Ask for a paper on information optics, such as the research progress of laser holography.

After the discovery of modern laser, optical holography developed rapidly. This paper will discuss some main research topics of optical holography and study some application topics. This paper will introduce the origin, development, characteristics and new applications of modern optical holography.

Based on the principle of interference, the specific light wave emitted by an object is recorded in the form of interference fringes, so that all the information before the light wave of the object is stored in the recording medium. The recorded interference fringe pattern is called a hologram. When the hologram is illuminated by light wave, due to the diffraction principle, the original light wave can be reproduced, thus forming a three-dimensional image realistic to the original. This process of wavefront recording and reconstruction is called holography or holographic beam.

Holography was put forward by Gabriel in 1948, when the holographic research of coherent radiation sources was still in its infancy. Holography at that time was a coaxial hologram with mercury lamp as the light source. The diffraction wave of +/- 1 order is inseparable, that is, there is a so-called "twin image" problem, and a good holographic image cannot be obtained. This is the first generation hologram.

With the appearance of laser in 1960 and 1962, American scientists Liz and Upatinix extended the concept of radio frequency in communication theory to airspace and proposed off-axis holography. He irradiated the hologram with off-axis reference light, so that the hologram produced three diffraction components separated from each other in space, one of which copied the original light, thus solving the two major problems of the first generation hologram and producing the second generation laser recording and laser reproduction.

The main topics in the development of contemporary optical holography are:

1. Spherical lens optical system

2. Light source and optical technology

3. Plane hologram analysis

4. Volume holographic diffraction

5. Pulsed laser holography

6. Nonlinear recording, speckle and negative particle noise

7. Information storage

8. Color holography

9. Synthetic hologram

10. Computer generated hologram

1 1. Reproduction, TV transmission and incoherent hologram

With the development of optical holography, some applications of optical holography have emerged, such as high-resolution imaging, diffuse medium imaging, spatial filtering, feature recognition, information storage and coding, precision interferometry, vibration analysis, contour measurement, three-dimensional image display and so on.

This paper will make academic research on two major topics of contemporary optical holography: research and application.

1. Research on Contemporary Optical Holography

Spherical lens can not only form an image of light amplitude distribution, but also easily form a Fourier transform diagram of this distribution. Therefore, with a simple lens, the object light can be transformed into the Fourier transform of an original figure on the holographic plane. The transform features stored in holograms have important applications in optical pattern recognition. In holography, lens, as an imaging device, can be used to form an image hologram. A lens can form: A. Fourier transform B. Input image with complex amplitude distribution.

Holography has become a practical subject because of the use of laser light source to make holographic images. The requirements for the light source used to form the hologram depend on the parameters determined by the arrangement of the object and the necessary optical elements.

There are two common methods to obtain object wave and reference wave from a single light source, as shown in the following figure:

A. Fractional wavefront method

B. Fractional amplitude method

The maximum optical path difference between the light source and the hologram (reflected by the object surface or reference mirror) must be less than the coherence length. The coherence of laser is related to the oscillation mode of laser. As far as holography is concerned, it needs the spatially coherent radiation of laser oscillating in any transverse mode. Because the oscillation of high dielectric mode is unstable, it often oscillates in two or more modes at the same time, so the best oscillation mode is the lowest mode.

The output power of laser beam must be divided into object illumination wave and reference wave. If you need to observe objects from multiple angles (in order to eliminate shadows), you need to divide the laser beam into several beams. Generally, the amplitude splitting method is adopted, because it can produce more uniform illumination, and has little requirement for beam broadening, which can be broadened before or after distribution.

Plane hologram analysis

The fringe spacing on the collinear hologram recorded by non-scattered light is wider than the thickness of emulsion. The light in the wavefront illuminating the hologram interacts with only one recording fringe before passing through the hologram. Therefore, the response of the hologram is similar to a plane diffraction grating with focusing characteristics. When analyzing these characteristics, Garber regarded these holograms strictly as two-dimensional. The results of two-dimensional model analysis are also in good agreement with experimental observations.

