Today we managed to observe the fundamental mode and stabilize the scan on it until we improved the alignment enough.

We see some coupling, but it is very week < 5% , We improved the alignment and the polarization, but there is still no explanation to why it is very low.

The mode shape is circular with radius = 0.89 mm at transmission point, ~ 40 cm from circular mirror.

One EOM was installed along the injection into the amplifier, we saw a drop in the power measured by the first stage monitor from ~ 8.2 mW to 7.2 mW

we improved the injection up to 7.6 mW, but it still fluctuates a lot. We need to be careful about it.

The error signal looks clean, but it is very week which is due to the weak coupling.

Today, we played on the CEP but using the I-tune input of the Menhir Laser (+/- 5V maximal range).

unfortunately, one only saw a very weak improvement of the transmission by 10-20%... and the coupling improvement is almost zero.

the best improvement was for the maximal I-tune range (+5V) which maybe means that we could improve more the effect if were able to get a full range of 2pi for the CEP (instead of the present pi/2 range).

We played on the CEP using the USB command "id0=xxxxx" of the Menhir.

we put id0=48650 and we improved A LOT the transmission and coupling (~ 60%)

here is an image of the first attempts to lock... but the locking is quite difficult.
yellow : transmission
green : reflection
amplifier is still with 2nd stage only (Pout=~1W)

we are adding an AOM in the loop...

We managed to lock the cavity by adding the AOM , but the lock is still difficult to stabilize.

There is some high frequency compensated by the AOM at ~ 170 kHz (yet to understand from where it comes from)

We wanted to increase the power of the amplifier to measure the transmission output at M2,

but locking the cavity again was very, very difficult.

We will try again tomorrow.

We locked the cavity, and it is stable using the Transmission,

the high frequency that we thought could have a reason for instability, is due to the high power on the photo diode of the PDH box which can cause non-linearity effects in the signal.

We also closed the fan of the third stage and there was no significant change on the error signal and the piezo+AOM compensation signal.  

image of the lock with Coupling of ~ 60%

Alignment was improved to ~ 1.2 V on the photodiode.

Only the second stage was on with ~ 1 W output from amplifier

we measured the transmission from S2 to be 2.3 mW

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

At transmission power of 42 mW, coupling 60%, we see fundamental mode with a degeneracy

The D-shape motors were moved to a position 2 000 000 steps (in theory they should be the max position)

but no change appeared on the 00 mode or the higher order mode.

We will break the vacuum and check the position of the D-shaped mirrors.

This afternoon we opened the cavity and put the D-shape mirrors at their correct place, close to the beam.
we checked the relative position of the mirrors to the beam using the 2nd stage of the amplifier (<1W) and with the sensitive (and cleaned) orange optical card.
with this configuration, we can see very clearly the beam inside the cavity (~ 100µW) and we can check easily if the D-shape mirrors are correctly placed.

the motors used to move the D-shape are the Newport Picomotors 8303-V
with roughly 30nm/step sensitivity and 50mm of range (~1 600 000 steps)
the 4 axis controller used ot move these motors is the Newport 8742.

For both Vertical and Horizontal D-shape mirrors:
* when you do +N steps on the controller, you retract the D-shape mirror from the beam
* when you do -N steps on the controller, you push the D-shape mirror to the beam

the 0 position on the controller corresponds to the D-shape close to the beam.

now, the FP cavity is closed and pumped to go back to vacuum.

The cavity box is vacuum pumped at 6*10^-2 mbar.

Power increased in the amplifier

reflective filters added at the transmission point to be able to have signal on the Beam Profiler, Transmission diode and power meter

at max stage we reached at 2.5 A on the diodes we have

8000 gain , coupling ~ 60% , with power inside the cavity 50 kW

The higer order mode that was observed today,

it was also observed on Friday  along the other higher order mode recorded on the logbook.

after cutting it with the D-shaped mirror we recorded the shape (image attached)

Following the storage of ~ 50 kW inside the cavity and a sudden drop in transmitted power from the cavity 

damage to the mirror surface was suspected.

We broke vacuum and took images of the surface of the 2 mirrors in the cavity, the spherical and the planar mirror 

image 1 , spherical reflective surface (no visible damage with the UV light, and no visible damage under the microscope)

image 2 , planar coupler mirror reflective surface (no visible damage under UV light, but under the microscope there is a damaged spot close to the center)

image 4 is the planar surface reflective surface at zoom 8 on the microscope.

Tomorrow will try to shift the injection mirror to avoid hitting the damaged spot.

After discussing, we have decided against shifting the mirror to avoid the time lost.

We changed the injection mirror to a different mirror from Mighty Laser set, Transmission of mirror 80 ppm. (no visible damage at the center of the mirror, only a small scratch on the back)

mirror cleaned using pure ethanol and pure water with spin coater, also the spherical mirror was cleaned again.

After changing the mirror M1 and cleaning the 2 mirrors

They were placed in the cavity box (avoiding touching the mounts to not affect the alignment and placing the mirrors as close to the previous position)

we saw horizontal higher order modes immediately after injecting power,  horizontal misalignment!!.

We aligned the injected beam, adjusted the cavity length and saw the modes 01 (high transition) and 00 (ok but still much lower than 01)

The image attached shows some coupling < 5% , we adjusted the CEP, but it was the max coupling.

we will need to align better tomorrow to increase 00 modes.

The cavity box is closed and placed under vacuum again.

The cavity was locked under vacuum, coupling is approximately < 10%

The lock was relatively easy and the usual sign (oscillations on the transmission tail) of high finesse on the transmission signal was not seen.

We improved the lock using the alignment, polarization and CEP , the maximum we got was ~ 12% coupling

only the second stage is on !

Decision : this is the maximum we can obtain using the M1 from mighty laser

tomorrow we break the vacuum and try with shifting M1 from ThomX position to avoid the damaged spot