Tomorrow’s article Burundi Sugar Daddy, let’s talk about an “Internet celebrity” technique – related optical communication.
Related optical communication, the full English name is Coherent Optical Communication, which is a technology in the field of optical fiber communication.
Compared with traditional non-correlated optical communication, correlated optical communication has the technical advantages of longer transmission distance and larger transmission capacity. Therefore, it has been widely followed and paid attention to by all walks of life in the industry, and research enthusiasm continues to rise.
What is correlated light
Before introducing correlated light communication, let us briefly understand what correlated light is.
Everyone knows that the “correlation” we often talk about in our actions means “interconnection or involvement”.
Light correlation (coherence) means that during the transmission process of two light waves, the following three conditions are met at the same time:
1. The frequency (wavelength) is the same;
2. The vibration direction is the same;
3. The phase difference is constant.
Correlated light
Such two beams of light can have a stable interference with each other during transmission.
This kind of intervention can be either constructive (increasing intensity) or destructive (offsetting).
As shown below:
Apparently, constructive intervention can make light waves (electronic signals) stronger.
Master, you can recall the famous Yang’s double-slit intervention experiment
What is correlated optical communication
OkayOkay, let’s get down to the topic and talk about what coherent optical communication is.
Many people may think that coherent optical communication means using coherent light for transmission and communication.
In fact, this statement is wrong. Correlated optical communication and non-correlated optical communication basically use lasers, and there is no substantial difference.
The reason why correlated optical communication is called “correlated optical communication” does not depend on the light used in the transmission process, but on the use of correlated modulation at the transmitting end and the use of related technologies at the receiving end for detection. BI Escorts: Non-correlated optical communication
Pictured below: Correlated optical communication
The difference is in the middle, not in the transmission path
The receiving end technology is the core of all correlated optical communications and an important reason for its awesomeness.
We can start with the conclusion: under the same conditions as Burundi Sugar, compared with traditional non-correlated optical communication, the receiver of coherent optical communication can increase the sensitivity by 20db.
What is the concept of 20db? 100 times!
This improvement is amazing, close to the shot noise limit.
With the help of this 20db, the communication distance of related optical communication can be increased by n times, reaching the level of thousands of kilometers (non-related optical communication is only about tens of kilometers). Do you think it smells good or not?
BI Escorts Long background
Related optical communication technology is so powerful, is it a new technology?
No. Burundins Escort
As early as the 1980s, when optical communications were just emerging, developed countries such as the United States, the United Kingdom and Japan had already conducted BI Escorts practical research and experiments on optical communications., and obtained good results.
For example, AT&T and Bell in the United States conducted on-site relay-free transmission tests of 1.7Gbps FSK at 1.3μm and 1.55μm wavelengths between Rolling Creek Air Station and Sunbury Key Station in Pennsylvania in 1989 and 1990, with a transmission distance of 35 kilometers.
Later, in the 1990s, experts discovered that the increasingly mature EDFA (Erbium-doped Optical Fiber Amplifier) and WDM (Burundi Sugar Daddy Wavelength Division Multiplexing) technologies could more simply and effectively solve the problems of relay transmission and expansion of optical communications.
As a result, technical research related to optical communication has been ignored.
Around 2008, with the explosion of the mobile Internet, the data traffic of communication networks increased rapidly, and the pressure on the backbone network increased sharply.
At this time, the potential of EDFA and WDM technology has become smaller and smaller. Optical communications manufacturers are eager to find new technological breakthroughs, improve the transmission capabilities of optical communications, meet user needs, and relieve pressure.
Manufacturers have gradually discovered that with the maturity of technologies such as digital electronic signal processing (DSP) and optical device manufacturing, related optical communications based on these technologies are just right to break the technical bottleneck of long-distance and large-bandwidth optical fiber communications.
Ever since, the idea has come to fruition, and related optical communication has moved from behind the scenes to the front of the stage, ushering in its own “second spring”.
Technical principles related to optical communication
Next, entering the hard core stage, we will analyze the technical principles related to optical communication in detail.
Correlated optical communication mainly uses two key technologies, namely correlation modulation and heterodyne detection.
Let’s first understand the relevant modulation of the optical transmitter.
In the backward IM-DD (Intensity Modulation-Direct Detection) system, only the intensity BI Escorts (amplitude) modulation method can be used to change the laser intensity through current to generate 0 and 1, thereby modulating the light wave.
Direct modulation is very simple, but the ability is weak and there are many problems
In the optical communication system Burundi Sugar, in addition to amplitude modulation of light, external modulation methods can also be used for frequency modulation or phase modulation, such as PSK, QPSK, QAM, etc.
