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Everything You Need to Know About Coherent Optical Modulation
- Benefits of complex modulation
- No more limits to spectral efficiency
- Shannon-Harley -theorem
- Complex Coding Concepts for Increased Optical Bit, Transfer Efficiency
- Which Modulation Scheme Best Files My Application?
- Time Domain Pulse Shaping for Increased Spectral Efficiency
- An Optical Transmitter for Every Need
- How to Detect Complex Modulated Optical Signals
- Coherent Optical Receivers – The Complete Answer
- Quality Rating of Coherent Measurement
- The Future of Coherent Data Transmission
New data centers are being built across the globe, while today’s CPUs and RAM ensure latencies so low that it’s no problem to map immense amounts of data spread over several servers within a fraction of a second. The more critical question is whether the rest of the infrastructure can keep pace. Explosively growing amounts of data have become an enormous challenge. To avoid bottlenecks in the near future, the bit-rate efficiency needs to increase at every stage of the data journey.
Benefits of complex modulation
Optical data transport started out like its electronic counterpart, with the simplest and therefore cheapest digital coding schemes: return-to-zero (RZ) or non-return-to-zero (NRZ) on/off-keying (OOK). The signal is ideally a rectangular sequence of ones (power on) and zeros (power off). But this concept faced a limitation when transfer rates reached 40 Gbps.
Due to the high clock rate at 40 and 100 Gbps, the bandwidth occupied by the OOK signal becomes larger than the bandwidth of a 50-GHz ITU channel. As can be seen in Figure 1, spectrally broadened channels start to overlap with their neighboring channel and the signals are shaped by the wavelength filters, resulting in crosstalk and degradation of the modulated information.
For this reason, a move away from OOK to more complex modulation schemes, such as differential quadrature phase-shift keying (DQPSK), is necessary for high-speed transmission. Complex modulation reduces the required bandwidth, depending on the symbol clock rate, and enables higher data rates to be transmitted in the 50-GHz ITU channel plan.
These new concepts also support compensation for chromatic dispersion (CD) and polarization mode dispersion (PMD) via digital signal processing when paired with coherent detection, which provides complete optical field information. Dispersion – an effect caused by the fact that light waves travel at different speeds depending on their frequency and polarization – leads to pulse broadening that degrades the signal if not compensated.
Dispersion is especially an issue for long fiber spans. Complex modulation schemes improve spectral efficiency by using all the parameters of a light wave for encoding information: amplitude and frequency or phase. Radio engineers have profited from this approach for many years; now it can be leveraged in the optical world.
The use of coherent detection means that complex optical modulation relieves from the need for PMD compensators or dispersion-compensating fibers and from the increase in loss and latency these elements induce.
In addition to coherent detection, complex modulation schemes can be combined with other transmission methods to transmit a data signal more efficiently over a fiber link. For example, in polarization division multiplexing (PDM), a second lightwave signal, which is polarized orthogonally to the first, carries independent information and is transmitted over the same fiber (see Figure 2). That’s like adding a second channel and doubling the transmission speed without the need for a second fiber.
Other types of multiplexing (like wavelength division multiplexing (WDM)) continue to be used. The use of pulse shaping filters, which reduce the bandwidth occupied by the signal, completes the toolset.
No more limits to spectral efficiency?
In the 1940s, the American mathematician and electronics engineer Claude Shannon, the ‘father of Information Theory’, found, that in any communication channel the maximum speed at which data can be transferred without errors can be described as independence of noise and bandwidth. He called this maximum bit rate ‘channel capacity which is largely known as ‘Shannon limit.
The channel capacity can be increased by either increasing bandwidth or by optimizing the signal-to-noise ratio (SNR = S/N). In fact, the theorem provides a theoretical maximum without giving any information about which signal concept allows to get closest to this limit.
In practice, the SNR is the fundamentally limiting factor. It is and will also in the future be the subject of ongoing optimization efforts because, for data rates beyond 100 Gbps, a better SNR performance is needed for long distances to reach the Shannon limit at a given bandwidth.