preparation for 1310nm laser power stabilization control loop

I checked the manual of the LD and Temperature Controller - CLD1015 
It is possible to modulate the current with an external source.
The input is a SMA connector on the rear panel of the controller.
Details on the modulation parameters are in the attached table (from the manual)

manual link: https://www.thorlabs.com/drawings/a2689db57be706ee-B083F81C-0501-84ED-9463F9E7DF4E37B5/CLD1015-Manual.pdf

1310nm probe - noise characterization

I checked the noise on the pd of the 1310nm probe laser. with and without sample. then I tried to switch off the chopper, to switch off the pump and to put the PD in the crossing point where the waist of the probe is inside the size of the detector. In all the cases the noise is around 100 uV. This means that the noise comes from the laser. So we are planning to make a power stabilization control loop on the laser.

Details and plots of the noise follow:

note that in the case of LMA sample with power=960mW, the SNR is about 1

with lma#15033 sample at the crossing point:
laser current 400mA; OD2 filter
pump on 960mW; chopper on
filename: lma15033crossing_point.txt

with lma#15033 sample at the crossing point:
laser current 400mA; OD2 filter
pump on 34mW; chopper on
filename: lma15033crossing_point-2.txt

with lma#15033 sample at the crossing point:
laser current 400mA; OD2 filter
pump off; chopper on
filename:  lma15033crossing_point-3.txt

with lma#15033 sample at the crossing point:
laser current 400mA; OD2 filter
pump off; chopper off 
filename: lma15033crossing_point_internal1kHz.txt

with lma#15033 sample at the crossing point:
laser current 400mA; OD2 filter
pump off; chopper off
filename: lma15033crossing_point_internal380Hz.txt

no sample
laser current 300mA; OD2 filter
pump off; chopper off
filename: 1310nmno_sample_no_chopper_internal380Hz

Beam characterization after the first BS before the first lens

I measured the beam dimension of it.

The reference point is the first lens(after the first BS). The beam waist is 60um and position is -0.576m.

According to this result, we designed the telescope. The purpose it to get a beam collimated and approximate 2mm. The simulation is in attached picture 2.

Galvo used for Automatic Alignment in TAMA

[Raffaele, Eleonora]

We have found in Tama storage room a box containg 9 so-called galvo, used to keep the beam centered on the quadrants in TAMA automatic alignment system. (Pic 1-2-3)

We have also found the NIM modules used to drive them (Pic 4) , which take as input the DC values of the four quadrants sectors.

In the attached pdf files the functiong of the system and the scheme of the board are explained in detail.

Note that the electronic can be used both for "plus" and "cross" disposition of the quadrant sectors, as both the configurations were used for TAMA quadrants. To switch from one to the other, some jumpers need to be added/removed (PIC 5). More details can be found in the document attached.

The box (pic 3)  has been currently stored in the bottom shelf placed near TAMA IMC end chamber.

The electronic modules are on the shelf in the entrance container.

Two galvos are still installed on the TAMA injection bench and still connected to their drivers.

AUX2 laser PZT gain characterisation

Participants: Marco, Eleonora

We have characterized the PZT gain of the laser AUX2 as function of the frequency.

The measurement has been done by monitoring (with the spectrum analyzer HP 8563E) the width of the beating note of the two axiliary lasers, when a modulation was sent to the PZT of AUX2.

The first attached plot shows the PZT gain as a function of the frequency from 1 kHz to 170 kHz. Its value is almost constant up to100 kHz. After that, the piezo resonanaces start to appear.  We remark that the gain at low frequency is about 1.8 MHz/V while from the laser spec we expected 1 MHz/V.

As an exemple, in the second attachment is shown the enlarged beating note observed when sending to AUX2 piezo a modulation at 60 kHz. The width of the beating corresponds to twice the PZT gain at that modulation frequency.

Test lock of green mode cleaner and the mechanical transfer function of it

Participant: Marco, Yuhang and Eleonora

During the last few days, we installed the RF photodiode for mode cleaner reflection. We achieved the lock and measured the mechanical TF.

For the power supply of RF PD, we used a T to seperate the power from CMS 100-05.

For coupling the beam into the PD, we put a 75.6mm lens before PD. In order not to saturate the PD we put a OD 2 to attenuated power. After the OD, we got power around 200uW. Now, we can get the sideband information(78MHz) and create the error signal.

