O4a: Fourth Observing Run Upgrades

• O4a is the first half of the fourth observing run by LVK, marked by significant instrumental upgrades that boosted sensitivity and enabled advanced quantum noise reduction.
• Detector enhancements, including frequency-dependent squeezing and laser power increases, improved gravitational-wave detection with a BNS range reaching up to ~175 Mpc.
• Advanced noise characterization and refined data quality pipelines provided robust signal validation and expanded the network's multi-messenger follow-up capabilities.

The fourth observing run, specifically the first half (“O4a”), of the LIGO–Virgo–KAGRA (LVK) gravitational-wave detector network represents a milestone series of instrumental, methodological, and infrastructure upgrades. O4a incorporated substantial advances in interferometer sensitivity, data quality, pipeline design, computational strategy, alerting infrastructure, and multi-messenger follow-up capability, as well as yielded an expanded gravitational-wave catalog carrying implications for astrophysics and cosmology. This article synthesizes the technical underpinnings and scientific outcomes characterizing O4a, integrating leading results from detector upgrades, search methodologies, data releases, and the landscape of gravitational-wave astrophysics.

1. Optical Configuration and Instrumental Upgrades

The O4a run was enabled by major enhancements to the optical configuration of LIGO’s 4-km dual-recycled Fabry–Perot Michelson interferometers, including components central to the “A+” upgrade pathway (Cahillane et al., 2022, Capote et al., 21 Nov 2024). Notable instrumental advances and their consequences include:

• 300 m Filter Cavity: Installation of a filter cavity enabled frequency-dependent rotation of the squeezed vacuum’s quadrature, providing broadband quantum noise reduction. The squeezing ellipse rotation reduced both shot noise at high frequency and quantum radiation pressure noise at low frequency.
• Laser and Optics Upgrades: Input laser power was doubled (from ~34–38 W to ~125 W), increasing circulating arm power from ~200 kW to 400 kW and thereby reducing shot noise and enhancing the GW signal. Replacement and cleaning of core optics (notably ITMY at Hanford and ETMs at Livingston) resolved power-absorbing defects, enabling improved recycling gain.
• Active Mode Matching: Introduction of deformable mirrors and other actuated elements reduced spatial-mode mismatch, minimizing losses in the squeezed-light injection path and facilitating up to 6 dB quantum noise reduction.
• Sensor and Controls Commissioning: Upgrades to wavefront sensors, photodetector readouts, and feedforward cancellation significantly improved low-frequency (<50 Hz) sensitivity.

The interferometric GW channel remains defined by:

$\Delta L = h\,L$

where $h$ is the GW strain and $L = 3995\,\mathrm{m}$. The detection bandwidth and noise performance are governed by the coupled recycling cavities and the frequency response of the arm resonators.

2. Lock Acquisition and Detector Operation

The lock acquisition protocol was redesigned for O4a to ensure robust and repeatable transitions to the high-sensitivity operating configuration:

• Pre-Stabilized Laser (PSL) improvements yielded better isolation and calibration of the input beam.
• Arm Length Stabilization (ALS) using green (532 nm) auxiliary lasers, phase-locked to the main infra-red, enabled robust initial acquisition via the Pound–Drever–Hall (PDH) technique. The resonance condition is enforced by:

$2kL = 2\pi n$

• Dual–Recycled Michelson (DRMI) Locking with 3f error signals avoided lock-loss from sign flips as arms enter resonance.
• Thermal Compensation and power ramping required automated tuning (using ring heaters, $CO_2$ projectors, wavefront sensors) to mitigate thermal lensing and minimize drift during high-power operation.
• Squeezer Activation and subsequent low-noise mode engagement systematically lowered the quantum noise floor with minimal control-induced technical noise.

The response of the antisymmetric port to GW-induced length changes is described by:

$$\frac{dP_\mathrm{as}}{d(\Delta x)} \propto k\,P_\mathrm{in}\,\sin(2k\,\Delta L_0)$$

where $k$ is the laser wavenumber, $P_\mathrm{in}$ the incident power, and $\Delta L_0$ the static operating point.

