Li-Wei Wei

I am an optical physicist specialized in optics and photonics, high-power fiber lasers, laser power stabilization, precision interferometry with optical cavities, and the numerical modelling of electromagnetic waves. I have been applying my optics background to large-scale experiments that address some of the most fundamental questions in physics, the detection of gravitational waves, on earth and in space, and the nature of the Dark Matter.

I am currently leading the optical verification activities of the European Space Agency (ESA)’s Laser Interferometer Space Antenna (LISA) space mission at the Max Planck Institute for Gravitational Physics in Hannover, Germany: the optical fiber backlink (phase reference distribution system) engineering model, and the quadrant photo-receiver — phase meter (signal detection and processing chain) engineering model with a hexagon interferometer. With a planned launch in 2035, LISA will be the first space-based observatory dedicated to studying gravitational waves.

Previously I have held a fellowship position to work on the Any Light Particle Search II (ALPS II) experiment at the German Electron Synchrotron (DESY) in Hamburg that features high-power lasers, high-finesse optical cavities, single-photon counting detectors and superconducting dipole magnets. The ALPS II experiment searches for hypothetical axion-like particles that can shed light on the strong CP problem in particle physics as well as the nature of the Dark Matter if detected.

For my doctoral researches, I worked on the high-power laser system for the Advanced Virgo gravitational wave detector that features the coherent combination of fiber laser amplifiers and laser power stabilization with high-photocurrent photodiode arrays. I am the co-author of the paper reporting the first direct observation of gravitational waves in 2015 and am among the awardee of the 2016 Special Breakthrough Prize in Fundamental Physics.

selected publications

2025

Design and performance of the ALPS II regeneration cavity

Todd Kozlowski, Li-Wei Wei, Aaron D. Spector, and 12 more authors

Opt. Express, Mar 2025

Abs

The regeneration cavity (RC) is a critical component of the Any Light Particle Search II (ALPS II) experiment. It increases the signal from possible axions and axion-like particles in the experiment by nearly four orders of magnitude. The total round-trip optical losses of the power circulating in the cavity must be minimized in order to maximize the resonant enhancement of the cavity, which is an important figure of merit for ALPS II. Lower optical losses also increase the cavity storage time, and with the 123 meter long ALPS II RC we have demonstrated the longest storage time of a two-mirror optical cavity. We measured a storage time of 7.17\textpm0.01ms, equivalent to a linewidth of 44.4 Hz and a finesse of 27,500 at a wavelength of 1064 nm.

2024

Temperature effects in narrow-linewidth optical cavity control with a surrogate quasi-second-harmonic field

Li-Wei Wei, Jan Hendrik Põld, Dennis Schmelzer, and 2 more authors

Appl. Opt., May 2024

Abs

Fabry–Perot cavities are widely used in precision interferometric applications. Various techniques have been developed to achieve the resonance condition via the direct interrogation of the cavity with the main laser field of interest. Some use cases, however, require a surrogate field for cavity control. In this study, we construct a bichromatic cavity to study the surrogate control approach, where the main and the surrogate fields are related by the second-harmonic generation with nonlinear optics. We experimentally verify the temperature dependence of the differential reflection phase of a dielectric coating design optimized for the surrogate control approach of the optical cavities of the light-shining-through-a-wall experiment Any Light Particle Search II and develop a comprehensive cavity model for quasi-second-harmonic resonances that considers also other important factors, such as the Gouy phase shift, for a detailed analysis of the surrogate control approach.
Optimized dielectric mirror coating designs for quasi-harmonic cavity resonance

Li-Wei Wei, Harold Hollis, Benno Willke, and 2 more authors

Appl. Opt., May 2024

Abs

High-finesse optical cavities have a wide range of applications, some of which are bichromatic. The successful operation of high-finesse bichromatic cavities can demand careful control on the temperature dependence of the wavelength-dependent reflection phase from the dielectric mirror coatings that constitute the optical cavity. We present dielectric coating designs that are optimized for minimal differential change in the reflection phase between a quasi-second-harmonic field and its fundamental field under temperature changes. These designs guarantee cavity resonance at a wavelength of interest via the control of its quasi-harmonic field. The proposed coating designs are additionally examined for their sensitivity to manufacturing errors in the coating layer thickness with promising results.
Towards the first axion search results of the Any Light Particle Search II experiment

