In fact, many of the mounts need to be adapted to 4": the beefy steering mirrors going into the PMC, the PMC RFPD, the ISS AOM, the Faraday between the NPRO and the AOM, the NPRO itself, the ISS PDs.

Also for the FSS: the 21.5 MHz EOM, the PBS, the AOM, the refcav periscope, and the RFPD.

Its obvious, in retrospect, that we would have to do this, but it somehow didn't occur to me until actually trying to put things on the table...

The NPRO itself is already tapped with 3 (metric) M3 holes. It also has 4 (un-tapped) holes at its 4 corners which look like they are for feet. Anyone have a mount design for the Innolight NPRO already?

We also started labeling the table with the new coordinate system. In this system, the NE corner is the origin. The screw hole which is most NE is 1,1. The numbering increases in the south (+X) direction and goes negative in the west (-Y) direction.

Jenne and Koji

Last week Jenne has put the accelerometers on and under the PSL table immediately after the plastic sheets were removed.

So I took the same measurement as I did on 9th Aug.

Here is the comparison of the vibrational performance of the table before and after the modification.

Basically the table is now stiffer and more damped than it was before.
We don't find any eminent structure below (at least) 70Hz.
This result is obtained despite elevating of the table.

1) Attachment 1

For the horizontal comparison (top),  it is clearly seen that the large resonant peak at 20Hz was eliminated.
At least the new resonances went up to 70-90Hz region. Y is basically equivalent to X.

For the vertical comparison (bottom), it is clearly seen that the resonant peaks at around 50 & 70Hz were eliminated. 
At least no new resonance is seen.

2) Attachment 2

All-in-one plot for the measurement --- spectra, coherences, transfer functions --- after the upgrade. I put the same plot for the one before the upgrade.

The lifting and resetting of the BLUE PSL enclosure has made it unstable somehow. When I push on it a little it rocks back and forth a lot.

Steve, please look into what's happening and stiffen it if you can. Its too unstable right now.

To test out this website - emachineshop.com, Jenne and I are designing some of the mounts for the new beam height.

It took me a few hours to figure out how to do it, but the software is easy enough for simple stuff. This is a brass mount with M4 clearance holes which are countersunk and a lip so that it can be dogged down to the table.

1. Steve is handling the mount height increase for the PMC and RC steering mirrors, as well as a mount (non-steerable) for the ISS' AOM.
2. Rana is working on the laser mount.
3. Jenne is drawing and getting the PMC mount made.
4. We got the lenses from CVI for the mode matching, but not the metric screws for the laser mounting. I am tempted to tap holes in the laser base.

I am feeling that it is ok to carefully make new holes and threads as far as the holes do not penetrate the plate.
The thickness of the plate can be measured by the four holes at the corners.

1. I can not see whether the attaching surface is flat or not.
It should have ~1mm step to avoid "the legs" of the laser at the four corners.
Otherwise we will have ~0.5mm space between the block and the laser
and will squish this gap by the screws => cause the deformation of the block and the laser.

2. The countersinks for the M4 screws can be much deeper so that we can use the existing M4 screws.
In any case, the long M4 screws are not rigid and also not common.

[Rana, Jenne]

More PSL progress. 

The new laser was raised to a 4 inch beam height using basically the most randomly thrown together method possible.  (It'll work just fine for aligning things, but we seriously need to get a nice block made.)  The PMC and the nice Osamu-mirror mount to go into the PMC also have temporary risers, so we'll need to replace them with the real deal as soon as we get things back from the shop.

So far we've got (1) the lens after the laser, (2) a Half Wave Plate (no quarter wave plate yet), (3) steering mirror that will go after the EOM, (4) 2 steering mirrors to get into the PMC, in addition to all of the stuff that we did the other day.  With all of this stuff we've got the beam hitting the 1st PMC mirror. We still don't have the EOM and AOM in the beam path however.

To get the rough alignment that we did, we turned on the new 2W NPRO, operating at the minimum power we could see on a card.  We turned it off after use, so it is still off.  Steve, we left the cable for the interlock sitting on the PSL table on the NW corner....can you please hook it up tomorrow?  Also, after the interlock is installed we should go back to regular running laser hazard mode. 

