The intensity noise that I have noticed after the EAOM installation disappeard with the use of a PBS before it.

Has to be mentioned that the light coming from the EAOM becomes eliptical with following power ratios at the PBS after theEAOM:

Pinc = 1.59mW (p-polarized);

Ptrans = 1.30mW (p);

Prefl = 0.25mW (s);

We were steering the beam transmitted from the cavities with use of lenses. I have realigned (not easy thing with such space constrains).

Intensity noise is way better, although the overall noise did not improved as the intensity noise is not the limiting noise. I think we need to suppress

more free-running noise. Actually I will go back to our FSS servo because I think we need different performances (not for now).

I have accidentally misaligned the north cavity, better not to say how :-);

The north cavity is now in place with 48% visibility! 

Table height

Currently the height of the optical table is 27.5" (see attached PDF). We would like to increase its height.

The legs are provided with a spacer http://www.newport.com/Pneumatic-Vibration-Isolators-with-Automatic-Re-Leveling/844255/1033/info.aspx 

which allow us to increase the height just by replacing the spacer (2.5").

The new spacers bring the height of the legs up to 28" (or 23.5") and the total height of the table up to 40"-41" (or 35"-36").

I ahave asked a quote for spacers which make the legs height equal to 28". They can come with flanges for horizontal bars. I have asked a quote

for this too.  We need to decide if this height is fine and if we want horizontal bars.

This morning I sent an email to Evan in order to obtain PMC documentation. 

For now the only documents found is a mechanical drawing E1400332. I am aware of additional 2 documents that he wrote for this design. I also asked for the electronic/servo

instructions as I am not aware if he built/bought parts for the PMC servo.

This morning I have also poked Rich, in order to have advices on what type of epoxy we need to buy for pzt gluing. Not sure about that. I am going to investigate

on what to buy and especially what should be the best procedure for gluing.

We have been collected the materials bought from Evan:

1. Spacers;

2.  Mirrors;

3. PZTs;

Things to buy:

1. O rings;

2. Epoxy for PZT;

3. Screws;

I am considering to use the table in the TCN lab, once we have a functionally lab. All the information about the PMC will be collected in a section on the PSL wiki page.

O rings and screws have been ordered!!!

This is my short time plan rough proposal for the period left of Tara's stay.

1. PMC number 1

- Locking

- Transfer function measurement

2. PMC number 2

- Find the 00 mode

- PZT test

- Finesse measurement (roughly)

- Locking

- Transfer function

3. Glass PMC (pzt broken)

Need to take a decision. How to remove the pzt? Do we want to give a try?

4. Additional PMC

- Do we want to assemble other PMCs?

One of the last few steps left was to put back the sprinkler. Much more complicated and longer than I thought, but

It is Done!!!

Following the previous elog entry today we succeeded in locking the PMC.

As a reminder this is only a temporary setup. The optical path need to be re-design in order to allow

the implementation of the PMC together with mode matching lenses.  

This is the list of things we did today:

• Setup RFPD on the table for the PMC
• Added the EOM on the path (may be temporary, as the mount is not ready)
• Connected the LO to EOM and the PMC card
• Checked Error signal
• Locked the cavity

The PZT of the glass PMC does not seem broken (at least looking at the FSR).

I have implemented the PMC glass again, in order to verify if the PZT has enough range and or if it was functional.

We thought that it may have been broken and we were in the process of removing it from

the glass spacer. It is not the case yet.

Once I have got the 00 mode (South path) I have scanned the PZT of the PMC and I have seen 2 entire FSR. 

However I have noticed that the South laser drifting is very high. I had hard time keeping the

scanning around the desired mode. The other PMCs have beeen tested in the North path

and the laser drift was way less than what I saw in the South path.

Summary

Analysis of the frequency modulation for the FSS loop has been done (may be I have repeated it )

as we want to increase the frequency modulation from the current 21.5MHz to something

bigger than something like 30MHz.

Higher order mode analysis

I have considered a cavity of length L=3.45’’=3.68cm and radius of curvatures R1=R2=1m.

1.

Plot 1 shows the frequency distance vs frequency modulations. I care that HOM (Carrier and

SBs) do not overlap with carrier and SBs TEM00. The closest mode is the number 23 which

interfere with the positive SB around 31MHz. I do not consider this to be a problem, but I go head

anyway. At that frequency we also see the mode 46 crossing the the SB (TEM00).

2.

In order to better visualize the distances shown in the previous plot I use the minimum distances from the TEM00 (carrier or SBs).

The minimum distances are referred to the closest mode which may change along the SBs scan.

We see that on 31.7mHz we have a minimum as noticed previously and it is because of the mode 23.

From here if we would like to avoid the mode 23 I would choose a frequency modulation of ~39MHz.

3.

I fix the frequency modulation at 39MHz

I wanted to check if the mode 23 can be “avoided”. I consider the tolerance on L as L+- 0.0005 and Rc of +-1cm

There is a chance that the mode 23 overlap depending on what the variations from the nominal values are.

However the same is going to happen if we fix the Frequency modulation at 35MHz. So between  35MHz and 39MHz

I do not see substantial differences. (Please note that the color are normalized at 1MHz detuning from carrier).

4. While the scan of L and Roc has been done I have checked that no other HOMs come into play. The mode 23 and 46 are

the modes which determins the red lines at the point 3.

5. Mode distances at 35MHz and 39MHz.

From what I see here 35MHz is fine as mode 23 is the closest one to TEM00. As a reminder the pole of the cavity is ~180kHz. In principle 35MHz would be 4MHz

apart considering the nominal values L=3.68cm and RoC=1m.

