Ronald W. P. Drever

Laser phase and frequency stabilization using an optical resonator

We describe a new and highly effective optical frequency discriminator and laser stabilization system based on signals reflected from a stable Fabry-Perot reference interferometer. High sensitivity for detection of resonance information is achieved by optical heterodyne detection with sidebands produced by rf phase modulation. Physical, optical, and electronic aspects of this discriminator/laser frequency stabilization system are considered in detail. We show that a high-speed domain exists in which the system responds to the phase (rather than frequency) change of the laser; thus with suitable design the servo loop bandwidth is not limited by the cavity response time. We report diagnostic experiments in which a dye laser and gas laser were independently locked to one stable cavity. Because of the precautions employed, the observed sub-100 Hz beat line width shows that the lasers were this stable. Applications of this system of laser stabilization include precision laser spectroscopy and interferometric gravity-wave detectors.

LIGO: The Laser Interferometer Gravitational-Wave Observatory

The goal of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Project is to detect and study astrophysical gravitational waves and use data from them for research in physics and astronomy. LIGO will support studies concerning the nature and nonlinear dynamics of gravity, the structures of black holes, and the equation of state of nuclear matter. It will also measure the masses, birth rates, collisions, and distributions of black holes and neutron stars in the universe and probe the cores of supernovae and the very early universe. The technology for LIGO has been developed during the past 20 years. Construction will begin in 1992, and under the present schedule, LIGOs gravitational-wave searches will begin in 1998.

Passive and active seismic isolation for gravitational radiation detectors and other instruments

Some new passive and active methods for reducing the effects of seismic disturbances on suspended masses are described, with special reference to gravitational radiation detectors in which differential horizontal motions of two or more suspended test masses are monitored. In these methods it is important to be able to determine horizontal seismic accelerations independent of tilts of the ground. Measurement of changes in inclination of the suspension wire of a test mass, relative to a direction defined by a reference arm of long period of oscillation, makes it possible to carry this out over the frequency range of interest for earth-based gravitational radiation detectors. The signal obtained can then be used to compensate for the effects of seismic disturbances on the test mass if necessary. Alternatively the signal corresponding to horizontal acceleration can be used to move the point from which the test mass is suspended in such a way as to reduce the effect of the seismic disturbance and also damp pendulum motions of the suspended test mass. Experimental work with an active anti-seismic system of this type is described.

Gravitational Wave Detectors Using Laser Interferometers and Optical Cavities: Ideas, Principles and Prospects

In these two lectures we will discuss principles underlying the development of some laser interferometer gravitational radiation detectors, some of the techniques being devised to overcome the many experimental problems, and results obtained so far in experimental work at the University of Glasgow and the California Institute of Technology. We also hope to give some indication about possible long-term prospects, as well as presenting some new ideas bearing on these problems.

Search for intermediate mass black hole binaries in the first observing run of Advanced LIGO

During their first observational run, the two Advanced LIGO detectors attained an unprecedented sensitivity, resulting in the first direct detections of gravitational-wave signals produced by stellar-mass binary black hole systems. This paper reports on an all-sky search for gravitational waves (GWs) from merging intermediate mass black hole binaries (IMBHBs). The combined results from two independent search techniques were used in this study: the first employs a matched-filter algorithm that uses a bank of filters covering the GW signal parameter space, while the second is a generic search for GW transients (bursts). No GWs from IMBHBs were detected; therefore, we constrain the rate of several classes of IMBHB mergers. The most stringent limit is obtained for black holes of individual mass 100u2009u2009M⊙, with spins aligned with the binary orbital angular momentum. For such systems, the merger rate is constrained to be less than 0.93u2009u2009Gpc^(−3)u2009yr^(−1) in comoving units at the 90% confidence level, an improvement of nearly 2 orders of magnitude over previous upper limits.

