The Advanced initial LIGO subsystem will be an extension of the initial LIGO Input Optics design to the higher specified power and lower noise level of Advanced LIGO. The IO will consist primarily of beam conditioning optics including Faraday Isolators and phase modulators, a triangular input mode cleaner, and an interferometer mode-matching telescope.

The functions of the IO subsystem are to provide the necessary phase modulation of the input light, to spatially and temporally filter the light on transmission through the mode cleaner, to provide optical isolation as well as distribution of interferometer diagnostic signals, and to mode match the light to the interferometer with a beam-expanding telescope. Table 1 lists the requirements on the output light of the IO II subsystem.

Requirement	Value
Optical throughput	0.67 (net input to TEM00 out
Non-TEM00 power	<5%
Frequency Noise	3x10-3 Hz/Hz1/2 (10 Hz)
Beam jitter	1x10-9 rad RMS

The Input Optics has to deliver 120 W of conditioned power to the advanced LIGO interferometer. The optical throughput requirement ensures that the required TEM₀₀ power will be delivered. The cavities of the main interferometer will accept only TEM₀₀ light, so the IO must remove the higher-order modes and its beam-expanding telescope must couple 95% of the light into the interferometer.

The IO reduces the frequency, and beam-jitter noise of the laser. The suspended mode cleaner serves as an intermediate frequency reference between the PSL and interferometer. Beam jitter (pointing fluctuation) appears as noise at the interferometer output signal through optical misalignments and imperfections. The nominal optic alignment error of 1×10-9 rad imposes the requirement in Table 1. Further details can be found in the IO Design Requirements document.

Concept/Options

The schematic layout of the IO is displayed in Figure 1, showing the major functional components. The development of the IO for Advanced LIGO will require a number of incremental improvements and modifications to the initial LIGO design. Among these are the needs for larger mode cleaner optics and suspensions to meet the Advanced LIGO frequency noise requirement, and increased power handling capability of the Faraday Isolator and phase modulators.

Figure 1 Shematic diagram of the Advanced LIGO Input Optics (IO) subsystem

Phase modulation for use in the length and angle sensing systems is applied using electro-optic crystals. Faraday isolators are used to prevent parasitic optical interference paths to the laser and to obtain information for the sensing system.

The mode cleaner is an in-vacuum suspended triangular optical cavity. It filters the laser beam by suppressing directional and geometric fluctuations in the light entering the interferometer, and it provides frequency stabilization both passively above its pole frequency and actively through feedback to the PSL. Noise sources considered in design studies include sensor/actuator and electronic noise, thermal, photothermal and Brownian motion in the mode cleaner mirrors, and radiation pressure noise. The mode cleaner will use 15-cm diameter, 7.5-cm thick fused silica mirrors. The cavity will be 17 m in length, with a finesse of 2000, maintaining a stored power of ~100 kW. A triple pendulum (part of the suspensions subsystem) will suspend the mode cleaner mirrors so that seismic and sensor/actuator noise does not compromise the required frequency stability.

Finally, the mode-matching telescope, which brings the beam to the final Gaussian beam parameters necessary for interferometer resonance, will be similar to the initial LIGO design, but will use two (rather than three) reflective spherical mirrors. The third element will consist of an adaptive optical lens that will allow for in situ adjustment of mode matching without the need for vacuum excursions. This design allows for optimization of mode-matched power by having independent adjustment of two degrees of freedom, waist size and position, over a wide range of modal space.

Further documentation of the design can be found in the Input Optics Conceptual Design Document.

R&D Status/Development Issues

The IO subsystem has completed its Design Requirements and Concept Review and is now in preliminary design. Development of the IO focuses on the need for power handling at the 180 W level and the corresponding development of the Faraday Isolators and phase modulators. For the Faraday Isolator, both wavefront distortion and depolarization effects need to be addressed. A new design providing compensation for polarization distortion has shown good isolation up to the maximum test power of 85W. For modulators, we are studying 5 different materials: potassium titanyl phosphate (KTP), potassium titanyl arsenate (KTA), rubidium titanyl arsenate (RTA), rubidium titanyl phosphate (RTP), and lithium niobate (LiNbO₃. Initial testing suggests that several of these are good candidates, potentially using a compensation approach similar to that for the Faraday Isolator.

Work Plan

Development of high power Faraday Isolators and phase modulators is proceeding under the University of Florida Advanced R&D program, and the subsystem lead role will remain with the University of Florida as for initial LIGO. A complete end-to-end test of the IO will be performed at the LASTI facility in conjunction with the mode cleaner suspension testing and the pre-stabilized laser testing in 2005. Installation will commence in 2007.

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 Estimates Sheets

Baseline Plan