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Holography (hologram) interference and diffraction to record and reproduce objects in a light wave technology. The first step is to take advantage of the interference principle record objects lightwave information, namely shooting process: subject diffuse object beam formed under laser irradiation (Fig. a); another part of the laser as a reference beam incident on the hologram, superimposed interference occurs and the object beam, the phase and amplitude of each point on the object lightwave convert spatially varying intensity, thereby using the interference fringes between the contrast and the interval will record all of the information of the object lightwave.
Holography - Introduction
Holography recording the interference fringes of the film after developing, fixing, etc. handler became a hologram or a holographic photo; The second step is to use diffraction theory reproducing the lightwave information of the object, which is the imaging process: The hologram is like a complex grating diffraction waves of coherent laser irradiation, a linear recording of the sine-type hologram can generally be given the two images, i.e., the original image (also known as the initial image) and the conjugate image. The reproduced image three-dimensional real visual effects. Each part of the hologram recorded in the optical information of each point on the object, and hence in principle each of its portion can reproduce the entire image of the original, a plurality of different image by the multiple exposure can also be recorded on the same one backsheet, and can without interfering with each other are displayed.
The hologram can be divided into the amplitude-type (also called an absorption-type) and the phase type two categories, respectively, to change the amplitude or phase of the lighting lightwave in a manner corresponding to the amount of exposure is recorded. If classified according to the spacing of interference fringes and the relative size of the thickness of the photosensitive layer, there is a thin (two-dimensional or planar type) and thick (three-dimensional or volumetric type) two types of holograms. Thick holographic photo shooting, the object beam and reference beam is incident on the same side of the photosensitive film, is divided into a transmission type hologram and a reflection type hologram. Such as by the different classifications of road layout of recording holograms time, a coaxial-type hologram, and the off-axis hologram.
Holography - Development introduced
The holography principle applies to the fluctuations of the various forms, such as X-ray, microwave, sonic, electronic waves. Have sufficient coherence to these fluctuations in the formation of the interference pattern. The optical holography expected stereoscopic film, TV, exhibition, microscopy, interferometry to learn projection lithography, military reconnaissance and surveillance, underwater detection, metal internal probe, to save precious historical relics, works of art, information storage, remote sensing, research and record changes in the physical state of fast transient phenomena, instantaneous process (such as an explosion and fire), is widely available.
Also known as holography. Record fluctuations in amplitude and phase distribution of the interference and subsequently reproduce technology. Widely used as a three-dimensional optical imaging, and can also be used for acoustic wave (see acoustic holography) and radio frequency waves. The "holographic" is from the Greek word "holos" change to the meaning the complete information - - includes not only the amplitude of the light also includes the phase information.
A brief history of 1947 D. Gabor engaged to improve the resolving power of electron microscopy. By WL Bragg in the X-ray Metallography and F. Zernike phase of work inspired about the introduction of a coherent background, Gabor proposed the idea of ??holography to improve the resolving power of the electron microscope. In 1948 he used mercury lamp for the first time hologram again phenomenon, which was founded holography, for which he won the Nobel Prize in Physics in 1971.
GL Rogers et al's work in the 1950s greatly expanded wavefront reproduction theory. However, due to the restrictions twins like "and coherent light source, holography into the doldrums after the 1955 stage. The emergence of the laser, the development of holography has opened up broad prospects, from 1961 to 1962, EN Leith et al Gabor hologram improved twin like "problem solved in one fell swoop, the introduction of the" the oblique reference beam method ", with a helium-neon laser successfully shot the first practical laser hologram. This makes holography in 1963 to become one of the most active branch in the field of optics. Leith et al in 1964 proposed the concept of the diffusion hologram, and the reproduction of the three-dimensional object. At the same time, the Soviet physicist Lippmann color photographic method and Gabor holography method proposed the concept of a reflection hologram. Since 1965 an important branch of holography - - pulse holography has been developed, which makes dynamic holographic interferometry practical application.
Holography - basic principles
In 1948 Gabor proposed a new two-step lensless imaging method - - holography, also known as wavefront reproduction technique. Record of the whole process consists of two steps - - wavefront and wavefront reproduction - - accomplished.
The wavefront recording this process, to introducing appropriate coherent reference wave, so that it can be obtained by a hologram with interference by the object diffraction (scattering) of light (object light), this interference field recorded. Holograms are no similar interference pattern and the object, it is not only recorded in the amplitude of the information of the object light and the loss of phase information in the ordinary photographic process recorded. The recording is shown in Figure 1a. Is provided on the recording medium such as a dry plate at the object light and the reference light wave fronts complex amplitude expressions known by the principle of superposition of waves, respectively, the total light intensity distribution recorded photographic dry plate is:
Photographic plates (or other recording medium) on the plane (x, y) exposure, after developing, fixing, will I (x, y) to complex amplitude transmittance t (x, y) in the form of recorded. Under certain conditions t (x, y) of aI (x, Y), where to (x, y) only and with reference to the light intensity relevant; second object light of the light intensity (or amplitude) relevant ; The third is determined by the phase of the reference light and object light. So that the complex amplitude transmittance of the hologram t (x, y) is the objective optical amplitude and phase of the completely recorded.