On the hologram obtained by off-axis technique first adopted by Liz and Panic, the fringe frequency is higher than that of collinear hologram, which is proportional to the included angle between the object beam and the reference beam. The typical value of fringe spacing can be considered by the interference of two plane waves.

The period d of the sinusoidal intensity distribution can be determined by the following formula:

2dsinθ=λ, where θ is the included angle between the wave normal and the interference fringes, the wavelength λ, and the fringe spacing d.

Where when θ = 15 and λ=0.5 micron (green light), d= 1 micron. The emulsion thickness for recording off-axis holograms is usually 65438 0.5 microns. In fact, the hologram recorded in this emulsion is no longer considered to be two-dimensional. Therefore, it is important to record that the analysis results of plane holograms can only be accurately applied to holograms formed by using relatively thin media.

Volume holographic diffraction

The response of basic volume hologram to coherent illumination can be described by coupled wave theory.

Suppose two plane waves propagate in the yz plane with unit amplitude, enter the recording medium and interfere. According to the law of refraction, there are

Sin, sin

N is the refractive index of the recording medium; Respectively indicating the included angle between the two waves and the Z axis in the air; And is the included angle between two waves in the medium and the Z axis.

Bragg's law can be written in the form of wavelength in air and refractive index of holographic medium:

2dsinθ= /

The characteristics of volume hologram are determined by Bragg's law, so it shows a selective response to illumination.

2. Typical application of optical holography

High resolution imaging

When the hologram is irradiated by a beam conjugated with the reference beam used to make the hologram, the object wave without aberration and distortion can be reproduced theoretically, and the resolution of the projected real image is only limited by the diffraction of the hologram boundary. Because the resolution will increase with the increase of hologram size. Because the hologram can be made very large, it can be predicted that the spatial frequency will be as high as 1000 lines /mm when it is as large as 5×5 cm in the field of view. Obviously, in this case, the magnification is 1, but the high-resolution projection imaging of 1: 1 has important potential applications in the lithography process of integrated circuits. At present, the work of accurately imaging lithography mask on semiconductor wafer is completed by contact imprinting method. But this method will quickly damage the template. It is an ideal alternative to transfer the image to the film by projection, but a very good and expensive lens is needed to make the projected mask image reach the required resolution and field of view.

When making holograms with coherent light source illumination, the shrinkage of photographic emulsion, surface deformation, nonlinearity and negotiation noise source have great influence. They will lead to spots, reduced contrast and blurred edges in the image, which are not allowed to be made into integrated circuits by photolithography. New and more stable materials may be the answers to these questions.

Trick recognition

Many features of Fourier transform holograms formed by spatially modulated reference waves have been used for feature recognition by van der Rohe et al. They used the spatial filter made by holographic method to complete the application of "matched filtering" in feature recognition.

The following figure can illustrate the concept, formation and application of matched filtering.

When the formed spatial filter is used for feature recognition, the upper part of the Z axis in the input plane is a transparent sheet, which is transparent to plane waves and contains m transparent characters on an opaque background. We express the transmittance of this set of character arrays as

Here all the characters are symmetrically distributed around the point, which is a typical character in the array, and its center is on the point. In addition, on the input plane, there is a bright spot light source with the light intensity of δ, and a Fourier transform hologram is formed on the spatial frequency plane ε η. This hologram can be regarded as a record of plane wave interference formed by t and δ functions. When the hologram completes the recognition function, it is only illuminated by a small part of T, that is, one or several characters on the incident surface. We will see that the reproduction we care about on the output plane is the bright image point representing the recognition result.

Information storage and coding

Holograms can store both two-dimensional information and three-dimensional information. Information can be colored or coded, graphic or alphanumeric; Can be stored on the surface or the whole volume of the hologram; Can be separated or overlapped in space; It can be permanently recorded or erased. The recorded contents can be irrelevant or paired with each other; It can be a recognizable image or a seemingly meaningless figure.

The development prospect of modern optical holography is very broad, but practical technology will be popularized, and people of insight should work together to promote social progress.