NPRO

Koji and I inspected and photographed the laser after opening up its case. I then drilled the clearance holes in the 4 corners and tapped holes for 1/4-20. I was careful to tap with the laser sideways, to avoid shavings getting into the laser and suctioned out as much of the pieces as I could. The laser is now mounted on some bad 1/4-20 based NewFocus style pedestals. The riser block can now be made with 1/4-20 through holes and the laser will sit on its for corner feet. We'll make the base aluminum to avoid differential CTE based stress in the laser base.

We checked the level of the laser. With the new mounting the beam is level to within ~1 mrad and has a 4" beam height.

Faraday

I've mounted the Faraday Rotator from the old MOPA. It has 8-32 mounting holes (who's shafts are curiously not parallel). We need an aluminum block of the proper height (2 3/4" ??) to make a permanent solution.

I've also mounted the thin-film polarizer. This works well, but it also needs a block machined to get the mounting to be less Mickey Mouse.

Pockel Cell for phase correction and 35.5 MHz PMC modulation

The EOM is mounted as before on the angle bracket to align it for P-pol light. The beam now goes cleanly through there. No further mounting hardware required.

Lenses

The 2 lenses in the 'mode matching telescope' between the laser and the PMC are in place, but not placed with any accuracy.

By sheer luck, I saw the PMC flashing in the TEM27 mode without any alignment from me. Next step is to get the lens positions tuned and then do the beam scan on the beam going towards the PMC to verify the approximate mode matching. This is all crude, but I just want to get the beam going into the vacuum as fast as possible.

Rana and I were poking around on the PSL table today, getting a few more items raised to the correct height.

I checked the polarization state of the new NPRO by using a HWP to minimize the transmission through a PBS cube, and then compared the power transmitted through the cube vs. reflected.  When the NPRO current was 0.772 \pm 0.001 (as read on the LCD), the transmission through the cube was 1.44mW, while the reflected was 10.53mW.  The reading of the Ophir power meter with no incident light was 0.03mW.  This factor of 10 means that the NPRO beam is ~10% circularly polarized and ~90% linearly polarized.  In order to improve the beam, we need a Quarter Wave Plate, which it turns out we don't have.  We need a QWP!

After that, using the linearly polarized part of my beam (maximizing the transmission through the PBS by rotating my HWP by 45deg), I tried to tune the angle of the polarizers that Rana pulled out of the MOPA.  I think I'm confused / too tired, because I can only get the polarizer to reflect a bunch of light, and I can't get it to pass any significant amount of light through, no matter where in its actuation range I put it (It's on a rotation stage with a few degrees of range).  It should just be a Brewster's Angle thing, and since I already have P-pol coming through the BS cube, this shouldn't be so hard.....

In any case, it may not be useful to do the final fine tuning of these polarizers until they are in their final places.  The hacky stack of mounts that I have has some slop in the position / alignment of the base of the polarizer, so no matter what we'll have to redo the tuning after the mounts are finalized.

To bypass the polarizer issue, I just used cubes. One I took from the FSS-Refcav path and the other from the power control part of the old MOPA, just downstream of the MOPA's periscope.

We'll swab these out with the thin-film polarizers after we get the mounts made.

With the cubes in, I also installed the Faraday + its 1/2-wave plate. The transmission looks good and we're getting into the PMC and its flashing a TEM00 mode sometimes. I set up a signal generator to drive the SLOW actuator by 1 FSR at 0.1 Hz.

I have set up a PMC transmission camera and transPD so that its easy to align. The flashing mode already allows us to align most of the rest of the table (except FSS).

Our next step should be to run the cables for locking the PMC:

1. RF cables to the PMC_REFL
2. Dsub for the PMC RFPD
3. HV cable between the PMC servo board and the PMC PZT (why is this not RED? we have to make sure to abide by the cabling color code).
4. RF cable from 35.5 MHz Frequency Reference card to the PMC EOM via the RF summing box on the table.

On Tuesday, we need to make sure that all of our mounts' drawings are in the cue for the shop. I'll put the list of mounts onto the PSL upgrade wiki page.

We also have to come up with a plan for wiring some of the 2W NPRO's channels into the cross-connect so that we can have some laser channels recorded by EPICS.

We ran the cables for the PMC: The RF cable for the 35.5 MHz drive was cheap and so we swapped the 29.5 MHz cable for it.

