Showing papers by "Karsten Danzmann published in 2007"

Upper limits on gravitational wave emission from 78 radio pulsars

Abstract: We present upper limits on the gravitational wave emission from 78 radio pulsars based on data from the third and fourth science runs of the LIGO and GEO 600 gravitational wave detectors The data from both runs have been combined coherently to maximize sensitivity For the first time, pulsars within binary (or multiple) systems have been included in the search by taking into account the signal modulation due to their orbits Our upper limits are therefore the first measured for 56 of these pulsars For the remaining 22, our results improve on previous upper limits by up to a factor of 10 For example, our tightest upper limit on the gravitational strain is 26×10-25 for PSR J1603-7202, and the equatorial ellipticity of PSR J2124–3358 is less than 10-6 Furthermore, our strain upper limit for the Crab pulsar is only 22 times greater than the fiducial spin-down limit

Searches for periodic gravitational waves from unknown isolated sources and Scorpius X-1: Results from the second LIGO science run

Abstract: We carry out two searches for periodic gravitational waves using the most sensitive few hours of data from the second LIGO science run. Both searches exploit fully coherent matched filtering and cover wide areas of parameter space, an innovation over previous analyses which requires considerable algorithm development and computational power. The first search is targeted at isolated, previously unknown neutron stars, covers the entire sky in the frequency band 160–728.8 Hz, and assumes a frequency derivative of less than 4×10^(−10) Hz/s. The second search targets the accreting neutron star in the low-mass x-ray binary Scorpius X-1 and covers the frequency bands 464–484 Hz and 604–624 Hz as well as the two relevant binary orbit parameters. Because of the high computational cost of these searches we limit the analyses to the most sensitive 10 hours and 6 hours of data, respectively. Given the limited sensitivity and duration of the analyzed data set, we do not attempt deep follow-up studies. Rather we concentrate on demonstrating the data analysis method on a real data set and present our results as upper limits over large volumes of the parameter space. In order to achieve this, we look for coincidences in parameter space between the Livingston and Hanford 4-km interferometers. For isolated neutron stars our 95% confidence level upper limits on the gravitational wave strain amplitude range from 6.6×10^(−23) to 1×10^(−21) across the frequency band; for Scorpius X-1 they range from 1.7×10^(−22) to 1.3×10^(−21) across the two 20-Hz frequency bands. The upper limits presented in this paper are the first broadband wide parameter space upper limits on periodic gravitational waves from coherent search techniques. The methods developed here lay the foundations for upcoming hierarchical searches of more sensitive data which may detect astrophysical signals.

Quantum engineering of squeezed states for quantum communication and metrology

Abstract: We report the experimental realization of squeezed quantum states of light, tailored for new applications in quantum communication and metrology. Squeezed states in a broad Fourier frequency band down to 1 Hz have been observed for the first time. Nonclassical properties of light in such a low frequency band are required for high efficiency quantum information storage in electromagnetically induced transparency (EIT) media. The states observed also cover the frequency band of ultra-high precision laser interferometers for gravitational wave detection and can be used to reach the regime of quantum non-demolition interferometry. Furthermore, they cover the frequencies of motion of heavy macroscopic objects and might therefore support attempts to observe entanglement in our macroscopic world.

Searching for a stochastic background of gravitational waves with the laser interferometer gravitational-wave observatory

Abstract: The Laser Interferometer Gravitational-Wave Observatory (LIGO) has performed the fourth science run, S4, with significantly improved interferometer sensitivities with respect to previous runs. Using data acquired during this science run, we place a limit on the amplitude of a stochastic background of gravitational waves. For a frequency independent spectrum, the new Bayesian 90% upper limit is ΩGW × [H0/(72 km s−1 Mpc−1)]2 < 6.5 × 10-5. This is currently the most sensitive result in the frequency range 51-150 Hz, with a factor of 13 improvement over the previous LIGO result. We discuss the complementarity of the new result with other constraints on a stochastic background of gravitational waves, and we investigate implications of the new result for different models of this background.

Search for gravitational-wave bursts in LIGO data from the fourth science run

Abstract: The fourth science run of the LIGO and GEO 600 gravitational-wave detectors, carried out in early 2005, collected data with significantly lower noise than previous science runs. We report on a search for short-duration gravitational-wave bursts with arbitrary waveform in the 64–1600 Hz frequency range appearing in all three LIGO interferometers. Signal consistency tests, data quality cuts and auxiliary-channel vetoes are applied to reduce the rate of spurious triggers. No gravitational-wave signals are detected in 15.5 days of live observation time; we set a frequentist upper limit of 0.15 day−1 (at 90% confidence level) on the rate of bursts with large enough amplitudes to be detected reliably. The amplitude sensitivity of the search, characterized using Monte Carlo simulations, is several times better than that of previous searches. We also provide rough estimates of the distances at which representative supernova and binary black hole merger signals could be detected with 50% efficiency by this analysis.