Wavefronts reproducing wavefront recorded result is a mind there was light amplitude and phase information of the hologram. Wavefront reproduction process is the use of appropriate coherent reproduction light irradiates the hologram of the B (x, y) to obtain a real image or virtual image of the object. The hologram is irradiated with coherent reproduction light B (x, y) through the hologram optical µ (X, Y),
Usually reproduction light B (x, y) is selected as the A (x, y), or A * (x, y) of, when the B (x, y) = A (x, y), if suitably selected ACT | A ( x, y) | 2 in the entire uniform distribution µ4 represents O (x, y) the reproduction of the object light, i.e. to obtain a three-dimensional virtual image. When B (x, y) = A * (x, y) A (x, y), also suitably selected so that | A (x, y) | 2 in the entire distributed evenly, µ3 (x, y ) on behalf of the object light conjugate light, to obtain a three-dimensional real image. In both cases the other in a uniform background or distortion like appears. Technically you can think of a way to eliminate them or reduce their impact.
Holography - classification introduced
Holography hologram classification of species range, there are many different classification methods. For example, based on the ratio of the thickness of the recording medium and the fringe spacing, can be divided into thin holograms and thick hologram; depending on the modulation variables of the complex amplitude transmittance can be divided into amplitude holograms and phase type hologram; according to the recording the orientation of the object and reference beams, can be divided coaxial hologram and the off-axis hologram; according to the time of recording the object light and reference light in the ipsilateral or both sides of the stem plate can be divided into a transmission hologram and the reflection hologram ; can also be based on the distance of the recorded object and dry plate, divided into Fresnel type and Fraunhofer type hologram; according to the production of all the nature of the light source can be divided into the hologram and the pulsed laser, a continuous wave laser hologram etc. The following is a brief introduction to the various types of holograms:
Coaxial hologram and off-axis hologram
1948, the transmittance of the transparent body Gabor use light as the reference light, the scattered light of the object light, the recording of the first holographic FIG. The - - coaxial hologram. Since this hologram reproduction twin like problems, Leith et al introduced the oblique reference beam, to obtain the off-axis hologram, to overcome the twin like problems.
Thin holograms and thick hologram
When the hologram recorded by the interference fringe spacing is greater than the thickness of the recording medium, it can be regarded as a two-dimensional grating structure, referred to as a thin hologram or planar hologram. Otherwise, the hologram can be seen as a three-dimensional grating structure, called thick holograms or volume holograms. In fact, a hologram usually contain the different intervals stripes structure, so it may also show the characteristics of the thin structure and thick structure of the two. For example, for the the Kodak 649F stem plate (having a thickness of ~ 16 microns, n ~ 1.5), and only in the object beam, the reference light angle is less than 10 degrees, the production of the hologram is a thin hologram.
Transmission hologram and the reflection hologram
For the most general terms of hologram, the image of an object are transmitted light by the diffraction by the hologram formed, this type of hologram is called transmission hologram, which is light and by the objects in the recording medium is the ipsilateral formed by the reference light. If the recording medium is a relief type, in connection with the transmission of the relief surface of the hologram on the reflective film of the plated layer can be formed a reflection hologram. For the non-relief type, the thickness of the recording medium, using, respectively, in the medium on both sides of the object light and the reference light can be obtained more complex reflection hologram, commonly referred to as a reflection hologram multi-fingered this latter. Figure 4 shows a hologram recording position of the source of several points. When the object light and the reference light angle close to 180 ° (in position), Preferably, the wavelength selectivity of the thick reflection hologram can be reproduced with white. Actually like the color reproduction latex shrinkage (such as silver halide dry plate) or expansion (as ammonium board) offset to the short-wave or long-wave direction.
Amplitude hologram and a phase-only hologram
Based on the formation mechanism of the hologram can know that it in some manner to the standing wave field of the object and reference light interference is formed on the hologram surface of the light intensity distribution I (x, y), and converted into the hologram dry plate ( or other recording medium) of the complex amplitude transmittance t (x, y). Usually t (x, y) can be represented by the following formula: (1) for the recording medium, a silver salt photographic dry plate after treatment enables dth (x, y) is constant, can make to 0. Then (2) having the formula (2) is absorbed by the hologram size determines the amplitude transmittance distribution, called amplitude hologram. For bleached silver salt dry plate, the medium in terms of dichromate gelatin plates, iron-doped lithium niobate, the t (x, y) of ~ 1, then the complex amplitude transmittance of (3) of this type of hologram is only phase dth (x, y) is modulated by the I (x, y), called a phase-type hologram. Phase type hologram having a uniform transmittance, but because of a different thickness or refractive index variation caused by the phase change of the incident light. It is characterized by the high diffraction efficiency. Table 1 shows the theoretical maximum diffraction efficiency of the hologram ?.
