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.
Four substrates from University Wafers (76.2mm / 0.5mm) in chamber
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.
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.
As decided at out Red Door meeting, we're going to clean the vacuum chamber and move it to the large table, which will be enclosed in a clean room.
So today we disassembled and packaged the vacuum chamber, which is now ready to be trasnspoted to be cleaned and baked.
This afternoon we opened the tall belljar vacuum chamber, and took everything out of it. All the stuff that was inside the chamber is "temporarily" stored on the floor beside the optical table.
We installed a "spacer" into the chamber, made from one of the optical table legs that were sitting in the hallway. We installed one of the aluminum base plates on top of it, so that the optical components will be at the level of the viewport. Another leg and a thinner base plate are installed out of the chamber, at a similar level.
After this we closed the chamber with one of the flats used for the old chamber, and a rubber o-ring. We started the roughing pump, quickly reached a pressure below 1 mTorr and switched on the turbo pump. Unfortunately, it seems that the low pressure gauge is not working properly, so we don't know what's the pressure right now. We'll check the gauge and controller tomorrow morning and swap it out if needed.
Fixed the 307 gauge controller (a missing contact on the rair panel). The low pressure gauge was connected to 1G port and has measured 1.7E-6 torr. We are not sure since how long the turbo was operating (no vacuum logger yet).
Installed a gate valve between the roughing and tubbo pump. See below a pump down curve. The convection gauge is not calibrated. the turbo started at 14th min (at about 3 torr)
At 5pm the pressure was 6.5e-6 torr.
Checked again at:
We connected the analog output of the vacuum gauge controller to one of the ADC channels. The signal is calibrated so that the pressure is 10^(X3:CR1-PRESSURE_LOGTORR_OUT). Unfortunately the RTG does not know how to compute 10^x...
I added three 1"x1" viton pads below the base plate, and realigned the entire optical system to the horizontal reference
The pads seem to reduce a bit the vibrations in the QPD_X direction, but significantly improve the situation in the QPD_Y direction, see below:
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 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.
While I was working, the network connection went down. I tried to reboot the workstation, but I won't boot anymore.
Working on it...
Reinstalled Debian 8, all packages and CDS software.
Everything seems to be working fine now.
We were confused a bit about how the camera image changes when you move the arm that holds the camera and lens around the mirror. It seems that scattering centers move in ways that cannot be explained by a misaligned rotation axis. So we wanted to make sure that the mirror surface is actually imaged as we intended to. We generated a white grid with 0.7cm spacing and black background on a monitor. The image that we saw is exactly how we expected it to be. So the image mystery has other reasons.
My bad, a cable was disconnected.
Finally, I tried to excite the disks, but I couldn't get any motion of the optical lever beams. To be investigated, there might simply be some cables disconnected.
Here a little purple video. It starts with scattering angle around 15deg and stops at about 80deg.
There are some clear point defects visible especially at small angles.
I will not start to think about some other interesting details of this video before I got the new lens.
Ed: The AVI did not run on Mac. I posted it on youtube. Koji
We have the new 405nm laser pointer. The image to the left shows the scattered light from the red laser, the image to the right scattered light from the purple laser. Both images were taken 30deg with respect to the normal of the mirror surface. Also, we got a new gallon of Methanol. After cleaning the mirror multiple times, the scattered light became significantly weaker. So the purple images look very different from red and green. It could be that the lens that we use to image the mirror surface is the problem since it is specified for the wavelength range 1000nm-1550nm. Could it also be the CCD camera? Anyway, to be sure I will order another broadband lens.
Today we improved alignment of the lens-camera arm. We discovered earlier that this alignment affects the amount of "snowfall" on the scattering images. Looking at the latest 405nm video (see attachment), one can still see snowfall, but it is considerably weaker now and the true scatter image is clearly visible. We took a set of scatter images at certain scattering angles and produced BSDF curves. The shape of these curves has partially to do with the snowfall contribution, but one also has to keep in mind that the mirror quality is much worse than what has been used in the Fullerton measurement. We still need to calibrate these curves. The calibration factor is different for the two images so that you cannot even compare them at the moment except for their shape.
