A 10V applied on the laser piezo was found to induce a frequecny change of about 5kHz, compatible with expectations from the piezo sensitivity. this was measured by a freqeuncy analysis of the signal produced by the laser itself on a photodiode det10a

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.

MIKAN phase noise and RIN measurements:

The NF_inj was calibrated with the USB microscope (1.4um/pixel on microscope image). The 5th ring of the lens is about 1.89 mm in diameter.

NF_inj: acA1920-40gm
    pixel size (real): 5.86um
    Magnification = 1.54
    pixel size (image): 9um

After check a mistake has been found on the magnification. This seems to be the good calibration (feel free to cross check). The projection of the 2mm hex is attached

the calibration are :

NF_Refl: acA1920-40gm
    pixel size (real): 5.86um
    Magnification = 1.71
    pixel size (image): 10um
    image donne on input plan mirror M1 (accuracy about few mm)

NF_Trans: acA1920-40gm
    pixel size (real): 5.86um
    Magnification = 0.68
    pixel size (image): 4um
    image donne on output plan mirror M2 (accuracy about few mm)

The NF_inj was calibrated with the USB microscope (1.4um/pixel on microscope image). The 5th ring of the lens is about 1.89 mm in diameter.

NF_inj: acA1920-40gm
    pixel size (real): 5.86um
    Magnification = 1.54
    pixel size (image): 9um

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].

05 April  2021 :  A rough alignment of the cavity was done using the Helium Neon Laser.

Following the Helium Neon Alignment + change in the distance between the mirrors to be; M3-M4 = 90.5 mm , M1-M2 = 80.2 mm  -→ The alignment using the Koheras CW laser is done.

• Additional components used:
  • for monitoring beam :  Photodiode (power of beam), Beam Profiler (shape, position, power , ... ) 
  • for Koheras frequency scan: function generator, Amplifier or use lase-lock (had some issues to be checked)
  • Telescope: made using 1 m focal length to match the beam shape of the cavity
• Observed during:
  • The alignment is fairly similar to the previous one, placed two irises to preserve it.
  • Fundamental mode observed (beam profiler after M2) was circular
  • when the frequency scan was fine-tuned around the fundamental mode we could see the mode pulsing in the cavity, but there was a bit of instability.
  • when doing a very wide frequency scan (50 V ~ 1.5 GHz), multiple modes where showing inside the cavity

Photos attached show:

• some resonating modes in the cavity 
• Fundamental mode resonating in the cavity, with its properties (2D shape, 1D shape, position) ** The picture is taken after subtracting the background **
  • from 2D it is very circular
  • from 1D it is confirmed to be circular (715.00 um - 704.00 um)
  • From position, we have a reference to compare with tomorrow morning.
• Diffraction that can be observed in the cavity (you can clearly see the edges of the mirrors in the photo)
• The temperature curve is attached for the duration of the exp. (before starting Ronic switched something off and then put it on !!!!!! it seems to be for temp regulation )

** Notes for tomorrow morning :  first : switch on the laser and check if the beam 00 mode is still observed and check its position

this is to see the stability and the effect of the temperature.

** Following up from yesterday an observation about the TM00 mode : it has been seen with similar dimensions and its position is the same, this was before restarting the temperature regulator. 

                 --> after restarting the temp. regulator  there was a slight shift in the position (likely it is caused by the temperature variation, a temperature graph for during the morning is attached)