There now remain 1 RG-174 cable to drive the FSS PC (21.5 MHz) and 3 Heliax for the Kiwamu Tri-Mod EOM (11, 29.5, and 55 MHz).

We also changed the BLACK HV drive cable for the RED one (previously used for the MZ). All HV cables MUST be RED.

The BLACK cable is now used for the PMC_REFL DC.

The Heliax cables are routed onto the table - it remains a Alberto/Kiwamu job to strain relieve them and attach them to the TriMod box and EOM in the morning.

The PMC is locked and we did some partially bootless alignment and mode-matching. It locks easily on a TEM00 mode (with very poor visibility), but the

rest of the beam train can now be aligned while Valera does the PMC matching mambo.   

Also, Kiwamu has modified the layout drawing to add the green PLL stuff. This has collapsed the reference cavity's wave function placing it close to its original position.

WE (maybe Valera and Steve) can now put the reference cavity back on the table.

I put some green stuff on the layout drawing.

I continue to refine the positions of these stuff.

Notes :

 1. I flipped the reference cavity. So now the cavity is sitting on the left hand side of the layout.

  2. I removed the ISS stuf. We should think about where ISS should be.

 I started mode matching of the beam going to IMC. The work is still going on.

According to Rana's calculation (see here), I put the first lens (f=200mm) in between two steering mirrors after PMC.

The distance from PMC to the first lens was adjusted by using a metal ruler.  So I believe the accuracy is something like 1mm.

I aligned the beam path going through the broadband EOM and the mode matching lenses.

  I could find the optimum position for the second lens  (f=-150mm) by sliding the position of the lens and measuring the mode after it.

But the optimum position looked a bit far from the EOM. It's off by about 3-4 inch from the designed position.

Somehow I feel that the beam before the second lens goes with a smaller divergence angle than that of designed.

So tomorrow I am going to restart the work from checking the mode before the second lens.

Maybe at first I should measure the mode without going through the EOM because it changes the waist position and makes the system not straightforward.

Over the last couple nights we got the beam into the FSS path and all the way to the IMC and out onto the AP table.

Tara and Valera have calculated a mode-matching solution for the reference cavity. It utilizes only a single lens between the AOM and the reference cavity. Valera and Steve will move the reference cavity into place in the morning.

We noticed that the layout was too tight on the end going into the MC and so we adjusted the angle of the final zig-zag. This will put the final mode-matching lens in between the final steering mirrors (which is generally undesirable) but the lens in this case is only f=400 mm. In addition, this lens may provide some more decoupling between the steering mirrors.

The whole layout has to be a little adjusted because of a calculation mistake I made in the mode-matching. I used only the nominal focal lengths from the CVI catalog and not the effective one. For the UV-grade fused silica lenses, the effective focal length is actually 20-30% longer. Today we measured that the "f=200 mm" lens we got is actually f = 238 mm. The BK7 lenses are much closer to the nominal.

We also replaced the Klinger mount ahead of the PMC with a Polanski style so that we could get the PMC REFL beam out without hitting the mount. Valera will continue to refine this section on the weekend.

Tomorrow, we will lock the MC using feedback only to the NPRO. The 0-150 V piezo driver is on the PSL table ready for action.

I also got a LCD video monitor from Frank and hooked it up on the PSL table. If we like this kind of thing, we can get many of them. They are pretty cheap. It would be handy to have 3-4 of them on the PSL and one on every of the ISC tables. They take the standard video for input and need +12V for power. Right now the one in there is looking at the PMC transmission.

The Omnigraffle layout as of tonight is attached.

- The PMC REFL PD was moved from the temporary location to the one called for by the PSL layout (picture attached). The leakage beams were dumped.

- The FSS reference cavity was aligned using temporary periscope and scanned using NPRO temperature sweep. The amplitude of the sweep (sine wave 0.03 Hz) was set such that the PMC control voltage was going about 100 V p-p with. With rough alignment the visibility was as high as 50% - it will be better when the cavity is locked and better aligned but not better than 80% expected from the mode astigmatism that Tara and I measured on Thursday. The astigmatism appear to come from the FSS AOM as it depends on the AOM drive. We reduced the drive control voltage from 5 V to 4V beyond that the diffraction efficiency went below 50%. The FSS REFL PD was set up for this measurement as shown in the attached picture. There is also a camera in transmission not shown in the picture.

