What is Coherent Optical Communication?

Coherent optical communication is a technology in the field of optical fiber communication. Compared with traditional non-coherent optical communication, coherent optical communication has the technical advantages of longer transmission distance and larger transmission capacity. Therefore, it has received wide attention from the industry, and the research interest in it has continued to rise.

What is Coherent Light?

Before introducing coherent optical communication, let’s briefly introduce what coherent light is. We often talk about “coherence”, and everyone understands that it means “interrelated or involved”. Coherence of light means that two light waves meet the following three conditions at the same time in the process of transmission:

1. The frequency (wavelength) is the same;

2. The vibration direction is the same;

3. The phase difference is constant.

Coherent Light

Such two beams of light can produce stable interference with each other during transmission. This interference can be either constructive interference (strengthening) or destructive interference (cancellation). As shown below:

It is obvious that constructive interference can make light waves (signals) stronger.

What is Coherent Optical Communication?

Well, let’s get to the point next and talk about what is coherent optical communication. Many people may think that coherent optical communication is the use of coherent light for transmission communication, which is incorrect actually. Coherent optical communication and non-coherent optical communication basically use lasers without any essential difference in terms of light.

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 transmitting end and the use of coherent technology at the receiving end for detection.

Non-coherent Optical Communication

Coherent Optical Communication

The difference between the two is at both ends, not on the transmission path. The technology of the receiving end is the core of the entire coherent optical communication, and it is also the main reason why it is so powerful. Under the same conditions, compared with traditional non-coherent optical communication, the receiver of coherent optical communication can improve the sensitivity by 20db– 100 times more sensitive than the non-coherent communication! With the help of this 20db, the communication distance of coherent optical communication can reach the level of thousands of kilometers (non-coherent light is only about tens of kilometers).

Development Background of Coherent Optical Communication

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, in 1989 and 1990, AT&T and Bell in the US successively carried out a 1.7Gbps FSK on-site coherence transmission experiment with 1.3μm and 1.55μm wavelengths without any relay between the Rolling Creek ground station and Sunbury hub in Pennsylvania in 1989 and 1990,  and the transmission distance reaches 35 kilometers.

Later, in the 1990s, experts found that the increasingly mature EDFA (Erbium-Doped Fiber Amplifier) and WDM (Wavelength Division Multiplexing) technologies could solve the problems of relay transmission and capacity 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. Optical communication manufacturers urgently need to find new technological breakthroughs, improve the transmission capacity of optical communication, meet user needs, and relieve pressure.

Manufacturers found that with the maturity of digital signal processing (DSP), optical device manufacturing, and other technologies, coherent optical communication based on these technologies is just a good choice to break the technical bottleneck of long-distance high-bandwidth optical fiber communication. As a result, it is logical that coherent optical communication has moved from behind the scenes to the front of the stage.

Technical Principles of Coherent Optical Communication

As mentioned earlier, coherent optical communication mainly utilizes two key technologies, namely coherent modulation and heterodyne detection. Let’s first look at coherent modulation on the optical transmitter side. In the backward IM-DD (Intensity Modulation-Direct Detection) system, only intensity (amplitude) modulation can be used to modulate the light wave by changing the laser intensity through current to generate 0 and 1.

Direct modulation

Direct modulation is very simple, but it’s with a weak ability and many problems. However, 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. Additional 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 is a schematic diagram of an external modulation:

As shown in the figure, at the transmitting end, the external modulation method is adopted, and the IQ modulator based on the Mach-Zehnder modulator (MZM) is used to realize the high-order modulation format, and the signal is modulated on the optical carrier, and sent out. 

It is the key link when entering the receiving end. First, a laser signal generated by local oscillation (local oscillator light) 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 rules as the signal light.

An enlarged version of the optical receiver structure

In a coherent optical communication system, the size 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 higher than the power of the signal light, the output photocurrent is greatly increased, and the detection sensitivity is also improved.

In other words,non-coherent optical communication uses a lot of amplifiers to continuously relay and amplify the signal during the transmission process, while the essence of coherent optical communication is to mix and amplify the weak arriving signal directly at the receiving end.

