What Are Holographic Discs?

The holographic universal optical disc can also be used as a holographic recordable optical disc. By changing the incident angle of the reference light, multiple interference patterns can be recorded at the same position of the medium to record holographic information of the optical disc.

1. Holographic disc records all information of light;

2. 2. Holographic discs span the storage of two-dimensional surfaces of optical information to three-dimensional volume storage;

3. 3. The storage of holographic disc information bits is not bit-by-bit storage, but page-by-page storage. Each page contains about bits.

4. The information is shown in Figure 1. Therefore, holographic discs not only have high storage density, but also have high data transmission rates.

5. Through various multiplexing technologies, the holographic memory makes the capacity of the holographic optical disc and the data transmission rate increase year by year.

The platter structure is shown in the figure, in which each layer is:

1. A polycarbonate substrate with address information pits, which together with the servo system of the holographic disc provide the addressing function of the holographic disc and ensure the interchangeability of the holographic disc;

2. Aluminum reflective layer, which is used to reflect the red light reading the address information;

1. Holographic universal disc 1. Wave characteristics of light

Light waves are essentially electromagnetic waves, which are propagated by the alternating changes of electric and magnetic fields in space. Usually, the change of the electric vector with space and time is used to describe the wave characteristics of light. For plane waves, E = E (x, t), and for spherical Wave E = E (r, t).

For plane waves, suppose a series of plane waves propagate in the X direction in space. As shown in the figure, the wave equation of plane waves is expressed as:

For spherical waves, the wave equation is expressed as:

Where K = 2 / represents the wave vector, is the wavelength; = 2, is the frequency of the light wave, and the frequency determines the color of the light;

or

/ r is the amplitude of the wave,

Figure 2: Waveform of light

Amplitude is directly related to light intensity. V in FIG. 2 represents the wave velocity of the light wave. The cosine function term in the above formula represents the phase of the light wave. Frequency, amplitude and phase are the three elements of the wave characteristics of light waves. Examples of the three elements of the wave characteristics of light waves in specific applications are as follows: general black and white photos, the amplitude information of light waves (that is, light intensity information) is recorded on the negative; general color photos, the frequency and amplitude of light waves are recorded on the negative Information; only holograms, all information such as the frequency, amplitude, and phase of light waves are recorded on the dry board, so it is called "holography". The amplitude and phase of light waves are important parameters when two (or more) rows of waves are superimposed.

2. Holographic universal disc 2. Light interference characteristics

If two columns of light waves meet in space, whether it is a plane wave and a plane wave, a spherical wave and a spherical wave, or a spherical wave and a plane wave

Figure 5: Principle of Light Interference

(As shown in Figure 5), as long as the two waves meet the coherence conditions of the light, that is, the two waves have the same frequency, the same vibration direction, and a certain phase difference at the meeting place, the two waves are superimposed Interference will occur in the area and interference fringes appear between light and dark, as shown in Figure 5 (a).

In this case, if a piece of a substrate coated with a photosensitive medium is placed in the coherent area, a photosensitive effect will be produced. Because the interference fringe light intensity distribution is different, the absorption in the medium is different, so the refractive index (or other optical constants) of the medium also changes differently, as shown in the optical constants, as shown in Figure 5 (b ) Other optical constants) also change differently, as shown in the figure. As shown in FIG. 5 (c), if the carrier is taken out, light interference information is retained on the carrier. This is what is commonly referred to as information being recorded or written to a medium.

If the point light source in FIG. 5 (a) contains signal information, the interference fringes containing the signal information are recorded on the medium. If a desired optical signal is to be extracted from the interference fringes recorded on the above-mentioned medium, a signal reproduction process described below is also required.

As shown in Figure 6,

Figure 6: Signal Light Reproduction

Signal light can be reproduced by using a beam of light having the same frequency as the reference light frequency used in writing information as the read reference light and irradiating the interference information of the recording medium. The reproduced signal light can be imaged in a detector to detect a single bit. One bit of information can be stored as a hologram. Similarly, a hologram in the detector corresponds to one bit when read out.

3. Holographic universal disc 3. Recording and reading of holograms

Figure 7 shows the hologram generation process: a beam of green or blue light is split into two laser beams by a beam splitter:

Figure 7: Holographic imaging of light

Signal and reference beams. The signal beam passes through the spatial light modulator and converges to a certain position on the recording medium, while the reference light is reflected by the reflector and enters the same position of the recording medium at a certain angle. Because the two beams come from the same laser and have the same frequency The amplitude and vibration directions are the same, and there is a certain phase difference at the meeting point, which meets the coherence conditions of light. Therefore, an interference pattern is formed after being superposed on this position of the recording medium, which is called a hologram, as shown in FIG. 7. This means that a hologram, which is a mark written by signal light and reference light, is stored in the recording medium. A hologram is essentially an interference pattern of signal light and reference light at a convergence point. By changing the incident angle of the reference light, multiple interference patterns can be recorded at the same position on the medium. The optical disc used to record holographic information is called a holographic disc.

Figure 8: Reading of holographic imaging

Unlike CD / DVD discs, holographic discs are recorded on the surface of the disc, so they can be read by light reflection. The hologram is stored in the entire recording medium of the holographic optical disc. It cannot be read out by the light reflection method, but can only be "taken out" by optical imaging methods. This method is, in principle, as long as the spatial light modulator in the hologram imaging device is removed, as shown in FIG. 8, the reference light is irradiated to the recording position on the recording medium at each angle corresponding to the time of writing, The holograms recorded at each point are read out one by one. Figures 7 and 8 are taken from the relevant materials of In Phase.

4. Holographic universal optical disk 4. Hologram recording and reading system

Figure 9 (a) shows a hologram recording and reading system. The spatial light modulator is a device that modulates the spatial distribution of light waves. It can perform real-time two-dimensional parallel input and processing of input signals. The spatial light modulator contains many independent units, which are arranged in a two-dimensional array in space. Each unit can independently control the optical signal and change its optical constant according to this signal.

Figure 9: Schematic diagram of hologram writing and reading system

Such as refractive index, reflectance, transmittance, etc., so that the light waves passing through it are modulated. During recording, the spatial light modulator converts the binary data 1 or 0 to be stored into a page formed by black and white pixels, which is used as a coherent optical signal to be incident on the holographic storage medium through a Fourier transform lens (FT). At the same time, the angle-coded reference light is incident on the same position of the storage medium in order to achieve the recording of multiple images. One angle code corresponds to one hologram, and multiple angle codes correspond to multiple holograms. This is commonly referred to as "angle multiplexing", that is, the use of angle coding to achieve multiple recording of holograms at one position on the recording medium. . In addition to angular multiplexing technology, holographic storage also has other multiplexing technologies, such as wavelength multiplexing.

When reading information, the recording medium is scanned with a reference light that is coded corresponding to the time of writing. The read optical signal is irradiated onto the CCD detection array through an inverse Fourier transform lens (FT-1) and is detected by the CCD image beam detection system. Receive and send to subsequent processors. In FIG. 9 (b), different colors of light are used to distinguish the optical paths of the hologram during writing and reading.