coherent optical communication(Optical Bridge)

1、 The basic working principle of coherent optical communication

　　Coherent modulation and heterodyne detection techniques are mainly utilized in coherent optical communication. The so-called coherent modulation is to use the signal to be transmitted to change the frequency, phase, and amplitude of the optical carrier, which requires the optical signal to have a certain frequency and phase (rather thanlikeLike natural light, which has no definite frequency and phase, it should be coherent light. Laser is a type of coherent light. The so-called heterodyne detection is to use a laser generated by local oscillation to mix with the input signal light in an optical mixer, and obtain an intermediate frequency signal that changes in frequency, phase, and amplitude according to the same law as the signal light.At the transmitting end, external modulation is used to modulate the signal onto an optical carrier for transmission. When the signal light transmission arrives

At the receiving end, coherent coupling is first performed with a local oscillator optical signal, and then balanced byreceiverConduct detection. Coherent optical communication can be divided into heterodyne detection and homodyne detection based on the difference or equality between the local oscillator frequency and the signal frequency. The former optical signal is obtained after photoelectric conversionIntermediate frequency signalIt still requires secondary demodulation to be converted into a baseband signal. The latter optical signal is directly converted into a baseband signal after photoelectric conversion, without the need for secondary demodulation, but it requires strict matching between the local oscillator frequency and the signal light frequency, and requires phase locking between the local oscillator light and the signal light.

Coherent optical communication systems can divide the optical frequency band into many channels, thereby fully utilizing the optical frequency band, that is, multi-channel fiber optic communication. We know that coherent communication in radio technology has the advantage of high receiving sensitivity, and coherent optical communication technology also has this characteristic. The receiving sensitivity using this technology can be 18dB higher than that of direct detection technology. In the early days, when studying coherent optical communication, it was required to use polarization maintaining fibers as the transmission medium because the phase and polarization plane of optical signals would randomly change when transmitted in conventional fiber optic lines. To maintain the phase and polarization plane of optical signals unchanged, polarization maintaining fibers were needed. However, it was later discovered that when optical signals are transmitted in conventional optical fibers, their phase and polarization plane changes slowly, which can be corrected by using a polarization controller in the receiver. Therefore, coherent communication can still be carried out using conventional optical fibers. This discovery has brought bright prospects to coherent optical communication.

The coherent fiber optic communication system adds a local oscillation light source required for heterodyne or homodyne reception in the optical receiver. The light wave output by the light source is optically mixed with the received modulated light wave under the conditions of wavefront matching and polarization matching. The square of the sum of the signal light wave field strength and the local oscillator light wave field strength output after mixing is proportional, from which the difference frequency signal between the local oscillator light wave and the signal light wave can be selected. Due to the fact that the variation pattern of the difference frequency signal is the same as that of the signal light wave, unlike the direct detection communication method where the detected current only reflects the intensity of the light wave, various modulation methods such as amplitude, frequency, phase, and polarization can be achieved. Coherent optical communication systems can be divided into two categories based on whether the frequency of the local oscillator light wave is equal to the frequency of the signal light wave: when the difference between the frequency of the local oscillator light wave and the frequency of the signal light wave is a non-zero constant, the system is called a heterodyne receiving system; When the frequency and phase of the local oscillator light wave are the same as those of the signal light wave, it is called a homodyne receiving system. But regardless of the receiving method used, its fundamental point is heterodyne detection.

2、 Advantages of coherent optical communication systems

Coherent optical communication fully utilizes the mixing gain, excellent channel selectivity, and tunability of coherent communication methods. Based on the analysis of the basic principles of coherent optical communication systems introduced above and compared with IM/DD systems, it is concluded that coherent optical communication systems have the following advantages:

（1） High sensitivity and long relay distance

One of the main advantages of coherent optical communication is the ability to perform coherent detection, thereby improving the sensitivity of the receiver. In coherent optical communication systems, 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.

（2） Reduce the impact of fiber dispersion on the system

Using electronic equalization techniques to compensate for the dispersion effect of optical pulses in optical fibers. The transmission function of the intermediate frequency filter in heterodyne detection coherent optical communication is exactly opposite to that of the optical fiber, which can reduce the impact of fiber dispersion on the system.

（3） Good selectivity and large communication capacity

Coherent optical communication can fully utilize the low loss spectral range (1.25-1.6nm) of optical fibers, improving the information capacity of fiber optic communication systems. If coherent optical communication is used, dense frequency division multiplexing with channel spacing less than 1-10 GHz can be achieved, fully utilizing the transmission bandwidth of optical fibers and enabling ultra-high capacity information transmission.

(IV)Has multiple modulation modes

In traditional optical communication systems, only intensity modulation can be used to modulate light. In coherent optical communication, in addition to amplitude modulation of light, it is also possible to usePSK、 Multiple modulation formats such as DPSK and QAM are beneficial for flexible engineering applications. Although this increases the complexity of the system, compared to traditional optical receivers that only respond to changes in optical power, coherent detection can detect all the information carried by the amplitude, frequency, phase, and polarization state of light. Therefore, coherent detection is a holographic detection technology that traditional optical communication technology does not possess.

3、 The main key technologies in coherent optical communication systems

（1） Light source technology

The requirements for signal light sources and local oscillator light sources in coherent fiber optic communication systems are relatively high, requiring narrow spectral lines and high frequency stability. The linewidth of the light source itself will determine the low bit error rate that the system can achieve, and should be minimized as much as possible. At the same time, the frequency of the semiconductor laser is very sensitive to changes in operating temperature and injection current, with changes generally around tens of GHz/℃ and GHz/mA. Therefore, in order to maintain frequency stability, in addition to stabilizing injection current and temperature, other active frequency stabilization measures should also be taken to keep the optical frequency stable.

