R. Amin

Compensation of thermally induced modal distortions in Faraday isolators

Two methods of compensation of thermal lensing in high-power terbium gallium garnet (TGG) Faraday isolators have been investigated in detail: compensation by means of an ordinary negative lens and compensation using FK51 Schott glass possessing a negative dn/dT. Key thermooptic constants for TGG crystals and FK51 glass were measured. We find that the contribution of the photoelastic effect to the total thermal lens cannot be neglected for either TGG or FK51. We define a figure of merit for compensating glass and show that for FK51, an ordinary negative lens with an optimal focus is more efficient, but requires physical repositioning of the lens for different laser powers. In contrast, the use of FK51 as a compensating element is passive and works at any laser power, but is less effective than simple telescopic compensation. The efficiency of adaptive compensation can be considerably enhanced by using a compensating glass with figure of merit more than 50, a crystal with natural birefringence or gel.

Method for compensation of thermally induced modal distortions in the input optical components of gravitational wave interferometers

The next generation of interferometric gravitational wave detectors will employ laser powers approaching 200 W to increase shot-noise limited sensitivity. Optical components that transmit the laser light will exhibit increased thermal lensing induced by bulk absorption and concomitant changes in the material refractive index, resulting in significant changes in the modal characteristics of the beam. Key interferometer components such as electro-optic modulators and Faraday isolators are particularly at risk, since they possess relatively large absorption coefficients. We present a method for passive correction of thermally induced optical path length (ΔΛ) changes induced by absorption in transmissive optical components. Our method relies on introducing material in the optical path that possesses a negative index temperature derivative, thereby inducing a compensating opposite ΔΛ. We experimentally demonstrate a factor of 10 reduction in higher order spatial mode generation for terbium gallium garnet, a Faraday isolator material.

Adaptive control of laser modal properties

An adaptive optical system for precise control of a laser beams mode structure has been developed. The system uses a dynamic lens based on controlled optical path deformation in a dichroic optical element that is heated with an auxiliary laser. Our method is essentially aberration free, has high dynamic range, and can be implemented with high average power laser beams where other adaptive optics methods fail. A quantitative model agrees well with our experimental data and demonstrates the potential of our method as a mode-matching and beam-shaping element for future large-scale gravitational wave detectors.

LIGO-Virgo searches for gravitational waves from coalescing binaries: A status update

Coalescing compact binaries of neutron stars and/or black holes are considered as one of the most promising sources for Earth based gravitational wave detectors. The LIGO-Virgo joint collaborations Compact Binary Coalescence (CBC) group is searching for gravitational waves emitted by these astrophysical systems by matched filtering the data against theoretically modeled template waveforms. A variety of waveform template families are employed depending on the mass range probed by the search and the stage of the inspiral phase targeted: restricted post-Newtonian for systems having total mass less than 35M?, numerical relativity inspired complete inspiral-merger-ringdown waveforms for more massive systems up to 100M? and ringdown templates for modeling perturbed black holes up to 500M?. We give a status update on CBC groups current efforts and upcoming plans in detecting signatures of astrophysical gravitational waves.

Gravitational-wave detector-derived error signals for the LIGO thermal compensation system

Thermal lensing has been one of the sensitivity-limiting factors for the LIGO detectors since their inception. Although estimates of such lensing were assumed when LIGOs core optics were specified, in practice a thermal lensing compensation system (TCS) was installed in 2004 in order to improve mode matching to the injected beam and ultimately detector sensitivity. This subsystems primary purpose was to induce a corrective thermal lens in the input test mass mirrors or LIGOs 4 km Fabry–Perot arms. A few empirically motivated means of monitoring the focal parameters of the input couplers were employed for the 2005–2007 science run, S5. We discuss results of a numerical model study, a set of signals, focal discriminants, that could have been used during S5 to set TCS compensation levels. Most of these signals would not have needed the installation of any new equipment or software. If investigated further, these focal discriminants may find utility in pathfinder projects as the next-generation LIGO detectors are commissioned.

Hierarchical Hough all-sky search for periodic gravitational waves in LIGO S5 data

We describe a new pipeline used to analyze the data from the fifth science run (S5) of the LIGO detectors to search for continuous gravitational waves from isolated spinning neutron stars. The method employed is based on the Hough transform, which is a semi-coherent, computationally efficient, and robust pattern recognition technique. The Hough transform is used to find signals in the time-frequency plane of the data whose frequency evolution fits the pattern produced by the Doppler shift imposed on the signal by the Earths motion and the pulsars spin-down during the observation period. The main differences with respect to previous Hough all-sky searches are described. These differences include the use of a two-step hierarchical Hough search, analysis of coincidences among the candidates produced in the first and second year of S5, and veto strategies based on a χ2 test.

Comparison of band-limited RMS of error channel and calibrated strain in LIGO S5 data

Aidan Brooks, David Hosken , Damien Mudge, Jesper Munch and Peter Veitch and are members of the LIGO Scientific CollaborationMany LIGO data analysis pipelines use either the DARM ERR or AS Q channels as the data source and use a response function R(f) generated from time-dependent calibration measurements to convert to strain in the frequency domain. As calibration varies on a timescale of tens of seconds, the response function must be updated frequently. An alternative is to use time-domain calibrated strain h(t). During the recent year-long LIGO science run (S5), preliminary strain data was published alongside raw interferometer output, typically within half an hour of the raw data being produced. As strain data is now available in highly-reduced form within the LIGO data archive, it represents a convenient alternative for LIGO search pipelines. This paper examines a measure of quality for calibrated strain data by calculating the band-limited RMS (BLRMS) difference between h(t) and strain he(t) as calculated directly from DARM ERR in the frequency domain.Many LIGO data analysis pipelines use either the DARM ERR or AS Q channels as the data source and use a response function R(f) generated from time-dependent calibration measurements to convert to strain in the frequency domain. As calibration varies on a timescale of tens of seconds, the response function must be updated frequently. An alternative is to use time-domain calibrated strain h(t). During the recent year-long LIGO science run (S5), preliminary strain data was published alongside raw interferometer output, typically within half an hour of the raw data being produced. As strain data is now available in highly-reduced form within the LIGO data archive, it represents a convenient alternative for LIGO search pipelines. This paper examines a measure of quality for calibrated strain data by calculating the band-limited RMS (BLRMS) difference between h(t) and strain he(t) as calculated directly from DARM ERR in the frequency domain.

Setting upper limits on the strength of periodic gravitational waves from PSR [Formula Presented] using the first science data from the GEO 600 and LIGO detectors

Data collected by the GEO600 and LIGO interferometric gravitational wave detectors during their first observational science run were searched for continuous gravitational waves from the pulsar J1939+2134 at twice its rotation frequency. Two independent analysis methods were used and are demonstrated in this paper: a frequency domain method and a time domain method. Both achieve consistent null results, placing new upper limits on the strength of the pulsars gravitational wave emission. A model emission mechanism is used to interpret the limits as a constraint on the pulsars equatorial ellipticity.