What is coherent optical communication
Coherent optical communication, the English full name is called Coherent Optical Communication, is a technology in the field of optical fiber communication. Compared with traditional incoherent optical communication, coherent optical communication has the technical advantages of longer transmission distance and larger transmission capacity, so it has attracted wide attention from all walks of life in the industry, and the research enthusiasm has continued to rise.
█What is coherent light
Before introducing coherent optical communication, let’s briefly understand what coherent light is .
The “coherent” we often say verbally, everyone understands that it means “interrelated or involved”.
The coherence of light refers to the fact that two light waves satisfy the following three conditions at the same time during transmission:
- The frequency (wavelength) is the same;
- The vibration direction is the same;
- The phase difference is constant.
coherent light
Such two beams of light can generate stable interference with each other during transmission.
This kind of interference can be either constructive (strengthening) or destructive (cancelling).
As shown below:
Apparently, constructive interference can make light waves (signals) stronger.
You can recall the famous Young’s double-slit interference experiment
█What is coherent optical communication
Well, let’s get to the point and talk about what coherent optical communication is.
Many people may think that coherent optical communication is the use of coherent light for transmission and communication.
In fact, this statement is wrong. Coherent optical communication and incoherent optical communication basically use lasers, and there is no essential difference.
The reason why coherent optical communication is called “coherent optical communication” does not depend on the light used in the transmission process, but on the use of coherent modulation at the sending end and the use of coherent technology for detection at the receiving end.
Above: Incoherent Optical Communications
Bottom: Coherent Optical Communications
The difference is at both ends, not in the transmission path
The technology at the receiving end is the core of the entire coherent optical communication, and it is also the main reason for its awesomeness.
We can start with the conclusion: under the same conditions, compared with traditional incoherent 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 limit of shot noise.
With the help of this 20db, the communication distance of coherent optical communication can be increased by n times, reaching the level of thousands of kilometers (incoherent light is only about tens of kilometers).
█Development background of coherent optical communication
Coherent optical communication technology is so powerful, is it a new technology?
Not really.
As early as the 1980s, when optical communication was just emerging, developed countries such as the United States, the United Kingdom, and Japan had already carried out theoretical research and experiments on coherent optical communication, and achieved good results.
For example, AT&T and Bell in the United States successively carried out 1.7Gbps FSK on-site non-repeater coherence at 1.3μm and 1.55μm wavelengths between Rolling Creek Ground Station and Sunbury Junction Station in Pennsylvania in 1989 and 1990. In the transmission experiment, the transmission distance reaches 35 kilometers.
Later, in the 1990s, experts discovered that the increasingly mature EDFA (Erbium-doped Fiber Amplifier) and WDM (Wavelength Division Multiplexing) technologies could solve the problems of relay transmission and expansion of optical communication more simply and effectively.
As a result, the technical research of coherent optical communication has been neglected.
Around 2008, with the outbreak of the mobile Internet, the data traffic of the communication network 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 communication manufacturers urgently need to find new technological breakthroughs to improve the transmission capacity of optical communication, meet user needs, and relieve pressure.
Manufacturers have gradually discovered that with the maturity of technologies such as digital signal processing (DSP) and optical device manufacturing, coherent optical communication based on these technologies is just suitable for breaking the technical bottleneck of long-distance and large-bandwidth optical fiber communication.
So, logically, Coherent Optical Communication moved from behind the scenes to the front of the stage, ushering in its “second spring”.
█Technical principles of coherent optical communication
Next, enter the hard core stage, let’s analyze the technical principles of coherent optical communication in detail.
Mr. Xiaozao told everyone earlier that coherent optical communication mainly uses two key technologies, namely coherent modulation and heterodyne detection .
Let’s first look at the coherent modulation on the optical transmitter side.
In the previous article ( link ), Mr. Xiaozao introduced the content of optical carrier modulation.
As I said, in the backward IM-DD (intensity modulation-direct detection) system, only the intensity (amplitude) modulation can be used to change the laser intensity through the current to generate 0 and 1, so as to modulate the light wave.
