Max Finesse obtained @ 4 mirror setup was ~ 19 300 

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

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 !!!!!!!!!!!!!

Closing Series for Alignment.

Beating observed, beam is in the locking stages

Today, SBOX mirrors have been cleaned with spin coater on HR face and isoprop on back face. They show similar spots as before.

They also have been installed in the SBOX. The M1 mount has been displaced so the beam doesn't go in its center (spot).

Yesterday, after installation and alignment of mirrors and cavity, the Finesse has been measured with cavity at air pressure, with Koheras and modulation technique.
the measurement has been done 7 times with quite different fits for the Finesse : 21.5k, 21.5k, 23.7k, 21.3k, 22.3k, 22k, 21.5k

But, as the cavity is at air pressure, the lock is not very stable.
we will pump the cavity and make the measurement again.

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

Measurement made on 03/10/18 (nothing has been done since there).

Stable power in the cavity of 225 kW.

I recorded the pulse shape of the MENHIR 216MHz in its fiber injection setup.
the fiber is then connected to the Labbuddy photodiode (~ 15GHz BW) and then to the R&S RTO2000 4GHz scope (~ 100ps rise time).

1) I put a mirror just before the CVBG.
I'm able to get >8mW in the fiber.

as the pulses coming from the laser are not streched, they should be close to Menhir pulse width (~ 200fs).
then, what I measure here is only the pulse response of the measurement system.

2) I remove the mirror before the CVBG to stretch the pulse inside the CVBG
I'm able to get >6mW in the fiber.

the 1st plot is the comparison of the 2 measurements:
in blue, the pulse measurement w/o the CVBG
in red, the pulse measurement with the CVBG

the 2nd plot is the comparison of the pulse streched by the CVBG with the pulse w/o CVBG convoluted with a tp=77.4ps sech² pulse (sech²(t/tp)).
in red, the pulse measurement with the CVBG
in dashed black, the convolution.
(as the spectrum is changed by the CVBG, it is normal that both pulse shapes are not exactly the same).
tp=77.4ps corresponds to t_FWHM = 1.76*77.4ps = 136 ps

both pulses exhibit a FWHM duration of 194 ps.

with the present spectrum after CVBG (see 3rd plot), if the pulse was Fourier Transform limited (no stretched), the product of the FWHM (BW x duration) should be 0.315 (for sech² pulses).
FWHM BW wavelength ~ 1.1nm => FWHM BW ~ 311GHz => FWHM Tau ~ 0.315/311GHz ~ 1 ps
with 136 ps of FWHM duration, one can consider the CVBG has properly streched the pulse.

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.

We start of power output from amplifier of 1W with only the first and second stage on then we start with the third stage power increase,

Note: when changing the current on the third stage 4 diodes, better to do it step by step for each one with a step of ~ 0.5 A

• we placed at the transmission point a splitter the transmitted power before was 35 mW after placing it we had 26 mW
  • 97 % transmission , 3% reflected which goes to the beam profiler

• We see degeneracy modes at the last step of 42.16 mW , so we started to test the D-shaped mirrors
  • the full range of the motors is 50.8 mm
  • each step =  30 nm , 30 000 steps = 1 mm