LCGT IFO Mirror Size

Contents

LCGT IFO Mirror Size

Mirror size requirements from the beam spot size

The minimum mirror sizes are calculated requiring less than 1ppm diffraction loss. This translates into the mirror sizes 2.7 times larger than the beam spot size. The situation is a bit more complicated for BS. Hiro Yamamoto calculated the loss of the light power in the PRC by the finite size of the BS using FFT simulation. BSLoss.pdf. Now we have to check what is the acceptable loss level for PRC ? 1000ppm ?

Optics	Beam Spot Size (1/e^2 radius)	Minimum Mirror Size (diameter)
ITM	3.5cm	19cm
ETM	3.5cm	19cm
BS	3.5cm	73sqrt(2)+2t/sqrt(2n^2-1), where t is the thickness and n is the refractive index. For t=10cm, d=40cm. Probably this value is a bit overkill. See Hiro Yamamoto's Calculation.
PR3	3.5cm	19cm
SR3	3.5cm	19cm
PR2	less than 1cm	10cm is enough
SR2	less than 1cm	10cm is enough
PRM	less than 1cm	10cm is enough
SRM	less than 1cm	10cm is enough

Mirror Size Plans Considering Suspension Varieties

Plan A

For the 3rd year (iLCGT) configuration

All the mirrors except for BS are 25cmx15cm fused silica of 16kg. They are all suspended by the Type B suspension platform with the IP and filter stages fixed.
BS is 37cm x 6.85cm fused silica to make the weight 16kg.

For the bLCGT

The ITMs and ETMs are replaced with 25cmx15cm sapphire mirror of 30kg. The suspension platforms are also replaced with Type A.

Plan B

This was proposed by D. Tatsumi.

For the iLCGT

ITMs, ETMs, BS, PR3 and SR3 (the mirrors which handles large beam) are all 38cmx12cm fused silica of 30kg, suspended by the Type A SAS platform.
PRM, PR2, SRM, SR2 are 10cm class TAMA sized mirror. These are suspended from a smaller simple suspensions, probably in common with MC suspensions.Issues

For the bLCGT

ITMs and ETMs are replaced with 25cmx15cm sapphire mirrors.

Advantages

No need to replace suspensions between iLCGT and bLCGT.
PRM, PR2, SRM, SR2 may be easily movable (depending on their suspension design). This is important to adjust the folding telescope in the PRC and SRC.
Further reduce the number of suspension types by using the same design for MC mirrors and the small IFO mirrors.

Disadvantages

38cmx12cm BS will have a larger loss than 37cmx6.85cm one. Need to be checked.
PR3 and SR3 are unnecessarily large.

Mirror requirement summary from the IFO group

The minimum mirror sizes are as shown in the table above.
It is necessary that PRM, SRM, PR2 and SR2 can be moved by about 50cm after installation to compensate for the fabrication error of the ROCs of those mirrors.