Optical methods to measure physical quantities
- Realisation of a pressure standard through the measurement of the refractive index of a gas
The pressure standard is based on the measurement of the refractive index of a gas by means of a homodyne Michelson interferometer in which the measurement arm is formed by a double mirror multiplication set-up. The research aims to obtain a primary pressure standard in the range between 100 Pa and 150 kPa, with a target relative standard uncertainty lower than 10 ppm at atmospheric pressure; currently the study is focused on the accurate measurement of the unbalance of the two arms of the interferometer and the realisation of an active thermal stabilisation system.
- Realisation of a density/pressure standard through the measurement of the Rayleigh scattering
The experiment is based on a typical manifestation of the interaction of radiation with matter, the Rayleigh scattering in the presence of gas molecules, in particular by exploiting its dependence on gas pressure.
Recently the realised system has been characterised in the pressure range between 15 kPa and 900 kPa. The relation between scattered light and gas pressure resulted strongly linear (R2 > 0.9999) allowing the future development of sensors based on this technique.
Furthermore, an accurate determination of the efficiency of the photon collection system could allow an absolute measurement of the density/pressure of a gas.
Hyperspectral imaging (HSI) is a powerful technique of analysis where to each pixel of the image is associated the spectral content of the radiation coming from the scene in the spectral band of interest. We have realised a hyperspectral imager based on a Fabry-Perot interferometer where the camera acquires the sequence of frames carrying the interference fringe information, synchronised with the scan of the optical path delay (OPD) between the mirrors of the FPI, from contact to the maximal distance of the mirror. For each pixel of the image, the interferogram is extracted from the acquired video and the spectral content is calculated with a Fourier Transform based algorithm.
The key points of this hyperspectral imager are:
- this interferometer could be inserted in any existing optical set-up, from a telescope to a microscope, creating therefore a hyperspectral image from an image acquired by a CCD.
- the resolution could be lowered by increasing the mirror distances
The volume occupied by a Si atom in the crystal lattice is one eighth of the cubic elementary cell, whose edge a0 is the lattice parameter. The measure of a0 with a relative uncertainty of less than 2 10-9, requires the coupling of optical and X-ray interferometry. The measure of the lattice parameter is obtained by dividing the displacement measured by the optical interferometer, that is directly linked to the definition of metre, by the number of lattice plains counted by the X-ray interferometer, while the lattice planes are translated.
In order to observe the X-ray fringes, each representing a single lattice plane, the crystal is split into two separate pieces and then recombined in space in such a way that the lattice plane are restored to their former relative position, before the crystal is split. The positioning tolerances of the two crystals forming the X-ray interferometer are extremely small: 1 pm for the relative motion; 1 nm for the two transversal displacements; 1 nrad for the pitch and yaw angles; 1 urad for the rolling angle. These requirements have imposed to develop specific technological and control strategies.
The ring laser is used as a high level, self-calibrating, transportable angle standard (uncertainty less than 50 nrad) and as a tool for the measurement of angular accelerations for scientific purposes. The instrument is built on a carbon fiber optical bench, installed on a precision rotating platform. The instrument is developed in collaboration with the INFN laboratories in Pisa.
The RL is used at INRIM as a novel and independent instrument for the calibration of highest level angular encoders and for the validation of novel measuring methods. The instrument will be circulated among main Metrology Institutes as a transfer standard. The RL, as a very sensitive gyroscope, will be used for the measurement of seismic accelerations for geologic measurements and other (like the characterisation of Earth based gravitational antennas). Thanks to the auto-calibration concept is a demonstrator for the possible realisation of a Earth based experiment for the measurement of the relativistic Lense-Thirring effect (Ginger project).
INRIM collaborates with the European Space Agency (ESA) and with the aerospace industry (Thales Alenia Space) for the development of novel measurement techniques, the realisation of demonstrator prototypes and the characterisation of instrument to be mounted on satellites. In particular INRIM realises interferometers for long distance measurements (e.g. for new generation gravity missions, NGGM), miniaturised interferometers with picometre resolution for accelerometers, absolute interferometers (e.g. for formation flight missions) and optical attitude sensors for synthetic aperture radar (SAR) missions.