The aim of the proposed project is to develop a working prototype of a laser interferometer of a novel construction and open the way of its industrial production. Such an instrument allows high accuracy length measurements, which is the key demand of machine-tool industry of CCE/NIS countries during the process of conforming to EC norms.
The central element in the laser frequency stabilisation system will be a ferroelectric liquid crystal cell acting as a fast optical polarisation switch and control unit. Such a design assures a very high accuracy of the frequency stabilisation and reduces the production costs of the instrument.
The project joins five partners, each having outstanding experience within their field of research, which assures that the objectives of the proposed project will be achieved. The project represents a significant step forward beyond the state of the art and includes substantial original work. International cooperation of researchers of different special fields (liquid crystals and optical interferometry) and necessary technology transfer allows to establish long-lasting relation between partners of project. Results of the research will be disseminated not only in laser interferometry but also in optical metrology and spectroscopy. The software for measurement of distance, angle and the static position uncertainty conforms to ISO 230 standard and will assist CIS industry in its introduction.
The key demand of the machine tool industry is a high-accuracy length standard allowing quick and reliable measurements of elements of any size. This requirement was satisfied by the development of a laser interferometer. The accuracy of the interferometer is determined by the laser wavelength, which is known to be better than 0.5 parts per million. This value compares favourably with best physical standards available and is certainly acceptable for the machine tool evaluation. The laser interferometer is easy to use and allows measurements to be done within minutes instead of the hours or even days as it was required before. The core element of the interferometer is the two frequency HeNe laser, which provides two wavelengths with a small frequency shift (about 2 MHz). These two wavelengths are circularly polarised in opposite directions with respect to each other and can, thus, be easily distinguished using the polarisation technique.
The stability of the laser frequency is here of the outmost importance. The two frequencies of Zeeman splitting laser line are symmetrically shifted from the centre of the laser transition. This property can be utilised in stabilisation of the laser at the centre of the emission line. Conventional techniques are either based on comparison of light intensities in two separate lines or utilise the beat frequency of two laser lines as an error signal in the stabilisation loop. Both methods have essential drawbacks affecting the accuracy and the cost of the instrument. They require additional optical elements and separate electronic channels, which are difficult to synchronise. A more recent approach utilises a nematic liquid crystal cell to switch between the two laser lines. In such a system no additional modulation of the laser frequency is needed. The main problem is here instead a very low switching rate of a nematic liquid crystal (about 1 Hz ), thus the frequency stabilisation loop is unable to follow fast changes of the laser frequency due to mechanical vibrations.
The goal of the project is to develop a new simple industrial laser interferometer which can be applied in machine tool industry. The main objective of the research will be the laser stabilisation system utilising a surface-stabilised ferroelectric liquid crystal (SSFLC) device having a switching rate beyond 1 kHz and the optical anisotropy sufficient for switching the circularly polarised light beams. Such a device should functionally correspond to a lambda quarter plate rotating 90 degrees. In order to prepare the liquid crystal polarisation switcher it is necessary to develop appropriate cell construction, find suitable liquid crystal mixture with corresponding alignment method, elaborate the method insuring the mechanical stability and measure all important parameters of the fabricated cells. Several other problems such as antireflection coatings and electronic control of the SSFLC cell have to be solved.
The proposed project is placed in the research sector 7.1. "Tools, techniques and systems for high quality manufacturing" . The machine-tool industry in CCE/NIS countries has initiated the process of conforming to EC norms such as ISO 9000. In order to fulfil the requirements of ISO, new measuring instruments certifying the machine tools are of immediate necessity. The position uncertainty measurements, determination of the straightness of bearing guides, determination of pitch and yaw errors in machine tools and calibration of surface plates are some of the most important problems which have to be solved to obtain high quality products. Laser interferometers are used as standard for these measurements. They are produced mainly by Hewlett Packard and Spindler & Hoyer. Because of their prohibitive price laser interferometers are rarely available in CCE/NIS countries. In the proposed project an industrial laser interferometer of high accuracy, simple construction and low price will be developed. It could then be widely used in the industry.
Laser frequency stabilization system using FLC switcher is a new idea which significantly simplifies the laser stabilisation loop, the system construction and decreases price of laser interferometer. The development of such a stabilisation system will be a considerable contribution to the applied science. The laser with stabilisation system using FLC cell will also be adequate for spectroscopy and metrology research laboratories.
The two frequency interferometer for machine tool industry will consist of the frequency-stabilised laser source of radiation and optical components.
As the source of radiation the two frequency Zeeman HeNe laser 0.63 mm will be used. The two wavelengths of the laser are circularly polarised in opposite directions. The intensity of the two laser beams changes when the length of the laser resonator is changing. The point where the intensities of both laser beams are equal corresponds to the centre of emission line. The principle of the frequency stabilisation system is to alter the length of the laser resonator in order to sustain the equality of the two laser beam intensities. These intensities will be measured in a single electronic channel utilising the ferroelectric liquid crystal (FLC) switcher and a photodetector. The FLC cell will be designed to have optical properties of a quarterwave plate. Hence, fixed quarterwave and halfwave plates normally present in laser stabilisation systems will no longer be needed. The liquid crystal switcher alternately delivers either of the light beams to the photodetector at a high repetition rate. The signal from the photodetector is then proportional to the difference in the intensities of the laser beams. Deviations of the laser frequency from the centre of transition line in opposite directions change the phase of the photodetector signal. The measurement of the difference of beam intensities (i.e. the error signal for the stabilisation loop) will be done using synchronous demodulation technique. The discrimination signal equals zero when laser frequency is exactly the same as the frequency of the centre of the emission line.