Upper Limit map of a background of gravitational waves

Abstract: We searched for an anisotropic background of gravitational waves using data from the LIGO S4 science run and a method that is optimized for point sources. This is appropriate if, for example, the gravitational wave background is dominated by a small number of distinct astrophysical sources. No signal was seen. Upper limit maps were produced assuming two different power laws for the source strain power spectrum. For an f^(−3) power law and using the50 Hz to 1.8 kHz band the upper limits on the source strain power spectrum vary between 1.2×10^(−48) Hz^(−1) (100 Hz/f)^3 and 1.2×10^(−47) Hz^(−1) (100 Hz/f)^3, depending on the position in the sky. Similarly, in the case of constant strain power spectrum, the upper limits vary between 8.5×10−49 Hz−1 and 6.1×10^(−48) Hz^(−1). As a side product a limit on an isotropic background of gravitational waves was also obtained. All limits are at the 90% confidence level. Finally, as an application, we focused on the direction of Sco-X1, the brightest low-mass x-ray binary. We compare the upper limit on strain amplitude obtained by this method to expectations based on the x-ray flux from Sco-X1.

Laser beam quality and pointing measurement with an optical resonator.

Abstract: We present a compact diagnostic breadboard that is based on an optical ring resonator for measuring beam quality and pointing of single-frequency continuous wave lasers at a wavelength of 1064nm. To determine the beam quality of the coherent test beam, this optical resonator is used to perform a mode decomposition into Hermite-Gaussian modes. For our laser system, a power fraction in the fundamental Gaussian mode of 97.2%±0.2% was measured. Residual misalignment and mis-mode-matching to the resonator as well as the astigmatism and/or ellipticity of the test beam have been determined. Numerical simulations showed that measurements of the M2 factor and transversal intensity distribution are not suitable for determining this power fraction. To measure the beam pointing, the fundamental mode of the optical resonator was used as a stable reference. The pointing of the test beam was measured with the differential wave front sensing technique up to Fourier frequencies of 1kHz with a sensitivity to relative pointing of ∣ϵ∣=1×10−6∕Hz. Pointing measurements with an alternative method were performed and showed good agreement.

Search for gravitational wave radiation associated with the pulsating tail of the SGR 1806-20 hyperflare of 27 December 2004 using LIGO

Abstract: We have searched for gravitational waves (GWs) associated with the SGR 1806−20 hyperflare of 27 December 2004. This event, originating from a Galactic neutron star, displayed exceptional energetics. Recent investigations of the x-ray light curve’s pulsating tail revealed the presence of quasiperiodic oscillations (QPOs) in the 30–2000 Hz frequency range, most of which coincides with the bandwidth of the LIGO detectors. These QPOs, with well-characterized frequencies, can plausibly be attributed to seismic modes of the neutron star which could emit GWs. Our search targeted potential quasimonochromatic GWs lasting for tens of seconds and emitted at the QPO frequencies. We have observed no candidate signals above a predetermined threshold, and our lowest upper limit was set by the 92.5 Hz QPO observed in the interval from 150 s to 260 s after the start of the flare. This bound corresponds to a (90% confidence) root-sum-squared amplitude h^(90%)_(rss-det) =4.5×10^(−22) strain Hz^(−1/2) on the GW waveform strength in the detectable polarization state reaching our Hanford (WA) 4 km detector. We illustrate the astrophysical significance of the result via an estimated characteristic energy in GW emission that we would expect to be able to detect. The above result corresponds to 7.7×10^(46) erg (=4.3×10^(−8) M_⊙c^2), which is of the same order as the total (isotropic) energy emitted in the electromagnetic spectrum. This result provides a means to probe the energy reservoir of the source with the best upper limit on the GW waveform strength published and represents the first broadband asteroseismology measurement using a GW detector.

Coherent control of broadband vacuum squeezing

Abstract: We present the observation of optical fields carrying squeezed vacuum states at sideband frequencies from 10Hz to above 35MHz. The field was generated with type-I optical parametric oscillation below threshold at 1064nm. A coherent, unbalanced classical modulation field at 40MHz enabled the generation of error signals for stable phase control of the squeezed vacuum field with respect to a strong local oscillator. Broadband squeezing of approximately -4dB was measured with balanced homodyne detection. The spectrum of the squeezed field allows a quantum noise reduction of ground-based gravitational wave detectors over their full detection band, regardless of whether homodyne readout or radio-frequency heterodyne readout is used.

