Scientific and technological problems, such as liquid and gas flow propagation, production of high purity medicine, heat and mass transfer, high accuracy temperature and pressure measurements, blood testing in medical treatment as well as non-destructive testing in machinery and microelectronics, can be successfully solved by holographic and optical technique.

Holography is a unique optical technique that has a number of advantages as compared to conventional photography. With holography, both the amplitude and phase are stored in a photosensitive recorder called a hologram. Since the hologram records all the information contained in a wave front from the object, the reconstructed images by means of holography are truly three-dimensional and extremely realistic.

The hologram making consists of the recording an interference pattern of the light scattered by an object and reference wave from the same source on a high-resolution light-sensitive material. To visualize an object image, it is necessary to illuminate the hologram by the reference beam. The light incident on the hologram will diffract on the recorded pattern as on a diffraction grating. An observer can see through the hologram a 3-D image of the object in the same place as it was during the recording.

The method known as holographic interferometry is based on the comparison between the wave front of the initial state of the object from a hologram and that propagating from the object at the current moment (real-time holographic interferometry) or on the comparison between the two wave fronts from the object recorded on the same hologram in its different states (double-exposure holographic interferometry). Such a comparison gives one very accurate data on the optical and mechanical properties of the object under investigation. There is also another method called time-average holographic interferometry. Time-average holography produces a hologram which is made while an object is driven in resonance. A visual image of the vibration pattern is obtained. The major advantage over the conventional interferometry methods is the possibility of studying and testing transparent and reflective light-diffusing objects.

1.2. Experimental techniques.

We have designed several devices for scientific and industrial application: Holographic camera REGINA for space research, Holographic microscope, Holographic camera for non-destructive testing of the outside surface of the spacecraft windows. The REGINA holographic camera with a photothermoplastic recorder and a TV-system can be mounted either on a table or directly on the set-up which is used to study the processes one has to test. The placing is arbitrary (vertical, horizontal, etc.)

The camera contains a He-Ne laser with the output power of nearly 2 mW, optical elements, a power supply and a hologram recorder. A remote control board is connected to the camera with a cable.

The investigations of the object can be made in 2 regimes:

1. Real-time holographic interferometry.

The hologram of an object is recorded on a photothermoplastic. Then one analyzes the interference pattern of object wave and the wave reconstructed from the hologram. The quantitative variation of the refractive index can be calculated with a computer, into which a signal from the TV-camera is fed. The temporal changes in the interference fringes can be recorded on a video-tape.

2.Double-exposure holographi interferometry.

An initial state of the object is recorded on a hologram. Then, after a certain time the second exposure is performed.

1.3. Scientific applications.

The REGINA holographic Camera is a compact optical device that permits visual observation and investigation of physical and chemical processes in liquids and gases, non-destructive testing, and different types of hologram production.

We have tested holographic interferometry with a holographic camera in the following fields:

1.3.1. Physical chemistry (crystal growth and dissolution).

Holographic Camera was used for the first time on board a space laboratory to investigate crystal dissolution processes.

Because gravity is much reduced in space, the rate at which the dissolved material is removed is decreased because convection currents are practically nil. This results in an increased concentration gradient in the near surface layers, and an overall retardation of the dissolution rate.

Differential holographic interferometry can be successfully employed to visualize the distribution of the materials in the solution in terms of the refractive index variations. In this method, the hologram records the difference between the two states of the object (Solution) at definite time intervals, i.e. the change in the refractive index in the cell interior is recorded as a function of time. When the wavefront is reconstructed, we get an interference pattern which can be interpreted by conventional methods to determine the quantitative changes in the refractive index and then the concentration distribution.

1.3.2. Biomedical research (blood testing, electrophoretic analysis) and cytological studies.

Holographic interferometry is successfully used in investigations of alive cells. A REGINA camera with a special microscope unit allows to examine plant cells, bacteria, etc. and to measure the water transport in plant cells and respiration of plants. To examine directly the plants cells, we use a special microscope system. The hologram is fixed on a photothermoplastic material. The reconstructed image is transmitted by a TV camera to an IBM PC for processing. The object is studied in real time. The experimental plants are placed in a special valve. They are kept there for some time without water. Then the hologram is recorded and after that the water is given to the plant. The interference pattern begins to change. Looking at the changes of the fringes on the display, one can observe the water transport in the plant.

Holography in electrophoresis experiments.

The successful use of the holographic camera in space flights has extended the range of applicability of holography for studying the dynamics of physical and chemical processes under reduced-gravity conditions. Two holograms of isotachophoresis were recorded with the REGINA Hologram Camera designed especially for the purpose. The experiment was motivated by the problem of producing highly pure biological drugs.The electrophoretic technique is more precise, powerful and convenient. Among the various methods of purification, separation, and analysis used for in biology. This method is extremely effective in the separation of biological preparations on the level of protein molecules, subcellular particles, and cells. Although the available electrophoretic apparatus and the methods for controlling the separation are highly advanced, they do not allow visual monitoring of the process dynamics. A precise understanding of the changes which occur in the course of electrophoresis is required for developing methods and apparatus for making pure preparation.

