R. Santagata

Interferometric length metrology for the dimensional control of ultra-stable ring laser gyroscopes

We present the experimental test of a method for controlling the absolute length of the diagonals of square ring laser gyroscopes. The purpose is to actively stabilize the ring cavity geometry and to enhance the rotation sensor stability in order to reach the requirements for the detection of the relativistic Lense-Thirring effect with a ground-based array of optical gyroscopes. The test apparatus consists of two optical cavities 1.32 m in length, reproducing the features of the ring cavity diagonal resonators of large frame He-Ne ring laser gyroscopes. The proposed measurement technique is based on the use of a single diode laser, injection locked to a frequency stabilized He-Ne/Iodine frequency standard, and a single electro-optic modulator. The laser is modulated with a combination of three frequencies allowing to lock the two cavities to the same resonance frequency and, at the same time, to determine the cavity Free Spectral Range (FSR). We obtain a stable lock of the two cavities to the same optical frequency reference, providing a length stabilization at the level of 1 part in

Optimization of the geometrical stability in square ring laser gyroscopes

10^{11}

Measuring general relativity effects in a terrestrial lab by means of laser gyroscopes

, and the determination of the two FSRs with a relative precision of 0.2 ppm. This is equivalent to an error of 500 nm on the absolute length difference between the two cavities.

Absolute control of the scale factor in GP2 laser gyroscope: Toward a ground based detector of the lense-thirring effect

Ultra sensitive ring laser gyroscopes are regarded as potential detectors of the general relativistic frame-dragging effect due to the rotation of the Earth: the project name is GINGER (Gyroscopes IN GEneral Relativity), a ground-based triaxial array of ring lasers aiming at measuring the Earth rotation rate with an accuracy of 10^-14 rad/s. Such ambitious goal is now within reach as large area ring lasers are very close to the necessary sensitivity and stability. However, demanding constraints on the geometrical stability of the laser optical path inside the ring cavity are required. Thus we have started a detailed study of the geometry of an optical cavity, in order to find a control strategy for its geometry which could meet the specifications of the GINGER project. As the cavity perimeter has a stationary point for the square configuration, we identify a set of transformations on the mirror positions which allows us to adjust the laser beam steering to the shape of a square. We show that the geometrical stability of a square cavity strongly increases by implementing a suitable system to measure the mirror distances, and that the geometry stabilization can be achieved by measuring the absolute lengths of the two diagonals and the perimeter of the ring.

Frequency comb-assisted QCL stabilization for high resolution molecular spectroscopy

GINGER is a proposed tridimensional array of laser gyroscopes with the aim of measuring the Lense–Thirring effect, predicted by the general relativity theory, in a terrestrial laboratory environment. We discuss the required accuracy, the methods to achieve it, and the preliminary experimental work in this direction.

Precise molecular spectroscopy using a stable and tuneable frequency comb

The sensitivity achieved by large laser gyroscopes opens the perspective of observing in a ground laboratories very thin relativistic effect related to the Earth rotating mass (gravitomagnetic effect or Lense-Thirring effect). The required accuracy asks for a strict control of the ring cavity geometry. Here we present a control procedure that can be applied in order to solve this task.

A network of heterodyne laser interferometers for monitoring and control of large ring-lasers

We present a frequency comb-assisted experimental setup which enables us to both coherently transfer the stability and accuracy of a remote near-infrared (NIR) ultrastable laser to a mid-infrared (MIR) quantum cascade laser (QCL), and widely tune the QCL frequency for spectroscopic applications. The QCL is stabilized at the level of 2×10−15 from 1 to 100 s, with an accuracy of 10−14 after 100 s. The achievable frequency tuning paves the way for high-resolution saturated absorption spectroscopy of molecular transitions, in particular in spectral regions that were previously inaccessible with standard stabilized MIR lasers. Preliminary results show a sensitivity at the kHz level on transition center frequencies.

Geometrical scale-factor stabilization of square cavity ring laser gyroscopes

Accurate molecular spectroscopy in the mid-infrared (mid-IR) region allows precision measurements with applications in fundamental physics. We present our on-going work towards measuring absolute vibrational frequencies of various polyatomic species — in particular methanol — around 10 μm, at an unprecedented level of accuracy, using a both ultra-stable and widely tuneable near-infrared frequency comb. We have recently been able to lock mid-IR radiation to a frequency comb stabilized to a 1.54 μm near-IR reference [1, 2]. This reference, generated at the French national metrology institute (LNE-SYRTE), is monitored against atomic frequency standards [3] and transferred to LPL via a 43-km long optical fibre [4] (see Fig. 1). This provides the ultimate frequency accuracy (potentially the 3x10−16 of the Cs fountain clock) and stability (∼10−15 after 1s of integration).

Measurements of Surface Waves Phase Velocity with a Large Ring Laser Gyroscope and a Seismometer

The sensitivity achieved by large ring-laser gyroscopes will make it possible to detect faint relativistic effects related to the rotation of the Earth’s mass. This task requires a strict control of the ring cavity geometry (shape and orientation), which can be performed by a novel network of portable heterodyne interferometers, capable of measuring the absolute distance betweeen two retro-reflectors with a nominal accuracy better than 1nm. First steps have been taken towards the realization of this device and a starting prototype of distance gauge is under development and test.

Experimental activity toward GINGER (gyroscopes IN general relativity)

Large frame ring laser gyros performances are ultimately limited by the instabilities of their geometrical parameters. We present the experimental activity on the GP2 ring laser gyro. GP2 is a ring laser gyro devoted to develop advanced stabilization techniques of the ring cavity geometrical scale-factor. A method based on optical interferometry has been developed for canceling the deformations of the resonator. The method is based on the measurement and stabilization of the absolute length of the cavity perimeter and of the resonators formed by the opposite cavity mirrors. The optical frequency reference in the experiment is an iodine-stabilized He-Ne laser, with a relative frequency stability of 10-11. The measurement of the absolute length of the two resonators has been demonstrated up to now on a test bench. We discuss the experimental results on GP2: the present performances as a ring laser gyro and the stabilization scheme to be implemented in the near future.