I have been working on the mode matching lenses which are sitting after the boradband EOM.

Last Friday I checked the mode profile after the first mode matching lens (f=-150mm). The measured mode was good.

According to the calculation done by ABCD software, the waist size is supposed to be 80.9 um after that lens.

The measured waists are 80.5 um for the vertical mode and 79.4 um for the horizontal mode.

The screenshot of the ABCD's result and the plot for the mode measurement are shown below.

I didn't  carefully check the mode after the last convex lens (f=200mm), but it must be already good because the beam looks having a long rayleigh range.

Now the beam is reflected back from MC1 and goes to the AP table since I coarsely aligned the beam axis to the MC.

/****  fitting result ****/

w0_v =  80.4615      +/- 0.1695 [um] 

w0_h =  79.4468      +/- 0.1889 [um]

z_v =  -0.115234        +/- 0.0005206  [m]

z_h =  -0.109722        +/- 0.0005753 [m]

The IR sensitive Olympus 570 camera gives us a really nice view of these IR beams. Its actually a lot better than what you can get with the analog IR viewers:

PSLAOMdogs

On Friday, Valera and I calculated the modematching for reference cavity from AOM.

We scan the beam profile where the spot should be.

The first beam waist in the AOM is 103 um, the lens (f= 183 mm, I'm not sure if I have the focal length right) is 280 mm away.

The data is attached. The first column is marking on the rail in inches,

the second column is distance from the lens, the third and fourth column are

vertical and horizontal spot radius in micron. Note that the beam is very elliptic because of the AOM.

- connected the TTFSS cables (FSS fast goes directly to NPRO PZT for now)

- measured the reference cavity 21.5 MHz EOM drive to be 17.8 dBm

-  turned on the HV for the FSS phase correcting EOM (aka PC) drive

- connected and turned on the reference cavity temperature stabilization

- connected the RefCav TRANS PD

- fine tuned the RefCav REFL PD angle

The RefCav is locked and aligned. I changed the fast gain sign by changing the jumper setting on the TTFSS board. The RefCav visibility is 70%. The FSS loop ugf is about 80 kHz (plot attached. there is 10 dB gain in the test point path. this is why the ugf is at 10 dB when measured using in1 and in2 spigots on the front of the board.)  with FSS common gain max out at 30 dB. There is about 250 mW coming out of the laser and 1 mW going to RefCav out of the back of the PMC. So the ugf can be made higher at full power. I have not made any changes to account for the PMC pole (the FSS is after the PMC now). The FSS fast gain was also maxed out at 30 dB to account for the factor of 5 smaller PZT actuation coefficient - it used to be 16 dB according to the (previous) snap shot. The RefCav TRANS PD and camera are aligned. I tuned up the phase of the error signal by putting cables in the LO and PD paths. The maximum response of the mixer output to the fast actuator sweep of the fringe was with about 2 feet of extra cable in the PD leg.

I am leaving the FSS unlocked for the night in case it will start oscillating as the phase margin is not good at this ugf.

Brilliant! This is the VERY way how the things are to be conquered!

Our new 2W Mephisto has a pretty zippy "SLOW" temperature input. Tuning the perl PID servo, I found that the best response came from setting

the "P" and "D" terms to zero. This is because the internal temperature stabilization servo has a fairly high UGF. In the attached

image you can see how the open loop step response looks (loop is open then the "KI" parameter is set to zero). The internal servo

really has too little damping. There is a 30% overshoot when it gets a temperature step. For this kind of servo Innolight would have done better

to back off on the gain until they got back some phase margin.

New SLOW parameters:

timestep = 1.9 s

KP = 0

KI = 0.035

KD = 0

The PSL out  2" OD beam guide tube was cut  1.5" shorter to 13.5"

The 10" OD  0.25" wall Al tube was replaced by a  lighter, not anodized and thinner  wall 0.094" tube of 15.5" lenght, that is 0.75" shorter.

The new position of the PSL table made these cuts necessary.

Big Johnny and I hacked a function generator output into the cross-connect of the 80 MHz VCO driver so that we could modulate the

amplitude of the light going into the RefCav. The goal of this is to measure the coefficient between cavity power fluctuations and the

apparent length fluctuations. This is to see if the thermo-optic noise in coatings behaves like we expect.