After mixing, detection is performed with a balanced receiver. Coherent optical communication can be divided into heterodyne detection, intradyne detection, and homodyne detection according to the relationship between the frequency of the local oscillator optical signal and the signal optical frequency.

Classifications of coherent optical communication

In the coherent optical communication of heterodyne detection, the intermediate frequency signal is obtained by the photoelectric detector. The second demodulation is also required before it can be converted into a baseband signal. Homodyne and intradyne detection bring less noise and reduce the power overhead of subsequent digital signal processing and the requirements for related devices, so they are most commonly used. In homodyne detection coherent optical communication, the optical signal is directly converted into a baseband signal after passing through a photoelectric detector without secondary demodulation. However, it requires that the frequency of the local oscillator light and the signal light frequency be strictly matched, and the phase locking of the local oscillator light and the signal light is required.

Next, is the digital signal processing(DSP) link of great importance.

 Digital Signal Processing(DSP)

Distortion occurs when an optical signal is transmitted in a fiber optic link. DCP technology takes advantage of the easy-handle characteristic of digital signals to combat and compensate for distortion, and reduce the impact of distortion on the system bit error rate. It has created the digital era of optical communication systems and become an important support for coherent optical communication technology. DSP technology can be not only applied to receivers, but also to transmitters.

As shown below:

DSP technology

Digital to analogue and analogue to digital

As can be seen from the above figure, DSP technology performs various signal compensation processing, such as chromatic dispersion compensation and polarization mode dispersion compensation (PMD).

Various Compensation and Estimation of DSP

Module	Function
IQ quadrature	Compensate for IQ under-quadrature caused by modulators and mixers
Clock recovery	Compensate for sampling error
Dispersion compensation	Compensate for dispersion
Polarization equalization	Compensate for polarization-dependent impairments, polarization demultiplexing
Frequency estimation	Carrier Frequency Shift Estimation and Compensation at Transmitter and Receiver
Phase estimation	Carrier Phase Noise Estimation and Compensation
Decision output	Soft/hard decision, channel decoding, source decoding, bit error rate estimation

The roles of each module of DSP

The traditional non-coherent optical communication performs dispersion compensation and other functions through optical path compensation devices, whose compensation effect is far inferior to that of the DSP. The introduction of DSP technology simplifies the system design, saves cost, and eliminates the original dispersion compensation module (DCM) or dispersion compensation fiber in the system, which makes the link design of long-distance transmission simpler. With the development of DSP, more algorithms and functions are added continuously, such as nonlinear compensation technology and multi-code modulation and demodulation technology.

Compensation	Corresponding algorithm
Quadrature Unbalance Compensation	"Gramm-Schmidt Orthogonal Process(GSOP) Ellipse correction method (EC)"
Dispersion compensation	Frequency Domain Dispersive Equalizer
Polarization equalization	Constant Modulus Algorithm (CMA)
Carrier frequency offset estimation	"Estimation Algorithm based on phase difference, FFT algorithm based on sign or sign phase"
Carrier Phase Estimation	Constellation Transformation (CT) Algorithm, Blind Phase Search Algorithm (BPS), Maximum Likelihood Estimation (ML) algorithm, etc.
Nonlinear compensation	Voltera algorithm, Some neural network nonlinear compensation algorithms, etc.
Channel Error Correction Coding Algorithm	LDPC encoding, Turbo encoding, etc.

Commonly-used compensation algorithms

After DSP processing, the final electrical signal is output. Next, we review the whole process through a case of 100G coherent transmission.

A case of 100G coherent transmission

The specific process is as follows:

1. After digital signal processing and digital-to-analog conversion, the 112Gbps signal stream, after entering the optical transmitter, undergoes “serial-parallel” conversion and becomes 4 channels of 28Gbps signals;

2. The signal emitted by the laser becomes an optical signal polarized in two vertical directions of x and y through the polarization beam splitter;

3. Through the high-order modulator composed of the MZM modulator, QPSK high-order modulation is performed on the optical signal in the x and y polarization directions;

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

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

6. Through coherent detection and reception, the X and Y vertically polarized signals become current/voltage signals;

7. Through ADC conversion, the current and voltage signals are turned into digital code streams such as 0101…;

8. Through digital signal processing, the interference factors such as dispersion, noise, and nonlinearity are removed, and the 112Gbps telecommunication number stream is restored, and it’s the end.