North Path

SUMMARY

Thigs done (Following the attached schematic):

1. The lens (77.3, f = 171.9mm) before the FI has been removed and placed after it;
2. The FI is closer and a bit tilted as the the back reflection was going straight to the laser;
3. lambda/4 and lambda/2 have been placed in the path before the FI and have been rotated in a way that at the output of the FI we have maximum power (~0.7W);
4. A lens (77.3) has been placed after the FI;
5. Lambda/2 has been placed in order to create P polarized light;
6. A high power PBS (PBSO borrowed from the 40m) has been implemented for power adjust;
7. Lambda/2 after the PBSO in order to create s-polarized light;
8. Beam has been measured in the area after the lambda/2 (distance reference) (as shown in the picture);
9. A BroadBand (BBEOM) has been placed with waist in the middle with resulting beam aligned in plus menus 10 micro meter range (This means that looking at a reference point with the ccd camera, after BBEOM was placed and aligned the beam was hitting the same point. ) The power of the incoming beam was 320uW while the power of the outcoming beam was 310uW.
10.  A lens (51.5, f = 114.5mm) after the BBEOM has been cleaned and placed (This lens has a little scratch);
11. A steering mirror (s) has been cleaned and implemented;
12.  A lambda/2 has placed after the mirror in order to have p-light;
13.  The AEOM has been implemented;
14.  Lambda/4 has been implemented and rotated at 10deg in order to have circular polarized light;
15.  PBS has been cleaned and implemented;
16.  Lambda/2 has been implemented in order to give s-polarized light (45deg);
17.  A lens (51.5, f = 114.5mm) has been cleaned and implemented in order to create the right beam for the resonant EOM to be used for the PMC sidebands;
18. Beam profile has been measured: the fit is not ok but the data are indicative enough (waist of ~ 100 um);
19.  A steering mirror (not labeled) has been power checked (“equal” for both s and p) and implemented;
20.  The resonant EOM (21.5MHz) to be used for the PMC sidebands has been implemented with alignment precision at the order of tens of microns (incoming power was 315 uW while the outcoming power was 306 uW);
21. Steering mirror implemented;
22.  Currently we are in the process of implementing two lenses to mode match the PMC;

Things to note:

1. The beam coming out of the laser is not perfectly at 3 inches height;
2. The resonant EOM used for the PMC will be removed while we take measurement without PMC;
3.  The beam coming out of the laser is very dirty;
4. There is a specific lens mount which fails to have the optics center at 1 inch;

Next step:

1. Mode matching for the PMC;

Attachment:

1. Schematic 

     2. Beam profile after l/2 (see point 8, zero is at l/2)

   3. Beam profile measured before EAOM (zero at the steering mirror)

4. Beam profile measure before EOM for PMC (zero reference at lens (51.5))

(to be updated: the fit is failing)

I have ultimated the work started few months ago about sealing the enclosure with the tape to avoid air air flowing from the top.

The work has been completed.

Summary

I have added two lenses in order to create a waist of 330um for when the PMC will be implemented.

Not satisfied about the size of the beam. This has to be improved, but we are close enough. I am thinking

to put the PMC in place and mode match the laser beam to the PMC, in order to have a better reference.

-------------------------------------------------------------------------------------------------------------------------------------

The two lenses are 114.5mm and 401mm focal length rispectively, as shown in the attached schematic.

At moment the beam for the PMC (after the two lenses) is the following:

In line with what we already started we decided:

1. Build the North and South optical paths by July 17th (12:59pm) (Antonio North path for now, Andi South path) 

• Both the paths must to be able to host the PMC, bb-EOM, AOM, and 2 resonant EOM in each of them

2. Week of July 18th beat noise measurements with what we have;

A summary with a slight different timing (written before the meeting) is in the following PDF (sorry for the weird layout)

Summary (Entry 1633 revised)

Analysis of the frequency modulation for the FSS loop has been done 

as we want to increase the frequency modulation from the current 14MHz to a

higher frequency in the range of 25 to 45 MHz in order to increase the FSS  unity

gain frequency. Below 25MHz we do not have too much benefit as currently we

are modulating at 14MHz. Above 45MHz we have photodiodes noise and we want to 

deal with that.

Higher order mode analysis

I have considered a cavity of length L=3.45’’=3.68cm and radius of curvatures R1=R2=1m.

1.

Plot 1 shows the detuning frequency vs frequency modulations. We care that HOMs (Carrier and 

SBs) do not overlap with the carrier TEM00. The closest mode is the number 23 which

interfere with the TEM00 around 31MHz. I do not consider this to be a problem. The morde order

is ment to be as the sum of n and m. The legend shows color of the modes in a dashed style. However

each mode order is showed with carrier (solid line) a positive SB (dash-dot) and negative SB (dashed).

From this plot we can see that in the range 25MHz to 45MHz we are pretty safe from HOMs overlaps.

If we care of the mode 23 than we should consider modulation frequencies above ~35MHz at least.

I consider a cavity pole of 135kHz (see elog 1480 https://nodus.ligo.caltech.edu:8081/CTN/1480)

2.

Plot 2 shows a zoom-in version of plot 1.

3.

In order to better visualize the difference between the detuning frequency from TEM00 and the closest HOMs (either carrier or sidebands) 

I provide the following plot. Please keep in mind that in general this way to plot the differences does not take in account of

which mode order is the closest. However in this case it is only the mode order 23.

We see that at 31.3MHz we have a minimum as noticed previously and it is because of the mode 23.

Choice of the Modulation Frequency

Currently in the TCN lab we have two oscillators; one is at 36MHz and the other one is at 37MHz; I was tempted to use them

but Rana suggested to have 2MHz separation between the two modulation frequencies in order to avoid to see noise in the beat note 

after the demodulation process.