To do this we disconnected the wire #2 (white wire) at the cross-connect for the 9-pin D-sub which powers the VCO driver. This is

called VCOMODLEVEL (on the schematic and the screen). In the box, this modulates the gain in the homemade high power Amp which

sends the actual VCO signal to the AOM.

This signal is filtered inside the box by 2 poles at 34 Hz. I injected a sine wave of 3 Vpp into this input. The mean value was 4.6 V. The

RCTRANSPD = 0.83 Vdc. We measure a a peak there of 1.5 mVrms. To measure the frequency peak we look in

the FSS_FAST signal from the VME interface card. With a 10 mHz linewidth, there's no peak in the data above the background. This signal

is basically a direct measure of the signal going to the NPRO PZT, so the calibration is 1.1 MHz/V.

We expect a coefficient of ~20 Hz/uW (input power fluctuations). We have ~1 mW into the RC, so we might expect a ~20 Hz frequency shift.

That would be a peak-height of 20 uV. In fact, we get an upper limit of 10 uV.

 Later, with more averaging, we get an upper limit of 1e-3 V/V which translates to 1e-3 * 1.1 MHz / 1 mW ~ 1 Hz/uW. This is substantially lower

than the numbers in most of the frequency stabilization papers. Perhaps, this cavity has a very low absorption?

We added the Thorlabs HV Driver in between the FSS and the NPRO today. The FSS is locking with it, but we haven't taken any loop gain measurements.

This box takes 0-10 V and puts out 0-150 V. I set up the FSS SLOW loop so that it now servos the output of FAST ot be at +5V instead of 0V. This is an OK

temporary solution. In the future, we should add an offset into the output of the FSS board so that the natural output is 0-10 V.

I am suspicious that the Thorlabs box has not got enough zip to give us a nice crossover and so we should make sure to measure its frequency response with a capacitive load.

I found that several linux libraries have been moved around and disabled today. In particular, I see a bunch of new stuff in apps/linux/ and ezca tools are not working.

Who did this and why is there no ELOG ???

Also found that someone has pulled the power cable to the function generator I was using to set the VCO offset. This is the one on top of the Rb clocks. Why?? Why no elog? This is again a big waste of time.

 We measured the Thorlabs HV Driver's TF today. It is quite flat from 1k to 10k before going up to 25 dB at 100k,

and the response does not change with the DC offset input.

The driver is used for driving the NPRO's PZT which requires higher voltage than that of the previous setup.

We need to understand how the driver might effect the FSS loop TF, and we want to make sure that the driver

will have the same response with DC input offset.

Setup

We used SR785 to measure the TF. Source ch was split by a T, one connected to Driver's input, another one connected to the reference (ch A). See fig2.

The driver output was split by another T. One output was connected to NPRO,

another was connected to a 1nF capacitor in a Pomona box, as a high pass filer (for high voltage), then to the response (ch B)

 The source input is  DC offset by 2V which corresponds to 38 V DC offset on the driver's output.

The capacitance of the PZT on the NPRO is 2.36 nF, as measured by LC meter.

 The result shows that the driver's TF is flat from 1k to 10k, and goes up at higher frequency, see fig1.

The next step is trying to roll of the gain at high frequency for PZT. A capacitor connected to ground might be used to roll off the frequency of the driver's output.

We will inspect the TF at higher frequency (above 100 kHz) as well.

Inside the FSS box, the FAST path has a ~10 Hz pole made up from the 15k resistor and the 1 uF cap before the output connector.

This should be moved over to the output of the driver to make the driver happy - without yet measuring the high frequency response,

it looks like to me that its becoming unhappy with the purely capacitive load of the NPRO's PZT. This will require a little surgery inside

the FSS box, but its probably justified now that we know the Thorlabs box isn't completely horrible.

(Rana, Yuta)

Motivation:
 We wanted to see thermal effects on the PMC.

What I did yesterday:
 Changed the current of the NPRO from 2A to 0.8A and measured the power of the reflected/transmitted light from the PMC when locked.
 I also measured the power of the reflected light when PMC is not locked (It supposed to be proportional to the output power of the laser).

Result:
 Attached. Hmmmm......
 At several points of the laser current, I could'nt lock the PMC very well. The power of the reflected/transmitted light depend on the offset voltage of the PZT.
 When the laser power was weak(~<0.9A), the power of reflected/transmitted light changed periodically(~ several minutes).

