Overview

The Advanced LIGO PSL will be a conceptual extension of the initial LIGO subsystem, operating at the higher power level necessary to meet the required Advanced LIGO shot noise limited sensitivity. It will incorporate a frequency and amplitude stabilized 180 W laser. The Advanced R&D program related to this subsystem will develop diode laser pumped slab or rod optical gain stages that can be used either in injection locked power oscillators or as a multipass power amplifier.

Functional Requirements

The main requirements of the PSL subsystem are output power, and amplitude and frequency stability. Table 1 lists the reference values of these requirements. Changes in the readout system allow some requirements to be less stringent with respect to initial LIGO; the extension to lower frequency provides the principal challenge.

Requirements	Value
TEM00 power	180 W
Non-TEM00 power	<5 W
Frequency noise	10 Hz/Hz1/2 (10 Hz)
Amplitude noise	2x10-9 /Hz1/2 (10 Hz)
Beam jitter	2x10-6 rad/Hz1/2 (10 Hz)
RF intensity noise	0.5 dB above shot noise at 25 MHz for mW

TEM₀₀Power: Assuming an optical throughput of 0.67 for the input optics subsystem, the requirement of 120 W at the interferometer input gives a requirement of 180 W PSL output.

Non-TEM₀₀ Power: Modal contamination of the PSL output light will mimic shot noise at the mode cleaner cavity, producing excess frequency noise. A level of 5 W non-TEM₀₀ power is consistent with the input optics frequency-noise requirements.

Frequency Noise: Frequency noise couples to an arm cavity reflectivity mismatch to produce strain noise at the interferometer signal port. The requirement is obtained based on a model with an additional factor of 10⁵ frequency noise suppression from mode cleaner and interferometer feedback, a 0.5% match in amplitude reflectivity between the arm cavities (a conservative estimate for the initial LIGO optics), and a signal recycling mirror of 10% transmissivity.

Amplitude Noise: Laser amplitude noise will cause strain noise in two main ways. The first is through coupling to a differential cavity length offset. The second and larger coupling is through unequal radiation pressure noise in the arm cavities. Assuming a beamsplitter of reflectivity 50±1%, the requirement is established.

Beam Jitter Noise: The coupling of beam jitter noise to the strain output is through the interferometer optics misalignment. Based on a model of a jitter attenuation factor of 1000 from the mode cleaner, a nominal optic alignment error of 10^-9 rad rms imposes the requirement on higher order mode amplitude.

RF Intensity Noise: The presence of intensity noise at the RF modulation frequency directly produces strain noise. The noise is limited with the requirement above.

Concept/Options

The conceptual design of the Advanced LIGO PSL is similar to that developed for initial LIGO. It will involve the frequency stabilization of a commercially engineered laser with respect to a reference cavity. It will include actuation paths for coupling to interferometer control signals to further stabilize the beam in frequency and in intensity. Three options for the laser design are under study: a slab injection-locked stable-unstable resonator, a rod injection-locked stable resonator, and a multipass power amplifier. The technology will be selected in early 2003. The control system of the Advanced LIGO PSL, including amplitude and frequency servos, will be largely adapted and extended from the initial LIGO design.

R&D Status/Development Issues

Three approaches to the development of the laser are being pursued. The target for the power from the laser head is 180 W to accommodate some losses to spatial mismatch from the source laser to the desired TEM₀₀ mode. Sketches of the proposed solutions are shown in Figures 1-3.

Figure 1 An injection-locked stable-unstable resonator (Adelaide)

Figure 2 End-pumped zig-zag amplifier (Stanford)

Figure 3 An injection-locked end-pumped rod system (LZH)

In one approach, the Adelaide University group is prototyping a system in which a low-noise, low power master oscillator injection locks a high power stage, formed with a diode-pumped slab crystal situated in a stable-unstable resonator.

An approach, undertaken by Stanford University, uses the master oscillator-power amplifier (MOPA) configuration. In this approach, the output of a master oscillator is passed one or more times through a series of gain elements. This is the laser configuration in use for the initial LIGO, developed by Lightwave Electronics Corporation based upon earlier Stanford work, which provides 10 W output power. The Stanford group is extending the MOPA design to 180W-output power by using the 10-W laser as a master oscillator and employing additional amplifier stages.

The third approach, pursued at the Max Planck Institute for Gravitational Wave Research/University of Hannover and the Laser Zentrum Hannover, is an end-pumped rod resonator that is injection locked to a master oscillator. It is based on experience with the GEO-600 laser, but taking the approach from ~25 W to ~200 W.

The overall goal of this advanced R&D effort is to develop the power laser technology to the point where industrial participation in engineering a reliable unit can begin. The Max Planck group will propose to German funding agencies to supply the laser system for Advanced LIGO, and is leading the downselect and conceptual design effort.

Work Plan

The parallel approach to the development of high power lasers is proceeding, with all three groups approaching the intermediate goal of a 100 W laser. Comparative tests of the three laser designs, with participation from LIGO, are planned for early 2003. After the selection is made, an effort with an industrial partner, the Laser Zentrum Hannover, similar to our practice in initial LIGO, will be undertaken to engineer a reliable unit that will meet the LIGO availability goal. Tests of a complete full-power PSL will be made in the LASTI installation in late 2005. The PSL subsystem design work will proceed in parallel with the laser fabrication, so that the complete subsystem will be ready for installation in early 2007.

Max Planck Institute for Gravitational Wave Research/University of Hannover and the Laser Zentrum Hannover are proposing to supply the PSL systems for Advanced LIGO as a German contribution to the partnership in Advanced LIGO. The University expects to propose project funding for this to the German funding agency in the next year. They are already supported for the development phase.

WBS Definition

This element includes all R&D, design, prototype testing, and hardware for the pre-stabilized laser subsystem upgrade (one operational and one spare per interferometer, and two prototypes). It includes all components of active elements including programmable controls items, and software specific to local control of this subsystem. It includes the final intensity stabilization system. It does not include general controls for the interferometer, nor shared controls infrastructure.

Design Requirements Document

Conceptual Design

R&D Activities

Detail Estimate Sheets

Baseline Plan