So if we want to use one of them we can pick 37MHz and buy a 40MHz one.

If we have to buy two of them anyway we can go for the same 37MHz and 40MHz or shift a bit the frequencies to 39MHz and 41MHz.

I would pick 37MHz and 40MHz leaving the option to buy or not buy the second 37MHz.

Before buying all the parts and do all the work which is consequent at this choice, it is good to have some comments/opinion!

In order to have a frequency stabilization system (FSS) working with a new frequency modulation (FM)

I collect some notes of things that need to be done;

In vacuum cavities

1. Choose the FM for both South and North cavities;
   1. Currently we are using ~14MHz;
   2. PMC are not into the path;
2. Buy the oscillators;
3. Modify the photodetectors to the right frequencies or build new ones;
4. Buy one BB EOM
   1. Currently we have a resonant ~14MHz and one BB with an external resonant circuit;
5. Build two resonant circuits for the BB EOMs: one applied to what we already have just replacing the external circuit and one to new BB EOM that we will buy;
6. We need to buy (perhaps) a HV driver;
7. Later we may need to modify the FSS servo board;
8. (...please add if something is missing...)

PMCs

In parallel and/or later in order to implement both the PMCs in the North and South path we need:

1. Buy one BB EOM;
   1. We currently have a resonant 21.5MHz;
2. Build a resonant circuit at 21.5MHz;
3. Build a resonant photodetector at 21.5MHz;
   1. We currently have one at that frequency;
4. Build two servo boards for both PMCs;
   1. We may have one available;

Summary

A couple of mode matching solutions for the north cavity that look promising have been found.
I'm thinking the following plan 2c to be implemented.

introduction

- We'll use the PMC in each path
- We also need to insert a FI in each path (to use the reflected light from the cavities)
- EOMs after the PMC will be inserted for the in-vacuum cavities PDH

These constrains the mode macthing to be done with the use of three lenses. One to focus the beam
into the BB EOM and the other two to mode match the North cavity.

Approach

Question1: Can we place the FI without changing the mode matching solution?

Answer: In Plan1 the profile caluclation is shown. As you can find there, considering that
the maximum distance from the waist of the PMC for the FI is ~ 1.32m (FI length ~ 10cm)
we have a diameter around ~2mm.

Question2: OK. We need to use some focusing lenses. How strong will they be?

Answer: In Plan2-a/b/c the profile caluclations with the sensitivity to the displacement of the lenses
 are shown. As you can find there, we have at the FI a beam radius that is around ~750um at most.
However the three setups show a different sensitivity with Plan2a and 2c being lens sensitive to lens 
displacement compared to 2b.

Conclusion

Along with the discussion above, I will use 2c. (or 2a looks applicable too)

For all the profiles the reference point is the waist at the PMCc (330um), zero point in the plots
and the cavity waist is located at 1.96m far from the PMC waist.

Plan 1: FI after the two MM lenses

Setup

In this case we have 3 lenses of focal "fi"(mm) located at "li"(m):

l1 = 0.16;

l2 = 0.762;

l3 = 1.107;

f1 = 0.1432;

f2 = 0.688;

f3 = 0.572;

Beam Profile

If we want to "risk" a 2 mm diameter beam into the FI.

The following plans 2(a,b,...) have all the FI in the between the two MM lenses. I just report

one figure with the setup:

Plan 2a:

Setup: 

l1 = 0.183;

l2 = 0.773;

l3 = 1.32;

f1 = 0.229;

f2 = 0.143;

f3 = 0.229;

Beam Profile

We can place the FI around 1.2m or further than the North cavity. The further we go and the smaller is the beam

until we reach the waist at 0.9m.

Sensitivity of lenses displacement

Dipending on which directions we move the lenses we may have 20% mismatch with a displacement of ~4cm for lens 2

while for Lens 3 only few percent.

Plan 2b:

Setup:

l1 = 0.117;

l2 = 0.951;

l3 = 1.313;

f1 = 0.229;

f2 = 0.143;

f3 = 0.143;

Beam Profile

Sensitivity

Plan 2c:

Setup

l1 = 0.148;

l2 = 0.683;

l3 = 1.325;

f1 = 0.1432;

f2 = 0.1719;

f3 = 0.229;

Beam Profile

Sensitivity

This shows to be less sensitive to the lens displacement, compared to the others. Furthermore we notice that the two lenses are somehow indipendent as the curve are more circular;

There is something not ok with the Gouy phase, I need to cross check what is wrong there. I should fix this doubt later, for now it is not important.

I would propose plane 2c

Data:

Data are currently stored in a shared Box Inc folder, but we may wanto create an svn folder for all our data.

Motivation
I have realized that I made a mistake measuring the distance of the North cavity waist from the PMC (waist).
I have done the same analysis presented in ID 1660 and adjusted the plan.

Conclusion
A new set of lenses has been found respecting all the constraints given in ID 1660
 

Setup
The waist of the North cavity is located at 1.701m far from the PMC waist. The size is 210um.
The chosen lenses focal lens f"i" (m) located at l"i"(m) are:

l1 = 0.136;

l2 = 0.489;

l3 = 1.041;

f1 = 0.114;

f2 = 0.114;

f3 = 0.229;



 

1. Beam profile

2. Sensitivity to lenses displacement






 

North Path

Summary
Following entry ID 1649, additional work along the North Path has been done

Conclusion
We have the light into the North cavity mode mateched and aligned to TEM00 
with a visibility {(Vis  = Vmax-Vmin) / Vmax} of ~71%.

