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Entry  Wed Apr 6 19:36:32 2011, Aidan, Summary, Green Locking, (In)sanity check of Green PD - some inconsistencies PD_measurement.pngplot_PD_RF_ratios.pdf
    Reply  Thu Apr 7 16:09:17 2011, Aidan, Summary, Green Locking, (In)sanity check of Green PD - some inconsistencies 
       Reply  Thu Apr 7 19:28:02 2011, Koji, Summary, Green Locking, (In)sanity check of Green PD - some inconsistencies SGD-444A.png
Message ID: 4494     Entry time: Wed Apr 6 19:36:32 2011     Reply to this: 4500
Author: Aidan 
Type: Summary 
Category: Green Locking 
Subject: (In)sanity check of Green PD - some inconsistencies 

I moved the Hartmut Green PD to the Jenne laser bench to try to determine if the response at RF was reasonable or somehow very much smaller than it should be. It was set up as shown in the attached diagram. The first pass at this was by comparing the ratio of the RF photocurrent of the green PD to the RF photocurrent of the New Focus 1611 InGaAs PD. That ratio (at a sufficiently low frequency) should be the same as the ratio the DC photocurrents of the two PDs.

Using the network analyzer I measured the ratio of the voltages of the two RF signals (and then scaled each of these by the respective transimpedances of the PDs: 700 Ohms for the 1611 and 240 Ohms for the Harmut PD). The resulting ratio is shown in the attached plot.

I measured the DC voltages from each PD and scaled those by the transimpedances to get the photocurrent (10 kOhm for the 1611 and 80 Ohm effective for the Harmut PD). The ratio of the DC photocurrents was 0.37. This is roughly 3x the ratio of the RF photocurrents at 500kHz (=0.14). This discrepancy is uncomfortably large.

 The full set of measurements is given in the table below:

Measurement Value
DC voltage from Hartmut PD 6.5mV (checked by turning laser on and off and measuring the difference)
DC voltage from 1611 InGaAs PD 2.20V
Transimpedance of Harmut PD at DC 80 Ohm (effective)
Transimpedance of Harmut PD at RF 240 Ohm
Transimpedance of 1611 InGaAs at DC 10 KOhm
Transimpedance of 1611 InGaAs at RF 700 Ohm
Incident Power on Hartmut PD (100% on PD area) 0.28mW (measured by Ophir power meter)
Incident Power on 1611 InGaAs (<100% on PD area) 0.64mW
Responsivity of Silicon PD at 1064nm 0.02 A/W (estimate)
Responsivity of 1611 New Focus PD at 1064nm ~0.8 A/W
   

There is one other troubling point: using the estimate of responsivity on the Harmut PD * incident power * transimpedance at DC = (0.02A/W) * (0.28mW) * (80 V/A) = 0.45 mV.

But the measured DC voltage is 6.5mV = inconsistent.

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plot_PD_RF_ratios.pdf
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