[Hard-core literacy] What exactly is coherent optical communication?

introduction:

Hello everyone, I am Xiaozaojun. In this article today, let’s talk about a “net celebrity” technology — coherent optical communication.

Coherent optical communication, whose full English name is 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. Therefore, it has attracted widespread attention from all walks of life in the industry, and research enthusiasm is constantly increasing.

█What is coherent light

Before introducing coherent optical communication, let's first briefly understand what coherent light is .

We often say "coherence", which everyone understands means "mutually related or involved".

The coherence of light refers to the fact that two light waves meet the following three conditions at the same time during transmission:

1. The frequency (wavelength) is the same;

2. The vibration direction is the same;

3. The phase difference is constant.

Coherent Light

These two beams of light can produce stable interference with each other during transmission.

This interference can be either constructive interference (reinforcement) or destructive interference (cancellation).

As shown below:

Obviously, constructive interference can make light waves (signals) stronger.

You can recall the famous Young's double-slit interference experiment

█What is coherent optical communication?

Okay, let’s get down to business and talk about what coherent optical communication is.

Many people may think that coherent optical communication is the use of coherent light for transmission communication.

In fact, this statement is wrong. Both coherent optical communication and incoherent optical communication basically use lasers, and there is no essential difference between them.

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 communication

Below: Coherent optical communication

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 the main reason for its awesomeness.

We can first state the conclusion: under the same conditions, compared with traditional incoherent optical communication, the receiver of coherent optical communication can improve the sensitivity by 20db .

What is 20db? 100 times!

This improvement is quite amazing and is close to the shot noise limit.

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). Isn’t that great?

█ 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 communications were just emerging, developed countries such as the United States, Britain and Japan had already conducted theoretical research and experiments on coherent optical communications and achieved good results.

For example, in 1989 and 1990, AT&T and Bell in the United States conducted field non-relay coherent transmission experiments of 1.7Gbps FSK at 1.3μm and 1.55μm wavelengths between the Rolling Creek ground station and the Sunbury hub station in Pennsylvania, with a transmission distance of 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 relay transmission and capacity expansion problems of optical communications more simply and effectively.

As a result, the technical research on coherent optical communication was neglected.

Around 2008, with the explosion of mobile Internet, data traffic in communication networks grew rapidly, and the pressure on backbone networks increased dramatically.

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 communications, 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 communications based on these technologies are just right for breaking the technical bottleneck of long-distance, high-bandwidth fiber-optic communications.

Therefore, it is only natural that coherent optical communication has stepped out from behind the scenes and ushered in its "second spring".

█Technical principles of coherent optical communication

Next, we will enter the hardcore stage and analyze the technical principles of coherent optical communication in detail.

As Xiaozaojun told you earlier, 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 ), Xiaozaojun 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 method can be used to change the laser intensity through the current to generate 0 and 1, thereby modulating the light wave.

Direct modulation is very simple, but weak and has many problems

In coherent optical communication systems, in addition to amplitude modulation of light, frequency modulation or phase modulation can also be performed using external 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 also are suitable for flexible engineering applications.

The following figure is a schematic diagram of external modulation:

Optical transmitter for coherent optical communication (polarization QAM)

As shown in the figure, at the transmitting end, external modulation is adopted, using an IQ modulator based on the Mach-Zehnder modulator (MZM) to implement a high-order modulation format, modulate the signal onto an optical carrier, and send it out. (For the specific principle, please refer to the article link just now: link )

When it reaches the receiving end, as mentioned earlier, it enters the critical link.

First, a laser signal (local oscillator light) generated by a local oscillator is mixed with the input signal light in an optical mixer to obtain an intermediate frequency signal whose frequency, phase and amplitude change in the same way as the signal light.

This is actually a process of "amplification".

In a coherent optical communication system, the magnitude of the output photocurrent after coherent mixing is proportional to the product of the signal light power and the local oscillator light power. Since the power of the local oscillator light is much greater than the power of the signal light, the output photocurrent increases significantly, and the detection sensitivity also increases accordingly.

In other words, incoherent optical communication uses many amplifiers to continuously relay and amplify signals during transmission. Coherent optical communication, on the other hand, directly mixes and amplifies weak arriving signals at the receiving end. This is the essence of coherent optical communication technology.

After mixing, detection is performed using a balanced receiver.

According to whether the frequency of the local oscillator light signal is unequal to or equal to the frequency of the signal light, 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, which needs to be demodulated twice before it can be converted into a baseband signal.

Both homodyne and intradyne detection methods bring less noise, reducing 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 passing through a photodetector, and no secondary demodulation is required. However, it requires that the frequency of the local oscillator light and the frequency of the signal light must be strictly matched, and that the phase of the local oscillator light and the signal light must be locked.

Next comes the equally important digital signal processing (DSP) step.

When optical signals are transmitted in optical fiber links, they are distorted, that is, they undergo unfavorable changes.

To put it simply, digital signal processing technology uses the characteristics of digital signals that are relatively easy to process to combat and compensate for distortion and reduce the impact of distortion on the system bit error rate.

It has ushered 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 in the following figure:

Here is another picture to help you understand:

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).

