[Paper Review] Gravitational wave experiments and Baksan project "OGRAN"

L. Bezrukov, С. Б. Попов+3 more|ArXiv.org|Nov 16, 2004

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper proposes the OGRAN project, a hybrid opto-acoustical gravitational wave detector combining a cryogenic bar resonator with laser interferometric readout for enhanced sensitivity. It presents a prototype demonstrating $1-2 \times 10^{-14}\,\text{cm}/\text{Hz}^{1/2}$ sensitivity, limited by thermal and optical noise, with plans to improve to $10^{-16}\,\text{cm}/\text{Hz}^{1/2}$ using higher-power lasers and high-reflectivity mirrors, aiming for $h \sim 3 \times 10^{-22}$ in a cryogenic version.

ABSTRACT

A brief sketch of the present status of gravitational wave experiments is given. Attention is concentrated to recent observations with the gravitational detector network. The project OGRAN for a combined optic-interferometrical and acoustical gravitation wave antenna planned for installation into underground facilities of the Baksan Neutrino Observatory is presented. We describe general principles of the apparatus, expected sensitivity and current characteristics of the antenna prototype; some ways for sensitivity improvement are also discussed.

Motivation & Objective

• To develop a hybrid opto-acoustical gravitational wave detector that combines the advantages of resonant bar detectors and optical interferometers.
• To achieve higher sensitivity than existing bar detectors by replacing SQUID readout with low-noise optical readout using high-finesse Fabry-Pérot cavities.
• To install the OGRAN detector in the underground Baksan Neutrino Observatory for reduced environmental noise.
• To improve sensitivity from room-temperature operation to cryogenic operation, targeting $h \sim 3 \times 10^{-22}$.
• To contribute to the global network of gravitational wave detectors by providing a complementary detection method with distinct signal response characteristics.

Proposed method

• The OGRAN detector uses a 2.5-ton niobium-titanium bar resonant at 5 kHz, with a Fabry-Pérot cavity attached to measure length changes via laser frequency modulation.
• A stabilized laser source is phase-locked to a high-finesse reference cavity to minimize frequency drifts, enabling precise measurement of bar length oscillations.
• The optical readout system employs a discriminator based on an external, thermally isolated cavity to detect frequency shifts induced by bar vibrations.
• A feedback loop with PZT actuators stabilizes the optical resonance by adjusting mirror positions in real time.
• The prototype uses a 50 kg pilot bar with $F \approx 800$ finesse, operating at room temperature with 2 mW laser power.
• Future upgrades include a 3 W single-mode stabilized laser, mirrors with $R = 0.9997$ and $50\,\text{ppm}$ losses, and a cryogenic version cooled to 10 mK.

Experimental results

Research questions

• RQ1 Can a hybrid opto-acoustical detector achieve sensitivity comparable to advanced interferometers while retaining the directional and spectral advantages of resonant bars?
• RQ2 What is the ultimate sensitivity limit of a room-temperature bar detector using optical readout instead of SQUID sensors?
• RQ3 How does the combination of a long bar and extended optical cavity affect the signal-to-noise ratio and system stability?
• RQ4 Can laser frequency stabilization techniques used in metrology be effectively adapted to detect gravitational wave-induced bar oscillations?
• RQ5 What improvements are required to reach the $3 \times 10^{-22}$ strain sensitivity target in a cryogenic configuration?

Key findings

• The OGRAN prototype achieved a sensitivity of $1-2 \times 10^{-14}\,\text{cm}/\text{Hz}^{1/2}$ for bar oscillations, limited by thermal and optical noise.
• The measured sensitivity is two orders of magnitude higher than the initial target of $10^{-16}\,\text{cm}/\text{Hz}^{1/2}$, though still short of the final goal.
• The prototype used a 50 kg pilot bar with $L = 50\,\text{cm}$ and $f_0 = 5\,\text{kHz}$, with $F \approx 800$ for both the bar and discriminator cavities.
• The optical system used a two-mirror Fabry-Pérot cavity with Faraday isolators due to limited laser power, instead of the more complex three-mirror design.
• The next phase includes a 3 W stabilized laser and high-reflectivity mirrors with $50\,\text{ppm}$ losses, expected to improve sensitivity by 1.5 orders of magnitude.
• The project plans to operate at $\sim 3 \times 10^{-19}$ strain sensitivity with a room-temperature bar starting in 2006, followed by a cryogenic version targeting $h \sim 3 \times 10^{-22}$.