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GEO 600 and the GEO-HF upgrade program: successes and challenges

K. L. Dooley, J. R. Leong, T. Adams, C. Affeldt, A. Bisht, C. Bogan, J. Degallaix, C. Gräf, S. Hild, J. Hough, A. Khalaidovski, N. Lastzka, J. Lough, H. Lück, D. Macleod, L. Nuttall, M. Prijatelj, R. Schnabel, E. Schreiber, J. Slutsky, B. Sorazu, K. A. Strain, H. Vahlbruch, M. Was, B. Willke, H. Wittel, K. Danzmann, H. Grote

arXiv:1510.00317v2physics.ins-det

TL;DR

GEO-HF sought to improve GEO 600’s high-frequency sensitivity while it continued collecting science data during other detectors’ upgrades. The program’s upgrades achieved up to a factor of 4 improvement in shot-noise-limited sensitivity, and GEO 600 also served as an observatory and technology testbed.

  • Problem

    GEO-HF addressed the goal of improving GEO 600’s high-frequency shot-noise-limited sensitivity while continuing science data collection during other detectors’ upgrades.

    Source: §1 · p. 2

  • Method

    GEO-HF pursued minimally invasive upgrades, including DC readout, an output mode cleaner, squeezed-vacuum injection, and increased circulating laser power.

    Source: §1 · p. 2, §2.1 · p. 4, §2.3 · p. 6

  • Results

    Up to a factor of 4 improvement in shot-noise-limited sensitivity and up to 3.7 dB of squeezing were achieved.

    Source: §5 · p. 15

  • Takeaways & Limitations

    GEO 600 collected more than four years of data and served as a testbed for technologies including squeezing, thermal compensation, and commissioning monitors.

    Source: §5 · pp. 15–16

    Higher-power operation remains constrained by a strong beam-splitter thermal lens, and 4 kW operation is not standard because of unknown noise below about 1 kHz.

    Source: §2.4 · pp. 8–9

Abstract

from arXiv · show

The German-British laser-interferometric gravitational wave detector GEO 600 is in its 14th year of operation since its first lock in 2001. After GEO 600 participated in science runs with other first-generation detectors, a program known as GEO-HF began in 2009. The goal was to improve the detector sensitivity at high frequencies, around 1 kHz and above, with technologically advanced yet minimally invasive upgrades. Simultaneously, the detector would record science quality data in between commissioning activities. As of early 2014, all of the planned upgrades have been carried out and sensitivity improvements of up to a factor of four at the high-frequency end of the observation band have been achieved. Besides science data collection, an experimental program is ongoing with the goal to further improve the sensitivity and evaluate future detector technologies. We summarize the results of the GEO-HF program to date and discuss its successes and challenges.

1. Introduction

GEO-HF sought to improve GEO 600’s high-frequency sensitivity while the detector continued collecting science data during other observatories’ upgrades. The introduction frames GEO 600 as both an operating observatory and a setting for testing new technologies.

1. Introduction

  • GEO 600’s 1200 m arms fold inside 600 m beam tubes near Hannover, and Figure 1 shows its simplified optical layout.The layout’s labels include the signal-recycling mirror, beam splitter, output mode cleaner, and photodiodes.Source: §1 · p. 1, Figure 1 · p. 2
  • GEO-HF began in 2009 to improve high-frequency shot-noise-limited sensitivity around 1 kHz and above, while demonstrating new technologies and continuing science data collection.GEO 600 was to serve as the community’s Astrowatch detector while other instruments were upgraded.Source: §1 · p. 2
  • From 2007 to 2009, GEO 600 continued data-taking with LIGO Hanford as LIGO and Virgo enhanced their larger instruments.The program was called Astrowatch.Source: §1 · p. 2
  • Operating as an observatory during commissioning required substantial effort, while providing an opportunity to test technologies in an observatory-style environment.The detector also continued collecting scientific data during this period.Source: §1 · p. 2
  • The article reviews GEO-HF’s goals and upgrades, then the detector’s noise budget, challenges, effectiveness as a GW detector, and observatory tools.It closes with a summary and outlook.Source: §1 · pp. 2–3

2. GEO-HF Upgrades

GEO-HF combined readout, output filtering, signal-recycling, squeezing, and power upgrades to pursue shot-noise-limited high-frequency sensitivity while maintaining or improving low-frequency sensitivity. The upgrades achieved up to a factor-of-four sensitivity improvement above 600 Hz, though increased power remained constrained by technical noise and thermal effects.

2. GEO-HF Upgrades

  • Up to a factor of 4 improvement in sensitivity above 600 Hz had been achieved, with each planned upgrade partially or fully implemented.The design goal was sensitivity limited by fundamental shot noise while maintaining or improving low-frequency sensitivity.Source: §2 · pp. 3–4

2.1. DC readout and implementation of an OMC

  • Changing from RF to DC readout reduced RF phase-noise coupling by a factor of 50, while the output mode cleaner lowered high-frequency noise but initially worsened low-frequency sensitivity.The OMC filters higher-order modes and RF sidebands; beam jitter coupling was later mitigated through improved output-optics suspension, vibration isolation, and thermal compensation.Source: §2.1 · pp. 4–5
  • Modulated differential wavefront sensing replaced low-frequency dithering and enabled alignment-control bandwidth up to several Hz, helping reduce non-stationary beam-jitter coupling.Earlier alignment schemes faced either higher-order-mode transmission or low signal-to-noise ratio.Source: §2.1 · p. 5

2.2. Signal recycling bandwidth increase

  • Replacing the signal-recycling mirror from 98% to 90% reflectivity widened cavity bandwidth from about 230 Hz to 1150 Hz and improved high-frequency sensitivity by more than 2x, but increased HOM-related jitter noise and 50–100 Hz feedback noise.The lower finesse reduced mode healing, increasing output-port HOM content by about 50%.Source: §2.2 · p. 6

