the M1 is not at 5.1045mm to put the fundamental on the scanning range of the laser piezo

M2 still at 5.104mm

M3/M4 at -2.9mm still

after 3hours (1pm --> 4pm) the cavity moved by 12um in total

new positions: +5.1065mm on M1 and M2 (5.1050 before)

the drift is 4um/hour which amounts to about 10kHz drift / hour+the temperature of the optics room also changed unfortunaltey (linked to extinction of air flow at around 11am this morning). the drift in the room temperature was about 0.5K from 1pm to 2pm and 3K since 11am.

maybe this measurement needs to be redone to more precisely correlate the drift to an overall temperature change

in addition to the standard locking scheme with the GHz laser PZT,

we added an AOM after the PDH modulation EOM and we drove it with an FM modulated signal generator (FMDev = 2.4MHz) seeded by the error signal.
(we didn't put a 50ohm plug to adapt the error signal coming from the PDH box, otherwise, it is too much smaller)

the result is a transmitted signal almost clean for some milliseconds... but we still have regular unlocks that the PZT loop is unable to drive.

the PZT resonant frequency around 30kHz seems much less present in the error signal.
todo list:

- take some data of the error/trans signals to make a post-mortem analysis (a windowed FFT could tell us if the 30kHz is more powerful just before an unlock)

- make an RLC model of the cable+resistor+PZT capacitor, to try to find a way to dump the 30kHz frequency.

We did the measurement of the Finess and coupling with the NKT, with 2 EOM, without AOM (so the lock signal is noisy), on the burst line.

For the coupling, we have the same value as with the GHz locked on the burst line too, which is around 20 percent.

it seems this low coupling comes from the geometrical coupling as we observed an incoming beam bigger than the cavity mode on the cameras.
(the same oscillator, locked on the lock line exhibit 50% coupling)

the FSR center value is 879.9MHz, and the FWHM is between 879.67MHz and 880.1MHz.

So, the linewidth is about 430kHz and then the Finess value is roughly the same as before about 2050.

To summerize the results of this week :

We made the setup in attachement 5 and align it to have the good mode in the cavity (TEM00).

With the good alignment, we found the lock of the GHz, in attachement 1. We used the motors in the cavity to position very precisely the mirrors to find this lock.

In attachement 2, we can see the error signal obtain thanks to the PDH, which is very clean.

But after that, we saw some modulations at 30 kHz frequency which represent the resonance frequency of the piezo in the GHz (attachement 3). To compensate this, we placed a fibered AOM to cut the high frequencies.

Finally, we observed a big difference in the transmission noise if the motor loop is open or closed (attachement 4). To have the best noise, we have to stop the closed loop and stay in open loop, after adjusting the mirrors with the cosed loop.

We have also measured the finesse (2000) and the coupling (20%), detailed in the previous post.

we have excited the PZT with a swept sinus wave from 1kHz to 10kHz and from 10kHz to 100kHz.
here are the 2 different spectrums:

- the 1kHz-10kHz is a standard spectrum where we see the impedance behavior of the PZT: Zpzt ~ 1/jCw

- the 10kHz-100kHz exhibit several PZT resonances and the 1st one is close to 28kHz.

(without PZT resonances, we should have the same behavior at higher frequencies than in the range 1kHz-10kHz)

3 phase noise measurements made on the Amplitude GHz oscillator with different PZT configurations :

- black curve: PZT connector is open
- green curve: PZT connector is shorted by 50 ohms
- blue curve: PZT is excited by 100mVrms of white noise coming from a generator.

on the blue curve, one can clearly see a phase noise increase in the region 10kHz - 1MHz but it is not evident the peaks seen with the PZT open or shorted are related to the peaks excited with the noise injected on the PZT.

with a PZT not excited, one can just observe that the phase noise is decreasing a lot around 10kHz to reach the reference oscillator phase noise floor and then increase again exactly when the PZT resonant frequencies appear, between 20kHz and 200kHz.... reaching at the end the phase noise detection floor.

I add below the measurements done on October 20th, the ones done in September which are very similar and on which one can see a peak around 26kHz.

MIKAN phase noise and RIN measurements:

Thorlabs PZT datasheet.
Reference: PC4QR

The optical spectrum of the GHz oscillator: FWHM ~ 4nm

Here the first part of all the presentations since the beginning of the project.

