The interferometric system with compensation of the refractive index fluctuation of the ambiance

Technical field

The solution is intended for measuring of lengths in the atmospheric conditions with high precision. It is an improvement of laser interferometry of geometrical quantities.

Background art

Metrology of lengths is derived from highly precise and stable etalon of optical frequency; the length measurement itself is than a counting of elementary lengths given by the wavelength of the light source. Wavelength of the source - the laser - is derived from its frequency through speed of light. This speed can be considered to be well known and precise in vacuum while in any other environment it is influenced by the refractive index of light. More, in gaseous and liquid environments it is influenced by the flow of the medium and by gradients of the major physical parameters. The most common environment - air - influences through the index of air and its fluctuations the precision of measurements quite significantly. This influence is much stronger than the limits given by the uncertainty of the primary etalons.

Relative uncertainty of interfrometric measurements is primarily given by stability (precision) of the laser source. In case of measurement based on simple counting of interference maximums (fringes) is this small uncertainty becomes considerable only when long distances are measured where the counting quantization error is negligible. The techniques of advanced digital signal processing allow resolving and interpolating very small fragments of the interference fringe. The limit here is technical given by the noise of the laser, dynamics of the analog-digital conversion and bandwidth. Modern systems reach the resolution down to several tens of picometers, values that cannot be achieved through any other optical method.

The value of refractive index of environment (air) is always the greatest limiting factor. The solution of this problem gave rise to the whole branch of metrology - refractometry. In case of commercial interferometric systems the traditional way of determination of the refractive index of air is so called Edlen formula. It is an empirical equation determining the value of the refractive index on the basis of the fundamental physical parameters of atmosphere - from temperature, pressure, humidity and eventually the content of carbon dioxide. The limits of specification of the refractive index of air are on the level of 10 ^"6 , in laboratory conditions close to 10 ^" . These limits

are given predominantly by the air flow and thermal gradients along with the practical impossibility to measure the parameters of atmosphere directly in the axis of measuring beam.

Dealing with the problem of index of refraction of air is years the metrological evergreen. In the laboratories of dimensional metrology the interferometric refractometer is an indispensable tool. It makes the direct measurement of refractive index possible and it is necessary for caslibration of the overall precisioin of industrial interference systems. The fundamental configuration of interferometric refractometer represents a differential interferometer with high resolution and with measurement of the difference between the air and vacuum path inside and along a cell of known length that can be evacuated. Consequently a set of refractometer designs emerged while the aim was to find a solution which would be compact and precise where the index of refraction would be available any time or at least more often than is allowed by periodic evacuating of the cell. The proposed system worked with shifting triangle cell, with a cell that can be elongated, etc.

Because the level of uncertainty that can be achieved by refractometer approaches 10 ^" limit, the problem of fluctuations of the refractive index of air caused by the air flow and thermal gradients is still there. The refractometer can be again only close to the measuring path. The use of refractometric compensation of the refractive index of air is again only dependent on laboratory conditions where the thermal stabilization and limited air flow can be secured.

The effort to unify an interferometer for measurement of lengths and a refractometer into one device which would evaluate the value of the refractive index of air and thus compensate the length measurement itself is not new. There are setups that emerged using two separated interferometers measuring index of air and length respectively. The compensation of the influence of refractive index of air is possible this way but the problem of determination of the value of refractive index of air in the axis of the measuring beam. An interesting approach represents the employment of the influence of dispersion where measurement on two different wavelengths generated by Nd: YAG laser with frequency doubling allows to determine the index of refraction on the basis of the difference between the values of the measured distance on these two wavelengths. The system of measurement using two wavelengths is able only to improve the noise parameters caused by the turbulences of air in the measuring path without direct compensation. An improvement in the suppression of the influence of the refractive index of air is also possible with a two-frequency laser with a generation of a second harmonic and with a phase-control in a heterodyne detection system. Another method offers a link of the wavelength of the laser to a mechanical length of a frame or a base plate. It means a set of two interferometers where one measures a fixed and non-varying length and serves as reference for the wavelength of the laser. The principle of laser stabilization and length metrology is reversed here, the etalon is non an

optical frequency of the laser source which is transferred through the speed of light and known refractive index into wavelength but the wavelength is fixed to mechanical and the fluctuations of the refractive index of air are suppressed. This needs a broadly and continuously tunable laser. It also does not allow solution of the problem with different paths of the compensating and measuring interferometers. This overview does not cover all presented solutions but represents an outline of various concepts and approaches.

