The LIGO Interferometer Sensing and Control System (ISC) is a large and highly distributed Multiple Input Multiple Output (MIMO) control system that is used to control the length and alignment degrees of freedom of the interferometers. The 4 kilometer Fabry-Perot cavity lengths are controlled to better than 10-13 meters (rms) and the angular degrees of freedom are controlled to better than 10-8 radians. This paper will describe the real-time digital servo control systems that have been designed, developed and implemented for the LIGO Length Sensing and Control (LSC) [1] and Alignment Sensing and Control (ASC) [2] systems. In addition, the paper will describe how these controls, along with the suspended optic controls [3], have been integrated into the overall LIGO control and data acquisition system [4].
The LIGO Control and Data system (CDS) features a tightly coupled and highly integrated control and data acquisition system. Control of the interferometers requires many Multiple Input Multiple Output (MIMO) control loops closed both locally and across the 4-kilometer interferometer arm lengths. In addition to providing the closed loop control, the control systems front end processors act as Data Collection Units (DCU) for the data acquisition system. Data collected by these front ends and the data acquisition system must be collected and time stamped to an accuracy of 1 microsecond and made a…
We discuss the influence of Young's modulus temperature dependence on the number of parametrically unstable modes in a Fabry-Perot cavity of Advanced LIGO interferometer. Some unstable modes may be suppressed by changing the mirror's temperature due to temperature dependence of Young's modulus. Varying the temperature of the mirrors we can change their frequencies of the elastic modes; it, in turn, can change the number of unstable modes, leading to nonlinear effect of parametric oscillatory instability. The determination of the optimal values of the temperature variations for some elastic mod…
To meet the strain sensitivity requirements [1], [2] of the Laser Interferometer Gravitational Wave Observatory (LIGO), the laser frequency and amplitude noise must initially be reduced by a factor of 1000 in the pre-stabilized portion of the interferometer [3]. A control system was implemented to provide laser noise suppression, data acquisition interfaces, diagnostics, and operator control inputs. This paper describes the VME-based analog and digital controls used in the LIGO Pre-stabilized Laser (PSL).
We describe an optical simulation program that models a complete, coupled-cavity interferometer like those used by the Laser Interferometer Gravitational-Wave Observatory (LIGO) Project. A wide variety of interferometer deformations can be modeled, including general surface roughness and substrate inhomogeneities, with no a priori symmetry assumptions about the nature of interferometer imperfections. Several important interferometer parameters are optimized automatically to achieve the best possible sensitivity for each new set of perturbed mirrors. The simulation output data set includes the …
We demonstrate feed-forward vibration isolation on a suspended Fabry-Perot interferometer using Wiener filtering and a variant of the common Least Mean Square (LMS) adaptive filter algorithm. We compare the experimental results with theoretical estimates of the cancellation efficiency. Using data from the recent LIGO Science Run, we also estimate the impact of this technique on full scale gravitational wave interferometers. In the future, we expect to use this technique to also remove acoustic, magnetic, and gravitational noise perturbations from the LIGO interferometers. This noise cancellati…
We present a compact heterodyne laser interferometer developed for high-sensitivity displacement sensing applications. This interferometer consists of customized prisms and waveplates assembled as a quasi-monolithic unit to realize a miniaturized system. The interferometer design adopts a common-mode rejection scheme to provide a high rejection ratio to common environmental noise. Experimental tests in vacuum show a displacement sensitivity level of 11 pm/rtHz at 100mHz, and as low as 0.6pm/rtHz above 1Hz. The prototype unit has a size of 20mm X 20mm X 10mm and weighs 4.5 grams, which allows s…
We describe the angular sensing and control of the 4 km detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO). The culmination of first generation LIGO detectors, Enhanced LIGO operated between 2009 and 2010 with about 40 kW of laser power in the arm cavities. In this regime, radiation pressure effects are significant and induce instabilities in the angular opto-mechanical transfer functions. Here we present and motivate the angular sensing and control (ASC) design in this extreme case and present the results of its implementation in Enhanced LIGO. Highlights of the ASC p…
We present the first demonstration of a three grating Talbot-Lau interferometer for electrons. As a proof of principle, the interferometer is used to measure magnetic fields. The device is similar to the classical Moire deflectometer. The possibility to extend this work to build a scaled-up charged particle deflectometer or interferometer for sensitive magnetic field sensing is indicated.
