FROSTI assembly was completed today. The RTD and power wires were terminated at the DB-25 connectors and the legs were put on. It is currently placed in front of the stand-in test mass (~5 cm away). The FLIR has also been moved back to it's nominal position. As of now, it appears there are some shorts within the power cabling. This will be a focus of tomorrow's work.

Upon finishing the FROSTI assembly last week, we ran into some electrical issues. An electrical short was found between two of the d-sub pins (2 and 8). It appears that the pins were somehow coming into contact with the aluminum surrounding them. This was causing the power supply to trip. The issue was seemingly fixed by adjusting the positioning of the cabling leading out of the reflector. When handling the device in the future, please make sure to keep the wiring as undisturbed as possible. The setup is rather fragile, and moving the cabling around could potentially reintroduce a short like this.

Below is the dark noise spectrum of the Red Pitaya, which was measured over the course of a weekend. Additionally, I have successfully measured a signal from the photodetectors with the FROSTI as the IR source, so it seems there shouldn't be any worry of these particular detectors not being feasible for the RIN measurement.

I tried adjusting the gain settings on the photodetectors to check if this would help improve the RIN spectra measurements. Overall, it doesn't look like it does, and if anything, looks worse. I assume this is so because as the gain is lowered, the amount of detectable signal from the FROSTI becomes smaller and smaller.

FLIR: After some adjustments, the plot generated from the FLIR measurements look much more symmetric (see attachment). There are more included grid points, which smooths out the curve as compared to last week.

Red Pitaya: It looks like a new OS update was released for the RP, which includes a new Python API (was previously only available in C). I'm going to try and update the one we have currently running in lab.

I have been looking at the feasibility of an alternative heater element design for FROSTI that replaces the original ring-like heater elements with n rectangular elements with straight edges. They form an n-sided regular polygon that could well approximate the original annular ring if n is large enough. This eliminates curved surfaces requirement for the heater elements, which was the source of the many month production delay for the prototype parts.

This design was implemented in COMSOL, shown in the attached. From the face on view, each element has a trapezoid shape with straight edges. The edges between neighboring elements are parallel, with a space of 2 mm in between them.

The ray tracing and thermal analysis obtained from COMSOL are shown in the attached pdf.

In particular, the 2D irradiance profiles were obtained from the ray tracing (so far from the front heating surfaces only). The 1D radial profiles were integrated and shown in the attached. The power delivery efficiency for the original ring-like heater element design is integrated to be roughly 65%, for comparison. The plot also shows the radial irradiance profiles for three different straight-edge designs, which correspond to 16 edges, 18 edges, and 24 edges. We see that with the straight-edge designs, the irradiance profiles stay in a good Gaussian shape. In addition, with a larger number of edges, the power efficiency increases, but is always less than the case for the optimized ring-like design.

The thermal distortions for the TM were also obtained from COMSOL, using the irradiance profiles at the TM HR. As shown in the attached, with the straight-edge design, the effects on the thermal lens OPD and the HR surface deformation are similar to the ring design, but with less severe edge roll-off for instance.

In the previous post, we saw that for the heater element design with straight edges in replacement of the current eight-element ring-like design, it provides the similar Gaussian-like irradiance profiles, but with smaller power delivery efficiencies, as shown in the plot. This turned out to result in similar but less prominent thermal effects.

They only differ from the original baseline design by a source power rescaling, however, as shown in the plot, where we see the power-rescaled irradiance profiles for the straight edge designs are close to that for the ring design. The resulting temperature profiles and thermal distortions are shown in plot and plot. The thermal effects for the 16 straight-edge design with renormalized source power for instance are strikingly similar to that for the original ring design.

An alternative straightened heater element design has also been investigated with COMSOL FEA simulation. As shown in the attached, in this new design each heater element component is cut with multiple straight edges but remains connected, shown in the same colors (green and red). In the example, four straight edges are cut from each of the four heater components (4x4=16 edges in total). There is no spacing between the neighboring edges from the same element component, but the edges from different components are separated by 2mm, as can be seen in the attached. This new N-in-one straight edge design offers similar irradiance compared to that for the evenly-spaced N-sided regular polygon straight edge design with the same number of edges, as shown in the plot. It however has fewer heater components, four in this case, which makes it easier to implement in assembly and wiring, and less vulnerable to electrical and thermal shorts with their fewer heater element pins.

Drawings and CAD models of the straight-edge designs are exported, and are visualized in SOLIDWORKS. Two are attached. One is a single edge of the evenly spaced polygon design with 16 edges, and the other is the 8x2 design, with two neighboring edges grouped together to replace the original single curved heater.

For the straight edge design in COMSOL, ray power detectors were placed at the heater's front surface. The irradiance is shown in figure. The amount of light rays deposited back to the heater is small when close to the center, where it is closer to the original ring. The ray power increases as we move further away from the center toward the edges. In addition, the total power integrated at the heater's front surfaces is about 21% of the original heater's emitted power. This could account for the power efficiency difference between the straight edge design and the ring design, as shown in plot for instance.

