Coherent optical transceivers use more sophisticated technology to heighten transmission speeds from optical signals. Where direct detection systems simply measure on and off signals of incoming data streams, coherent systems modulate amplitude, phase and polarisation of light to enable high speed, long distance data transmission with enhanced efficiency and reliability. They are used primarily in Dense Wavelength Division Multiplexing (DWDM) applications and are common in long haul data centre interconnects (DCIs) and metro networks. Here is how it works:
Coherent detection
Where traditional technologies use only the amplitude or intensity of light signals to transmit data, coherent transceivers also use the phase and polarisation of light waves to encode more data onto the same signal, allowing for transmission of more bits per clock cycle.
- Amplitude
- The intensity of the light.
- Imagined as a wave, the peaks and troughs may represent one piece of data (1 or 0); however, with different intensities more data can be encoded depending on the height of the wave e.g. a small peak may be 11, while a high peak may represent 00.
- Phase
- The position of the light wave, achieved through changing the delay between pulses
- Each section of the wave may represent a different piece of data e.g. if the wave begins at the peak it may represent 11, while partly down the wave may represent 10.
- Polarisation
- The orientation of the light wave
- The wave may be oriented differently. Information can be encoded depending on the alignment of the wave e.g. a vertical wave may be 11 whereas a horizontal wave may be 10.
- The light is not restricted to four polarisations; in fact, many coherent transceivers use 8 or 16 polarisations to further transmission speeds.
By combining these parameters, 3 bits (e.g. 010, 111, 100) can be encoded in the same time frame where direct detection would encode only one. This improves data rates eight-fold as eight combinations can be encoded with three bits.
Advanced Modulation Formats
Coherent transceivers utilise advanced modulation formats to cope with their complex design and functionality. QPSK, 8-QAM and 16-QAM are commonly used in these modules, allowing for higher capacity data transmission. Modulation of the above variables enable coherent transceivers to reach speeds of up to 800Gbps (as of November 2024), leaving space for higher speeds in the future.
Digital Signal Processing
In order to manage the computational power requirements of using these three parameters, Digital Signal Processing (DSP) is needed. DSP is a core component of coherent technology used for error correction and signal recovery, ensuring integrity of the signal. This helps coherent transceivers reach hundreds, and even thousands of kilometres (with the help of amplification) while maintaining a fast and stable signal.
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