In order to solve the problem of the pattern on the surface reference sample I had to move the pump probe by 2°. The resulting path was outside the periscope mirror, so I had to move the periscope. It also went outside the chopper, so I had to move the chopper as well. First I aligned the pump beam without the focusing lens and measured the transmitted peak from the pinhole at different Z
  Y                 Z
121.384100  55
120.684361  35
119.984621  15
angle 0.034987rad = 2.004deg

Then I put the focusing lens again and  after putting the lens maximize the AC signal on the surf ref sample. Then measured again the transmitted peak position:

  Y                 Z
121.923354 55
121.28          35
120.57          15
angle = 1.94deg

and moving the pinhole around the crossing point I maximized the transmitted power of both pump and probe at the following coordinates:
X 327.432
Y 121.255
Z   34.900

this is the new position of the crossing point

The first screenshot is the surface reference scan along Z.
The second screenshot is the surface reference scan along Y, that shows that the pattern amplitude is now of about 5%
The third screenshot is the bulk reference scan along Z.

I also made a map of the surface reference sample
Please note that the map Absorption[ppm] axis is not calibrated.

Manuel, Eleonora

I checked the pump power vs laser current with the power meter PM100D with the head S145C ( Integrating Sphere Photodiode Power Sensor, InGaAs, 800 - 1700 nm, 3 W ).
It doesn't work with the chopper modulation, so I turned off the chopper. The angle of Input Power Controller (IPC) is set around the minimum of transmitted power because the power meter head has 3W of range.
Result: see the first plot.
Every time the current is changed, the laser needs some time (about 20 min) to stabilize. Since I changed the current without waiting for this time, the plot is noisy.

Then I set the laser current to 1.5A and rotated the angle of the IPC to change the power in a more stable way. I measured the power for different angles, without the chopper modulatoin, with two power meters: the PM100D-S145C and the Ophir-Vega (that has a larger range). Then we repeated the measurement with the chopper on and measured it with the Ophir-Vega to check if the ratio with and without chopper is 0.5.

Participant : Eleonora, Marco, Yuhang and Matteo.

After put the first lens(f = 62.9mm), I checked the beam diameter by beam profiler. The result is 101um and 88um. According to the simulation, it should be 88um. It looks fine, so I procced.

For Faraday:

    1. Rotate lambda/4 to find the maximum, then rotate 180 degree, compare to find a better angle.(146 degree)

    2. Rotate lambda/2 to find the maximum.(279 degree)

    3. Before FI the power is 272mW, after FI the power is 253mW. The coupling is 93%.

After putting the second lens, we measured the beam is collimated and has a beam diameter as 2500um.

Then we rotate the half wave plate(163.5 degree), we have s-polarization 202mW while p-polarization 100uW. Means we have 99.95% s-polarization.

After put the 98:2 beam splitter, we got 1.9mW as transmission and 217mW as reflection. The ratio between them becomes 99 : 1. The ratio change is because of the polarization.

For the purpose of having 2.4mm collimated beam in diameter(as required by entry 801), we did the design.

This is for AUX 1, the laser that will be used for the coherent control. A first characterization was done and reported in entry 666. It seems compatible with the new measurement. 

The expected beam diameter for our case is calculated like this:

D = 4*lambda(1064nm)*f(11mm)/(pi*MFD(6.2um)) = 2.4mm

We also received some suggestions from Tomura-san,

1. Check to meet the NA's requirement. (Tomura-san told us basically that is all)

        NA of lens: lambda/(pi*(MFD/2)) = 0.11

        NA of fiber(HI-1060-Flex): 0.14 (from manual)

   From this point of view, we are fine.

2. He also thinks we need to notice if we damage the fiber, if we bend it too much. Our complicated collimator (more complicated than Tomura-san's) can make couple more difficult.

Since we find the beam parameter changed because we change the laser power, we did the beam parameter measurement again. Now the power of laser is 277mW.

result 1: 183um at -0.19m

result 2: 201um at -0.20m

mean: 192um at -0.195m

Seris number: BM-3 UV.

Without filter, it is 0.1W/cm2.

With filter, it is 20W/cm2.

After check everything is fine, I proceed to put 98:2 and total reflect mirror.(As attached Fig. 1)

1. I checked the beam parameter, we want beam raidus as 1450um. But we have only 1100um now. (See Fig. 2) I am wondering why it doesn't match the simulation in e-log 791. Here the power should not influence a lot. Because I just increase it from 10mW to 30mW. This is confusing.

2. I tried to align the beam to fiber. I got only 30uW (total is 11mW), means only 0.3% couple. I have tried my best. On the multi-meter, the shown number is very stable(I didn't use 50Om). But the shown number on the power meter is changing a lot. (See Fig3 and 4)

Improved the simulations. Instead of calculating the temperature at fixed times, I calculated the real part and the imaginary part of the temperature. In this way, the time dependence is given by the rotation of the phasor on the complex plane.
Then I calculated the propagation of the probe through the real part an through the imaginary part of the temperature distribution. These two separate propagations give two values on the photodetector. One is the real part of the signal and one is the imaginary part. Putting these two values together in the complex plane we get the modulus and the phase of the signal. Doing this for each position of the sample we get the scan of the AC signal and of the phase. See the plot.

