IFO Modeling

Modeling of the interferometer is always useful for many purposes:
- Interferometer design
  - Optical configurations
  - Signal extraction and control schemes
  - Noise budget
- Trouble shooting / Noise hunting
Once upon a time analytical calculations were almost the only way for any meaningful interferometer calculation. But that sort of folkloric era has been over. The interferometer configuration have got much more complicated since recycling techniques were committed to practical use. Now we are facing on the dual recycling issues. None of the design could be done without IFO modeling. The modeling is now one of the indispensable skills of a real interferometer person.

Key points of the IFO Modeling

Optical configuration
- Recycling cavity lengths and effect of arm cavity resonances
- Effect of optical losses
- Power Budget / Recycling gains
Signal extraction
- Length signal extraction
  - Sensing Matrix
  - 3f demodulation for lock acquisition
  - Detuning
  - Signal handing off procedure
- Alignment signal extraction
  - MC WFS
  - Dither alignment
- Aux Laser injection
Noise budget
Higher order mode (HOM) studies
- Contrast defect by the curvature mismatch
- HOM cleaning/enhancement by recycling
- Misalignment and alignment sensing

IFO modeling tools can be found in the svn directoryhttps://nodus.ligo.caltech.edu:30889/svn/trunk/ifomodeling/.

By Matt Evans.
The Optickle modeling package is part of the ISCmodeling tools. The SVN contains the latest version as of Feb-02-2010.
It is independent to looptickle. Optickle Wiki

The 40m model file is contained under the config40m directory of looptickle.
The model represents the interferometer according to this layout:
- attachment:IFO_diagram.png
A dual recycled interferometer could be set in different configurations (for a discussion on the topics see: Kokeyama K. et al. Class. Quantum Grav. 25 (2008) 235013 (12pp)).
The 40m will follow the choices for Advanced LIGO.
The single sub-cavities of the model are individually set in the following ways:
Arm Cavities: resonant for the carrier; non-resonant for the sidebands
Michleson: resonant for the carrier; Schnupp asymmetry of 0.0307 m
PRC: anti-resonant for the carrier; resonant for both sidebands
SRC: resonant for the carrier; resonant for the f2 sideband; non-resonant for f1
In particular, since the model adds no microscopic offset to the SRM position, and the cavity length is set equal to c/f2, the SRC is by default resonant for the carrier. That means that when the arms are locked, the carrier becomes anti-resonant and f2 resonant.
(By adding an optional microscopic offset of lambda/4 the SRC would become resonant for the carrier when the arms are locked. In that case f2 would be anti-resonant. - not desirable)
[attachment:IFOtuning.pdf In this document] I discuss the details behind the choices.

By Stefan Ballmer. The documentation file for it is [attachment:looptickle_README.txt this].
Next time Stefan comes visit us, we can try to ask him to explain Looptickle a bit more.
It is crucial for the demo files in the looptickle package to run, to have the Matlab path set properly.
Especially since the iscmodeling directory often contains different copies of the same functions,
some of which are obsolete and just cause conflicts.
Also looptickle gets confused by other junk optickle and looptickle structures, or tickle matrices left around in the matlab workplace. It's highly recommended to clear the workplace before starting to work with a new structure. Otherwise error messages are very likely to interrupt you.
Here's how the command lines for the first run should be:
- ```
   1      clear all
   2      restoredefaultpath
   3      cd <your path to the iscmodeling package>/iscmodeling/looptickle
   4      addpath(genpath('../Optickle'));
   5      addpath(genpath('./'));
   6      addpath(genpath('../gwinc'));
   7      addpath(genpath('../40m'));
```

By Alberto. It is a semi-analythical modeling package. The two building blocks of the models are a Fabry-Perot function and a Michelson function. Each function replaces either a cavity or a Michelson, with an effective compound mirror. The functions return complex reflectance and transmittance. The input parameters are the most generic. The surfaces of all mirrors are treated individually, i.e. reflectance and transmittance can be set independently for each side of a mirror.
Macroscopic length and microscopic offsets are passed to the functions as parameters.
By nesting any combination of cavities in the proper way and order, one can build up a compound mirror representing coupled cavities or the whole interferometer. Then the reflectivity and transmissivity of the compound mirror will be the Bright and Dark port outputs, respectively.
All functions contain a help description on how to use it.

SIS is an FFT-based simulation program written and maintained by Hiro. It is useful for simulation of non-ideal optical surfaces (surface roughness, mesa beams) and thermal effects (lensing, ROC errors).
Introduction to SIS