Today, I installed a power meter at the transmission point of the 1st spherical mirror , transmission 3 ppm (direct from cavity window beam profiler, no filter)

We locked the cavity at 2 different resonances of the fundamental mode, the lock in both cases was stable for a round 1 minute.

at different transmitted power of 33.85 uW ( 11 W average inside cavity) and then locked again at 82.65 uW (27.5 W average inside cavity) transmitted power 

Note : the coupling is almost zero for both of the resonances locked !!!!

Oscillator : 33.33 MHz ( 33.356 MHz , frequency read on LAL software)

power injected into amplifier (after injecting into fiber and an EOM) : 3.886 mW (on LAL software)

Amplifier power : 0% (injected power into cavity ~ 300 mW)

Both images of oscilloscope have same voltage scales, only difference timescale and the color code is : 

yellow : transmission

blue    : reflection

green : piezo voltage

red     : error signal

Yesterday, with Ronic we locked ThomX cavity on the second resonance

Laser : 33.36 MHz , Amplifier : 0 % (worked with no errors all day)

adding image for the lock on the second resonance (locked), 00 Mode image, fit, Beam measurements

and image of the first resonance signal (not locked)

Info : when locking on the secondary resonance, we aligned better and adjusted on the oscillator CEP and increased the transmission signal 

Controllers connected to computer

through Ethernet :

• Cavity Mirror motors
• Lase lock
• oscillator CEP and Frequency motors (easier to follow the drift with this controller, and we can see it moving)

by USB :

• Amplifier controller

finding the right modulation/demodulation PDH phase is very difficult on the main resonance because the we get non stationnary signals with a lot of oscillations.
changing the phase, in this condition, does not really change the error signal.
Then, we moved on the first secundary resonance with less gain and less coupling.
Thus, the error signal is more similar to the theoretical PDH signal => one can adjust the modulation/demodulation PDH phase to get the maximum error signal.

then, we locked pretty easily on this first secondary resonance, with a coupling around some % when we adjust the CEP motor.

we tried to lock on the main resonance but it is too noisy and unstable.
it seems we really need high BW feedback.

I tried to add a fast analog loop on the laser intra-cavity EOM but without a clear effect.
the problem is the gain of this loop : it is difficult to produce a "high voltage" (above 10Vpp) on this EOM.
I put "my" amplifier but the voltage output is limited... commercial amplifiers will have the same issue.
we can add HV amplifiers but it takes place and it will add some noise on the signal.

A loop with an AOM could be easier to install and manage... but at the price of a loss of power before the laser amplifier...
 

the last tries to lock the 33MHz + amplifier to the 30k Finesse FP-cavity were unsuccessful.

during a laser Frep scan using the Laselock, one observes that the main cavity resonance is not able to stay inside the PZT scan range from one scan to another (500ms-1s period)
is it the effect of a large and slow phase noise ?

some informations:

- The 33MHz laser came back at lab from repair on March 2018.
- it has been sent to Alphanov in May 2020.
- it failled and has been sent to NKT/OneFive for repair in September 2021
- it came back to lab from repair in June 2022.
- on post #92 (Feb. 2020), it seems that we already locked the 33MHz laser + CELIA amplifier to the ThomX FP-cavity.

- The PZT sensitivity for the 33MHz laser is given to 0,3Hz/V for Frep <=> 2.6MHz/V for optical frequency.
=> 10V on PZT is equivalent to 26MHz of optical frequency shift which is less than FSR !

- by comparison, the PZT sensitivity for the 133MHz laser is given to 3.9Hz/V for Frep <=> 8.5MHz/V for optical frequency.

- by comparison, the PZT sensitivity for the NKT CW laser is given 10pm/100V for Wavelength <=> 30MHz/V for optical frequency

- by comparison, the PZT sensitivity for the ThomX FP cavity (Z20H38x40C) is 4nm/V for length expansion => 8nm/V for round-trip expansion <=> 0.03Hz/V for FSR expansion <=> 260kHz/V for optical frequency !!!
the PZT expansion estimation is in attached file.

This morning, while trying Guillaume solution of disconnecting the 24 V necessary to the second and third stage and raise the threshold of Temp preamp 1 (to only operate the ampli with first stage only)

Ronic disconnected the 24 volt connection, turned on amplifier LAL software but no MMD3 error or a big temp on Temp preamp 1 , software worked fine with no issue. 

we connected the 24 V again and the software worked with no issues.