Next Step
If FOR THE MOMENT we accept this mode matching I will hook up the electronics
and work on the North cavity locking, leaving modematching improvments for later.

Things done
1. PMC has been placed in the path and aligned. Visibility was about ~90%;
2. PMC has been removed;
3. A PLCX-51.5-UV lens has been cleaned and placed in the optical path soon after the PMC;
4. A resonant EOM (14.5MHz) for PDH sidebandshas been placed and aligned:

• Power at the input was ~3mW, at the output ~2.95mW

5. Two steering mirrors have been cleaned and placed in the path;
6. Two mode matching lenses have been placed:

• A PLCX-51.5-UV lens has been placed in a location that few inches above the nominal
  found in the design (ID 1661) ;
   
• A PLCX-103.0-UV lens has been placed in its nominal location;

7.  In between the two lenses a FI (IO-5-1064-HP) has been aligned:

• May be just for now, I have rotated the input polarizer in order to have light rejected from the cavity parallel to the table;
• The output polirizer has rotated while the FI was in reversal mode in order to optimize the isolation;
• A lambda/2 has been placed before the FI to maximize the output light;
• Input power ~ 2.98mW and output power~2.83mW;
• Some info about the FI: 
  https://www.thorlabs.us/newgrouppage9.cfm?objectgroup_id=4915&pn=IO-5-1064-HP
  https://spie.org/samples/PM200.pdf

8. The TEM00 has been found by scanning the PZT of the laser with a triangular wave (+-1V) at 10Hz through an HV amplifier set at ~40V;
    Additionally a voltage calibrator has been used to actuate on the crystal temperature of the laser in order to localize the are to be scanned with the PZT.
 

9. A Lambda/2 has been placed after the FI in order to have one linear polirized light:

• With light at ~45degree I have seen 2 TEM00 resonance not exactly the same;
• By rotating the waveplate we can have purely one or the other resonance (not sure yet at which angle);

10. A lens and a steering mirror have been placed in the path of the rejected light from FI to monitor the visibility/resonances with the DC output of the photodiodes;
    For now the visibility is at ~71%

11. This is the current setup:

That is perfectly in agreement with my analysis.

36MHz and 37MHz are good according with our analysis if 1MHz separation between the two modulation frequencies is good enough;

If 1MHz separation is not enough than we should consider 34.5MHz and 37MHz. Here the risk is to get close to the order 23 (n+m) at

a nominal detuning frequency that is around 2.2MHz.

Summary
Following ID 1649 we start the implementation of the South cavity path.
Some optics has been placed along the path.

Conclusions 
A quick beam profile measurement and the implementation of the BB EOM
is the next step.
 

Things done:

• Optics as shown in the setup have been placed;
• References of the beam along the path have been aligned  (Those should be kept in place)

Notes:

• The FI is mounted on a base which brings it NOT at 3" height; Two steering mirrors have been used to overcome this issue;
• There is a second beam coming from the FI; I did not have a chance to play with it, but Andrew did and the origin of that is not totally understood;

Setup

Summary
Following the previous entry the South path construction was at the point where mode matching was needed for the South Cavity.
We also needed to install a resonant photodiode for the PDH lock and start to prepare the optic in transmission.

Coonclusions

• The light is resonating into the South Cavity with a visibility = (Vmax-Vmin)/Vmax = ~ 0.75. 
• We also have the resonant PD for the light reflected from the cavity.
• Camera and ISS PD in transmission of the South cavity have been aligned too.
• All the cabeling are hooked, but they need to be tidied up.

Note:

1. Lenses have been adjusted and slightly changed compared to the previous entry.
2. The alignment of the cavity took me a while. The mounts for the steering mirrors on the
   periscope are very sensitive, here the alignment requires a bit of more attention. 
3. All power supply and cables are connected. I just need to verify the HV power supply settings.

Summary
Today I have aligned the South cavity with the beam prepared and described in the previous ID entry.

Conclusion
The South cavity is aligned and mode matched with a visibility (Vmax-Vmin)/Vmax of ~75%.

Things done today:

• Aligned the beam in order to get the TEM00 mode resonating into the cavity;
  The resulting visibility is of 0.75;
• Prepared and connected all the power supplies to their components for the south FSS lock;
• South Cavity locked; The lock happened this morning and it is still locked; (1mW Pin).
• Realigned (vis = ~ 0.7) and reset all the polarization in the North path, because for some unknown reason
  the North cavity was not anymore in the same place of two days ago, not even close; the waveplates too.
• Worked on the locking of the North Cavity as it was not showing robust; The North cavity stays locked for few hours now.
• Started to preapare the Optics for the PLL:
  - Photodiode for the beat note, mirrors and beam splitters alignment;
  - Mirrors and beam splitters alignment;
  - Alignment of the transmitted beams from the cavities into the beat note PD together with Andrews
    The two beams are visually overlapped now;
  - We checked the polarizations at which the two cavities are locked currently:
  North is locked on P;
  South mostly on S (not totally);
  - I and Andrew decided to use 2 lambda/2 at the output in order to be able to change the linear polarization whenever
  we need. The optics at the output are mainly made for s-light;
• Photodiodes and optics for the ISS are aligned too;
• I have connected the power supply for the temperature (output=10V);
• Started some tyding up the table;

We can start to look at the beat note!

Summary

• Today we have worked reying to improve the beat note signal in its amplitude.
• We also connected all the parts for the PLL loop and tryied to lock the PLL but we did not succeded;
• We noticed at some pint a noise at the input of the sr560 which pheraps prevents the PLL lock
  
  
  Conclusions

           We need to improve the lock of the cavities in order to get a more stable signal and solve the origin
           of the noise at the input of the sr560 in order to lock the PLL.

