to find the fundamental mode we moved the cavity mirrors to new positions:

+5.104mm for both planar mirrors in closed loop

spherical unchanged (-2.9mm)

a cavity length measurement gave 880.9Mhz. The m1/M2 have been put to +5.1000mm both M3/M4 srill at -2.9mm

doing the sensitivty measurement of the piezo we observed satellite peaks around 800 to 900 Mhz. Removing the phones from the experience room(in the cupboard in the sas) removed that effect immediately.

A 10V applied on the laser piezo was found to induce a frequecny change of about 5kHz, compatible with expectations from the piezo sensitivity. this was measured by a freqeuncy analysis of the signal produced by the laser itself on a photodiode det10a

following the measuremetn at 879.9 MHZ of the laser theoptical cavity length has been adjusted to that value from 876MHz setting of the 5th of may 21.

new motor positions:

M3/M4 : -2.9mm

M1/M2 : +5.196mm

the ip adress of the smaract driver is now 10.0.53.10:5000

please think about putting such kind of changes in the logbook

With the help of Ronic the cavity was locked in preparation to measure the polarization of the reflection line when the cavity is locked (measurement when it is not locked was done before)

the purpose is to compare the two measurements (locked  vs not locked)

General details:

• when not locked : measurements of the polarization was taken from the point where the photodiode is placed in the picture enclose
• When Locked: we can't measure it from that point as we need the line to split into two, one goes to PDH to maintain the lock of the cavity and the other one we use for our measurement.
  • The reflection line is split at the point where the beam splitter is (BS, behind the photodiode in picture), we intended to take the measurements from this point.

Observation:

before starting the measurement of polarization, we observed

• the power measured for the reflection line (point at end of red arrow in picture, after BS) is really sensitive to the polarization, it shows when rotating the half-wave plate
• but, when measuring the power at the point shown in the picture (before BS, where the photodiode is placed) it is not sensitive to polarization.

This tells us that the dielectric BS placed in the reflection line affects the polarization.

This could affect the stability of the locking of the cavity, as the PDH is sensitive to polarization.

** Further investigation is needed before proceeding **

Footnotes:

• BS: Beam splitter.
• Dielectric component's sensitivity to polarization
• most of the components placed in the transmission line are dielectric.

After data processing, I finally find the polarization in transmissin of the FPC. As you can see below, the polarisation is not linear but elliptical.

The four ellipsies are here due to having not enougth parameters to extract with certainty one ellipse.

We put a diaphragm in the input beam but it doesn't work to suppress the ring light only but it attenuates the whole transmitted beam (cavity mode and ring light) viewed on the camera.

To have the polarisation, we must have 1 value because the laser in entry of the system has 1 polar. Due to that, the graphe of the ellipticity is not true, it is the mean value on the following table which shows the ellipticity and the polarization.

We change the setup a bit to have the same polarization on the photodiode and the PDH (in reflexion of the cavity). CF: schematics (futur).

Thanks to that, the symetry between reflexion and transmission is better now (as you can see on the plot).

We also put more power on PHD to have a better SNR by changing splitters. (We had 100 µW and now we have more than 1 mW, at the expense of the power on the camera).

Thanks to that we obtained a good locking, The coupling is quite good (roughtly 50%, on the green line)

In the following plot, the green line is the reflexion and the yellow lin is the transmission.

I made many tests about polarization and we can see some points thanks to the following figures :

1- The ellipsicity of the NKT is near to 0 (between -0,2 and 0,2) which means the polarization is rectilinear horizontale (attachement 1).

2- The power of the NKT has no influence on the polarization, verticale or horizontale (attachement 2 and 3).

3- The polarization is not changed by the type of mirror (Ag or Diélec) (attachement 4)

The next step is to measure the polarization after the cavity to know the ellipsicity.

we observed that the transmitted light coming from the cavity is made of the cavity mode light and the scattered light on the borders of the mirrors.
the scattered light on the borders of the mirrors is triggered by the cavity mode itself when the cavity is locked.
we didn't put an iris to cut this scattered light coming from the cavity.

Question:

could it be possible that the wrong Finesse value could come from the fact that the transmission was measured with the whole light coming from the cavity and not only the cavity mode light?

Cavity is lock and optimized with the NKT. Input power is maximized (10mW).

Cavity mode is in attachement. The reference on camera with BeamProfiler Matlab code is the following:

FF_refl: [2.0883    1.1606]

NF_refl: [5.3869    3.9327]

FF_inj: [1.9491    1.3980]

NF_inj: [5.6431    3.5234]

New configuration is made. The new mode observed in NF_Trans is the following :

After installing cameras the actual calibration are :

NF_Refl: acA1920-40gm
    pixel size (real): 5.86um
    Magnification = [0.53,0.56]
    pixel size (image): 3.22um
    image donne on input plan mirror M1 (accuracy about few mm)

NF_Trans: acA1920-40gm
    pixel size (real): 5.86um
    Magnification = 1.32
    pixel size (image): 7.73um
    image donne on output plan mirror M2 (accuracy about few mm)
 

need to adjust the NF transmission as the Magnification is greater than 1.

For spherical mirrors M3 and M4 (batch C117I054) the reflectance (R) is around 99.9987% (if T+R=1 => T=13ppm)

For plan mirror M2 (batch C217G054) the reflectance (R) is around 99.9977% (if T+R=1 => T=23ppm)

(1) => For plan mirror M1 (batch C217H023) the reflectance (R) is around 98.96% and transmittance (T) is around 1.14% (T=11400 ppm)
(2) => For plan mirror M1 (batch C217H027) the reflectance (R) is around 99.9385% (if T+R=1 => T=615ppm)

*************************************
case (1) :
Finesse = 546
Gain = 346
Coupling = 1.7%
=> it seems we don't use this mirror for M1
**************************************
case (2) :
Finesse = 9460
Gain = 5578
Coupling = 27%

if one adds 10ppm of losses due to dust on each mirror :
Finesse = 8923
Gain = 4963
Coupling = 44%
**************************************

New update on the position of the Motors for the cavity

***** we moved the motors to set the cavity at 876MHz, and checked it right after with the RF modulation at FSR.

       So compared to the expected setting we had to move inwards the two planar mirrors by 0.9 mm each.

Planar     : M1-M2 motor = + 4.815  mm    ------>    M1-M2 =  78.399 mm

Spherical : M3-M4 motor = -2.9 mm          ------>     M3-M4 =  90.4895 mm

Ronic just suceeded in obtaining a reasonably good lock on the cavity. Air flow is switched off.

We stopped all movements (closed loop, click STOP in the PTC interface) and saw immediately a fair but not excellent lock.

We then switched off the smaract motors and the obtained lock was good. Switching on again the smaract means tthat the references are lost.

We futher saw a drift of the locking point, probalby suggetsing that the thermal load in the cavity slightly changed after switching off the motors.

We then added the second 1GHz BW EOM to add sidebands thanks to the MARCONI RF generator. We observed that the FSR is aroung 867.6MHz (in air). We then looked at the points where the transmission signal related to the sideband is halved. We observed that the corresponding frequencies are 867.296 and 867.776MHz. the corresponding FWHM of about 500kHz corresponds approximately to a 2000 finesse.

Picture color code:
TRANS : Green
REFLECT : Yellow
PZT : Blue