Direct modulation, very simple, but weak ability and many problems
In a coherent optical communication system, in addition to amplitude modulation of light, external modulation can also be used to perform frequency modulation or phase modulation, such as PSK, QPSK, and QAM.
More modulation methods not only increase the information carrying capacity (a single symbol can represent more bits), but are also suitable for flexible engineering applications.
The picture below is a schematic diagram of external modulation:
Optical Transmitters for Coherent Optical Communications (Polarized QAM)
As shown in the figure, at the sending end, an external modulation method is adopted, and an IQ modulator based on a Mach-Zehnder modulator (MZM) is used to realize a high-order modulation format, and the signal is modulated onto an optical carrier and sent out. (For the specific principle, please refer to the link of the article just now: link )
At the receiving end, as mentioned above, it has entered a critical link.
First, a laser signal (local oscillator light) generated by a local oscillator is used to mix with the input signal light in an optical mixer to obtain an intermediate frequency signal whose frequency, phase and amplitude change according to the same law as the signal light.
The general structure of the optical receiver
is zoomed in
This is actually a “magnification” process.
In a coherent optical communication system, the magnitude of the output photocurrent after coherent mixing is proportional to the product of the 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 signal light, the output photocurrent is greatly increased, and the detection sensitivity is also improved accordingly.
In other words, incoherent optical communication uses many amplifiers to continuously relay and amplify signals during transmission. In coherent optical communication, the weak arrival signal is mixed and amplified directly at the receiving end. This is the essence of coherent optical communication technology.
After mixing, detection is performed with a balanced receiver.
According to the unequal or equal frequency of the local oscillator optical signal and the signal optical frequency, coherent optical communication can be divided into heterodyne detection, intradyne detection, and homodyne detection.
In heterodyne detection coherent optical communication, the intermediate frequency signal is obtained by the photoelectric detector. It also needs to be demodulated twice before it can be converted into a baseband signal.
The two methods of homodyne and intradyne detection bring less noise, which reduces the power overhead of subsequent digital signal processing and the requirements for related devices, so they are the most commonly used.
In homodyne detection coherent optical communication, the optical signal is directly converted into a baseband signal after being passed through a photoelectric detector, without secondary demodulation. However, it requires a strict match between the frequency of the local oscillator and the frequency of the signal light, and the phase locking of the local oscillator light and the signal light.
Next, is the equally important digital signal processing (DSP) link.
When an optical signal travels through a fiber optic link, it is subject to distortion, that is, an unfavorable change.
Digital signal processing technology, to put it bluntly, is to use the characteristics of digital signals that are relatively easy to process, to counteract and compensate distortion, and reduce the impact of distortion on the bit error rate of the system.
It ushers in the digital era of optical communication systems and is an important support for coherent optical communication technology.
Digital signal processing (DSP) technology is used not only in receivers but also in transmitters. As shown below:
Another picture to help understand:
Digital to Analog, Analog to Digital
As can be seen from the above figure, DSP technology performs various signal compensation processes, such as chromatic dispersion compensation and polarization mode dispersion compensation (PMD).
Various compensations of DSP and estimation
of the function of each module of DSP
In traditional incoherent optical communication, it is necessary to perform work such as dispersion compensation through optical path compensation devices. Its compensation effect is far inferior to that of DSP.
The introduction of DSP technology simplifies system design, saves costs, and eliminates the original dispersion compensation module (DCM) or dispersion compensation fiber in the system, making the link design of long-distance transmission easier.
With the change and development of DSP, more algorithms and functions are constantly added, such as nonlinear compensation technology and multi-code modulation and demodulation technology.
Common Compensation Algorithms
After DSP processing, the final electrical signal is output.
Next, we review the whole process through a case of 100G coherent transmission .
The picture comes from the Internet
In this case, the sending end adopts ePDM-QPSK high-order modulation, and the receiving end adopts coherent detection receiving technology.