Today we have tested the second PMC that we have assembled.

We have obtained the 00 mode;

We measured the finesse (~300);

We have scanned the PMC PZT and we see 3 FSR;

We also noted again the presence of a second 00 mode and we confirmed that it was due to laser mode hopping. This has been checked

by injecting a voltage (0.2V) in the temperature input of the laser. Once the voltage was applied the second mode disappeard.

We think that it is enough PMC testing for now. I am tempted to assemble a third PMC, while we should try to fix the glass PMC.

We need to work on the lab organization too.

Yesterday the DC transmission (ISS photodiode, North cavity) channel to acromag has been added and the viewer has been set on the Ubuntu machine. The channel has been also calibrated (against voltage externally injected). We are going to add other channels from the VME crates and work on the PDH board interface in order to remote control this unit.

Additionally the floor has been sweeped and mopped.

We figured out that when we sweep the laser PZT and we want to look at the error-signal, this can be monitored at the "mixer" output of the FSS interface board while the switch test/off/ramp is on ramp. However we need to understand what this switch does when it is OFF.

Summary
Following ID 1649 & 1663 electronics for the FSS of the North path has been hooked up.
The North cavity was ready for locking today.

Conclusion
The error-signal of the North PDH does not look right. The cavity can be locked but it is
not an easy operation for now as it was before. We need to investigate on the error-signal
and nail down the reason.

Notes:

• Initially the err-sig had a weird shape and a low SNR (~2), later it became for unknown reasons a proper
  PDH with still the same SNR.
• Attempts to lock the North cavity have been done anyway. Without driving the BBEOM 
  cavity stayied locked for half an hour. 
• The error signal was taken at the monitor of the TTFSS and at the FSS box on the table (out1 and out 2); they were similar

Motivations
We had problems with the error-signal monitor from the TTFSS board. We thought that something 
was wrong with it because the SNR was very small. Before starting the process of nailing down what was
wrong with the electronics I wanted to put some effort to reach a good lock again because I suspected
that the monitor was wrong. We need to check it (later)!

Conclusions
The cavity now it can be stably locked for half an hour.

The cavity was initially easy to lock without the EOM. Few crucial things have been done to make the lock:

• Lower the power entering the cavity from 3mW to 1mW;
• Inverter on "-" on the FSS box on the table;

With this changes the lock was possible but the control-signal was very noisy;
 

Thanks to Andrew, he added a lambda/4 before the BB EOM to remove additional circular light (after the PBSO).
The quality improved considerably.

I have then twicked the gain Knobs in order to have a better lock. The lock is satisfying (fo now!). Aidan will
provide us of a feedback on the temperature too. Then the lock will stay longer.

Some settings:

• It cannot be locked with boost ON.;
• Inverter on FSS box on "-";
• Common = 600; Fast = 750; Offset = 550;
  The Offset has been adjusted in a way that after the lock the cavity was on resonance (monitoring the DC channel of the PD in reflection);
   
• 

Summary
In the last two days we have been searching for beat note, in order to get 
our "first" noise measurement of the PLL. While we monitor the beat note of the lasers
at the input side we search the same beat note at the transmission side.

Conclusions
We did not succeded yet. I start to believe that what we see at the input side are
not the beat note we are looking for. Some mode hopping could be happening around
the temperature region we have chosen.
 

Things done

1. We built a Mach Zehnder interferometer at the input side of the lasers, in order to fined the beat
directly from the two lasers; The beat note were found around 450MHz.

2. Once we found them we changed the temperature of the vacuum chamber and we locked the cavities
whit laser beat note of 78MHz.

3. Trasmitted beam have been checked for their spatial overlap and polarizations;

4. Photodiodes have been changed in order to see if some problem was coming from there;

5. Network analizer have been changed in order to check if there were wrong settings.

6. PLL rialigned from beginning;

NO RESULTS

PLAN 
We may want to know where mode hopping happen; We need to scan the temperature and  first measure
the power and see where they are. 

Looking for other beat note at the lasers.

Today Eric provided his Python scripts (and installed them) needed to connect the SR785 and the AG4395A devices with our lab computer through the GPIB interface. With these scripts we are able to download measurement data that we take by using the two above mentioned devices, plot them and set the measurement settings directly from the computer. Mainly we need to use two of them, i.e. with following commands:

1. AGmeasure: AGmeasure --getdata -i 10.0.0.13

2. SRmeasure: SRmeasure --getdata -i 10.0.0.13

These scripts can run from any folders on the computer. These and some other features will be explained in the TCN wiki page, which I am going to write soon.

We now need to make the lab computer accessible from other computers. The SSH protocol is iinstalled, but the modem need to be configured. Less attaractive but a possible option is to have a svn folder on the lab machine.

The work that we need to do for the TCN experiment is at the state where we cannot easily go further without improving the lab organization.

I have collected the optics (1064nm) and made an inventory which is on dokuwiki: https://nodus.ligo.caltech.edu:30889/ATFWiki/doku.php?id=main:experiments:psl:optics. At moment is little but I hope we can keep this updated and we can follow the policy that is written there.

The inventory is shown in the picture.

• Yesterday and today some cleaning has been done by Eric and Aidan, removing big useless parts. The rack on the left is free now.

Oi mate, seems like you've gone and got stuck in the FSS. Get out and start buzzing those mounts!

Hey Bono do you like awade's song

Grating choices:

In the literature, people seem to use 1800/mm spacing. Attached are efficiency graphs found on Thorlabs for 1200/mm and 1800/mm spacing. At 1064nm (which is where we are interested in tuning to), the efficiency drops off for the 1800/mm spacing grating. This could be fixed if we use an angle close to perpendicular, or we may be better off using the 1200/mm grating, which has an average efficiency of about 35% at 1064nm, vs an average of 20% for the 1800/mm grating. It is a question of efficiency vs. resolution. 