First cross-correlation analysis of interferometric and resonant-bar gravitational-wave data for stochastic backgrounds

Abstract: Data from the LIGO Livingston interferometer and the ALLEGRO resonant-bar detector, taken during LIGO’s fourth science run, were examined for cross correlations indicative of a stochastic gravitational-wave background in the frequency range 850–950 Hz, with most of the sensitivity arising between 905 and 925 Hz. ALLEGRO was operated in three different orientations during the experiment to modulate the relative sign of gravitational-wave and environmental correlations. No statistically significant correlations were seen in any of the orientations, and the results were used to set a Bayesian 90% confidence level upper limit of Ω_(gw)(f)≤1.02, which corresponds to a gravitational-wave strain at 915 Hz of 1.5×10^(−23) Hz^(−1/2). In the traditional units of h^2_(100)Ω_(gw)(f), this is a limit of 0.53, 2 orders of magnitude better than the previous direct limit at these frequencies. The method was also validated with successful extraction of simulated signals injected in hardware and software.

Three-port beam splitters-combiners for interferometer applications

Abstract: We derive generic phase and amplitude coupling relations for beam splitters-combiners that couple a single port with three output ports or input ports, respectively. We apply the coupling relations to a reflection grating that serves as a coupler to a single-ended Fabry-Perot ring cavity. In the impedance-matched case such an interferometer can act as an all-reflective ring mode cleaner. It is further shown that in the highly undercoupled case almost complete separation of carrier power and phase signal from a cavity strain can be achieved.

Charge measurement and mitigation for the main test masses of the GEO 600 gravitational wave observatory

Abstract: Spurious charging of the test masses in gravitational wave interferometers is a well-known problem. Typically, concern arises due to the possibility of increased thermal noise due to a lowering of the quality factor of modes of the test-mass suspension, or due to the potential for increased displacement noise arising from charge migration on the surface of the test masses. Recent experience gained at the GEO 600 gravitational wave detector has highlighted an additional problem. GEO 600 uses electrostatic actuators to control the longitudinal position of the main test masses. The presence of charge on the test masses is shown to strongly affect the performance of the electrostatic actuators. This paper reports on a measurement scheme whereby the charge state of the GEO 600 test masses can be measured using the electrostatic actuators. The resulting measurements are expressed in terms of an effective bias voltage on the electrostatic actuators. We also describe attempts to remove the charge from the test masses and we show that the use of UV illumination was the most successful. Using UV illumination we were able to discharge and re-charge the test masses.

Local readout enhancement for detuned signal-recycling interferometers

Abstract: High power detuned signal-recycling interferometers currently planned for second-generation interferometric gravitational-wave detectors (for example Advanced LIGO) are characterized by two resonances in the detection band, an optical resonance and an optomechanical resonance which is upshifted from the suspension pendulum frequency due to the so-called optical-spring effect. The detector's sensitivity is enhanced around these two resonances. However, at frequencies below the optomechanical resonance frequency, the sensitivity of such interferometers is significantly lower than non-optical-spring configurations with comparable circulating power; such a drawback can also compromise high-frequency sensitivity, when an optimization is performed on the overall sensitivity of the interferometer to a class of sources. In this paper, we clarify the reason for such a low sensitivity, and propose a way to fix this problem. Motivated by the optical-bar scheme of Braginsky, Gorodetsky, and Khalili, we propose to add a local readout scheme which measures the motion of the arm-cavity front mirror, which at low frequencies moves together with the arm-cavity end mirror, under the influence of gravitational waves. This scheme improves the low-frequency quantum-noise-limited sensitivity of optical-spring interferometers significantly and can be considered as an incorporation of the optical-bar scheme into currently planned second-generation interferometers. On the other hand it can be regarded as an extension of the optical-bar scheme. Taking compact binary inspiral signals as an example, we illustrate how this scheme can be used to improve the sensitivity of the planned Advanced LIGO interferometer, in various scenarios, using a realistic classical-noise budget. We also discuss how this scheme can be implemented in Advanced LIGO with relative ease.

