closing series

• We started measuring the phase noise on the OEwaves CW laser.
  • Class 3b
  • wavelength 1.5 um
• The procedure is done using self coupling of the laser
  • splitter 50%-50%\
  • delay line 100 m 
  • all fibers are PM type (polarity maintained)
  • Photodetector is "lab buddy", very fast diode.
  • Note: différance from schematic (we did not use a low pass filter)

a correction on the wavelength of the laser  it is 1030 um 

correction on unit 1030 nm

The SBox cavity setup was changed to have only 2 mirrors M1 plane and M2 spherical, both from ThomX

Distance between the mirror ~ 72 cm , increased from 70 cm to take into account the thickness of the ThomX mirrors

Two lenses (300 mm @ 50 cm , 200 @ 104 cm) were placed to have the beam radius ~ 0.55 mm.

The cavity was locked with a coupling of 60 %, for Finesse measurement the sweep was taken over 100 KHz of 2 seconds.

FSR ~ 210.00 MHz, line width ~ 8.56 KHz, Finesse ~ 24 500 .

The cavity was realigned using irises instead of pinholes, gave a better alignment.

The inside of the box, the spherical and the injection mirror were cleaned and placed back inside the box.

we see beating of fundamental mode, previously at the transmission point we placed a wedge to split the beam which resulted in an elliptical mode

we removed it and placed a very thin beam splitter, the beam is circular now.

The cavity was locked in air at a coupling of ~ 60-70 %

Finesse and line width measured five readings with a Finesse average 25095.08884  of a Gain ~ 8000

FWHM (KHz) = 8.2928
Finesse = 25323.0544

FWHM (KHz) = 7.9202
Finesse = 26514.4395

FWHM (KHz) = 8.5834
Finesse = 24465.8636

FWHM (KHz) = 8.4571
Finesse = 24831.2419

FWHM (KHz) = 8.6275
Finesse = 24340.8448

Theoretical and expected Finesse for the 2 mirror setup with the losses is calculated by Ronic for comparison between four and 2 mirror setup.

Update for Finesse measurement, The cavity was put under vacuum ~ 1.1*10^-1 mbar

and the alignment and coupling improved.

FSR = 210.1 MHz

Average Finesse = 25686.46222

 FWHM (KHz) = 8.2387
Finesse = 25501.5659

FWHM (KHz) = 8.2028
Finesse = 25613.2858

FWHM (KHz) = 8.0978
Finesse = 25945.3289

FWHM (KHz) = 8.1744
Finesse = 25702.3142

FWHM (KHz) = 8.1847
Finesse = 25669.8163

Concluded from Ronic's calculations, this could be the maximum finesse we might be able to obtain with this setup

with Gain ~ 8000

On Monday we adjust the frequency to match 2160.66 MHz and lock the Pulsed,

at the same time start we start with the CELIA amplifier.

The FSR of the 2 mirror (plan-spherical) Cavity was adjusted from 210 MHz to reach 216.643 MHz

it was done by having two reference irises, one at the injection point and one at the reflection 

then changing the position of injection plan mirror to slightly closer distance and monitoring the reflection on the oscilloscope to be max.

The cavity modes were still seen, and we had to only improve the injection alignment after.

Me and Ronic locked in air and measured the Finesse, which was bigger by ~ 20%

average Finesse = 30208.53614

FWHM (KHz) = 7.0179
Finesse = 30869.9522

FWHM (KHz) = 7.1257
Finesse = 30403.005

FWHM (KHz) = 7.1287
Finesse = 30390.4014

FWHM (KHz) = 7.2884
Finesse = 29724.5531

FWHM (KHz) = 7.3055
Finesse = 29654.769

Yesterday evening the cavity was Vacuum pumped up to pressure of 5.5*10^-2 and locked 

changed FSR to be 216.662 MHz and alignment a little and measured the Finesse

in Vacuum we have average Finesse = 30341.6265

FWHM (KHz) = 7.0592
Finesse = 30692.1961

FWHM (KHz) = 7.2186
Finesse = 30014.556

FWHM (KHz) = 7.1051
Finesse = 30493.7635

FWHM (KHz) = 7.1079
Finesse = 30481.9812

FWHM (KHz) = 7.1413
Finesse = 30339.2695

FWHM (KHz) = 7.1624
Finesse = 30249.776

FWHM (KHz) = 7.0239
Finesse = 30846.2719

FWHM (KHz) = 7.2614
Finesse = 29837.6477

FWHM (KHz) = 7.1935
Finesse = 30119.1768

a Finesse of 30k with the present mirrors :

T1=120 ppm ;A1=(2.6+1) ppm;
T2=1.5 ppm ;A2=(4.5+0.27) ppm;

corresponds to 39 ppm of additional losses for each mirror and a theoretical gain of about 11k.

