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In one of our previous articles we looked at key standards and form factors for optical transceivers. Here we will take a look at coherent transceivers. Coherent optics use modulation of the amplitude and phase of the light, as well as transmission across two polarizations, to enable the transport of considerably more information through a fiber optic.

Developments in materials for Photonic Integrated Circuits (PIC), lasers and Central Processing Units (CPUs) have benefited coherent detection. These advances have contributed to shape the foundation for higher bandwidth and greater reach applications, e.g. subsea and transcontinental. Knowledge developed in these high cost, high value applications is changing the economics of coherent detection so the advantages of coherent optics can be applied to cost sensitive and short reach applications.

Encoding & Decoding

There are two key approaches for encoding and decoding links for optical networks, direct, and coherent detection. These methods share some basic properties, fundamentally optical transceivers contain a transmitter and a receiver, the transmitter and receiver encode 0s and 1s as light pulses. For example, the absence of laser light (off) is shown by 0 and the presence of light (on) is shown by a 1. In a direct detect transceiver the transmitter state is either on or off.

The 1 or 0 is decoded by the receiver and to send more data the laser needs to flash on and off faster. As data rates and distances increase it then starts to become challenging for the receiver to decide if the broadcasting laser is on or off. This is due to a number of causes such as the ability to control the laser at higher data rates and also due to dispersion. The requirement for greater distances and increased data rates is and ever greater need and this is pushing advancements in coherent detection.

Greater pressure for increased bandwidth has also pushed the design and acceptance of Dense Wave Division Multiplexing (DWDM). Initially DWDM networks used detect transceivers, with each one given a unique wavelength. Numerous wavelengths are transmitted on the same fiber without obstructing each other. This technique has been the main approach to increase bandwidth to networks across cities and regions.

With data rates increasing and additional wavelengths added to fibers, there becomes a time when limits of physics are reached for both laser receiving / transmitting and the number of DWDM wavelengths. Coherent detection contributed to resolve this problem by increasing the bandwidth open in every wavelength. They are now the main technology for improving bandwidth on DWDM networks for both short and long reach.

Three qualities of light to improve bandwidth

With Coherent detection the benefits are increased date rates and greater transmission distances. It does this by carrying three dimensions of light as listed below, so it is possible to send more bits (1s, 0s) per second:

1. Amplitude

This can be thought of as a wave, it will have a height and a depth, measured from the central point between the height and the depth. The top of the wave might signify a 1 and the bottom a 0. Waves can be small or large and the depth or height of the wave can signify a different symbol. A small depth my be a 00 and a small height may be a 11. Through changing the amplitude (depth or height) of the wave numerous bits (symbols) can be broadcasted (encoded) per second. Although one key constraint is that just one symbol is transmitted at a time.

2. Phase

Phases of a wave are utilized to include additional symbols per second. A phase is a way to split a wave into separate parts, each one represents a separate symbol. The top of the wave may be a 11 and the next part down may be a 10, close to the bottom a 01 and the bottom of the wave a 00. One wave can then correspond with four symbols to increase the bandwidth on the data link

3. Polarization

This is the third feature of light which can be applied to improve the symbol rate per second to expand the bandwidth. Think of directions on a compass from north, east, south and west. Aligning the wave in a way so the peaks and troughs are in these four directions with each one signifying a different bit stream, with distinct information separate to the others. All four can exist at the same time which then expands the number of symbols per second. This is a simple example; numerous applications of coherent transceivers use up to 16 or more polarizations to transmit even more data per second.

Digital Signal Processors

Blending amplitude, phase and polarization then provides the capability to encode a higher number of symbols per second, although this needs a substantial amount of computational capacity. The pace and difficulty of these computations has forced the development of Digital Signal Processors (DSP) with specialized chips. They use the most recent in 7nm chip technology. The algorithms required to encode and decode the light are also key as they need to blend amplitude, phase and polarization in distinctive ways to provide increased higher symbol rates per second.

Conclusion

Advancements in coherent detection is delivering higher complexity. With rigorous in-house testing and quality control at AddOn we ensure our products are capable of reaching the high data rates and long reaches required by our customers. Our teams spend a high amount of time and resources so our customers can be confident they have the highest performing products for their applications. Network design and applications, fiber characteristics, existing equipment from companies such as Cisco, Nokia, Juniper all contribute to deciding which coherent transceivers match the conditions. With our in-house experts AddOn knows the challenges to deliver our customers solutions that work now as well as the future.

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