Change in the optical layout

The major change in the optical layout is the change of the cavity lengths for PRC and SRC. Since the modulation frequencies of 11MHz and 55MHz are used for the control of the interferometer, the lengths of PRC and SRC are significantly streched.

The actual modulation frequency is 11.065399MHz and its integer multiples. (based on the measurement of the MC length on 29th Sept, 2009)

This corresponds to the following lengths of the cavities:

• PRC: 6.773 m

• SRC: 5.419 m

This change result in numbers of modifications in the optical configuration:

• In order to accomodate longer PRC and SRC, they have to be folded across the several optical tables.
• The curvature radii of PRM and SRM are recalculated accordingly.
• We are going to employ 2inch flat folding mirrors suspended with Enhanced LIGO/Advanced LIGO style Tip-Tilt suspensions with passive damping using eddy current.

New Optical Layout plan

The optical layout in the central part will be changed significantly to house folded recycling cavities. The current layout plan is posted here. attachment:40mUpgradeOpticalLayoutPlan01.pdf And zoomed in to the important parts: attachment:layout-zoom.pdf

The original CAD file is in the VariCAD format. The file can be found in /cvs/cds/caltech/users/aso/doc/CAD40m/2D/40mUpgradeOpticalLayoutSRMinITMX.dwb

VariCAD is installed on rosalba and allegra. (Now KA is switching VariCAD/dwb to AutoCAD/dwg based work.)

The beam size indicated by the three parallel lines in the drawing is 1" diameter. This is larger than the actual beam size. The waist size of the arm cavity is about 3mm (1/e^2 radius) and the Rayleigh range is about 27m. The beam path length inside the central part is less than the Rayleigh range. So the beam size is less than 4.2mm. If the beam is not clipped within the 1" diameter, the power loss is far less than 1ppm.

Here is the Mathematica notebook I wrote to calculate various parameters for the new recycling cavities. [attachment:NewRecyclingCavities.zip attachment:NewRecyclingCavities.zip]

The PDF version is here: [attachment:NewRecCav.pdf attachment:NewRecCav.pdf] [attachment:GaussianOpticsTutorial.pdf attachment:GaussianOpticsTutorial.pdf]

This notebook explains how the cavity lengths, recycling mirror ROCs, PRM reflectivity are chosen.

Effect of the wedge

All of the main optics have wedge angles of several degrees in order to avoid undesirable interference of back-surface reflections into the main beam. Because of these wedges, we need to design the optical path slightly shifted from the normal angles.

In order to incorporate the effect of the wedges, a Mathematica code to calculate the beam deflections has been developed. attachment:wedge_analysis.nb

During the installation we like to align the beam spot onto ETMs while the beam goes through the center of ITMs. However, if we put the actual ITMs at their correct positions, we will not be able to see the spot at the end. If we remove the ITMs, the deflection of the beam is not provided, and the correct alignment is not obtained.

One possible solution is to put a wedged plate which has no coating. If the wedge angle is the same as the final optics, the resulting beam has the desired deflection angle. Even though the difference of the thickness shifts the beam in parallel, the amount of shift is small and will not be a significant problem. The multiple reflections will allow us to align the pick off beams as well. The candidate of the wedge plates are:

attachment:ITMY_wedge_1deg.png

attachment:ITMY_wedge.png [attachment:ITMY_wedge.pdf attachment:ITMY_wedge.pdf]