How the Optical Transmitter is developed and working
The use of light waves to transmit television signals and data information is a new science and technology developed at the end of the 20th century. Its appearance has enabled the world's information industry to develop rapidly. Now the optical fiber transmission technology is developing at a speed beyond people's imagination. Its optical transmission speed is 100 times higher than 10 years ago, and it is estimated that it will increase by about 100 times in future development. With the continuous development of optical fiber transmission technology, multiplexing, demultiplexing, routing, and switching can be performed in the optical domain. The network can use the huge bandwidth resources of optical fiber to increase the network capacity and realize the "transparent" transmission of multiple services.
The optical transmission system is mainly composed of optical transmitters, optical receivers, optical splitters, fiber optic cables, and other components.
I. A basic principle of optical fiber transmission of optical signals
Optical transmission is a technology that transmits in the form of optical signals between the sender and the receiver. The working process of optical transmission of TV signals is carried out between the optical transmitter, the optical fiber, and the optical receiver; the optical transmitter in the central computer room converts the input RF TV signal into an optical signal, which is composed of an electric/optical converter ( Electric-Optical Transducer (E/O) is completed, and the converted optical signal is received by the optical fiber transmission guide receiving device (optical receiver), and the optical receiver converts the optical signal obtained from the optical fiber into an electrical signal. Therefore, the basic principle of the optical transmission signal is the whole process of electrical/optical and optical/electrical conversion, which is also called an optical link.
The current optical transmission method uses light intensity modulation. For example, a laser-based light-emitting device emits so-called coherent light with the same phase. Therefore, a modulation method that changes the overall luminous intensity is adopted. It uses the linear change of the output optical power corresponding to the change of the input signal current of the electrical/optical converter. characteristic.
In the Optical-Electric Transducer (O/E), the output current is proportional to the intensity of the input optical signal. The output current waveform of the optical/electric converter is therefore similar to the input current waveform of the electrical/optical converter, achieving The purpose of signal transmission.
So, how does the optical fiber guide the optical signal? At present, the optical fiber used in the cable television system is a cylindrical optical fiber, which is composed of an optical fiber cylinder and a cladding and is a quartz glass material. The cladding plays the role of tightly enclosing the light in the optical fiber, protecting the core, and enhancing the strength of the optical fiber itself. The role of the fiber core is to transmit optical signals. Although both the core and the cladding are made of quartz glass materials, there are differences in the doping composition of the two during production, which leads to different refractive indexes (the core is 1.463~ 1.467, and the cladding is 1.45~1.46), of course, is also related to the different materials used. When the light source emitted by the laser enters the core of the fiber, when the light enters the interface of the cladding, as long as the incident angle is greater than the critical angle, the total reflection will occur in the core, and the light will not leak into the cladding. The optical signal in the core will continue to propagate uninterrupted until it is directed to the optical receiver. This process is the basic principle of optical signal transmission in optical fiber.
II. Distortion in optical transmission
When light is transmitted in an optical fiber, some distortion will also occur. The reasons for the distortion are as follows:
(1) In the optical fiber transmission system, due to the nonlinearity of the electrical/optical conversion characteristics of the semiconductor laser, the output optical signal is inconsistent with the change of the excitation current, resulting in distortion, which is called modulation distortion. The value of modulation index M is not allowed to be too large. It is necessary to choose an optical transmitter with high performance and strong pre-distortion processing technology. The pre-distortion processing technology uses artificial design to generate pre-distortion to improve the modulation linearity, so as to eliminate and reduce the optical fiber transmission system. The purpose of CSO and CTB.
(2) In the optical transmission system, since the driving RF amplifier and the receiving RF amplifier have little chance of distortion, the linear PIN photodiode can ignore the slight distortion because the signal level is not too high. The main reason is Distortion of semiconductor laser modulation characteristics and fiber dispersion.
(3) When the laser is modulating the intensity of the light, the wavelength of the light will change, and additional frequency modulation will appear, which will broaden the signal frequency and cause a chirp effect, which is mainly manifested as CSO distortion.
(4) The dispersion characteristics of the optical fiber will cause differences in the group delay of different wavelengths, resulting in distortion caused by inconsistent arrival times at the terminal, mainly CSO distortion.
The distortion produced in the optical fiber transmission system is mainly CSO distortion, and the degree of CTB distortion is much smaller than CSO distortion. In order to ensure the transmission quality of the system and make the system carrier-to-noise ratio and distortion performance within a reasonable range, the measures taken are general Use CNR indicators to balance CSO and CTB indicators. If you increase or decrease the CNR value by 1dB, then the CSO will deteriorate or improve by 1dB, and the CTB index will deteriorate or improve by 2dB.
