Today we put the cavity to vacuum (~ 0.1 mBar) and we measured again the Finesse with Koheras and modulation technique.
the measurement has been done 3 times with Finesse of 23.2k, 23.8k, 23k

with this Finesse around 22-23k, the technique comparing TRANSMISSION and REFLECTION signals doesn't work, even in taking into account individual photodiode time responses.
It seems that the cavity is not completely stationnary and the shapes are not comparable easily with just a Low Pass Filter related to the Finesse.
Below, an example of the best fit of filtered REFLECTION signal compared to TRANSMISSION signal.... it is clear that the shapes don't fit....

As we said, after glow discharge and spin coater we recovered a low power finesse ~23000 (aligning away from the M1 crater).

Today, we have made some power up to 48kW (3A on 3rd stage). At this power, there was no important effect on the transmission which can be compared with the coupling (see image "3A_136mWtrans"). We waited ~20min without aligning and the power dropped slowly (misalignment) to 39kW. 

Then we increased the amplifier power to 4A and the power went up to 44kW. At this moment, the strange transmission behavior that we observed before mirror crater, appeared again (see image "4A_126mWtrans"). We also observed a mode deformation with the camera, see image "mode_strange". This shape was not depend of the camera/filter rotation angle and still appear with another camera.

On 10th of december 2020 we cleaned the SBOX mirrors and took microscope images (the name of the images indicates what they are).

There are 7 mirror, the initial M1 (spot in the center), M2 (spot on the edge), M3 and M4 which made the 200-400kW and the M2, M3 and M4 SPARE. The difference we make between M3 and M4 SPARE is the number on the box (11 or 13).

We used 3 different cleaning methods : 1st, one spin coater on HR, 2nd one, tissu wipe on AR (wipe with the optical tissu and isoprop) or 3rd one, mirror wiped on tissue (put isoprop on tissu and press AR face of the mirror doing "8" shape 3 times).

The second method is far les efficient as a cleaning method. The image "M3_M4_spare_11_after_cleaning_back.tif " shows the traces let by it and removed by the 3rd method on image "M3_M4_spare_11_after_cleaning_back_second_time_on_tissu.tif".

We can also notice that the spin coater let some trace on the HR face, round shaped, see Image "M3_M4_spare_13_after_cleaning_back.tif". We can propose to use the third method with Acetone on HR face before using spin coater to remove oil or organic particles.

It also lets a trace on the AR face, this is why we clean the AR face with the 2nd method after cleaning it with the spin coater.

Note : The position of the mirrors in the microscope is always the same here. Meaning mirrors are directed so that the arrow (which shows the HR face and is placed on the side of the mirror) is placed on the top of the images.

• Thursday 10 December 2020:   First Test Measurement - Inside Box
  •  The transmission ( T ) for the mirrors of SBOX and ThomX was measured over a range from the center of the mirror up to 5mm.
  •  
  • The first setup was initially done while the mirrorr is installed inside the metal box, when measuring T we observe that there is an erregular  change in the power measured as we move azay from the center of the mirror, this was due to reflection of light inside the box. The reflection from the mirror was reflected on the metal inside the box and it was measured by the power meter.

               (so for this setup we only took measrements for T along the center of the mirror)

The file stating the measurements is : Test Cavity _ Mirrors transmission _ Inside box

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• Tuesday 15 December 2020 :  Second Test - Outside Box
  • The setup for T test was moved outside the metal box with the same distances between the injection beam and mirror ~ 50 cm. and the distance between the injection laser and the mirror next to it ~ 9 cm.
  •  
  • Observation for T is done at full angle and at half angle of reflection ( the mirror was tilted to achive this )
  •  
  • we observed that after each change in the distance from the center the power meter needed to be aligned to have the max power.
  • after doing all the measurements for the range from the center we made it again for the center only and the consistance of the transmission was good.

The file stating the measurements is : Test Cavity _ Mirrors transmission _ outside box

note: the laser used for the test is the Koheras

details on the pwer used, mirrors, power meter, filters, ....etc are shown in the Excel data sheets

Done By : Loic , Manar and Ahmed

we received from NKT the Origami SN2440 133MHz oscillator from repair on 2020/12/17
and we measured the power trend from 2020/12/21 until the whole winter Hollyday, during 14 days and 17h (1 273 334 s).
the power was quite constant, about 58mW (the measurement has been done with the OD2 filter and using a (x20) factor in the software to compensate for it),
except for some peaks at the beginning of the measurement to 60.7mW.
I tried to reproduce these peaks by switching on several lights in the room and the airflow ceil but the effect is neglectable,
thus these peaks seem to really come from the oscillator power.

at the beginning of January, the oscillator suffered a back reflection from the rugged anodized convex surface of the power meter (apparently not from the OD2 filter itself) and the laser stopped immediately to modelock.
the laser power dropped to 0 and the laser controller led started to blink red. NKT has been contacted and they think it could be the laser pump diodes that have been damaged.

for the future, we will have to install a fixed optical isolator from the start, even for short operations.

Today, we received the Origami SN2440 133MHz oscillator from repair (NKT mentioned a defective wire inside the controller....)

We immediately installed it behind an isolator (Faraday rotator+PBS+halfwave plate).

the output power is around 60mW.
the spectrum is around 1030nm
the repetition rate is around 133.33MHz

Starting with the alignment of the KBox using CW Koheras.

The size of the CW output from the collimator at 1 meter from is ~ 2.4 mm, to be adjusted with lenses to match the cavity !!! (attached image)

For the 2 alignment mirrors, I am only able to control 3 axes for now (motor issue for the fourth to be fixed !!) , Horizontal and vertical for Ma1 and Horizontal for Ma2(closes to injection window).