Current Controller:

We are currently examining a current controller designed by Libbrecht and Hall (1993), since it has been shown to have lower noise than before. There is a commercially available controller based on the design from their paper (http://www.vescent.com/products/electronics/d2-105-laser-controller/). I did some literature search and it seems that there was a design by Erickson (2008) which is an improvement on the design by Libbrecht and Hall. This paper is attached... we may try to use this design instead. Tara and I will meet on Thursday to determine if the requirements we want to have on our experiment to have low noise, and then choose whether to pursue the Erickson controller further. 

 Updates from the past few days...  (sorry I forgot to elog but I've been keeping notes in my notebook)

Monday 6/17

I met with Tara to discuss how to get started. In order to create a useful ECDL, it must be capable of meeting the noise requirements of the experiments it is being used for. Thus, we need to see the possible experiments that we could perform to test our ECDL and choose which would be the best goal to aim for this summer. I created a power spectral density (PSD) noise plot of various experiments performed at LIGO and some noise plots from ECDLs created by other research groups. We decided to use the crackle experiment as our standard since it has the highest noise requirement and thus will be easiest to achieve at first. 

List of things to order (links):

• laser diode
• laser diode mount with TEC built in
• diffraction grating
• diffraction grating mount: need to figure out
• current driver

Tuesday 6/18

General SURF requirements: I attended laser safety training for class 3b and class 4 lasers in the morning. I attended orientation activities for SURF for the latter half of the afternoon. 

In terms of my project: I remade the PSD noise plot in Matlab since this is the language I will be using (attached). The next step is to determine how to design the ECDL so that the noise requirements from the crackle experiment can be met. I met with Tara and discussed how to start doing noise calculations using the 1982 Saito paper as reference. 

Wednesday 6/19

I attended the LIGO internal orientation and general safety training as part of the SURF requirements. Then, I continued to try to perform the noise calculations. My noise calculations will need to take into account intrinsic noise of the laser due to current and temperature fluctuations. The noise is then decreased by some factor determined by parameter X (from Saito paper) by the external cavity. I am starting with the parts from Monday's list to see if they can be arranged in a way that meets the noise requirements. I was able to determine that parameter X for a setup with 10 cm between the laser and the grating will be about 71. 

I sat in on Rana's weekly talk to the graduate students, since Tara thought he might discuss phase lock loops (PLL) which I may need to measure my frequency stabilization later on. 

Thursday 6/20

Dr. Weinstein gave a talk for the LIGO SURF students today about the basics of LIGO. He discussed the general significance of gravitational waves, how gravitational waves are generated by pulsars, and some basic interferometry to explain how LIGO works.

I spent the morning trying to figure out how to convert the calculations I have into a final linewidth and average PSD. Tara wasn't sure how to do this so I emailed Dmass about how to do this... he suggested I estimate the bandwidth of my noise (where the signal from noise would disappear) and go from there. Using a bandwidth of about 1 GHz, I find that the linewidth after going through the external cavity should be about 12 kHz (which represents a noise level of 0.15 Hz/rtHz). This is well below the 300 Hz/rtHz that is required for the crackle experiment. Assuming that there will be noise from other mechanical sources, this seems reasonable... I think I can maybe order parts soon? I will check my calculations again tonight to make sure there aren't any glaring errors. Maybe the bandwidth of the noise signal I used was too small, I'm not positive yet. I will write this up nicely so that I can post it here, maybe later tonight. 

Today, I wrote up my noise calculations in a nicer way so that I have it on hand in the future. They are attached here. I found a mistake from the last elog entry, so it turns out that we are looking at a linewidth of about 440 kHz (or a PSD noise of 191 Hz/rtHz). This still meets the requirements of the experiment, and can be adjusted as necessary, since I can still increase the length of the cavity to decrease the noise. 

LIGO students had another presentation by Dr. Weinstein about the general progress of the LIGO project this afternoon. There was some discussion of decreasing noise level and aLIGO. 

Just musing: There is some discussion of using a lens to focus the beam (e.g. Saito paper, Saliba paper). I'm not sure how this will affect the noise levels; it might be possible to decrease the noise on the ECDL a lot if we include a lens that focuses the beam? In the Saito paper, they talked about having the beam aligned to focus at 10 m, with a lens of 15 mm focal length. Tara hasn't been around today but I want to talk to him about the idea, because it could affect the noise level. Alternately, multiple mirrors to redirect the beam and make the cavity longer before hitting the grating, although this sounds more complicated. 

 There was another mistake in my calculations (sorry!) so we are actually looking at a possible linewidth of about 5.3 kHz (or noise level of 0.03 Hz/rtHz). This is very low, so we are looking at using a current driver from Thorlabs since we have room in our noise budget, and this would make the assembly much easier since the parts fit together. Waiting for a response from Thorlabs about their noise levels for current drivers.

Attached are the corrected calculations. 

I added some additional explanation for the fact that the noise reduction was not dependent on frequency (see attached). I also added references for the data that I used. 

I spent awhile dealing with Thorlabs customer service trying to figure out about their lowest noise current driver (250 mA), which is the LDC205C. They are still working on getting an answer as to the noise level of this current driver. 

The next step is to start considering the mechanical setup of the ECDL and see if we need to order any special mounts for this. Tomorrow I will draw up more of a sketch for the mechanical apparatus, since Tara had talked about possibly getting some parts machined. 

