Principles of holography
Through the study of university physics, we know that in general, when the phases of two beams of coherent light are the same, the vibration (corresponding light intensity) of the synthetic light source will increase, otherwise, the vibration of the light wave will decrease. The phase of light changes with position, so the vibration increase and decrease of light wave also change with position. In this way, strong and weak interference fringes are generated at the intersection of the two beams. The distribution of the fringes reflects the change of the phase of the synthesized light wave at different positions. Therefore, the interference fringes generated by the interference of the two beams can effectively record the phase changes. Holography uses the interference of light to record the scattered light waves of the scene in the form of interference fringes, that is, the amplitude and phase of the light waves are recorded in On the photosensitive material, that is, all the information of the object is recorded, so it has many advantages to obtain a stereoscopic image.
Holography is divided into two steps. The first step is to use the interferometry method to take a hologram (hologram), as shown in Figure 1 (a). The coherent light beam emitted from the laser is split into two beams by the beam splitter. One beam illuminates the object, and the reflected or scattered object light from the object hits the photosensitive film. Another part of the light beam is projected onto the mirror, and the reflected light wave directly illuminates the photosensitive film. This light is called reference light. The object light and the reference light interfere with each other on the film, and the resulting interference pattern records all the information about the amplitude and phase of the object. This film with interference pattern is a hologram (hologram) after proper exposure and processing. This shooting process is a process of recording or storing information (or wavefront).
The second step is to use the principle of diffraction to reproduce (reproduce) the object. Since the hologram records the result of the interference of the two coherent lights, there is no similarity to the original object. However, when the hologram is put back and the hologram is illuminated with coherent reference light (referred to as a reproduction beam at this time), as shown in Figure 1 (b), this hologram with an interference pattern is like a complex grating Diffraction will occur. These diffracted light waves contain the original object light wave. The observer can observe a realistic, highly stereoscopic object reproduction image in the direction of the reproduced light wave. This is a process of object light wavefront reproduction or imaging. However, if the reproduction beam is in the same direction as the original reference beam, the resulting object image is a virtual image. If the hologram is reversely irradiated with the original coherent light, the object image obtained is a real image. If you do not use laser light and irradiate with white light, because white light is composed of light of multiple wavelengths, the interference fringes on the hologram must diffract light of various wavelengths at the same time. Therefore, many overlapping and misaligned images will appear on the hologram, making it impossible for people to see clearly. Of course, if we use recording technologies such as rainbow holography and reflective Fourier transform holography in the hologram shooting process, we can obtain white light holography that reproduces the original image with white light illumination.
Fig.1 Principle of holographic recording and reproduction (a) Recording optical path of hologram (b) Reproducing optical path and image of hologram
Classification of holograms
The types of holograms can be classified from different points of view. According to the needs of the discussion, they are classified from only four aspects:
1. According to the hologram complex amplitude transmission coefficient (or reflection coefficient) classification
If the amplitude transmission coefficient of a hologram is a real function, that is, this hologram is called an amplitude hologram; if the amplitude transmission coefficient of a hologram is a complex number, that is, this hologram is called a mixed hologram; The amplitude transmission coefficient of the graph is only a function of the phase factor, which is called the phase hologram. The actual recording material has three types: amplitude type, phase type and mixed type. Phase-type recording materials are divided into relief type and refraction type. If the thickness of the recording medium changes after exposure and processing and the refractive index does not change, it is called a relief type; otherwise, if the thickness of the recording material does not change and the refractive index changes, it is called a refractive type. Holograms made with holographic dry plates are amplitude holograms after development processing; if they are bleached, they become phase holograms or hybrid holograms.
2. Classification by hologram structure
The structure of interference fringes in the hologram is closely related to the direction and waveform of the reference light. Taking the plane reference and object waves as an example, as shown in FIG. 2, the object light wave and the reference wave are incident from the same side of the recording medium, and the hologram recorded in this way is called a transmission hologram. At this time, the stripe surface in the recording medium is nearly perpendicular to surface. When the object light wave and the reference light wave are incident on the recording medium from both sides, as shown in FIG. 2, the stripe surface is parallel to the surface in the recording medium, and thus the recorded hologram is called a reflection hologram. Reflective holograms require a thicker recording medium to record multiple stripes.
Figure 2 Structure of transmission hologram
3. According to the direction of the main light of the reference light wave and the object light wave
According to the direction of the chief rays of the parameter and object light waves, there are also on-axis and off-axis holograms. When recording the coaxial hologram, the object center and the reference light source are located on the same straight line passing through the center of the hologram [see Figure 3 (a, b)]. Taking spherical wave parameters and object waves as an example, suppose the object is a point light source. The reference light wave is a spherical wave, and the interference fringe of its coaxial hologram is a set of concentric rings or ellipses. When recording off-axis transmission holograms, the object and the reference light are not on the same straight line as the center of the hologram [see Figure 3 (c)], and its stripe shape is part of the revolving conic curve.
Figure 3 Recording optical path of three holograms
(a) coaxial, the optical axis is perpendicular to the hologram (b) coaxial, the optical axis is not perpendicular to the hologram (c) off-axis hologram
4. According to the positional relationship between the object and the photosensitive material (film or dry version), Fourier lens
Holograms can be divided into Fraunhofer holograms and Fresnel holograms. Let the coordinates on the object plane be that the distance between the object plane and the hologram satisfies the condition: the one is Fraunhofer hologram.
And if the condition is satisfied: the Fresnel hologram, as shown in Figure 1. In addition, it can be divided into Fourier transform hologram, lensless Fourier hologram, image plane hologram, two-dimensional hologram, three-dimensional hologram according to the presence or absence of lens and the positional relationship between the object and photosensitive material (film or dry version), Fourier lens Figures, 2D / 3D holograms, etc.
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