The performance of coherent optical communication is powerful, but the system is very complex and it’s hard to make the technology happen.
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.
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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:
Optical Transmitter (Coherent 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.
Optical receiver (heterodyne detection)
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:
digital signal processing(DSP) technologyDSP 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
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.
After DSP processing, the final electrical signal is output. Next, we review the whole process through 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.
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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.
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.