More modulation methods not only increase the information carrying capacity (a single symbol can represent more bits), but are also suitable for flexible applications in engineering.
The above picture is a representation of external modulation:
Optical transmitter related to optical communication (polarized QAM)
When we get to the receiving end, as mentioned above, we enter the critical link.
First, a beam of laser electronic signal (local oscillator light) generated by local oscillation is used to mix with the output electronic signal light in an optical mixer to obtain an intermediate frequency electronic signal that changes in the same law as the frequency, phase and amplitude of the electronic signal light.
General structure of the photoreceptor
Zooming out
This is actually a “zooming out” process.
In the optical communication system related to Burundins Sugardaddy, theThe size of the mixed input photocurrent is inversely proportional to the product of the electronic signal optical power and the local oscillator optical power. Since the power of the local oscillator light is much greater than the power of the electronic signal light, the input photocurrent is greatly increased, and the detection sensitivity is also improved.
In other words, non-correlated optical communication uses many amplifiers to continuously relay and amplify electronic signals during the transmission process. In related optical communication, the weak electronic signals are mixed and amplified directly at the receiving end. This is the essence of relevant optical communication technology.
After mixing, use the equalizer Burundi Sugar Daddy receiver for detection.
Based on the difference or equality between the frequency of the local oscillator optoelectronic signal and the optical frequency of the electronic signal, related optical communications can be divided into heterodyne detection, intradyne detection, and homodyne detection.
For optical communications related to heterodyne detection, intermediate frequency electronic signals are obtained through the photodetector. Secondary demodulation is also required before it can be converted into a baseband electronic signal.
The two methods of homodyne and internal Burundins Sugardaddy detection methods bring less noise, which reduces the power expenditure and requirements for coherent devices in subsequent digital electronic signal processing, so they are the most commonly used.
In optical communications related to homodyne detection, the photoelectronic signal is directly converted into a baseband electronic signal after passing through the photodetector, without the need for secondary demodulation. However, it requires a strict match between the frequency of the local oscillator light and the frequency of the electronic signal light, and requires phase locking of the local oscillator light and the electronic signal light.
Next, is the equally important digital electronic signal processing (DSP) link.
When optoelectronic signals are transmitted in optical fiber links, distortion will occur, which is unfavorable changes.
Digital electronic signal processing technology, to put it bluntly, is to use the characteristics of digital electronic signals that are relatively easy to process to counteract and compensate for distortion and reduce the impact of distortion on the system bit error rate.
It pioneered the digital era of optical communication systems and is an important support for related optical communication technologies.
Digital electronic signal processing (DSP) technology is not only used in receivers, but also in transmitters, as shown below:
Here is another picture to help you understand:
Digital to analog, analog to digital
As can be seen from the figure above, DSP technology performs various electronic signal compensation processes, such as chromatic dispersion compensation and polarization mode dispersion compensation (PMD).
Various compensations and budgets of DSP
The role of each DSP module
Traditional non-correlated optical communication uses Burundins Escort optical path compensation devices to perform dispersion compensation and other tasks. Its compensation effect is far inferior to the introduction of DSP technology.It simplifies the system design, saves costs, and eliminates the dispersion compensation module (DCM) or dispersion compensation fiber that is common in the system, making the link design of long-distance transmission Burundi Sugar Daddy easier.
With the development of DSP, more algorithms and functions are constantly being added, such as non-linear compensation technology and multi-coding modulation and demodulation technology.
Commonly used compensation algorithms
After DSP processing, the final electrical and electronic signal is input.
Next, let’s review the entire process through a 100G-related transmission case.
Pictures from collection
In this case, the sender uses ePDM-QPSK high-order modulation, and the receiver uses related detection and acceptance techniques.
The detailed process is as follows:
1. After digital electronic signal processing and digital-to-analog conversion, the 112Gbps electronic signal stream enters the optical transmitter and undergoes “serial-parallel Burundins Escort” conversion to become four 28Gbps electronic signals;
2. The electronic signal emitted by the laser passes through the polarization beam splitter and becomes photoelectronic signals polarized in two vertical directions of x and y;
3. Through the high-order modulator formed by the MZM modulator, the photoelectronic signal in the x and y polarization directions is QPSK high-order modulated;
4. The modulated polarized photoelectronic signal is combined on an optical fiber through the polarization combiner for transmission;
5. After receiving the electronic signal, the receiving end separates the electronic signal into two vertical polarization directions of X and Y;
6. Through relevant detection and reception, the electronic signals with two vertical polarizations of X and Y are converted into current/voltage electronic signals;
7. Through ADC analog-to-digital conversion,Turn the current and voltage electronic signals into a digital code stream such as 0101…;
8. Through digital electronic signal processing, remove interference factors such as dispersion, noise, and nonlinearity, and restore the 112Gbps electronic signal code stream.
