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

• 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)

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

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.

• 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

=======================================================================

• 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

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.

I just had a phone call with Jérome and he told me 2 things :

* Be carefull ! the MightyLaser amplifier is not designed to work with 33MHz laser : the streching level is not sufficient !
One could worsen the phase noise by self-modulation due to peak power or even distroy the amplifier !
One should use it only at low power !!!

* He thinks we should more or less find back the same DC levels than before even with lower seeding power and lower repetition rate.
He thinks we should look at the optical spectrum to check if we don't have some ASE in the 1st stage and 2nd stage signal !
We can send him plots or call him to discuss these points.

Onefive output power is 24mW now, and 2.41mW(after EOM) injected into fiber,

a injection power monitor added, 99% (2.06mW)  injected into amplifier, 1% (16.1uW) monitored with photodiode DET36A/M, which gives ~500mV DC signal on oscilloscope with 1Mohm impedanc;

First stage amplifier works good, monitoring phtodiode gives more than 200mV DC signal with 50ohm impedance on oscilloscope (as attached photo);

Second stage, the old monitoring photodiode is broken, a new monitoring photodiode is connected, which we don't have reference data for it,

on the optical output port of the monitoring signal, it's written 150mW, but at where we measured 40mW.

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.

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....

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

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, 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).

The glow discharge cleaner has been tested on the SBOX mirrors.

I've put them 1 by 1. Each run was 15min long at 15mA. The mirrors HR face was always away from the electrode and ~35° angle with the support. These values have been choosen thanks to the reference:

• https://doi.org/10.1364/AO.26.003884

We have learned that Air can be compared to Azote and Azote to Oxygen in glow discharge. The main difference is H2O in air which make the glow discharge less stable as say several papers.

The CFBG has been removed successfully and the amplifier is closed now.

The soldering machine had a calibration trouble and was sent to the company for review during holidays.

The output power has been checked and is the same as before (tested until 30W ouput power).

We had enough time to proceed a quick scan of M2 which has also a hole but not centered on the mirror.

The hole is larger and higher than the one on M1. But the vertical range was to high for the AFM. Then we cannot see if there are sparkles or not on this image. Further study with microscope would be welcome.

AFM has been proceeded on M1 and M2.

The pdf shows the first images taken with the PLIC room Leica microscope (zoom x10 and x80).

Then the hole has been studied with a handmade microscope. It brought a better resolution. We can now see the hole has an edge, a center structure and and extra-hole sparkle (pailleté) structure.

The AFM shows these 3 structures are real. The top of the hole is at ~+1µm and the center ~-2µm compare to the coating surface. The sparkles are ~10nm high and we also found kind of "explosion" desposit while zooming on the sparkles.

The 3D view and profil show perfectly the "crater".

A chart which summarizes the data we have or we can estimate.

in orange, the case 1, where we suppose the initial cold Finesse is the one measured by modulation technique in December 2019 (F=20.8k).
and in green, the case 2, where we suppose the initial cold Finesse is the one measured by "zero compensation" technique between transmission and reflection signals during the power-up measurements (F~11k).

clearly, the case which matches better the only one data (written in red) of input power and then of cavity gain, is the Finesse estimated by the "zero compensation" technique. it matches also better the gain of the cavity measured after M1 had its hole and for which the estimated Finesse of 4k, and then estimated gain of 277 by "zero compensation" technique is not so far from the measurement of 185 (the gain is may be higher than 185 as it is possible we had some additional misalignment which reduced the gain).
 

the 10th of January, we increased the power of the amplifier to study the cavity transmitted and reflected power signals.
analyzing the noise transfer functions of transmitted and reflected power one could deduce the Finesse of the cavity.
the power of this technic (if it is confirmed) does not depend on the decay time of one signal which depends on the speed of the cut off but on the difference between reflected and transmitted transfer functions,
and then is independant of the cut off speed.

here are 6 analysis of the Finesse when the cavity is cold, depending only on short lock periods.
5 of them agrees on a Finesse around 11k.
the 6th estimation at 40kW stored in the cavity is about 4k but now, we know that the M1 mirror had suddenly a hole for this power... thus the Finesse value is reasonable.

we can then, use the non conservation of TRANS+REF signal to estimate the FInesse decrease when the cavity is hot... to be done

The mirrors went in the cavity the 28th of november (We did several power up to 30kW stored and only one to 40kW then the power went down to 2kW during the run).

Microscope study shows that mirrors get some dust during the handling [travel from microscope to SBOX --> installation --> in SBOX for +1month and power up --> travel to microscope].

Almost all of these dusts can be removed with cleaning.

There is only one important difference between 28th of november and today, a large spot on M1.