A quick update on the effective emissivity analysis for the CIT FROSTI testing:

I was able to (roughly) match the OPD data to a referenced COMSOL model, with an applied power of 12.6 W (as seen below). However, when changing the emissivity of the ETM in COMSOL, the dT profiles do not seem to change much. I am not sure as to why this is the case at the moment, and will continue to look further.

Additionally attached are the current RIN measurements of the FROSTI prototype. Shown is the PSDs of both channels, in reference to their individual backgrounds.

We have tested the heater to find emissivity, mounted the heater system to the optical table, and have taken irradiance maps of the heater projected onto the screen.

The heater's emissivity was determined by using a thermocouple in conjunction with the FLIR's temperature calibration. To attach the thermocouple to the heater initially, I used Kapton tape and ran both the wires of the heater and the thermocouple through the heater bridge. This allowed for the heater to rest on an optical post and be observed without anyone directly holding it, but there were some measurement issues. The thermocouple had a very wide range of temperatures it was reading, which may have been due to intermittent contact or a short between the two legs of the thermocouple. To solve this and make the temperature measurements more stable, we pried apart the two ends of the thermocouple (to ensure there was no short) and put tape on either side, leaving the end connection bare. This was then taped to the heater, and the thermocouple was much more stable. We also used a K-type thermocouple that has an adhesive tape on it already, which assisted with the intermittent contact as well. With the thermocouple measuring the temperature of the heater, we could point the FLIR directly at the heater and calibrate the emissivity until the FLIR and the thermocouple agreed. Cassidy's emissivity calculator was also used, as I could input a temperature and observe what the emissivity of an area was based on that temperature. We found the emissivity of the heater to be 0.57.

As a note, when observing the heater with the FLIR, it appeared that there was a hot spot in the center, where the Kapton tape sat. Because the Kapton has a different emissivity than the 304 stainless steel of the heater, the FLIR will read it as having a different temperature than it actually does. When using the FLIR in the future, be sure to ascertain whether there is a temperature difference somewhere or if there may be different emissivities.

Additionally, the first heater that I used was taken to a very high temperature and oxidized. The emissivity of this oxidized heater is not known, but could be good information for knowing how oxidation affects these heaters specifically.

To mount the heater system in front of the screen, I used 1/2'' optical posts and the mount I designed using COMSOL's CAD program. The heater was originally 2.5 inches away from the screen, and has since been moved back by an additional two inches so that we could observe the heater side of the screen with the FLIR. We wanted to see what temperature the heater side of the screen was when irradiated by the heater, and how that compared to the camera side of the screen. When the heater ran at 1.12 W of input power, the heater side of the screen had a max temperature of around 29.7 C, and the camera side of the screen read at about 29.5 C. This means that there is very little thermal loss between the two sides of the screen, and any insulation that the screen's adhesive may have is largely negligible. Additionally, the camera was placed at an angle and undetermined distance for these tests, confirming that the temperature measurements compensate well/don’t depend on changes in angle or distance between the camera and the screen. However, there was spots on the back of the screen that the camera was measuring as hot spots where there shouldn’t have been any. I have included an example below. It would be useful to run a test where the camera is directly on the back of the screen without the heater to characterize the screen and see if the hot spots are physically present on the screen or if this is an artifice of the camera because of something like angle of viewing.

Taking irradiance maps of the screen was straightforward. After checking that the emissivity of the screen is 0.99 by viewing it at room temperature, we monitored the max temperature while slowing increasing the wattage the heater was running at. There is not a large change until the heater is at around 95 C, at which point the screen began to rise in temperature from 27 C to 28 C. We took measurements of this while the heater was 2.5 and 4.5 inches away from the screen. The irradiance map has a very symmetrical and circular shape, but does not have the ring pattern that we expected. There may be a few reasons for this: there could be some conduction between the two sides of the screen that is causing the pattern to spread further, the heater setup may not be as ideal as it was modeled to be, or there could be a different, unknown issue.

TO DO:

- It would be useful to run a test of the camera in multiple different positions to ensure our conclusion that the camera’s measurements don’t depend on angle or distance (or that these factors are well accounted for in the current temperature calculations) is correct.

- Measure the back of the screen straight on to identify bright spots and possible reasons as to their appearance.

- Recalibrate camera to ensure it is still correct after testing in multiple positions.

- Take another irradiance map of the screen at a higher input power, as well as moving the heater close/further away to try and replicate the COMSOL irradiance maps. It would be useful to also redo the COMSOL modeling at lower powers and variable distances.

Pictures included of full table setup, the heater mount, the heater with Kapton tape attaching the thermocouple as well as FLIR's measured irradiance map.

