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We present the design, implementation, and characterization of a heterodyne laser interferometer for nanometer displacement metrology. The purpose is to monitor the 3-D shape of a large optomechanical structure planned for future general relativity experiments. Reaching the target 10(-11)-m displacement uncertainty over 7-m distances and many days' integration periods is a challenging task. The solution here investigated consists of a nonpolarizing Mach-Zehnder layout, featuring an optical cancelable circuit and a holey folding mirror. The instrument working principle and the method for online phase reconstruction are presented, as well as the complete hardware configuration used. The several sources of noise are investigated mathematically and, whenever possible, verified experimentally. The displacement gauge was tested up to one day of continuous data acquisition, showing nanometer-level performance down to 100 mHz, while air index variations and mechanical instabilities are currently the main limiting factors at lower frequencies. This experiment has brought into light many technical issues that will constitute precious "lessons learned" for the future improvements of the system.

Palladium thin films of different thicknesses have been processed by oxidation and subsequent reduction in hydrogen atmosphere. Hydrogen optical sensing properties of as-deposited (Pd) and processed (r-Pd) samples have been experimentally tested with a H2 concentration of 5% and 1%, discovering that the reduced films show improved performances in term of response/recovery time. As reveled by SEM images, the oxidation/reduction process modifies the surface appearance, which assumes a nano-islands structured morphology. The porosity of the processed films may explain the reduction of the response/recovery time, while the larger effective sensing surface in thinner samples justifies the responsivity performances.

Optical components such as mirrors, filters and windows need to be tested and qualified to verify their resistance in space environments. Future space missions, such as ESA JUICE and SOLO, will operate in harsh environments, rich of ions and electrons. Experiments and development of appropriate protocols are needed to develop proper radiation-hard components and to qualify them.

One main focus of high precision heterodyne displacement interferometers are the means of splitting and merging for the reference (R) and measurement (M) beams when a cancelable circuit is implemented. Optical mixing of R and M gives birth to a systematc error called cyclic error, which appears as a periodic offset between the detected displacement and the actual one. A simple derivation of the cyclic error due to optical mixing is proposed for the cancelable circuit design. R and M beatings are collected by two photodiodes and conveniently converted by transimpedance amplifiers, such that the output signals are turned into ac-coupled voltages. The detected phase can be calculated as a function of the real phase (a change in optical path difference) in the case of zero-crossing detection. What turns out is a cyclic non-linearity which depends on the actual phase and on the amount of optical power leakage from the R channel into the M channel and vice versa. We then applied this result to the prototype of displacement gauge we are developing, which implements the cancelable circuit design with wavefront division. The splitting between R and M is done with a double coated mirror with a central hole, tilted by 45° with respect to the surface normal. The interferometer features two removable diffraction masks, respectively located before the merging point (a circular obscuration) and before the recombination point (a ring obscuration). In order to predict the extent of optical mixing between R and M, the whole layout was simulated by means of the Zemax ® Physical Optics Propagation (POP) tool. After the model of our setup was built and qualitatively verified, we proceeded by calculating the amount of optical leakages in various configurations: with and without the diffraction masks as well as for different sizes of both the holey mirror and the diffraction masks. The corrisponding maximum displacement error was then calculated for every configuration thanks to the previously derived formula. The insertion and optimization of the diffraction masks greatly improved the expected optical isolation inside the system. Data acquisition from our displacement gauge has just started. We plan to experimentally verify such results as soon as our prototype gauge will reach the desired sub-nanometer sensitivity.

Metallic films of palladium (Pd) and palladium-tin (Pd-Sn) have been deposited by evaporation technique. They were used as sensitive material for optical sensor by measuring the variation of absorbance. All samples were then oxidized by annealing at 500°C in low vacuum atmosphere. All the films were investigated by X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM) to observe the influence of the structure and morphology on the optical properties of the films, carrying useful information for the sensing properties of the different sensing materials. Furthermore, the sensing performances were tested by monitoring the variation on the optical absorbance induced during the absorption / desorption of hydrogen gas. While the use of Pd for gas sensing has been widely covered for electrical and SPR sensors, this work aims to extend our comprehension of the optical sensing behavior, especially in absorbance-mode, of the thin films of PdO, Pd-Sn and PdO-SnO 2 .

