Robert B. Hurst

Experiments with an 834 m2 ring laser interferometer

An ultralarge ring He–Ne ring laser gyroscope, UG-2, with area 834 m2 and dimensions 39.7×21 m2, has been built underground at Cashmere Cavern, Christchurch, New Zealand (latitude −43.575°). Earth rotation is sufficient to unlock it, giving a Sagnac frequency of 2.18 kHz. Supermirrors are used with transmission ∼0.18 parts per million (ppm) and optical loss unexpectedly high at ∼200 ppm per reflection. The cavity Q is 1.5×1012. Residual Sagnac frequency error caused by backscatter coupling is measured as 1000 s, mechanical movement of the mirror assemblies, which act to change the geometrical dimensions and tilt. At all averaging times the residual rotational noise is well above the li...

An elementary proof of the geometrical dependence of the Sagnac effect

We offer an elementary proof, within a classical model, that the beat frequency produced in an active optical Sagnac interferometer, in which clockwise (CW) and counterclockwise (CCW) waves are mixed, is proportional to its vector area A divided by its perimeter P. We aim to fill a gap in the literature, since alternative proofs are usually incomplete or rather more complicated. Our observations in various large ring laser interferometers support this A/P dependence and allow the observed Sagnac frequencies from Earth rotation to be predicted to very high accuracy.

Mode behavior in ultralarge ring lasers

Contrary to expectations based on mode spacing, single-mode operation in very large He-Ne ring lasers may be achieved at intracavity power levels up to approximately0.15 times the saturation intensity for the He-Ne transition. Homogeneous line broadening at a high total gas pressure of 4-6 Torr allows a single-peaked gain profile that suppresses closely spaced multiple modes. At startup, decay of initial multiple modes may take tens of seconds. The single remaining mode in each direction persists metastably as the cavity is detuned by many times the mode frequency spacing. A theoretical explanation requires the gain profile to be concave down and to satisfy an inequality related to slope and saturation at the operating frequency. Calculated metastable frequency ranges are > 150 MHz at 6 Torr and depend strongly on pressure. Examples of unusual stable mode configurations are shown, with differently numbered modes in the two directions and with multiple modes at a spacing of approximately 100 MHz.

Correction of backscatter-induced systematic errors in ring laser gyroscopes.

In ring laser gyroscopes, backscatter coupling between the counterpropagating beams leads to systematic errors in measured rotation rates. We show the errors can be derived from the nearly independent optical frequency perturbations of the separate beams. In a process of double backscatter each beam is scattered first into the other beam direction, then rescattered back into itself, giving phase and amplitude perturbations. The analysis proceeds from a purely passive ring cavity, through inclusion of a gain medium and gain saturation. The rotation rate errors may be estimated and corrected from the modulations of the counterpropagating beams. The method is demonstrated with real data.

Mode selection in an ultralarge ring laser gyro

A significant operational difficulty with very large ring laser gyroscopes is the length of time required to achieve the desired single-mode configuration. A control technique has been developed where the order of mode splitting between corotating beams is alternated. Theoretical advantages to this are the elimination of noise caused by variations in perimeter and systematic error caused by Adler pulling. External seeding of mode configurations has been proposed to allow the technique to work fast enough to eliminate known sources of perimeter perturbations. While investigating the intensity requirements for this concept, we found that the operating mode of a large ring laser can be successfully self-seeded with seed beams of near (6±3) single photon cavity mode population.

Large ring laser gyroscopes: towards absolute rotation rate sensing

Ring laser gyroscopes have increased in sensitivity by six orders of magnitude over the last several decades such that they are poised to make valuable contributions to geodesy and terrestrial tests of general relativity. To fully exploit their capabilities, residual (time varying) read out errors arising from backscatter coupling must be physically minimized or otherwise compensated. We present the results of a backscatter correction process for a 12.25 m2 gyroscope with a vast improvement in long term rotational sensitivity.

Crystalline coatings for near-IR ring laser gyroscopes

Substrate-transferred crystalline coatings represent an entirely new concept in high-performance optical coatings. This technology was originally developed as a solution to the long-standing thermal noise limitation found in ultrastable optical interferometers, impacting cavity-stabilized laser systems for precision spectroscopy and optical atomic clocks, as well as interferometric gravitational wave (GW) detectors [1]. The ultimate stability of these systems is currently dictated by coating Brownian noise, driven by the excess mechanical losses of the materials that comprise the highly reflective elements of the cavity end mirrors. Compared with state-of-the art ion-beam sputtered dielectric reflectors, crystalline coatings, comprising substrate-transferred GaAs/AlGaAs heterostructures, exhibit competitive reflectivity together with a significantly enhanced mechanical quality, resulting in a thermally-limited noise floor consistent with a tenfold reduction in mechanical damping at room temperature [2]. Building upon this initial demonstration, we have recently developed high-performance crystalline supermirrors with parts-per-million levels of optical losses, including both absorption and scatter, at wavelengths spanning 1000 to nearly 4000 nm, with experimentally verified absorption coefficients below 0.1 cm-1 in the near infrared [3]. These advancements have opened up additional application areas including the focus of this work. Here we demonstrate the first implementation of crystalline supermirrors in an active laser system, expanding the core application area of these low-thermal noise cavity end mirrors to inertial sensing systems and specifically next-generation high-sensitivity ring-laser gyroscopes [4,5].