（2） Receiving technology

The receiving technology of coherent optical communication includes two parts, one is the receiving technology of light, and the other is the demodulation technology of various standards after intermediate frequency. The demodulation technology is actually the demodulation technology of electronic ASI, FSK, PSK, etc. There are three main types of light receiving technologies:

1. Balanced reception method. In the FSK system, due to the modulation process of semiconductor lasers, additional amplitude modulation noise is inevitably present. The use of balanced reception methods can reduce amplitude modulation noise. The main idea of the balance method is that when an optical signal enters from an optical fiber, the local oscillator light is polarized to ensure compatibility with the polarization state of the signal. The local oscillator light and the signal light are simultaneously split into two paths through a directional synthesizer and input into two identical PIN photodetectors, so that the two photodetectors output envelope signals of equal amplitude and opposite phase. After combining these two signals, the frequency modulation signal is doubled, and the parasitic amplitude modulation noise cancels each other out. The DC component also cancels out, achieving the requirement of eliminating the influence of amplitude modulation noise.

2. Phase diversity receiving method. In addition to amplitude modulation noise, if there is relative fluctuation between the phase of the local oscillator light and the phase of the signal light, phase noise will be generated, which seriously affects the reception effect. To address this impact, phase diversity method can be used to overcome phase noise. The three-phase phase diversity method mainly divides the signal and the local oscillator light into three paths. The phase of the three paths of the local oscillator light is 0, 120 °, and 240 °, respectively. Therefore, although there are random fluctuations in the relative phase between the signal and the local oscillator light, after synthesizing the three signals, they can still remain constant, which can reduce the influence of phase noise. At the same time, this technology can be used in zero difference receiving systems without using optical phase locking.

3. Polarization control technology. As mentioned earlier, the receiving end of coherent optical communication systems must require the polarization of the signal light and the local oscillator light to be the same in order to achieve good mixing effect and improve reception quality. After long-distance transmission through single-mode fiber, the polarization state of signal light fluctuates randomly. To overcome this problem, methods such as polarization maintaining fiber, polarization controller, and polarization diversity reception can be used. When light is transmitted in ordinary optical fibers, the phase and polarization plane will randomly change. Polarization maintaining fiber is a special type of fiber that maintains the same phase and polarization through process and material selection. However, this type of fiber has high losses and is very expensive; The polarization controller mainly polarizes the signal light and the local oscillator light, which has a relatively slow response speed and high requirements for loop control; Polarization diversity reception mainly utilizes the mixing of signal light and local oscillator light, and the mixed light is divided into two mutually perpendicular polarization components by a polarization beam splitter element. The two perpendicular polarization components of the local oscillator light are controlled by a polarization controller to make the two components equal in power. In this way, when the polarization randomly fluctuates in the signal light, it may cause the intermediate frequency signal of one branch to fade, but the intermediate frequency signal of the other branch still exists. Therefore, the demodulated signal obtained by this system is almost independent of the polarization of the signal light. This technology has a fast response speed and is practical, but the implementation is relatively complex.

4、 Widely used

Coherent optical communication has developed rapidly, especially for ultra long wavelength (2-10 μ m) fiber optic communication. Coherent optical communication toolsattractive force. Because in the ultra long wavelength range, the inherent properties of optical fibers are determined by Rayleigh scatteringlossIt will be further significantly reduced, so theoretically, fiber optic transoceanic communication without relay can be achieved in ultra long frequency bands. In the ultra long frequency band, the performance of direct detection receivers is poor, so coherent detection naturally becomes a better choice.

Ultra long wavelengthFiber optic communication systemIt uses ultra long wavelength optical fibers as transmission media and coherent optical communication technology to achieve ultra long distance communication. Ultra long wavelength fibers are crucial in this system. It is a more ideal transmission medium, characterized by extremely low loss and onlyquartzOne millionth of the material. Therefore, ultra long wavelength optical fibers can achieve transmission over tens of thousands of kilometers without the need for relay stations. It can significantly reduce communication costs, improve system stability and reliability, and is particularly important for underwater communication and desert areas.

With the development of fiber optic communication technology, utilizing ultra long wavelength fibers to achieve ultra long distance communication is one of the important directions for the future development of fiber optic communication. However, there are still many technical issues that need to be further addressed in ultra long wavelength fiber optic communication systems, such as material purification and drawing of ultra long wavelength fibers, the use of ultra long wavelength light sources and ultra long wavelength dry photodetectors required for coherent optical communication technology, and so on.

In addition to the above applications, coherent optical communication has also been widely used in frequency division multiplexing (FDM) cable distribution networks due to its excellent channel selectivity and sensitivity.

V. Summary

Coherent optical communication, with its advantages, has been widely used in fiber optic communication. It not only continues to develop towards higher speeds and longer distances in point-to-point systems, but also has huge market potential in underwater communication. In addition to the new high-efficiency laser, the new coherent detection technology is also the key to the development of the system. The use of new detection technology reduces the impact of the light source on the overall performance of the system. The introduction of new receiving methods such as adaptive optics and polarization diversity improves the system response speed and further improves its application.

References

[1] Mu Daosheng. Modern Fiber Optic Communication System. Science Press

[2] Yu Yang. Coherent Optical Communication and Its Applications. Science, Technology and Society

[3] Lei Zhaodi. Fundamentals of Fiber Optic Communication. University of Electronic Science and Technology of China Press

[4] Zheng Dapeng. Principles of Fiber Optic Communication. People's Posts and Telecommunications Press

[5] Yang Tongyou. Fiber Optic Communication Technology. People's Posts and Telecommunications Press