• 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 ???

Closing Series for Alignment.

Beating observed, beam is in the locking stages

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.

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

Closing series of 00 mode for FSR @ 133.33 MHz

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

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)

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.

The "ThomX" CELIA amplifier is installed on the optical table.
I added all the rubber pieces available between the 2 racks to isolate as best as possible the top rack, which embeds some fans, from the bottom rack from which the laser beam is going out to the FP cavity.

1ST STAGE
-----------------

if I put 1mW (minimum input power) on the input fiber of the amplifier and I switch ON the first stage, one can measure 7mW on the 5% output tap on the front panel.
then it is mandatory to check this power before swtiching ON the other stage.
this 5% output tap on the front panel MUST BE ABOVE 7mW

input power : 1mW => 5% output tap : 7mW => amplifier output : 260µW

with the present setup, I can reach 6.8mW of input power, but the 5% output tap seems to saturate at ~ 8mW.
in this condition, the amplifier output is around 800µW

the SMA connector on the rear panel does not output any signal with the 1st stage ON.

2ND STAGE
-----------------

then, one can switch ON the 2nd stage : amplifier output is around 1.4 W (without any iris or dichroïc mirror).

the SMA connector on the rear panel does not output any signal with the 2nd stage ON.

Beam size at the amplifier output:

to make this measurement, I switched ON only 1st and 2nd stage.

the dichroïc mirror seems to work the best in reflection at normal AOI !!!
it's strange as most dichroïc mirrors seem to be specified at AOI = 45° ! => to be checked ! => the dichroïc mirror was set on its wrong face !!! => problem solved

the 2 images correspond to the beam measured at 24cm from the amplifier output.
we cannot use the gaussian fit due to the pump beam shape which perturbate the measurement.
I used the FWHM measurement => DX = 2.6 mm, DY = 1.8mm

Pulse shape model:

dP(x,y,t) = DP * exp( - ln(2) * ( (x/DX_fwhm)² +  (y/DY_fwhm)² ) * sech²( t / tp )

=> can we use safely the Newport 20Z40DM.10 mirrors to transport the amplifier beam ?
they are specified for 500 W/cm² CW and 4 J/cm² for 10 nsec pulses @ 1064 nm.

• if I am correct, for the previous shape model, the average power is :

Pmoy (W) = 2pi / ln(2) * tp * DP * DX_fwhm * DY_fwhm * Frep

with DP=500W/cm2, DX_fwhm=2.6mm DY_fwhm=1.8mm, tp=77ps, Frep=216MHz => Pmoy = 3.5 W !!!
this means that we should not exceed this average power with these mirrors !?!

• if I am correct, for the previous shape model, the maximal energy density (in the pulse center) is :

DE (J/cm2) = tp * DP => Pmoy = 2pi / ln(2) * DE * DX_fwhm * DY_fwhm * Frep

with DE= 4J/cm2, DX_fwhm=2.6mm DY_fwhm=1.8mm, Frep=216MHz => Pmoy = 366 MW !!!
if one applies a safety factor due to the pulse duration ratio (77ps / 10ns) => Pmoy = 2.8 MW !
this specification seems much less restrictive !

=> can we use safely the Thorlabs BB1-E03 mirrors to transport the amplifier beam ?
they are specified for 10 kW/cm CW (linear power density) and  0.5 J/cm² for 10 nsec pulses @ 1064 nm.

• the linear power density (LPD) is defined as the average power divided by the beam diameter (1/e²)

LPD = Pmoy / DXY = Pmoy / (1.7 DXY_fwhm) => Pmoy = LPD * 1.7 DXY_fwhm

with LPD=10kW/cm and DY_fwhm=1.8mm => Pmoy = 3 kW

• for maximal energy density:

with DE= 0.5J/cm2, DX_fwhm=2.6mm DY_fwhm=1.8mm, Frep=216MHz => Pmoy = 45 MW !!!
if one applies a safety factor due to the pulse duration ratio (77ps / 10ns) => Pmoy = 350 kW !

this morning, I locked the FP cavity with the OEwaves CW laser and the "Fred fiber amplifier" used at 500mA of pump current.

the lock was much more easy than with the Koheras.

I had to change the 10GHz EOM which seems damaged as the modulation depth is very low and does not allow a Finesse measurement by modulation technique.
I changed it by a recently buyed 2GHz EOM... the modulation depth is large enough and we can make the Finesse measurement.

I took several sets of data and the average Finesse is 25.5k !