To prepare for the amplifier. I installed the D-shapped mirrors after cleaning (using aceton and ethanol) between the cavity 2 mirrors.

Before closing, I observed the back reflection from M1 (injection mirror). both the injection and reflection lines were off, due to that there was an internal reflection hitting the walls of the cavity.

I tried to correct it by slightly adjusting on M1 without losing the mode, but unfortunately we lost it. 

Tried to go back to the original position using the reflection iris reference, with no success.

Cavity axis is lost, and we need to align again.

The D-shaped mirrors are installed properly and not cutting the path of the beam.

The cavity has been aligned again and 00 modes beating observed, and external reference points has been placed.

I locked the Koheras CW on the FP cavity but the lock was pretty noisy and very difficult to acquire.

I tried to produce some modulation sidebands close to 216MHz to measure the Finesse but the power loss was very small so, the signal to extract the Finesse would be unreadable !
(generator voltage was at maximum Vout = 0.5Vrms on 50 ohms... is it normal ? or the used 10GHz EOM suffers some problems ?)

At one moment, I lost the lock and was not able to find it again... it seems the Koheras is too noisy for this cavity (may be is it a good news for the Finesse ?).
Tomorrow, I will try to use the OEwave + amplifier to lock the cavity.

We measured again the 100W CELIA laser amplifier with a pump current until 8A.

as the first current pump of the amplifier has a Peltier issue, we don't exceeded 6A on this stage and we compensated with the 3 other stages.

7A average current is obtained with 6A / 7.3A / 7.3A / 7.4A
7.5A average current is obtained with 6A / 8A / 8A / 8A
8A average current is obtained with 6A / 8.6A / 8.7A / 8.7A

we did the power measured either with the "big" powermeter which is able to handle 100W
and with a smaller powermeter after a wedge, in the reflection path, which is multiplied by 39 to match the big powermeter measurement.

a fit a 12W/A from the cut-off current of 2A is a good estimation until 5A.

After installing the 2nd EOM, we had some trouble to be able to lock again.
One possible reason was the very low signal level in transmission, which is important to trigger the locking system (and stop it).

See the Alice post for details, but we were able to measure only once the Finesse of the cavity at around 2600.

After the Finesse measurement, we opened the box to change the M1 mirror... so the box is at ambient pressure now.

I took back the avalanche photodiode from the Minicav room and installed it on the setup to replace the FPC transmission photodiode.
Now, the transmission peaks are at the 1V level, and it's very easy to trigger on...
The system locked very easily, even without being under vacuum.
It will help if we need to inject very low power laser (e.g. OEwaves after 2x EOM and AOM).

Migthylaser amplifier has been moved from the SBox table to the PLIC table.

Measurments of Finesse with the 2 polarization states, let's call them H (higher) and L (lower): 24500 for the H and 23500 for the L.

We checked the polarization states in transmission of the FP cavity after a PBS. The H was stronger in PBS trans and the L stronger in PBS ref.

We measured the power in reflection of the PBS and added a half WP that we aligned with the PBS polarization. Then, to get the maximum power we had to tilt the half WP of 22° for the H and 18° for the L.

Finally we checked the extinction through half WP and PBS for H and L. 

- For H : max 75 mW min 5 mW. Ratio 6.66%

- For L : max 70 mW min 4 mW. Ratio 5.7%

Right after Koheras : max 3.5 mW min 47 uW. Ratio 1.3%

2 measurement.

First one locked, in transmission of M2 with 2nd stage 0A.Total 89 cm from the big waist (planar mirrors). 2 wedges used.

Second one at input beam with the same power. Datas taken at the same equivalent position than the first one. 3 wedges used.

* Loic has to fix the number of files (3) regarding the number of measurements (2)

* splitted Intensity and phase HASO files

* image for each file

with just intensity, the coupling is 98%, and with phase, x direction coupling is 95%, y direction 97%, so the telescope is good.