Finesse: Frequency domain INterfErometer Simulation SotfwarE Andreas Freise 21.10.2010 afreise@googlemail.com README ------------------------------------------------------------ Finesse [1] is a numeric simulation for laser interferometers using the frequency domain and Hermite-Gauss modes. This document gives a short overview of the main features of Finesse. Please see the file INSTALL for information on installing and running Finesse. 1. Interferometer signals 2. Beam geometry and imaging 3. Documentation, testing and examples 4. Examples of the impact of Finesse on commissioning tasks 5. Finesse and MATLAB 6. References ------------------------------------------------------------ 1. Interferometer signals ------------------------------------------------------------ Finesse can be used to compute a great variety of interferometer signals for control systems, including longitudinal control, alignment control and thermal compensation, for example: - transfer functions and error signals using up to five demodulations per photodiode. - detectors for amplitude, phase, intensity (all three can be given integrated over the beam or as CCD-like images), user- defined split detectors - noise propagations, such as laser frequency noise or oscillator phase noise 2. Beam geometry and imaging ------------------------------------------------------------ One of the main features of Finesse is the extensive integration of physics related to the beam shape. This makes it possible to study interferometer signals in the presence of defects such as misalignments and mode mismatch, mirror surface defects, thermal deformations and mis-centred, split photo detectors. Finesse can also model imaging properties of optical systems, for example, it automatically determines eigenmodes of cavities and interferometers. Gouy phases and beam waist positions can be plotted as functions of positions of optical elements. 3. Documentation, testing and examples ------------------------------------------------------------ The simulation code has been developed and improved continuously over the last ten years. It has been frequently and successfully tested against experimental data from GEO 600 (and sometimes using LIGO and Virgo data). The code is under version control and is executed within a nightly test-suite to maintain stability during the ongoing development. The program is easy to use for students: For the basic use, including graphical output, no commercial software is required. The implemented physics is well documented in a 180 pages manual. Simple examples are provided as well as detailed input files for all main interferometric gravitational-wave detectors. 4. Examples of Finesse usage for commissioning tasks ------------------------------------------------------------ The following examples highlight Finesse analyses of pressing problems in detector commissioning. Finesse predictions have been used to improve the detector performance and the Finesse results have been shown to match experimental results: - lock acquisition of the power-recycled GEO 600 interferometer: a suspension tilt instability was discovered as the source of severely distorted interferometer error signals [2] - thermal compensation of GEO 600: a wrong radius of curvature was discovered to cause unexpected beam patterns in the dark fringe. Finesse results for the beam pattern as a function of heater power were used to find the current operating point of the thermal compensation. [3] - detector characterisation of the Virgo arm cavities: Finesse has been used to characterise all details of the Virgo north arm cavity from the cavity Finesse to the astigmatism of the mode matching telescope. [4] - thermal lensing induced bi-stable operating point in Virgo: Finesse results were crucial in understanding the origins of a double zero crossing in a longitudinal error signal of Virgo. [5] - RF modulation induced change in interferometer noise couplings: Detailed measurements and Finesse simulations were used to understand the laser power noise coupling due to RF sidebands in higher order modes and how it limits the detector sensitivity. [6] 5. Finesse and MATLAB ------------------------------------------------------------ Finesse is a stand-alone executable written in C. It is, however, well interfaced with MATLAB. A number of MATLAB tools (m files and mex files) are provided to run Finesse simulations from MATLAB or to communicate with a running Finesse process from within MATLAB. 6. References ------------------------------------------------------------ [1] A. Freise, G. Heinzel, H. Lueck, R. Schilling, B. Willke and K. Danzmann, "Frequency-domain interferometer simulation with higher-order spatial modes", Classical and Quantum Gravity, Vol.21, (2004), available at http://www.rzg.mpg.de/~adf/ [2] A. Freise: "The Next Generation of Interferometry: Multi-Frequency Optical Modeling, Control Concepts and Implementation", Ph.D. Thesis, University of Hannover (2003), http://www.amps.uni-hannover.de/dissertationen/freise_diss.pdf [3] H. Lueck, A. Freise, S. Gossler, S. Hild, K. Kawabe and K. Danzmann, "Thermal correction of the radii of curvature of mirrors for GEO 600", Classical and Quantum Gravity, Vol.21, (2004) [4] A. Freise, M. Loupias : "The VIRGO north arm cavity: Examples for the use of the interferometer simulation Finesse", VIRGO note VIR-NOT-EGO-1390-269 (2004) [5] J. Marque: 'Input mirrors thermal lensing effect Frequency modulation PRCL length in Virgo' talk at LSC meeting, LIGO document number LIGO-G070338-00-Z (2007) [6] J. R. Smith, J. Degallaix, A. Freise, H. Grote, M. Hewitson, S. Hild, H. Lueck, K. A. Strain and B. Willke, "Measurement and simulation of laser power noise in GEO 600", Classical and Quantum Gravity, Vol.25, (2008)