What is Coherent Optics?

With the advancement of science and technology and the needs of market development, coherent light technology has developed from being difficult, high-cost, and high-power consumption in the early 1990s to becoming a hot topic in the field of optical communications. If you want to learn more about the development and technical characteristics of coherent light, this article will be very suitable for you.

Coherent Optics definition

Coherent light refers to two beams of light with the same frequency, stable phase difference and vibration components in the same direction.

Development Background of Coherent Optical Communication

The historical development of coherent optical communication technology can be traced back to the 1980s. Developed countries such as the United States, the United Kingdom, and Japan have begun theoretical research and experiments on coherent optical communications and have achieved remarkable results. For example, AT&T and Bell in the United States conducted relevant experiments in Rolink, Pennsylvania in 1989 and 1990. However, due to the complexity of coherent optical communication technology, high power consumption and high cost, it has not been widely used.

In the 1990s, the application of DWDM technology increased the capacity of optical fibers. At the same time, the emergence of erbium-doped fiber amplifiers (EDFA) further promoted the development of optical communication systems. Its low loss and high gain characteristics made C-band and L-band signals Being able to pass through has greatly improved the system capacity and transmission distance, and traditional optical communications have entered a stage of rapid development.

In recent years, the rise of data centers and AI has caused a surge in the amount of communication data. The potential of DWDM and EDFA technologies in traditional optical communications has been exhausted. After years of research on coherent optical communications, the cost, volume, and power consumption have also increased. Continuously reducing, it began to become a hot spot and gradually entered the stage of widespread application.

What is a Coherent Optical Fiber Communication System? 

The coherent optical communication system is a technology that uses the principle of coherent light to modulate the amplitude and phase of the two polarization states of light at the transmitting end, and demodulates it through coherent technology at the receiving end to transmit more information through optical fibers.

How Coherent Optics Deliver More Data  

In traditional optical communication technology, if you want to increase the communication capacity of optical fiber, there are two ways: one is to increase the channel bandwidth and increase the baud rate of single-channel data; but to use more fiber channels or wavelength channels. In the coherent optical communication system, in addition to the two methods of traditional optical communication, the communication rate can also be improved from two aspects: phase and polarization state.

The traditional modulation method is OOK modulation, which uses the on and off of light to represent 0 and 1, so that each baud optical signal only carries 1 bit of information. Light vector modulation uses the phase and intensity of light to simultaneously represent information. Taking QPSK as an example, there is a point every 90°, so that each baud signal carries 2 bits of information. When phase modulation is used in combination with amplitude modulation, for example, if each phase is coupled with high and low level amplitude modulation (in the x and y axis directions), then each point is split into 4 points, and It becomes 16 QAM.

Coherent optical communication uses two orthogonal polarization states of the same wavelength to transmit data respectively, doubling the transmission capacity of a single channel wavelength.

 

Coherent optics vs DWDM

DWDM divides wavelengths into different channels, increasing the number of channels and transmission capacity of a single optical fiber. However, after nearly 20 years of development, the potential of DWDM has been fully tapped. 

To continue to increase the capacity of optical fiber communications, we need to start from other aspects.

Coherent optical communication is a modulation and demodulation technology that improves single-channel transmission capacity. DWDM can also be applied to coherent optical communication systems. 

Coherent optical communication samples narrow linewidth lasers, and the optical indicators of the laser fully meet the requirements of the DWDM system; at the same time, the wavelength of the laser in coherent optical communication is adjustable, allowing the DWDM system to be used more flexibly.

