It is difficult to quickly control the heat radiation process of the light source as the information carrier, but the light beam emitted by it can be modulated, filtered or otherwise processed, so that the light beam carries information during its propagation. Luminous light sources other than thermal radiation can naturally carry information in the propagation process, but more importantly, they carry information in the emission process. Semiconductor PN junction light emitting diodes that can be driven at low voltage are usually used, especially high brightness semiconductor light emitting diodes and semiconductor lasers. They have the advantages of fast response, easy modulation, small volume and light intensity. Laser has good monochromaticity, coherence, directivity and high intensity, which is beneficial to optical communication and other applications. That is, photoelectric and optical-optical converters can be divided into photoelectric effect and thermal effect.
① photoelectric effect: it can be divided into external photoelectric effect and internal photoelectric effect. The external photoelectric effect is the photoelectron emission effect, and the devices using this effect are all vacuum electronic devices. For example, a photomultiplier tube, whose photocathode can convert an optical signal into a one-dimensional (time) electrical signal, and after repeated secondary emission, the signal is enhanced by the electron multiplier electrode and then output from the anode. The sensitivity of this device is so high that it can even be used to form a photon counter to detect a single photon. A two-dimensional (spatial) photon counter has been developed to detect extremely weak optical information. Another example is the image intensifier tube, which converts X-rays or ultraviolet rays into light sensitive to photocathode, or uses photocathode sensitive to infrared rays to make the light image on the imaging photocathode emit corresponding photoelectrons. After accelerated imaging, these photoelectrons bombard the fluorescent screen, output visible light and emit brighter light images. It is an optical-optical conversion device. This is the working principle of X-ray or ultraviolet image intensifier and infrared image converter. This device can expand the sensitive range of human eyes to electromagnetic wave bands. Devices using internal photoelectric effect are all semiconductor devices. Its main principles are photoconductivity and photogenerated electromotive force. The photoconductive detector is made of a single semiconductor or diode, which is called a semiconductor photodiode. When exposed to light, its resistance will change. Wherein the photodiode usually works under reverse bias condition. If the reverse bias voltage is high enough, the carrier current passing through the PN junction directly reflects the light energy received by the detector in unit time. Photodiodes can also work without bias. At this time, the radiation will produce electromotive force at both ends of the PN junction, and its short-circuit current is proportional to the received radiation power. The detector of infrared thermal imaging system is usually photoconductive. Commonly used are mercury cadmium telluride, lead tin telluride and germanium-doped mercury detectors. They all have to work at low temperature to reduce the thermal noise of the detector.
(2) Thermal effect: A detector using thermal effect is generally called a thermal detector, which mainly uses the effects of resistance change, thermoelectric electromotive force generation and spontaneous polarization change caused by the temperature rise of an object after radiation irradiation to measure radiation power. These detectors are all used in infrared band, with the advantage that their responsivity is independent of wavelength, and they can also detect long-wave radiation at room temperature, but the response time is much longer than that of photoelectric detectors. The main characteristics of light are intensity, spectrum, polarization, luminous time and coherence. When a light beam propagates, it has the characteristics of directionality, divergence or convergence. The function of the control element is to change these characteristics of light. In order to deflect, focus and collimate light beams, mirrors, lenses, prisms and beam splitters are often used. The reflector is usually made of metal film or dielectric film with high reflection coefficient and selectivity. The reflector can be made of total reflection, which is used for image inversion, image conversion, beam splitting and total reflection. In order to change other characteristics of light beam, commonly used components include filters, prisms, gratings, polarizers, photointerrupters, electro-optic crystals controlled by electric fields and liquid crystals.
Electro-optical switch can not only change the intensity and polarization of light, but also control the duration of light passing, and it is a widely used device. Its structure is that a birefringent crystal is placed between two orthogonal polarizers, and an electric field is applied to the crystal, so that the polarization direction of light passing through the crystal will rotate, and the rotation angle depends on the intensity of the electric field. Therefore, the intensity of transmitted light can be changed by adjusting the intensity of electric field; Changing the action time of electric field can modulate the duration of light.
Using the diffraction effect of sound wave on light, the frequency, intensity and propagation direction of light beam can be controlled. Under the condition of near Bragg diffraction, the interaction of acousto-optic makes the beam deflect. When the audio changes, the deflection angle changes proportionally. When the diffraction effect is small, the intensity of diffracted light is directly proportional to the intensity of sound wave. By modulating the intensity of sound waves with information, the intensity of diffracted light can be modulated by this proportional relationship. This control method has been widely used in the fields of optical transmission, display and information processing.
In the optical digital processing system, the key is to develop optical transistors or optical bistable devices. The developed optical bistable devices can be divided into two categories: intrinsic type or all-optical and photoelectric hybrid type. Generally speaking, this device consists of nonlinear medium, feedback system and light source. The high and low states of emitted light intensity can be regarded as "on" and "off" states respectively. Optical transistors can perform optical amplification, modulation, limiting and shaping, and can form optical logic gates.
Optical storage, including optical disks and holographic ultramicro-storage negatives, can be used for optical video recording and large-capacity information storage, as well as books and materials storage. Used to generate optical analog signals, digital symbols and optical images, which can be divided into vacuum devices and non-vacuum devices. The former includes electron beam tubes, low-voltage cathode fluorescent lamps and incandescent bulbs. The latter includes light emitting diodes, electroluminescent screens, plasma and liquid crystal display devices. Except for LCD, which needs ambient lighting and belongs to passive display, others can emit light and belong to active display. There are two display modes: ① Use line segments to combine numbers, symbols or patterns to be displayed. For example, spell numbers and symbols with seven pictures. Most light emitting diodes or liquid crystal displays are used in calculators, digital instruments, etc. With this method. (2) Select some suitable cells in the multivariate array to form the required characters or patterns. These units can be incandescent lamps, light emitting diodes, electroluminescent screens and liquid crystals. This is a matrix cross screen with no gray scale.
Black-and-white and color TV picture tubes are widely used in imaging technology. CRT bombards the fluorescent screen with scanning electron beams to produce black-and-white or color images. The aforementioned optical-optical conversion devices, such as image intensifiers and image converters, are also imaging devices. In addition, multiple arrays with brightness levels can also be used, for example, in a solid-state flat panel display or an imaging screen, two sets of orthogonal electrodes are used. When a sufficiently high potential difference is applied to the intersection of two orthogonal electrodes, a luminous point is formed. It is a pixel, and many pixels with different light and dark make up a picture. Using this structure, electroluminescent screen, liquid crystal display screen and plasma display screen are made.