The demodulation frequency(78MHz) of mode cleaner is the same with the demodulation of filter cavity. The demodulation signal(LO) is provided by DDS2 board. We also maximize the error signal by select the best phase (175 degree).

We use SR560 for a temporarily lock. We set a first order pole at 3Hz with a gain of 10. Then we measured the mechanical transfer function of green mode cleaner (cavity + piezo). It is attached as Fig 1. As expected, it is flat at low frequecy. The structures due to the piezo resonances start from about 7 kHz.

Polarization of beam going to green-mode-cleaner

I used the PBS to check the polarization. The reflection of PBS is 12mW while the transmission is 11uW. This means the polarization is good s-polarization.

Swap AOM RF input amplification to the amplifier box

Partecipants: Marco, Eleonora, Yuhang

Before:  the AOM inpu RF signal was amplified by a +15dB ZHL-2 Minicircuit amplifier. Its input signal is generated by a Tecktronix AFG351 Frequency generator and its power level was set to +11 dBm

After:  we connect the RF generator output to the first channel of the amplifier box. This contains a +37dB amplifier, thus its input power level was reduced from +11 dBm to -10 dBm.

Swap to DDS 2 Board

participants:  Marco, Eleonora, Yuhang

- Filter cavity modulation:

Before Tektronix AFG3102 Fout=78MHz Ampl=1Vpp (Measured with HP 8563E Power 2.83dBm Fout+78.017MHz)

After DDS2 Board CH0 Fout=78MHZ Full scale output (Measured wit HP: 8563E Power after amplificator 13.6 dBm)

I put a 6 dBm SMA attenuator and I set the output at 1/2 scale after it I measure 4.6 dBm at 78.3 MHz

 

- Filter cavity PDH demodualtion:

Before provided by Tektronix AFG3102 Fout=78MHz Ampl=9.5 Vpp (the mixer was a class 23 Minicircuit ZAY-3)

After DDS2 board CH1 Fout = 78MHz Full scal output. I measure 6.43 dBm after the amplifier.

We replace the mixer with a Class 7 Mixer Minicrcuit ZAD-1-1+)

 

- GrMC reflection PDH demodulation:

Now DDS2 Board CH3 Fout=78MHz Full scale output. I measure 6.43 dBm after the amplifier.

 

Note that the CH2 of the DDS 2 Board is not working

 

Phase optimization

CH0 = 0

Ch1 = 135 deg (before was 125 deg)

CH2 = not working

CH3 = 175 deg

RF amplifier box

Just to remind the amplification values of the RF amplifier box realized by Pierre Prat.

AOM channel: AMPA-B-34-20.425 -> +37dB
EOM channels: ZFL-500HLN+ -> +20dB
demod channels: ZFL-1000+ -> +13dB
PLL channels: ZFL-1000+ -> +6dB

IR beam realigned into the cavity

We have checked and quickly recovered the alignement of the IR beam into the filter cavity.

Characterization of the main laser beam for PLL

Participants: Yuhang, Eleonora

We have characterized the IR beam from the main laser transmitted by the first BS. We have measured the beam starting from the 48th hole (which is about 6 cm after the lens with f = 500 mm).

The beam is quite collimated: it has a waist of 0.67 mm ad a distance z = +2.9 m from the 48th hole. In the region around the 50th hole it has a diamiter of about  3 mm.

We plan to take a pickoff of the beam in this region and use it for the PLL. We will inject it into a fiber through a collimator. Accoring to the collimator characterization reported here, we need to prepare a collimated beam with a diameter of about 2 mm.

A possible solution for the telescope is to use two lenses with f = 75 mm and f = 50 respectively,  placed at a distance of about 12.3 cm from each other.  See in the attachement the jammt simulation.

Tests on the LMA coated samples

I did some tests with the 633nm probe on the LMA samples that I already measured some years ago with the original setup. elog entry: http://www2.nao.ac.jp/~gw-elog/osl/?r=141

