Enabling Space-Based Gravitational Wave Detection

A Major Step Toward Detecting Gravitational Waves from Space

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In September 2019, researchers unveiled a new laser prototype that brings us significantly closer to realizing the Laser Interferometer Space Antenna (LISA) mission — a space-based observatory designed to detect gravitational waves from outer space.

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What Is LISA and Why It Needs Exceptional Lasers

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LISA is a joint mission by the European Space Agency (ESA) with support from NASA. The observatory consists of three spacecraft separated by millions of kilometers, forming a giant triangle in space. By sending laser beams between them and interfering those beams, LISA aims to detect the faint ripples in spacetime known as gravitational waves.

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Detecting gravitational waves in space imposes extremely strict requirements on every component, and especially on the lasers. The laser must maintain high output power, ultra-narrow linewidth, low noise, stability, reliability, and spectral purity — all in the harsh conditions of space over long mission durations.

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The Prototype Laser System

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The newly developed laser prototype is one of the first to meet — or come close to meeting — the demanding specifications for LISA. Key features include:

• Seed laser: The prototype begins with a packaged self-injection-locked laser tuned to the mission’s required wavelength of 1064 nm. This portion of the system was developed by OEwaves.
• Amplifiers: The light from the seed laser is boosted first by a core-pumped Ytterbium-doped fiber amplifier (YDFA), increasing the power from about 12 mW up to 46 mW. Then, after passing through further components including modulators, another core-pumped YDFA plus a large-mode area double-clad YDFA drive the output up to almost 3 watts.
• Phase modulation and stability: Part of the amplified light feeds into an optical reference cavity to improve spectral purity and stability by orders of magnitude. A phase modulator is included for implementing the necessary signal comparisons among the three spacecraft (interferometry). Additional components handle stabilization of power output.

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Performance Tests and Shortcomings

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The research team tested the prototype with a dedicated station using several high-quality references: an optical frequency comb, cavity-stabilized ultra-narrow 1560 nm laser, an H-maser, and low-drift temperature-controlled photodetectors.

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The results show that the prototype meets most of LISA’s specifications across the relevant frequency and noise ranges. There are however small deviations:

• Below about 1 MHz and above ~5 MHz, there are some performance shortfalls versus the specification.
• Noise at higher frequencies was one area identified for improvement. The team suggests technical refinements such as adding a drop port to the resonator to help reduce high-frequency noise.

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Why This Matters

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Developing a laser that satisfies LISA’s demands is a milestone. The mission’s success hinges on extremely sensitive measurements over vast distances. Even tiny instabilities, drift, or noise in the laser would degrade the ability to detect the subtle distortions in spacetime caused by gravitational waves. This prototype demonstrates that it is possible to nearly reach the threshold required for space-based operation. Also, by using space-compatible or space-grade components, the work makes sure the design can survive the environmental challenges of space: vibration, temperature variation, radiation, and so on.

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What’s Next

While the prototype comes very close, more refinement is needed. The research group has already pinpointed some sources of residual noise and deviations from spec. Over the coming development period, they will incorporate those improvements. LISA is planned for launch in the early 2030s, so the timeline still allows for these refinements.

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Read more here.