Interferometer Sensing and Control

Overview

This subsystem comprises the length sensing and control, the alignment sensing and control, and the overall controls infrastructure modifications for the Advanced LIGO interferometer design. The infrastructure elements will be modified to accommodate the additional control loops in the reference design. The single most significant difference in the Advanced LIGO subsystem is the addition of the signal recycling mirror and the resulting requirements on the controls.

Functional Requirements

Table 1 lists significant reference design parameters for the interferometer length controls.

Configuration	Signal and power recycled Fabry-Perot Michelson interferometer
Controlled lengths	- differential arm length (GW signal) - near-mirror Michelson differential length - common-mode arm length (frequency control) - power recycling cavity resonance - signal recycling mirror control
Controlled angles	2 per DOF above, 12 in total
Main differential control requirement	10-14 m rms
Short noise limited displacement sensitivity	4x10-21 m/Hz1/2
Angular alignment requirements	10-9 rad rms

The requirements for the readout system are in general more stringent than those for initial LIGO. The differential control requirement is a factor of 10 smaller, as is the angle requirement, and the additional degrees of freedom add complexity. Integration with the thermal compensation system and the gradual transition from a "cold" to a "hot" system will be needed.

In spite of the increased performance requirements for Advanced LIGO, significant simplification in the controls system is foreseen because of the large reduction in optic residual motion afforded by the active seismic isolation and suspension systems. Reduced core optic seismic motion can be leveraged in two ways. First, the control servo loop gain and bandwidth required to maintain a given RMS residual error can be much smaller. Second, the reduced control bandwidths permit aggressive filtering to block leakage of noisy control signals from imperfect sensor channels into the measurement band above 10 Hz. While control modeling is just getting started, this latter benefit is expected to significantly relieve the signal-to-noise constraints on sensing of auxiliary length and alignment degrees of freedom.

Concept/Options

The signal-recycled configuration is chosen to allow tunability in the response of the interferometer. This is useful for the broadband tuning to control the balance of excitation of the mirrors by the photon pressure, and the improvement in the readout resolution at 100-200 Hz. A narrow-band instrument (to search for a narrow-band source, or to complement a broad-band instrument) can also be created via a change in the signal recycling mirror transmission. An example of possible response curves for a single signal recycling mirror transmission is shown in Figure 1.

Another important advantage of the signal recycled configuration is that the power at the beamsplitter for a given peak sensitivity can be much lower; this helps to manage the thermal distortion of the beam in the beamsplitter, which is more difficult to compensate due to the elliptical form of the beam and the significant angles in the substrate.

Figure 1 Possible sensitivity curves for a narrowband interferometer.

Most length sensing degrees-of-freedom will be sensed using RF sidebands in a manner similar to that in initial LIGO. There are two options for the main gravitational readout. One is to use an RF system similar to initial LIGO, in which variants of the Pound-Drever-Hall scheme are used to derive zero-crossing error signals. The other is to shift the output of the interferometer slightly away from the dark fringe and to use deviations from the setpoint as a measure of the strain. This approach considerably relaxes the requirements on the laser frequency; the nominally more stringent requirement on the baseband intensity fluctuations appears tractable. Two considerations will inform the choice of approach: (i) A complete quantum-mechanical analysis of the two readout schemes to determine which delivers the best sensitivity; and (ii) Requirements imposed on the laser and modulation sources due to coupling of technical noise.

Alignment sensing and control will be accomplished by wavefront sensing techniques similar to those employed in initial LIGO.

The much lower seismic noise in Advanced LIGO will allow smaller control bandwidths for the test-mass actuators; on the other hand, forces to keep the system stable against photon pressure will need to be exerted. In general, the active isolation system and the multiple actuation points for the suspension provide an opportunity to optimize actuator authority in a way not possible with initial LIGO, but will also lead to a more complex system for initial acquisition of operation ("locking") as well as during operation.

R&D Status/Development Issues

The signal-recycled optical configuration chosen for Advanced LIGO challenges us to design a sensing and control system that includes the additional positional and angular degrees of freedom introduced by the signal-recycling mirror. Several straightforward extensions of the sensing system for initial LIGO have been considered. Mason, Delker and Shaddock have demonstrated locking of signal-recycled tabletop interferometers using variants of the initial LIGO asymmetry method, adapted in more or less radical ways to accommodate the additional signal recycling cavity degrees of freedom.

These tabletop experiments and their associated simulations have shown that it is not difficult to arrive at non-singular sensing schemes by adding an additional RF modulation which, through selection of resonant internal lengths, preferentially probes the new cavity coordinates. However there is a great deal of subtlety in choosing parameters to decouple the coordinate readouts adequately to establish a simple, robust control design while realizing the high strain signal-to-noise required.

A detailed prototype test of the control system is underway in GEO (Glasgow), with results expected in early 2003. An engineering control demonstration is in preparation in the LIGO 40 Meter Interferometer (Caltech); it will be fed with information from the GEO effort, and will strive to make a complete emulation of the control system using the target control hardware and software. Locking and operation of the system will be studied.

The selection of the readout scheme involves a trade-off between optimal signal detection and sensing noise (of both fundamental quantum origin and technical noise). The signal-recycling mirror, detuned from perfect resonance, generates a coupling between the shot noise and the mirror motion induced by radiation pressure noise. This causes the GW signal to appear simultaneously in both the phase and amplitude quadratures of the output field (a significant departure from initial LIGO and other first-generation detectors). The DC and RF readout schemes respond to the frequency-dependent optimal signal quadrature differently and the goal is to find a best compromise.

To accommodate the needs for wideband multi-frequency auxiliary length readouts, the DC strain readout, and high-frequency wavefront sensing, characterization of photodiodes will be undertaken. As for initial LIGO detectors, the first steps will be surveys of commercial devices and those developed by colleagues in other projects. This phase will likely be followed in one or more cases by development work to customize or to improve performance and to optimize the electronic amplifiers that mate to these detectors.

Though not necessarily required, lower noise analog-to-digital and digital-to-analog converters would be of great benefit in the design of the sensing and control signal chain. We will prototype board circuitry and software to integrate these converters into our VME-based digital control environment. We also will experiment with new topologies and circuits for the critical analog signal conditioning filters that match the dynamic range of the converters to that of the physical signals they deal with.

Work Plan

The controls configuration will be developed based upon the experience gained from the use of signal recycling in the GEO 600 interferometer, experiments conducted at several institutions in the LSC including pivotal work at the GEO 10 meter prototype from which results are due in early 2003. The final test takes place in the Caltech 40 Meter Interferometer for which the construction will be complete in late 2003; it will inform the design in mid-05, and fabrication can start shortly thereafter. The LIGO Laboratory will manage the design and fabrication of the controls subsystem as it did during initial LIGO construction.

WBS Definition

This element includes all R&D, design, prototype testing, and hardware for the sensing, signal conditioning and digital conversion electronics, programmable items, computers, and software for the servocontrol of the Advanced LIGO interferometer systems. These include control and coordination of all degrees of freedom of the interferometer up to the interface points with the PSL, AOS, SUS, and SEI subsystems, and sensing and readout of lengths and angles of optical elements.

Design Requirements Document

Conceptual Design

R&D Activities

Detail Estimate Sheets

Baseline Plan