SCANS

calibration 633nm: z=34.5mm  power 33mW; AC=0.2017V; phase=-103.8deg; DC=3.38V freq=375Hz filename=Fri, Jun 08, 2018 6-20-48 PM.txt

median filter order 5, average filter order 5

lasr current 1.3A, angle maximum IPC

lma15033 power=960mW z=34.5;  AC=0.000651 phase=-104; DC=4.52V  freq=375Hz filename=Tue, Jun 12, 2018 11-22-12 AM.txt
absorption = 16.4ppm (nominal 12.8ppm)

lasr current 4A, angle maximum IPC

lma15033 power=5090mW z=34.5;  AC=0.00305 phase=-105; DC=4.62V  freq=373Hz filename=Tue, Jun 12, 2018 3-46-17 PM.txt
absorption = 14.2ppm (nominal 12.8ppm)

lma15032 power=5130mW z=34.5;  AC=0.001407 phase=-105; DC=4.7V  freq=378Hz filename=Tue, Jun 12, 2018 4-12-54 PM.txt
absorption = 6.4ppm (nominal 4.5ppm)

median filter order 20, average filter order 20

lma15034 power=5130mW z=34.5;  AC=90uV phase=-110; DC=4.28V  freq=375Hz filename=Tue, Jun 12, 2018 4-49-39 PM.txt
absorption = 0.47ppm (nominal 0.65ppm)

laser current 7.5A

lma15034 power=10W  filename= Tue, Jun 12, 2018 5-18-22 PM.txt
absorption = 0.44ppm (nominal 0.65ppm)

MAPS

lma15033 power=960mW  filename = Tue, Jun 12, 2018 11-46-14 AM.txt
mean (std) on the map =  15.3 +/- 1 ppm

lma15034 power=10W  filename = Tue, Jun 12, 2018 6-08-48 PM.txt
mean (std) on the map = 0.43 +/- 0.08 ppm

Collimator alignment

Participants : Yuhang, Eleonora

By sending the fiber output to the collimators (PAF-X-7-C) and (PAF-X-11-PC-C) we got the output beam and we could adjust the z of the collimator to make the beam collimated, and x and y to make it go straight.

 We used beam profiler to measure the dimension of the collimated output beam:

•  PAF-X-7-C  has a diameter of  2000um
•  PAF-X-11-PC-C has a diameter of  3300um

We put a label on the collimator holders to specify the beam diameter that we measured  (that is the dimension of the collimated beam that they need in input).

They are now well aligned and should not be touched.

More info on the collimator mechanics and aligment procedure can be found here:

https://www.thorlabs.com/drawings/95fa854d421b0d3c-A5221257-C242-648D-DA41342A6D75B16A/PAF-X-11-PC-C-Manual.pdf

Useful tip to adjust the z of the collimator: if the beam is diverging the z screws should be turned counter clock wise, if it is converging they should be turned  clockwise.

Comment to Broken collimator (Click here to view original report: 809)

A screw identical to the broken screw have been found in the collimator box. This screw is likely the lock screw for the XY position of the collimator lens.
This spare screw has been used in place the broken one and the collimator has been riassembled and it seems to work.

Filter cavity lock recovered

Partecipants : Yuhang, Marco, Eleonora

During the auxiliary lasers installation performed in the past week  the green beam got a bit misaligned and  the filter cavity was not locked since then.

Today,  in order to recover the lock, we have at first realigned the green beam, than we have zeroed the error signal for the local control of the input mirror since the stanford was saturating.

Note that the last time we had to zeroes the local control was more than 2 month ago.

The SHG temperature control was set to 3.175 kOhm and we injected about 8.5 mW of green light  into the viewport.

The local control offsets for the optimal alignment were found to be

	pitch	yaw
BS	0.3	-0.07
PR	-0.76	-0.34
IM	-0.95	0.45
EM	-1.73	2.82

The transmitted power is about 1.6 V.

The IR beam on the other hand is not aligned anymore, since we are going to take a pick off to be used to the aux lasers PLL.

Aux2 (p-polarization) installation and fiber coupling

Participant: Eleonora, Marco, Yuhang and Matteo

Input beam: waist of 192e-6 m at a distance -0.2 m from the laser head. Characterized here.

The input power is 232mW. After put two waveplates, the power becomes 228mW.

Telescope

First lens (f = 50) at a distance 13.5 cm from the laser head.

after the lens we measured  a beam with a  waist  of  33.6e-6 m at z = 0.188 m from the laser head

Second lens (f=175) at a distance 39 cm from the laser head.

after putting the second lens, we adjust it and make it collimated. The beam diameter now is 3800e-6m.