It was a bit difficult to comprehend the result.
Is it good? or bad? Have you seen the thermal effect? or not?

- Put linear lines to show the visibility of the cavity.

- Calibrate the incident power and make another plot to show the visibility (%) vs the incident power (W).

We got two small RF filter for the PMC from Valera  They are made by http://www.larkengineering.com/   "MC35.5-3-AB" sma, 29300-01

Result2:
 Attached is the visibility vs incident power(assuming output of the PD is proportional to the input laser power).
 Ideally, the graph should be flat. (In another words, attached graph in the elog #3667 shoud be linear.)
 But the visibility reduces with higher laser power in this graph. This is maybe because of the thermal effect. I'm thinking about how to confirm this.

Innolight PSL laser is set to @2.1A , ~1.6W output ! Please scan out when finished! 

The output monitoring pick up window W2-LW-2-2050-UV-1064-45S is in place. IOO_ANG_OPD and IOO_POS_QPD roughly aligned.

The PMC alignment and/ or mode matching is bad. PMC reflected is 50%, throughput 600mW

Remember to block PSL output ! into IFO

 The NPRO at the PSL table still can generate 2W laser !  He is still alive.

  When I reduced the temperature to  25 deg, the output power increased to 2W successfully.

  As Steve wrote down in his last entry (see here), the NPRO output was at 1.6 W currently, which is supposed to be 2W.

We were suspicious about the laser crystal's temperature because the current temperature looks a bit high.

In fact the setpoint of the temperature was 45.9 deg instead of 25 deg that is the previous setpoint.

Background:
 I measured the PMC's visibility vs incident power relation last week to see the thermal effect, but I didn't calibrated the laser power(see elog #3672).
 So, I calibrated it on Oct 12.

Setup:
 Attachment #1
 The laser grade window PW-1025-UV-1064-45P had power reflectivity of about 0.5% in this setup.

What I did:
1. Calibrated the laser power(Attachment #2).
  To measure the laser power, I put the Ophir power meter at just in front of "PMC REFL PD".

2. For the calculation of the visibility K, I used the following formula;
 K= [1-(R1-R0)/(R2-R0)]*100
where R0, R1 and R2 are the PD outputs in voltage when laser is off, PMC locked and not locked respectively.

3. Plotted the visibility vs the incident power(Attachment #3).

Result:
 Attachment #2
  From the linear fit by least squares, the calibration turned out to be 1.12±0.07 mV/uW. The error of this value is calculated from assuming PD output error~1mV and laser power error~3uW for all measured value.
  The largest error was from the position and the angle of the power meter probe.

 Attachment #3
  I used the same data I took last week(see elog #3672), but better plot.
  I put the error bars for just several points. When the laser power is weak, the errors are large because of the cancellation error. When the laser power is high, the errors are estimated to be so small that you can't see it in the plot(~1%).
  At the several points, I couldn't lock the PMC well and  the power of the reflected light depended on the offset voltage of the PZT.
  The horizontal axis has about 6% error because of the calibration error.

Note:
 Now the condition is a bit different from this measurement(NPRO temperature changed, optics moved slightly), so the visibility may be changed.

I cleaned up the scattered tools, optics, and mounts of the PSL table. I gathered those stuffs at the two coners.

At the end of the work I scanned the table with an IR viewer. (This is mandatory)
I put some beam block plates to kill weak stray beams.

One thing I like to call the attention is:

I found that some beam blocks were missing at around the PBSs just after the laser source.
Those PBSs tend to reject quite a lot of beam power
--- no matter how the HWPs/QWPs are arranged.
--- even at the backward side. (remember that we have a faraday there.)

Particularly, there was no beam block at the forward rejection side of the first PBS where we dump the high power beam.

Be careful. 

[Suresh and Kevin]

We placed the quarter wave plate in front of the 2W laser and moved the half wave plate forward. To make both wave plates fit, we had to rotate one of the clamps for the laser. We optimized the angles of both wave plates so that the power in the reflection from the PBS was minimized and the transmitted power through the faraday isolator was maximized. This was done with 2.1 A injection current and 38°C crystal temperature.

Next, I will make plots of the reflected power as a function of half wave plate angle for a few different quarter wave plate rotations.

