Advancing Gravitational Wave Detectors by Increasing their Resilience to Scattered Light Induced noise

29 April 2026

Photo: Voigt et.al (2025)

Working principle and sketch of the experimental setup. For the experiment, we used a Michelson interferometer with intentional stray light injection to demonstrate its suppression. The upper part illustrates the concept of using pseudo-random-noise sequences by showing how the correlation between two identical sequence copies decreases depending on their relative delay.

Interferometers are a widely used tool to measure for example rotation or changes in distance. Depending on the level of sophistication, they can be extremely sensitive to the smallest fluctuation in the lengths of their arms.

The currently largest and most sensitive interferometers are used for detecting gravitational waves, tiny ripples of spacetime itself. A passing gravitational wave leads to a change in arm-length on the order of a thousands of the size of a proton. In order to achieve the sensitivity needed to measure this, the detectors like the Laser Interferometer Gravitational Wave Observatory (LIGO) have to reduce and mitigate all kinds of technical and quantum mechanical noise sources.

Among the technical noise sources is stray light, which makes it cumbersome to identify limitation especially at lower frequencies. However, especially at these lower frequencies, interesting gravitational wave signals are expected. This is why current improvement efforts and future detectors are aiming to increase the sensitivity here. In our paper, we demonstrated a new concept to ease stray light constraints to make these increases more achievable.

The main problem with stray light lies in the laser’s coherence which is actively enhanced to achieve the needed stability of the interferometer. This, however, also allows stray light to still interfere with the measurement, even after it traveled a completely different path through the detector. Along this different, unintended path, it picks up path-length fluctuations which create non-linear, and thus challenging to remove, noise in the readout. These fluctuations can be vibrations of any reflective surface along the path and are therefore very challenging to identify. If they only occur occasionally, this can be even harder. One entertaining example for this are vibrations caused by ravens picking on a pump on the outside of the detector. These vibrations propagated through the tubes and caused a reflective surface to transfer them onto a stray light beam, thereby creating glitches in the measurement.

Our approach, as demonstrated in this paper, tries to tackle the stray light problem at its core. By carefully controlling the coherence of the laser, we created a pseudo-white light interferometer. In an interferometer with incoherent (white) light, interference can only occur if the two arms are exactly identical in length, meaning any stray light interference coming from a different path is strongly suppressed. To transfer this property to the highly coherent laser light used in the interferometers for gravitational wave detection, we manipulated the light wave emitted by the laser at very fast rates. By inverting the amplitude of the laser light at GHz-frequencies following a pseudo-random-noise sequence, its coherence or ability to interfere with itself, exhibits similar characteristics as white light. Only light having the same sequence with the same delay imprinted can coherently interfere while any delay and thus path-length mismatch leads to a mismatch in the sequence. This mismatch then causes the phase relation between the two beams to be almost fully randomized, which means interference is strongly suppressed.

Using this approach, we were able to demonstrate a suppression of stray light noise by 40 dB. In the context of tolerable stray light power in the detector, this means about a thousand times higher power in the stray light beam could be tolerated while keeping the noise at the same level.

However, in order to be usable in high precision interferometry, any technique must be compatible with the more complex interferometer layouts used. To achieve the needed sensitivity of a gravitational wave detector like LIGO, several optical resonators are used to e.g. increase the light power stored inside the interferometer. Reaching such high power reduces, for example, the relative noise stemming from intrinsic fluctuations of the laser power itself. Currently it is thus inconceivable to have a detector without at least four main optical cavities.

This poses the problem that we not only needed to demonstrate the functionality and stray light suppression of a pseudo-white light interferometer but also that the modulated laser can still be resonant in an optical cavity. To achieve this, we made use of the fact that the modulated sequence repeats itself. This allows light having a delay of one or multiple integer repetitions of the sequence to see the same modulation and thus interfere with itself again. Thus, by designing and matching the length of the used cavities to the repetition length of the sequence, we were able to show that the modulation had no negative influence on the cavity performance. Further, it introduced a new, macroscopic resonant condition, which could be exploited in future experiments.

Our work acts as a first experimental demonstration of this powerful technique, which we call tunable coherence, to suppress stray light noise in high precision interferometers. It shows that it not only works as predicted but is also compatible with optical resonators, key parts of gravitational wave detectors. Further work has been and will be done on demonstrating that also complex interferometer layouts can be equipped with tunable coherence and investigating possible limitations of the technique. An important aspect here is to show that the high-speed phase modulation, even though it is too fast to be seen by the photodetectors, does not introduce additional noise or negatively impacts other important noise reduction schemes like e.g. squeezed light injection to reduce quantum noise.

Fig. 2: Simplified illustrations of typical scattered light sources and couplings in gravitational wave detectors. a) shows light scattered at a mirror bouncing back from the vacuum tube walls and thus coupling back into the interferometer. This is typically shielded by the implementation of so-called baffles as shown in b), however, these themselves can reflect the scattered light and can lead to noise coupling. In c) incoming and outgoing laser beam are spatially identical, making it easier to block most of the scattered light from the mirror. In d) however, the two beams are separated which makes blocking scattering from one beam into the other impossible to block with a baffle. This is an important weak point of ring-topologies like Sagnac interferomters used as e.g. gyroscopes. Credits: Voigt et.al (2025)