Started with only irises placed on both the M1 and M2. Placed a beam profiler at the window behind M2 to observe an output beam.

Motors position for the 3 axes and beam shape and position after M2 attached.

SBox alignment, using CW laser Koheras power 1 mW

• Finished M1 and M2 alignment using the diffraction patterns seen on the beam profiler,  the beam fit is not very good ?? (image taken)
• Positions of Ma1 and Ma2 motors are registered (image taken)
• Placed Fixed 2 irises at M1 window and M2 window (Image taken while both are closed)
• Distance between the collimator and beam profiler ~ 2 m
• Distance between collimator and M1 diaphragm   ~ 1 m
• Distance between collimator and M2 diaphragm ~ 1.6 m 

After pointers and Advice from Aurélien,

improvement of the beam alignment through M1 and M2 irises was done.

To improve the image on the beam profiler, I cleaned the alignment mirrors and the windows using pure ethanol and optical lens cleaning tissue,

a significant improvement after cleaning the inside of M2 Window. Images have less dust diffraction rings.

Attached new images of the beam:

• New Ma1 and Ma2 motor position
• After M1 pinhole optimized (saturated)
• After M2 and M1 pinhole optimizing (saturated and not)

Remember to update the position of the reference Irises tomorrow !!!!

• Adjust the placement of the Irises  (done at the start of the day, adjusted at the end when M1 mirror was placed)
• Improvement on the Ma1 and Ma2 (The Alignment Mirrors)
  • Improves the alignment a bit. There is an observation where there is no symmetry in the image, concluded to be because of the pinholes that are usually used for the alignment the circle cut is not symmetric (took images on phone of pinholes)
  • This explains why, for the pinhole used on M1, we see a little more symmetry than the pinhole used on M2. 
  •  
  • Placed M1 mirror from Sbox (with damage spot in the middle), after cleaning it using the spincoater, only used to adjust the injection into the mirror where it will give Horizontal displacement due to the thickness of the mirror.
  • Last position of the motors attached
• Installed the 2 dielectric mirrors for the reflection line
• Did a rough alignment for the reflection line to have it exit through the reflection window.

Note about mirrors : They were cleaned a year a go and placed in the boxes. When opened the box and seen under microscope they were very dirty with dust particle 

Specially from the back, I believe the boxes have dust in them (to be verified under the UV light /  Microscope  !!!!!!!! )

Finished the alignment of the SBox with M1 of SBox placed, to be changed to gamma factory mirror later

Observed what seems to be transmission through M2 mirror, to be confirmed. image attached shows it

a PowerPoint attached showing the details of today's work.

Last week In FP cavity we had all SBox mirrors including the damaged M1 mirror.

The beating was observed with just a slight adjustment on M4 alignment.

In addition, it was also observed while driving the piezo of the CW laser.

In the attached image of the beam at transmission point at M2 mirror, the shape of the beam is deformed.

one of the factors could be related to the damaged spot on M1 mirror. The deformation position can also be seen in the higher order modes.

after changing the injection mirror to Gamma factory M1, this deformation should disappear.

Today; replaced the M1 SBox mirror with M1 from gamma factory , following the previous alignment.

Removed all mirrors and cleaned them one by one.

Started with no M1, placed M2 did its alignment through M3 pinhole, then placed M3 aligned it through M4 pinhole, then placed M4 aligned it through M1 pinhole.

Placed M1 Gamma factory at the end

Note: mirror is new was not used before, took image on microscope there was no dust in the center, but there was little on the edges (cleaned it using spin-coater)

beating of fundamental mode observed (at M2 transmission) immediately with very higher order modes, slight adjustment on M4 suppressed them.

The beam is not circular and still has some deformation, M1 was changed, so it could be a damaged point at one of the other mirrors (to be investigated tomorrow)!!

No significant damage seen on the other mirrors.

Yesterday, to find the source of why the mode is deformed, I tried to rotate the mirrors to see if there is a change.

I started with M2 and rotated it by ~ 90 degrees, there is a change in the shape of the beam.(image attached)

Will see if changing M2 to the spare M2 from SBox can fix the issue.

At the end of the day, alignment was lost.

will be redone today while changing M2 mirror.

M2 Sbox changed to spare. Alignment of M2 was redone. 

With the help of Ronic , We observe fundamental mode of the cavity with higher order modes. A frequency scan on the piezo was done to find the beating frequency , the shape is elliptical with no clear deformation.

The beam seems to be hitting something, which causes a reflection seen on the beam profiler.

We started a slow scan to improvement of the outside alignment.

At the end of the day, I can only get the TM01 and TM10 modes,

alignment to be continued tomorrow + the scan offset

Note: observation on TM01 and TM10 modes, we clearly see that the beam position is different from the mode center position !!

The external and internal alignment was redone.

External Alignment : the collimator position moved so in order to have a good start I placed 2 metallic mirrors and did a rough retrace of it's line through the cavity box.

It helped as I did not have to do a lot of adjustment on Ma1 and Ma2 position (new position attached)

Internal Alignment: I replaced the 2 pinholes inside the cavity box with another set where the holes are more symmetric, it shows in the alignment.

Images attached of the beam diffraction pattern after each mirror pinhole.

The injection mirror was placed at the end of the alignment.

Alignment Done !!!!!!!!!!!!!

The current setup for the SBox has a beam viewer and 2 photodiodes placed at reflection and transmission.

The CW laser piezoelectric crystal was driven using the lase lock system to observe the 00 mode.

Mode Observed and has no reflection (from what was previously seen)

higher order modes are resonating with 00 mode , example 11 mode (shown in image attached)

the coupling is yet to be seen and measured.

Adding images from the transmission after the spherical mirrors M3 and M4 window for a resonating mode inside the cavity.

There appears to be some misalignment from the center of the mirror ???