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 Fri Jan 4 19:04:29 2019, awade, DailyProgress, RFAM, Time to look at RF AM and RIN Wed Jan 9 17:25:38 2019, anchal, DailyProgress, RFAM, Time to look at RF AM and RIN Wed Jan 9 19:05:56 2019, anchal, DailyProgress, RFAM, Time to look at RF AM and RIN Thu Jan 10 11:38:53 2019, awade, DailyProgress, RFAM, Time to look at RF AM and RIN Thu Jan 10 17:30:24 2019, anchal, DailyProgress, RFAM, Time to look at RF AM and RIN Thu Jan 10 18:52:20 2019, anchal, DailyProgress, RFAM, Time to look at RF AM and RIN Sat Jan 12 13:11:02 2019, awade, DailyProgress, RFAM, Time to look at beatnote after RF AM and RIN tuneup Wed Jan 16 11:07:41 2019, anchal, DailyProgress, NoiseBudget, Beatnote after RF AM and RIN tuneup Thu Jan 10 14:19:46 2019, anchal, DailyProgress, RFAM, Time to look at RF AM and RIN
Message ID: 2283     Entry time: Wed Jan 16 11:07:41 2019     In reply to: 2282
 Author: anchal Type: DailyProgress Category: NoiseBudget Subject: Beatnote after RF AM and RIN tuneup

## Intensity noise to the beatnote transfer function

• I resurrected the AEOM power control lines with SMA cables to junction plate on the north side.
• AEOM was driven in linear amplitude modulation regime by aligning input polarization to s (vertical) and making output polarization circular by a quarter waveplate and dumping the s (vertical) polarization through a PBS.
• $I_o = \frac{I_i}{2} \left( 1 + \sin(\frac{V_i}{V_\pi}\pi) \right )$
• For the New Focus 4103 models, we have, $V_\pi = 300 V$ . I used source signal for amplitude modulation of $V_i = 5 V$ . This gives modulation index $\beta = 0.052 \, rad$ and maximum deviation from linearity by $I_i \times 1.17 \times 10^{-5}$ .
• The measurement was done as a swept sine with input at reference signal as the modulation signal to the AEOM and output as PLL control voltage.
• The TF is then converted to units of Hz/W using PLL actuation slope of 5 kHz/V. The gain in the PLL loop was set to 100.
• Beat note frequency during the measurement was near 135 MHz.
• For the measurement in the North path, Settle Cycle was set to 10 and integration cycle was set to 10.
• During the measurement in the South path, the beatnote started drifting faster so Settle Cycle was set to 2 and integration cycle was set to 4 and average of 3 measurements is taken.
• PFA transfer functions. Note, transfer functions are mentioned from different initial points for future use. They were converted by scaling down by the ratio of the DC power measured at Cavity input, PMC output, and AEOM output.

## Transmission RIN:

I measured transmission RIN before the beatnote measurement. Unfortunately, we can not compare with earlier intensity noise measurement I took because I didn't take transmission power measurements then.

These measurements were taken in 4 steps of frequency ranges and stitched together. See the attached notebook for further details.

## Beat Note Frequency Noise Spectrum

I used the same measurement method as explained in (PSL:2272). Following are some experiment state measurements I made:

Experiment State
Parameter North Path South Path Common Unit
PLL Gain     100 -
PLL Actuation Slope     5 kHz/V
BN Frequency

~ 130 MHz

drifting down

-
PMC Input Power 1.82 2.404   mW
PMC Output Power 1.615 1.619   mW
PMC Overlap 89% 67%   %
Incident Power on Cavity 1.497 1.497   mW
Transmitted Power from Cavity 0.96 0.82   mW
Cavity overlap 64% 55%   %

PMC Reflection RFAM

(@ PMC loop RFPM frequecy)

-52 dBm

@ 21.5 MHz

<-71 dBm

@ 14.75 MHz

dBm

Cavity Reflection RFAM

(@ PMC loop RFPM frequency)

<-87 dBm

@ 21.5 MHz

< -77 dBm

@ 14.75 MHz

dBm

Cavity Reflection RFAM

(@ FSS loop RFPM frequency)

-76 dBm

@ 36 MHz

-75 dBm

@ 37 MHz

dBm

Cavity Reflection RFAM

(@ 2x PMC loop RFPM frequency)

< -87 dBm

@ 43 MHz

-54 dBm

@ 29.5 MHz

dBm

Note: I also found that half waveplate behind North FSS EOM had no marked order and was probably multi-order. I replaced it with a zero-order half waveplate and found significant improvement in RFAM at Cavity Reflection RFPD. Above mentioned numbers were taken after this change.

## Updated Noise Budget:

I'm still using the old code here as there are some unresolved questions with the new code I want to be sure about first.

• The new spectrum is lower at noise at lower frequencies. This might be due to the unmeasured amount of light falling on the cavities in the previous measurement.
• RFAM and RIN tuneup decreased noise slightly at 2 and 3 kHz.
• However, the method used for taking these measurements keeps the uncertainty high. So maybe the effects of our changes are actually negligible and something else is more pressing.
• Clearly, the growth of noise beyond 1 kHz is still happening and this is one major difference between earlier noise measurements and new measurements.
• I think installing ISS should be the next step in the queue because these high-frequency bump is probably coming from there.
• I will also work on tuning up the FSS loop parameters to see if that changes anything.

 Attachment 1: LaserPowerNoise_to_BNFreqNoise_TF.pdf  47 kB  Uploaded Wed Jan 16 12:12:32 2019
 Attachment 2: IN_to_BN_TF.zip  298 kB  Uploaded Wed Jan 16 12:53:46 2019
 Attachment 3: Transmission_RIN_Jan15_2019.pdf  70 kB  Uploaded Wed Jan 16 13:44:18 2019
 Attachment 4: RIN_Measurement_20190115.zip  70 kB  Uploaded Wed Jan 16 13:44:36 2019
 Attachment 5: 20190116_130937noiseBudget.pdf  934 kB  Uploaded Wed Jan 16 13:50:11 2019
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