[Koji Suresh]

We found the beat at 1064nm. T(PSL)=26.59deg, T(X-end)=31.15deg.

The X-end laser was moved to the PSL table.

The beating setup was quickly constructed with mode matching based on beam profile measurements by the IR cards.
We used the 1GHz BW PD, Newfocus #1611-FS-AC.

As soon as we swept the Xtal temp of the X-end laser, we found the strong beating.

[Koji / Suresh]

We worked on the 1064 beating a bit more.

- First of all, FSS and FSS SLOW servo were disabled not to have variating Xtal temp for the PSL.

- The PSL Xtal temp (T_PSL) was scanned from 22deg-45deg while we search the Xtal temp (T_Xend) for the Xend laser to have the beat freq well low (f<30MHz).
The pumping current for each laser was I_PSL = 2.101 [A] and I_Xend = 2.000 [A]

For a certain T_PSL, we found multiple T_Xend because the freq of the laser is not a monotonic function of the Xtal temperature. (see the innolight manual).

T_Xend to give us the beating was categorized in the three sets as shown in the figure. The set on "curve2" is the steadiest one. (Use this!)
The trends were quite linear but the slope was slightly off from the unity.

- T_PSL was scanned to see the trend of the PMC output.

The PMC was sometimes locked to the mode with lower transmission (V_PMCT ~ 3.0V).
When T_PSL ~ 31deg we consistently locked the PMC at higer transmission (V_PMCT ~ 5.3V).

At the moment we decided the operating point of T_PSL = 32.25 deg, V_PMCT = 5.34, where we found the beat at T_Xend=38.28deg.

- We cleaned up the PSL table more than how it was. Returned the tools to their original places.
The X-end laser was shut down and was left on the PSL table.

NEXT:
Kiwamu can move the X-end laser to the Xend and realign it.
Then we should be able to see the green beating quite easily.

[Koji and Kevin]

We measured the reflection from the PBS as a function of half wave plate rotation for five different quarter wave plate rotations. Before the measurement we reduced the laser current to 1 A, locked the PMC, and recorded 1.1 V transmitted through the PMC. During the measurements, the beam was blocked after the faraday isolator. After the measurements, we again locked the PMC and recorded 1.2 V transmitted. The current is now 2.1 A and both the PMC and reference cavities are locked.

I will post the details of the measurement tomorrow.

I measured the reflected power from the PBS as a function of half wave plate rotation for five different quarter wave plate rotations.

The optimum angles that minimize the reflected power are 330° for the quarter wave plate and 268° for the half wave plate.

The following data was taken with 2.102 A laser current and 32.25° C crystal temperature.

For each of five quarter wave plate settings around the optimum value, I measured the reflected power from the PBS with an Ophir power meter. I measured the power as a function of half wave plate angle five times for each angle and averaged these values to calculate the mean and uncertainty for each of these angles. The Ophir started to drift when trying to measure relatively large amounts of power. (With approximately 1W reflected from the PBS, the power reading rapidly increased by several hundred mW.) So I could only take data near the minimum reflection accurately.

The data was fit to P = P0 + P1*sin^2(2pi/180*(t-t0)) with the angle t measured in degrees with the following results:

where V is the visibility V = 1- P_max/P_min. These fits are shown in attachment 1. We would like to understand better why we can only reduce the reflected light to ~150 mW. Ideally, we would have V = 1. I will redo these measurements with a different power meter that can measure up to 2 W and take data over a full period of the reflected power.

Q1. Suppose the laser beam has a certain (i.e. arbitrary) polarization state but contains only TEM00. Also suppose the PSB is perfect (reflect all S and transmit all P). What results do you expect from your expereiment?

Q2. Suppose the above condition but the PBS is not perfect (i.e. reflects most of S but also small leakage of P to the reflection port.) How are the expected results modified?

Q3. In reality, the laser may also contain some thing dirty (e.g. deporarization in the laser Xtal, higher order modes in a certain polarization but different from the TEM00's one, etc). What actually is the cause of 170mW rejection from the PBS? Can we improve the transmitted power through the PBS?

Q4. Why is the visibility for the lambda/4 with 330deg better than the one with 326deg? Yes, as I already explained to Kevin, I suppose it was caused by the lack of the data points in the wider angle ranges.