2.3. Squeezing

  • Squeezing improved sensitivity above approximately 1 kHz by 3.5 dB in 2010; it later operated during 85% of science-mode time, with observed squeezing improving by several tenths of a dB per year.The cumulative duty factor in Figure 3 tracks the fraction of squeezing-on time at each level; the median corresponds to 0.5.Source: §2.3 · pp. 6–7, Figure 3 · p. 7
  • Squeezer alignment and phase controls were commissioned for stable operation, while approximately 40% optical loss limited maximum observed squeezing to 4 dB from 10 dB possible.The record sensitivity improvement from squeezing was 3.7 dB; phase noise reduced the best observed squeezing by about 0.2 dB.Source: §2.3 · p. 8

2.4. Power increase

  • Stable operation reached 4 kW circulating power against a 17 kW goal, but unknown-origin noise below about 1 kHz kept this state out of standard running; beam-splitter thermal lensing remained the main power limit.Indirect compensation was explored up to 3 kW; beyond about 3.5 kW, excess output-port power increased auxiliary-sensor shot noise and contributed to control breakdown and lock loss. Direct thermal compensation demonstrated a 30% reduction of thermally induced HOMs.Source: §2.4 · pp. 8–10

3. Noise budget and challenges

GEO 600’s noise is well understood above 1.5 kHz and mostly understood below 100 Hz, but significant excess noise between 100 Hz and 1.5 kHz remains unexplained. Control noise, readout electronics, and power-dependent noise constrain sensitivity in different frequency regions.

3. Noise budget and challenges

  • The noise budget explains sensitivity above 1.5 kHz and partially below 100 Hz, but significant excess noise remains unexplained from 100 Hz to 1.5 kHz.The measured strain sensitivity is compared with modeled fundamental noise and actual noise sources.Source: §3 · p. 10, Figure 5 · p. 11
  • Below 100 Hz, alignment and signal-recycling-cavity control noises are severe, while feed-forward cancellation is difficult because coupling varies with output-beam alignment.Limited seismic pre-isolation requires comparatively high control-loop bandwidths.Source: §3 · pp. 10–11
  • At high frequencies, shot noise dominates; readout-electronics dark noise is a factor of 4.5 below shot noise with squeezing, versus 7 below without it.A new electronics design aims to reduce dark noise by a factor of 2 to 3.Source: §3 · p. 11
  • Investigations have ruled out several potential technical-noise sources, but the excess noise is nonstationary and the revised coating thermal-noise curve remains too low to explain it.The coating thermal-noise curve was adjusted 20% higher than in the prior update article.Source: §3 · pp. 11–12
  • Standard operation uses 2 kW to 2.5 kW because higher laser power increases noise below 600 Hz; the remaining power-increase goal depends on thermal compensation and identifying that noise source.Operation at 4 kW is possible during commissioning periods.Source: §3 · p. 12

4. GEO 600 as a GW detector

GEO 600 combined science-quality observing with commissioning, while new monitoring tools tracked astrophysical reach and transient noise. Its reach improved, but glitches and configuration changes kept data quality and sensitivity variable.

4. GEO 600 as a GW detector

  • During the 2011 S6e/VSR 4 run, GEO 600’s sensitivity above 2 kHz equalled Virgo’s.The run took place from June to August 2011.Source: §4 · p. 12
  • 63% of the time from 2008 to 2015 was spent collecting science-quality data, with a median lock of 11.5 hours and a longest stretch of 102.5 hours.Automatic relocking required human intervention on average once every five days, while maintenance and commissioning reduced observation time.Source: §4 · p. 12
  • The non-Gaussian supernova range combines the time-averaged noise floor with transient-noise information, helping commissioners assess possible analysis reach and target transient noise.The Gaussian range assumes Gaussian noise and uses a detection threshold of SNR 8.Source: §4 · pp. 13–14
  • The Gaussian supernova range improved about 20% to 220 pc, while the non-Gaussian range remained below 150 pc and varied substantially.The ranges cover 500 Hz–4 kHz, with stronger emphasis on lower frequencies, so the large sensitivity increase above 2 kHz is not reflected in them.Source: §4 · p. 14
  • New glitch families limited non-Gaussian reach in 2012, while reduced filtering after an input mode cleaner upgrade saturated laser-frequency detection electronics until the issue was fixed.Commissioning effort was needed to track down glitches and other emerging noise sources and maintain astrophysical sensitivity.Source: §4 · pp. 14–15
  • No GW detections were reported, and analysis of 129 GRBs using GEO 600 and a LIGO or Virgo detector found no evidence for signals individually or across the sample.The analysis was the first GW analysis using squeezed-light states to improve detector sensitivity.Source: §4 · p. 15

5. Summary and outlook

Most planned GEO-HF upgrades were implemented, improving shot-noise-limited sensitivity by up to a factor of four, while GEO 600 continued collecting data and testing technologies. Its longer-term future remained undetermined.

5. Summary and outlook

  • Shot-noise-limited sensitivity improved by up to a factor of 4, and up to 3.7 dB of squeezing was achieved.Further work remained to reach higher squeezing levels and potentially increase circulating laser power.Source: §5 · p. 15
  • GEO 600 collected more than four years of data since 2008 while serving as a testbed for technologies including thermal compensation, squeezing, and commissioning monitors.These technologies could be tested in an observatory environment.Source: §5 · pp. 15–16
  • The near-term plan was to maintain a reasonable observing fraction and continue technology tests, while GEO 600’s longer-term future remained undecided.Source: §5 · p. 16
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