An improved design is attached. I modified the input telescope to avoid using shor focal length lenses, to make it less critical, and to reduce the beam spot radius at the QPD to 0.5 mm.

[Massimo Granata (LMA), Quentin Cassar (LMA), Gabriele] 

This week I'm visiting LMA to learn how their Gentle Nodal Suspension system works and to measure the quality factors (Q) of one of Mark Optics disks. First of all we annealed the disk for 9 hours at 900 degrees (plus 9 hours warm up and 9 hours cool down).

Then we installed the disk into the measurement system and started by searching for all the resonances.

My COMSOL simulation proved to be good enough to give us the frequencies, especiallty after a small fine tuning of the disk thickness (within specs). We identifies a total of 32 modes of different families, and measured the ring down of all of them. Since our disk has no flats, each mode is actually a doublet with very small frequency separation. The analysis software has a bandwith of 1 Hz to find the peak amplitude, so it can't resolve the two modes. When both are excited to a significant amplitude by the electrostatic actuator, we see a clear beat in the ring-down. I had to write a new fitting code to take this into account. More details will follow in a DCC document. However, here I can say that the fit works remarkably well for all modes. 

A couple of examples:

Here is a summary plot of the quality factor and loss angle for all modes. We measured Q as high as 10e6, in line with other LMA samples (2") we tested in these days. In conclusion, the Mark Optics disks, as they are, are good enough for our coating tests.

Small modifications to the optical setup:

• added a 1m focal lenght lens (AR coated for 1064nm...) right before the beam enters the viewport, to reduce the beam spot size on the QPD
• added a 0.3 ND right after the laser, to avoid saturation of the QPD (about 75 microwatts total on the QPD)

The equations below are wrong. Please refer to https://nodus.ligo.caltech.edu:8081/CRIME_Lab/116 for the correct results

I measured the properties of the beam on the QPD. The total power is 31 uW. The beam shape is not gaussian, since we are seeing the interference of the reflection from the two surfaces:

The X and Y diameters are 1400 and 1300 microns, so I take the average of the two as an estimate of the beam size: 1300 +- 100 um. I also estimated the lever arm length to be 1.03 +- 0.02 m.

This allows me to esitmate the response of the normalized QPD signal to a tilt of the disk surface:

Plugging in the numbers gives a gain of (1900 +- 300) /rad for the normalized signals. I implemented those numbers in the filter banks: now X_NORM and Y_NORM have units of radians, and measure the disk surface angular motion. I also calibrated the SUM channel in microwatts, using the nominal responsivity of 0.45 A/W and the transimpedance of 200k (gain 11.1 uW/V)

Here's teh calibrated spectrum: note that the background noise is much larger than the real one because of the signal jumps.

I assembled the disk suspension sytem and installed into the chamber. Although I don't have the magnets and coils, I installed the movable retaining disk, and used it to center the disk.

I first aligned the input laser using the reflection off the black glass, which turns out to be quite bright and very well visible. Tomorrow I'm going to measure how much power we have in the black glass.

The reflection from the disk is slighlty separated from the reflection from the black glass, so I can block it using an iris.

At 6:50pm I closed the chamber and started the roughing pump. At 7:05pm pressure was below 1 Tor so I started the turbo pump. When leaving pressure is about 1.6e-5 Tor.

The last two ring downs I measured today showed a weird behavior of the lowest modes:

Although I'm not 100% sure, I suspect this is related to the fact that the beam reflected from the black glass was so close to the beam reflected by the disk that I could see interference.

So I broke vacuum and improved the setup, adding a peek washer below one edge of the black glass, to wedge it. In this way the reflection from the black glass is largely separated: it misses the upper periscope mirror and it is dumped on a black panel (together with the viewport reflection).

I realigned everything, installed back the disk and started pumping down at 1:30pm.

We set up a test facility for laser polishing the disk edges, using the CO2 laser in the TCS laboratory. We focused the beam with a 10" focal length lens, and installed the disk on a "rotation stage" that we motorized with a hand drill. We used a HeNe optical lever and a small container with water to define the horizontal plane and adjusted the disk as well as we could.

We first tested the procedure on the MO02 disk, which is the one already scared with the electrostatic drive burn mark. This disk is now definitely in bad shape. However, we felt confident in our procedure, so we took out the MO03 disk that was into the measurement system and proceeded to laser polish the edges. Things went quite smothly. Unfortunately we added some small damages to the disk surface in a couple of spots where the CO2 laser went out of alignment and melted the fused silica support of the disk. The edge however looks quite good now.

Q measurement is on-going at the timw of writing

Here are the nominal parameters of the disk with flats

A COMSOL simulation gives the frequencies and mode shapes shown in the attached PDF file. Following the list of frequencies and a classification of the mode family (numer of radial nodes, number of azimuthal nodes in a half turn):

Goal

Improve the optical setup, by increasing the response of the QPD to disk motion.

The old configuration

In all my previous measurement the optical lever was as simple as possible: no lenses were used, and therefore the beam was free to expand over all its path. The estimated arm lever from the disk to the QPD was 1030 mm.