Traditional incoherent optical communication requires the use of optical path compensation devices to perform dispersion compensation and other tasks, but 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 simpler.

With the development of DSP, more algorithms and functions are constantly being added, such as nonlinear compensation technology and multi-coding modulation and demodulation technology.

After DSP processing, the final electrical signal is output.

Next, we review the whole process through a 100G coherent transmission case .

In this case, the transmitter uses ePDM-QPSK high-order modulation and the receiver uses coherent detection reception technology.

The specific process is as follows:

1. After digital signal processing and digital-to-analog conversion, the 112Gbps signal code stream enters the optical transmitter and undergoes "serial-to-parallel" conversion to become 4 channels of 28Gbps signals;

2. The signal emitted by the laser passes through the polarization beam splitter and becomes a light signal polarized in two perpendicular directions, x and y.

3. Perform QPSK high-order modulation on the optical signals in the x and y polarization directions through a high-order modulator composed of MZM modulators;

4. The modulated polarized light signal is combined into one optical fiber through a polarization combiner for transmission;

5. After receiving the signal, the receiving end separates the signal into two perpendicular polarization directions, X and Y;

6. Through coherent detection reception, the signals polarized in two vertical directions, X and Y, are converted into current/voltage signals;

7. Through ADC analog-to-digital conversion, the current and voltage signals are converted into digital code streams such as 0101...;

8. Through digital signal processing, interference factors such as dispersion, noise, and nonlinearity are removed, and the 112Gbps telecommunications number stream is restored. End.

█Other supporting technologies for coherent optical communications

Coherent optical communication has powerful performance, but the system is highly complex and the technology is difficult to implement.

Incoherent light vs. coherent light (Image from Communications Encyclopedia)

To realize the practical application of coherent optical communication, the following technologies are also required:

• Polarization Maintaining Technology

In coherent optical communication, coherent detection requires that the polarization directions of the signal light and the local oscillator light are the same, that is, the directions of the electric vectors 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 mixing.

In order to ensure high sensitivity, measures must be taken to stabilize the polarization of light waves.

There are 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 optical fiber will change the polarization state of the light wave due to factors such as mechanical vibration or temperature change of the optical fiber. )

Second, use ordinary single-mode optical fiber, but adopt polarization diversity technology at the receiving end.

• Frequency stabilization technology

In coherent optical communications, the frequency stability of semiconductor lasers is very important. However, the frequency of lasers is very sensitive to changes in operating temperature and current.

If the frequency of the laser drifts depending on the operating conditions, it will affect the intermediate frequency current and thus increase the bit error rate.

• Spectrum Compression Technology

In coherent optical communications, the spectrum width of the light source is also very important.

Only by ensuring the narrow line width of the light wave can the influence of quantum amplitude modulation and frequency modulation noise of the semiconductor laser on the sensitivity of the receiver be overcome. Moreover, the narrower the line width, the smaller the phase noise generated by 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.

█Application of coherent optical communication

After reading this, everyone should have a very 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 and larger capacity information transmission.

In long-distance transmission of optical fiber, EDFA ( erbium-doped fiber amplifier ) ​​is generally used every 80km span.

EDFA

This thing is not cheap, and it is easy to break in the wild environment

With coherent optical communication, long-distance transmission is much easier. Moreover, coherent optical communication transformation can directly reuse existing optical fiber cables, and the cost is controllable.

In real 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 mid-haul scenarios. Even metropolitan FTTx fiber access has begun to study the introduction of coherent optical communication.

Currently, the hottest discussion about coherent optical communications focuses on the "data center interconnect" scenario, which is what we now commonly call DCI (Data Center Interconnect).

data center

DCI interconnection has a strong demand for long-distance coherent optical modules. Especially this year, the country has vigorously promoted "Eastern Data West Computing", which has a significant stimulating effect on the coherent optical communication market.

It is also worth mentioning that coherent optical communication is also a hot research topic in the field of inter-satellite free-space optical link communication (that is, satellite communication).

Optical carriers have large transmission bandwidth, small mass and volume, low power consumption, and strong anti-interference and anti-interception performance, making them very suitable for satellite communications. Coherent optical communication technology has become a "potential stock" in the field of satellite communications.

Conclusion ​

In summary, the return and popularization of coherent optical communication technology will help further tap the performance potential of optical communication, improve the ultimate bandwidth, and reduce deployment costs.

At present, the research on coherent optical communication technology is still ongoing. The problems of complex process, large size and high power consumption of coherent optical modules have not been completely solved. There is still a lot of room for technological innovation in each key link of coherent optical communication.

Where will coherent optical communications go in the future? Let us wait and see.

—— The End ——

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references:

1. Coherent optical communication technology, Xu Fei;

2. What are coherent optical communication and incoherent optical communication? Communication Encyclopedia;

3. Do you know what coherent optical communication is? Keysight Technologies;

4. Research on polarization multiplexing and tracking technology in PM-16QAM coherent optical communication, Zhang Manli;

5. The mission and basic principles of coherent optical communication, Zhihu, Silver Witch;

6. Discussion on coherent optical transmission technology beyond 100G rate, Gigalight Communications;

7. What is coherent optical communication system? CSDN;

8. Coherent optical communication, Baidu Encyclopedia entry;