THE LASER TECHNOLOGY FOR EARTHQUAKE'S FORECAST AND FOR DIFFERENT APPROACHES OF SEISMIC HAZARD ASSESSMENT

Historically, the first laser-deformograph was developed by group of Geophysical Observatory of the Tavria National University named after I. Vernadsky (formerly Simferopol State University named after M.V. Frunze) and started to work in 1981. This laser complex allowed to carry out the measurements of the Earth’s long time deformation. The measuring volume of the observatory was located in an adit (depth of about 20 meters), which connects the right rangefinder post with the main battery structure and has a series of sealed baffles (doors, hatches) that isolate it from external influences. In the capacity of the main tools for studying oscillatory processes in the environment, the Geophysical Observatory used two-beam laser interferometers of the Michelson type with spaced beams, which have very high metrological characteristics and use the wavelength of a frequency-stabilized laser as a reference. Engineering support of the interferometric complexes’ functioning in the Geophysical Observatory was carried out by: F.N. Pankov, A.V. Buklersky, V.I. Tokarev [5].

Sinusoidal Phase-Modulated Angle Interferometer for Angular Vibration Measurement

Primary angular vibration calibration devices based on laser interferometers play a crucial role in evaluating the dynamic performance of inertial sensing devices. Here, we propose a sinusoidal phase-modulated angle interferometer (SPMAI) to realize angular vibration measurements over a frequency range of 1–1000 Hz, in which the sinusoidal measurement retro-reflector (SMR) and the phase generation carrier (PGC) demodulation algorithm are adopted to track the dynamic angle variation. A comprehensive theoretical analysis is presented to reveal the relationship between demodulation performance of the SPMAI and several factors, such as phase modulation depth, carrier phase delay and sampling frequency. Both the simulated and experimental results demonstrate that the proposed SPMAI can achieve an angular vibration measurement with amplitude of sub-arcsecond under given parameters. Using the proposed SPMAI, the frequency bandwidth of an interferometric fiber-optic gyroscope (IFOG) is successfully determined to be 848 Hz.

Investigation and Mitigation of Noise Contributions in a Compact Heterodyne Interferometer

We present a noise estimation and subtraction algorithm capable of increasing the sensitivity of heterodyne laser interferometers by one order of magnitude. The heterodyne interferometer is specially designed for dynamic measurements of a test mass in the application of sub-Hz inertial sensing. A noise floor of 3.31×10−11m/Hz at 100 mHz is achieved after applying our noise subtraction algorithm to a benchtop prototype interferometer that showed a noise level of 2.76×10−10m/Hz at 100 mHz when tested in vacuum at levels of 3×10−5 Torr. Based on the previous results, we investigated noise estimation and subtraction techniques of non-linear optical pathlength noise, laser frequency noise, and temperature fluctuations in heterodyne laser interferometers. For each noise source, we identified its contribution and removed it from the measurement by linear fitting or a spectral analysis algorithm. The noise correction algorithm we present in this article can be generally applied to heterodyne laser interferometers.

Application of spaced system of laser interferometers and tiltmeters for sharing the earthquakes precursory events

Our previous investigations evidently show that synchronous observation of global atmosphere and lithosphere disturbances by means of 130-1,600 km spatially distributed precise instruments is the effective method of detecting precursors of large seismic events and other dangerous natural phenomena. This study expands the spatial range of our search up to 7,000-8,000 km and allows regional and global disturbances to be shared.

Single-Element Dual-Interferometer for Precision Inertial Sensing

Tracking moving masses in several degrees of freedom with high precision and large dynamic range is a central aspect in many current and future gravitational physics experiments. Laser interferometers have been established as one of the tools of choice for such measurement schemes. Using sinusoidal phase modulation homodyne interferometry allows a drastic reduction of the complexity of the optical setup, a key limitation of multi-channel interferometry. By shifting the complexity of the setup to the signal processing stage, these methods enable devices with a size and weight not feasible using conventional techniques. In this paper we present the design of a novel sensor topology based on deep frequency modulation interferometry: the self-referenced single-element dual-interferometer (SEDI) inertial sensor, which takes simplification one step further by accommodating two interferometers in one optic. Using a combination of computer models and analytical methods we show that an inertial sensor with sub-picometer precision for frequencies above 10 mHz, in a package of a few cubic inches, seems feasible with our approach. Moreover we show that by combining two of these devices it is possible to reach sub-picometer precision down to 2 mHz. In combination with the given compactness, this makes the SEDI sensor a promising approach for applications in high precision inertial sensing for both next-generation space-based gravity missions employing drag-free control, and ground-based experiments employing inertial isolation systems with optical readout.