Demonstration and comparison of tuned and detuned signal recycling in a large-scale gravitational wave detector

Abstract: The British/German gravitational wave detector GEO 600 located near Hannover in Germany is the first large-scale gravitational-wave detector using the advanced technique of signal recycling. Currently the instrument operates in detuned signal recycling mode. Several problems arise due to the fact that the signal recycling cavity changes amplitude and phase of all light fields (carrier and sidebands) present at the dark-port. In addition, in the case of detuned signal recycling this leads to unbalanced sideband fields at the detector output. The large amplitude modulation caused by this asymmetry does not carry any gravitational wave information, but might be the cause of saturation and nonlinearities on the main photodiode. We developed and demonstrated a new control method to realize tuned signal recycling operation in a large-scale gravitational wave detector. A detailed comparison of tuned and detuned signal recycling operation is given. The response function of the system (optical gain) was measured and compared, as was the size of amplitude modulation on the main photodiode. Some important noise couplings were measured and partly found to be strongly reduced in the case of tuned signal recycling operation.

Detuned Twin-Signal-Recycling for ultrahigh-precision interferometers.

Abstract: We propose a new interferometer technique for high-precision phase measurements such as those in gravitational wave detection. The technique utilizes a pair of optically coupled resonators that provide identical resonance conditions for the upper as well the lower phase modulation signal sidebands. This symmetry significantly reduces the noise spectral density in a wide frequency band compared with single-sideband recycling topologies of current and planned gravitational wave detectors. Furthermore, the application of squeezed states of light becomes less demanding.

Photon-pressure-induced test mass deformation in gravitational-wave detectors

Abstract: A widely used assumption within the gravitational-wave community has so far been that a test mass acts like a rigid body for frequencies in the detection band, i.e. for frequencies far below the first internal resonance. In this paper, we demonstrate that localized forces, applied for example by a photon pressure actuator, can result in a non-negligible elastic deformation of the test masses. For a photon pressure actuator setup used in the gravitational-wave detector GEO 600, we measured that this effect modifies the standard response function by 10% at 1 kHz and about 100% at 2.5 kHz.

On the mechanical quality factors of cryogenic test masses from fused silica and crystalline quartz

Abstract: Current interferometric gravitational wave detectors (IGWDs) are operated at room temperature with test masses made from fused silica Fused silica shows very low absorption at the laser wavelength of 1064 nm It is also well suited to realize low thermal noise floors in the detector signal band since it offers low mechanical loss, i e high quality factors (Q factors) at room temperature However, for a further reduction of thermal noise, cooling the test masses to cryogenic temperatures may prove an interesting technique Here we compare the results of Q factor measurements at cryogenic temperatures of acoustic eigenmodes of test masses from fused silica and its crystalline counterpart Our results show that the mechanical loss of fused silica increases with lower temperature and reaches a maximum at 30 K for frequencies of slightly above 10 kHz The losses of crystalline quartz generally show lower values and even fall below the room temperature values of fused silica below 10 K Our results show that in comparison to fused silica, crystalline quartz has a considerably narrower and lower dissipation peak on cooling and thus has more promise as a test mass material for IGDWs operated at cryogenic temperatures The origin of the different Q factor versus temperature behavior of the two materials is discussed

Real-time phase-front detector for heterodyne interferometers

Abstract: We present a real-time differential phase-front detector sensitive to better than 3 mrad rms, which corresponds to a precision of approximately 500 pm. This detector performs a spatially resolving measurement of the phase front of a heterodyne interferometer, with heterodyne frequencies up to approximately 10 kHz. This instrument was developed as part of the research for the Laser Interferometer Space Antenna Technology Package interferometer and will assist in the manufacture of its flight model. Because of the advantages this instrument offers, it also has general applications in optical metrology.

Mechanical Q-factor measurements on a test mass with a structured surface

Abstract: We present mechanical Q-factors (quality factors) of a crystalline quartz test mass with a nano-structured surface, measured in the temperature regime from 5 to 300 K. The nano-structure was a grating with a period of 2 µm and a depth of about 0.1 µm. Comparative measurements were performed on the plain substrate and on the structured test mass with different numbers of SiO2/Ta 2O5 coating layers. The measurements at different stages of the test mass fabrication process show that the surface distortion induced by the nanostructure does not severely lower the mechanical Q-factor of the substrate. Damping due to a multi-layer coating stack was found to be orders of magnitude higher. The results provide vital information concerning the potential usage of low- thermal noise nano-structured test masses in future generations of high-precision laser interferometers and in current attempts to measure quantum effects of macroscopic mirror oscillators.