Disclosure of the invention

The above mentioned disadvantages of the known solutions are eliminated by the device according to this invention, which substance is a use of two interferometers measuring the same distance. Their function is not determined to be measuring and compensating and both of them measure the distance by counter propagating beams in the same axis. To unify the function of the interferometers for measuring and compensation allows solving the fundamental problem — the compensation of the fluctuations of the refractive index in the axis of the displacement measurement itself. The core of the solution is the arrangement with two interferometers measuring the same distance in a differential way from two opposing directions and with a common light source (laser) where its wavelength is stabilized to the sum of the values from the two interferometers. This sum is constant during motion of the bi-directional reflector of both interferometers when the index of refraction of air is constant. When the index of refraction fluctuates it varies with it. The link of the wavelength of the laser source to the total length of the measuring range eliminates the influence of the fluctuations of the index of refraction and ensures the constant wavelength in the measuring axis.

The interferometric system with compensation of the changes of the refractive index of environment consists of a source of radiation and two counter measuring interferometers with a fixed length when their beamsplitters are adjusted into an arrangement when the axes of both measuring arms of both interferometers were identical. The distance between the beamsplitters thus determines the measuring range within which the common reflector can move. In front of the first interferometer there is a beamsplitter to divert part of the radiation into the second interferometer. Detectors of the first as well as the second interferometer are linked to a controller which is connected with the source of radiation. Output values from the two interferometers are processed and their sum evaluated. This sum serves as a quantity pro stabilization of the optical frequency of the laser.

The above mentioned reflector can be a bi-directional flat mirror or it can consist of two corner cube reflectors oriented with their tips to each other. The arrangement with corner cube reflectors is advantageous especially because it is independent to angle deviations of the reflector during its motion along the measured axis. The beamsplitter can be a semireflecting mirror at best with a 1 : 1 ratio and without the dependence on the polarization of the light passing through or it can be a beamsplitting prism. This version is designed for a bulk optics setup, the mirrors are highly reflective. Another version represents a system where the beam delivery into both interferometers is done through fiber optic components and the beamsplitter is thus a fiber optic one. This fiber optic splitter is thus connected with the other interferometer through optical fiber. Next version represents a system the same as in the previous case where the whole light delivery is made through optical fibers starting with the fiber coupled laser, fiber optic beamsplitter attached to fiber and following fibers delivering the light radiation to both interferometers ended up with fiber optic collimators. Next version represents a system where the bulk optics and fiber optics is combined, the beam delivery is partially free-space from the laser up to the beamsplitter and fiber coupling follows behind the beamsplitter, than the beam delivery is arranged the same way as in the previous examples with collimators at the ends of the fibers. Next version represents a system which can be a combination of all the previously mentioned arrangements and the resolution of the interferometers is enhanced through a multipass configuration.

Figure overview

The fundamental configuration is described in Fig. 1.

Examples

The interferometers Ia, Ib for measuring of displacements are the Michelson type interferometer and in the setup there are two and they are oriented in opposite. The beamsplitter 7 and reference arms of the interferometers are attached to the base plate (frame) made of material with the smallest thermal expansion coefficient. The moveable bi-directional reflector 5 is common to both interferometers Ia, Ib. Both interferometers Ia, Ib are supplied from a single light source 2, which is a continuously tunable laser with a tuning range large enough to cover the changes of wavelength proportional to the changes of the refractive index of air within the range of expected operating conditions. The light beam is split by a beamsplitter 7 and its part is through mirrors steered into the second interferometer. The outputs from the detectors 6 of the interference signals

representing the measured distance are instantly during the measurement (motion) as well as during rest summarized and in the controller 4 the optical frequency of the laser is controlled the way to keep the value of this sum constant. Thus the instant control of the wavelength in the whole measuring path and in the axis of the measuring beam is ensured regarding the mechanical length of the setup and the influence of changes of the refractive index of air is instantly eliminated by the controller.

Industrial applicability

The system will find its application wherever the precision of interferometric incremental measurement is crucial and where it is impossible to put the measuring system into vacuum.