The AOS for Advanced LIGO is an extension of this subsystem for initial LIGO, and will accommodate the planned higher laser power and additional signal-recycling mirror. The AOS is responsible for transport of interferometer output beams and for stray light control. It includes beam reducing telescopes, and beam dumps and baffles. An additional element of this subsystem is active optics thermal compensation, where compensatory heating of an optic is used to cancel thermal distortion induced by absorbed laser power. It also includes the photon actuator, which uses light pressure to adjust the length of the interferometer arms. AOS also covers the addition of an output mode cleaner.

Functional Requirements

The conventional subsystem requirements relate to control of interferometer ghost beams and scattered light, delivery of interferometer pickoff beams to the ISC subsystem, and maintenance of the surface figure of the core optics through active thermal compensation. While the requirements on these elements are somewhat more stringent than for the initial LIGO design, no significant research and development program is required to meet those requirements.

There are elements which are new to the Advanced LIGO design for which the requirements will be numerically determined as part of the systems flowdown:

• Active Thermal Distortion Compensation: The axisymmetric thermal lens must be corrected sufficiently to allow the interferometer to "cold start"; the compensation may also be required to correct for small (cm-) scale spatial variations in the substrate absorption.
• Photon Actuator: Forces must be applied to the test mass during the operation of the interferometer to maintain the operating length without compromising the mechanical losses of the system. The photon actuator must have sufficient authority to perform the actuation, without adding noise above a negligible level.
• Output Mode Cleaner: The length sensing system requires that non-TEM00 light power at the antisymmetric output port be reduced substantially to allow a small local-oscillator level to be optimal and thus to maintain the efficiency of the overall shot-noise-limited sensing.

Concept/Options

The AOS conventional elements consist of low-aberration reflective telescopes that are placed in the vacuum system to reduce and relay the output interferometer beams out to the detectors, and baffles of absorptive black glass placed to catch stray and "ghost" beams in the vacuum system. The elements must be contamination free and not introduce problematic mechanical resonances. Because of the increased interferometer stored power, the AOS for Advanced LIGO will involve careful attention to control of scattered light, and will require greater baffling and more beam dumps than for initial LIGO.

The thermal compensation approach involves adding heat, which is complementary to that deposited by the laser beam, using two complementary techniques: a ring heater that deals with circularly symmetric distortions, and a directed laser that allows uneven absorption to be corrected.

The frequency-dependent transmission and filtering properties required of the output mode cleaner depend on the ISC readout scheme chosen (DC or RF) and will be determined in an integrated manner with the choice of the readout scheme. ACIGA, with their expertise in sensing systems, will aid in the design of the output mode cleaner, and ACIGA is proposing to contribute materially in the fabrication and installation of an output mode cleaner. This complements their efforts to study variable transmission signal recycling mirrors.

The photon actuator employs an auxiliary laser beam that is reflected from the optic to be actuated upon; the laser amplitude is modulated to control the radiation force. Lasers of several watts can deliver the very small forces required.

R&D Status/Development Issues

Development of active optic thermal compensation is proceeding under the LIGO advanced R&D program. A model of the thermal response of the interferometer in a modal basis has been developed and used extensively to make predictions for the deformations and of the possible compensation. A prototype has successfully demonstrated thermal compensation, in excellent agreement with the model, using both the ring heater and directed laser techniques. A detailed characterization of the spatial distribution of absorption in Sapphire is needed to quantify the correct approach for Advanced LIGO; this will be available from the Core Optics Components test articles in early 2003. This will be complemented with a physical optics model using FFT beam propagation techniques, using these phase maps as input.

The photon actuator will require a more complete systems model for the dynamic range and frequency response to be precisely defined. The intensity stabilization of the source laser is likely to present the only challenge, but present models do not indicate difficulty with the design.

There are two potential designs for the output mode cleaner, dependent on the chosen gravitational wave readout technique. If RF sidebands are used, then the output mode cleaner will be effectively a copy of the input mode cleaner, as it must pass efficiently both the carrier and sidebands. If DC readout were used, the output mode cleaner would be a short, rigid cavity, mounted in one of the output HAM chambers. Both the VIRGO Project and GEO600 use output mode cleaners in their initial design. We plan to start with a study of their approach and the experience with those systems. The principal design challenges lie in the interface to the Interferometer Sensing and Control. The cavity must be aligned with the nominal TEM00 axis of the interferometer, but the bulk (by several orders of magnitude) of the output power will be in higher-order modes; determining the correct alignment is thus non-trivial. The length control, in particular the lock acquisition sequence, also adds complexity.

Work Plan

Work on the active optics thermal compensation is proceeding under the advanced R&D program. A complete prototype thermal compensation system will be tested in the ACIGA Gingin facility in 2003. A prototype photon actuator is being developed with a test on the Caltech 40 Meter Interferometer prototype planned for 2004. The output mode cleaner will be studied using the modeling tools developed for the Mode Cleaner cavity (to which this may bear a strong resemblance) and overall interferometer controls models; a small-scale tabletop prototype will be developed if indicated to ensure that the models are complete to support the ISC design schedule (with a Preliminary Design Review in mid-2004), with the design and fabrication profiting from the suspension and core optics groups. The design process for the beam dumps, baffles, reducing telescopes will resemble that for the initial LIGO design with a planned installation starting in 2007.

ACIGA is proposing to contribute materially in the fabrication and installation of an output mode cleaner. This complements their efforts to study variable transmission signal recycling mirrors.

WBS Definition

This element includes all R&D, design, prototype testing, and hardware of the output optics subsystem (OO) (all telescopes, output mode cleaner, and miscellaneous steering optics), the stray light control (SLC) subsystem (beam dumps and baffles), the photon actuator for the test mass suspensions (PHO), and the active optics thermal compensation subsystem (AOC). Controls are designed by the interferometer sensing and controls subsystem.

Design Requirements

Conceptual Design

R&D Activities

Detail Estimate Sheets

Baseline Plan