Optical modulation for High Baud Rate networks…40G/100G Speed Networks……

Last Updated: August 16, 2025

A range of newly developed fundamental communications technologies must be employed in order to reliably transmit signals of 40/43 Gbps and even 100 Gbps in the near future using telecommunications networks.

One of these technologies involves the use of higher-level modulation methods on the optical side, similar to those which have been used for many years successfully on the electrical side in xDSL broadband access technology, for example.

Until now, just one modulation method was used for transmission rates of up to 10 Gbps, namely on/off keying or OOK for short.

Put simply, this means that the laser light used for transmission was either on or off depending on the logical state 1 or 0 respectively of the data signal. This is the simplest form of amplitude modulation.

Additional external modulation is used at 10 Gbps. The laser itself is switched to give a continuous light output and the coding is achieved by means of a subsequent modulator.

Why are higher-level modulation methods with their attendant complexity needed at 40/43 Gbps? There are many reasons for this.

Every method of modulation broadens the width of the laser spectrum. At 10 Gbps this means that about 120 pm bandwidth is needed for OOK. If the transmission rate is quadrupled to 40 Gbps, the necessary bandwidth also quadruples, i.e. to around 480 pm. The greater bandwidth results in a linear increase in the noise power level in the communications channel. A four-fold increase in the noise power level corresponds to 6 dB and would result in a decrease in the minimum sensitivity of the system by this same factor. This results in a much shorter transmission range at 40 Gbps, and a consequent need for more regenerators.

Increasing the laser power in sufficient measure to compensate for the missing balance in the system compared to 10 Gbps is not possible. Nonlinear effects in the glass fiber, such as four-wave mixing (FWM), self-phase modulation (SPM), and cross-phase modulation (XPM) would also adversely affect the transmission quality to a significant degree.

Higher-level modulation methods reduce the modulation bandwidth and thus provide a way out of this dilemma.

One absolute necessity is the need to integrate the 40/43 Gbps systems into the existing DWDM infrastructure. The bandwidth required by OOK or optical dual binary (ODB) modulation only allows a best case channel spacing of 100 GHz (= approx. 0.8 nm) in a DWDM system. Systems with a channel spacing of 50 GHz (= approx. 0.4 nm) have long been implemented in order to optimize the number of communications channels in the DWDM system.

For both technologies to be integrated into a single DWDM system, the multiplexers/demultiplexers (MUX/DEMUX) would have to be reconfigured back to a channel spacing of 100 GHz and the corresponding channel bandwidths, or hybrid MUX/DEMUX would have to be installed. Both these solutions are far from ideal, since they either result in a reduction in the number of communications channels or the loss of flexibility in the configuration of the DWDM system.

Here, too, the answer is to use higher-level modulation methods that reduce the required bandwidth.

As well as other factors, the transmission quality of a communications path also depends on polarization mode dispersion (PMD) and chromatic dispersion (CD).

CD depends on the fiber and can be compensated for relatively simply by switching in dispersion-compensating fibers. However, this once again degrades the loss budget. This is within acceptable limits for realizing the usual range distances in 10 Gbps systems. But this is not the case with 40 Gbps, where the system budget is already reduced anyway. For this reason, other compensation methods must be used, subject to the additional requirement for exact compensation at all wavelengths of a DWDM system because the maximum acceptable value for CD is a factor of 16 lower than that for 10 Gbps.

The maximum acceptable PMD value for 40 Gbps is reduced by a factor of four. The PMD value is largely affected by external influences on the fiber, such as temperature and mechanical stress, and is also dependent on the quality of manufacture of the fiber itself. A requirement for any new modulation method would be a corresponding tolerance to PMD and CD.

When you take a look at the data sheets issued by systems manufacturers or in other technical publications, it is easy to be confused by the number of abbreviations used for new modulation methods.

How do these methods differ, and which of them are really suitable for future transmission speeds? Unfortunately, there is no easy answer to that either. Apart from the technical requirements, such as