power meter. This also proves that the responsivity of the two photodiodes is the same within 1%.

Balancing the HD would require to change the BS splitting ratio to reduce the power on PD1 by about 2% and increase the power on PD2 by the same amount.  I checked that, within the rotation range of the BS holder, the splitting ratio changes by much less than 1%.

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sorrentino - 23:12 Wednesday 22 September 2021 (53231) Print this report

The effect of improved CMRR after balancing is clearly visible on the audio channel as well. In the attached plots a comparison of unbalanced (magenta curves) and balanced (ble curves) conditions using the BAB at maximum power (2 mW). First plot, the  noise in the audio band is about 10% higher with the unbalancing due to BS spliting ratio (~4%); this proves that in such condition the correlated noise is about 40% of the shot noise in differential audio spectrum. Second plot: the coherence between differential and sum audio channels is largely suppressed after balancing.

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