Still not sure what happened for it to work correctly !!!!!!!!!!

• We used the amplifier at 0% all day with no errors (to test , I turned it for 10 minutes @ 10% with no error too)
• We changed the lase lock box with ThomX one (having Ethernet connection), it was connected to computer and works ok
• Ronic installed a signal amplifier for the PDH error signal. (yet to be fully optimized)
• we see a better coupling than before in the cavity.
• The dominant mode is 00 mode, but we see many higher order modes, need to alight better.

Today with Ronic and Daniele we attempted to lock the cavity, but the alphanov amplifier did not turn on because of an error in MMD3 (related to the third stage)

the new error is having the MMD3 error show on the alarm window of the LAL software, and having the Alarms on the line 3 of the alphanov software red for both EXT/CPU and Laser T Max (never been red before)

this is after a 2-week work stop, but the last time when Daniele changed the fiber connector it worked for 3 hours with no errors.

Ronic, is in contact with Guillaume from Alphanov to fix the issue !!!

Work scheduled all Wednesday, in hopes the issue is fixed by then.

Added Note: the coupling we get is at low power, only preamplifier is on (~ 300 mW)

was done at the end due to excitation of higher order modes.

Wanted to improve the fundamental first, then increase the power.

• On Monday with Daniele,
  • we did full characterization of amplifier beam @ 10% amplification, adjusted the telescope accordingly and injected into the cavity aligned and improved on the fundamental mode.
• On Tuesday with Daniele and Kevin
  •  I added a low pass filter between 1 - 1.9 MHz on the reflected signal, to reject the oscillator signal (33.33 MHz)
  • removed the external resistors on the signal, only the internal resistance of the oscilloscope used ( transmission : 1 M ohm  , Reflection : 50 ohm  )
  • We connected the motors to control the CEP and adjusted on them until we reached ~ 5 -10% coupling
• Images:
  • 1st : showing the size of the beam just before injecting into the cavity @ 10%
  • 2nd: signal with only improved alignment (not CEP adjustment yet), no low pass filter added yet
  • 3rd : zoom on the signals (reflection and error) while adjusting on the CEP
  • 4th :attempting to lock the cavity , after getting the max coupling using the CEP motors

Note : the voltage ranges are not the same between the images

Yesterday,

• we installed the second high power reflecting mirrors at the output of the amplifier.
• We aligned and installed a telescope that has adjustable distance between the 2 lenses of -100 mm, and + 200 mm, with approximate diameter at the injection window of  ~ 4.5 mm.
• Amplifier was turned on only at the first stage only (output power ~ 300 mW) aligned and injected into the cavity.
• Measured the repetition frequency of the One Five oscillators, Frep = 33.326239 MHz
• Changed the FSR of the cavity to match it.
• Using the Piezo scan on the oscillator, we observed 00 Mode, Fundamental. the shape is the same 
• We see transmission on the diode, but there was zero coupling observed (yet to be investigated experimentally )
  • a reason could be the CEP of the oscillator ,
  • bad matching of the beam waist position,
  • noise from the amplifier (as we were operating it at very low power)
  • additional alignment needed,

This morning, with Ronic and Daniele, we locked the cavity again (The lock was relatively easy and stable during the reading of the finesse)

We took 2 reading of finesse

1st   : @ 33.2808 MHz (what the cavity length was at)           :  Finesse = 27 024

2nd :  @ 33.3382 MHz (adjusted to match the pulsed laser)  :  Finesse = 30 433

The difference between the 2 is the FSR, during the change we also did some alignment to counter the reduced transmitted power.

We took an image of the beam profile before and after changing FSR and there was ~ 70 um change in the beam position on the Horizontal axis of the beam.

This morning with Daniele and Viktor,

- we confirmed the "potato" mode was well centered on the injected beam and was not due to some misalignment of the injected beam compared to the cavity axis.
we look at the reflected beam during a lock. we can see the direct reflected beam superposed with the cavity mode (cf attached image).