Here the second part of all the presentations since the beginning of the project.

Here the last part of all the presentations since the beginning of the project.

M1 and M2 at 5.1085mm and M3 and M4 at -2.9mm

Input power of the AOM :

150mW

output power of the AOM :

125mW

Generator output :

250mVpp, 50ohm, 240MHz

I realign once again the oscillator GHz because it was not mode lock.

- On a observé que le fondamental du Tangor et celui de la Lockline n'étaient pas situés à la même fréquence. Avec la différence de tension donnée par le piezo entre les deux et le déplacement du piezo en fonction de la tension appliquée donnée par sa datasheet (150 V pour 20 µm), on obtient un écart de fréquence de 121 MHz [Image 1].

- On a tout d'abord testé si les drivers qui contrôllent les AOMs 200 MHz et 40 MHz dans le tangor envoyaient la fréquence voulues, soit respectivement 200 MHz et 40 MHz, ce qui est le cas.

- Sachant que le fondamental de la lockline est initialement à 240 MHz, on a shifté la fréquence de celui-ci pour le superposer au fondamental du Tangor, ce qui a donné un écart de fréquence de 80 MHz environ. Cela montre en reprenant les calculs que le déplacement du piezo est de 20 µm pour 227 V [Image 2]. De plus cela correspondrait à un ordre 1 sur l'AOM 200 MHz et -1 sur l'AOM 40 MHz.

- Mercredi 11/05/2022 et jeudi 12/05/2022, la température de la salle est montée à 33°C. On a dû réaligner le Tangor et la lockline qui avaient bougés.

- Ajout sur la ligne de transmission d'une lambda/2 et d'une lambda/4, déplacement de la caméra et de la photodiode en transmisson, imagerie refaite. Alignement fin à finir sur la photodiode en transmission. Ajouter un TPBS pour envoyer le train du Tangor sur une photodiode rapide. Prendre les références des caméras.

- On a vu du couplage sur plusieurs modes dans la cavité avec le Tangor [Image 3].

Position Cam :

692;363 NF_inj
520;193 FF_inj

Position Moteurs :

5.085435
5.085435
-2.9
-2.9

Spectre de l'oscillateur ~6 nm largeur (correct, cf image)

Durée d'impulsion : 3.35 ps (cf image)

On peut également voir que la forme du mode TEM00 a changé (cf image sur la caméra de transmission), dû à un endommagement d'un/des miroirs de la cavité. On voit également que rien n'est visible sur la caméra de réflection.

AOM 100 Hz , t_burst = 5 µs, PP 50 kHz : P_burst = 0.15 W, P_burst_PhD = 0.47 mW

AOM 100 Hz , t_burst = 5 µs, PP 50 kHz : P_burst = 0.25 W, P_burst_PhD = 0.75 mW

AOM 100 Hz , t_burst = 5 µs, PP 50 kHz : P_burst = 0.8 W, P_burst_PhD = 2.09 mW

J'ai également mis les mesures de long-term, ainsi que les burst correspondants.

Mesures de puissance fibres lockline :

AVANT POLISSAGE

injection premier étage : 15 mW

sortie premier étage : 63 mW

Sortie AOM à 250 MHz : 2.5 mW

Sortie EOM : 1.3 mW

Sortie : 20 =>40 mW

APRES POLISSAGE

injection premier étage : 15 mW

sortie premier étage : 63 mW

Sortie AOM à 159 MHz : 2.9 mW            Sortie AOM à 250 MHz : 15 mW

Sortie EOM : 1.5 mW                              Sortie EOM : ?

Sortie : 47 mW                                        Sortie : 54 mW

Même après polissage de la fibre de sortie du premier étage, on observe que la puissance diminue d'un facteur 4 à travers l'AOM drivé à sa fréquence nominale.

Changement d'AOM pour passer sur l'AOM 200 MHz.

Le schéma suivant récapitule les mesures prises.

Il y a eu une chute de la puissance en sortie de la fibre bleue de la lockline.

1) à la vue des mesures, il semble que le deuxieme etage de la lockline soit bien fatigué

2) La fibre bleue s'est déteriorée d'un seul coup...

Par contre il n'y a plus les modulations observées précédement sur la photodiode grâce au changement d'AOM, à la fois à 159 MHz et 200 MHz.