Faraday isolator: input power 227mW output power: 202 mW  Throughput 89%

Semireflective mirror: input: 180 mW  reflection: 174 mW trransmission 2.7 mW

Fiber collimator: input power 2.6 mW output power 2* 0.88 mW.  Power coupled 68%

Assembly of OPO (part 1)

In this report the first part of the assembly of the OPO cavity. For the drawing refer to file "assiemepdopov3asm.pdf". The pictures have been uploaded on drive at the following link: https://drive.google.com/open?id=1bjSdCoMSOHeWpJarAu0ZhKGuFZZME19h

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Step 1: check peltier (21) hot/cold side. In order to check which side of the peltier is the hot or cold one, the peltier has been powered up and the sides have been checked manually to determine which is the hot and which is the cold one. For this particular type of peltier, the cold side is the one with text (XF29) while the hot one is blank. See picture 1 and 2. Since we need the peltier to heat our crystal up, the cold side has been mounted in contact with the OPO assembly baseplate (6) while the hot one will be mounted in contact with the copper lower L (8). In order to ensure a good thermal contact between the components, a sheet of 0.1mm indium has been placed in between the peltier and the copper L.

Step 2: mount the thermometers (24) on the copper lower L (8) with the retainer (22, 23). See picture 3 and 4. After mounting the thermometers, bend the wiring of the thermometers and ensure the wires go in the groove, see picture 5.

Step 3: apply indium on the copper lower L and on the macor upper L (9). See picture 6.

Step 4: place the PPKTP crystal (7) in position. See picture 7. The PPKTP crystal mounted is the one with part number 7-11159316418, see picture 8. The crystal has a black dot on one side. I suppose this indicates the HR side, so the crystal has been mounted accordingly, see picture 9.

Step 5: assemble the lower and upper L on the peltier element and close the assembly with the POM bridge (5), see picture 10. Before doing this step, make sure the two POM tipped set screw (27) have been completely relased. While mountin the POM bridge be very careful about the crystal positioning. During this part one issue has been found and solved, see "Problem 1" part.

Step 6: fix the crystal position acting on the POM tipped screws. See picture 11. On this step be very careful not to brake the crystal. Alternate acting on X and Y screws, starting from the Y one. If you see the macor L starts tilting up, while acting on X, act on Y to fix the tilt. The final position of the assembly is when the peltier, and both the lower and upper L touch the left side of the POM bridge.

Step 7: mount the crystal holder retiner (25). See picture 12.

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Problem 1: during the step 5 of the assembling procedure an issue has been found. The Hexagon Socket Low Head Cap Screws (26) was not a "Low head cap screw", therefore this screw was interacting in a bad way with the POM tipped screw for X alignment, and was preventing the complete relese of this screw, necessary to start step 5 (see picture 13). This problem has been solved substituting this screw with a different screw with lower head cap (see picture 14 and 15). This solution allowed the release of the POM tipped screw for X alignment without problems.

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List of picture:
01 - peltier cold side
02 - peltier hot side
03 - copper lower L, thermometers and retainer before assembling
04 - copper lower L, thermometers and retainer after assembling
05 - lower L with thermometers in place and termometers wires bended
06 - indium applied to the lower and upper L
07 - crystal on the lower L
08 - crystal part number
09 - black dot on the crystal side indicating the HR (??) side
10 - POM bridge assembled
11 - crystal positioning using the POM tipped screws
12 - part 1 of OPO assembly finished
13 - problem with compenetrating screws
14 - new screw compared to old one
15 - compenetrating screw problem solved

Broken collimator

Few days ago we noticed that one of the two 11mm EFL collimators was behaving in a strange way. In particular the lens was not moving along X direction. I unmounted the collimator and found that the screw for X movement was broken (see picture 1 and 2).


NOTE: if you look for the collimator into the Thorlabs site the part number is PAF2P-11C. Thorlabs changed very recently the part number of this item from the one written on top of the collimator to this new one.

damping the pump reflection

- Since the pump is now at 2° incidence, the reflection from the sample was hitting the mount of the periscope. Without moving the periscope, I collected the reflection with a mirror and I redirected it outside with another mirror through a hole in the enclosure. Then I put a blackhole damper to absorb the power. I decided to bring the power outside to avoid heat generation inside the enclosure that may cause temperature fluctuations and noise at low frequency on the absorption signal.

- I also added a shutter after the chopper, to be able to close the pump for few seconds (not more otherwise the shutter melts) and be able to move the sample away from the pump path without burning the sample mount.

- The pump path at the ground level of the optical table was covered with a black paper enclosure. I replaced it with black anodized aluminum panels to reduce the generation of dust.