Li-Wei Wei

PoS, May 2024

2022

Design of the ALPS II optical system

M. Diaz Ortiz, J. Gleason, H. Grote, and 16 more authors

Physics of the Dark Universe, May 2022

Abs

The Any Light Particle Search II (ALPSII) is an experiment currently being built at DESY in Hamburg, Germany, that will use a light-shining-through-a-wall (LSW) approach to search for axion-like particles. ALPSII represents a significant step forward for these types of experiments as it will use 24 superconducting dipole magnets, along with dual, high-finesse, 122m long optical cavities. This paper gives the first comprehensive recipe for the realization of the idea, proposed over 30 years ago, to use optical cavities before and after the wall to increase the power of the regenerated photon signal. The experiment is designed to achieve a sensitivity to the coupling between axion-like particles and photons down to gαγγ=2×10−11GeV−1 for masses below 0.1meV, more than three orders of magnitude beyond the sensitivity of previous laboratory experiments. The layout and main components that define ALPSII are discussed along with plans for reaching design sensitivity. An accompanying paper (Hallal et al., (2021)) offers a more in-depth description of the heterodyne detection scheme, the first of two independent detection systems that will be implemented in ALPSII.

2021

A Gravitational-wave Measurement of the Hubble Constant Following the Second Observing Run of Advanced LIGO and Virgo

B. P. Abbott, R. Abbott, T. D. Abbott, and 1188 more authors

The Astrophysical Journal, Mar 2021

Abs

This paper presents the gravitational-wave measurement of the Hubble constant (H 0) using the detections from the first and second observing runs of the Advanced LIGO and Virgo detector network. The presence of the transient electromagnetic counterpart of the binary neutron star GW170817 led to the first standard-siren measurement of H 0. Here we additionally use binary black hole detections in conjunction with galaxy catalogs and report a joint measurement. Our updated measurement is H 0 = km s−1 Mpc−1 (68.3% of the highest density posterior interval with a flat-in-log prior) which is an improvement by a factor of 1.04 (about 4%) over the GW170817-only value of km s−1 Mpc−1. A significant additional contribution currently comes from GW170814, a loud and well-localized detection from a part of the sky thoroughly covered by the Dark Energy Survey. With numerous detections anticipated over the upcoming years, an exhaustive understanding of other systematic effects are also going to become increasingly important. These results establish the path to cosmology using gravitational-wave observations with and without transient electromagnetic counterparts.
Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences

R. Abbott, T. D. Abbott, S. Abraham, and 1598 more authors

The Astrophysical Journal Letters, Jun 2021

Abs

We report the observation of gravitational waves from two compact binary coalescences in LIGO’s and Virgo’s third observing run with properties consistent with neutron star–black hole (NSBH) binaries. The two events are named GW200105_162426 and GW200115_042309, abbreviated as GW200105 and GW200115; the first was observed by LIGO Livingston and Virgo and the second by all three LIGO–Virgo detectors. The source of GW200105 has component masses and , whereas the source of GW200115 has component masses and (all measurements quoted at the 90% credible level). The probability that the secondary’s mass is below the maximal mass of a neutron star is 89%–96% and 87%–98%, respectively, for GW200105 and GW200115, with the ranges arising from different astrophysical assumptions. The source luminosity distances are and , respectively. The magnitude of the primary spin of GW200105 is less than 0.23 at the 90% credible level, and its orientation is unconstrained. For GW200115, the primary spin has a negative spin projection onto the orbital angular momentum at 88% probability. We are unable to constrain the spin or tidal deformation of the secondary component for either event. We infer an NSBH merger rate density of when assuming that GW200105 and GW200115 are representative of the NSBH population or under the assumption of a broader distribution of component masses.
Laser power stabilization for Advanced VIRGO

Frédéric Cleva, Jean-Pierre Coulon, Li Wei Wei, and 5 more authors

In 2021 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, Jun 2021

Abs

We present the laser power stabilization in Advanced VIRGO using very high photocurrent photodiodes with excellent spatial uniformity. The RIN is currently 2.5E-9 Hz-12 and will be able to reach 1.2E-9 Hz-12 for the most sensitive configuration

2020

Optics mounting and alignment for the central optical bench of the dual cavity enhanced light-shining-through-a-wall experiment ALPS II

Li-Wei Wei, Kanioar Karan, and Benno Willke

Appl. Opt., Oct 2020

Abs

Any Light Particle Search II (ALPS II) is a light-shining-through-a-wall experiment seeking axion-like particles. ALPS II will feature two 120 m long linear optical cavities that are separated by a wall and support the same photon mode. The central optical bench at the core of the experiment will be equipped with a light-tight shutter and two planar mirrors for the cavities. We show that the mounting concept for ALPS II provides sufficient angular stability and verify that a simple autocollimator assisted alignment procedure for crucial components of the ALPS II optical cavities can lead to the required overlap of the cavity eigenmodes. Furthermore, we show that mounted quadrant photodiodes added to the optical bench can have sufficient stability to maintain this overlap even without a clear line of sight between the two optical cavities.
Prospects for improving the sensitivity of the cryogenic gravitational wave detector KAGRA

