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Message ID: 2351     Entry time: Thu May 23 03:00:24 2019
Author: Anjali 
Type: Update 
Category: 2micronLasers 
Subject: Frequency noise measurement of 2 micron source using PLL 
  • The delay line length is measured to be 15.7 m. 
  • As suggested by Prof.Rana, I used a DC block after the photodetector and the output of it was then fed into a RF amplifier (ZHL-3 A+). Attachment #1 shows the output of the detector (a) MZI output- we see the beat note at 78.8 MHz and SNR is about 37 dB (b) when light is bloked- we see  radiation from RF amplifier with a small amplitude of about -63 dBm at 78.8 MHz  (c) when RF is turned off. 
  • It can be seen that, after the RF amplifier, the noise level is also increased with the signal.
  • The RF level that is now fed into the mixer is about -23 dBm. But the PLL was not getting locked-oscilloscope trace is oscillating. A video is attached, which is captured when the there were no gain in SR 560 (G=1) and actuation slope was 10 kHz/V. Attachment 2 and 3 shows the corresponsing traces from SR 785 in full span mode and short spn mode respectively. It is observed that , the oscillation strength was increasing as I increased the gain value in SR 560. I doubt, this is happening because of the higher noise level at the RF port of the mixer in this case. The trace on SR 785 was more stable and it was not swinging as much as we observed before. 
Attachment 1: Detector_output.pdf  54 kB  Uploaded Thu May 23 04:02:16 2019  | Show | Hide all | Show all
Attachment 2: FM_noise_full_span.pdf  24 kB  Uploaded Thu May 23 04:12:22 2019  | Hide | Hide all | Show all
FM_noise_full_span.pdf
Attachment 3: FM_noise_short_span.pdf  24 kB  Uploaded Thu May 23 04:12:33 2019  | Hide | Hide all | Show all
FM_noise_short_span.pdf
Attachment 4: oscilloscope.mp4  13.836 MB  Uploaded Thu May 23 04:20:53 2019
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