H. Grote, K. Danzmann, K.L. Dooley, R. Schnabel, J. Slutsky, H. Vahlbruch
First Long-Term Application of Squeezed States of Light in a Gravitational-Wave Observatory
H. Lück et al
The upgrade of GEO 600

GEO High Frequency and Squeezing

Since 2009 the GEO-HF upgrade program commenced at GEO600 to once more extend the technologically possible. Amongst the next generation techniques implemented in GEO600 since 2009 are the implementation of squeezed light, the inclusion of an output mode cleaner, an increase of the employed laser power, a compensation of thermal warping of optics, a changed readout of the detector, and a change in the signal-recycling.

After extended periods of data-taking, GEO600 has started a sequential upgrade program called 'GEO-HF' in summer 2009. The main points of this program are:

  1. Changing the operating point of GEO600 from detuned signal recycling to tuned signal recycling, and changing the readout method from heterodyne readout to DC readout.
  2. Implementing an output mode cleaner (OMC) in vacuum, together with in-vacuum readout of the gravitational-wave signal.
  3. Implementing squeezed vacuum states, to be injected into the anti-symmetric port of GEO600.
  4. Increasing the bandwidth of the signal recycling cavity.
  5. Increasing the laser power incident on the power recycling cavity from 3 to 20W.

Squeezed light at GEO600

Fig. 1: Delivery of the squeezed light container. Zoom Image
Fig. 1: Delivery of the squeezed light container.

The application of squeezed light injection at GEO600 has raised particular interest in the international research community. Its use is advised since GEO600 has pushed the limits in its measurements so far that it is limited by the quantum nature of light itself. The laser beam used to measure the differential arm length in GEO600 is not truly continuous, but consists, as any beam, of a large number of light particles (photons). The very nature of light itself implies that these photons are not arranged like a string of pearls but are travelling in irregular groups, and it is this grouping that creates noise in the gravitational wave measurement.

GEO600 squeezed light source Zoom Image
GEO600 squeezed light source

In 2010 GEO600 was the first gravitational wave detector to apply squeezed light injection to reduce this quantum noise and to this day GEO600 is the only gravitational-wave detector routinely using squeezed light injection. The squeezed light source was developed and built at the Albert Einstein Institute, and currently the optimal application of squeezed light injection is being researched at GEO600. All of the following ongoing squeezing related research items of GEO600 are unique in the world [1]:

  • Development of an automatic alignment system for squeezed light
  • Development of a new control signal for squeezing angle detection
  • Automated locking of the squeezed light source
Fig. 3: Effect of squeezed light application to GEO600 as of September 2012. Zoom Image
Fig. 3: Effect of squeezed light application to GEO600 as of September 2012.

With all these techniques combined, the first long-term application of squeezed light at a GW detector has been achieved. Squeezing can typically be applied for more than 90% of the time, and we could show that squeezing is compatible with a low glitch rate of the h-channel [1].  Figure 3 shows the effect of squeezing application on the strain sensitivity of GEO600. Squeezing is clearly observed broadband above approximately 400Hz.

The output mode cleaner

Beam profile of the GEO600 laser before (left panel) and after (right panel, zoomed in) the output mode cleaner. The mode cleaner removes the irregular, higher-order mode beam content, which is caused by imperfections of the detector mirrors. Zoom Image
Beam profile of the GEO600 laser before (left panel) and after (right panel, zoomed in) the output mode cleaner. The mode cleaner removes the irregular, higher-order mode beam content, which is caused by imperfections of the detector mirrors. [less]

The implementation of an output mode cleaner is another field of international interest. In a gravitational wave detector the gravitational wave signal is encoded in the interferometer’s output beam. Due to microscopic defects in GEO600’s mirrors there is also light in the output beam that does not carry a gravitational wave signal. This light conceals the signal GEO600 is looking for and therefore has to be removed. The output mode cleaner that was implemented in 2009 is an arrangement of mirrors, a so-called optical resonator, which only lets that light pass through which carries the gravitational-wave signal. The exact design choice of such a resonator and the best system to control the resonators alignment are an active field of research to which GEO600 has made important contributions.

The laser

GEO600 needs a laser for the measurement: In 2011 GEO600 switched from a 14 W laser system to a more powerful 35 W laser system. Both systems were developed and built at the Albert Einstein Institute. The increased laser power emphasizes another challenge in the operation of gravitational wave detectors. Modern gravitational wave detectors use a technique called power-recycling in which the laser power that travels along the two arms is increased manifold. In the case of GEO600 the circulating power is increased by about a factor of 1000 over the power that the laser system delivers. A small fraction of these several kilowatts of light power is absorbed in the mirrors that form GEO600’s interferometer. This absorption heats the mirrors which warps them into an incorrect shape. Compensation of this warping is an active area of research. The existing thermal compensation system at GEO600 has recently been enhanced, evaluating a possible solution of the thermal warping problem.

DC readout

A particularly challenging task in operating a gravitational wave detector is to extract the information about the relative arm length fluctuations from the interferometer’s output port. This is because gravitational wave signals are encoded in the output beam as phase fluctuations on the order of 10−12 radians. To extract this information GEO600 implemented a new readout technique, the so-called DC readout, simultaneously with the LIGO detectors. As expected this new readout technique gave a modest increase to the detectors’ sensitivities, among some other technical advantages.


In the past, GEO600 has been the only gravitational wave detector employing signal-recycling. It provides a profound sensitivity improvement and is used in the Advanced LIGO detectors and planned to be used in the Advanced Virgo detector. GEO600 researched different signal-recycling configurations in detail, which resulted in a considerable sensitivity improvement at high frequencies.


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