Thermal diagnostic of the optical window on board LISA Pathfinder

Abstract: Vacuum conditions inside the LTP gravitational reference sensor must be under 10−5 Pa, a rather demanding requirement. The optical window (OW) is an interface which seals the vacuum enclosure and, at the same time, lets the laser beam go through for interferometric metrology with the test masses. The OW is a plane-parallel plate clamped in a titanium flange, and is considerably sensitive to thermal and stress fluctuations. It is critical for the required precision measurements, hence its temperature will be carefully monitored in flight. This paper reports on the results of a series of OW characterization laboratory runs, intended to study its response to selected thermal signals, as well as their fit to numerical models, and the meaning of the latter. We find that a single-pole ARMA transfer function provides a consistent approximation to the OW response to thermal excitations, and derive a relationship with the physical processes taking place in the OW. We also show how the system noise reduction can be accomplished by means of that transfer function.

Detuned Twin-Signal-Recycling for ultra-high precision interferometers

Abstract: We propose a new interferometer technique for high precision phase measurements such as those in gravitational wave detection. The technique utilizes a pair of optically coupled resonators that provides identical resonance conditions for the upper as well the lower phase modulation signal sidebands. This symmetry significantly reduces the noise spectral density in a wide frequency band compared with single sideband recycling topologies of current and planned gravitational wave detectors. Furthermore the application of squeezed states of light becomes less demanding.

Thermal diagnostic of the Optical Window on board LISA Pathfinder

Abstract: Vacuum conditions inside the LTP Gravitational Reference Sensor must comply with rather demanding requirements. The Optical Window (OW) is an interface which seals the vacuum enclosure and, at the same time, lets the laser beam go through for interferometric Metrology with the test masses. The OW is a plane-parallel plate clamped in a Titanium flange, and is considerably sensitive to thermal and stress fluctuations. It is critical for the required precision measurements, hence its temperature will be carefully monitored in flight. This paper reports on the results of a series of OW characterisation laboratory runs, intended to study its response to selected thermal signals, as well as their fit to numerical models, and the meaning of the latter. We find that a single pole ARMA transfer function provides a consistent approximation to the OW response to thermal excitations, and derive a relationship with the physical processes taking place in the OW. We also show how system noise reduction can be accomplished by means of that transfer function.

Electromagnetically induced transparency, electromagnetically induced absorption and giant Kerr effect in closed degenerate two level systems

Abstract: Spontaneous transfer of coherence has proven to be a good interpretative scheme for the explantation of the spectra of a probe laser probing a closed degenerate two-level systems. With its aid it was possible to give an explanation for the emergence of electromagnetically induced absorption and also to make predictions on the associated coupling laser spectra. Here we extend our work on the role of the coupling laser in electromagnetically induced absorption presenting new measurements of the probe and coupling laser absorption and dispersion spectra - taken in the D2 line of caesium - which reassert the importance of spontaneous transfer of coherence in the generation of electromagnetically induced absorption spectra. All measurements were performed with linearly polarised coupling and probe laser of orthogonal polarisation, acting on a perpendicularly propagating caesium atomic beam to minimise the Doppler broadening of the lines. For sake of completeness we compared the electromagnetically induced absorption spectra with electromagnetically induced transparency spectra obtained in another two-level system within the same line. The measured probe spectra were used to calculate the refractive index of caesium in the presence of electromagnetically induced transparency and absorption. On that basis we could calculate the effect of Kerr nonlinearities and measure nonlinear Kerr coefficients of the order of n 2 ≈ 10-5 cm2/mW with absorption coefficients of the order of α ≈ 0.1 cm-1.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Narrow linewidth diode laser system for coherent precision spectroscopy

Abstract: A new type of diode laser system for precision spectroscopy is presented. Its excellent passive stability eases locking to fs-frequency combs, which is demonstrated with high resolution spectroscopy of cold HD+ ions.

Giant Kerr effect in degenerate closed transitions

Abstract: Giant Kerr nonlinearities about twelve orders of magnitudes greater than in glass were measured under negligible absorption conditions within two different closed transitions of the cesium D2 line characterized y electromagnetically induced transparency or absorption.

Exploration of electromagnetically induced absorption with circular polarised lasers in a degenerate two-level system

Abstract: The authors presented the corresponding measurements obtained with a modified version of our heterodyne interferometer. The measurement were obtained in the 6s 2S1/2 F=4 rarr 6p 2P3/2 F=5 transition of the caesium D2 line. As it is the case for double linear polarisation the probe absorption signals showed electromagnetically induced absorption. However, the amplitude of the induced absorption peak over the broader regular absorption signal resulted much lower than the measured one for the double linear case. This may be due to the fact that in the double circular case no compact N subsystems can be easily built between the sublevels. The resulting spectra are therefore similar to those measured in open two-level systems.