** Coupling of the cavity and koheras is observed (photo attached)

** Error curve measurement to be done.

• What we used before to do a frequency scan and lock the cavity was  a function generator and an amplifier
  • There is noise from the amplifier causing the sweep to pass through the cavity frequencies a lot.
  • we shift to using LaseLock, it gives a very clean sweep ( a clean sweep meaning we have a resistance at the output of laselock combined with the capacitance in the Piezo input behind the koheras, this gives the time constant which affects the signal shape)
• Changes in the temperature of the koheras affects its wavelength and in turn the mode resonates in the cavity.
  • note** when the mode is at the top peak of the sweep  (Piezo )  meaning we are observing the resonance at the ends of the sweep, we increase the temperature of the laser very slightly to shit the sweep a little ( temp increase ~ 0.010 c , very small increase step by step )
• For observing the Mode Lock of the cavity, there are a few points to be aware of :
  • Coupling Frequency -----{convention represented in blue curve}
  • The reflection from the mirror M1 (cavity reflection) -----{convention represented in yellow curve}
  • The transmission from the nirrors M2,M3,M4 ( what is resonating in the cavity) -----{convention represented in green curve}
  • :When the laser is in resonance with the cavity "locked" the reflection decreases and power is stored momentarily in the cavity meaning the transmission increase that is why when the cavity is locked we see a decrease in the reflection (yellow curve) and at the same moment an increase in the  cavity transmission (green curve)
• To be done : do a measurement of the error signal curve, what is needed :
  • the reflected beam (split it into two ) -- one for coupling measurement done before and the other for the error signal curve
  • PDH -- photodiode with bandwidth amplifier
  • Function generator
  • EOM
  • ...

For spherical mirrors M3 and M4 (batch C117I054) the reflectance (R) is around 99.9987% (if T+R=1 => T=13ppm)

For plan mirror M2 (batch C217G054) the reflectance (R) is around 99.9977% (if T+R=1 => T=23ppm)

(1) => For plan mirror M1 (batch C217H023) the reflectance (R) is around 98.96% and transmittance (T) is around 1.14% (T=11400 ppm)
(2) => For plan mirror M1 (batch C217H027) the reflectance (R) is around 99.9385% (if T+R=1 => T=615ppm)

*************************************
case (1) :
Finesse = 546
Gain = 346
Coupling = 1.7%
=> it seems we don't use this mirror for M1
**************************************
case (2) :
Finesse = 9460
Gain = 5578
Coupling = 27%

if one adds 10ppm of losses due to dust on each mirror :
Finesse = 8923
Gain = 4963
Coupling = 44%
**************************************

New configuration is made. The new mode observed in NF_Trans is the following :

I made many tests about polarization and we can see some points thanks to the following figures :

1- The ellipsicity of the NKT is near to 0 (between -0,2 and 0,2) which means the polarization is rectilinear horizontale (attachement 1).

2- The power of the NKT has no influence on the polarization, verticale or horizontale (attachement 2 and 3).

3- The polarization is not changed by the type of mirror (Ag or Diélec) (attachement 4)

The next step is to measure the polarization after the cavity to know the ellipsicity.

To have the polarisation, we must have 1 value because the laser in entry of the system has 1 polar. Due to that, the graphe of the ellipticity is not true, it is the mean value on the following table which shows the ellipticity and the polarization.

We put a diaphragm in the input beam but it doesn't work to suppress the ring light only but it attenuates the whole transmitted beam (cavity mode and ring light) viewed on the camera.

After data processing, I finally find the polarization in transmissin of the FPC. As you can see below, the polarisation is not linear but elliptical.

The four ellipsies are here due to having not enougth parameters to extract with certainty one ellipse.

With the help of Ronic the cavity was locked in preparation to measure the polarization of the reflection line when the cavity is locked (measurement when it is not locked was done before)

the purpose is to compare the two measurements (locked  vs not locked)

General details:

• when not locked : measurements of the polarization was taken from the point where the photodiode is placed in the picture enclose
• When Locked: we can't measure it from that point as we need the line to split into two, one goes to PDH to maintain the lock of the cavity and the other one we use for our measurement.
  • The reflection line is split at the point where the beam splitter is (BS, behind the photodiode in picture), we intended to take the measurements from this point.

Observation:

before starting the measurement of polarization, we observed

• the power measured for the reflection line (point at end of red arrow in picture, after BS) is really sensitive to the polarization, it shows when rotating the half-wave plate
• but, when measuring the power at the point shown in the picture (before BS, where the photodiode is placed) it is not sensitive to polarization.

This tells us that the dielectric BS placed in the reflection line affects the polarization.

This could affect the stability of the locking of the cavity, as the PDH is sensitive to polarization.

** Further investigation is needed before proceeding **

Footnotes:

• BS: Beam splitter.
• Dielectric component's sensitivity to polarization
• most of the components placed in the transmission line are dielectric.

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.

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.