III. Working principles of the optical transmitter
The most important optical device in the optical transmitter is the semiconductor laser. In fact, it is a laser diode (LD). Of course, some do not use laser diodes but use semiconductor light-emitting diodes (Light Emitting Diode, LED). of.
1310nm optical transmitter generally adopts direct modulation mode (vestigial sideband-amplitude modulation, VSB-AM mode). Its function is to convert electrical signals into optical signals, which can be achieved by changing the power supply of the injected laser through an external circuit. The bias circuit it sets can provide the best bias power supply for the laser. The laser will have different power output when the bias current is different. To ensure a stable output of optical power, an automatic control circuit for optical power and laser temperature should be designed, Such as the use of microcomputers to achieve the best working state of automatic control of the optical transmitter.
Lasers are widely used as optical oscillators (ie, light-emitting devices), which rely on the interaction between the energy state of the laser medium material and light.
In order for the laser to work, there must be a certain amount of current. There is a certain relationship between the size of this current and the light intensity. When the current is increased, the light intensity increases sharply. This indicates that the laser has started to work. This makes the laser work. The current is called the threshold current. The smaller it is, the better, because it has already enabled the laser to work. If the threshold current continues to increase, the output saturation zone will be formed. When the saturation zone current reaches a certain value, the signal will be transmitted. In terms of the power required for optical fiber transmission, the output power of several megawatts in the linear region can meet the requirements of long-distance transmission of signals and information. In addition to the amount of light intensity, the transmission quality of light is also related to problems such as spectrum and noise.
The multi-wavelength spectrum is not suitable for the transmission of high-quality analog signals. Even if it works in a single-mode, its emission spectrum has a width. The narrower the width, the purer the light wave becomes and the more time-coherent it becomes. That is light waves with good coherence. The light wave with good coherence does not need lenses and other devices to converge it into a small spot, and it is more suitable for the incidence of optical fibers.
IV. Working principles of the optical receiver
The main component of the optical receiver is the photodetector, that is, the high-sensitivity photodiode (PIN). The photodiode uses the photoelectric effect of the semiconductor to complete the detection of the optical signal so that the optical signal is restored to the RF TV signal, and then the RF signal After amplification and AGC level control, the qualified RF signal is output for the network distribution.
The main technologies of optical receivers are C/N, C/CTB, and C/CSO. These three technical indicators are all determined by the performance of the photoelectric conversion module. In the case of the same optical power input, the RF level of the conversion output is different. When the conversion efficiency of the photoelectric module is high, its output power Even if the level is high, the C/N value index brought by it is good, and vice versa, the C/N value index becomes worse. The two technical indicators of C/CSO and C/CTB are determined by the linearity of the photoelectric module. High-quality photoelectric modules allow a wider receiving power range under the same C/CSO and C/CTB indicators.
V. Development prospects of optical devices
With the continuous update of optical fiber transmission technology in broadband networks and the continuous improvement of multi-functional services, the requirements for the transmission characteristics of optical devices and optical fibers are getting higher and higher. The era of optical fibers replacing copper wires is finally coming. With the footsteps of the information age With the advent, the development prospects of optical transmission technology are very broad.
Selection and use of optical transmitter
The optical transmitter is the core equipment of the optical cable transmission system. Its function is to optically modulate the radio frequency cable television electrical signal input to the optical transmitter to achieve electrical and optical conversion (E/O), and to send continuous, stable, and reliable optical signals to the optical cable system. The types of optical transmitters currently on the market: according to their different modulation methods, they are divided into two types: directly modulated optical transmitters and externally modulated optical transmitters. Directly modulated optical transmitters are mostly used in 1310nm optical fiber systems, and externally modulated optical transmitters are mostly used in 1550nm optical fiber systems. Regardless of whether it is a directly modulated or externally modulated optical transmitter, its core component is composed of lasers.
Directly modulate the laser transmitter
1. Composition
The composition of the direct modulation optical transmitter, in addition to the core component DFB laser components, there are power supply, a laser bias circuit, laser slow start circuit, overload protection circuit and drive protection circuit, power control and cooling control circuit, light detection circuit, Distortion compensation circuit, photodetector (PIN) chip (for optical power detection and automatic power control), semiconductor refrigerator and thermistor for two-way automatic temperature control (ATC), etc.
2. Working process
The input signal of the optical transmitter is the TV radio frequency (RF) signal. At the front end, multiple RF signals are mixed into one signal by a multiplexer and then sent to the input of the optical transmitter. After being amplified by a preamplifier, it is electronically controlled attenuation, distortion compensation, and automatic power level control. , And then drive the laser chip to perform electrical/optical modulation, and convert the electrical signal into an optical modulation signal. Adding an optical isolator to the output end can greatly reduce the influence of the reflected light wave from the optical cable on the laser. The optical signal is sent to the optical cable through the optical movable joint, and the optical signal is transmitted to each optical point through the optical cable.