GINGER (Gyroscopes IN General Relativity) is a project aiming at measuring the Lense-Thirring effect, at 1% level, with an experiment on earth. It is based on an array of ring-lasers, which are the most sensitive inertial sensors to measure the rotation rate of the Earth. The GINGER project is still under discussion; the experiment G-GranSasso is an R&D experiment financed by INFN Group II, it is studying the key points of GINGER and at the same time developing prototypes. In the following the signal coming out of a ring-laser and the present sensitivity are described.The prototypes GP2 and GINGERino and the preliminary results are reported. This project is inter-disciplinary since ring-lasers provide informations for the fast variation of the earth rotation rate, they are used for the rotational seismology and for top sensitivity angle metrology.

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.

Two ring laser gyroscopes are being developed by the INFN in view of fundamental Physics applications. The scope of this activity is to ‘fill the gap’ between the present state of the art technology and the requirements in sensitivity and accuracy needed for general relativity tests. The first prototype, called GP2 and located at INFN Pisa, is dedicated to the interferometric control of the ring laser cavity form factor. The second prototype, called GINGERino, is a larger ring laser located deep underground (INFN LNGS) and has the scope of characterizing the low frequency rotational noise of the site. We show the most recent results of the two experiments.

An optical setup able to switch between the surface-plasmon resonance (SPR) angular resolved spectroscopy in Kretschmann’s configuration and SPR imaging is presented. The system’s concept is based on an adaptive optical module which allows to perform the two complementary measurements by sharing all optical elements and avoiding any moving part. Among the main advantages given by this switching feature there are both the fast calibration capability of the SPR image by using the angular resolved spectroscopy and the optimization of the SPR chip probing area.

GINGER (Gyroscopes IN GEneral Relativity) is a proposal for measuring in a ground-based laboratory the Lense-Thirring effect, known also as inertial frame dragging, that is predicted by General Relativity, and is induced by the rotation of a massive source. GINGER will consist in an array of at least three square ring lasers, mutually orthogonal, with about 6-10 m side, and located in a deep underground site, possibly the INFN - National Laboratories of Gran Sasso. The tri-axial design will provide a complete estimation of the laboratory frame angular velocity, to be compared with the Earths rotation estimate provided by IERS with respect the fixed stars frame. Large-size ring lasers have already reached a very high sensitivity, allowing for relevant geodetic measurements. The accuracy required for Lense-Thirring effect measurement is higher than 10-14 rad/s and therefore Earth angular velocity must be measured within one part in 10-9. A 3.6 m side, square ring laser, called GINGERino, has been recently installed inside the Gran Sasso underground laboratories in order to qualify the site for a future installation of GINGER. We discuss the current status of the experimental work, and in particular of the GINGERino prototype.

Active control of the geometrical parameters of large ring-lasers arrays is a challenging task, requiring the development of an accurate length monitoring system acting on the edges of these devices. The proposed solution is based on what was outlined by NASA JPL for its discontinued Space Interferometric Mission (SIM). It will be made up of a network of compact laser heterodyne interferometers, working together to keep the geometrical frame constantly well known (and fixed) to nanometer accuracy. We took the first steps towards the realization of this device by focusing on its fundamental element, i.e. the compact heterodyne laser interferometer. A starting simplified prototype has been conceived and is now under development. Its preliminary design will let us to evaluate parts behavior, alignment issues and links between single components and overall gauge performances. Concerned distance is investigated by means of a reference and a measurement beam, obtained by spatial splitting of the laser source. The phase shift between two heterodyne beatings will provide a reliable signal for precise monitoring of a linear distance.