Holographic analysis

Holographic principle

Through the study of university physics, we know that under normal circumstances, when the two beams of coherent light have the same phase, the vibration of the synthetic light source (corresponding light intensity) is enhanced, whereas the vibration of the light wave is weakened. The phase of the light changes with the position, and therefore, the increase and decrease of the vibration of the light wave also changes with the position. In this way, strong and weak phase interference fringes occur at the intersection of the two light beams. The distribution of the fringes reflects the change of the phase of the synthetic light wave at different positions. Therefore, the interference fringes produced by the interference of the two light beams can effectively record the changes of the phase phase. Holography is the use of light interference to scatter the light waves 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, to record all the information of the object, there are many advantages in obtaining a stereoscopic image.

Holography is divided into two steps. The first step is to use a method of interferometry 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, and a beam of light illuminates the object, and light reflected or scattered from the object hits the photographic film. The other part of the beam is projected onto the mirror and the reflected light is directed onto the photographic film. This beam is called the reference beam. The object light and the reference light are superimposed on the film, and the resulting interference pattern records all the information of the object's amplitude and phase. This film with an interference pattern is a hologram (hologram) after proper exposure and processing. This shooting process is a process of recording or storing information (or wavefronts).

The second step is to use the principle of diffraction to reproduce (reproduce) the object. Since holograms record the results of mutual interference between two coherent lights, they have nothing in common with the original subject. However, when the hologram is returned to its original position and the hologram is illuminated with coherent reference light (called the reproduction beam at this time), as shown in FIG. 1(b), this hologram having an interference pattern looks like a complicated grating. Diffraction will occur. These diffracted light waves contain the original object light wave, and the observer can observe the reproduction light wave direction to observe a realistic and highly stereoscopic object reproduction image. This is a process of rehearsing, that is imaging, an optical wavefront. 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 inversely illuminated with the original coherent light, the resulting image is a real image. If white light is used to irradiate without using a laser, since white light is a mixture of light of various wavelengths, the interference fringes on the hologram must be diffracted at the same time for various wavelengths of light. As a result, there will be many overlapping and misplaced images on the hologram, making it impossible to see clearly. Of course, if we use recording techniques such as rainbow holograms and reflection Fourier transform holograms in the photographic process of a hologram, white light holography in white light illumination to reproduce the original image can be obtained.

Fig. 1 Principles of holographic recording and reproduction (a) Recording light path of a hologram (b) Reproduction light path and image of a hologram

Holographic classification

The types of holograms can be categorized from different perspectives. According to the needs of the discussion, the following are only categorized from four aspects:

1. According to hologram complex amplitude transmittance coefficient (or reflection coefficient) classification

If the hologram amplitude transmission coefficient is a real function, that is, the hologram is called an amplitude hologram; if the hologram amplitude transmission coefficient is a complex number, that is, the hologram is called a hybrid hologram; if the hologram The amplitude transmission coefficient of a graph is only a function of the phase factor and is called a phase hologram. The actual recording materials are amplitude type, phase type and hybrid type. The phase-type recording material is further divided into an embossed type and a refractive type. If the thickness of the recording medium changes after exposure and processing, and the refractive index does not change, it is referred to as an embossed type; conversely, if the thickness of the recording material does not change and the refractive index changes, it is referred to as a refractive type. A hologram made with a holographic dry plate is an amplitude hologram after the development process, and a phase hologram or a hybrid hologram after the bleaching process.

2. Classification by hologram structure

The structure of the interference fringes in the hologram is closely related to the direction and waveform of the reference light. Taking the plane reference and the object wave as examples, as shown in FIG. 2 , the object light wave and the reference wave are incident on the same side of the recording medium. The recorded hologram is called a transmission hologram, and the stripe surface in the recording medium is close to perpendicular to the 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. Reflection holograms require thicker recording media to record multiple layers of striped surfaces.

Figure 2 Structure of a transmission hologram

3. According to the direction of the reference light and the object light

According to the classification of the direction of the principal light of the reference and object light, there are points of coaxial and off-axis holograms. In the recording, the center of the object and the reference light source are located on the same line passing through the center of the hologram [see Figure 3(a, b)]. Take the spherical wave type reference and object wave as examples. Let the object be 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. Off-axis transmission hologram When recording, the object and reference light are not on the same line as the center of the hologram [see Figure 3(c)], and its stripe shape is part of the conic conic curve.

(a) Coaxial, optical axis perpendicular to hologram (b) coaxial, optical axis not perpendicular to hologram (c) off-axis hologram

4. Classification according to the positional relationship between the object and the photosensitive material (film or dry plate) and the Fourier lens

Holograms can be divided into Fraunhofer holograms and Fresnel holograms. Let the coordinates on the object surface be such that the distance between the object plane and the hologram satisfies the condition: the Fraunhofer hologram.

However, if the condition is satisfied, the result is a Fresnel hologram, as shown in Figure 1. In addition, the Fourier transform hologram, the lensless Fourier hologram, the image plane hologram, the two-dimensional hologram, and the three-dimensional hologram can also be classified according to the presence or absence of the lens and the position of the object and the photosensitive material (film or dry plate) and the Fourier lens. Graphs, 2D/3D holograms, etc.