Fresnel and Fraunhofer-type hologram
Fresnel hologram is formed when the two-dimensional object from the hologram plane to the finite value, reproduction, the diffracted wave complex amplitudes for the object wave complex amplitude of the Fresnel transform. If the object is a three-dimensional distribution, the reproduction to obtain the three-dimensional image of the object, like a hologram can be considered as a Fresnel hologram, which is formed by the image of an object hologram. Obtained when the size of the object as compared to its distance away from the hologram plane is much smaller, Fraunhofer holograms. Reproduction diffracted wave complex amplitudes for the complex amplitude of the Fourier transform of object waves. Under such conditions the formation of the hologram called Fraunhofer hologram. Fourier hologram is a Fraunhofer hologram, it is using the lens of the two-dimensional image an object at infinity (was placed on the focal plane of the lens), and coherent plane wave as the reference wave, This is equivalent to infinity as interference with the reference wave, to obtain the Fourier hologram. There are also lensless Fourier hologram.
Holography - calculate the hologram
Based the advanced acoustic holography noise measurement system
Usually holograms are produced by optical methods. But the demanding requirements of technical conditions caused by the distortion of the image as well as the manufacturing process due to the nonlinearity of the recording medium, so that the quality and production of optical holograms repeatability there are many problems. With the development of computer technology, people started to use the computer to create a vision objects (no matter how complex, in principle, the hologram can be) - - hologram.
Its many advantages, such as a computer and grayscale plotter used together, particularly in the calculated hologram often used in black and white, a hologram or called binary holograms, the nonlinear effects of the recording medium can be reduced to a relatively small extent; In addition, due to the reliability of the computer and plotter, so that the calculated hologram repeating quality guaranteed; addition, is difficult to obtain optically complex objects, using a computer to make a hologram according to the mathematical expression and be reproduced like, which can be The computer as a generalized optical element to use. Calculated holographic attracted universal attention appeared to have gained more and more applications, such as optical spatial filtering, testing optical surfaces, three-dimensional computer display.
Calculating the hologram production mainly includes two steps: The first step is to calculate, using the mathematical model of the envisaged object to calculate the intensity distribution of the object wave and the reference wave coherent light in the hologram surface superimposed. This step can also be without reference wave, not the reference wave is calculated from the distribution of the object wave. The second step is the drawing, the computer calculates the hologram complex amplitude transmittance distribution was plotted by the plotter, the optical microfilm or the electron beam is directly controlled by an electronic computer plotter for drawing, and to obtain the calculated hologram.
Holography - rainbow hologram
Based the advanced acoustic holography noise measurement system
The rainbow holography initially S.A. Benton. It is the hologram is recorded with a laser, with a white light transmittance reproducing. According to the characteristics of the human eye is arranged horizontally, the imaging only in the horizontal direction a parallax effect. The rainbow hologram retains the object information in the horizontal direction, and the expense of the vertical direction of the object information, which can reduce the requirements of the coherence time of the illumination light source. It will spread to different wavelengths of light along the vertical direction of color, to false color perspective view of the different colors can be seen at different heights reconstructed image.
The rainbow hologram diffracted light converging performance, high brightness of the reconstructed image. White light illumination source, coherent scattered markings effects caused by the effects of noise can be avoided. The rainbow hologram recorded near the real image of the object. According to the real image can be divided into one-step rainbow holography and two-step rainbow holography. Two-step real rainbow holography such as general master hologram.
One-step rainbow holography real like Act Rainbow holography records generated by the imaging lens optical path diagram. Objects through the slit by a lens imaging. The reference light in the above (or below) of the slit is oblique to the dry board. Reproduction, a white point light source located in the recording, the reference position of the light. The white dispersion thus like into the slit in a different vertical position. Eyes to see different colors at different heights like. Eye movement, in the horizontal viewing range can see the stereoscopic effect of the reproduction image.
Holography - Introduction of Application
Gabor invented holography soon pointed out that the application prospects of the three aspects that holographic interferometry measuring technique, holographic optical elements and holographic information storage. With the advent of the laser, the three different extent practical. Later extended to three-dimensional holographic displays, holographic transform feature recognition. Holography in the field of science and technology, culture, industry, agriculture, medicine, the arts, business, the application of a certain degree. However, due to various technical reasons, the most effective applications are still holographic interferometry technique and the holographic optical element.