Current Issues of the IFO design

This page summarizes the current issues of the LCGT IFO design and provides links to pages discussing the details of each problem.

Error assessments

To realize the design sensitivity, all of IFO parameters should be close to the designed nominal values. Error assessments aim to evaluate that how large tolerances of the IFO parameters are acceptable.

Arm Cavity Parameters

Arm Cavity Design


1550 is the default value decided by the IFOBW working group.


g-factor (mirror ROC)

g1 = g2 = sqrt(1/3) = 0.57735 is the conventional number to avoid HOM resonances. For L = 3000.0 m, it gives ROC = 7098.08 m.

There are several factors to determine the g-factor.

Thermal noise and beam radius

Coating Brownian and thermoelastic noise are the largest noise sources; coating is the biggest at f<500 Hz and TE is the biggest at 500 Hz<f. Coating TN is inversely proportional to the beam radius while TE noise is almost independent from the beam radius at low temperature.

Sidles-Sigg instability

With the current design of the arm cavity power (420 kW), the two eigen-frequencies of the radiation-pressure-induced angular springs are 1.7 Hz and 0.88 Hz for the Sapphire mirror of 30 kg (moment of inertia = 0.173 [kg*m2]). For positive g-factor, 1.7 Hz becomes unstable whereas it is 0.88 for negative g-factor.

Mirror Size

Mirror size requirements are first set by the beam spot size on each mirror. The mirror radius should be larger than 2.7*(beam radius of 1/e^2) so that the diffraction loss is less than 1ppm.

For LCGT, we do not want to have too many variations of suspensions. This is another factor to be considered when choosing mirror sizes. Different mirror size require different suspension design. So we don't want to have many different sizes of mirrors.

Mirror size issues are discussed here. Mirror Size.

Detection Ports

MC length


Optical Layout

The optical layout of the main IFO is related to the choice of the wedge angle, arm tunnel slope ans so on.

Details are discussed in LCGT Optical Layout

Original Slides

Technical issues on the control scheme

Disscussion is here.

LCGT/subgroup/ifo/ISC/Issues (last edited 2011-06-17 00:41:19 by OsamuMiyakawa)