Yuta Michimura, Kentaro Komori, Yutaro Enomoto, and 18 more authors

Phys. Rev. D, Jul 2020

2017

A gravitational-wave standard siren measurement of the Hubble constant

Nature, Nov 2017

Abs

The astronomical event GW170817, detected in gravitational and electromagnetic waves, is used to determine the expansion rate of the Universe, which is consistent with and independent of existing measurements.
Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

B. P. Abbott, R. Abbott, T. D. Abbott, and 1153 more authors

The Astrophysical Journal Letters, Oct 2017

Abs

On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB 170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory. The probability of the near-simultaneous temporal and spatial observation of GRB 170817A and GW170817 occurring by chance is . We therefore confirm binary neutron star mergers as a progenitor of short GRBs. The association of GW170817 and GRB 170817A provides new insight into fundamental physics and the origin of short GRBs. We use the observed time delay of between GRB 170817A and GW170817 to: (i) constrain the difference between the speed of gravity and the speed of light to be between and times the speed of light, (ii) place new bounds on the violation of Lorentz invariance, (iii) present a new test of the equivalence principle by constraining the Shapiro delay between gravitational and electromagnetic radiation. We also use the time delay to constrain the size and bulk Lorentz factor of the region emitting the gamma-rays. GRB 170817A is the closest short GRB with a known distance, but is between 2 and 6 orders of magnitude less energetic than other bursts with measured redshift. A new generation of gamma-ray detectors, and subthreshold searches in existing detectors, will be essential to detect similar short bursts at greater distances. Finally, we predict a joint detection rate for the Fermi Gamma-ray Burst Monitor and the Advanced LIGO and Virgo detectors of 0.1–1.4 per year during the 2018–2019 observing run and 0.3–1.7 per year at design sensitivity.
GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral

B. P. Abbott, R. Abbott, T. D. Abbott, and 1121 more authors

Phys. Rev. Lett., Oct 2017

2016

Coherently combined master oscillator fiber power amplifiers for Advanced Virgo

Li-Wei Wei, Frédéric Cleva, and Catherine Nary Man

Opt. Lett., Dec 2016

Abs

Stable low-noise high-power lasers are indispensable in advancing the strain sensitivity of interferometric gravitational wave detectors. Advanced LIGO and Advanced Virgo are currently under commissioning and require about 200 W of single-frequency laser power, while the future detector design may require up to the order of 500 W. In this Letter, we present the design and, to the best of our knowledge, the first experimental demonstration of the laser system for Advanced Virgo that is based on coherently combined fiber laser amplifiers. We show the long-term performance of two 40 W fiber laser amplifiers, as well as their characterization in terms of beam quality, power noise, phase noise, and beam pointing. Moreover, a simple and compact setup utilizing fibered modulators and actuators for the coherent beam combination of these two fiber laser amplifiers is reported. A combination efficiency of about 96% was achieved, and no spurious noise was observed.
Observation of Gravitational Waves from a Binary Black Hole Merger

B. P. Abbott, R. Abbott, T. D. Abbott, and 1008 more authors

Phys. Rev. Lett., Feb 2016

2014

Advanced Virgo: a second-generation interferometric gravitational wave detector

F Acernese, M Agathos, K Agatsuma, and 231 more authors

Classical and Quantum Gravity, Dec 2014

Abs

Advanced Virgo is the project to upgrade the Virgo interferometric detector of gravitational waves, with the aim of increasing the number of observable galaxies (and thus the detection rate) by three orders of magnitude. The project is now in an advanced construction phase and the assembly and integration will be completed by the end of 2015. Advanced Virgo will be part of a network, alongside the two Advanced LIGO detectors in the US and GEO HF in Germany, with the goal of contributing to the early detection of gravitational waves and to opening a new window of observation on the universe. In this paper we describe the main features of the Advanced Virgo detector and outline the status of the construction.

2012

Assessment on the applicability of finite difference methods to model light propagation in photonic liquid crystal fibers

Katarzyna Agnieszka Rutkowska, and Li-Wei Wei

Photonics Letters of Poland, Dec 2012
Full-Vectorial Description of the Light Guidance in Anisotropic Photonic Liquid Crystal Fibers

K. Rutkowska, and L. Wei

Acta Physica Polonica A, Nov 2012