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

After the installation of mode cleaner, we measure the contrast again. This method is like this

We use the high voltage driver of MZ as an offset adder. By giving different offset to it, we can have almost all light transmit or almost no light transmit.

Then we record the highest peak hight while almost all light transmit. Record the highest peak hight while almost no light transmit(actually, this is the time when TEM00 almost vanishes).

We got result like attached picture 1.

However, I don't know if this is the right calculation method. (Maybe I should do like Marc did for FC scan, intergrate all the peaks in-between a FSR?)

Corresponding to the comment of Eleonora, the bandwidth of filter cavity for infrared is 114Hz but not 55Hz. Then I think we can explain the result (almost).

bandwidth=FSR/Finesse=500000/4355=114

According to the signal we send to DDS board, the modulation frequency of we give to EOM is 15.2MHz. The FSR is 0.5MHz.

15.2/0.5=30.4

So we will have additional 0.4 of FSR. This exactly explains the result we have on the oscilloscope.

Participaint: Marc, Matteo

We used PSD(position sensitivite detector) to detect the beam dittering incured by AOM modulation.

The modulation information of AOM: 2MHz(pk-pk), 10Hz(50ms for a half period). This gives us AOM scanning velocity of 40MHz/s. (this is really a fast scan!)

We put PSD inbetween the telescope after AOM. Although this gives us diffcuilty, but this is the only proper place to put PSD. See attached picture 1 to know how can we propagate the dittering back through the lens.

The calibration we use the result of entry 276. It is 0.0071[m/V]. We use it to transfer the voltage change of PSD to the beam dittering in meter.(Then we got "x" in attached picture 1)

Conclusion: See attached picture 2. We have 10.7e-6[rad] angle change corresponding to frequency change of 2MHz.

Participants: Yuhang, Yuefan, Matteo

Today we tried again to couple the laser beam into the fiber.

But first, we moved the two steering mirrors and collimator closer to each other. Since the further the distance, every time the movement on the collimator input is larger.

Then by adjusting the two steering mirrors and the x, y screws of the collimator, we are able to reach 10V(maximum) on the photodiode which corresponds to 5.5mW which is even larger than the input laser power.

We found out this is because the oscilloscope has a high impedance which will amplify the power. So we add a 50ohm resistor and we could get 10% beam power coupled into the fiber and after that, it became difficult to improve more.

According to Thorlab website, this collimator should have 2.09mm at the output, but now with a 200mm focal length just after the laser output, we have a beam at the collimator around 400um in radius.

So we did a bit simulation, in order to get the right size of the beam we should combine one -100mm and one 200mm lens. We will try to change the setup tomorrow.