The specific process is as follows:
- After digital signal processing and digital-to-analog conversion, the 112Gbps signal code stream enters the optical transmitter and undergoes “serial-parallel” conversion to become 4-channel 28Gbps signals;
- The signal emitted by the laser passes through the polarization beam splitter and becomes an optical signal polarized in two perpendicular directions of x and y;
- Through the high-order modulator composed of MZM modulator, QPSK high-order modulation is performed on the optical signals in the x and y polarization directions;
- The modulated polarized light signal is combined to an optical fiber through a polarization combiner for transmission;
- After receiving the signal, the receiving end separates the signal into two vertical polarization directions of X and Y;
- Through coherent detection and reception, the signals polarized in the two vertical directions of X and Y become current/voltage signals;
- Through ADC analog-to-digital conversion, the current and voltage signals are converted into digital code streams such as 0101…;
- Through digital signal processing, remove dispersion, noise, nonlinear and other interference factors, restore the 112Gbps telecom number stream, and end.
█Other supporting technologies for coherent optical communication
The performance of coherent optical communication is powerful, but the system complexity is high, and the technical implementation is difficult.
Incoherent light VS coherent light (picture from Communication Encyclopedia)
To realize the practical application of coherent optical communication, it also depends on the following technologies:
Polarization Maintaining Technology
In coherent optical communication, coherent detection requires that the polarization direction of the signal light and the local oscillator light be the same, that is, the direction of the electric vector of the two must be the same, in order to obtain the high sensitivity that coherent reception can provide.
Because, in this case, only the projection of the signal photoelectric vector in the direction of the local oscillator photoelectric vector can truly contribute to the intermediate frequency signal current generated by frequency mixing.
In order to ensure the sensitivity, it is necessary to take measures to stabilize the polarization of the light wave.
There are currently two main methods:
First, use “polarization maintaining fiber” to keep the polarization state of the light wave unchanged during the transmission process. (Ordinary single-mode fiber will change the polarization state of light waves due to factors such as mechanical vibration or temperature changes of the fiber.)
Second, use ordinary single-mode fiber, but use polarization diversity technology at the receiving end.
Frequency Stabilization Technology
In coherent optical communication, 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 coherent optical communication, the spectral width of the light source is also very important.
Only by ensuring the narrow linewidth of the light wave can the influence of semiconductor laser quantum amplitude modulation and frequency modulation noise on receiver sensitivity be overcome. Moreover, the narrower the linewidth, the smaller the phase noise caused by the phase drift.
In order to meet the requirements of coherent optical communication on the spectral width of the light source, spectral width compression technology is usually adopted.
█Applications of coherent optical communication
Seeing this, everyone should have a good understanding of the characteristics of coherent optical communication technology.
In short, it is an advanced and complex optical transmission system suitable for longer distance, higher capacity information transmission.
In the long-distance transmission of optical fiber, EDFA (erbium-doped fiber amplifier) is generally used for every 80km span.
EDFA
The price of this thing is not cheap, and the wild environment is easy to break
With coherent optical communication, long-distance transmission is much easier. Moreover, the transformation of coherent optical communication can directly use the existing optical fiber and cable, and the cost is controllable.
In practical applications, coherent optical communication can be used to upgrade the existing backbone network WDM wavelength division multiplexing system, and can also be used in 5G medium and backhaul scenarios. Even metropolitan FTTx optical fiber access has begun to study the introduction of coherent optical communication.
At present, the hottest discussion on coherent optical communication focuses on the “Data Center Interconnect” scenario, which is what we often call DCI (Data Center Interconnect).
data center
DCI interconnection has a strong demand for long-distance coherent optical modules. Especially this year, the country vigorously promotes “counting in the east and calculating in the west”, which has a great stimulating effect on the coherent optical communication market.
It is also worth mentioning that coherent optical communication is also a research hotspot in the field of inter-satellite free space optical link communication (that is, satellite communication).
The optical carrier has large transmission bandwidth, small mass and volume, low power consumption, strong anti-interference and anti-interception performance, and is very suitable for satellite communication. Coherent optical communication technology has become a “potential stock” in the field of satellite communication.
