Installation of the MIKAN.
powermeter (with OD1) just after the oscillator shows 440mW for 4A of the MIKAN pump current.

Installation of a periscope as the oscillator beam position is very close to the table... not easy to put devices at this height (be aware to use proper wavelength range mirrors: Thorlabs BB1-E03)
Installation of a HALF waveplate to align beam polarisation on the isolator axis
Installation of a High power isolator (the one of ThomX): Newport ISO-FRDY-05-1030-N
powermeter (with OD1) just after the isolator shows 427mW

Coupling into a 50-50% fiber coupler using the Thorlabs XYZ table NanoMax TS.
We reached 117mW after the 50% arm which means we coupled about 230mW (the coupling better than 50%).

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%
**************************************

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 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 connections to control the motors of the mirrors are connected in the order stated :

                 M1        -      M2       -       M3     -    M4

            bottom right - bottom left -  top right   -   top left

* The cables on the back of the box are connected as shown in attached photo

as they are connected they show on the software (Precision Tool commander) as

                 M1        -      M2       -       M3     -    M4

                Ch0        -      Ch1      -      Ch2     -   Ch3

Added the cabling to the categories 

A document attached that describes the procedure needed to:

- connect the motors

- configure the Ethernet connection

- Calibrate and reference the software used to control the motors.

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

•   At the reference zero the distance between the mirrors is (taken from a reference presentation "status9nov2020" attached):

                   M1-M2 = 88.029 mm

                   M3-M4 = 84.6895 mm

• The distances between the spherical mirrors ( M3-M4 ) was set to take into account the stability of the mirror ( M3-M4 > Spherical mirror focal length = 85.3 mm)

                 M3-M4 = 90.5 mm

                 we increased the distance between them by 5.8 mm, and moving the mirrors symmetrically, M3 and M4 moved by -2.9 mm (negative defines outer motion)

• Following the definition of M3-M4, fixing of the angle = 2.55637 degrees and frequency (Frep = 876.3636 MHz). Distance between planners (M1-M2)

                 M1-M2 = 80.2 mm

                we decreased the distance between them by 7.83 mm, and moving the mirrors symmetrically, M1 and M2 moved by +3.915 mm (positive defines inward motion)

**** Photo attached is the values on the software at the time.

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
  • ...

New update on the position of the Motors for the cavity

***** we moved the motors to set the cavity at 876MHz, and checked it right after with the RF modulation at FSR.

       So compared to the expected setting we had to move inwards the two planar mirrors by 0.9 mm each.

Planar     : M1-M2 motor = + 4.815  mm    ------>    M1-M2 =  78.399 mm

Spherical : M3-M4 motor = -2.9 mm          ------>     M3-M4 =  90.4895 mm

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

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.

After installing cameras the actual calibration are :

NF_Refl: acA1920-40gm
    pixel size (real): 5.86um
    Magnification = [0.53,0.56]
    pixel size (image): 3.22um
    image donne on input plan mirror M1 (accuracy about few mm)

NF_Trans: acA1920-40gm
    pixel size (real): 5.86um
    Magnification = 1.32
    pixel size (image): 7.73um
    image donne on output plan mirror M2 (accuracy about few mm)
 

need to adjust the NF transmission as the Magnification is greater than 1.

Cavity is lock and optimized with the NKT. Input power is maximized (10mW).

Cavity mode is in attachement. The reference on camera with BeamProfiler Matlab code is the following:

FF_refl: [2.0883    1.1606]

NF_refl: [5.3869    3.9327]

FF_inj: [1.9491    1.3980]

NF_inj: [5.6431    3.5234]