Coherent Optical Communication

What is Coherent Optical Communication?

Coherent optical communication differs from regular optical communication in that coherent modulation is used at the transmitting end, and coherent technology is applied for detection at the receiving end. However, the lights during transmission are the same as in regular optical transmission. Coherent solutions do not require expensive EDFA (often deployed every 80 kilometers in long-haul fiber transmission) and can reuse existing fiber cabling, which saves system operation costs.

How Does Coherent Work?

Coherent modulation used at the transmitting end and heterodyne detection applied at the receiving end are the two key technologies of Coherent optical communication. 

In addition to Amplitude Modulation, the transmitter in coherent optical communication systems can also deploy external modulation to conduct frequency and Phase modulation, such as QAM, PSK and QPSK. The added modulation techniques extend information-carrying capability and adapt to deployment agility. 

In the transmitting end, coherent optics use the IQ modulator based on MZM and apply the external modulation to achieve advanced modulation formats. The signals are modulated to the optical carrier for output.

When the optical signals reach the receiving end, the pivotal Phase of coherent communication technology is set in motion. By mixing the laser signals from local oscillating (LO) with the input signals in the optical mixer, IF signals (that share the exact change regularly with the source signal frequency, Phase, and amplitude) are generated. After mixing, detection is performed with the balancing receiver. Then, the digital signal processor (DSP) functions to compensate for the distortion of optical signals during transmission over optical fiber links. After DSP processing, the final electrical signal is output.

Simply put, coherent amplifies the signals when they reach the receiver. However, in un-coherent communications, the signals are amplified and repeated in the transmitting process.

What are the Types of Coherent?

By Local Oscillator (LO) optical signal frequency and the equal or unequal signal frequencies, coherent optical communication can be classified into three types: Heterodyne detection, intradyne detection, and homodyne detection.

Heterodyne detection

The photodetector acquires intermediate frequency signals through heterodyne detection, which requires further demodulation to convert them into baseband signals.

Homodyne detection and Intradyne detection

Homodyne and intradyne detection bring less noise, lowering the power consumption for subsequent DSP and requirements for related components. These two approaches are more common in coherent optical communication.

Homodyne-detected signals through photodetector are baseband signals, which save secondary demodulation. However, this detection method requires the frequency of LO lights to match the signal lights strictly, and the Phase of the two should be fixed. 

What are the Technologies of Coherent?

Digital Signal Processing (DSP)

DSP technology simplifies system design, saves costs, and eliminates the Dispersion Compensation Module (DCM) and Dispersion Compensation Fiber (DCF), simplifying long-haul transmission links. Further, more algorithms and functions are added as the DSP develops, such as Nonlinear Compensation Technology and Multi-code modulation and demodulation technology.

Polarization Maintenance Technology

To maintain high sensitivity and help efficient detection, coherent optical communication requires consistent polarization direction of signal lights and LO lights, making the two electric vectors matched. Two methods can achieve Polarization Maintenance, they are: 

Using Polarization Maintaining Fiber to stabilize the polarization state of light waves, protecting them from interference in the transmission environment.

Applying standard single-mode fiber and polarization diversity technology to react polarization changes.

Frequency Stabilization Technology

Lasers’ frequency stability is vital since coherent optical communication relies on stable frequency to reduce Bit Error Rate (BER). However, the frequency of lasers drifts with temperature and current. This is where the Frequency Stabilization Technology comes in.

Spectrum Compression Technology

In coherent optical communication, the narrow spectral width of the light source is also essential. Narrow-linewidth can help reduce the sensitivity loss caused by Quantum noise and lower the Phase noise. It is common to adopt Spectrum Compression Technology to optimize signal reliability to meet the requirements of light source spectral width for coherent optical communication.

What are the Applications of Coherent Optical Communication?

Coherent is an advanced but complex optical communication system used for data transmission over longer distances and with greater capacity.

Coherent optical communication is widely used in data center interconnect (DCI), backbone network WDM system upgrades, and 5G mid-haul and backhaul scenarios. Other niche fields like FTTx and satellite communications are also being researched.