The main advantage of the proposed laser stabilisation system is its tolerance to optical misalignment and the use of a single optical and electrical channels for both beams. The stabilisation loop is far less sensitive to the component parameter distribution than in conventional systems with two separate channels.
Modulators for laser interferometry must meet special requirements concerning the optical quality of all parts in the laser light path. First of all antireflection coatings have to be used to maximise the transmission coefficient and to minimise the influence of the reflection beams on the laser. For FLC modulators this point is important not only for external, but even more for internal interfaces. Due to the reflection of light at the internal interfaces of the modulator, the interference fringes are observed which distort its optical properties. The problem is aggravated by the fact that an FLC cell has several internal layers - indium-tin-oxide (ITO) electrodes, ion barrier layers, liquid crystal alignment layers, etc. It is planned to develop antireflection coatings for glass-ITO and for ITO-aligning layer interfaces. These coatings have to be effective for light of either polarisation.
The second problem which will be investigated is the thermal stability of the FLC modulator. The refractive indices of FLC's change significantly with temperature variations, which affects the retardation of the device and distorts the efficiency of the internal antireflection coatings. Several possible solutions, such as temperature stabilisation, variation of the cell thickness to stabilise the retardation and stabilisation of the retardation by the electric field will be studied.
Another important problem concerning adaptation of FLC devices into laser optics is their mechanical instability. Sufficient rigidity can be provided in several ways. Modulators with small apertures can be made of thick (up to 10 mm) glass plates separated by the spacers in the peripheral part. For apertures larger than few centimetres in diameter the increase of the glass thickness is inefficient and spacers should be deposited in the entire area of the modulator. In this case the glass plates can be about 1 mm thick independently of the aperture of the modulator and the robustness of the modulator is provided by the spacers covering not less than 1% of the active area. To ensure sufficiently low light scattering and to provide optical parameters of the modulator, the spacers must be made non-transparent. This will be achieved by photolithographic processing and etching.
For the distance and velocity measurements the optical remote interferometer will be used. The remote interferometer is essentially the same type of optical system as used by Michelson to measure the meter bar. It consists of the polarisation beam splitter and two retroreflectors; one fixed as a reference and one movable. A family of optical modules will be also developed for the use with the remote interferometer to make a variety of linear and angular measurements, including pitch, yaw and flatness.
The proposers are well qualified to successfully realise the proposed joint research project since their research activities are complementary and actually represent a perfect match
has done the pioneer work on the surface-stabilised ferroelectric liquid crystals (discovery of SSFLC device concept!). It is definitely one of the world's strongest research laboratories working in the area of FLCs and have made a number of original contributions to para-, ferro-, antiferro- and pyroelectric materials. The group has good cleanroom facilities and semi-industrial equipment permitting the production of device prototypes. It is further very well equipped in electronic addressing. It shares considerable scientific interest with the Minsk group and has collaborated with this group on the unique spacer technology for rigid displays. The major scientific responsibility of Göteborg's group will be the development of suitable FLC cell construction.
will develop the laser frequency stabilisation system using the ferroelectric liquid crystal cell as an optical polarisation switcher. The parameters of the developed systems will be investigated and optimised. On the base of the developed stabilised laser the industrial interferometer for distance measurements will be constructed. The optical elements used in the interferometer will be designed and their parameters will be measured. A working prototype of the interferometer will be produced and software for distance, angle measurements and positioning according to ISO norms will be developed.
has gained expertise in evaluating performance of FLC displays in terms of director evolution through FLC cells, disturbing memory effects, such as hysteresis of the optical response in addressing, temperature, time and voltage operating range of several addressing modes purposely developed by the group. Moreover, Rome researchers gained scientific credit in designing, fabricating and evaluating performance of FLC flat panel displays. The laboratory is equipped to perform cell construction and electrooptical measurements.
The project will gain knowledge and experience about the criteria and techniques by which to evaluate and choose materials and construction techniques for best device performance in terms of high switching speed, ability to separate different circular polarisations, device stability over wide and different operating temperature ranges (different because this measurement system must work both at the equator and poles) insensitivity to noisy environments, mechanical robustness, reproducibility and reliability. An optical bench will be set up to precisely measure all relevant parameters of FLC cells as a function of temperature and different liquid crystals and aligning techniques will be compared.
has specialists which were focused on development of a national display technology using a variety of different LCD techniques. This program has already resulted in a Bielorussian industrial production of flat panel displays. The laboratory is equipped to perform cell construction, spectroscopic and electrooptical measurements and evaluation of electrooptical parameters and device performance. The group has also gained considerable expertise in FLC technology and they have recently developed a unique method for achieving absolute mechanical rigidity for LCDs.
The Minsk group will investigate materials to fabricate FLC cells, such as FLC mixtures in combination with different alignment and antireflection coatings. They will test various spacing techniques. Several cells will be designed and produced in collaboration with Göteborg and Rome groups. Specific problems which have to be solved are to suppress the influence of reflections on the laser frequency and to enhance mechanical insensitivity of the cell.
has accumulated significant experience in the investigation and certification of frequency-stabilised lasers and laser interferometers. Preparation of optical components for metrological lasers and laser interferometers have been conducted in Kharkov over the last 20 years.
The group will be responsible for the measurements of the frequency stability of the developed prototype of the laser interferometer. The optical elements for the prototype will be produced using documentation prepared by the Wroclaw group. Testing the software of the prototype of the measurement interferometer and checking its conformity with the CIS norms will be done.
top of the page
back to Tomasz Matuszczyk's homepage