Gravitational wave astronomy — Potential and possible realisation

Abstract Since the pioneering work of Joseph Weber more than a decade ago there has been a continuing effort towards the development of more sensitive gravitational wave detectors. There are a number of interesting astrophysical sources of gravitational waves including coalescing compact binary star systems, stellar collapses and rotating neutron stars, and to detect all of these is likely to require a strain sensitivity better than 10−22 over a bandwidth of a few hundred Hz at frequencies at or below 1kHz. To achieve such sensitivity requires considerable experimental ingenuity; however work in a number of laboratories suggests that such performance should be attainable using laser interferometry between freely suspended masses separated by a distance of the order of a kilometre. This paper includes a review of possible sources and outlines methods of detection currently being developed or planned, with particular emphasis on long baseline laser interferometers.

Optical Cavity Laser Interferometers for Gravitational Wave Detection

Most of the techniques being developed for detection of gravitational radiation involve sensing the small strains in space associated with the gravitational waves by looking for changes in the apparent distance between two (or more) test masses. In many of the experimental searches performed so far the detectors consisted of massive aluminium bars, the metal near the ends of the bars acting as the test masses, and impulsive strains induced in the bars were searched for. Thetrain sensitivity of such experiments has been in the range 10−16 to 10−18 for pulses of duration of order 1 millisecond, the limits usually being set by thermal noise in the bar, and transducer and amplifier sensitivity. Current predictions of gravitational waves to be expected from various types of astrophysical sources suggest that strain sensitivities some three orders of magnitude better than these are likely to be required for detection of gravitational wave bursts from known types of sources at a useful rate, although indecd signals may be present over a wide frequency range — from 10 kHz to 10−4 Hz or lower. (A good summary is given in the proceedings of a conference on “Sources of Gravitational Radiation” [1]). Work on bar gravity wave detectors is continuing; but an alternative approach is to use widely separated and nearly free test masses, and monitor changes in their separation by optical interferometry techniques. This method shows considerable promise for both high sensitivity and wide bandwidth and frequency coverage. At the sensitivity levels required absolute length measurements would be difficult, but a comparison of two baselines perpendicular to one another, which may be affected in opposite senses by a gravitational wave travelling in a suitable direction, provides a practical alternative. Early experiments of this type were carried out at Hughes Laboratories [2] using a simple Michelson interferometer to monitor separations between ree test masses suspended in vacuum. The displace- ment sensitivity of such an arrangement may be improved by causing the light in each arm of the interferometer to travel back and forth many times between mirrors attached to the test masses, and a multireflection system of this type using Herriott delay lines was proposed by R. Weiss [3]. Experimental work on multireflection Michelson interferometers for gravity wave detection has been carried out at MIT, the Max-Planck Institute at Munich, and the University of Glasgow.

Gravitational Wave Detectors Using Laser Interferometers and Optical Cavities: Some Practical Aspects and Results

As was explained in the last lecture the gravitational radiation detector being developed at Glasgow University contains two resonant optical cavities at right angles to each other. These are three mirror ring cavities which prevent light reflected from the input travelling back into the laser, and each triangle has sides of length 10 m, 10 m and 0.05 m. The test masses each of 8 kg on which the mirrors are mounted are hung like pendulums (to reduce the effect of seismic noise) at three corners of a square. The system is illumxadinated with approximately 50 mw of single mode light at 514.5 nm from an argon ion laser as is shown in Figure 1.

Extension of gravity-wave interferometer operation to low frequencies

Experiments relating to concepts for extending interferometer operation, particularly at low frequencies, are discussed. This includes work with suspensions connected by a suspension-point interferometer. A new concept for achieving similar frequency extension without requiring an additional interferometer between suspensions is outlined, as well as a technique for improving positioning of laser beams relative to centres of gravity of test masses in gravity-wave interferometers and other instruments.

Recycled light improves gravity-wave detection

The detection of gravitational waves from coalescing binary neutron stars and other astrophysical sources presents an outstanding challenge to experimental physics. Several major experimental facilities are being built to observe the tiny ripples in space caused by the waves. A team of researchers from the Max Planck Institute for Quantum Optics at Garching in Germany, Glasgow University in the UK and Hannover University, also in Germany, has recently demonstrated some ingenious optical techniques on a prototype detector (G Heinzel et al. 1998 Phys. Rev. Lett. 81 5493). The work is a significant step towards achieving the nearly incredible measurement sensitivity aimed for in gravity-wave research-equivalent to detecting a change less than the radius of an atom in a distance as large as that from the Earth to the Sun.