100g Coherent Optics

100G CFP2-DCO Product Features：

• Available in hot-swappable CFP2 package
• Working voltage 3.3V
• Dual LC optical ports
• C-Band full-band tunable ultra-narrow linewidth laser
• TX transmit power adjustable -6.5 to + -0.5dBm
• 100G DP-QPSK modulation mode
• Integrated silicon photonic modulator and demodulator
• 100GE/OLT4 client interface
• Typical power consumption 22W (MAX 24W)
• Commercial grade operating temperature range from 0 to 70 degrees Celsius

Application:

•  100G Ethernet
•  OTN OTU4
•  Access network, metropolitan area network, long-distance DWDM system

Digital coherent optics Coherent optical modules include analog coherent ACO and digital coherent DCO. Integrate the three optical elements required for coherence (tunable laser, modulator, ICR (Integrated Coherent Receiver)) and DSP into one module to form a complete coherent transceiver. This module is called DCO, which stands for Digital Coherent Optics . Since the early optical devices and DSP technology are not yet mature, the power consumption and size are very large, and the size requirements of optical modules are getting smaller and smaller, so the three optical elements required for coherence are separately packaged into a module, and the input and output are It is an analog signal, called ACO, and the DSP is moved outside the module, and the host computer implements the DSP function. PAM4 vs. Coherent Optics Pulse Amplitude Modulation (PAM pulse amplitude modulation) is a basic pulse modulation method. PAM4 means that there are 4 types of pulse modulation amplitudes. Compared with the traditional OOK modulation method, each level of OOK only represents 1 bit of information, and each level of PAM4 represents 2 bits of information. Under the same bandwidth, the transmission rate of PAM4 can be doubled compared to OOK. Compared with coherent optical devices, PAM4 signals are more sensitive to noise. For the same system noise, the PAM4 signal has a signal-to-noise ratio loss of about 9.5dB. The PAM4 signal has 16 switching states, which will cause distortion on the rising and falling edges when the signal state transitions. Nonlinear problems in the eye diagram are also more likely to occur, resulting in bit errors. For the above reasons, PAM4 is not suitable for long-distance transmission and is only suitable for short-distance applications. Coherent Optics Advantages & Disadvantages Advantages of coherent optical communications The main advantages of coherent optical communication are higher modulation efficiency; reception uses heterodyne demodulation, which greatly improves reception sensitivity; long-distance support is possible without the need for a third-party system, and the transmission distance can reach 1,000km. Disadvantages of coherent optical communications Coherent optics are more powerful and cost more, and these disadvantages can impact overall operating costs. Another disadvantage of coherent optical modules is that at both ends of the link, two DSPs from the same vendor will be required. Different DSPs cannot work together, and in some cases the line cards must be identical. Other Supporting Technologies for Coherent Optical Communication In order to realize the practical application of coherent optical communication, the following technologies need to be relied on: 1. Coherent modulation: Use the signal to be transmitted to change the frequency, phase and amplitude of the optical carrier. 2. Heterodyne detection: A laser beam generated by local oscillation is mixed with the input signal light in an optical mixer to obtain an intermediate frequency signal that changes according to the same rules as the frequency, phase and amplitude of the signal light. 3. Polarization maintenance technology: Keep the polarization state of light unchanged and ensure the coherence of light, which is very important for long-distance transmission. 4. Frequency stabilization technology: ensure the frequency stability of the optical carrier and avoid signal distortion caused by frequency fluctuations. 5. Spectrum compression technology: Compress the spectrum through technical means to improve signal transmission efficiency. 6. Nonlinear crosstalk control technology: Controls nonlinear crosstalk between signals to ensure pure transmission of signals. Applications Due to its large communication capacity, coherent optical communication is widely used in data center interconnection to meet the growing bandwidth and distance requirements of ultra-large-scale data centers. Its transmission distance is long and it is suitable for long-distance and remote communication with metropolitan area networks, backbone networks, submarine optical cables, etc. Ultra long distance transmission. Coherent optical communications can be used directly on traditional optical networks. To expand the capacity and speed of traditional optical communication networks, it is a good choice to replace them with coherent optical communications. Conclusion Coherent optical communication is a higher-order form of optical communication technology. It improves the communication capacity and distance of optical fibers from various dimensions, such as increasing single-channel rate, amplitude modulation, phase modulation, polarization multiplexing, WDM, EDFA, etc., covering almost all possible applications. to optical communication technology. With the current application of data centers and AI, the demand for data transmission is increasing, and the market demand for coherent optical communications has also experienced explosive growth. Coherent optical communications will enable faster, more efficient and more flexible networks to support the growing needs of our connected world.