I spent this morning looking at the mounts and other mechanical parts necessary for the ECDL. This afternoon, I met with Tara to discuss how I should run some noise calculations for including a servo to reduce frequency noise. I will deal with the mechanical logistics later while we are waiting for the diode, etc. from Thorlabs. 

I corrected this, since the paper did have an equation about how the power spectral density is reduced by frequency. This is in the updated noise pdf attached. We no longer have a low enough noise level to do the crackle experiment below 100 Hz or above 10 MHz using our original estimates. This makes running calculations including a servo important. 

I also played around in Mathematica trying to see what value of X would be sufficient to reduce the noise level. Uploading the notebook isn't working right now. It shows that in order to reduce the noise level to meet the requirements for the Crackle experiment, we need a parameter X of about 3000. This is quite large, and would require a cavity of length 30 m. Alternatively, we could reduce the noise by:

• A different laser diode that had low enough noise to begin with, or a very small reflectivity
• A diffraction grating that had a very high reflectivity
• Finding a very good TEC, which would reduce thermal noise (most websites don't seem to offer this data...)
• Note that at this time, it seems unfeasible to go with any current driver besides the one designed by Libbrecht and Hall, since the current noise limits how low the diode's noise can be at high frequencies

Tonight or tomorrow, I will try to shop around to see if other laser diodes have slightly nicer specs. I will also look to see if other papers encountered the same problem. 

Today I spent the morning searching the literature on Web of Knowledge to see if anyone had ways to reduce the noise level of an ECDL further by tweaking the parameters of the Littrow configuration (our current plan, where first order beams coming off the diffraction grating go back into the laser diode). It may be worth examining configurations with more mirrors to lengthen the cavity, but otherwise my search was not particularly helpful. We may need to start looking at the Littman-Metcalf configuration??? This theoretically reduces linewidths more but has lower efficiency. The diffraction grating is immovable, and an adjustable mirror is used instead to reflect light back onto the diffraction grating. 

Tara got me the information for me to calculate how a servo would reduce the noise of the ECDL further. I worked most of the afternoon to understand the principle behind the feedback, and ran calculations after searching the literature for reasonable numbers. Using a piezoactuator, we can reduce the noise at low frequencies, but it does not solve our problem at high frequencies (above 10 MHz), where there is essentially no noise reduction. See the attached pdf with the updates included (pages 5-8). 

Tomorrow I will see if I can find a piezoactuator that has a large actuator gain, which would cause more noise reduction at higher frequencies. Otherwise, building an ECDL will not be very useful for us to use at LIGO... 

Yesterday I spent awhile reading literature, then met with Rana and Tara. Rana wanted us to produce a sketch of the physical layout of our ECDL and generate some graphs comparing different parameters (diodes, gratings, cavity lengths) so we could determine exactly what parameters we'd need to order parts. Last night I made the plots in Mathematica (BAD). This morning I did the sketch of the mechanical layout of the ECDL. Will make a nicer sketch with the changes made today and post here this weekend. 

I calculated a few values: the grating should be placed at an angle of 39.7 degrees in order for the first order diffraction go back into the cavity. I checked the tuning range, and concluded for a frequency change of 100 MHz - 4 GHz, we will see a change of output beam angle on the order of microradians. This means we will not need a mirror to make sure the output beam is directed correctly for frequency changes. 

Tara and I met with Rana again. I got mocked excessively for using Mathematica, and will remake all the plots in Matlab this weekend. We decided on some parts to order, which are listed at the end of this entry. Other things to do:

• We want our apparatus to be in a metal box to lower noise. Will build inside the box. Check the 40m lab to see any metal boxes laying around, and check online (Newark, Allied). We want a setup with a lid that can be very rigidly screwed on. 
• Ask Dmass about using the current driver in the Cryo lab. We don't want to have to order this online, it's expensive!
• Talk to the companies that sell laser diodes to make sure that the laser diodes are precisely 1064 nm. If not, see if we can arrange something where they preselect a diode, since we need very high accuracy. 
• We will design the TEC/current driver connections to the laser diode once we have parts, since it'll likely require some fiddling around and this is a prototype. We will prefer BNC cables to D-Sub. 
• Eventually make some of the more unique parts of the apparatus in Solidworks so we can get it machined. 

Parts to order (bolded): 

I redid the plots from my meeting on Friday with Rana and Tara in Matlab, comparing different components. They are attached here. I'm still trying to get the minor gridlines to show up. 

Plot 1: Comparing noise levels of different experiments to determine which we will use as our standard. 

Plot 2: Comparing noise levels after the ECDL and servo of different diodes. Different diodes have different sizes, which affects the value of parameter X. They are all made of GaAs so other parameters are not affected. We have decided to order the Thorlabs and QPhotonics diodes. The Lumics diode has suspiciously low noise - perhaps the theoretical approximation breaks down in this case. 

Plot 3: Comparing noise levels after the ECDL and servo of different gratings. The gratings are only affected by the efficiency. We will go with the Thorlabs 1200/mm 1um blaze wavelength grating, since we want a blaze wavelength close to the wavelength of light we are selecting for (see Tara's ECDL note on the SVN), and we want as many grooves possible for maximum resolution. 

Plot 4: Comparing noise levels after the ECDL and servo of different cavity lengths. This plot is much better than the Mathematica one; we can see that longer cavities have lower noise, but a smaller FSR. We will likely go with 60-10 cm. 

Also attached is a sketch of our mechanical setup, agreed upon during the meeting on Friday with Rana and Tara. 

This week, I will get a draft of my first report done before the long weekend for Tara to look over. This will probably involve looking over some old concepts to write up something comprehensive. I will also be waiting for a response from QPhotonics and Thorlabs about preselecting diodes, and I need to talk to Dmass about using a current driver. Start looking at metal boxes in the 40m  and building the parts in Solidworks if I have time. 