QPD response to disk angular motion

The response of the QPD can be characterized with its optical gain in 1/rad, which is how much the normalized signal (difference / sum) changes for one radians of motion of the disk. This is the product of two parts:

1. the gain from angular motion of the disk to beam spot motion on the QPD. In the simple case of free propagation this is 2L, where L is the distance from the disk to the QPD, and the factor 2 is due to the fact that the beam deflection is the double of the disk angular motion. If there is a telescope in between the disk and the QPD, it is easy to compute the total ray transfer matrix:
   
   Then the gain is simply the B element of the matrix.
2. the response of the QPD normalized signal to beam motion. This depends only on the beam spot radius w on the QPD. It can be computed by simple gaussian integration, and in the approximation of small beam motion, it is given by the following expression:
   

In the case of the old configuration, the beam spot size on the QPD was measured to be about 1.5 mm in radius, so the optical gain is of the order of 1900 /rad.

Laser beam profiling

Since I wanted to improve the optical setup, I first needed to measure the beam coming out of the HeNe laser. I used the WinCam beam profile and a Newport rail to measure the beam X and Y sizes at different positions.

The measurements are not the best ever, but I can still get a fit for the evolution of the gaussian beam, as shown in the plot below. The beam waist is 254 um, located 340 mm behind the laser output (inside the laser tube).

Design of the improved setup

I decided to try a brute force algorithmic optimization for the optical gain. I allow two lenses between the laser and the disk and two lenses between the disk and the QPD. I wrote a MATLAB script that picks the four lenses from a list of all those available (I have a Thorlabs LSB02-A lens kit). For each combination of lenses, MATLAB moves them around into pre-defined ranges, and try to find the maximum value of the QPD total optical gain, which is the product of the factor g above and of the B element of the ray tracing matrix.

It turned out that the best optical gains could almost always be obtained by making the beam huge on the disk (5-10 mm radius) and tiny on the QPD (tens of microns). This is not a good solution. So I decided that the beam on the disk must be smaller than 2mm in radius and the beam on the QPD must be larger than 200 microns. I enforced those limits into the optimization code by weighting the gain with a function which is one in the allowed range, and then quickly drops to zero when either of the beam sizes fall out of the allowed range.

The script ran for about half hour and gave me a lot of possible options. After some inspections, I decided to use the following one, which uses only one lens between laser and disk, and two between the disk and the QPD. Distances and focal lengths are shown below. Note that the first distance (laser to first lens) is from the laser beam waist to the lens, so the actual distance must take into account that the waist is estimated to be 340mm into the laser.

With this configuration the optical gain is computed to be 17000 /rad, or about 9 times larger than the original setup. The beam radius on the disk is 1 mm and on the QPD is 0.23 mm.

Implementation

First of all I measured some distances:

• from the inner side of the viewport to the disk: 420 mm
• viewport thickness: 12 mm, which is about 18 mm optical length considering n~1.5
• so from the input to the chamber to the disk: 438 mm
• from the viewport to the upper external periscope mirror center: 110 mm
• distance between the periscope mirror centers: 275 mm

Using these distanced I build the designed optical setup. Some remarks on the procedure

• I first aligned the laser beam to be horizontal, then added the first lens and centered it by ensuring no beam shift far away from the lens
• I first aligned the periscope to get the beam roughly centered on the inner 45 degrees mirror, and then roughly centered on the black glass
• Then I put a small container with water inside the chamber, on top of the black glass. I aligned the inner mirror and the periscope so that the beam coming back from the horizontal water surface was perfectly overlapped with the input beam. I used an iris on the input beam path
• Then I removed the water container and installed a test disk. I moved the disk around until I got the same beam position in output. This tells me that the disk is horizontal
• Finally I moved the upper periscope mirror to separate horizontally the beam coming back, at the level of the table. The separation is large enough to allow me to pick up the outgoing beam with a mirror.

Here's a picture of the setup, with the optical path highlighted. 

Today I measured the amount of space available on the table for the new (4-fold) C.Ri.Me. setup. It's 1050 x 1220 mm, with the table hole in it. 

So I updated the optical layout to fit into this space, and optimized the telescope to have a beam spot on the QPD of the order of 350 um. The average lever arm length is 1.5 m, so the optical gain will be about 7000 /rad.

We have a few motorized mounts (with New Focus picomotors) and one controller (an old New Focus 8753, six axis total) that I connected with a makeshift null modem cable to the laboratory workstation (better cabling and power supply coming soon).

I wrote a couple of python scripts that can be used to continuosly read out the QPD values and move the picomotors if needed. It's wortking quite well, so we should be able to use it in the future to keep the QPD centered during the measurement. 

The scripts are in the ~/CRIME directory. Launch the function center() in the autocenter.py script.

I measured the beam profile of the new Thorlabs HeNe (21.8 mW measured). The beam waist is 355 microns, very close to the laser output port.

