Hugh, Evan
Over the course of several locks, we moved the HAM4 HEPI in the y direction by several hundred microns in order to see the interferometer behavior at 2 W with the SRC locked on different fringes.
For a 1.1 mm shift in the HEPI position (i.e., a 2.2 mm shift in the one-way SRC length), there is no discernible trend in the rf sideband buildups or in the DARM pole frequency.
We would like to repeat this test with SR2 offsets as well.
Craig C, Rana
We've been looking at the Gouy phase in the LHO SRC to understand the mode hopping, matching, etc.
The attached PDF shows something like the diagonal elements of the Jacobian: the change in the round trip Gouy phase as a function of each of 4 distances and 4 Radii of Curvature. The goal for the RT Gouy was 38 deg assuming a 50 km thermal lens.
In the plots the zero position on the x-axis is the nominal position of each optic according to (E1400205) the IAS as-built numbers OR the RoC according to the specs/measurements in optics database (galaxy.ligo.caltech.edu/optics).
The first page of the PDF shows the situation in the nominal warm state (ITM thermal lens with 50 km focal length). The second page is with no thermal lens. I have ignored the curvature of the CP as well as the cold lens in the ITM due to index inhomogeneity; assuming for now that these effects are small.
As you can see by flipping between pages 1 & 2, the ITM thermal lens stabilizes the SRC by increasing the round trip phase shift by ~15 deg.
So, is the LHO mode hopping problem due to a 0.05% RoC increase of SR3? Or is the LLO SRC more stable because it has a short RoC?
Is it possible that the alignment trouble with the LHO SRC can be mitigated by increasing the Gouy phase shift? If so, perhaps we could determine this by translating the HAM4/5 HEPI as well as pushing on the M1 stages of the SUS. If it goes in the right direction, perhaps we can make a bigger correction using the screws on HAM4 and get more like a 1 cm motion of the SR3-SR2 length without venting the main volume.
Uncertainties:
- The placement of the SR2 and the SR3 has a small uncertainty in the x & z axes, but the y-direction (the important one for the SRC) has an uncertainty of +/- 1.5 mm since the measurement method is different.
- The RoC uncertainty on the SR3 is ~15 mm or dRoC/Roc ~ 0.05%. This corresponds to a Gouy phase deficit of ~15 deg.
- The RoC uncertainty of the SR2 is the next most important parameter, but even a 0.1% error (standard off-the-shelf tolerance) would not be so bad.
- We are very insensitive to the distance between the ITM and SR3, as well as the SR2-SRM distance.
- Similarly, there is no conceivable distortion of the SRM temporary mirror which could destabilize the SRC.