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Entry  Wed May 22 15:07:09 2019, awade, DailyProgress, WOPO, Demodulation and subtraction for homodyne detector with Zurich box IMG_6099.JPGSweep_DemodLPFValues.pdfSingleHDChannel_RelaxOscillations_CompairNoiseEaterONOFF.pdfDemodFreqSweep_median100Hzto1kHz_vsdemodFreq.pdfCompairSubtractedSignalToDarkNoise.pdf
    Reply  Thu May 23 15:29:58 2019, awade, DailyProgress, WOPO, Making time domain measurement of noise power in homoydne with WOPO pump on RMSFunctionTimeCompairingSNWithInjected532nmpump.pdf
       Reply  Sun May 26 21:24:38 2019, awade, DailyProgress, WOPO, Excess noise when pumping: checklist for Monday. 
          Reply  Wed May 29 15:24:33 2019, awade, DailyProgress, WOPO, Update to checklist 20190528_ProperIQReconstructionAndLPFilteringData.zip
Message ID: 2352     Entry time: Thu May 23 15:29:58 2019     In reply to: 2349     Reply to this: 2354
Author: awade 
Type: DailyProgress 
Category: WOPO 
Subject: Making time domain measurement of noise power in homoydne with WOPO pump on 

In order to see squeezing I want to scan the phase of the homodyne relative to the squeezed light and see the variations in noise power as a function of time. 

Excess 532 nm dumped on detector A

From my initial scans it seems like there is excess noise of a factor of 1.5 above shot noise (see below).  This is only present with 532 nm pump injected. It could be anit-squeezing washing out with some phase noise bluring across the sample time of 0.1 seconds or maybe RIN from residual 532 nm present at one of the photo detectors.

After injecting light into the WOPO for some initial tests it was apparent that there was about 1.4 mW of waste 532 nm light exiting the fiber launch on the detector A path. About 16 µW gets through to the detector from the dichroic mirror reflection. A quick measurement of 532 nm power on detector B show that there was about 49 µW coming out of that fiber end with 0.2 µW making to the photodiode. The 50:50 splitting doesn't apply for non-design spec wavelengths of fiber splitter.   The InGaAs detectors have a pretty poor responsivity at this wavelength but the pumping light on detector A was enough to create a DC voltage of 1.36 mV. After dividing through by detector gain of 2kΩ this is equivalent to about 0.68 µA of DC power on the detector A.  This suggests a responsivity of order 0.014 A/W.  There isn't enough light on detector B to create any DC voltage. 

The imbalance here, with the 532 nm light, means that there is a mechanism for coupling in 532 nm RIN into the measured output signal.  Not sure if the RIN would be all that high at the 600 kHz (that I'm planning to mix down from) but it would be a good idea to remove it anyway.  I'll look for a another HR1064/HT532 dicroic mirror to attenuate this 532 nm component a little more.




For the detection of squeezed light the homodyne detector  is now configured for digital subtraction in post processing. The measurement scheme is now to take the output of the two TIA amplifiers (see QIL:2324 and QIL:2327), digitize​ them directly at 210 MSa/s (5 nV/rtHz input ref noise) with the Zurich box and demodulate both detector streams using its internal FPGA at about 1 MHz.  The Zurich box allows for direct sampling of the signal out of the FPGA which can then be downloaded either through the web interface or the python API. T

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