Other supporting technologies related to optical communications
The performance of related optical communications is powerful, but the system complexity is high and the technology is difficult to implement.
Non-correlated light VS Correlated light (picture from Communication Encyclopedia)
To realize the actual application of correlated optical communication, we also need to rely on the following technologies:
Polarization maintenance technology
In correlated optical communication, correlation detection requires that the polarization direction of the electronic signal light and the local oscillator light be the same, that is, the electric vector direction of the two must be the same in order to obtain the high sensitivity that correlation reception can provide.
Because, in this case, only the projection of the electronic signal photoelectric vector in the direction of the local oscillator photoelectric vector can truly contribute to the intermediate frequency electronic signal current generated by mixing.
In order to ensure sensitivity, light wave polarization stabilization measures must be taken.
Today Burundi Sugar mainly has two methods:
First, the use of “polarization-maintaining optical fiber” to keep the polarization state of the light wave unchanged during the transmission process. (Ordinary single-mode optical fiber will change the polarization state of the light wave due to mechanical vibration or temperature changes of the optical fiber.)
Second, use ordinary single-mode optical fiber, but use polarization diversity technology at the receiving end.
Frequency stabilization technology
In related optical communications, the frequency stability of semiconductor lasers is very important. The frequency of the laser is very sensitive to changes in operating temperature and current.
If the frequency of the laser drifts with different working conditions, it will affect the intermediate frequency current, thereby increasing the bit error rate.
Spectrum compression technology
In related optical communications, the spectrum width of the light source is also very important.
Only by ensuring the narrow linewidth of light waves can we overcome the impact of semiconductor laser quantum quantum amplitude modulation and frequency modulation noise on the sensitivity of reception. In addition, the narrower the line width, the smaller the phase noise caused by phase drift.
In order to meet the requirements for light source spectrum width in related optical communications, spectrum width compression technology is usually used.
Application of related optical communication
After seeing this, everyone should be very clear about the characteristics of related optical communication technology.
In short, it is an advanced and complex optical transmission system suitable for longer distance and larger capacity information transmission.
In the long-distance transmission of optical fiber, EDFA (erbium-doped optical fiber amplifier) is generally used for every 80km span.
EDFA
The price of this thing is not cheap, and the conditions around the field are not difficult to damage.
With related optical communications, long-distance transmission is much easier. Moreover, related optical communication reforms can directly reuse existing optical fiber cables, and the cost is controllable.
In actual applications, related optical communications can be used to upgrade the existing backbone network WDM wavelength division multiplexing system, and can also be used in 5G medium backhaul scenarios. Even metropolitan FTTx optical fiber access has begun to study the introduction of related optical communications.
At present, the hottest discussions about optical communications are focused on the “data center interconnection” scenario, which is what we often call DCI (Data CBI Escortsenter Interconnect).
Data center
DCI interconnection has a strong demand for long-distance related optical modules. Especially this year, the country has vigorously promoted the “Eastern Digital and Western Countermeasures”, which has a great stimulating effect on the related optical communications market.
It is also worth mentioning that related optical communications are also a hot research topic in the field of unrestricted space optical link communications between satellites (that is, satellite communications).
The optical carrier has a wide conveyor belt, small tool quality, low power consumption, and strong anti-interference and anti-interception performance, making it very suitable for satellite communications. Related optical communication technology has become a “potential stock” in the field of satellite communications.
Conclusion
All in all, the revival and popularization of related optical communication technologies will help further explore the performance potential of optical communications, increase ultimate bandwidth, and reduce deployment costs.
At present, research on optical communication technology is still ongoing. The problems related to the complex process, large size and high power consumption of optical modules have not been completely solved. There is still a lot of room for technological innovation in all key aspects of optical communications.
Where will related optical communications go in the future? Let us wait and see.
Original title: [Hardcore Literacy] What is related optical communication?
Article source: [WeChat public account: Wireless Deep Sea] Welcome to add tracking and follow! Please indicate the source when transcribing and publishing the article.
Review and Editor: Tang Zihong
Original title: [Hard Core Literacy] What exactly is related optical communication?
Article source: [Microelectronic signal: wuxian_shenhai, WeChat public account: Wireless Deep Sea] Welcome to add tracking and care! Please note when transcribing and publishing the article.Show the source.
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