Gravitational Wave Detection

Optical Truss Interferometer

Artist rendering of the LISA Space Telescope

The Laser Interferometer Space Antenna (LISA) is a mission led by ESA, in collaboration with NASA and an international consortium of scientists, to create a large-scale space-based gravitational wave observatory. The observatory will consist of a constellation of three spacecraft, each separated by 2.5 million kilometers to form an equilateral triangle in their formation. One of the main technological goals for LISA is to measure variations in the separation between each spacecraft with a sensitivity at the 10 level using interferometric methods. The spacecraft will relay laser beams between each other which are used to measure the separation between freely falling test masses that are shielded, but untouched by the spacecraft. An incoming gravitational wave will induce perturbations in the separation between each pair of test masses, and these perturbations can then be measured and analyzed to determine the source and origin of the wave.

One daunting challenge in gravitational wave interferometry is dealing with local noise sources and being able to distinguish gravitational wave sources from the noisy environment. For this reason, Earth-based gravitational wave detectors, such as LIGO, are mainly restricted to the audio frequency band where transient events are observed such as the merger event of a binary black hole system. A key advantage of LISA, a space-based detector, is the ability to measure gravitational waves in a bandwidth from 0.1 mHz to 1 Hz. Detection in this bandwidth has not yet been achieved for gravitational waves and would introduce valuable knowledge of the universe

One of the most critical components to LISA are the six telescopes (two per spacecraft) that will be used to relay the laser beams. Extensive design work and testing must be done on these telescopes before they can be used in LISA, as the structural and thermal properties as well as the optical properties of the telescopes must meet stringent requirements for the mission to be a success. One proposed way to monitor the dimensional stability of the LISA telescopes is to use a Fabry-Perot based optical truss interferometer (OTI). The OTI system will be composed of three optical truss cavities (OTCs), each built-in lengthwise at different positions around the telescope. Variations in the cavity lengths will in principle be solely due to variations in the telescope length along the three different lateral positions. Members of the LASSO team are working to develop a compact fiber-based PDH system that integrates a fiber-coupler, mode matching optics, and a cavity input mirror into a compact input stage for each OTC. These input stages will be attached around the primary mirror of the telescope, while the rear cavity mirrors will be attached around the secondary mirror of the telescope.

Diagram of the Optical Truss Interferometer

Shown above is a conceptual schematic portraying how the truss cavities will be mounted onto the telescopes. On the left is a side view, where the OTC input stage can be seen attached around the primary (leftmost) mirror of the telescope, and the back cavity mirror is attached on the other end of the telescope around the secondary mirror. On the right is a head-on view, where the three truss cavities are shown to be mounted around the telescope mirrors. The use of three truss cavities is key, as this allows for the measurement of not only lengthwise variations in the telescope, but more importantly, this allows for the measurement of differential length changes at three separate locations around the telescope mirrors. These measurements will give insight into the torsional stability of the telescope, which is the main goal of the OTI project.

Using a PDH-like frequency locking technique, variations in the OTC cavity lengths will change the frequency of the locked laser light. Thus, monitoring the frequency variations in the laser light will in effect monitor length variations in the telescope structure. The frequency variations will be monitored using heterodyne detection by measuring a beat frequency between the laser light from each OTC and a laser that is locked to a stable reference cavity. Current work is focused on the development of the compact input stage for the OTCs.

Researchers Involved:

Compact Sensors for LIGO

A rendering of the sensor being developed for use in LIGO. There are two fused silica resonators, each with a different resonance in order to increase the bandwidth of the sensor. The acceleration of each resonator is measured by observing the phase difference between two beams of light: one that is reflected off a mirror on the test mass and one that is reflected off a mirror on the frame of the sensor.

Gravimetry is vital for operations in the Laser Interferometer Gravitational-wave Observatory (LIGO) project. In order to detect gravitational waves, LIGO’s interferometers must be able to track the position of a test mass with an accuracy on the order of 10^-19 m. However, vibrations coming from outside the interferometer, such as seismic activity and cars driving along the road, would normally prevent LIGO interferometers from being stable enough to reach this displacement sensitivity. With the use of gravimeters, these outside vibrations can be detected and removed from the LIGO systems via active feedback, allowing LIGO researchers to achieve the desired displacement sensitivity.

The gravimeters employed by LIGO tend to suffer from one or more drawbacks, including being large, expensive, and incompatible with vacuum or cryogenic temperatures. One of our projects in LASSO is to develop a gravimeter that does not have these drawbacks while maintaining a competitive acceleration sensitivity. As shown in Figure 1, the sensor we envision creating for use in LIGO uses two optomechanical resonators with different frequencies. Such resonators are projected tot have acceleration noise floors that are competitive with the accelerometers already used by LIGO and using two different resonators increases the bandwidth of the sensor to meet LIGO’s requirements.