In the plot there is the comparison of two scans of the same sample, same thickness, same absorption rate, same incident power, but made of different materials: Silica and Sapphire. The sapphire sample looks shorter because the angle inside the sample is smaller (snell's law). The AC signal is smaller because the sapphire diffusivity is higher. The ratio between the two AC signals is 3.7. The phase difference is 33°. 

After found a half waveplate in TAMA(it is a square one as in attached Fig.1), I used it to rotate the polarization of the output of FI.

After increasing laser power around 300mW,

     I checked the power before and after FI(see Fig.2 and Fig.3). The ratio is 90.55%(=280.8mW(detect after FI)/310.1mW(detect after the first lens)).

     The light comeing from the side of FI is really not a point. So it is very diffcult to detect it. Roughtly there is 7mW coming out from output side port, 3mW from input side port.

     I checked the p-polarization portion is 113.4uW, s-polarization portion is 235mW.(The total power sending to PBS is 245.9mW, this means we lose a lot of light by half waveplate and focal lens)

The maximun power we can reach for our laser is around 600mW.

Today I check the beam shaking at another place, as shown in attached Fig. 1. I compared it with the calculation I did with ABCD matrix.(Fig. 2 The result is 0.00059) However, the result doesn't match the measurment(Fig. 3 this value should ccrrespond to 0.000142). I will try to find why my code is wrong.

I also measured the noise spectrum in low frequency, as shown in Fig. 4 and 5. As told by Matteo, our loop can sense the beam shaking as a length shaking. So we can see the noise level is increased.

So I decided to take the video of this shaking on the first iris in the vacuum tube between IM and EM.

(Video is here)https://drive.google.com/open?id=1jNG-MD1ATfqfCkNuEixQux5IXnzmwJTk

Participaint: Eleonora, Yuhang and Matteo

Motivation: Last week, we achieved the coupling of laser to fiber up to 50 percent. However, we find it has a large fluctuation. We guessed it might be caused by the back reflection laser. Anyway, we need to have this FI.

Simulation: I designed how to put them so that we can have a output beam which can match collimator and fiber. I also checked the beam going into FI is much smaller than the aperture of FI(5mm). See attached Figure. 1.

Installation: I collected all the necessary components and put them in sequency. I also checked the beam after the last lens seems fine. But I found the second half-wave plate is broken. I mean we cannot rotate it, so we cannot change polarization. I will change it tomorrow and make the light s-polarization. See attached Fig. 2.

Result:

The transmission of FI is 12.71/14.07 = 90.33%. See attached Fig. 3 and 4.

I used roughly 33 cm of space.

I find there is a mistake in the calculation, see attached picture 1. I used a wrong unit for a number. After the correction, I plot the result again(see attached picture 2)

Now the error on AOM is 1.07mrad.

By using ABCD matrix, I propogate the AOM dithering from AOM to filter cavity import mirror. Here we modulate AOM with a very large amplitude (2MHz) and small velocity.

In this case, I got the result of 0.012m dithering on AOM.

The calculation is attached.

The serial number of AOM is MT110-A1.5-VIS. I checked the manual today. The best input modulation voltage is 1V. There is also one information about the seperation angle's relationship with wavelength. However, the manual is so sketchy that I cannot understand clearly. 

Bad thing: The operating manual is only availabe for someone has account of that company!

Today, We tried to use smaller value of beam radius. I found that this can make coupling better. From 0.1mV to 0.2mV. The input laser is 5mW.

The fiber detector(DET01CFC/M) is 3.5V(5.5mW), that means we have 0.0003mW comes out from fiber now. This is definitly smaller than the case of last week. The coupling is only 0.01% now.

The procedure I did is like this:

1. Without inserting fiber, check the laser is almost going through the center of collimator.

2. Insert the fiber to collamitor, then adjust the x and y of collimator.

3. At the same time, use multi-meter to monitor the fiber detector(without using 50 Om). Until the voltage goes beyond the range of multi-meter, we start to use 50 Om.

4.Now you will 0.1mV on the multi-meter. Usually, this is the time to use two sterring mirrors to maximaze the coupling.

I did this procedure many times, but still the best result is 0.2mV. It seems something maybe wrong. 

(By the way, I spoke a wrong thing during today's optics meeting. When we can read 0.1mV on multi-meter, the output of fiber is very weak! I am so sorry that I remember something wrong!)

Yesterday, we installed a telescope in front of the collimator of fiber. The purpose is to make the output of this telescope have the same beam radius ( roughlty, r=1500um )with the output of fiber.(the simulation of telescope is in Fig.3)

However, during the simulation and installation of this telescope, we found the result doesn't agree with what we found in practise. We measured the beam parameter of AUX1 again.(the parameter of AUX1 is shown in attached Fig.1, we should say that the origin of this measurement is the head of laser box)

The total amount of power not coupled inside the FC is composed of the mode-mismatch and the sidebands.

The sidebands maximum are respectively : 0.1462 and 0.1422 mV (this takes into account the background value).

This means that 11.84% of the light is not coupled inside the FC.

I think there is a factor 2 missing in the formula: the pole of the cavity is FSR/(2*F) = 500000/(2*4355) = 57 Hz