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Message ID: 2394     Entry time: Tue Aug 13 16:53:57 2019
Author: Nathan Holland 
Type: Electronics 
Category: 2micronLasers 
Subject: Phase Noise of Brimrose AOM Driver (VCO). 

After reviewing the Brimrose AOM driver mnual, yesterday, it turns out I was previously using it incorrectly. It is a VCO with the frequency port accepting a DC voltage, between 0 and 10 V to control the frequency of the AOM - note that the mapping is not intuitive so one should refer to the manual. The modulation port is used for amplitude modulation, not frequency modulation. This port, modulation, has 50 ohm input impedance and accepts signals between DC and 10 MHz - modulating the power output. Table 1 below shows the operating parameters we should use:

Table 1: Recommended, constant, operating procedures for Brimrose AOM driver.
Port Value (V) Comments
Frequency +9.08 Gives 80 MHz modulation frequency.
Modulation +0.85 Gives 27 dBm of RF output, which is 90% of the maximum linear range of the Brimrose AOM.


Following this I characterised the phase noise of this VCO. Results are shown in attachment 1, for various powers. The setup is shown in figure 2, with the data for M = +0.85 V provided in attachment 3. These results show that this VCO has poor phase performance. A value of M = +0.02 V gives the same RF power as when a 0.5 Vpp signal @ 80 MHz was input into the modulation port - as I was previously doing.

This has a few implications for previous measurements:

  • The reason it was hard for the Mach Zehnder to remain locked for a substantial duration was due to the low input power to the AOM. This probably resulted in little signal in the Mach Zehnder LO arm.
  • Previously the amplitude modulation would have provided a 80 MHz signal in the Mach Zehnder LO arm


The script used to create the smoothed ASD can be found here.

Attachment 1: 20190813__Brimrose_AOM_Driver_phase_noise.pdf  36 kB  Uploaded Tue Aug 13 18:06:58 2019  | Hide | Hide all
Attachment 2: 20190813__VCO_phase_noise_setup.pdf  32 kB  Uploaded Tue Aug 13 18:07:26 2019  | Hide | Hide all
Attachment 3: brimrose_AOM_driver_VCO_phase_noise_spectra.hdf5  11 kB  Uploaded Tue Aug 13 18:07:44 2019
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