Optoelectronic devices mainly include light sources as information carriers, radiation detectors, control and processing components, optical fibers, and display devices.
The process of thermal radiation from a light source as an information carrier is difficult to control quickly, but the light beam it emits can be modulated, filtered or otherwise processed so that the light beam carries information during propagation. Luminous light sources other than thermal radiation can naturally carry information during the propagation process, but more importantly, they carry information during the emission process. Usually, semiconductor PN junction light-emitting diodes that can be driven at low voltage are used, especially high-brightness semiconductor light-emitting diodes and semiconductor lasers. They have the advantages of fast response, easy modulation, small size and powerful light. Lasers have good monochromaticity, coherence, directionality and high light intensity. These properties are beneficial to optical communications and other applications. That is, photo-electric and photo-optical converters, which are divided into two categories: those that utilize the photoelectric effect and the thermal effect.
① Photoelectric effect: divided into external photoelectric effect and internal photoelectric effect. The external photoelectric effect is the photoelectron emission effect, and devices that utilize this effect are vacuum electronic devices. For example, in a photomultiplier tube, its photocathode can convert light signals into one-dimensional (time) electronic signals. After multiple secondary emissions, the electron multiplier electrode enhances the signal and outputs it from the anode. This device is so sensitive that it can even be used to form a photon counter to detect single photons. A two-dimensional (spatial) photon counter has been developed to detect extremely weak light information. Another example is the image intensifier tube, which converts X-rays or ultraviolet rays into photocathode-sensitive light, or uses a photocathode that is sensitive to infrared rays. It causes the light image on the imaging photocathode to emit corresponding photoelectrons. These photoelectrons are accelerated and imaged. Bombards the fluorescent screen, outputs visible light, and emits a brighter light image. It is a light-to-light conversion device. This is how X-ray or ultraviolet image intensifier tubes and infrared image tubes work. This device can expand the sensitivity range of the human eye to electromagnetic wave bands. Devices that utilize the internal photoelectric effect are all semiconductor devices. Its main principles are two effects: photoconductivity and photoelectromotive force. Photoconductive detectors are made from a single semiconductor, or into a diode, called a semiconductor photodiode. When exposed to light, its resistance changes. Among them, photodiodes usually operate under reverse bias conditions. If the reverse bias voltage is high enough, the current of carriers through the PN junction directly reflects the light energy received by the detector per unit time. Photodiodes can also operate without bias. At this time, the irradiation of radiation will generate an electromotive force at both ends of the PN junction, and its short-circuit current is proportional to the received radiation power. The detectors of infrared thermal imaging systems are usually of the photoconductive type. Commonly used detectors include mercury cadmium telluride, lead tin telluride, and germanium-doped mercury detectors. They all must operate at low temperatures to reduce the detector's thermal noise.
② Thermal effect: Detectors that utilize thermal effects are generally called thermal detectors. They mainly use the change in resistance, the generation of temperature difference electromotive force, and spontaneous polarization caused by the increase in temperature of an object after being irradiated by radiation. Change equivalent effects to measure radiated power. This type of detector is used in the infrared band. The advantage is that the response rate has nothing to do with the wavelength, and it can also detect long-wave radiation at room temperature, but the response time is much longer than that of the photoelectric detector. The main characteristics of light include intensity, spectrum, polarization, luminescence time and coherence. When the light beam propagates, it has characteristics such as directionality, divergence or convergence. The function of the control element is to change these characteristics of the light. In order to deflect, focus and collimate the beam, mirrors, lenses, prisms and beam splitters are often used. Reflectors often use metal films or dielectric films, the latter of which have a high reflection coefficient and are selective. Total reflection can be used to make mirrors for inversion, image rotation, beam splitting and total reflection, etc. In order to change other characteristics of the light beam, commonly used components include filters, prisms, gratings, polarizers, choppers, electro-optical crystals and liquid crystals controlled by electric fields, etc.
Electro-optical switches can not only change the intensity and polarization of light, but also control the duration of light passage. They are a widely used device. Its structure is to put a birefringent crystal between two orthogonal polarizers, and add an electric field to the crystal, and the polarization direction of the light passing through the crystal will rotate. The size of the rotation angle is determined by the intensity of the electric field. Therefore, adjusting the intensity of the electric field can change the intensity of the transmitted light; changing the action time of the electric field can modulate the duration of the light.
Using the diffraction effect of sound waves on light, the frequency, light intensity and propagation direction of the light beam can be controlled. The interaction of sound and light deflects the beam under conditions close to Bragg diffraction.
When the audio frequency changes, the deflection angle also changes proportionally. When the diffraction effect is small, the intensity of the diffracted light is proportional to the intensity of the sound wave. By using information to modulate the intensity of sound waves, the intensity of diffracted light can be modulated through this proportional relationship. This control method has been widely used in light propagation, display and information processing.
In optical digital processing systems, the key is to develop optical transistors or optical bistable devices. The optical bistable devices that have been developed can be roughly divided into two categories: intrinsic type or all-optical type and optoelectronic hybrid type. Generally speaking, this device consists of three parts: nonlinear medium, feedback system and light source. The high and low states of the emitted light intensity can be regarded as "on" and "off" states accordingly. Phototransistors can perform light amplification, modulation, limiting and shaping, and can form optical logic gates.
Optical storage includes optical disks and holographic ultra-micro storage films, which can be used for optical video television and large-capacity information storage, and can also be used for book and material storage. Used to generate optical analog signals, digital symbols and optical images, they are divided into two categories: vacuum devices and non-vacuum devices. The former includes electron beam tubes, low-voltage fluorescent tubes and incandescent light bulbs, etc.; the latter includes light-emitting diodes, electroluminescent screens, plasma and liquid crystal display devices, etc. Except for liquid crystal displays that require ambient lighting and are passive displays, all others can emit light and are active displays. There are two display methods: ① Use line segments to form the numbers, symbols or patterns that need to be displayed. For example, use seven pictures to spell out the various numbers and symbols. Most of the light-emitting diodes or liquid crystal displays used in calculators, digital meters, etc. use this method. ② Select a part of the units with appropriate positions in the multi-element array to form the required characters or patterns. The units can use incandescent lamps, light-emitting diodes, electroluminescent screens, liquid crystals, etc. This is a matrix cross screen without grayscale.
In imaging technology, black and white and color television picture tubes are widely used. The picture tube uses a scanning electron beam to bombard a fluorescent screen to produce a black and white or color picture. The light-to-light conversion devices mentioned earlier, such as image intensifiers and image change tubes, are also imaging devices. In addition, a multi-element array with brightness levels can also be used, such as using two sets of mutually orthogonal electrodes in a solid flat panel display or picture screen. When a sufficiently high potential difference is applied to the intersection of two orthogonal electrodes, a light-emitting point is formed. It is a pixel, and many pixels with different light and dark colors make up a picture. This structure has been used to make electroluminescent screens, liquid crystal screens and plasma display screens.