We measured again the 100W CELIA laser amplifier with a pump current until 8A.

as the first current pump of the amplifier has a Peltier issue, we don't exceeded 6A on this stage and we compensated with the 3 other stages.

7A average current is obtained with 6A / 7.3A / 7.3A / 7.4A
7.5A average current is obtained with 6A / 8A / 8A / 8A
8A average current is obtained with 6A / 8.6A / 8.7A / 8.7A

we did the power measured either with the "big" powermeter which is able to handle 100W
and with a smaller powermeter after a wedge, in the reflection path, which is multiplied by 39 to match the big powermeter measurement.

a fit a 12W/A from the cut-off current of 2A is a good estimation until 5A.

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
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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 afternoon, with Fabian, we did some measurements on the 3rd stage.

finally we used the Dichroïc mirror at AOI=45°C as first mirror and a Thorlabs BB1-E03 as 2nd mirror to send the beam to the powermeter.
we put an iris to cut the pump beam part.

with only the 2nd stage ON, the dirchroïc mirror and the iris : P=0.4W

3RD STAGE
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IT IS MANDATORY TO SWITCH ON THE CHILLER

once the Alphanov software is launched, 4 windows appears on the screeen, one for each diode.
once the chiller temperature reach the set value (23°C in our case), one can start to increase the pump current.
(set "voltage adj." to AUTO)

the TEC of the first diode is not operative, then we decided to stop its current to 4A to avoid a too large temperature (in this condition, the temperature of the diode reach ~45°C !)
the other diodes temperatures are stabilized around 25°C

1A for all diodes       =>   1 W
2A for all diodes       =>   10.7 W
3A for all diodes       =>   22 W
4A for all diodes       =>   33 W
5A for diodes 2-3-4  =>   40.5 W
6A for diodes 2-3-4  =>   48.1 W

we stopped the measurement at 6A and we did not notice any change in the optical spectrum

During the optical spectrum measurement we tried to use a NDUV20 Thorlabs reflective filter to reflect the high power beam to the powermeter and let a few power part be transmitted to the spectral measurement.
unfortunatelly, the ND filter coating has been completly removed => DO NOT USE Thorlabs ND filters at high power !