3. Quantum and Thermal Noise Reduction

O4a achieved significant reductions in fundamental noise sources:

• Quantum Shot Noise ($$\sqrt{S_\mathrm{shot}} = \sqrt{2\hbar\,\omega_0\,P_{dc}}$$) was reduced through increased power and optimal squeezed state phase.
• Quantum Radiation Pressure Noise (QRPN) was optimized with frequency-dependent squeezing, via:

$$\sqrt{S_{\rm QRPN}(f)} = \frac{1}{m\,L\,(2\pi f)^2}\sqrt{\frac{32\,P_{\rm bs}\,\hbar \omega_0}{\omega_c^2 + (2\pi f)^2}}$$

where $m$ is test mass, $P_{\rm bs}$ beamsplitter power, and $\omega_c$ the cavity pole.

• Thermal Noise in diagnostic components was minimized with material/cooling improvements, following

$$\sqrt{S_x}(f) = \sqrt{\frac{8 k_B T (1+\sigma)(1-2\sigma)\,d}{\pi\,w^2\,E}\,\frac{\phi}{2\pi f}}$$

with $T$, $\sigma$, $d$, $E$, $w$, $\phi$ representing physical and loss parameters.

Replacing defective optics and improving squeezed light injection optics reduced overall optical losses, enabling up to 5.2 dB (Hanford) and 6.1 dB (Livingston) quantum noise reduction.

4. Measured Sensitivity and Detection Reach

Upgraded detectors yielded BNS inspiral ranges approaching and exceeding 160 Mpc (Hanford median: 152 Mpc, maximum: 165 Mpc; Livingston median: 160 Mpc, maximum: 177 Mpc) (Capote et al., 21 Nov 2024). Compared to O3 sensitivity (Hanford: ~111 Mpc, Livingston: ~134 Mpc), this increased the available detection volume by more than a factor of two, given the scaling:

$N \propto R^3$

where $N$ is event rate and $R$ range.

The improved sensitivity also lowered the noise floor across the detection band, enhancing parameter estimation for detected mergers and increasing the number of observable events.

5. Data Quality, Noise Characterization, and Software Improvements

Substantial advances in both hardware and software influenced the data quality pipeline:

• Glitch and Artifact Mitigation: Scattered light from vibrating structures, induced by microseismic, anthropogenic, and mechanical resonances, was mitigated by damping measures (e.g., rubber Viton dampers on cryo-manifold baffles, adjusted suspensions) and electronics optimization (improved ESD bias, reduced grounding fluctuations).
• Noise Identification Tools: Upgraded tools such as Omicron, Gravity Spy (CNN-based glitch classification), Hveto (auxiliary channel correlation), and Fscan (FFT spectral scanning) enabled better glitch tracking and rapid flagging.
• Data Quality Reporting (DQR): Automated statistical testing with calibrated p-values streamlined issue flagging and event validation, interfacing detector-wide through standardized repositories (Soni et al., 4 Sep 2024).

These quality improvements substantially increased sensitivity, decreased non-Gaussian transients, and enhanced robustness for parameter estimation and event validation.

6. Impact on Multi-messenger Science and Observing Network

The increased sensitivity and duty cycle, in conjunction with robust instrument stability, enabled longer high-quality coincident data stretches among LVK detector sites. This improvement maximized the detection rate of CBCs and increased the probability of multi-messenger detections by other facilities (e.g., Virgo, KAGRA).

Enhanced duty cycles and a lower noise floor, combined with improvements in control (including feedforward configurations, advanced PDH locking, and optimized thermal compensation), enabled more reliable detector operation and streamlined integration with alert and EM follow-up networks.

7. Outlook and Broader Significance

O4a instrumental upgrades, especially frequency-dependent squeezed vacuum injection and improvements to the core laser and optical system, systematically reduced quantum and technical noise limits, enabling an average BNS range increase to ~175 Mpc. These enhancements have quantifiably increased the signal-to-noise ratio of detected signals, improved parameter recovery, and expanded the probability of coincident operation with the global GW detector network.

Formulas such as the cavity resonance condition

$2kL = 2\pi n$

and the cavity pole frequency

$i\,f_p = - \frac{1}{2\pi}\frac{c}{2L}\ln(r_i r_e)$

demonstrate how optical and mechanical parameters are tuned for optimal GW sensitivity.

The advancement of O4a provides a foundation for ongoing astrophysical measurements—including rigorous tests of general relativity, cosmological inference with standard sirens, and the pursuit of new gravitational-wave sources—while also highlighting the interplay between hardware innovation, interferometric control, and sophisticated data analysis in 21st-century gravitational-wave astronomy.