[Tyler, Shane, Mohak, Cynthia, Luke, Michael, Luis]

The tabletops have been attached to the workbench frames. Unfortunately, one of the tabletops came out of the box with a large scratch and small dent in the middle. One of the electric top shelves is ready to be attached to the undamaged table, but the other is yet to be opened. Assembly will be completed Wednesday morning.

The workbenches are now completely assembled and put into their final places. Additionally, the tool chest has been moved.

I reproduced Cao's CE beamsplitter code (see below for example plots). I received the current info on the beamsplitter parameters for A+ and A# from GariLynn also. The next steps are to perform a similar power loss analysis on the anticipated A# beamsplitter.

After meeting and discussing the current state of our work with Prof. Fulda a few weeks ago, we have decided that the best next step for the gtrace project is its integration into finesse work. Our first step towards this integration involved creating a sequential beam trace in contrast to the previous non sequential gtrace simulations. A sequential beam trace not only allows for faster runtimes of the simulation (<1 second) but also allows for more direct reading of certain beam parameters (beam size, gouy phase, and angle of incidence). The sequential model was created alongside a yaml output which provides values of parameters, now including the angle of incidence on a mirror.

Last Monday, Pooyan gave a report to the Cosmic Explorer optical design team on the current state of our project and the ultimate goal of our work. During the same meeting another group working in the optical design team presented their own work with gtrace and optical design, focusing more on optimization of parameters based on desired beam sizes at each mirror. It might be a good idea to begin attempting to bring our individual projects together to allow for collaboration and further developments.

Currently, only the crab1 layout has a sequential trace model. Pooyan is currently working on creating a finesse model for crab1 to serve as a proof of concept for how gtrace could be integrated with finesse by providing useful values such as angles of incidence.

Attached is a brief recap PDF file. A video file showing separate HOMs plots for the cavity scan with ETM08 surface map is also attached.

The codes are available at https://git.ligo.org/uc_riverside/hom-rh/-/tree/main/SIS

For this study, we looked at a simple case with an aLIGO-like test mass geometry (R=0.17m, H=0.2m) plus a barrel RH with 0.02m width at 0.03m from the AR surface with FEniCSx. The irradiance profiles are constant inside the width along the longitudinal direction, and zero outside the width. For the baseline non-astigmatic actuation with constant irradiance azimuthally. We have obtained roughly equal quadratic actuations along the x and y directions, as shown in figure. The total delivered power on the entire barrel is normalized to 1 W. The actuation on the curvature per power Delta S/Delta P in this non-astigmatic case thus is 0.835 uD/W.

For the astigmatic case however, the irradiance for the regions from one diagonal is increased by a given amount, compared to the non-astigmatic case, whereas for the other diagonal regions is decreased by the same amount (thus the total power is unchanged at 1 W). The HR deformation when the power is changed by 50%, for instance, is shown in picture, where the deformation along the x direction is larger than the y direction. The deformation in each direction however remains quadratic, with different curvature per powers for the x and y components, as shown in plot. The actuation on the curvature per power for an increasing amount of astigmatism is shown in plot. In terms of Zernike polynomials, the maximum amount of Z22 (astigmatism) for 1 W of total power is 2um while the remaining curvature content (Z20) is 6nm. This is shown in plot.

For a quick projection for the resonant frequencies going from aLIGO to CE, the height and width of the BS are increased assuming the mass is increased from 14 kg to 70 kg, while keeping the aspect ratio fixed. The resonant frequencies for the two mechanical modes as a result becomes smaller, to 1.43 kHz and 2.11 kHz respectively, risking getting in the detection band.

Next step is to implement a mechanical ring with high stiffness outside the BS barrel to combat the decrement of the resonant frequencies of the relevant mechanical modes.

This is to optimize the FROSTI heating profile for ETM, by minimizing the residual RMSE of the HR surface deformation after the beam size weighted curvature is removed by the current RH. The parameters of the profile being explored are the location, width, and total power for the Gaussian Annulus. As shown in the attached series of plots, the optimal location is 9.9 cm, with a width of 7.7 cm, and a total FROSTI power of 12.7 W (for 1 W of Gaussian beam absorption). The residual RMSE is 1.2 nm. About 0.5% of the FROSTI power is lost at the edges of the TM.

For comparison, without FROSTI, the residual RMSE after the beam size weighted curvature removed by the current RH is 44.5 nm. When the width of the Annulus is set to be 3 cm however, the residual RMS is 3.1 nm, with much smaller FROSTI power needed at 4.7 W, and less power loss at 0.02%.

Created a Google Slides presentation to summarize all the mirror surface map information that we use for simulating interferometers. 

A+ expected maps are based on correspondence with G. Billingsley. The estimate for the A+ ITMs will be to take the “as polished” data and add coating non-uniformity to it. (T2000398) Neither of these are scaled for the precise thickness of the Ti:Ge coatings.

Google Slides link: https://docs.google.com/presentation/d/1ge-ciAiEdNyyTvSShYdZz2JpACFRY2W3JDpxHRqMnOQ/edit?usp=sharing