Other Supporting Technologies for Coherent Optical Communication:

The performance of coherent optical communication is powerful, but the system is very complex and it’s hard to make the technology happen.

	Non-coherent communication	Coherent communication
Definition	Optical transmission system that do not require coherent local oscillator light	Optical Transmission System Using Local Oscillator for Coherent Detection
Modulation and demodulation technology	Transmitter: Intensity Modulation Receiver: direct detection	Transmitter: External modulation Receiver: local oscillator optical coherent detection
Optical format	Amplitude Modulation (RZ/NRZ/ODB) Differential Phase Modulation (DQBSK)	Phase Modulation (BPSK/QPSK) Quadrature Amplitude Modulation (QAM)
System structure	Simple Easy to implement and integrate	Complex High technical requirements
Spectral efficiency	Low The frequency and phase information of the optical carrier cannot be exploited; Single-channel bandwidth capacity is limited	High The information carried by the amplitude, frequency and phase of the optical signal can be detected;the single-channel bandwidth is high
Dispersion tolerance	Low DCM needs to be configured for dispersion compensation	High Using DSP technology to offset fiber dispersion, it can be used in very long distance to achieve DCM-free dispersion compensation
ROADM Architecture	Complex The receiving direction needs to use the demultiplexer to filter out the corresponding wavelength signal	Concise Coherent reception can select a specific wavelength from the multiplexed signal, without the need for a demultiplexing version

Non-coherent light vs. Coherent light

In order to realize the practical application of coherent optical communication, it is necessary to rely on the following technologies:

• Polarization PreservingTechnology

Coherent detection requires that the polarization directions of the signal light and the local oscillator light are the same in coherent optical communication, that is, the electric vector directions of the two must be the same, so as to obtain the high sensitivity that coherent reception can provide.

Because, in this case, only the projection of the signal light electric vector in the direction of the local oscillator light electric vector can really contribute to the intermediate frequency signal current generated by the mixing. In order to ensure high sensitivity, it is necessary to take lightwave polarization stabilization measures. There are two main methods currently:

First, the “polarization-maintaining fiber” is used to keep the polarization state of the light wave unchanged during the transmission process. (Ordinary single-mode fiber will change the polarization state of the light wave due to factors such as mechanical vibration or temperature change of the fiber.)

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

• Frequency Stabilization Technology

The frequency stability of semiconductor lasers is very important In coherent optical communication. The frequency of the laser is very sensitive to changes in operating temperature and current. If the frequency of the laser drifts with different operating conditions, it will affect the IF current, thereby increasing the bit error rate.

• Spectrum Compression Technology

The spectral width of the light source also matters in coherent optical communication. Only by ensuring the narrow linewidth of the light wave, can the influence of the quantum amplitude modulation and frequency modulation noise of the semiconductor laser on the sensitivity of the receiver be overcome. Besides, the narrower the line width, 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.

 Application of the Coherent Optical Communication

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 fibers, EDFAs (Erbium-Doped Fiber Amplifiers) are generally used for every 80km span.

EDFA

With coherent optical communication, long-distance transmission is much easier. Moreover, coherent optical communication can be transformed directly with the existing optical fiber and cable, whose cost is controllable.

Coherent optical communication can be used to upgrade the existing backbone network WDM system, and can also be used in 5G mid-backhaul scenarios. Even metro FTTx fiber access has begun to study the introduction of coherent optical communication. At present, the most heated discussion of coherent optical communication focuses on the “data center interconnection”(DCI) scenario.

Datacenter

DCI has a strong demand for long-distance coherent optical modules. Especially this year, the country vigorously promotes channeling more computing resources from the eastern areas to the less developed western regions, which has a great stimulating effect on the coherent optical communication market.

Conclusion:

All in all, the return and popularization of coherent optical communication technology are conducive to further tapping the performance potential of optical communication, increasing the limit bandwidth, and reducing deployment costs. At present, the research on coherent optical communication technology is still in progress. The problems of a complex process, large volume, 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.