- then we assumed the "potato" mode problem could come from the too short distance between spherical mirrors which introduces ellipticity and finally can lead to some instability.
we manage to move further spherical mirrors S2 and S3 by moving them from 1 350 000 to 800 000 steps.
=> we increased the distance by 2x 550 000 steps (with 6µm for 1000 steps) => we increased the distance between spherical mirrors by 6.6 mm
the new mode shrinked a lot and at the end was spherical at 73%.
the mode size is now about 4400 x 6000 µm but with the major and minor axis still at 45° !
 

Yesterday morning we locked the cavity on a strange mode in the cavity (we call it mode, but not sure what it is ??)

we also managed to lock on a higher order mode 02

we get an estimated 30% coupling (expected value), and note that we have not installed a telescope yet.

Today, we prepared lenses for a telescope for the CW laser injection into the cavity, yet to be installed

Image of the transmission at P4 attached, an estimation of the beam size of the mode see is ~ 5.5 mm 

The vacuum in the 2 optical cavities is decreasing but slowly. today it is 2.6 - 2.4 * 10^-7 mbar

Last week, with Ronic and Victor we aligned the cavity using CW laser (Koheras)

• we have also installed 4 mirrors in the cavity with the best theoretical qualities (serial number and images included)
  • The Plan ULE mirror P4 thickness was not compatible with the piezoelectric mount. We had to install a metal spacer behind the mirror to be able to fix it. the metal spacers in from ThorLabs made from INOX, It is 1 inch diameter (25.4 mm), we drilled a hole in the middle of 15 mm in the Workshop in bat 200. Then cleaned it using an ultrasound bath in pure water, using a small Ultrasound bath machine in Maverics platform Vide and Surfaces next to ThomX eglo.
  •  
  • This gives some difficulties in imaging the transmission at P4. To help with the imaging, we used the plastic #D printed irises that Yann made long time ago
  • we took images of the mode inside the cavity at P4 transmission.

• We also observed some oxidization on the metal bars of the controllers, more on the side of S2 and P4. we are yet to test the motion of the motors in the z-axis.
• Friday evening we closed the windows using a new copper rings (last one we have, Ronic placed a new order).
• The pump was turned on by Bruno and Eric yesterday afternoon. The current vacuum level is 6-7*10^-6 mbar

the pre and the turbo pimps are on so still too noisy to work, we have access all day tomorrow.

due to several "case temperature" issues, I change the "Max case temperature" in the "general config" of the "Alphanov Software" from 50°C to 55°C.
=> validated by Alphanov.

Today, I restarted all the equipements :

Chiller                                                 : ok, temperature converged quickly to the set temperature : 25°C
Cavity mirrors motors controller         : I had to configure the general network as 192.168.xxx.xxx in order to operate the software designed by Didier Jehanno
Computer                                           : ok, now, it IP address is 192.168.1.1
Laser amplifier controller                    : I had a LOT of issues in restarting the LAL software. I had to restart several times the amplifier controller in order to access it by software. I quickly check the power increase to 50% after it has been correctly seeded
CW oscillator                                     : ok, Koheras restarted without any issue.
OneFive oscillator                             : the power ON was ok but it did not modelock. I tried several gently knocks on the case but without success (one clearly see the effect on the scope, the oscillator "tries" to modelock).
                                                            fortunately, after about 1h, the oscillator modelocked alone (check with Buddy fiber photodiode + scope).... the power is the same as before. the Alphanov software measures 5.4mW (it was 5.5 mW the 2/1/2023)

Smaract controller for OneFive        : ok, I changed its IP address to 192.168.1.10. I tested the PC control in moving forth and back the "CEP" motor by 1mm... the OneFive was still modelocked.
 

As the general local network for the FP-cavity devices is 192.168.xxx.xxx, I changed the IP adress of the smaract laser motors (Frep and CEP) controller.

For that, I used the embedded web server as described in the manual in attached file.

the new adresses are :
IP :             192.168.1.10
Gateaway : 192.168.1.1
Subnet :      255.255.255.0
Port :          5000

All the equipments have been switched off on Friday 17th due to a power shut down on Saturday 18th :
CW and Onefive oscillators, computers, laser amplifier, cavity mirror motors, chiller, etc...

Today we tested a mirror from one patch

image is for the front face of the mirror

PowerPoint has more details about the images

Information about today's work is added to logBook !!