Using those numbers and the optical gain optimization algorithm, I tweaked the optical lever design. The simplest solution uses two lenses right after the laser to focus the beam down to about 300 microns on the QPD. The arm lever length is about 1.6 m, corresponding to an optical gain of about 18000/rad. I updated the DCC drawing in D1600213

Today I installed and aligned part of the optical components for the optical lever of the new setup. For the moment being I installed only the input components, and aligned the beams into the vacuum chamber. Since I don't have any in-vacuum optics yet, there's nothing more that can be done now.

Today I cabled and installed the four QPD, in a temporary position. I also assembled four picomotor mounts that will be used for the auto-centering.

The four optical levers are completely installed and aligned to a horizontal reference.

The wandering line I mentioned in my previous elog, which is spoiling most of the sensitivity, turns out to be power noise of the laser. 

I used a Thorlabs PDA100 and a SR785 to measure the power noise out of the laser directly, and saw a huge forest of peaks above 20kHz. Among them, a couple of peaks are moving up and down in frequency very fast. The plot below compares two different times of the Thorlabs HNL210L laser (the new one, 21 mW) with the old JTSU laser we are using for the test setup:

The noise of the new laser is cleary much larger (even after the laser has been on for some time) and non stationary. This is a big issue for us. I will contact Thorlabs to inquire if this behavior is normal.

The attached video file shows the peaks dancing around on the SR785 screen.

The Thorlabs laser has been misbehaving for the whole weekend. Even after many days being continuosly on, the wandering line is still moving all over the frequencies.

So this morning I swapped in a JDSU 1125P borrowed from the 40m lab, which provides about 6.8 mW of power. I tested it over the weekend on a separate test table, and after one day or so of operation the power looks reasonably stable. Now it's been on for a few hours: there is still a line moving around, but it's slowing down and hopefully setting down in a good place.

I started a series of test measurements on the samples that were already installed.

• Quiet time before excitation: 1170443490
  Excitation (broad band) at 1170443523 (60 s)
  Quiet time after excitation: 1170443586

The high power lasers I tested so far (the Thorlabs 21mW and the JDSU 1125P) are noisy: they both have wandering lines that from time to time are alised down into the base band, destroing the measurement. 

I have three JDSU 1103P units: two of them dlived about 2.5 mW, the third one delivers about 1.4 mW. One of the 2.5mW was installed in the test setup. I swapped it out with the 1.4 mW, so now I have two good 2.5 mW laser. My plan is to modify the new setup to use those two lasers in parallel, splitting each one in two, for a total of four beams of about 1.2 mW each.

The new optical layout is atttached.

Today I swapped out the 8mW laser and installed two 2.5 mW lasers. I rebuilt the input part of the optical levers and re-aligned everything. See below for a picture of the new setup: red beams are input, yellow beams are output. I also installed a protective screen all around the table, to abvoid any suprios beam to get out.

The lasers are behaving well, there is no high noise or wandering lines. The spectrum below is taken in air: that explains the excess of noise in the few kHz region.

This morning I measured the beam profiles at the QPD positions. As expected, the beams are not gaussian there, since we are seeing the interference of the reflections from the two faces of the disks. Nevertheless, the measured sizes are

Here are the images of the beams:

The arm lever lengths (distance from the disk surface to the QPD) can be estimated from the optical drawing:

The optical gain (normalized QPD signal over disk surface angle) is given by

Since I had recurrent problems with the picomotors used for QPD3, I swapped them with another Newport motorized mirror that was previously used in the Crackle1 experiment. This is the same model used for the other three QPD centering. Everything looks to be working fine now.

I also realigned all optical levers and swapped out an iris with a smaller one, to avoid beam clipping. All beam paths look clear now.

This afternoon I removed the old periscope from CR0 and installed a new one with finely adjustable mount, like those in the new chamber. I realigned the optical lever to the horizontal refererence.

The JDSU HeNe laser 1103P that I was using is dead. I swapped it with a JDSU 1125P borrowed from the 40m.

This afternoon I installed the new Lumentum (former JDSU) HeNe laser, model 1103P in CR0.

Realigned everything.

I installed a sample in the chamber to reflect a beam back inot the QPD. Checking the QPD signals over a hour and more did not show any sign of excess noise or instability.

I realigned all optical levers to measure the 50mm disks. In brief, I moved the input 2" mirrors, the in-vacuum 2" mirrors and the PZT mirrors so that the beam hits the 50mm sample and gets back into the QPD. Re-aligned everything to the horizontal reference using water.

I realigned the entire CR1-4 setup

• ensured that beam out of the chamber are at 6" height everywhere. Moved some iris to center them
• ensured that the beam hits the 45 degrees in-vacuum mirrors at a height of 3.78" = 96 mm
• checked that all incoming beams are roughly centered on the 45 degrees in-vacuum mirrors. Move some of them in-vacuum mirror posts
• realigned the input steering mirros horizontally and the in-vacuum mirros so that the beams reflected from a water-level are not clippe anywhere
• realigned the horizontal reference to the center of all QPDs