FOA Guide 

Topic: Coherent Fiber Optic Communications

Table of Contents: The FOA Reference Guide To Fiber Optics

Coherent Communications Fiber Optic Data Links

The purpose of this document is to define a “coherent" fiber optic datalink, its purpose, design and performance. It is intended to provide guidance for the designer of datalinks or communications systems and the installer of fiber optic systems who must verify the performance of the datalink including the "fiber characterization" of the cable plant installed for its operation.

Coherent communications are used for many systems above 100Gb/s, and almost exclusively for OSP long haul links. Laser sources have a limit on the speed of direct modulation, typically around 25Gb/s, limiting the lower cost, simpler direct modulation data links to that speed. Higher speeds are obtained by wavelength division multiplexing of multiple 25Gb/s signals on singlemode fiber or, in the case of some premises multimode links, running parallel fibers of 25Gb/s signals. Coherent transmission is much more complicated but becomes cost effective for many OSP links because of its superior reach without amplification, although it is compatible with fiber amplifiers when necessary to extend link length.

Coherent transmission is compatible with all types of fiber, but on long links, the usual G.652 fiber is usually replaced with G.654 fiber with large area cores to obtain the lowest possible loss.

Datalinks - Direct Modulation vs. Coherent

A fiber optic datalink is a communications subsystem that connects inputs and outputs (I/O) from electronic subsystems and transmits those signals over optical fiber.
A fiber optic datalink consists of fiber optic transceivers or individual transmitters and receivers at either end that transmit over optical fibers.

Direct Modulation
fiber optic data link labelled
A direct modulation fiber optic datalink gets an electrical pulse input from an electronic system. In the transmitter, a source driver sends current through an optical source, typically a laser, which creates a pulse of light. The pulse of light from the source is coupled into an optical fiber that is part of a fiber optic cable plant. The pulse travels down the fiber where it is attenuated by the fiber and suffers loss from fiber joints created by splices or connections. As it travels along the fiber, it suffers dispersion, both chromatic dispersion caused by the fiber interaction with the wavelength and spectral width of the source, and polarization mode dispersion, caused by the variation in the polarization characteristics of the fiber. At the receiver, the light pulse is converted to an electrical pulse by a photodetector, amplified by the receiver circuitry and converted to an electrical pulse compatible with the communications equipment it connects.

fiber optic transceiver

Direct modulation datalinks are typically limited by the ability to directly modulate lasers, around 25Gb/s, and the bandwidth of receivers.
Higher speed links are obtained by wavelength division multiplexing of multiple 25Gb/s signals on singlemode fiber or, in the case of some premises multimode links, running parallel fibers of 25Gb/s signals. More on fiber optic transceivers and their components 
Coherent Transmission
Coherent transmission uses much more complex transceivers. Coherent transmitters overcome the limited bandwidth of lasers when directly modulated by leaving the laser on all the time and modulating it externally using two electro-optical modulators. The modulators are not just making ones and zeroes, but produce signals of 2 or 4 amplitudes to encode more than one bit of data in a single pulse. The outputs of the two modulators are coupled as two beams of different polarization which are multiplexed on the same fiber. Thus the coherent transmitter can encode optical data in the amplitude of the signal pulse and in the polarization in the fiber, making it possible to encode very high data rates.

coherent fiber optic transmitter

Coherent receivers use a technique used for electronic and wireless transmission called a hetrodyne circuit that uses a local oscillator to mix with an incoming signal to demodulate the signal using techniques related to the beats in acoustic mixed signals. Virtually all RF signal transmission uses hetrodyne techniques. Hetrodyne techniques are more complex with optics than electronics, so the development of coherent receivers took many years of R+D effort.

coherent fiber optic receiver

Two other technologies were needed to facilitate coherent optical communications, both electronic: high speed analog-to-digital (A/D)/digital-to-analog (D/A) conversion and digital signal processing (DSP). In the coherent transmitter, the incoming electronic signal is manipulated digitally to change the shape of the signal to make it remove some distortions that will occur in the transmission through the fiber. In the receiver, after the incoming signal is split into the two polarization components, mixed with the local oscillator and converted to the electrical domain, it is digitized and DSP algorithms are used to correct signal degradation due to chromatic dispersion (CD) and polarization mode dispersion (PMD) and extract the amplitude modulation to reconstruct the signal. Since the DSP must extract amplitude data from the waveform which can be affected by noise, optical signal to noise ratio (OSNR) is important. For that reason, coherent systems often use ultra-low loss fiber like G.654 with a larger mode field diameter. 

As can be seen from the block diagrams of coherent transmitters and receivers, they are much more complex than direct modulation transceivers. While a direct modulation transceiver can be miniaturized to the size of a SFP plug-in module, a coherent transmitter and receiver is typically the size of a line card. Coherent
transmitters and receivers are also much more costly than direct modulation transceivers but their advantages for higher speeds and/or longer distances make them the logical choice.

Coherent links not only provide for higher speeds and longer reach, but the DSP can also overcome some of the dispersion problems created by transmission over long lengths of fiber. CD is relatively easy to compensate since it is predictable and constant. PMD compensation is performed by the receiver DSP, but PMD is more problematic. Since fiber polarization has a dependence on the physical condition of the fiber and cable, PMD tends to be variable with external stress on the fiber, like wind on aerial cable, lighting strikes or large ground vibrations such as earthquakes. Coherent receivers can respond to changes in PMD within limits, but large short period variations in PMD can be a problem.

As with other high speed links, the performance of the cable plant is important to proper operation of the data link. For direct modulation high speed long distance links, the CD and PMD of the fiber are important because while CD can be compensated, PMD cannot. Comprehensive testing called fiber characterization is done on the cable plant for direct modulation systems to ensure the fiber is within spec for the planned datalinks. One might think that fiber characterization would be unnecessary for coherent links, but that is not true. A 100G coherent system can operate with fiber CD and PMD performance adequate for 10G direct modulation, but that still means many older fibers need the thorough testing required for fiber characterization, including, of course, inspection of all connectors to ensure they are in good condition and clean.

Coherent communications used on long distance high speed links work best on ultralow loss fibers which have large mode field diameters, e.g. G.654 fiber. Coherent  links are sensitive to optical signal-to-noise ratio (OSNR), so the lower loss in the cable plant is preferable. As a result, when characterizing fibers, accurate loss measurements of these links is important also.

Table of Contents: The FOA Reference Guide To Fiber Optics


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