The Advanced LIGO gravitational wave detectors are nearing their design sensitivity and should begin taking meaningful astrophysical data in the fall of 2015. These resonant optical interferometers will have unprecedented sensitivity to the strains caused by passing gravitational waves. The input optics play a significant part in allowing these devices to reach such sensitivities. Residing between the pre-stabilized laser and the main interferometer, the input optics is tasked with preparing the laser beam for interferometry at the sub-attometer level while operating at continuous wave input p…
The optical readout scheme for the length degrees of freedom of the LCGT interferometer is proposed. The control scheme is compatible both with the broadband and detuned operations of the interferometer. Interferometer simulations using a simulation software Optickle show that the sensing noise couplings caused by the feedback control can be reduced below the target sensitivity of LCGT with the use of feed forward. In order to improve the duty cycle of the detector, a robust lock acquisition scheme using auxiliary lasers will be used.
We present a compact, fibre-coupled interferometer with high sensitivity and a large working range. We propose to use this interferometer as a readout mechanism for future inertial sensors, removing a major limiting noise source, and in precision positioning systems. The interferometers peak sensitivity is $2 \times 10^{-{14}}$ m/${\sqrt{\rm{Hz}}}$ at 70 Hz and $8 \times 10^{-{11}}$ m/$\sqrt{\rm{Hz}}$ at 10 mHz. If deployed on a GS-13 geophone, the resulting inertial sensing output will be dominated by suspension thermal noise from 50 mHz to 2 Hz.
Single-Element Dual-Interferometer for Precision Inertial Sensing: Sub-picometer Structural Stability and Performance as a Reference for Laser Frequency Stabilization
We propose a method to track signals from quadrant photodiodes (QPD) in heterodyne laser interferometers that employ digital phase-locked loops for phase readout. Instead of separately tracking the four segments from the QPD and then combining the results into length and Differential Wavefront Sensing (DWS) signals, this method employs a set of coupled tracking loops that operate directly on the combined length and angular signals. Benefits are increased signal-to-noise ratio in the loops and the possibility to adapt the loop bandwidths to the different dynamical behavior of the signals being …
Concatenated modal interferometers based multipoint sensing system for detection of instantaneous amplitude, frequency, and phase of mechanical vibrations is proposed and demonstrated. The sensor probes are fabricated using identical photonic crystal fiber (PCF) sections and integrated along a single fiber channel to act as a compact and efficient sensing system. Individual probes operate independently producing a resultant signal that is a superposition of each interferometer response signal. By analyzing the resultant signals, information about the measurand field at each location is realize…
Advanced gravitational-wave detectors such as the Laser Interferometer Gravitational-Wave Observatories (LIGO) require an unprecedented level of isolation from the ground. When in operation, they are expected to observe changes in the space-time continuum of less than one thousandth of the diameter of a proton. Strong teleseismic events like earthquakes disrupt the proper functioning of the detectors, and result in a loss of data until the detectors can be returned to their operating states. An earthquake early-warning system, as well as a prediction model have been developed to help understan…
We discuss the design of quantum hybrid inertial sensor that combines an optomechanical inertial sensor with the retro-reflector of a cold atom interferometer. This sensor fusion approach provides absolute and high accuracy measurements with cold atom interferometers, while utilizing the optomechanical inertial sensor at frequencies above the repetition rate of the atom interferometer. This improves the overall measurement bandwidth as well as the robustness and field deployment capabilities of these systems. We evaluate which parameters yield an optimal acceleration sensitivity, from which we…
We report on our progress in the construction of a continuous matter-wave interferometer for inertial sensing via the non-destructive observation of Bloch oscillations. At the present stage of the experiment, around $10^5$strontium-88 atoms are cooled down to below 1$μ$μK. Pumped by lasers red-tuned with respect to the 7.6 kHz broad intercombination transition of strontium, the two counterpropagating modes of the ring cavity form a one-dimensional optical lattice in which the atoms, accelerated by gravity, will perform Bloch oscillations. The atomic motion can be monitored in real-time via its…
Using a network of seismometers and sets of optimal filters, we implemented a feed-forward control technique to minimize the seismic contribution to multiple interferometric degrees of freedom of the LIGO interferometers. The filters are constructed by using the Levinson-Durbin recursion relation to approximate the optimal Wiener filter. By reducing the RMS of the interferometer feedback signals below \sim10 Hz, we have improved the stability and duty cycle of the joint network of gravitational wave detectors. By suppressing the large control forces and mirror motions, we have dramatically red…
Isolating ground-based interferometric gravitational wave observatories from environmental disturbances is one of the great challenges of the advanced detector era. In order to directly observe gravitational waves, the detector components and test masses must be highly inertially decoupled from the ground motion not only to sense the faint strain of space-time induced by gravitational waves, but also to maintain the resonance of the very sensitive 4 km interferometers. This article presents the seismic isolation instrumentation and strategy developed for Advanced LIGO interferometers. It revie…