Today we also got the new broadband lens for the camera arm. First measurements show that image quality is better. Playing a bit around with distances between object mirror, lens and image plane, we also found that image quality becomes better when the lens and camera get closer to the mirror (which is only an issue for the 405nm measurement since 633nm and 532nm look very good anyway). So we are thinking to change the camera arm setup to make it much shorter.
Nothing has happened since Steve, the visiting highschool teacher, has left. Meanwhile, some parts of the multi-color BRDF setup were delivered. I assembled everything today and realigned the lasers. Everything is ready now for a three-color BRDF measurement (the previous Richter record was 2 colors). I will claim back my video capture device as soon as possible from my neighbors and then take new images.
M. D. Ediger, in PNAS (2014), pp. 11232–11233.
A. J. Leggett and D. C. Vural, arXiv cond-mat.dis-nn, arXiv:1310.3387 (2013).
L. Berthier and M. D. Ediger, arXiv cond-mat.mtrl-sci, 40 (2015). Phys. Today.
G. Parisi and F. Sciortino, Nature Materials 12, 94 (2013).
S. Singh, M. D. Ediger, and J. J. de Pablo, Nature Materials 12, 139 (2013).
Steve Maloney, a visiting highschool teacher, and I have started to set up a new scattering experiment in the Richter lab. The idea is to take images of large-angle scattered light using different lasers. We have one 633nm laser, and 532nm and 405nm laser pointers. The goal is to uniformly illuminate the same disk of about 1cm diameter on a silver-coated mirror with all three colors. We use a silver-coated mirror to make sure that the light is reflected from the same layer so that all colors are scattered from the same abberations.
The image shows one of the laser pointers and the HeNe laser. The first step is to widen the beam with a f=5cm broadband, AR coated lens (Newport PAC15AR.15). The diverging beam is then aligned through an iris to give it the right size on the mirror. In this way, illumination is almost uniform on the mirror surface.
The mirror is mounted over the rotation axis of a unipolar stepper motor. For the moment we only took images from fixed direction (initially with a commercial digital camera, later with a monochromatic Sony XT-ST50 CCD camera. The problem with the commercial camera was that you cannot completely control what the camera is doing. Also it would have been very difficult to calibrate the image once you start comparing scattering with different colors. A f=7.5cm lens is used to image the illuminated disk on the CCD chip to make maximal use of its resolution. The CCD signal is read out on a Windows machine with an EasyCap video capture device connected to a USB port. Standard software can then be used to take images or record videos. For some reason the capture device reduces the image size to 640x480 pixels (a little less than the size of the CCD chip).
Eventually the camera and lens will be mounted on a metal arm whose orientation is controlled by the stepper motor. The stepper motor was part of the Silicon Motor Reference Design (Silicon Laboratories). It comes with all kinds of cables and a motor control board. Software is provided to upload compiled C code to the board, but for our purposes it is easiest to use primitive communication methods between the PC and the board. We are working with HyperTerminal that used to be part of Windows installations, but now it has to be downloaded from the web. This program can send simple commands through TCP/IP and COM ports. These commands allow us to position the motor and define its rotation speed. Since our PC does not have a serial port, we purchased a Belkin USB Serial Adapter. You will have to search the web to find suitable drivers for Windows 7 x64. Luckily, Magic Control Technology has similar products and the driver for their U232-P9 USB/serial adapter also works for the Belkin product.
So our goal for the remaining weeks is to take many images from various angles and to set up the experiment in a way that we can VNC into our lab PC and control everything from the Red Door Cafe.
We played around with Matlab today. The first step was to convert light wavelengths into RGB colors. In this way we can combine images taken at different colors. The picture shows the purple and red images (stored in gray scale) in heat colormap. Then the sum of these two images is calculated in their natural RGB colors.
Please see prosidures for pumping down and venting with air for the test vacuum chamber here https://dcc.ligo.org/T1600304
The following two pictures were taken from the same angle with green (left) and red (right) incident laser at an angle of 15deg from the incident beam (reflected to about -5deg). Some scattering centers are collocated. The green laser power is about 5 times as high as the red laser power, but this factor does not seem to calibrate the image well (the green image becomes too dark dividing all pixel values by 5). So there seems to be a significant difference in the divergence of the two lasers. We will have to use a photodiode to get the calibration factor. These images were taken after cleaning the mirror. Before cleaning, there was way too much scattering and the images were mostly saturated.