Today, I spent the day working on the first progress report for SURF, which Tara would like by Wednesday. I have everything complete except a nice figure of the planned experimental setup, which I will do tomorrow. 

I also emailed Dmass about getting a current driver from the Cryo lab that we can modify and use. 

I've also been corresponding with the companies that sell the laser diodes. It seems like neither can guarantee exactly 1064 nm wavelength, nor does either offer any way to preselect a diode. However, we should be able to tune the wavelength using temperature, since both diodes change at 0.3 nm/degree C. Tara is placing an order for several of the items today. 

Today, the last of the parts we've decided on were ordered. 

Dmass said they do not have any extra current drivers, nor does Eric Gustafson. Eric said that if we can find a commercial board, I can ask Alex Cole (one of his SURF students) to show me how to put the commercial board into a standard LIGO module. Not sure if we'll do this or not. 

I spent the day finishing up my first progress report for SURF and uploaded it to the ECDL folder on the 40m SVN. Tara wants to look at this before I submit it next week. 

I've also started reading about how to measure the frequency noise, so I can start planning for making measurements when the laser diode arrives. 

To Do:

• Figure out what to do for a current driver
• Decide how to measure the frequency noise (read up on this)
• Figure out how to wire the TEC to the laser diode so everything is ready when parts arrive. Read up a lot online about the theory behind this. 
• Look for metal boxes to house the ECDL

Today I made some edits to my progress report for SURF, mainly with the graphs in Matlab. I tried to make them look better. The updated version is on the SVN. 

I spent the rest of the day reading papers about how to measure frequency noise and some basics on the Peltier effect so I could understand how the TEC will work. I'm going to start figuring out the wiring and stuff so that we will be prepared when the parts arrive. 

I made more edits to my SURF progress report. I need to remember to make good looking graphs in Matlab without being reminded by Tara. I just submitted the pdf of the final version. It is also on the SVN in the ECDL documents folder. 

I spent more time trying to understand the difference between heterodyne and homodyne detection, and trying to figure out which method I would want to use for my ECDL measurements. My understanding is that homodyne detection involves superimposing the output beam with a modified version of itself, and measuring the beat frequency spectrum. Heterodyne detection involves superimposing the ECDL signal with a reference signal and measuring the beat frequency spectrum. I believe we will be using heterodyne detection because we have a very good reference laser at 1064 nm and this saves the trouble of having to modify the output beam. However, the literature has not been super descriptive for a beginner, and the exact mechanism of making this happen still confuses me. I will continue looking into this. 

I also spent some time figuring out how we will wire the TEC and TEC controller. It seems fairly straightforward. See http://www.thorlabs.com/Thorcat/15900/TED200C-Manual.pdf, page 13. This explains how we will wire things. We will use an LED that can signal to us when the TEC element is on. I still need to figure out the thermistor we will use as a temperature sensor... 

For future reference, the 2 parts that Tara ordered are:

• laser controller: Thorlabs TEC200C
• Peltier element: Thorlabs TEC3-2.5

I spent the morning figuring out how to wire the TEC/thermistor/TEC controller together. After reading through the manual from last time (http://www.thorlabs.com/Thorcat/15900/TED200C-Manual.pdf), I also have a pretty good idea of how to change the settings of the controller so that it will work. I have some notes of my own but it's mostly self contained to the manual. When the parts arrive, I should be prepared to make the TEC work. 

(Note from last time: we will be using a 10 kilo ohm thermistor as a temperature sensor, which Tara ordered as well)

I downloaded Solidworks on my computer, which turned out to be more complicated than it would seem. Haven't used the program in a long time, so it took awhile to get reacquainted. I have a prototype of the final design we will send to the machine shop (attached), but some measurements still need to be figured out. In particular, I need to figure out the dimensions of the PZT we will be using so I can design the hole in the apparatus properly to mount the PZT. I also need to figure out how this will be screwed down onto a table or into a box. Possibly have more holes to contain screws?

In my free time, I'm still reading about how to measure the frequency noise. 

I attached the Solidworks parts that I built. I put these together with the parts we are ordering from Thorlabs (they have the Solidworks parts on their website) and have an image of the assembly attached as well. 

 I spent today building the elements we want machined in Solidworks. We have a few pieces we need to get machined: 

• Laser diode mount: This will hold the socket that Tara ordered (Thorlabs S8060), and we can easily replace the laser diodes in this socket since they are both the same package. 
• Grating mount: The grating will be glued to the front of this, and a PZT will be used to adjust the distance. The PZT I am looking at is here (http://www.physikinstrumente.com/en/products/prdetail.php?sortnr=703300). Still need to check that this is our best choice, I'm shopping around more. 
• Base/enclosure: We could either design our own base where the elements screw in solidly and put a lid on the structure, or we could use a commercial box (this one could work http://www.alliedelec.com/search/productdetail.aspx?SKU=70166638). Using a commercial box will only require minor modifications to the design. I want to talk to Tara about how easy it would be to modify a commercial box. The TEC will be attached to the base or box like the Birmingham group. 
• I'm waiting on dimensions of the collimator so I can also build a collimator mount that the adjustable tube will sit in. 

We're seeing if the current driver Dmass uses is from Thorlabs. If it is, it means the commercial driver is good enough and we can purchase this. 

I haven't looked at tolerance values for shop processes yet because I'm not sure how important this is, or exactly how to do it. I know the general idea, but not sure how to deal with the actual calculations yet. I'll work on this more tomorrow once I talk to Tara again.