I copy section 6.2 into here to share with you all what the diagnostic capabilities of the new NPRO are. Its not a lot.

We'll need to record a sample of the NPRO output beam on a regular photodiode in order to get a real power monitor. My plan is to use a regular 25-pin Dsub and run it fron the NPRO controller over to the PSL rack and hijack the old MOPA monitoring channels (3113 and 3123 ADCs).

The laser power was attenuated to 40 mW yesterday for ensuring that the power built up within the MC does not damage the optics. 

This however stopped us from the doubling work and besides also reduced the power available for locking the PMC. 

Therefore, today the laser attenuation was removed and once again 500mW is available at the exit of the PMC .  

However the power sent to the MC has been reduced to 60mW by changing one of the mirrors in the zig-zag to a 33% beam splitter.  Though about 450mW is incident on the beam splitter the reflected beam is only about 55mW since the mirror reflectance is specified for P polarised light incident at 45deg while ours is S-polarised incident at less than 45deg.   The light transmitted through the beam splitter has been blocked by a beam dump.

[Rana and Kevin]

I made a low pass filter for the piezo driver for the 2W laser that is now installed. The filter has a pole at 2.9 Hz. The transfer function is shown in attachment 1.

Attachment 2 shows the outside of the filter with the circuit diagram and attachment 2 shows the inside of the filter.

[Koji and Kevin]

Since there was still a lot of power being reflected from the PBS before the Faraday rotator, I placed another PBS at the reflection from the first PBS to investigate the problem. If everything was ideal, we would expect the PBS to transmit P polarization and reflect S polarization. Thus, if the laser was entirely in the TEM00 mode, with the quarter and half wave plates we should be able to rotate the polarizations so that all of the power is transmitted through the PBS. In reality, some amount of P is reflected in addition to S reducing the power transmitted. (We are not sure what the PBS is since there are no markings on it but CVI says that their cubes should have less than 5% P reflection).

For the following measurements, the laser crystal temperature was 31.8° C, the current was 2.1 A, the half wave plate was at 267° and the quarter wave plate was at 330°. I first measured the power reflected from the first PBS then added the second PBS to this reflected light and measured the transmitted and reflected powers from this PBS with the following results:

This shows that approximately 81 mW of P polarization was being reflected from the first PBS and that there is approximately 48 mW of S polarization that could not be rotated into P with the two wave plates. Attachment 1 shows the shape of the reflected (S polarization) beam from the second PBS. This shows that the S polarization is not in TEM00 and can not be rotated by the wave plates. The transmitted P polarization is in TEM00.

We then rotated the first PBS (in yaw) to minimize the amount of P being reflected. Repeating the above measurement with the current alignment gives

Thus by rotating the cube to minimize the amount of P reflected, ~70 mW more power is transmitted through the cube. This adjustment moved the beam path slightly so Koji realigned the Faraday rotator and EOM. The PMC was then locked and the beam was realigned on the PMC. At 2.1 A, the transmission through the PMC is 6.55 V and the reflection is 178 mV. With the PMC unlocked, the reflection is 312 mV. This gives a visibility of 0.43.

Note by KA:
We realigned the beam toward the PMC at 1.0A at first so that we don't cook any parts. Once we get the TEM00 resonance, the steering mirrors were aligned to maximize the PMC transmission. Then the pumping current was increased to 2.1A.

At the PSL table I aligned the wideband EOM more carefully.

Amplitude modulation (AM) components in the main beam at 29.5MHz were successfully diminished by 24 dB. 

Last night when we were locking the MC, we noticed that the reflected light had AM which somewhat disturbes the Pound-Drever-Hall locking of the MC.

So I aligned the wideband EOM to reduce the AM components.

(method)

  In order to observe AMs I put a photodiode PDA255 whose bandwidth is 50MHz after the wideband EOM.

Before the PD I also put a convex lens together with a stack of ND filters and put a steering mirror to control the beam spot on the PD.

First I aligned the EOM such that the DC voltage from the PD was maximized. This process corresponds to a coarse alignment.

And then I tried reducing a peak at 29.5MHz seen in the spectrum analyzer. 

(results)

At the beginning the AM peak in the spectrum analyzer was at about -48 dBm.

After the alignment of the EOM it went down below the PD's dark noise floor of -72 dBm.

I checked the alignment also with an IR viewer, it looks quite good.