Images of the Transmitted beam at M2 mirror at different days. 

The first one was from last week, the second from yesterday.

There is misalignment (drift due to temperature) from the position where the beam was initially. ( reference taken between centers of mode and direct injected beam) 

it is most clear in the vertical position.

Start with the cavity lock images attached, taken from last week, at start we have a coupling ~ 5%  and started improving on the alignment .

The image of 00 modes attached corresponds to the coupling seen on the oscilloscope.

We have a higher coupling with the 11 mode due to the size of the injected beam in comparison to mode

lenses were added to improve it.

Divergence of the CW beam out of the collimator measured after 1 meter to be 

• 2 x = 0.7425  mrad  --> x = 0.37125  mrad
• 2 y = 0.7115  mrad  --> y = 0.3575    mrad

two lenses were placed to decrease the diameter of the injected beam

Current setup at SBox for locking the cavity

After spacial matching, the coupling increased to ~ 15 - 20 % 

The locking was done for the fundamental mode, tried to improve the coupling with the motors, but the lock is very unstable to be investigated. (Note: the cavity in under normal pressure)

Cavity locked today under vacuum and no airflow, it is stable over a long time

the alignment was improved and an increase of the transmission by 20 % from its original value

unfortunately,  data from the oscilloscope was saved, but it was not in the correct format (image) to show.

Attached ; the mode shape, the new motor positions. 

The SBox Length (FSR), was changed to match the repetition rate of the Menhir pulsed laser Frep = 216.6625 MHz

It was changed by removing the 2 D-shaped mirrors placed between M1 and M2, distance between M1-M2 ~= 169 mm 

it was still needed to be adjusted with M1 motor to reach the correct FSR.

The pulsed laser is placed on the able with injection into a fiber

The Finesse of the cavity has progressed from ~2500  to ~ 18 000

** Note: PowerPoint attached shows the progress over the last two weeks on the laser and 

The Sbox currently has the injection from ThomX installed T ~ 120 ppm , M2 planar from Sbox spare and M3 / M4 spherical mirrors from Sbox installed.

The last finesse measurement that was done gave a Finesse of ~ 19300 (After cleaning M2 using ethanol + aceton + pure water)

on Friday the whole optical table was cleaned again and the inside of the box was cleaned (even with a lot of dust inside the box the Finesse was 19300 )

all the mirrors were cleaned after using the three steps, M3 and M4 had clear visible dust on them.

D-shaped mirrors were installed again between M1 and M2

The alignment was redone and the beating observed ,  There is still a need for alignment improvement.

the cavity is under vacuum ~ 4.8*10^-2 mbar on Friday evening (~ 19h00) , The cavity is yet to be locked and to measure its Finesse

A PowerPoint showing the last finesse measurement and the error signal of the menhir laser lock

Closing the series for the setup

The cavity was only locked @ 133.33 MHz but the Finesse was not measured

The FSR was changed to match the new laser MENHIR 216.6625 MHz 

Closing series of 00 mode for FSR @ 133.33 MHz

Closing the series of the telescope for the beam @ 133.33 MHz

Max Finesse obtained @ 4 mirror setup was ~ 19 300 

With M1 from ThomX , M2/M3/M4 from SBox

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 .

Under the microscope,

the spherical mirrors show the deposit of metal dots on the reflecting surface and not on the back of the mirrors

The mirror with fewer spots near the center was used in the setup.

Mirror images in the previous log showing the metal deposit of the coating of the spherical mirrors

(not sent by mail due to large size of images)

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.