Introduction to Coherent Optics

Development Of Coherent Optics

The development of coherent optics must mention from the emergence and development of optical fiber communication. The beginning of optical fiber communication dates back to 1966, Charles K. Kao, the father of fiber optic communications, proposed quartz glass fiber as the transmission carrier, thus 

laying the groundwork for the high-speed optical fiber communication system.

And since the 1980s, optical communication has got explosive development with the reduction of fiber loss and the continuous development of optical communication components such as semiconductor lasers and photodetectors.

In the early 1990s, the most significant invention of the optical communication system is the erbium-doped fiber amplifier (EDFA), which can amplify the signal at the relay or the receiving end, laying the foundation for long-haul optical communication. Up to now, EDFA is still recognized as the most widely used, and most reliable optical amplifier technology. Around the same period, dense wavelength-division multiplexing (DWDM) technology was also developed, which fully utilizes the low absorption region of the fiber, greatly increasing the transmission capacity of optical communications. 

Based on the combination of EDFA and DWDM technology, the intensity-modulated/direct-detection (IM/DD) optical system had been widely used. IM/DD provides a low-cost and simple solution for large capacity and long-distance optical fiber transmission, so coherent optics progressed slowly for some time, because of its high cost and complex technologies.

After entering the 21st century, with the surge of network traffic, the optical communication system has higher demands for transmission capacity and quality. EDFA and WDM technology are limited, optical communication servicers urgently need to find new technological breakthroughs to improve the transmission capacity. To meet the demands, the optical communication system needs to further overcome the fiber dispersion effect, improve the transmission capacity within the limited bandwidth, and improve the receiver sensitivity to achieve a longer transmission distance. Therefore, coherent optical technology enters into a rapid development stage. Compared with the traditional IM/DD optical system, the coherent optical communication system has the advantages of high sensitivity, long transmission distance, and large communication capacity. Coherent technology improves the transmission capacity of optical communication system to a higher level.

Coherent Optics Definition

Coherent optics uses coherent modulation on the transmitter and coherent detection technology on the receiver. Compared to the conventional IM/DD communication system, coherent optical communication enables longer transmission distance and better communication quality.

The following figure shows the basic working principle of coherent optical communication. At the transmitter, using external modulation, the signal is modulated to the optical carrier in the form of amplitude modulation, phase modulation, or frequency modulation, and then send out. At the receiver, the signal is mixed with the local oscillation (optical signal generated by local optical oscillator) on the optical mixer and then detected on a balanced detector. Finally, the signal is processed by digital signal processing (DSP) and output.

Working Principle of Coherent Optical Communication

The following table concludes the major differences between noncoherent optics and coherent optics.

Coherent vs Noncoherent Optics

Advantages of Coherent Optics

• Improves the sensitivity of the receiver. In the coherent optical communication system, the output photocurrent after coherent mixing is proportional to the product of signal optical power and local oscillator optical power. Since the local oscillator power is much larger than that of the signal light, the output photocurrent increases significantly, and the detection sensitivity increases accordingly. Under the same conditions, compared with traditional incoherent optical communication, the receiver of coherent optical communication can improve the sensitivity by 20dB, equivalent improves 100 times than that of traditional incoherent optical communication.

• Enables longer transmission distance. Enhanced receiver sensitivity enables the relayless transmission distance to increase from dozens of kilometers to thousands of kilometers.

• Multiple modulation types. In traditional optical communication systems, there is only one modulation format, intensity modulation. However, in coherent optical communication, in addition to amplitude modulation, PSK, DPSK, QAM, etc. can also be used, which brings flexibility to applications. Moreover, the traditional optical receiver only responds to the optical power changes, whereas coherent detection can detect all the information carried by the amplitude, frequency, phase, and polarization state. 

• Powerful DSP technology. The coherent technology uses digital signal processing (DSP) to balance and compensate for the dispersion, polarization mode dispersion (PMD), and nonlinear effects of the optical fiber line, and improve the line dispersion tolerance and ROADM filter tolerance so that it can minimize the relay in the ultra-long range transmission.

Coherent Optical Technology

The key attributes of coherent optical technology include:

• External modulation technology: When a certain parameter of the optical carrier of the semiconductor laser is directly modulated, there will always be parasitic oscillation to other parameters. For example, ASK direct modulation is accompanied by phase change, and the modulation depth will be limited. Therefore, in the coherent optical communication system, except FSK which can use direct injection current for frequency modulation, all other methods use external optical modulation.

• Polarization maintaining technology: In order to achieve a good mixing effect and improve the receiving sensitivity, the coherent optical communication system requires the signal light and the local oscillator to have the same polarization direction. After long-distance transmission through single-mode fiber, the polarization state is random. Therefore, many polarization control methods have been proposed, such as polarization maintaining fiber, polarization controller, and polarization diversity reception.

• Frequency maintaining technology: In coherent optical communication, the frequency stability of semiconductor lasers is very important. The frequency of the laser is very sensitive to the change in working temperature and current. If the frequency of the laser drifts with different working conditions, it will affect the if current and thus improve the bit error rate.

• Spectrum compression technology: The spectral width of the light source is also very important in coherent optical communication. To overcome the influence of FM noise on receiver sensitivity, it is necessary to ensure the narrow linewidth of light waves. The narrower the line width, the smaller the phase noise caused by the phase shift.

Coherent Optical Transceiver

Coherent optical transceivers are the major solution for long-haul backbone networks and metropolitan area networks (MANs). In the very beginning, coherent optical transceivers adopted CFP form factor, which is also referred to as CFP-DCO (digital coherent optics). CFP-DCO coherent transceiver is 82mm wide and the max consumption is up to 24W. However, low consumption, small form factor, higher data rate, and low cost are always the pursuit of optical communications, so CFP2 coherent optical transceiver developed, which is also referred to as CFP2-ACO (analog coherent optics). It is 41.5m wide, and the max power consumption is 12W. There is no stopping technological progress. in recent years, smaller, higher speed, and lower power consumption form factors are introduced, such as 400G OSFP DCO and QSFP-DD DCO, as the following figure shows.

In summary, the development trend of coherent optical modules can be concluded as follows.

• High speed: From 100G/200g to 400G, and even 800G.

• Miniaturization: from 100G MSA to CFP/CFP2DCO/ACO. Today, 400G OSFP DCO and QSFP-DD DCO are also introduced.

• Low power consumption: Considering the power consumption requirements of the overall system, for example, the power consumption of QSFP-DD form factor coherent optical module products has been controlled by less than 15W.

• Standardization and interconnection: Traditionally, various equipment manufacturers use their own developed special interface boards and use private high-order modulation methods and FEC algorithms, leading to the interfaces of different manufacturers cannot be interconnected. The interconnection of coherent optical modules is the future direction of the industry.

The coherent optical modules will get a blowout growth in the next few years. QSFPTEK can provide coherent optics solutions and transceivers, welcome to visit www.qsfptek.com and contact QSFPTEK tech support.