Generally, electrophoresis uses staining, which is quite suitable for a preliminary analysis but must be abandoned when a preparation is produced in a pure form.

An alternative method is to control electrophoresis by holographic interferometry. This method allows observation of the separation of unstained preparation, and makes it possible to determine the changes in the refractive index of the solution as a function of the changing concentration of the fraction being separated. Finite-fringe holographic interferometry has been used in experiments. A hologram permits reconstruction of the isotachophoresis of a preparation.

1.3.3. Molecular physics.

Investigation of free convection in a closed space by real-time holographic interferometry.

By using the holographic interferometry method with hologram REGINA Camera was used in investigations of a free convection jet in real time in a closed space filled with various liquids. The optical layout is the same as in the previous section. A closed liquid gate with a small point-heater at the bottom was placed in the camera and the hologram of the initial state of the liquid (without heating) was recorded.

By applying voltage to the electric heather which is placed at the bottom of the gate one can view the kinetics of the heat-mass transfer.

1.3.4. Aero- and hydrodynamics (flow visualization, speed measurements, turbulence research, vibration convection analysis, shock wave propagation diagnostics, etc.)

1.4. Industrial application.

The REGINA camera is also effective for non-destructive testing in industry. For example, we have used it for testing units and parts of precision tools, products in microelectronic industry, optical fibers, optical components, etc.

1.4.1. Non-destructive testing of artificial heart valves (AHV).

Nowadays many people have cardiovascular diseases. For some of them the only chance to stay alive is a surgical implantation of an artificial heart valve (AHV).

The patients's life directly depends on the quality of the AHV. AHV must open and close for several million cycles during the 20 year operation. Thus, the quality control during the manufacturing and testing of the final products is the main guarantee of reliability of the subsequent operation of valves. Therefore, testing operations make up over 40% of all production operations. With the help of modern effective testing methods we can make AHV more reliable and durable. Quantitative and qualitative testing of the mechanical properties of AHV and also testing of their hydrodynamic parameters can be made by REGINA. We have designed methods for testing AHV under dynamical loads. The deformation of "EMIKS" AHV was explored. We also use hydrodynamic testing of AHV parameters in a special device which simulates the conditions under which the heart can work normally. We also have a special device for visualization of liquid flows which travel through the valve and for measurement of the velocity distribution of the flows.

1.4.2. Non-destructive holographic testing of quality of metallization in the holes of electronic boards.

There are many methods of testing defects of the holes in boards. All of them take a lot of time. This method allows to speed up this procedure and to obtain better results than by conventional methods.Also we can see what type of defect is in the hole.

Experiment is quite simple.The board is illuminated by spherical coherent laser waves.With the help of a diode and TV camera the distribution intensity is registered. When the hole is illuminated by the laser beam, one can see the diffraction picture on the monitor. It looks like a set of circular zones. The analysis of the picture gives the possibility of estimation of the quality of the metallized surface of the holes.

A hologram is a record from which one can obtain information about the geometry and optical properties of the object and the quality of the surface. Diffraction pattern contains a central maximum with 5 or 6 circles. This picture allows to estimate the diffraction on the holes and also the type of metallisation defect.

Data analysis shows that the test hole has a symmetrical pattern. Defective holes have asymmetrical ones, also there are some differences in the distribution of illuminated and dark zones. Some circles can be absent in defective holes.

These experiments were supported theoretically. The results of investigations are in good agreement. Special testing devices can be created on this basis.

 

2. EDUCATION

The Camera REGINA is an ideal device for teaching students of any specialty; it provides a vivid idea and practical skills for those studying mechanics, physics, hydroaerodynamics, chemistry, physical optics; illustrates and provides the possibility to carry out laboratory work in molecular physics (studying of gas processes, physical characteristics of liquids, gases and solid bodies); assists in studying mechanics, physical oscillations, strength and plastically theory; visualizes gas and liquid flows and provides the idea of turbulence; is of great use in acoustics.

In the physical optics REGINA allows to study geometrical optics (properties of optical elements); light diffraction and interference, coherence of emission; gives the idea of holography, holographic interferometry and optical processing of information.

 

Optical elements and holographic cameras were tested and used at the Zero-gravity conditions in space flights and under the severe loading at the underwater measurements. It tolerates chocks up to 50 g, strong vibrations and changes of temperature in wide range. "REGINA" camera is new word in the optical instrumentation. It allows to apply the sophisticated technology developed in specially protected and very expensive environment for everyday use in the simplest laboratory and industrial conditions.

Dr. Vladimir B.KONSTANTINOV

Solid State Electronics Division Executive Secretary

Lead Research Scientist

A.F.Ioffe Phisico-Technical Institute Russian Academy of Science

26, Politekhnicheskaya, St. Petersburg 194021, Russia