It can be seen that the transmission power and nonlinear distortion of the laser depend on the bias current (IO), so the optical transmitter is equipped with the bias circuit and distortion compensation circuit of the laser to ensure the stability of the nonlinear index and the transmission output.
When the temperature of the laser increases, the threshold will increase, the saturated output light intensity will decrease, and the linear range of the PI curve will decrease (that is, the 2 self-dynamic range will decrease). In order to ensure that the optical transmitter always works normally, it must be ensured The laser works at a constant temperature (generally 25°C). The semiconductor cooler and thermistor used for the two-way automatic temperature control (ATC) of the optical transmitter are guaranteed to work at a constant temperature of 25°C.
There is a microprocessor in the optical transmitter, and the best working state data of the laser is stored in the chip. The laser can be slow-started and the RF TV drive current can be automatically disconnected to protect the laser. The various switches on the front panel of the optical transmitter are controlled by a microprocessor.
Temperature changes and device aging will cause changes in the laser threshold current and photoelectric conversion efficiency. If you want to accurately control the optical output power of the laser, you should solve it from two aspects: one is to control the bias current of the laser so that it automatically tracks the threshold. The change of current ensures that the laser always works in the best bias state; the second is to control the amplitude of the laser modulation current to automatically follow the change of the electrical and optical conversion efficiency. Automatic power control completes the above two tasks to ensure that the laser outputs accurate optical power.
Externally modulated optical transmitter
The externally modulated optical transmitter is composed of the external modulator, laser, laser control circuit, modulation control circuit, microprocessor, pre-distortion circuit, photodetector, RF signal attenuator, amplifier, power supply, etc.
3. Comparison of direct modulation and external modulation optical transmitters
Direct modulation transmitters are mostly used for DFB lasers. DFB lasers have good linearity and can obtain better CTB and CSO values without the compensation of pre-distortion circuits. However, due to direct modulation, there is additional frequency modulation and nonlinear distortion indicators (especially CSO value) is difficult to be very high.
DFB transmitter has stable performance, a simple structure, and a low price, so it is widely used.
The power of the direct modulation optical transmitter is generally not too large, within 18nw, therefore, the transmission distance is limited, and it is generally used in local distribution networks and township-level optical cable transmission networks. This type of optical transmitter is mostly used in 1310nm optical fiber networks, and the 1310nm optical fiber attenuation is 0.35db/km, so the maximum transmission distance does not exceed 35 kilometers.
Externally modulated optical transmitter: high output power, up to 2×20mw or more (two outputs), low noise, and no cso distortion caused by the combination of additional frequency modulation and fiber dispersion characteristics similar to LD. Therefore, it is often used in long-distance transmission of large-scale wired systems. Externally modulated optical transmitters generally use YAG lasers. After YAG lasers are externally modulated, the linearity is very poor, and pre-distortion circuits must be used to compensate. Because of its less dispersion, the YAG optical transmitter is very suitable for 1550nm wavelength optical fiber, mostly used in 1550nm optical fiber networks. YAG light is transmitted in the 1550nm optical fiber network, which can be used for amplification and relay. The 1550nm optical fiber has a small attenuation (0.25db/km), so the YAG optical transmitter can be used for ultra-long distance transmission. The externally modulated optical transmitter is used in the 1310nm optical fiber network, and the transmission distance can reach 50 kilometers, which is also faster than the transmission distance of the direct-modulated optical transmitter. However, externally modulated optical transmitters are expensive, and optical fiber networks for short-distance transmission rarely use externally modulated optical transmitters
4. The technical indicators of the optical transmitter
The technical indicators of the optical transmitter are the basis for selecting the optical transmitter, and the good performance parameters of the optical transmitter directly affect the good technical indicators of the entire cable television system.
5. The choice of the optical transmitter
It is very important for cable TV technicians to understand and master the composition, working principle, and performance parameters of optical transmitters, because only by mastering the basic working principles and technical performance indicators of optical transmitters, can optical transmitters be used effectively and reasonably. Good daily maintenance.
At present, there are many foreign and domestic manufacturers of optical transmitters. There are more types of optical transmitters, and the performance indicators and stand-alone prices are also very different. Reasonable selection is of great benefit to ensuring the quality of the optical fiber network and reducing the cost of network construction. High performance-price ratio, reliable quality assurance system, and good after-sales service guarantee are the choice of optical equipment
