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Introduction to Coherent Optics



With the improvement of single channel transmission rate, coherent optics is used in more and more application scenarios in modern optical communication. This article will tell what is coherent optics, its development history, features, and advantages, also discuss the development trend of pluggable coherent optical transceivers.

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


Working Principle of Coherent Optical Communication


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


Coherent vs Noncoherent Optics



Non-coherent Optics

Coherent Optics

Modulation and Detection  Technology

Transmitter: intensity modulation

Receiver: direct detection 

Transmitter: external modulation

Receiver: Local oscillator coherent detection

Modulation Code

Amplitude modulation(RZ/NRZ/ODB)

Differential phase modulation(DQPSK)

Phase modulation(BPSK/QPSK)

Quadrature amplitude modulation(QAM)

Spectral Efficiency


the frequency and phase information of optical carrier cannot be utilized, and the bandwidth capacity of single channel is limited


It can detect the amplitude, frequency and phase of the optical signal, and the single channel bandwidth is large

Dispersion Tolerance


DCM is required for dispersion compensation


Using DSP to offset fiber dispersion


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 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.



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 and contact QSFPTEK tech support.



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